Excited-State Dynamics and Thermally Activated Delayed Fluorescence in the Classic Electron Acceptor Tetracyanoquinodimethane.

Tetracyanoquinodimethane (TCNQ) is identified as one of the most important and classic constituents for the synthesis of organic conductors and shows an acute response of the fluorescence quantum yield to subtle changes in the polarity of solvents. Here, we report on characterization of the excited-state dynamics of TCNQ using time-resolved fluorescence and femtosecond transient absorption (TA) measurements in various solvents. Fluorescence decay and TA dynamics reveal that the fluorescence emissive and nonemissive states reach equilibrium within the fluorescence lifetime in carbon tetrachloride. Thermally activated delayed fluorescence of TCNQ is also revealed. The fluorescence in the polar solvents is quenched by the forward relaxation to the nonemissive state within a few picoseconds and the subsequent rapid de-excitation of the nonemissive state within a few tens of picoseconds. The nonemissive state is probably assigned to the triplet state, and the change in the forward and reverse intersystem crossing rates can be responsible for the response of the fluorescence to the polarity of solvents.