Charge-transfer state and state mixing in tetracyanoquinodimethane probed using electroabsorption spectroscopy

Tetracyanoquinodimethane (TCNQ) is an important constituent of organic conductors and a versatile electron acceptor. TCNQ exhibited thermally activated delayed fluorescence and an unusually long fluorescence lifetime. In this study, we studied the Stark effect on the absorption spectrum of TCNQ using electroabsorption spectroscopy to gain insights into its photophysics. The electroabsorption spectrum was simulated using multiple absorption bands for different electronic states, which were characterized by different dipole moments and polarizabilities. These electronic states are identified as a locally excited (LE) state with a high oscillator strength and zero dipole moment, and an intramolecular charge transfer (ICT) state with a nonzero dipole moment. The mixing of the LE state with the ICT state is augmented when the molecule is perturbed by an electric field. We provide tangible experimental evidence establishing the key role of mixing between the emissive LE and nonemissive ICT states in the deactivation pathway of electronically excited TCNQ. The dipole moment of the ICT state suggests symmetry breaking of the structure belonging to the D_2h point group.