![]() ![]() 1– 4 This renewed interest has been driven mainly by the possibility that it could provide a physical mechanism to surpass the Shockley-Queisser limit on solar energy conversion efficiency. The study of SF has enjoyed a renaissance over the last 15 years, after its initial discovery and characterization in the 1960s-1980s. Converting one singlet into one triplet (intersystem crossing (ISC)) is spin-forbidden and typically slow, but the ability of a triplet pair to exist in an overall singlet state, usually denoted 1(TT), removes this constraint and allows SF to be very efficient. Singlet fission (SF) is an excited state relaxation channel in which a spin singlet state (S) converts into a pair of triplet states (TT). When considered in the context of other results, our data suggest that the production of triplets in tetracene for temperatures below 250 K is a complex process that is sensitive to the presence of structural defects. ![]() A kinetic analysis shows that the redshifted emission seen at lower temperatures cannot be an intermediate in the triplet production. At very long times (≈1 μs) a delayed fluorescence component corresponding to the original S 1 state can still be resolved, unlike in polycrystalline films. For temperatures in the range 20 K to 250 K, the singlet exciton continues to undergo a rapid decay on the order of 200 ps, leaving a red-shifted emission that decays on the order of 100 ns. All the data for T>250 K are consistent with direct production of a spatially separated 1(T.T) state via a thermally activated process, analogous to spontaneous parametric downconversion of photons. The damping rate of the triplet pair spin quantum beats in the delayed fluorescence also exhibits an Arrhenius temperature dependence with an activation energy of 165☗0 cm −1. The fission process is insensitive to this localization and exhibits Arrhenius behavior with an activation energy of 550±50 cm −1. Over the temperature range 250 K to 500 K, the vibronic lineshape of the emission indicates that the singlet exciton becomes localized at 400 K. The temperature dependent fluorescence spectrum, decay rate and spin quantum beats are examined in single tetracene crystals to gain insight into the mechanism of singlet fission. ![]()
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