Distinct properties of the triplet pair state from singlet fission

Abstract

The triplet pair from singlet fission is characterized by distinct spectroscopic signature and can be difficult to break apart.

Striking the right balance of intermolecular coupling for high-efficiency singlet fission

“…Global target analysis has been applied to model singlet fission dynamics of a number of chemical and material systems, including those that are disordered. 3 – 6 , 50 , 54 , 83 , 86 , 111 – 117 Unlike previous models of singlet fission dynamics in disordered systems which have exclusively accounted for sequential dynamics (and have thus ignored potential sample heterogeneity), 3 , 4 , 6 , 25 , 68 , 81 – 83 , 85 , 91 , 114 , 117 , 118 the six-component kinetic scheme presented here accounts for the parallel decay of two sets of sub-populations. Parallel decay pathways can especially be manifested in systems where multiple packing arrangements or conformations exist within the potential energy landscape 119 , 120 and additionally where the underlying sub-populations exhibit overlapping ground-to-excited-state absorption spectra (see e.g.…”

“…6b and c and 7 thus highlight the so-called “dual singlet-triplet optical character” of triplet pairs, originally reported in concentrated solutions of a tetracene derivative 88 and nanoparticle suspensions of pentacene derivatives, 5 that has also been observed in covalently-tethered tetracene and pentacene pairs. 3 , 6 , 81 A simple, coherent and unifying explanation of the origin of this dual singlet-triplet optical character, however, has yet to be provided.…”

“…More recently, this view has been developed further to acknowledge a unique, although unresolved, aspect of the triplet pair which is that it has “dual singlet-triplet optical character”, as has been reported in concentrated solutions, 88 in the solid state, 5 and in covalently-tethered molecular pairs. 6 In contrast to the view that the triplet pair comprises two individual triplet excitations, ab initio calculations have indicated that the triplet pair is well described as a single entity with substantial doubly-excited electronic character that can effectively couple to the ground state. 66 , 68 , 89 While differences between experiment and theory remain, the studies above could be unified by considering a more fundamental view where losses can be explained by considering how molecular packing influences the extent to which the constituent triplets act independently, either through ground-state intermolecular packing geometries 9 , 66 or through those introduced via excited-state processes such as excimer relaxation.…”

Singlet fission in a hexacene dimer: energetics dictate dynamics

“…They represent the fundamental smallest number of chromophores required for SF and varying the connectivity between the chromophores can lead to insightful structure-property relationships of the constrained excitons, from the generation, [19][20][21][22] separation, [23][24][25][26] and recombination [27][28][29][30] of triplet states, to the elucidation of the bound triplet pair state. [31][32][33][34] It has been established that molecular vibrations play a key role in mediating singlet ssion in both oligoacene molecular crystals (intermolecular singlet ssion, xSF) and in acene intramolecular singlet ssion (iSF) compounds. 17,[35][36][37][38] In crystals, it is now commonly accepted that both coherent and incoherent formation of triplet pairs is possible, even within the same system.…”

“…Contiguous dimers have shown that the singlet and triplet pair energy manifolds are mixed, with allowed TT-S n optical transitions. 32,62 Furthermore, radiationless decay of triplet pairs has been shown to slow down as chromophore proximity decreases. From this, we suggest that the triplet pair decays through coupling to singlet vibronic modes, permitting a rapid and spin-allowed route back to the ground state.…”