There are growing interests to increase solar cell efficiency by overcoming the Shockley-Queisser limit, which is based on one electron generated per photon absorbed. Singlet fission (SF), where one photon generates two excited electrons, is one of the most promising approaches. A recent study showed that a quinoidal bithiophene equipped with terminal cyano groups (QOT2) undergoes intramolecular SF. In this work, a two-electron receptor, anthraquinone (AQ), is coupled to QOT2 (QOT2/AQ) to demonstrate multi-electron transfer (ET).<br/>To enable this photochemistry, constrained density functional theory calculation first showed the feasibility of spontaneous multi-ET in QOT2/AQ with the thermodynamic driving force of 9.4 kcal/mol. Theoretical predictions are then confirmed by various experimental observations. Time-resolved fluorescence anisotropy measurement gives a faster emissive decaying of QOT2/AQ mixture than the one of pure QOT2, which implies the existence of an additional decaying path for the excited singlet state. Transient absorption spectroscopy results of QOT2/AQ show the additional excited state absorption bands, and these are assigned to absorption of ET products with the help of high-level ab initio theory, extended-multi state complete-active-space second-order perturbation theory. Measurement of transmission and spin-flip restricted active space calculations are used to quantify the yield of two ET to be 23 %. Infra-red spectra confirm the two-electron reduction to a stable reduced species is viable in protic solvent with a small amount of acid.<br/>The combined investigation of theory and experiment demonstrates, for the first time, the feasibility of multi-ET from an intramolecular SF chromophore, which can be used to open a way to construct highly efficient solar-to-chemical energy storage materials.

Multi-electron transfer via photo-excited quinoidal bithiophene to anthraquinone

254th National Meeting (2017)

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