Geminate Delayed Fluorescence by Anisotropic Diffusion-Mediated Reversible Singlet Fission and Triplet Fusion

Abstract

Singlet fission (SF) occurs in molecular solids and is expected to increase the conversion efficiency of photons into electrons by transferring excitons or charges into low-band-gap semiconductors. The fundamental processes of SF and its reverse process of triplet fusion (TF) would have to be elucidated to characterize the effect of SF on the conversion efficiency. In situations in which TF occurs subsequent to SF within the same pair, a certain fraction of the excited singlets could be regenerated by TF and this may even happen repeatedly. Consequently, the excited singlets exhibit delayed fluorescence by geminate fusion of the triplets. In this study, we show that the exponent in the asymptotic power law decay of the geminate delayed fluorescence reflects the diffusional anisotropy. In molecular crystals, the difference in the ratio between the largest and smallest exciton diffusion constants can be orders of magnitude. By comparing the theoretical results and the fluorescence decay in tetracene and rubrene, we estimated the range of anisotropy in the triplet diffusion constants in these materials. We also show that the exponent of the power law decay changes as a function of time for certain values of the anisotropy ratio in the triplet diffusion constants.

In optimized rubrene-based nanoparticle blends for photon upconversion, singlet energy collection outcompetes triplet-pair separation, not singlet fission

“…The intermediate power law dynamics that we observe in the NP films are characteristic of geminate fusion of associated and separated triplet-pair states, 43,44,74 i.e. 1 (TT) and (TÁ Á ÁT).…”

“…The power-law behaviour is characteristic of geminate fusion of 1 (TT) and (TÁ Á ÁT). 43,44 We find an excitation density dependent enhancement to the delayed PL beyond 100 ns, that corresponds to the onset of bimolecular TTA determined by monitoring the triplet transient absorption signal at 510 nm (B). (C) Magnetic field effect on fluorescence at two different time delays.…”

“…28 Previous studies of rubrene-DBP TTA-UC systems include in some form the assertion that FRET from rubrene to DBP outcompetes singlet fission. 18,[26][27][28] This simple statement appears to overlook the complexity of singlet fission and triplet fusion that occurs in solid rubrene, dynamics that have been extensively studied in vapour-grown orthorhombic single crystals [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] and to a lesser degree in more complex thin films. 25,30,[46][47][48][49][50][51] Furthermore, transient absorption spectroscopy of rubrene single crystals has shown that S 1 -1 (TT) occurs extremely rapidly, with reported time constants varying from 25 fs 35,36 to 2 ps.…”

“…39,41 Geminate and non-geminate fusion of triplet-pairs and triplets respectively results in pronounced delayed PL. 38,40,43,44 Geminate fusion of separated triplet-pairs gives rise to distinct power-law dynamics in the delayed PL, with exponents that depend on the dimensionality and anisotropy of the triplet diffusion. 43,44 Overall, triplet fusion in rubrene crystals is extremely efficient and can contribute more than 90% of the total photoluminescence quantum yield (PLQY).…”

“…38,40,43,44 Geminate fusion of separated triplet-pairs gives rise to distinct power-law dynamics in the delayed PL, with exponents that depend on the dimensionality and anisotropy of the triplet diffusion. 43,44 Overall, triplet fusion in rubrene crystals is extremely efficient and can contribute more than 90% of the total photoluminescence quantum yield (PLQY). 40 The long diffusion length of the triplet excitons, coupled with dominant contribution of triplet fusion to the total PL, means that the PLQY of rubrene single crystals is extremely sensitive to triplet-quenching defects.…”