Rational design and optimization of photocatalytic systems can only be achieved through understanding the reaction mechanisms involved. Time-resolved optical spectroscopy has been employed to resolve the complexities involved in photocatalytic reaction systems by identifying transient reaction intermediates and measuring the key kinetic parameters. In this Perspective, we showcase three systems that were systematically investigated as examples to demonstrate that well-designed time-resolved spectroscopic experiments can play a vital role in mechanistic investigations of photocatalytic systems while it is necessary to combine them with other analytical methods to fully resolve the complexities in these reaction processes. We summarize the commonly used methodologies and indicate the critical dynamic information that should be addressed in spectroscopic analysis. We also discuss the utilization of mechanistic insights to improve reaction performances and inspire the invention of novel photocatalysts. We foresee that the close collaboration of physical, synthetic, and materials chemists will mutually promote progress in the rapidly developing fields of photocatalysis and spectroscopy.

The innovation of photocatalytic processes requires a mechanistic understanding to provide design principles for improving the reaction performances as well as developing novel catalytic processes. Time-resolved optical spectroscopy has been serving as an essential toolbox in this collaborative exploration and has greatly promoted the field. Systematic applications of spectroscopic methods have been integrated into photocatalytic research in the past decades. It is time to summarize the research methodologies and philosophy as a guide for future investigations and motivate efforts in this area to continuously promote the implementation of new spectroscopic techniques.

Harnessing sunlight as a sustainable energy source for manufacturing valuable chemicals is one of the frontline areas of current research in chemistry and materials science. In the most ambitious explorations, researchers aim to produce solar fuels through artificial photosynthesis as a response to the grand challenge of global climate change while satisfying our increasing energy demands.1–4 Intense research efforts have also been made in the field of light-driven synthetic chemistry, which employs photogenerated intermediates to discover new bond formation approaches and construct molecular structures that are difficult to realize via conventional methods.5–10 

A light-driven reaction system involves multiple components, and its performance depends on a series of fundamental photophysical/chemical processes.11 Since most reactants cannot effectively harvest photon energy, light absorbers that carry out energy conversion are introduced into the reaction systems as photocatalysts or photosensitizers. A simplified scheme of a photocatalytic process is illustrated in Fig. 1. After excitation, photon energy is temporarily stored in the electronically excited state of the photosensitizer. Next, the energy or the photogenerated charge carrier is transferred to the reaction system to promote reactants to the active intermediate that can cross the reaction barrier. This sensitization process must compete with the intrinsic relaxation and other deactivation pathways of the transient catalytic species in its excited state manifold. Cocatalysts are commonly used in the reaction system to form a separated catalytic cycle after photosensitization to help deliver the desired product. Their roles include regulating energy/charge carrier transfer to facilitate selective activation, directing reaction intermediates through the correct reaction routes, and reducing deleterious side reactions.

FIG. 1.

Schematic illustration of processes in a simplified photocatalytic system consisting of a photosensitization cycle and a reaction cycle regulated by the cocatalyst. PS: photosensitizer, R: reactant, Cocat: cocatalyst, I: intermediate, P: product, EnT: energy transfer, CT: charge transfer. “∗” denotes species in their excited states.

FIG. 1.

Schematic illustration of processes in a simplified photocatalytic system consisting of a photosensitization cycle and a reaction cycle regulated by the cocatalyst. PS: photosensitizer, R: reactant, Cocat: cocatalyst, I: intermediate, P: product, EnT: energy transfer, CT: charge transfer. “∗” denotes species in their excited states.

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The design of a photocatalytic system must accord with the purposes of the reaction. For instance, the primary aim of solar fuel production is to achieve efficient energy conversion. Therefore, the design should focus on utilizing every charge carrier generated in the excitation event while minimizing energy loss during the downhill charge carrier transfer processes.1,12 In contrast, for the photocatalytic synthesis of organic compounds, although energy efficiency is still a general concern, compared to the cost of raw materials and the great added value of the fine chemical products, photons can be regarded as an unlimited reagent. It is more important to ensure selective generation of the correct intermediates leading to chemo- and configurational controls.13,14 Given the complexities involved in photocatalytic processes, building optimal reaction systems requires precisely manipulating various elementary steps, which can only be achieved after thoroughly understanding the reaction mechanisms involved.

From the perspective of physical chemists, photocatalytic reactions provide an exciting and also challenging platform to implement various time-resolved spectroscopic methods. Based on spectral features collected from different channels, the identity and configuration of the reaction intermediates can be determined, and the changes in their population over time provide the relevant kinetic information for each reaction step. The mechanistic information obtained from spectroscopic data paves the way for the design and optimization of novel photocatalytic reaction systems. This is a great opportunity for spectroscopists, synthetic chemists, and materials scientists to collaboratively solve grand challenges in the field of photocatalysis.

Time-resolved spectroscopy has been successfully integrated into the research of various photocatalytic systems; but here, limited by the length of this Perspective, we only select the works associated with three systematically investigated systems/scenarios as examples to introduce the research methodology and philosophy. These sample systems/scenarios are (1) photocatalytic proton reduction sensitized by semiconductor nanocrystal-based heterostructures—we will demonstrate how critical kinetic information can direct the optimization of photocatalytic systems; (2) the development of photocatalysts that employ multiphoton absorption to attain high redox potentials—this section will address how mechanistic insights from excited state dynamics contribute to the design of novel photocatalytic processes; (3) mechanistic investigation of Ni-based metallaphotocatalysis for C-X (X = N, S, O, etc.) coupling reactions—this section aims to show the necessity for integrating various spectroscopic and chemical methods to unveil reaction mechanisms in complicated systems.

In this Perspective, we will focus on time-resolved optical spectroscopic methods that cover the time regime from sub-picoseconds to a few milliseconds, which can reveal the kinetics of elementary photochemical steps occurring in the excited state manifold. As shown by the simplified model in Fig. 2(a), after photoexcitation, an isolated chromophore relaxes back to its ground state through radiative and non-radiative pathways. The lifetime of the excited species can be obtained by monitoring the decay of the ensemble sample’s emission intensity using various time-resolved methods. In photocatalytic processes, photogenerated excitons and/or charge carriers migrate among donor–acceptor pairs formed by photosensitizers, cocatalysts, and reactants. When the chromophore couples with an acceptor, the energy or charge carrier transfer processes create external pathways to depopulate the emissive excited state and cause an accelerated decay of the emission intensity. Similarly, if the emissive excited state is populated by accepting excitonic energy from a photoexcited donor, the process will be shown as growth in the kinetic trace. By fitting kinetic traces, one can extract the most basic dynamic information associated with the generation and deactivation of excited species in a photocatalytic process. The most widely applied time-resolved photoluminescence (TR-PL) technique is time-correlated single-photon counting (TCSPC),15 whose setup is included in many commercial fluorometers to provide nanosecond time resolution for monitoring changes in the emission intensity. Instruments based on photoluminescence upconversion16,17 and optical Kerr effect18 gating techniques can push the time resolution to 50–100 fs for observing rapid kinetics. Using streak cameras as the detector, the global spectral evolution can be captured in the ps time scale. TR-PL measurements provide basic kinetic information of one or two (such as a donor–acceptor system) excited species and require the sample to have appreciable emission intensities. For complicated photocatalytic systems that consist of multiple parties and involve “dark” transients, more comprehensive techniques are needed to reveal the reaction mechanisms.

FIG. 2.

(a) Right: A two state-system with non-radiative and radiative relaxation pathways interacts with the energy donor and acceptor. Left: Simulated excited population reflected by the PL intensity evolution in different cases. (b) Right: Schematic illustration of a pump–probe transient absorption experiment. The pump pulse produces molecules in their excited state at Δt = 0, and the excited state population decays over time until observed by the broadband probe pulse. Left: TA spectra with positive and negative signals evolve over time.

FIG. 2.

(a) Right: A two state-system with non-radiative and radiative relaxation pathways interacts with the energy donor and acceptor. Left: Simulated excited population reflected by the PL intensity evolution in different cases. (b) Right: Schematic illustration of a pump–probe transient absorption experiment. The pump pulse produces molecules in their excited state at Δt = 0, and the excited state population decays over time until observed by the broadband probe pulse. Left: TA spectra with positive and negative signals evolve over time.

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Pump–probe approaches provide various means for elucidating comprehensive pictures of reaction intermediates over different reaction stages. The actinic pump pulse can conveniently set off a photo-driven reaction.19,20 Then, researchers can choose different probe techniques, from x rays to radio waves, to obtain information from the perspective of chemical structures,21,22 electronic transitions,23 vibrational patterns,24,25 and electron spin states.26 A schematic diagram of a pump–probe experiment is shown in Fig. 2(b) using the most commonly employed technique, transient absorption (TA), as an example. After pump excitation, the probe radiation will take snapshot spectra at different time delays to track the evolution of the system. TA spectra are generated by taking the difference between probed absorption signal with/without the pump excitation: where the initial absorption is reduced upon pump excitation shows negative signals or bleach, while where the photogenerated species have stronger absorption than the original sample shows positive signals. TA in the spectral window spanning the UV to near-IR region can capture a broad range of electronic transitions of various transient species and track their evolutions with a sub-50 fs resolution.27 However, as we will see in the case study, in complicated photocatalytic systems, assigning broad and often overlapped absorption features to each transient species and extracting reaction rates for the processes involved can be challenging.

Time-resolved vibrational spectroscopy is a good complement to TA based on electronic transitions because vibrational fingerprints are important pieces of evidence for the precise assignment of excited intermediates. Time-resolved IR spectroscopy shares the same TA approach. Both the narrow bandwidth of the mid-IR probe light source and the solvent absorption limit the spectral window of the technique in regard to observing more vibrational modes. Time-resolved Raman spectroscopy uses a narrow bandwidth laser pulse to generate Raman scattering covering a broad vibrational frequency range. Introducing the resonance enhancement effect can further increase the sensitivity of Raman spectroscopy for probing reaction species with low concentrations in the solution phase.28 In particular, compared to the method based on spontaneous Raman emission, femtosecond stimulated Raman spectroscopy can handle samples with strong emission background29,30 and can provide time response competitive with TA (<50 fs),31,32 hence has broader applications. The selective resonance of Raman probes with the electronic transition of particular transient species can help capture vibrational signals only belonging to the on-resonant species and, therefore, has the unique advantage of disentangling the complexity involved.

Although, in practice, nuclear magnetic resonance spectroscopy (NMR) is a powerful technique for measuring reaction kinetics in ensemble systems, the time-response of this technique is often too slow to capture elementary photochemical steps happening in the excited state manifold. Time-resolved x-ray spectroscopy is less commonly used due to the requirement for extensive instrumentation. These techniques will not be explicitly discussed unless they provide supportive evidence for interpreting the results from time-resolved optical experiments.

Directly reducing protons to generate H2 via artificial photosynthesis features the most abundant resource and the minimum environmental cost for solar fuel production.1,33 The first demonstration was reported by Fujishima using a TiO2–Pt electrochemical cell in 1972.34 Since then, many light absorbers, including semiconductors and molecular dyes, have been applied to sensitize the reaction in aqueous or mixed solutions, with the aid of proton reduction catalysts.12,35–37 As shown in Fig. 3, the reaction pathways can be classified as reductive or oxidative according to the direction of electron flow in the initial charge transfer event of the photosensitizers.12 As the half-reaction of water splitting, the catalytic process relies on electron suppliers, often a sacrificial electron donor, for its operation. Within its catalytic cycle, photosensitizers serve as the medium to pass electrons to the H2 generation catalyst, where protons are reduced. The whole process involves a series of electron transfer events across interfaces of multiple reactive species. The kinetics of these processes determines the final efficiency of the reaction.

FIG. 3.

Reductive and oxidative pathways of photocatalytic hydrogen generation.

FIG. 3.

Reductive and oxidative pathways of photocatalytic hydrogen generation.

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Colloidal semiconductor nanocrystals (NCs) have attracted intense research interest in the field of photocatalysis due to their superior optical and electronic properties, including strong absorption over a broad spectral range and tunable band energy via dimensional control, exceptionally long-lived excitons, and remarkable photostability.38 Surface engineering approaches have been developed to allow NCs access to various solvent systems, including water, and to facilitate chemical coupling with other functional structures, including proton reduction catalysts. In the past decade, NCs have been successfully employed to drive proton reduction with the assistance of a variety of cocatalysts.39,40 In this Perspective, we select a series of studies on NC-driven photocatalytic proton reduction systems to demonstrate how kinetic investigation with time-resolved spectroscopy can guide the design and optimization of photocatalytic systems.

In 2012, Eisenberg and Krauss developed an extremely robust photocatalytic system using CdSe quantum dots (QDs) and a nickel–dihydrolipoic acid complex (Ni-DHLA), as shown in Fig. 4(a), which can stably work in an aqueous solution under 520 nm excitation for >360 h and achieve a quantum yield >35%.41 Krauss and co-authors later revealed the electron transfer dynamics between CdSe QDs and Ni-DHLA with transient absorption (TA) spectroscopy.42 The time constant of electron transfer (eT) from CdSe QDs to Ni-DHLA was extracted by comparing the decay of the exciton state bleach (XB) signal of CdSe with and without the presence of Ni-DHLA [Figs. 4(b) and 4(c)]. Ultrafast eT (τeT = 69 ± 2 ps) outcompetes other relaxation pathways and leads to a 90% quantum yield (QY), which explains the excellent proton reduction performance of the CdSe/Ni-DHLA catalytic system. As revealed in Fig. 4(c), the eT process is even faster than the Auger recombination of biexciton states in CdSe QDs (180 ± 6 ps), implying that the CdSe/Ni-DHLA photocatalytic system has the potential to make use of the carrier multiplication in CdSe QDs. A general relationship between the electron transfer rate from QDs to Ni-DHLA and the turnover number of H2 reduction was established. TA kinetics revealed that the eT rate decreased with increase in the QD size. For larger-sized QDs with extended absorption towards the red, more Ni-DHLA must be applied to facilitate the eT in order to improve the turnover number.43 In a later study, using CdTe QDs as the photosensitizer, analysis of the kinetics revealed rapid charge trapping that impedes the desired eT process when the surface of QDs is oxidized, which explains the decrease in turnover number when using small CdTe QDs that are more susceptible to oxidation.44 The study identified the detrimental competitive process and suggested further improvement in efficiency could be achieved by mitigating the deleterious electron trapping effect.

FIG. 4.

(a) Cartoon illustrating the catalytic system. TA kinetics extracted at 540 nm indicated the evolution of ground state bleach signals of QDs with photon fluence ⟨N0⟩ equal to (b) 0.54 and (c) 1.92 per QD. The solid lines in panels (b) and (c) are multiexponential fits of the kinetic traces. Panel (a) was reproduced with permission from Han et al., Science 338, 1321 (2012). Copyright 2012 American Association for the Advancement of Science. Panels (b) and (c) were reproduced with permission from Liu et al., J. Phys. Chem. B 119, 7349 (2015). Copyright 2015 American Chemical Society.

FIG. 4.

(a) Cartoon illustrating the catalytic system. TA kinetics extracted at 540 nm indicated the evolution of ground state bleach signals of QDs with photon fluence ⟨N0⟩ equal to (b) 0.54 and (c) 1.92 per QD. The solid lines in panels (b) and (c) are multiexponential fits of the kinetic traces. Panel (a) was reproduced with permission from Han et al., Science 338, 1321 (2012). Copyright 2012 American Association for the Advancement of Science. Panels (b) and (c) were reproduced with permission from Liu et al., J. Phys. Chem. B 119, 7349 (2015). Copyright 2015 American Chemical Society.

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In light of the importance of charge transfer dynamics to photoenergy conversion and other applications, Lian’s group conducted systematic investigations on charge transfer phenomena in low-dimensional semiconductor nanocrystals.45 A unique Auger-assisted electron transfer mechanism was proposed to explain the lack of a Marcus-inverted regime for electron transfer from cadmium chalcogenide quantum dots (QDs).46 The Newns–Anderson model was applied to understand electron transfer between QDs and strongly coupled semiconductor acceptors.47 Through wavefunction engineering, Lian’s group was able to mediate the rate of charge transfer in QD–molecule heterojunctions and maximize the extraction of photoelectrons from these systems.48,49 These fundamental studies as well as the methodologies developed during the investigation, pave the way toward analyzing and rationally designing photocatalytic systems based on nanocrystal materials.

Lian and co-authors investigated charge transfer dynamics in a promising proton reduction photocatalyst, the nano-heterostructure composed of CdS nanorods (NRs) and Pt nanoparticles.50 Based on the TA kinetic features (Fig. 5), the authors observed rapid electron transfer (τ1/2 = ∼3.4 ps) to the Pt tip decorated on the CdS NR and a long lifetime (τ1/2 = ∼1.2 ± 0.6 µs) of the charge-separated state, which ensures efficient electron transfer for proton reduction in the next step. The authors assigned photoinduced absorption (PA) in the 550–700 nm region as the signal of the trapped holes in CdS NRs, and the rapid rise of this signal in ∼0.7 ps represents ultrafast hole trapping. The time constants for these key processes are listed in Fig. 5(c). Contrary to electron trapping, this rapid hole trapping phenomenon facilitates the efficient formation of a long-lived charge-separated state, making the CdS–Pt NR heterostructure a good photocatalyst for proton reduction. A later work conducted by Stolarczyk and co-authors also argued that it is not electron transfer from NR to Pt but the following transfer step from Pt to the proton that limits the reaction performance.51 Therefore, increasing the lifetime of the charge-separated state (NR+-catalyst) by slowing down backward charge recombination is critical for enhancing efficiency.

FIG. 5.

(a) TA spectra of CdS NRs at indicated time delays after 400 nm excitation: 0–1 ps (top) and 3–3000 ns (bottom) with marked spectral components. Inset: Expanded view of the broad photoinduced absorption (PA) signal. (b) TA kinetics were extracted at 456 nm for XB (exciton band bleach, black, top), 550–700 nm for PA (red, middle), and at 470 nm for XA1 (hot-exciton induced shift)/SE (Stark effect signal, green, bottom) of CdS–Pt NRs. The kinetics of CdS NRs are shown in gray dashed lines for comparison. (c) Schematic energy level and exciton quenching pathways in CdS–Pt NR heterostructures. In addition to the intrinsic exciton decay within CdS NRs, the presence of Pt introduces interfacial electron transfer (ET) and hole transfer (HT) pathways. Time constants for several key processes, such as electron transfer, hole trap (Htr), and charge recombination (CR), obtained from TA, are shown. Reproduced with permission from Wu et al., J. Am. Chem. Soc. 134, 10337 (2012). Copyright 2012 American Chemical Society.

FIG. 5.

(a) TA spectra of CdS NRs at indicated time delays after 400 nm excitation: 0–1 ps (top) and 3–3000 ns (bottom) with marked spectral components. Inset: Expanded view of the broad photoinduced absorption (PA) signal. (b) TA kinetics were extracted at 456 nm for XB (exciton band bleach, black, top), 550–700 nm for PA (red, middle), and at 470 nm for XA1 (hot-exciton induced shift)/SE (Stark effect signal, green, bottom) of CdS–Pt NRs. The kinetics of CdS NRs are shown in gray dashed lines for comparison. (c) Schematic energy level and exciton quenching pathways in CdS–Pt NR heterostructures. In addition to the intrinsic exciton decay within CdS NRs, the presence of Pt introduces interfacial electron transfer (ET) and hole transfer (HT) pathways. Time constants for several key processes, such as electron transfer, hole trap (Htr), and charge recombination (CR), obtained from TA, are shown. Reproduced with permission from Wu et al., J. Am. Chem. Soc. 134, 10337 (2012). Copyright 2012 American Chemical Society.

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Lian’s group attempted to introduce an electron shuttle, methyl viologen (MV2+), between the Cd-chalcogenide NC and the Pt catalyst to mediate proton reduction.52 The distinct absorption signal of the reductive intermediate, MV, at 605 nm was employed to track the electron transfer steps. Using TA and time-resolved photoluminescence (TR-PL), the authors assessed the influence of various charge carrier migration processes [Fig. 6(a)] on the final proton reduction performance. As shown in Fig. 6(b), the charge separation rates (τCS = 0.4–20 ps) in all the tested systems are much greater than the intrinsic exciton recombination (τRX > 1000 ps), which guarantees the unity initial quantum yield of MV. However, in the long term, the amount of MV that can effectively pass the electron to the Pt reaction center is limited by charge recombination between NCs and MV. The authors further discovered that the extent of charge recombination is determined by the efficiency of hole-filling from the sacrificial electron donor, mercaptopropionic acid (MPA), based on the comparison of kinetics in Figs. 6(c) and 6(d). It is worth noting that the kinetics of hole-filling must be monitored using time-resolved photoluminescence because the bandgap XB signal of Cd chalcogenide is almost entirely contributed by the electron filling effect. The CdSe/CdS dot-in-rod (DIR) nanostructure features comparatively fast hole-filling vs DIR+–MV charge recombination and therefore shows a near-unity steady state MV yield and the best proton reduction performance in the study [Figs. 6(e) and 6(f)]. Since the hole-filling process was identified as the rate-limited step for proton reduction with cadmium chalcogenide NCs, further efforts have been made to facilitate hole transfer. A later work from the group demonstrated that sulfite was a more effective hole scavenger in the aqueous solution for the CdS NR-Pt nano-heterostructure, using TA and TR-PL, and improved the proton reduction yield.53 To further accelerate hole-filling, an improved reaction system based on the CdS NR and Ni catalyst employed the redox OH·/OH as a hole shuttle. The relayed hole transfer efficiently eliminated holes from the CdS NR surface and enhanced the reaction performance.54 

FIG. 6.

(a) Schematic depiction of relevant processes in a solar-to-fuel conversion system using MV2+ as intermediate electron transfer and containing a sacrificial electron donor. (b) Comparison of the formation and decay kinetics of MV radicals generated by 400 nm excitation of aqueous solution containing different NCs: CdSe seed (black), CdSe/CdS core/shell QDs of similar lowest exciton energy (CS-SE) (blue), CdSe/CdS CS-similar volume (SV, dark red) as the nanorod, CdS rod (red), and CdSe/CdS dot-in-rod (DIR) (green). Comparison of hole-filling and charge recombination kinetics in different NCs: (c) CdSe seed and (d) CdSe/CdS DIR. The hole-filling time is measured by fluorescence decay kinetics of MPA–NCs in water (blue line). The charge recombination time is monitored by the MV decay kinetics of NC–MV2+ complexes in chloroform (red line). Also shown for comparison is the MV decay kinetics of MPA–NC–MV2+ complexes in water (green line). (e) Initial quantum yields of MV radical generation using different sensitizers (bars). Also plotted are the transient quantum yields (open triangles) at 10 µs obtained from TA measurements. (f) Internal (left axis) and external (right axis) quantum yields of H2 evolution using different sensitizers. Reproduced with permission from Zhu et al., J. Am. Chem. Soc. 134, 11701 (2012). Copyright 2012 American Chemical Society.

FIG. 6.

(a) Schematic depiction of relevant processes in a solar-to-fuel conversion system using MV2+ as intermediate electron transfer and containing a sacrificial electron donor. (b) Comparison of the formation and decay kinetics of MV radicals generated by 400 nm excitation of aqueous solution containing different NCs: CdSe seed (black), CdSe/CdS core/shell QDs of similar lowest exciton energy (CS-SE) (blue), CdSe/CdS CS-similar volume (SV, dark red) as the nanorod, CdS rod (red), and CdSe/CdS dot-in-rod (DIR) (green). Comparison of hole-filling and charge recombination kinetics in different NCs: (c) CdSe seed and (d) CdSe/CdS DIR. The hole-filling time is measured by fluorescence decay kinetics of MPA–NCs in water (blue line). The charge recombination time is monitored by the MV decay kinetics of NC–MV2+ complexes in chloroform (red line). Also shown for comparison is the MV decay kinetics of MPA–NC–MV2+ complexes in water (green line). (e) Initial quantum yields of MV radical generation using different sensitizers (bars). Also plotted are the transient quantum yields (open triangles) at 10 µs obtained from TA measurements. (f) Internal (left axis) and external (right axis) quantum yields of H2 evolution using different sensitizers. Reproduced with permission from Zhu et al., J. Am. Chem. Soc. 134, 11701 (2012). Copyright 2012 American Chemical Society.

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2D CdS nanoplatelets (NPLs) were employed to replace 1D NR in the CdS–Pt nano-heterostructure to improve the efficiency of H2 generation.55 Similar to the NR system, in the NPL systems, TA and TR-PL measurements showed that after photoexcitation, the initial electron transfer (eT) step to Pt and the hole trapping (hTrap) step outcompete the intrinsic exciton recombination [Figs. 7(a)7(d)]. Therefore, both eT and hTrap have yields close to unity, and the reaction efficiency is again limited by the lifetime of the charge-separated state. The authors introduced MV2+ to substitute Pt as the electron acceptor to illustrate the charge recombination kinetics by monitoring the evolution of the representative ∼600 nm absorption feature of MV. As shown in Figs. 7(e) and 7(f), the decay of the MV signal indicates the lifetime of the charge-separated state. The lifetime increases from 0.23 ± 0.09 to 9.07 ± 0.64 µs when the pH increases from 9 to 14, corresponding to the increased yield of H2, suggesting that the system needs to quickly eliminate the OH· radical to prevent charge recombination. At pH < 13, the charge-separated state lives longer in NPLs than in NRs, which explains the advantage of the 2D morphology in H2 generation over 1D nanorods.

FIG. 7.

(a) Average TA spectra at indicated delay time windows of CdS NPL-Pt heterostructures in hexane. Inset: Expanded view of the TA spectra at long delay times (0.1–0.2, 0.8–1, and 4–5 µs). (b) Comparison of exciton bleach (XB) kinetics at 416 nm of CdS NPLs (blue circles) and CdS NPL-Pt heterostructures (red squares) and the scaled charge-separated state (CS) kinetics (green triangles). (c) Static-PL spectra of CdS NPLs dispersed in hexane and aqueous solution with different pH. X: bandgap, Tr1 and Tr2: hole traps at the NPL edges and basal plane, respectively. (d) PL decay kinetics (circles) and fits (solid lines) of the emission of the band edge excitons and the emission of Tr1+X (upper panel) and Tr2 (lower panel) in CdS NPLs dispersed in hexane and aqueous solution. (e) Comparison of normalized MV radical kinetics of CdS NPL-MV2+ complexes at different pH. (f) Comparison of MV radical kinetics of CdS NR-MV2+ (red line) and NPL-MV2+ complexes (black line) at pH 9. Reproduced with permission from Li et al., J. Am. Chem. Soc. 140, 11726 (2018). Copyright 2018 American Chemical Society.

FIG. 7.

(a) Average TA spectra at indicated delay time windows of CdS NPL-Pt heterostructures in hexane. Inset: Expanded view of the TA spectra at long delay times (0.1–0.2, 0.8–1, and 4–5 µs). (b) Comparison of exciton bleach (XB) kinetics at 416 nm of CdS NPLs (blue circles) and CdS NPL-Pt heterostructures (red squares) and the scaled charge-separated state (CS) kinetics (green triangles). (c) Static-PL spectra of CdS NPLs dispersed in hexane and aqueous solution with different pH. X: bandgap, Tr1 and Tr2: hole traps at the NPL edges and basal plane, respectively. (d) PL decay kinetics (circles) and fits (solid lines) of the emission of the band edge excitons and the emission of Tr1+X (upper panel) and Tr2 (lower panel) in CdS NPLs dispersed in hexane and aqueous solution. (e) Comparison of normalized MV radical kinetics of CdS NPL-MV2+ complexes at different pH. (f) Comparison of MV radical kinetics of CdS NR-MV2+ (red line) and NPL-MV2+ complexes (black line) at pH 9. Reproduced with permission from Li et al., J. Am. Chem. Soc. 140, 11726 (2018). Copyright 2018 American Chemical Society.

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The examples demonstrated that time-resolved spectroscopy, such as TA and TR-PL, are effective tools for extracting kinetic information of the key processes involved in photocatalytic reactions. However, tracking the reaction system with these spectroscopic methods might not be sufficient, for some of the elementary steps do not have prominent spectral features. A well-designed model reaction that can mimic the dynamics of the actual reaction step using a spectral probe with detectable signals can help reveal the kinetics.

By comparing the rate constants of various processes, reaction steps that limit the efficiency can be identified, improved catalytic systems can be designed, and more favorable reaction conditions can be selected. Taking charge carrier trapping processes as an example, although, in general, both electron and hole trapping steps are detrimental in the application of nanocrystals,56 they have different influences in the case of proton reduction. Electron trapping, which directly competes with electron transfer to the reaction center, is obviously a detrimental process, as suggested by Ref. 44, whereas hole trapping, which hinders charge recombination and increases the lifetime of the reactive charge-separated state, can promote proton reduction. However, hole trapping still causes energy loss and can retard the photooxidation step, which can be problematic when designing photocatalytic systems for the complete water-splitting cycle.57 This brings back the ultimate challenge of the field: Proton reduction comprises only one-half of the reaction cycle for solar fuel generation, and the other half, for example, water oxidation, has at least a similar level of complexity that needs to be resolved. More comprehensive mechanistic studies that can provide insight into balancing the requirements from both half-cycles are needed.

The upper limit of redox potentials in regular photoreaction processes is constrained by the energy that one single photon can offer. Given that additional energy losses can occur in subsequent electronic transition steps during photocatalytic processes, visible light activation for inert compounds can be challenging to realize.6,58 Employing photosensitizers that absorb in the UV range to raise the energy is a straightforward solution. However, high-energy UV illumination increases the risk of photodamage and other side reactions while overcoming the thermodynamic limit.13 To solve this dilemma, researchers seek photosensitization mechanisms that can accumulate energy from two or more visible-to-near-IR photons, as illustrated in Fig. 8. The additional advantage of using long-wavelength photons, especially those in the infrared range, for excitation is that they suffer less scattering loss and hence can activate reactions in large-scale reactors and special reaction media.59 A detailed summary of applications of various multiphoton sensitization strategies can be found in a specialized review.60 

FIG. 8.

Simplified schematic comparison of photocatalytic processes driven by absorbing one short-wavelength photon and two long-wavelength photons.

FIG. 8.

Simplified schematic comparison of photocatalytic processes driven by absorbing one short-wavelength photon and two long-wavelength photons.

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Spectroscopists have long been curious about multiphoton photochemical processes,61–65 but practical applications of multiphoton processes in benchtop synthesis require the use of affordable light sources, such as LED lamps. The output power densities of these sources are much lower compared to short-pulse lasers commonly used in spectroscopic studies. Nevertheless, earlier spectroscopic studies have garnered experience in enhancing the yield of multiphoton excitation. A recent breakthrough in multiphoton photocatalysis, which achieved IR-driven photoredox reactions,59 benefits from mechanistic research on triplet–triplet fusion upconversion and singlet fission.66–68 Time-resolved spectroscopy can not only identify key parameters for applying multiphoton redox processes but also reveal novel photophysical mechanisms that can be utilized to invent new multiphoton catalytic systems.

The simplest multiphoton photocatalytic process relies on the consecutive absorption of two photons by one photosensitizer to generate a high-energy excited state that can initiate the reaction. This mechanism requires the photoinduced species formed after the first excitation to have an appreciable absorption cross section and live long enough to encounter the second photon. For most molecule-based photocatalysts, the lifetime of the optically allowed singlet excited state is too short, so a triplet excited state formed via intersystem crossing (ISC) or a radical doublet generated from photoinduced charge transfer can be a better candidate for carrying out the second absorption. It is also necessary to consider if charge transfer from the targeted high-energy excited state can compete with their relatively fast intrinsic relaxation.

Wenger and co-authors overcame the difficulty with regard to lifetime using a water-soluble derivative of tris(2-phenylpyridine) iridium(III), Irsppy.69 The compound has a unity yield of the long-lived T1 state (τ = 1.6 µs), and the second excitation eventually produces hydrated electrons as the super-reducing reagent [Fig. 9(a)]. The authors used a pump–pump–probe approach to observe the two-photon sensitization process with TA spectroscopy. Irsppy was first excited at 430 nm, settled for a time delay of 450 ns to allow the formation of 3∗Irsppy, and then excited with the second pulse at 530 nm. The selection of the wavelength and time delay of the secondary pump pulse is based on the absorption spectrum and the formation kinetics of 3∗Irsppy, measured with regular pump–probe TA experiments. As shown in Fig. 9(b), the secondary excitation caused instant diminishment of the 3∗Irsppy signal and the rise of a new absorption feature corresponding to the hydrated electron with a lifetime of 1.4 µs. This long lifetime as well as the extremely negative reduction potential of the hydrated electron can activate challenging substrates like chloroacetate. To quantify the yield of hydrated electrons, the authors employed a ruthenium dye as an external standard of actinometry. Using the same experimental setup, the authors further determined the rate constant for the reaction between hydrated electrons and chloroacetate as 1.15 × 109 M−1 s−1 [Fig. 9(c)].

FIG. 9.

(a) Catalytic generation of eaq•− through visible light-driven ionization of the photocatalyst (PC) Irsppy with subsequent catalyst regeneration using a sacrificial donor. (b) TA kinetic traces extracted at the absorption of 3∗Irsppy (upper) and the hydrated electron (lower) with (cyan) and without (blue) second excitation. Inset: Excited Irsppy bleaching/hydrated electron formation (blue) at different intensities of the second laser pulse. The orange trace represents the signal of [Ru(bpy)32+] used as a reference for relative actinometry. (c) Decay of the hydrated electron signal in the presence of different concentrations of chloroacetate. Inset: Stern–Volmer plot of the kinetic data. Reproduced with permission from Kerzig et al., J. Am. Chem. Soc. 141, 2122 (2019). Copyright 2019 American Chemical Society.

FIG. 9.

(a) Catalytic generation of eaq•− through visible light-driven ionization of the photocatalyst (PC) Irsppy with subsequent catalyst regeneration using a sacrificial donor. (b) TA kinetic traces extracted at the absorption of 3∗Irsppy (upper) and the hydrated electron (lower) with (cyan) and without (blue) second excitation. Inset: Excited Irsppy bleaching/hydrated electron formation (blue) at different intensities of the second laser pulse. The orange trace represents the signal of [Ru(bpy)32+] used as a reference for relative actinometry. (c) Decay of the hydrated electron signal in the presence of different concentrations of chloroacetate. Inset: Stern–Volmer plot of the kinetic data. Reproduced with permission from Kerzig et al., J. Am. Chem. Soc. 141, 2122 (2019). Copyright 2019 American Chemical Society.

Close modal

Using the Irsppy sensitizer, Wenger’s group demonstrated an interesting reaction system in which the illumination photon density controls the reaction results.70 High illumination photon density promotes two-photon excitation of Irsppy, which generates hydrated electrons that have enough potential to reduce aryl chloride and conduct hydrogen abstraction with olefin. There is a quadratic relationship between the illumination photon density and the probability of two-photon absorption. At low photon density, most photocatalysts are left in their first triplet excited state, which only reduces aryl bromides or triggers cis-to-trans isomerization of olefins via triplet–triplet energy transfer.

Besides the triplet excited state, the long-lived photogenerated radical state can absorb the second photon to form the excited state radical, PS·, and act as potent redox species. In this strategy, the precursor radical, PS·, is prepared via photoinduced charge transfer after the first excitation. Since both the triplet and optically allowed singlet excited states of the photosensitizer can be used to generate PS·, the sensitizer design is more flexible compared to processes that rely on high ISC yield. König and Ghosh also reported a reaction system sensitized by the excited radical state of Rhodamine-6G and demonstrated chromatic dependence on the photosensitization of the second excitation.71 After being excited at 530 nm, the singlet excited state was reduced by N, N-diisopropylethylamine, and formed anion radical Rh-6G•− that can activate only one C–Br bond on the aryl ring. Without the 450 nm blue light, the reaction stops after the formation of products with a single substitution. The Rh-6G•− radical can absorb 450 nm blue light to form more reductive ∗Rh-6G•−, and the reactivity of the system is turned up to obtain twofold substituted products.

In 2014, the König group prepared a stable radical of perylene diimide (PDI•−) via photoreduction by triethylamine.72 The radical can absorb the second photon to gain enough reduction potential to convert aryl chloride (Ar–Cl) to reactive aryl radicals [Fig. 10(a)]. More recently, Schanze and co-authors evaluated the reactivity of PDI•− using the kinetic data measured by transient absorption.73 The lifetime of the excited state of the PDI anion radical (PDI•−) was measured as 160 ± 2 ps. The authors conducted a Stern–Volmer quenching analysis of PDI•− using electron acceptors with different reduction potentials and obtained the relationship between quenching rates and the driving force [Figs. 10(b)10(d)]. The results were fitted to the Rehm−Weller model for estimating the reduction potential of PDI•− as −1.87 eV vs SCE. The potential guarantees efficient reduction of electron acceptors with ERe > −1.7 V vs SCE, and the author found that Ar–Cl with more negative potentials can also be activated. One hypothesis is that the endothermic electron transfer processes to Ar–Cl can be thermally activated, and the subsequent reaction, dehalogenation of Ar–X−·, is so rapid that the entire reaction is irreversible.

FIG. 10.

(a) The two-photon catalytic mechanism employs ∗PDI•− to reduce aryl halide for generating aryl radicals. (b) TA spectra show the quench of ∗PDI•− using 0.2M 4-nitrobenzaldehyde. (c) Stern–Volmer plots for lifetime quenching of ∗PDI−· vs various concentrations of aryl quenchers. (d) Rehm–Weller analysis of the quenching rate constant for PDI−· vs the reduction potentials of various quenchers. Panel (a) was reproduced with permission from Ghosh et al., Science 346, 725 (2014). Copyright 2014 American Association for the Advancement of Science. Panels (b)–(d) were reproduced with permission from Zeman et al., J. Am. Chem. Soc. 142, 2204 (2020). Copyright 2020 American Chemical Society.

FIG. 10.

(a) The two-photon catalytic mechanism employs ∗PDI•− to reduce aryl halide for generating aryl radicals. (b) TA spectra show the quench of ∗PDI•− using 0.2M 4-nitrobenzaldehyde. (c) Stern–Volmer plots for lifetime quenching of ∗PDI−· vs various concentrations of aryl quenchers. (d) Rehm–Weller analysis of the quenching rate constant for PDI−· vs the reduction potentials of various quenchers. Panel (a) was reproduced with permission from Ghosh et al., Science 346, 725 (2014). Copyright 2014 American Association for the Advancement of Science. Panels (b)–(d) were reproduced with permission from Zeman et al., J. Am. Chem. Soc. 142, 2204 (2020). Copyright 2020 American Chemical Society.

Close modal

Wasielewski and co-authors explored the possibility of creating a super-photooxidant based on the excited state of 10-phenyl-10H-phenothiazine radical cation (PTZ).74 In this work, PTZ is prepared by chemically oxidizing PTZ with “magic blue,” but the PTZ can also be generated through photoinduced charge transfer.75–77 The authors investigated charge transfer in covalently linked dyads consisting of PTZ and hole acceptors with different oxidation potentials (Eox = 1.04–1.6 V vs SCE) [Fig. 11(a)]. With 517 and 900 nm excitation, the PTZ•+ accessed different excited states. Singular value decomposition (SVD) was employed to analyze the TA data and extract the evolution dynamics of different excited states of PTZ and their interactions with the tethered hole acceptors. One example is shown in Figs. 11(b) and 11(c) for the system of PTZ and 9-anthracenecarbonitrile (ACN). Global fitting of the species-associated spectral components provides the oxidation rates of the acceptors. The authors also employed femtosecond stimulated Raman spectroscopy (FSRS) to complement information gathered from TA analysis. FSRS confirmed the lifetimes of the PTZ excited states measured by TA [Figs. 11(d) and 11(e)]. Moreover, FSRS distinguished the relaxation event along the potential surface of the D1 state by capturing subtle shifts in the vibrational frequency of the ring deformation mode, which explained the consecutive decay kinetics observed in the SVD analysis of TA data. The full pictures of the excited state evolution and the charge transfer processes are shown in Figs. 11(f) and 11(g).

FIG. 11.

Super-photooxidant based on the excited state of the PTZ cation radical. (a) Design of the PTZ/hole acceptor dyad with experimental (top) and computed (bottom) UV–vis spectra of PTZ indicates using 517 and 900 nm excitation to access different excited states. (b) and (c) Species-associated spectra obtained by global fitting of the TA spectra of PTZ-ACN dyad under 517 and 900 nm pump excitation, respectively. (d) Time-resolved FSRS spectra of PTZ. The arrow denotes the evolution of the centroid for the Raman band in the 1300–1390 cm−1 region. (e) Decay kinetics of the integrated area of the Raman band in the 1300–1390 cm−1 region. Jablonski diagrams summarizing the photophysics and charge transfer reactions of (f) PTZ•+, and (g) the dyads formed by PTZ and 10-phenyl-9-anthracenecarbonitrile (ACN), following excitations at λex = 517 nm (green) and λex = 900 nm (red). Reproduced with permission from Christensen et al., J. Am. Chem. Soc. 140, 5290 (2018). Copyright 2018 American Chemical Society.

FIG. 11.

Super-photooxidant based on the excited state of the PTZ cation radical. (a) Design of the PTZ/hole acceptor dyad with experimental (top) and computed (bottom) UV–vis spectra of PTZ indicates using 517 and 900 nm excitation to access different excited states. (b) and (c) Species-associated spectra obtained by global fitting of the TA spectra of PTZ-ACN dyad under 517 and 900 nm pump excitation, respectively. (d) Time-resolved FSRS spectra of PTZ. The arrow denotes the evolution of the centroid for the Raman band in the 1300–1390 cm−1 region. (e) Decay kinetics of the integrated area of the Raman band in the 1300–1390 cm−1 region. Jablonski diagrams summarizing the photophysics and charge transfer reactions of (f) PTZ•+, and (g) the dyads formed by PTZ and 10-phenyl-9-anthracenecarbonitrile (ACN), following excitations at λex = 517 nm (green) and λex = 900 nm (red). Reproduced with permission from Christensen et al., J. Am. Chem. Soc. 140, 5290 (2018). Copyright 2018 American Chemical Society.

Close modal

In the above-mentioned donor–acceptor dyads, neither the excited states of the PTZ radical, PTZ, nor the charge-shifted states formed after charge transfer with the hole acceptors live long enough to effectively carry out photosensitization in practical reaction conditions. A donor–bridge–acceptor triad consisting of peri-xanthenoxanthene radical cation (PXX), 9,10-bis(trifluoromethyl)-anthracene (TMFA), and 9,10-diphenylanthracene (DPA) was designed to slow down the decay of the charge-shifted state.78 TA spectroscopy, along with SVD analysis, elucidates a cascade hole transfer after the formation of the excited state of PXX. With the TMFA bridge, the quantum yield of the charge-shifted state, PXX-TFMA-DPA, is 46%. The subsequent charge transfer processes cause energy loss, and the newly formed charge-shifted doublet has a lower oxidative potential compared to the initial PXX. However, the charge-shifted doublet in the triad has a lifetime of 11.5 ± 0.6 ns, much longer than PXX (τ = 124 ps), and the charge-shifted state in the dyad (τ = 2.4 ps) (Fig. 12), and therefore can be incorporated into photocatalytic systems.

FIG. 12.

(a) fsTA spectra in CH3CN (left column) and (b) species-associated spectra obtained by global fitting analysis (right column) of the triad of PXX–TFMA–DPA. (c) Jablonski diagram summarizing the photophysical dynamics and charge transfer reactions of PXX–TFMA–DPA. Reproduced with permission from Christensen et al., J. Phys. Chem. C 122, 23364 (2018). Copyright 2018 American Chemical Society.

FIG. 12.

(a) fsTA spectra in CH3CN (left column) and (b) species-associated spectra obtained by global fitting analysis (right column) of the triad of PXX–TFMA–DPA. (c) Jablonski diagram summarizing the photophysical dynamics and charge transfer reactions of PXX–TFMA–DPA. Reproduced with permission from Christensen et al., J. Phys. Chem. C 122, 23364 (2018). Copyright 2018 American Chemical Society.

Close modal

The absorption of multiple photons can also be carried out by separated absorbers within a molecular complex. Wasielewski’s group discovered the formation of high-energy long-lived charge-separated state through annihilation of charge transfer (CT) excitons in the cocrystal of peri-xanthenoxanthene (PXX) and N,N-bis(3-pentyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis(dicarboximide) (Ph4PDI).79 Polarization-dependent TA identified spectral features of Ph4PDI−· and PXX radicals formed after excitation [Figs. 13(a) and 13(b)]. When fitting the decay kinetics of the charge transfer exciton based on an exciton hopping annihilation model [Fig. 13(c)], a long-lived species was identified by observing that the kinetic trace deviated from the extrapolation of the fit from the early time kinetics. This long-lived species was assigned to the separated free-charge carriers, Ph4PI−·-PPX-Ph4PI-PXX, generated through CT exciton annihilation after hopping along the axis of the 1D crystal, as summarized in Fig. 13(d). This free electron–hole pair has an energy that is 0.84 eV higher than that of the individual vertical charge transfer exciton formed by absorbing one photon and has the potential to be applied in photocatalytic systems.

FIG. 13.

Transient absorption spectra of a PXX–Ph4PDI cocrystal. Arrows denote probe polarizations relative to the cocrystal long axis: (a) Vertical and (b) parallel. (c) Average of eight normalized kinetic curves in the wavelength range of 665–700 nm. The data over the 0.3 ps < t < 500 ps time range were fit to the exciton hopping/annihilation model, where after normalization, ΔOD0 was set to 1. The data for times >500 ps were not fit. The blue trace is the early time fit extrapolated to longer times to highlight the deviation from the model. Inset: Expanded view of the difference between the data and the fit at longer times. (d) Charge transfer (CT) biexciton annihilation model. Following photoexcitation, two photons create two singlet Frenkel excitons on Ph4PDI acceptor molecules, which then rapidly charge separate to create two CT excitons with a rate constant of kCS. The transition dipole moment of Ph4PDI, μA, is perpendicular to the CT exciton transition dipole moment, μCT. CT exciton migration with a site-to-site rate constant, khop, and collision results in CT biexciton annihilation, which produces spatially separated electron–holes pairs with a rate constant kA. Alternatively, the CT biexciton can be produced on adjacent D–A pairs resulting in annihilation. The electron–hole pair recombination rate is kCR. Reproduced with permission from Schlesinger et al., Chem. Sci. 11, 9532 (2020). Copyright 2020 The Royal Society of Chemistry.

FIG. 13.

Transient absorption spectra of a PXX–Ph4PDI cocrystal. Arrows denote probe polarizations relative to the cocrystal long axis: (a) Vertical and (b) parallel. (c) Average of eight normalized kinetic curves in the wavelength range of 665–700 nm. The data over the 0.3 ps < t < 500 ps time range were fit to the exciton hopping/annihilation model, where after normalization, ΔOD0 was set to 1. The data for times >500 ps were not fit. The blue trace is the early time fit extrapolated to longer times to highlight the deviation from the model. Inset: Expanded view of the difference between the data and the fit at longer times. (d) Charge transfer (CT) biexciton annihilation model. Following photoexcitation, two photons create two singlet Frenkel excitons on Ph4PDI acceptor molecules, which then rapidly charge separate to create two CT excitons with a rate constant of kCS. The transition dipole moment of Ph4PDI, μA, is perpendicular to the CT exciton transition dipole moment, μCT. CT exciton migration with a site-to-site rate constant, khop, and collision results in CT biexciton annihilation, which produces spatially separated electron–holes pairs with a rate constant kA. Alternatively, the CT biexciton can be produced on adjacent D–A pairs resulting in annihilation. The electron–hole pair recombination rate is kCR. Reproduced with permission from Schlesinger et al., Chem. Sci. 11, 9532 (2020). Copyright 2020 The Royal Society of Chemistry.

Close modal

The separated charge carriers can be formed in molecular complexes through a biomimicry Z-scheme mechanism to accumulate energy from two absorbed photons. Odobel and co-authors developed a Z-scheme molecular complex by carefully aligning the molecular light absorbers and electron donors/acceptors with appropriate redox potentials.80 This molecular system can store 2.03 eV energy in the charge-separated state from 4.26 eV of the total input energy of the two absorbed photons. Most recently, Wenger and co-authors demonstrated that the molecular Z-scheme setup, consisting of a CuIbis(α-diimine) complex (Cu(dap)2) and 9,10-dicyanoanthracenyl (DCA) radical anion, can combine energy from two red photons to activate dehalogenation and detosylation reactions, which require energy in the UV range.81 TA data yielded both the anion radical and the triplet excited state of DCA in the system [Figs. 14(a) and 14(b)]. According to the decomposed TA spectral features, the relative population of the two active intermediate species, 2DCA−· and 3∗DCA, varied in different solvents. Nevertheless, this did not affect the overall reactivity since both intermediates can generate the super-reductant, 2∗DCA−·, through different reaction paths, as illustrated in Fig. 14(c).

FIG. 14.

TA spectra at different time delays for [Cu(dap)2]Cl (100 µM) in deaerated acetonitrile (a) and in deaerated acetone (b) were excited at 532 nm (30 mJ) in the presence of DCA (500 µM). The radical and triplet excited state signals of DCA were noted. (c) Two different reaction pathways initiated from charge transfer or triplet energy transfer from Cu(dap)2 to DCA. Reproduced with permission of Glaser et al., JACS Au 2, 1488 (2022). Copyright 2022 American Chemical Society.

FIG. 14.

TA spectra at different time delays for [Cu(dap)2]Cl (100 µM) in deaerated acetonitrile (a) and in deaerated acetone (b) were excited at 532 nm (30 mJ) in the presence of DCA (500 µM). The radical and triplet excited state signals of DCA were noted. (c) Two different reaction pathways initiated from charge transfer or triplet energy transfer from Cu(dap)2 to DCA. Reproduced with permission of Glaser et al., JACS Au 2, 1488 (2022). Copyright 2022 American Chemical Society.

Close modal

Most of the multiphoton photoredox systems described in this section are still in the proof-of-concept stage. In these investigations, time-resolved spectroscopy was employed to identify the intermediate states of photosensitizers after each absorption event. Important spectral and kinetic data, including the absorption wavelength, the formation and the elimination of active intermediates, and the rates of final charge transfer, were determined, which laid the foundation for efficiently accumulating the redox potential from multiphoton processes. The spectroscopic experiments also clarified the intertwined routes for generating high redox potential species in molecular complexes, which provided the basis to design systems using novel photochemical mechanisms. Future development in the field should apply these super-energetic systems to real reaction systems. Two challenges need to be addressed. (1) How to further increase the lifetime of the doubly excited states? A few examples in this section bypass the lifetime issue by indirectly generating active species, such as hydrated electron69 and the charge-shifted state.78 These strategies suffer from energy drop and could constrain the application scenarios. A universal solution will require mechanistic innovation for producing durable active species through direct multiphoton excitation. (2) How to selectively activate the desired reactants with the super energetic states? With high redox potential, these species can sensitize not only the target substrates but also other substances, such as solvent molecules. In fact, the formation of hydrated electrons in Refs. 69 and 70 set the upper limit of reduction potential that a system can reach in an aqueous environment. The risk of activating undesired reaction pathways increases with the increase in redox potential. After finding the mechanism for generating high-energy species, future investigations need to focus on interactions between reactive excited states and different molecular acceptors to realize controlled activation of desired reactions. Close collaboration between laser spectroscopists and synthetic chemists will open the door to new chemistry that can practically resolve the complexities of challenging reactions using energy from multiple photons.

The merging of photochemistry and transition metal catalysis invoked the revolutionary development of synthetic methods that led to unconventional chemical bond formation strategies.82,83 In these “dual” catalytic systems, photon energy is collected by a photosensitizer, e.g., polypyridyl complexes of metals, organic dyes, and semiconductor nanocrystals,83 most of which have well-known excited state behaviors.11 The photoactivation of catalytic reactions involves electron and energy transfer processes from the sensitizer, which can also be conveniently tracked using various steady state and time-resolved spectroscopic methods.11 However, the dual catalytic reaction just terminates the initial stage after photoactivation: Most bond breaking/formation occurs in subsequent reaction steps involving various intermediates that formed with the metallic cocatalysts.84 These “dark” steps directly mediate the product formation and are challenging to study.84 For instance, Ni-based catalysts have been widely used in photoredox carbon–carbon and carbon–heteroatom coupling reactions. There is a strong interest in understanding the role of multiple interconvertible oxidation states of Ni complexes in the redox cycle.85 In this section, we summarize the recent progresses made in the mechanistic investigation of photosensitized dual catalytic systems with Ni(II) aryl complexes.

In photocatalytic processes, the excited state of the Ni complex can be accessed through sensitization or direct photoexcitation.86,87 Doyle and co-authors observed the metal-to-ligand charge transfer triplet excited state (3MLCT) of the Ni (II) aryl halide complex [Fig. 15(a)] using transient absorption.88,89 Unlike the lower-row transition metals, Ni(II) aryl complexes do not possess a long-lived 3MLCT.90 As shown in Figs. 15(b) and 15(c), the short-lived state is distinguished from the long-lived state by a subtle excited state absorption (ESA2) feature at ∼600 nm. The lifetime of the state represented by the ESA1 between 350 and 400 nm is insensible with the solvent polarity [Fig. 15(d)], providing indirect evidence to disfavor the assignment of this feature to a 3MLCT. A decisive proof for the assignment of relatively long-lived species to the 3d-d state was provided by the time-resolved IR (TR-IR) measurement of this complex with a carbonyl-decorated bipyridine ligand. Because the C=O stretch frequency is sensitive to changes in the electron distribution of the bipyridine ligand under different electronic states, a transition from the downshift to the upshift of vC=O in Fig. 15(e) indicates the evolution from the 3MLCT to the 3d-d states. The spectral feature of the latter state is predicted by DFT calculation. Therefore, the behavior of this Ni(II) complex in its excited state manifold can be summarized as the ultrafast formation of a 3MLCT state upon excitation, which quickly relaxes to the 3d-d excited state with a lifetime of a few nanoseconds. The geometric deformation associated with the evolution of the excited states of Ni(II) complexes was illustrated using NMR and interpreted with the assistance of DFT calculations. The full picture of the configurational change of the Ni complex during the excitation and relaxation processes is illustrated in Fig. 15(f). The authors speculated that the 3d-d state of the Ni(II) complex participated in the photoredox reaction by generating the aryl radical. The hypothesis was consistent with DFT calculations and verified by the EPR spin trapping experiment.

FIG. 15.

(a) Chemical structure of the Ni(II) complex. TA spectra of the Ni complies (R = H) in deoxygenated THF upon 530 nm pulsed laser excitation in (b) 0.6–2.2 and (c) 14–7000 ps time delay. (d) Excited state lifetimes measured by TA upon MLCT excitation in deoxygenated solvents. (e) Experimental TR-IR difference spectra of Ni(CO2Etbpy) (o-Tol)Cl in deoxygenated THF upon 610 nm pulsed laser excitation (3 µJ per pulse, 100 fs FWHM). Inset: DFT-calculated IR difference spectrum for the tetrahedral 3d-d state. (f) Relaxation pathway of Ni(t-Bubpy)(Ar)X following MLCT excitation. The structures shown were calculated by DFT (M06/TZVP//B3LYP/TZVP). Reproduced with permission from Ting et al., J. Am. Chem. Soc. 142, 5800 (2020). Copyright 2020 American Chemical Society.

FIG. 15.

(a) Chemical structure of the Ni(II) complex. TA spectra of the Ni complies (R = H) in deoxygenated THF upon 530 nm pulsed laser excitation in (b) 0.6–2.2 and (c) 14–7000 ps time delay. (d) Excited state lifetimes measured by TA upon MLCT excitation in deoxygenated solvents. (e) Experimental TR-IR difference spectra of Ni(CO2Etbpy) (o-Tol)Cl in deoxygenated THF upon 610 nm pulsed laser excitation (3 µJ per pulse, 100 fs FWHM). Inset: DFT-calculated IR difference spectrum for the tetrahedral 3d-d state. (f) Relaxation pathway of Ni(t-Bubpy)(Ar)X following MLCT excitation. The structures shown were calculated by DFT (M06/TZVP//B3LYP/TZVP). Reproduced with permission from Ting et al., J. Am. Chem. Soc. 142, 5800 (2020). Copyright 2020 American Chemical Society.

Close modal

Although there is clear evidence that the triplet excited state of the Ni complex can be an active intermediate for C–N coupling reactions,86,87 the fate of these reaction intermediates in the organometallic catalytic cycle remained unclear. In fact, a “dark” reaction mechanism was proposed, in which the redox reaction does not involve the excited states of the Ni(II) complex.91,92 After carefully estimating the quantum yield of each reaction step, Macmillan and co-authors found that it is the reductive elimination reaction of the Ni intermediates cycling between Ni(I) and Ni(III) that facilitates C–N coupling. The process does not need photoexcitation.91 By comparing the spectroelectrochemistry result with the TA spectra [Fig. 16(a)], they found that the Ir(III) photosensitizer was reduced by DABCO to form Ir(II). Interestingly, neither the Ir(II) species nor the DABCO·+ radical strongly reacted with the Ni(II) complex. The reduction of Ni(II) to the reactive Ni(I) species accounted for a minor portion of the removal of the Ir(II) species, as shown by the TA kinetics in Fig. 16(b). However, the small amount of Ni(I) species produced in this process is enough to compensate for the loss of the reactive Ni(I) species and to support the reaction. Time-resolved pulse radiolysis experiment that directly generates reductive Ni(I) species in the reaction system supports this argument.92 

FIG. 16.

(a) Difference spectrum taken from TA at 1000 ns pump–probe delay (blue), difference spectrum taken from spectroelectrochemistry [A(E = −1600 mV) − A(E = 0 mV)] (orange) normalized at λ = 850 nm, and the resulting difference spectrum from these two traces (blue–orange, inset). (b) Pump–probe difference spectra and temporal evolution profile monitored at λ = 540 nm (inset) for a mixture of Ir(III) photosensitizer, DABCO (486 mM), aryl bromide substrate (270 mM), hexylamine (405 mM), and NiBr2·3H2O (13.5 mM). (c) Schematic representation of photochemical pathways for the Ir(III) photosensitizer and Ni(II) precatalyst. (d) General Ni(I)/Ni(III) cross-coupling mechanism showing elementary steps involved in oxidation state changes. Reproduced with permission from Sun et al., J. Am. Chem. Soc. 142, 15830 (2020). Copyright 2020 American Chemical Society.

FIG. 16.

(a) Difference spectrum taken from TA at 1000 ns pump–probe delay (blue), difference spectrum taken from spectroelectrochemistry [A(E = −1600 mV) − A(E = 0 mV)] (orange) normalized at λ = 850 nm, and the resulting difference spectrum from these two traces (blue–orange, inset). (b) Pump–probe difference spectra and temporal evolution profile monitored at λ = 540 nm (inset) for a mixture of Ir(III) photosensitizer, DABCO (486 mM), aryl bromide substrate (270 mM), hexylamine (405 mM), and NiBr2·3H2O (13.5 mM). (c) Schematic representation of photochemical pathways for the Ir(III) photosensitizer and Ni(II) precatalyst. (d) General Ni(I)/Ni(III) cross-coupling mechanism showing elementary steps involved in oxidation state changes. Reproduced with permission from Sun et al., J. Am. Chem. Soc. 142, 15830 (2020). Copyright 2020 American Chemical Society.

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Nocera and co-authors reported a self-sustained Ni(I)/Ni(III) catalytic cycle for C–S cross-coupling reactions that share similar features in the mechanism as the C–N coupling reaction, in which the Ir photosensitizer pumps the metalorganic reaction system through a pyridinium electron shuttle.93 TA spectroscopic experiments tracked the reaction step by step and measured the rate constants, as shown in Fig. 17. In the photoactivation step, the excited Ir(III) photosensitizer reacted with iodide to generate Ir(II) species and I2•−, which have TA features at 525 and 700 nm [Fig. 17(a)], respectively. By tracking the evolution of these two spectral features with and without the presence of pyridinium, the author concluded that pyridinium could oxidize Ir(II) back to Ir(III). The rate of this reaction outcompetes the back electron transfer from I2•− to Ir(III), leaving the remaining I2•− to react with the thiol via a proton-coupled electron transfer (PCET) mechanism. Evidence of PhS· radical was found in the difference spectrum obtained by subtracting the TA spectrum collected without pyridinium from the one collected with pyridinium, and the rate constant of the PCET step (kPCET) is (2.6 ± 0.1) × 105 M−1 s−1 [Fig. 17(b)]. After identifying the processes involved in photoactivation, the Ni(II) complex was added for investigating the following reactions in the metallic catalysis cycle. Spectral features at 425 and 600 nm extracted from the TA difference spectrum were assigned to Ni(I) [Fig. 17(c)], which was primarily generated via reducing Ni(II) by the pyH· radical. Though Ni(I) can also be produced by reducing Ni(II) with Ir(II), the rate is much slower compared to the reaction with pyH·. From the decay of the Ni(I) signal with the presence of aryl iodide at different concentrations, the rate of oxidative addition can be extracted [Fig. 17(d)]. The final mechanism of the processes with the rate constants for various steps is shown in Fig. 17(e). This self-sustainable nature of the Ni catalytic cycle of C–S coupling is similar to that of mechanisms of the C–N and C–O coupling reactions.91,94 Nocera and co-authors compared the TA spectral feature of the C–O coupling system with the spectra of species in the same system generated via photochemical, electrochemical, and synthetic approaches. The solid spectral evidence in this work clarified the photochemical formation of a mixed-valent Ni(I/II) dimer in their system, as shown in Fig. 18. The Ni(I) species were identified to be the reactive intermediates, while the comproportionation of Ni(I) and Ni(III) that brings back Ni(II) was found to be a deactivation pathway.

FIG. 17.

(a) TA spectra of a solution containing 150 µM Ir(III) and 25 mM TBAI or pyHI (λexc = 430 nm). The inset shows that the corresponding TA spectrum at 30 ns can be deconvolved into contributions from Ir(II) (red) and I2•− (blue). (b) TA kinetic trace probed at 700 nm for the solution of S2 (150 µM Ir(III) and 25 mM pyHI) with 150 mM thiophenol, 150 mM thiophenol in the presence of 200 mM pyridine, and 150 mM 4-methoxybenzyl mercaptan in the presence of 200 mM pyridine (λexc = 430 nm). The faster decay for solutions containing pyridine is due to PCET between I2•− and thiol with pyridine as a base. The inset shows the difference TA spectrum at 6.3 µs (gray) for solutions of S2 with 150 mM thiophenol in the absence and presence of 200 mM pyridine; this difference spectrum matches the TA spectrum of thiophenoxyl radical (red) obtained independently from directly exciting diphenyl disulfide at λexc = 355 nm. (c) The difference between TA spectrum at 7.5 µs for solution S3 (S2 + 200 mM pyridine and 150 mM 4-methoxybenzyl mercaptan) and S4 [S3 + 10 mM Ni(II)] (parent spectra shown in the inset), revealing the presence of a putative Ni(I) intermediate. (d) TA kinetic trace measured at 600 nm for solution S4 with 0.1 and 0.5M 4-iodotoluene, 1a. The faster decay for solutions with higher concentrations of aryl iodide implies the oxidative addition of aryl iodide to Ni(I). (e) Complete reaction mechanism and key rate constants for photoredox-mediated nickel-catalyzed aryl thiolation. Reproduced with permission from Sun et al., J. Am. Chem. Soc. 143, 2005 (2021). Copyright 2021 American Chemical Society.

FIG. 17.

(a) TA spectra of a solution containing 150 µM Ir(III) and 25 mM TBAI or pyHI (λexc = 430 nm). The inset shows that the corresponding TA spectrum at 30 ns can be deconvolved into contributions from Ir(II) (red) and I2•− (blue). (b) TA kinetic trace probed at 700 nm for the solution of S2 (150 µM Ir(III) and 25 mM pyHI) with 150 mM thiophenol, 150 mM thiophenol in the presence of 200 mM pyridine, and 150 mM 4-methoxybenzyl mercaptan in the presence of 200 mM pyridine (λexc = 430 nm). The faster decay for solutions containing pyridine is due to PCET between I2•− and thiol with pyridine as a base. The inset shows the difference TA spectrum at 6.3 µs (gray) for solutions of S2 with 150 mM thiophenol in the absence and presence of 200 mM pyridine; this difference spectrum matches the TA spectrum of thiophenoxyl radical (red) obtained independently from directly exciting diphenyl disulfide at λexc = 355 nm. (c) The difference between TA spectrum at 7.5 µs for solution S3 (S2 + 200 mM pyridine and 150 mM 4-methoxybenzyl mercaptan) and S4 [S3 + 10 mM Ni(II)] (parent spectra shown in the inset), revealing the presence of a putative Ni(I) intermediate. (d) TA kinetic trace measured at 600 nm for solution S4 with 0.1 and 0.5M 4-iodotoluene, 1a. The faster decay for solutions with higher concentrations of aryl iodide implies the oxidative addition of aryl iodide to Ni(I). (e) Complete reaction mechanism and key rate constants for photoredox-mediated nickel-catalyzed aryl thiolation. Reproduced with permission from Sun et al., J. Am. Chem. Soc. 143, 2005 (2021). Copyright 2021 American Chemical Society.

Close modal
FIG. 18.

(a) TA spectrum obtained after exciting (λexc = 425 nm) a solution of 0.2 mM [Ir(dF–CF3–ppy)2-(dtbbpy)][PF6], 5 mM NiCl2(dme), 5 mM dtbbpy, and 50 mM quinuclidine in CH3CN. (b) Difference spectra observed from in situ monitoring of the photoredox reaction (top), from spectroelectrochemistry (middle), and from the comproportionation reaction (bottom). (c) Crystal structure of the synthesized dimer of the Ni complex collected at 100 K with thermal ellipsoids drawn at 50% probability. Reproduced with permission from Sun et al., J. Am. Chem. Soc. 141, 89 (2019). Copyright 2019 American Chemical Society.

FIG. 18.

(a) TA spectrum obtained after exciting (λexc = 425 nm) a solution of 0.2 mM [Ir(dF–CF3–ppy)2-(dtbbpy)][PF6], 5 mM NiCl2(dme), 5 mM dtbbpy, and 50 mM quinuclidine in CH3CN. (b) Difference spectra observed from in situ monitoring of the photoredox reaction (top), from spectroelectrochemistry (middle), and from the comproportionation reaction (bottom). (c) Crystal structure of the synthesized dimer of the Ni complex collected at 100 K with thermal ellipsoids drawn at 50% probability. Reproduced with permission from Sun et al., J. Am. Chem. Soc. 141, 89 (2019). Copyright 2019 American Chemical Society.

Close modal

Although the above-mentioned studies have illustrated that the reactions of C–N and C–S coupling undergo the Ni(I)/Ni(III) cycle, the role of the excited state of the Ni(II) complex in the reaction was not clarified. In the C–N coupling reaction, increasing the concentration of the excited state of the Ni(II) complex can increase the reactivity.86,87 It has been proposed that the excited state of Ni(II) species can generate the reactive Ni(I) species through an intramolecular electron transfer mechanism.87 On the other hand, a disproportionation mechanism in which the excited Ni (II) species reacts with another Ni(II) complex in its ground state to give the reactive Ni(I) species along with a Ni(III) species was proposed by Doyle and co-authors.88 In the dual catalytic C–O coupling reaction, spectral evidence did not support the formation of Ni(I) through the disproportionation mechanism.95 As shown in Figs. 19(a) and 19(b), in the photosensitization step, the Ir(III) sensitizer showed a concerted decay of TA kinetics of the GSB at 410 nm and the ESA at 520 nm, indicating energy transfer from the Ir photosensitizer to generate the excited state of the Ni(II) complex. The independence of the excited state lifetime of the Ni(II) complex to the concentration disfavored the disproportionation mechanism [Fig. 19(c)]. Therefore, a direct reaction pathway through the excited state of the Ni(II) complex via reductive elimination was proposed by MacMillan and the authors, as shown in Fig. 19(d). However, the authors also suggested the possibility of an existing oxidation reaction channel involving Ni(III) when the Ni center is coupled with different substrates. The Ni(III) species are likely formed through electron transfer with an Ir photosensitizer.

FIG. 19.

(a) Temporal evolution of photoinduced dynamics of a mixture of 96 µM Ir-photosensitizer (1), 192 µM Ni(II)(Ar) acetate (2), and 3.8 mM tert-butyl-isopropyl amine in DMF, λexc = 400 nm (500 nJ/pulse). (b) Single-wavelength kinetic traces of the corresponding mixture in (a) at 410 nm (τ = 0.94 ± 0.04 µs) and 520 nm (τ = 1.03 ± 0.01 µs). (c) Single-wavelength traces at 405 nm of TA spectra of 2 with different concentrations in DMF, λexc = 400 nm (1000 nJ/pulse). (d) Schemes of excitation-induced pathway and oxidation-induced pathway between excited state and ground state molecules of 2 eventually leading to product formation. Reproduced with permission from Tian et al., J. Am. Chem. Soc. 142, 4555 (2020). Copyright 2020 American Chemical Society.

FIG. 19.

(a) Temporal evolution of photoinduced dynamics of a mixture of 96 µM Ir-photosensitizer (1), 192 µM Ni(II)(Ar) acetate (2), and 3.8 mM tert-butyl-isopropyl amine in DMF, λexc = 400 nm (500 nJ/pulse). (b) Single-wavelength kinetic traces of the corresponding mixture in (a) at 410 nm (τ = 0.94 ± 0.04 µs) and 520 nm (τ = 1.03 ± 0.01 µs). (c) Single-wavelength traces at 405 nm of TA spectra of 2 with different concentrations in DMF, λexc = 400 nm (1000 nJ/pulse). (d) Schemes of excitation-induced pathway and oxidation-induced pathway between excited state and ground state molecules of 2 eventually leading to product formation. Reproduced with permission from Tian et al., J. Am. Chem. Soc. 142, 4555 (2020). Copyright 2020 American Chemical Society.

Close modal

Systematic work on Ni metallaphotocatalytic systems has demonstrated that through careful experimental design and in conjunction with other chemical and analytical methods, the insight provided by time-resolved spectroscopy investigations can go beyond the initial “bright” stage of photosensitization and help interpret the evolution of reaction intermediates in the following “dark” cycle in depth. The challenging complexities in the organic reaction systems involve side reactions that might not be relevant to the desired processes and the possible coexistence of multiple branches that lead to the same final product. Currently, such mechanistic investigations are still behind the development of novel reaction systems. Future works need to expand to metal photocatalytic systems based on other transition metals, such as Pd, Cu, Au, and Co. Moreover, most recent spectroscopic studies heavily rely on UV-to-visible TA spectroscopy, which is not very effective in distinguishing different transient species. It will be necessary to interrogate the mechanisms through multiple spectral channels and cross-check the hypothetical mechanisms using different analytic techniques. A comprehensive toolkit is necessary to figure out every jigsaw piece for completing the full picture. Finally, the time scale of elementary steps in these reaction systems span a broad range. Many time-resolved setups designed for facilitating fast response may not be compatible with reacting kinetics. Distinct kinetic evolution in sequential reactions may lead to low concentrations of intermediates, which requires sensitive probe techniques to conduct quantitative analysis. A recent comprehensive investigation on the photoredox reaction of α-aminoarylation set a successful example of disentangling subtle spectral features in the micro-to-millisecond time scale.96 

In conclusion, the mechanistic investigation of photocatalytic systems should clarify the primary reaction pathways involved, evaluate the kinetics of related reaction steps, indicate the possible limitations in regard to reaction efficiency, and provide a roadmap for improving the reaction performance. The first task for time-resolved spectroscopic studies is to identify unknown reaction intermediates based on their spectral features. This can be achieved by comparing the transient spectra with the standard spectra of known species prepared using other methods or with simulated spectra obtained computationally. The lifetimes and the transition rates between intermediates can also aid in the assignment of unknown species. The identified reaction intermediates outline the basic structure of the mechanistic model. Next, the kinetic data associated with the reaction intermediates are incorporated to reveal the evolution of reaction dynamics. Finally, through quantitatively comparing the rates of different steps involved in the reaction process, the rate-limited steps and the major detrimental processes need to be pinpointed, and the nodal points that determine the product selectivity should also be addressed, providing clues for optimizing the reaction performance.

Examples presented in this Perspective show that photocatalytic systems are generally more complicated compared to other photophysical/chemical systems that have been investigated with time-resolved spectroscopy. They comprise multiple reactive species, namely, photosensitizers, cocatalysts, substrates, and other active reagents, like charge carrier shuttles. The number of involved electronic transitions surges with the active components. The coexistence of parallel and reverse side reactions, as well as branched pathways that may be irrelevant to the primary pathways, confounds the search for correct reaction models. Therefore, good chemical intuition with openness to different possibilities is critical while sketching hypothetical models. Computational investigations that provide the energy landscapes of reaction intermediates can be a good prescreen of probable reaction routes.

Correctly assigning spectral features to different transient species is the first challenge. The commonly used TA approach is based on measuring broad electronic transitions that lack unique spectral features. Moreover, signals of different species often overlap with each other. Singular value decomposition with global kinetic fitting can help unravel the convolved spectral features and extract species-associated kinetics.97,98 The SVD method must start with a good hypothetical model, which largely depends on intelligent empirical guesses of the dynamic processes involved. Examples of employing SVD analysis are provided in Refs. 74 and 78.

The intermediates are mostly short-lived species and are rapidly consumed in the sequential reaction steps, so spectroscopic measurements must confront the difficulties associated with the low concentrations of targeting objects. A careful experimental design by stepwise increment of the number of reaction components is a practical approach to peel the complexity layer-by-layer. In Ref.93, the authors highlighted the active species in each reaction step by taking different spectra of samples with stepwise addition of reaction components.93 The kinetic evolution can also be extracted from the difference of kinetic traces. In some scenarios, directly generating large amounts of intermediates using non-photochemical methods is a feasible approach if the intermediates can be stabilized so that time-resolved spectroscopy can focus on the following steps after the initial intermediates.

Another difficulty is that some of the key components or processes may not have clear spectral signals in the probed channel. For instance, the ground state bleach signal of cadmium chalcogenide nanocrystals in the visible spectral range is dominated by the electron filling effect and thus cannot reflect the kinetics associated with hole transfer. In Ref. 52, the hole kinetics had to be inferred by comparing the TR-PL kinetics with the ground state bleach kinetics.52 It is often necessary to probe in multiple spectroscopic windows: In this case, the kinetics of photoinduced electrons and holes can be monitored separately based on their resolved intraband transitions in the near-IR window.99 Another common strategy to address the missing links is to reproduce the photochemical process of interest using probe molecules that have prominent spectral features. Spectroscopists have gathered a repertoire of exciton and charge carrier acceptors from earlier works to illustrate energy and charge transport processes. If none of the optical spectroscopic approaches can track the transition, then we must turn to non-real-time methods and design model chemical reactions that respond to other analytical tools, such as mass spectroscopy, NMR, and EPR experiments, to figure out the right course.

The complexities related to photocatalytic systems should motivate the application of more sophisticated spectroscopic tools in this field. We have highlighted the potential of vibrational spectroscopy to provide important complementary information. In a similar work, in Ref. 89, the authors employed the mid-IR spectral window that covers the carboxylic/carbonyl stretching bands as signatures to probe the excited states of metal complex photocatalysts100–104 and to monitor charge transfer to the surface adsorbed molecules.105,106 TR-Raman, with a broader frequency range, can provide a more comprehensive picture of the excited states of various organic molecules in photoreactions.107–110 With the assistance of quantum computation, these vibrational fingerprints are more suitable for identifying transient species than the featureless electronic transition bands in UV–vis TA spectra. We would like to suggest an experimental design to combine the advantages of UV–vis TA and TR-resonance Raman spectroscopy. TA spectra that capture electronic transitions of multiple transient states can serve as a roadmap for selecting resonance Raman pump wavelengths in the following stimulated resonance Raman (SRR) experiment. Using a Raman pump that resonates with a specific electronic transition observed by TA, one can selectively collect Raman spectra of the species of interest. It is possible to rely on TA to conveniently record the global kinetic information and use vibrational signatures captured by SRR to reconfirm the assignments of key intermediates. Resonance-enhanced Raman signals can also be employed to resolve the overlapping issue in TA spectra and selectively trace the kinetics of each species in reaction processes.

We also believe that surface/interface-specific spectroscopic methods, such as surface-enhanced IR/Raman spectroscopy and sum-frequency generation (SFG) spectroscopy, when integrated with the time-resolved approach, can be helpful in unraveling photocatalytic reactions that occur on the surface of catalytic materials. Many photocatalytic reactions using solid-state materials or nanoparticles require reactive species to be in close contact with these catalysts to ensure efficient transportation of excitons and charge carriers.111,112 Moreover, the asymmetric surface assembly of reaction intermediates can induce unique configurational selectivity.38,113–116 Surface-specific spectroscopic methods that monitor vibrational patterns can distinguish the adsorbed reaction intermediates from species in the bulk solution and determine their special conformations at the catalytic sites.

After many decades of development of time-resolved spectroscopy applications, researchers can now answer the increasing need for a mechanistic understanding of photocatalytic reactions by not only analyzing simplified model systems but also comprehensively interpreting the actual reaction courses in detail. With the advances made in the area of photocatalysis, plenty of challenging problems await solutions. Besides developing state-of-the-art techniques, carefully designing experiments and analyzing data by considering evidence from different perspectives will lay the foundation for a successful mechanistic investigation of these complicated systems.

The authors acknowledge Queens College and the City University of New York for financial and facility support. This work was partly supported by the PSC-CUNY award.

The authors have no conflicts to disclose.

Chen Wang: Conceptualization (equal); Investigation (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal). Aaron Malinoski: Writing – original draft (equal); Writing – review & editing (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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