Singlet fission

Singlet fission (SF) is a process whereby a photo-excited singlet exciton spontaneously splits into lower energy triplet excitons. This process has been known to occur in select organic materials for over 50 years but in the past was viewed as a curiosity and not as a problem worthy of focused attention. Over the past decade, this viewpoint has changed as the promising use of SF to improve photovoltaic efficiency has spurred vast activity, which has revealed the richness of the phenomenon. Currently, our understanding of the role played by various bright and dark states and their couplings to vibrations and to each other and by chemical composition and packing, among other factors, has enabled a large increase in the number of viable SF systems as well as a far deeper understanding of the process itself. These advances have allowed for the demonstration of proof-of-concept increases in the solar cell efficiency via the use of SF materials in simple tandem cells and as sensitizers in conjunction with conventional photovoltaics. Indeed, concepts surrounding the field have now invaded other fields, with a prominent example being the use of photon upconversion associated with triplet–triplet fusion (the inverse process to SF) in organic chemistry and photocatalysis. The 23 research articles in this special topic showcase recent cutting-edge advances in the field and highlight the diverse new directions being explored in the field of SF.

A major point of focus in the field is how packing and bulk morphology affect SF in a variety of systems. Both the local and long-ranged structural factors that enable efficient SF are subtle and sensitively depend on the molecular unit, its chemical modifications, and its local coupling to its neighbors. In addition, the long range order (as it exists in crystalline solids and films) or the lack of order (in amorphous systems) can play a major role in the overall efficiency of the process, in particular, with respect to the diffusion and spatial separation of correlated triplets, which form rapidly during the first steps of fission. The papers by Felter et al.,¹ Bae et al.,² Ryerson et al.,³ Datko et al.,⁴ and Wang et al.⁵ bring powerful modern characterization and experimental techniques to bear on the systematic explication of morphological features that influence all aspects of SF.

The role of coherence and a focus on spin angular momentum can reveal detailed information concerning the nature of the most important states involved in SF and their couplings to other states, since spin angular momentum plays a large role in intermediate states such as the triplet pair state in which triplet excitons are entangled into an overall state of singlet symmetry. Bardeen⁶ presents a detailed discussion of potential experimental routes for exploiting temporal fluctuations of entangled spin states to extract detailed information on the nature and coupling of states vital for SF. Collins et al.⁷ provide a theoretical study of generation of the quintet state analog of the standard triplet pair state, illustrating how ESR spectroscopy may sensitively unveil transitions between various spin-entangled states associated with the fission process. Wang et al.⁸ monitor quantum beats in delayed florescence to delineate the coupling of the triplet pair state to relaxed, spatially separated triplet pair states, as well as the original photoexcited singlet state in tetracene. All of these studies illustrate the potential power of targeting spin and coherence observables to further understand singlet fission.

Recently, the use of vibrational probes and the role of vibronic coupling have come to the fore as means to both investigate and understand SF. Grieco et al.⁹ experimentally targetted the C–H vibrational motion of the side groups in TIPS-pentacene to show that side group-mediated aggregation in solution, and not diffusive encounters of individual molecules, enables fission. Deng et al.¹⁰ combine theory and ultrafast transient infrared spectroscopy to elucidate how particular vibrational modes can couple to and help drive the fission process in hexacene.

In the past several years, the field of SF has expanded from studies of fission in bulk to molecular studies of fission that can occur within a single covalently bonded entity. Such “intramolecular” SF (iSF) is highly promising as a means of expanding the palate of potential fission systems and providing systems that may function as solar-cell sensitizers. Musser et al.¹¹ studied the role of heavy atom substitution on iSF in conjugated polymers, illustrating that such substitutions alter only the long-time kinetics of the fission process. Sandoval-Salinas et al.¹² and Reddy et al.¹³ present theoretical studies of different potential iSF platforms, demonstrating how a variety of tools, ranging from electronic structure theory to accurate quantum dynamics methods, may reveal constraints on design principles and the mechanisms of operation of iSF systems.

The above two papers highlight the power of theory to guide experiments, make predictions, and promote our understanding of SF. Indeed, for decades, theory has played a major role in the field. A variety of papers in this collection focus on the theory and modeling of time-dependent dynamical phenomena using a variety of techniques. Shushin¹⁴ focused on the role played by anisotropic triplet migration in SF kinetics. Nakano¹⁵ and Tao¹⁶ used the master equation approach and a novel non-adiabatic trajectory method, respectively, to understand the role played by the aggregate size and delocalization in driving SF. Schmidt¹⁷ presented a complimentary view to that of master equation modeling by placing SF kinetics in the language of Marcus–Hush theory. Takahashi et al.¹⁸ and Martinez et al.¹⁹ took varied theoretical approaches to illustrate how SF can be altered by external technology (via strong light–matter coupling in a cavity) or alter external technology (via driving catalytic water splitting). Sun et al.²⁰ and Kim et al.²¹ used quantum dynamical approaches in conjunction with simulated or actual laboratory-based optical experiments to understand the roles played by conical intersections and charge transfer states, respectively, in different SF platforms.

Our understanding has, in fact, advanced to the point where a synergy of theory, synthetic techniques, and experiment enable the rational design of SF platforms. Several papers in this special issue demonstrate the potential for theory for such theoretically based screening. Japahuge et al.²² used TD-DFT and NEV-PT2 (second order n-electron valence state perturbation theory) electronic structure approaches to optimize design of SF chromophores composed of cyclic carbines. Perkinson et al.²³ took a complimentary large-scale screening approach, considering over 4000 candidates to select 88 novel SF candidate systems based on anthracene derivatives. These works demonstrate the state-of-the-art in inverse design of SF chromophores and materials, an area that is expected to exponentially grow in the coming years.

In summary, this special topic issue highlights the diverse progress that has been made in the rapidly evolving and active area of SF research. Over the past decade, the field has evolved from focused and specific questions on highly circumscribed materials to a much more developed understanding of the process coupled to a rapidly growing number of SF systems and potential applications. We hope that this special topic illustrates the rapid developments in the field and points the way for new avenues of SF research.