We use native vibrational modes of the model singlet fission chromophore 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) to examine the origins of singlet fission in solution between molecules that are not tethered by a covalent linkage. We use the C—H stretch modes of TIPS side groups of TIPS-Pn to demonstrate that singlet fission does not occur by diffusive encounter of independent molecules in solution. Instead, TIPS-Pn molecules aggregate in solution through their TIPS side groups. This aggregation breaks the symmetry of the TIPS-Pn molecules and enables the formation of triplets to be probed through the formally symmetry forbidden symmetric alkyne stretch mode of the TIPS side groups. The alkyne stretch modes of TIPS-Pn are sensitive to the electronic excited states present during the singlet fission reaction and provide unique signatures of the formation of triplets following the initial separation of triplet pair intermediates. These findings highlight the opportunity to leverage structural information from vibrational modes to better understand intermolecular interactions that lead to singlet fission.

Singlet fission offers the potential to multiply excitons in conjugated organic materials to enable more efficient utilization of the solar spectrum.1 For example, singlet fission sensitized photovoltaics could exceed the Shockley-Queisser limit for single junction cells by reducing thermalization losses from higher energy photons, with theoretical power conversion efficiencies increased from ∼31% to 44%.2 Spectroscopic studies of model systems such as 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) have demonstrated that singlet fission to form proximal, spin-correlated triplet pair intermediates 1(TT) can occur on the hundreds of femtosecond time scale3,4 and with unit quantum yield in crystalline solids or dimers, resulting in the potential for 200% quantum yield of triplet excitons.5,6 Equation (1) highlights the sequence of intermediates that are believed to be involved in singlet fission.7 However, recent studies reveal that the time scale on which the separated triplet pairs 1(T⋯T) complete their separation process in crystalline TIPS-Pn can extend into the hundreds of picosecond time regime.8,9 This observation suggests that spin-allowed relaxation processes may compete with triplet pair separation even in systems like pentacenes that undergo exoergic singlet fission,10,11

S1S01(TT)1(TT)T1+T1.
(1)

Recent work has examined the dynamics of singlet fission to understand the interplay between intermolecular interactions and relaxation processes that can compete with triplet pair separation.10 It has been suggested that transient encounter complexes between TIPS-Pn molecules diffusing in solution are sufficient to enable quantitative singlet fission even without preforming dimers using covalently bonded architectures.5,12 Concentration dependent studies of the electronic spectra of TIPS-Pn in solution suggested that the molecules did not aggregate under conditions where singlet fission was observed. This led to the suggestion that brief collisional encounters within the excited state lifetime are sufficient for singlet fission to occur with unit quantum yield in TIPS-Pn.5,12 Other investigators used molecular architectures that permit control of intermolecular interactions including substitutions of peripheral groups that influence crystalline molecular packing arrangements,10,13 crystallization into different polymorphs,14–19 and cocrystallization with inert species.20–22 A particularly instructive method has been to use covalently bound dimers to precisely define intermolecular interactions so these can be correlated with the dynamics of singlet fission.6,11,13,23–27

In our prior work,8,28 we sought to identify new spectroscopic probes of separated triplet pair intermediates 1(T⋯T) that could directly examine their formation and separation dynamics to help clarify the influence that molecular structures and crystallinity have on triplet separation vs relaxation processes. Toward this end, we demonstrated that the alkyne (C≡C) stretch vibrational modes of the TIPS side groups of TIPS-Pn [Fig. 1(a)] provide unique signatures of triplet states that form following the initial separation of proximal triplet pairs 1(TT).28 The experiments utilized an ultrafast visible pump pulse tuned to the S0 → S1 transition of TIPS-Pn at 655 nm [Fig. 1(b)] to create an initial population of singlet excited states in crystalline films that underwent the sequence of steps represented in Eq. (1) leading to independent triplets. The dynamics of the electronic states involved in the singlet fission reaction were probed through the alkyne stretch modes using an ultrafast mid-infrared (mid-IR) probe pulse. The alkyne stretch modes include the formally symmetry forbidden symmetric stretch around νsym = 2090 cm−1 and a strongly allowed antisymmetric stretch around νas = 2130 cm−1 [Fig. 1(c)].

FIG. 1.

(a) Structure of TIPS-Pn with symmetric and antisymmetric alkyne stretch modes highlighted. (b) Beam geometry used for ultrafast mid-IR transient absorption measurements. (c) FTIR spectrum of the TIPS-Pn film in the region of alkyne stretch modes. (d) Mid-IR transient absorption spectra of TIPS-Pn films measured at 1 and 10 ps time delays. The data are overlaid with best fit and basis spectra describing the ground state bleach and triplet absorption features. Reprinted with permission from C. Grieco et al., J. Phys. Chem. Lett. 8, 5700–5706 (2017). Copyright 2017 The American Chemical Society.

FIG. 1.

(a) Structure of TIPS-Pn with symmetric and antisymmetric alkyne stretch modes highlighted. (b) Beam geometry used for ultrafast mid-IR transient absorption measurements. (c) FTIR spectrum of the TIPS-Pn film in the region of alkyne stretch modes. (d) Mid-IR transient absorption spectra of TIPS-Pn films measured at 1 and 10 ps time delays. The data are overlaid with best fit and basis spectra describing the ground state bleach and triplet absorption features. Reprinted with permission from C. Grieco et al., J. Phys. Chem. Lett. 8, 5700–5706 (2017). Copyright 2017 The American Chemical Society.

Close modal

We demonstrated that the resulting mid-IR transient absorption spectra consisted of a broad electronic transition superimposed on narrow vibrational features of the antisymmetric alkyne stretch mode [Fig. 1(d)].8,28 The broad absorption feature corresponded to an absorption of the singlet excited state to a low lying doubly excited state of TIPS-Pn that has been proposed to participate in singlet fission.29–32 This feature also provided a probe of the dynamics of the proximal triplet pair 1(TT) intermediate as it completed its separation into the 1(T⋯T) state on the hundreds of picosecond time scale.8 Comparison of the time-evolution of the vibrational features of the alkyne stretch mode in the transient spectra measured at 1 ps and 10 ps time delays in Fig. 1(d) revealed that a distinct alkyne stretch vibrational mode of the TIPS side groups appeared as an induced absorption feature around 2120 cm−1 on the few picosecond time scale.28 A spectral fitting procedure was utilized to show that the growth of this vibrational feature [labeled T in Fig. 1(d)] occurred synchronously with the decay of the 1(TT) broad electronic transition in the mid-IR and with the growth of the T1 → Tn transition in the visible spectra range.8 Both the decay of the broad electronic transition and the growth of the T1 → Tn have been assigned to separation of proximal triplet pairs.8,33 Therefore, the formation of the vibrational feature at 2120 cm−1 on the few picosecond time scale was assigned to a signature of triplet excited states that had separated from their proximal triplet pair 1(TT) intermediates.28 

In this work, we use the structural specificity of vibrational modes of TIPS-Pn to build on our prior results and explore how singlet fission occurs in solutions of untethered molecules. Our findings reveal that TIPS-Pn molecules actually form aggregates in solution at concentrations where singlet fission has been reported,5,12 but they do so through their side groups rather than through π-π interactions of their conjugated cores when dissolved in chlorinated solvents. We reveal that these interactions break the symmetry of the TIPS-Pn molecules, which modulates the amplitudes of the vibrational features of the alkyne stretch modes of triplet excited states in solution vs in crystalline films. These findings reveal that the sensitivity of vibrational modes to their local bonding and chemical environments provides new information about the interactions of molecules that enable singlet fission in both crystalline and amorphous environments.

Solutions of TIPS-Pn were prepared by dissolving appropriate amounts of TIPS-Pn in CCl4 sufficient to make 0.100 M (100 mM) concentration and then using serial dilution steps to access a range of concentrations between 6 × 10−6 M and 0.100 M. Depending on the concentration, solutions were loaded into a homemade liquid cell composed of sapphire or KBr optical flats sandwiching an optical spacer of varying thicknesses between 1.7 mm and 50 μm. For the most concentrated solutions, liquid samples consisted of a liquid film formed by pressing two optical flats together without an optical spacer. Surface tension then set the liquid film thickness to be a few micrometers as determined from etalon fringe spacings observed in the mid-IR absorption spectra. The most dilute solutions were measured using a 1 cm pathlength quartz cuvette in the visible to near-IR spectral regions where the quartz transmission was sufficiently high. Care was taken to maintain peak optical densities in the visible absorption spectra below unity for the spectroscopy measurements where possible. We note that this was not possible in the most concentrated 0.075 M and 0.100 M solutions of TIPS-Pn where the few micrometer thick liquid films had peak optical densities near 3. This introduced self-absorption artifacts in the fluorescence spectra that are discussed below.

Crystalline films of TIPS-Pn in the Form-I brickwork phase were prepared by spin-coating 20 mg/mL solutions of TIPS-Pn in dichloromethane onto 2.5 cm diameter CaF2 substrates at ∼800 rpm. The as-cast films were initially amorphous34 and were converted to the Form-I brickwork phase by annealing them in solvent vapor for ∼2.5 h as has been previously described.34 

Steady-state absorption spectroscopy measurements were conducted in the visible to near-IR and mid-IR spectral regions in the following ways. Some visible absorption spectra were collected using a commercially available spectrometer (Beckman, DU 520; Brea, CA). Other absorption spectra were collected using a home-built instrument constructed from a stabilized tungsten-halogen lamp (SLS-201, Thorlabs; Newton, NJ) that was collimated and passed through the sample as previously described.34 The transmitted beam was detected using a CCD spectrometer (USB-2000, Ocean Optics; Dunedin, FL). FTIR spectra in the mid-IR were measured using a commercially available instrument (Madison Instruments, Mattson Research Series; Middleton, WI) equipped with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector.

Nanosecond mid-IR transient absorption spectra were measured using an inspIRe nanosecond transient absorption system (Magnitude Instruments, State College, PA) that utilized a frequency doubled Nd:YAG nanosecond pulsed laser, which was used to pump a dye laser to make tunable excitation pulses in the visible spectral range as previously described.35 A MoSi2 infrared element was used to generate the probe light, which was collimated and transmitted through the sample, dispersed in a monochromator, and detected with a mercury cadmium telluride (MCT) detector.

Nanosecond visible and near-IR transient absorption spectra were measured using an enVISion transient absorption system (Magnitude Instruments, State College, PA) that also utilized the frequency doubled Nd:YAG nanosecond pulsed laser and dye laser combination described above.34,35 The probe source consisted of a tungsten halogen lamp that was collimated and transmitted through the sample, dispersed in the monochromator, and detected with appropriate detectors. Depending on the wavelength range, silicon or InGaAs photodiode detectors were used to detect the transmitted probe beam after the monochromator. Steady-state fluorescence spectra were also measured with the enVISion spectrometer with the probe beam blocked during the experiment. The time-resolved fluorescence traces obtained from the fast digitizers in the instrument were integrated to produce the steady-state spectra.

Ultrafast visible pump/mid-IR probe spectroscopy measurements were performed using a home-built system as described elsewhere.36 The visible pump beam was tuned to 655 nm, while the probe beam was tuned over the range of ∼4.4–4.9 µm.

Time-dependent density functional theory (TDDFT) calculations employing the Tamm-Dancoff approximation37 were used to obtain the singlet excitation energies of TIPS-Pn molecules. The calculations were performed using the long-range corrected ω-PBE exchange-correlation functional (ω = 0.3)38–41 and with a cc-PVTZ expansion for the valence molecular orbitals within the Gaussian09 software package. These have previously been shown to be effective for describing both ground and excited state properties in pentacene molecules.42 These calculations were used to compute isosurfaces of the natural transition orbitals (NTOs) of the singlet state of TIPS-Pn to highlight the overlap with the alkyne groups.

Figure 2(a) highlights the antisymmetric alkyne stretch modes corresponding to the ground state bleach (GSB) and triplet absorption peak (T) of TIPS-Pn in the crystalline Form-I brickwork film. Figure 2(b) depicts the basis functions that were used in our earlier work to produce the fit to the TA spectrum measured at 10 ps that is represented in Fig. 2(a).28 The vertical lines serve as guides to the eye to show the relationship of the basis spectra to the experimental data. The FTIR spectrum of the sample measured at room temperature was used to represent the ground state bleach of the alkyne stretch. A skewed Lorentzian line shape was introduced to represent the triplet absorption peak corresponding to the antisymmetric stretch of the triplet state. A weighted sum of these basis spectra overlaid on the broad absorption offset produced the best fit spectrum describing the transient absorption data measured at 10 ps.28 The comparison of the basis spectra in Fig. 2(b) demonstrates that the triplet absorption peak possesses a vibrational frequency that is Δν ∼ 10 cm−1 lower than the corresponding antisymmetric alkyne stretch mode of the ground electronic state around 2130 cm−1. The origin of this shift in frequency in the triplet state is still under investigation and will be the subject of a forthcoming publication. We note that the isosurface of the NTO corresponding to the lowest unoccupied molecular orbital of TIPS-Pn that is involved in the S0 → S1 transition [inset of Fig. 2(b)] has significant overlap with the alkyne groups. This suggests that changes of electron density in the excited state or position of the nodes can influence the vibrational potential and, therefore, the frequency of the alkyne stretch modes. We speculate that subtle differences in the singlet vs triplet excited states, electron densities or node locations may modulate the vibrational frequency of the alkyne stretch modes and give rise to the ∼10 cm−1 shift to lower frequency.

FIG. 2.

(a) Mid-IR transient absorption spectrum of a TIPS-Pn film measured at 10 ps time delay. Reproduced from Fig. 1 for comparison. Reprinted with permission from C. Grieco et al., J. Phys. Chem. Lett. 8, 5700–5706 (2017). Copyright 2017 The American Chemical Society. (b) Basis spectra used to fit the mid-IR transient absorption spectra at 10 ps time delay. The FTIR spectrum represents the ground state bleach (GSB) of the antisymmetric alkyne (C≡C) stretch. A skewed Lorentzian function represents the triplet exited state absorption (T Abs.). These features measured in the crystalline film differ in frequency by Δν = 10 cm−1. Mid-IR transient absorption spectra of TIPS-Pn of 1 mM (c) and 25 mM (d) solutions in CCl4 measured at a range of time delays. The triplet absorption feature is visible at 2080 cm−1 and differs in frequency with the symmetric alkyne stretch by Δν = 10 cm−1. The FTIR spectrum of the solution appears at the bottom. Reprinted with permission from C. Grieco et al., J. Phys. Chem. Lett. 8, 5700–5706 (2017). Copyright 2017 The American Chemical Society.

FIG. 2.

(a) Mid-IR transient absorption spectrum of a TIPS-Pn film measured at 10 ps time delay. Reproduced from Fig. 1 for comparison. Reprinted with permission from C. Grieco et al., J. Phys. Chem. Lett. 8, 5700–5706 (2017). Copyright 2017 The American Chemical Society. (b) Basis spectra used to fit the mid-IR transient absorption spectra at 10 ps time delay. The FTIR spectrum represents the ground state bleach (GSB) of the antisymmetric alkyne (C≡C) stretch. A skewed Lorentzian function represents the triplet exited state absorption (T Abs.). These features measured in the crystalline film differ in frequency by Δν = 10 cm−1. Mid-IR transient absorption spectra of TIPS-Pn of 1 mM (c) and 25 mM (d) solutions in CCl4 measured at a range of time delays. The triplet absorption feature is visible at 2080 cm−1 and differs in frequency with the symmetric alkyne stretch by Δν = 10 cm−1. The FTIR spectrum of the solution appears at the bottom. Reprinted with permission from C. Grieco et al., J. Phys. Chem. Lett. 8, 5700–5706 (2017). Copyright 2017 The American Chemical Society.

Close modal

In our earlier work, we used a concentration dependent study of TIPS-Pn solutions to assign the alkyne stretch mode of the triplet state.28Figures 2(c) and 2(d) represent the mid-IR transient absorption spectra of TIPS-Pn in CCl4 solution at 1 mM and 25 mM, respectively. The spectra are reproduced from our earlier work in Ref. 28 to facilitate comparison of the vibrational features in solution with those in the crystalline film. The transient absorption spectra measured in the 1 mM solution exhibit the ground state bleach of the alkyne stretch modes superimposed on the broad electronic transition of the singlet excited state that decays with the fluorescence lifetime of TIPS-Pn.8 At this concentration, a combination of intersystem crossing (ISC) and possibly a limited amount of singlet fission5,12 produces a small population of triplet states on the microsecond time scale that gives rise to the triplet absorption feature around 2080 cm−1 in Fig. 2(c).28 

TIPS-Pn molecules in solution have been shown to undergo singlet fission in the 25 mM and higher concentration range in chloroform (CHCl3)5 and other solvents.12 For completeness, we verified that TIPS-Pn molecules also undergo singlet fission in CCl4 in this concentration range by monitoring the spectral signatures of the S1 → Sn and the T1 → T2 transitions in the near-infrared region.8 We note that we used CCl4 rather than chloroform in these studies because of its superior mid-IR transmission properties. Figure 3(a) represents near-IR transient absorption spectra of the S1 → Sn transition of 25 mM and 100 mM solutions of TIPS-Pn in CCl4 following optical excitation at 642 nm. The solutions were purged of oxygen by bubbling with nitrogen gas prior to the measurements to prevent triplet quenching. Figure 3(b) depicts transient absorption kinetics traces measured at the peak of the S1 → Sn transition at 1400 nm for a range of concentrations of TIPS-Pn. In the lower concentration solutions with 0.5 mM and 1 mM TIPS-Pn, the transient absorption kinetics do not depend on concentration and decay with the fluorescence lifetime of TIPS-Pn in CCl4.8 However, at the higher 25 mM and 100 mM concentrations of TIPS-Pn, the transient absorption signal at 1400 nm decays more rapidly and becomes instrument limited. This faster decay of the S1 excited state population results from singlet fission occurring rapidly in solution. The decay of the singlet population of TIPS-Pn in chloroform has been correlated with the growth of the triplet population and has been assigned to singlet fission in solution occurring on the nanosecond time scale.5 The data in Fig. 3(b) are consistent with singlet fission occurring on the same time scale in CCl4.

FIG. 3.

(a) Near-IR transient absorption spectrum of TIPS-Pn in CCl4 solution at 25 mM and 100 mM measured in the region of the S1 → Sn transition. (b) Transient absorption kinetics measured at 1400 nm of TIPS-Pn in CCl4 at various concentrations. (c) Near-IR transient absorption spectra of TIPS-Pn in CCl4 at 1 mM, 25 mM, and 100 mM measured between 100 and 500 ns after singlet transient feature decays completely in the region of the T1 → T2 transition. (d) Transient absorption kinetics traces measured at 970 nm of TIPS-Pn in CCl4 at various concentrations. The fast rise in the 0.5 and 1 mM solutions results from overlap of the S1 → Sn transition that dominates before 100 ns.

FIG. 3.

(a) Near-IR transient absorption spectrum of TIPS-Pn in CCl4 solution at 25 mM and 100 mM measured in the region of the S1 → Sn transition. (b) Transient absorption kinetics measured at 1400 nm of TIPS-Pn in CCl4 at various concentrations. (c) Near-IR transient absorption spectra of TIPS-Pn in CCl4 at 1 mM, 25 mM, and 100 mM measured between 100 and 500 ns after singlet transient feature decays completely in the region of the T1 → T2 transition. (d) Transient absorption kinetics traces measured at 970 nm of TIPS-Pn in CCl4 at various concentrations. The fast rise in the 0.5 and 1 mM solutions results from overlap of the S1 → Sn transition that dominates before 100 ns.

Close modal

Figure 3(c) represents transient absorption spectra measured between 100 and 500 ns in the region of the T1 → T2 transition after the higher frequency tail of the S1 → Sn transition decays to zero.8 The spectra exhibit a distinct vibronic progression. Figure 3(d) depicts transient absorption kinetics measured at the 0-0 vibronic peak of the T1 → T2 transition at 970 nm at several concentrations of TIPS-Pn in CCl4. The kinetics traces have been scaled to match the shapes of the decays at time delays greater than 2 µs. In the solutions with 0.5 mM and 1 mM concentrations of TIPS-Pn, a slow rise of the T1 population is observed, which results from ISC. The fast rise in the low concentration kinetics results from a combination of overlap of the S1 → Sn transition before 100 ns and possibly a limited amount of singlet fission among TIPS-Pn molecules. In contrast, at the higher 25 mM and 100 mM concentrations, triplets are formed in solution on the few nanosecond time scale [Fig. 3(b)].5 On the microsecond time scale plot used to represent the data in Fig. 3(d), this nanosecond growth of the triplet absorption feature appears as an abrupt rise around zero time delay.

We verified that TIPS-Pn molecules do not form aggregates through π-π interactions of their conjugated pentacene cores in CCl4 solutions under the concentration range examined here. Figure 4(a) displays visible absorption spectra of TIPS-Pn solutions in CCl4 at a range of concentrations. The spectra have been normalized to facilitate a quantitative comparison of their vibronic progressions and peak positions. Where studies of covalent dimers in solution have demonstrated red-shifts of the electronic absorption spectra of monomer units as a result of π-π interactions,43 no such red-shifts are observed among untethered TIPS-Pn molecules in CHCl3 solution.5 The comparison in Fig. 4(a) demonstrates that the absorption spectra are superimposable at all concentrations. Similar results were reported for studies of TIPS-Pn in chloroform solutions of a variety of concentrations.5 We note that TIPS-Pn molecules do form π-π interactions in solutions of nonhalogenated solvents.44,45

FIG. 4.

(a) Visible absorbance spectra of TIPS-Pn in CCl4 at various concentrations normalized by their 0–0 peaks. The fluorescence spectrum of the 1 mM solution is included for reference. Reprinted with permission from C. Grieco et al., J. Phys. Chem. C 122, 2012–2022 (2018). Copyright 2018 The American Chemical Society. (b) Fluorescence spectra of TIPS-Pn in CCl4 at various concentrations. The absorbance spectrum of the 1 mM solution is included for reference to highlight the overlap of the 0–0 vibronic peak in both spectra. The fluorescence spectra have been normalized by their 1–0 vibronic peaks.

FIG. 4.

(a) Visible absorbance spectra of TIPS-Pn in CCl4 at various concentrations normalized by their 0–0 peaks. The fluorescence spectrum of the 1 mM solution is included for reference. Reprinted with permission from C. Grieco et al., J. Phys. Chem. C 122, 2012–2022 (2018). Copyright 2018 The American Chemical Society. (b) Fluorescence spectra of TIPS-Pn in CCl4 at various concentrations. The absorbance spectrum of the 1 mM solution is included for reference to highlight the overlap of the 0–0 vibronic peak in both spectra. The fluorescence spectra have been normalized by their 1–0 vibronic peaks.

Close modal

The fluorescence spectra of the samples represented in Fig. 4(b) are also consistent with this lack of aggregation through π-π interactions of the conjugated pentacene cores in CCl4. The spectra have been normalized to the intensities of the 0–1 vibronic peaks. The comparison demonstrates that the 0–1 and 0–2 peak positions and intensities are independent of concentration. The concentration dependence of the 0–0 peak in the fluorescence spectra is a result of self-absorption because the 0–0 peak of the fluorescence spectrum overlaps the 0–0 peak in the absorption spectrum. We, therefore, conclude that the data in Figs. 3 and 4 are consistent with prior reports of singlet fission in TIPS-Pn solutions,5,12 which allows us to draw a quantitative comparison of the results observed here.

In Fig. 2(d), the mid-IR transient absorption spectra of the 25 mM TIPS-Pn solution in CCl4 exhibit the rapid formation of a much larger amplitude alkyne stretch triplet absorption feature at 2080 cm–1. Because we confirmed that TIPS-Pn molecules undergo singlet fission at this concentration, we assigned the 2080 cm–1 feature to the alkyne stretch mode of triplet states of TIPS-Pn.28 Comparison of the triplet absorption peak with the FTIR spectrum of the solution [Fig. 2(d), bottom] demonstrates that it is Δν = 10 cm−1 lower in frequency than the symmetric alkyne stretch of the ground electronic state at 2090 cm−1. This shift of the alkyne stretch mode in solution is the same as the shift of the antisymmetric alkyne stretch mode of triplet states in the crystalline film [Fig. 2(b)]. We note that the triplet absorption peak corresponding to the antisymmetric alkyne stretch of triplets around 2120 cm−1 may also be present in solution. However, this feature is obscured by the broader ground state bleach measured in solution. The data therefore indicate that the triplet states of TIPS-Pn molecules in both solution and crystalline film environments possess alkyne stretch modes that are shifted Δν = 10 cm−1 lower in frequency than their corresponding ground state vibrational modes. However, in solution, TIPS-Pn molecules appear to experience interactions that break their symmetry and enhance the amplitude of the symmetric alkyne stretch under conditions where singlet fission occurs.

We used the sensitivity of vibrational modes to their local environments to investigate the source of this break in symmetry in solution. FTIR spectra of TIPS-Pn in CCl4 are plotted in Fig. 5(a) over a range of concentrations from 6 μM to 13 mM. A variety of pathlengths were used for these measurements to obtain similar optical densities of the C—H stretch modes of the TIPS side groups to facilitate comparison of their absorption profiles. The spectra represented in Fig. 5(a) have been scaled by the integrated area of the C—H stretch modes of the isopropyl side groups for quantitative comparison. For the purpose of comparison, we assume that the spectrum measured at 6 μM concentration represents the absorption of isolated TIPS-Pn molecules. Unlike the visible absorption spectra in Fig. 3, the vibrational spectra of the isopropyl chains of the TIPS side groups change markedly with increasing concentration even at concentrations as low as 0.1 mM. We note that the C—H stretch modes of the conjugated pentacene core around 3050 cm−1 are also enhanced with increasing concentration.

FIG. 5.

(a) FTIR spectra showing the concentration-dependent evolution of the C—H stretching (area-normalized) vibrational absorbance spectrum of TIPS-Pn dissolved in CCl4. The spectra reveal aggregation of TIPS-Pn molecules in solution through their TIPS side groups. (b) FTIR spectra comparing dilute and concentrated solutions of TIPS-Pn with an amorphous film. Interactions between side groups in the concentrated TIPS-Pn resemble those in the amorphous film.

FIG. 5.

(a) FTIR spectra showing the concentration-dependent evolution of the C—H stretching (area-normalized) vibrational absorbance spectrum of TIPS-Pn dissolved in CCl4. The spectra reveal aggregation of TIPS-Pn molecules in solution through their TIPS side groups. (b) FTIR spectra comparing dilute and concentrated solutions of TIPS-Pn with an amorphous film. Interactions between side groups in the concentrated TIPS-Pn resemble those in the amorphous film.

Close modal

Figure 5(b) presents a comparison of FTIR spectra of the C—H stretch modes of the TIPS side groups that reveals the origins of their concentration dependent changes. The comparison demonstrates that the spectrum of the TIPS side groups in the 13 mM solution closely resembles their spectrum in an amorphous film of TIPS-pn where the side groups are in close proximity to neighboring TIPS-Pn molecules in the solid state. The small shift in frequency is likely due to the change in solvent environment from the CCl4 solution to the solid film. The data, therefore, demonstrate that TIPS-Pn molecules interact in CCl4 solution through their TIPS side groups in a manner similar to what is observed in solid films.

Spectral signatures of these interactions are visible at lower concentrations and increase smoothly at the higher concentrations where singlet fission is observed in solution.5,12 The data, therefore, indicate that it is the side groups that preassociate TIPS-Pn molecules in solution and enable singlet fission to occur. At lower concentrations around 1 mM and below, it is likely that very few molecules have on average formed such preassociated complexes through their side chains for singlet fission to occur to a significant extent within their excited state lifetimes.

Importantly, the evidence for aggregation through the TIPS side groups also provides an explanation for the broken symmetry that enhances the amplitude of the symmetric alkyne stretch mode under conditions where singlet fission can occur in solution [Fig. 2(d)]. Figure 6(a) depicts a cartoon illustrating conceptually how such side group aggregation can break the symmetry of the molecules. We note that recent studies of TIPS-Pn polymorphs have revealed that the TIPS side groups can interact with the conjugated cores of neighboring molecules.46 Such interactions could also occur in CCl4 solutions and would break the symmetry of the molecules.

FIG. 6.

(a) Illustration depicting side group interactions occurring in TIPS-Pn solutions that cause preassociation of the molecules and enable rapid singlet fission to be observed in solution. The interactions also break the symmetry of the TIPS-Pn molecules. (b) Molecular packing geometries of TIPS-Pn in the Form-I brickwork structure. The symmetry and crystalline order suppress the symmetric alkyne stretch but allow the antisymmetric stretch to probe triplet dynamics in the film.

FIG. 6.

(a) Illustration depicting side group interactions occurring in TIPS-Pn solutions that cause preassociation of the molecules and enable rapid singlet fission to be observed in solution. The interactions also break the symmetry of the TIPS-Pn molecules. (b) Molecular packing geometries of TIPS-Pn in the Form-I brickwork structure. The symmetry and crystalline order suppress the symmetric alkyne stretch but allow the antisymmetric stretch to probe triplet dynamics in the film.

Close modal

In either case, preassociation of TIPS-Pn molecules through their side chains increases the probability that pairs of molecules can adopt intermolecular geometries where singlet fission can occur. Because the ability of molecules to undergo singlet fission depends on the intermolecular coupling of their conjugated moieties,10,13–22 we expect that this preassociation is essential for singlet fission to be observed in the absence of covalent bridges.6,11,13,23–27 As a consequence, pairs of molecules capable of undergoing singlet fission also have broken symmetry because of their side chain interactions. This causes the symmetric alkyne stretch mode of triplet states to be prominent in the mid-IR transient absorption spectra represented in Fig. 2(d).

In contrast, the assembly of TIPS-Pn molecules in the Form-I brickwork crystalline film is driven by π-π interactions of the pentacene cores. This leads to the molecular packing geometries represented in Fig. 6(b), which were obtained from analysis of X-ray diffraction patterns measured in TIPS-Pn crystalline films.14 The molecular crystals are centrosymmetric (P1¯ space group), leading to symmetric interactions of the TIPS side groups with their molecular nearest neighbors.47 The symmetry of the molecular crystal also suppresses the symmetric alkyne stretch mode of TIPS-Pn molecules. The projection of the symmetric and antisymmetric alkyne stretch modes of TIPS-Pn onto the inversion operator of the P1¯ space group is discussed in detail in the supplementary material. This causes the antisymmetric stretch of the ground and triplet excited states to be the only prominent features in the mid-IR transient absorption spectra in the film [Fig. 2(a)]. The narrower linewidth of the ground state bleach measured in the film then makes it possible to observe the antisymmetric alkyne stretch of triplet states following the initial separation of proximal triplet pairs 1(TT). It will be interesting to explore the different intermolecular interactions that occur in ordered crystalline vs disordered solution or amorphous environments that enable singlet fission to occur. The sensitivity of vibrational modes to their local molecular environments suggests that vibrational studies will figure prominently in these efforts.

The comparison of the triplet absorption features in Fig. 2 that were measured in crystalline films and in CCl4 solution suggests that these are reporting vibrational features of the same types of electronic states. In crystalline films, the triplet vibrational feature does not appear until proximal triplet pairs 1(TT) dissociate into separated triplet pairs 1(T⋯T).28 We did not resolve the singlet fission dynamics in solution on ultrafast time scales in this study and so did not determine whether proximal triplet pairs form first in solution prior to triplet pair separation. However, the low electronic coupling between TIPS-Pn molecules in solution (evidenced by the lack of perturbation of the electronic states of the conjugated cores, Fig. 4) indicates that the triplet pair states that form on nearest neighbors in solution may already be separated by their TIPS side groups and may more closely resemble 1(T⋯T) states observed in crystalline films.28 For simplicity, these vibrational features have been labeled T in Fig. 2 with the understanding that the nature of the electronic states possessing these vibrational modes is still the subject of investigation.

We used the sensitivity of the vibrational modes of the model singlet fission chromophore 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) to their local molecular environments to examine the origins of singlet fission in solution when molecules are not tethered by a covalent linkage. We report distinct vibrational frequencies of alkyne stretch modes of side groups of TIPS-Pn molecules in their triplet excited states that permit direct examination of the separation of proximal triplet pair states 1(TT). Furthermore, the C—H stretch vibrational modes of alkyl chains of the TIPS side groups allowed us to detect aggregation of TIPS-Pn molecules in solution. The interactions between TIPS-Pn molecules were mediated through their side groups rather than through π-π interactions of their pentacene cores, which prevented detection of the aggregates through their electronic spectra in previous studies. These interactions through the side groups also broke the symmetry of the TIPS-Pn molecules and enhanced the absorption intensity of the formally symmetry forbidden symmetric alkyne stretch mode in solution. In contrast, the symmetry of the ordered crystalline environment of TIPS-Pn crystals suppressed the symmetric stretch but permitted the formation of triplet states to be probed via the higher frequency antisymmetric alkyne stretch mode in crystalline films. The symmetric alkyne stretch mode should be Raman active and enable investigations of the dynamics of triplet states in crystalline films using Raman spectroscopy methods.48,49

Our findings emphasize the importance of intermolecular interactions in mediating singlet fission even in solution, where preassociation of TIPS-Pn molecules through their side groups enables the rapid and efficient singlet fission that has been reported in solution. Furthermore, the difficulty of controlling interactions of molecules through their side groups highlights the importance of utilizing molecular systems with well-defined intermolecular geometries such as covalently bonded dimers or systems with crystalline order. Our findings also highlight the opportunities vibrational modes offer to gain new insights about intermolecular interactions and symmetry breaking that influence singlet fission processes.

See the supplementary material for the description of the symmetry of the alkyne stretch modes of TIPS-Pn and their associated computational details.

C.G., G.S.D., and J.B.A. thank the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences for support of this research through Grant No. DE-SC0019349. K.T.M. is grateful for support from the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1255832. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. I.D. acknowledges support from the Soltis faculty support award and the Ralph E. Powe junior faculty award from Oak Ridge Associated Universities. J.E.A. thanks the National Science Foundation (Grant No. CMMI-1255494) for supporting semiconductor synthesis.

C.G. and J.B.A. own equity in Magnitude Instruments, which has interest in this project. Their ownership in this company has been reviewed by the Pennsylvania State University’s Individual Conflict of Interest Committee and is currently being managed by the University.

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Supplementary Material