Diffusion-Controlled Singlet Fission in a Chlorinated Phenazinothiadiazole by Broadband Femtosecond Transient Absorption.

Singlet fission (SF) is a process by which one excited singlet state yields two triplet states upon close interaction with a ground-state chromophore of the same kind. This photoreaction was first observed in solid state and has important implications in organic photovoltaics. Singlet fission was also reported in concentrated solutions, where the need for diffusion of the reaction partners slows the dynamics. This helps to single out reaction stages and to identify the involved species. In this work, ultrafast transient absorption spectroscopy and time-correlated single photon counting are applied to the concentration-dependent (from 10 to 10 mM) photodynamics of a tetrachlorinated phenazinothiadiazole in toluene. Time-resolved emission shows a monoexponential decay, which is constant across the emission band. The corresponding decay rate depends linearly on the concentration of the phenazinothiadiazole. Femtosecond transient absorption demonstrates that a concentration-dependent singlet-to-triplet conversion hides behind the emission decay which is diffusion controlled. Contrary to previous reports on SF in pentacenes and tetracenes, no indication of intermediate states has been found. Efficient, direct and barrierless SF is concluded. The strong enhancement of the triplet yield at increasingly higher concentrations of the thiadiazole indicates very efficient singlet fission with a triplet yield up to 189 ± 5%.