A quantum chemical study of excited states and thermally activated delayed fluorescence in tetracyanoquinodimethane

Abstract

It has recently been reported that tetracyanoquinodimethane (TCNQ) shows thermally activated delayed fluorescence (TADF). Here, we report quantum chemical study using the second-order approximate coupled-cluster singles and doubles (CC2) calculations to understand the excitation energy and the optimized geometry for the excited singlet and triplet states. The ground state geometry is optimized with the CC2 and second-order Møller-Plesset perturbation theory. The spin–orbit coupling (SOC) matrix elements computed by time-dependent density functional theory show that the electronic coupling via SOC between the S1 state and the triplet state is small. The contribution of the intersystem crossing between the singlet and triplet states to the TADF process is not suggested by the calculations. The adiabatic energy gap between the ground and T1 states for TCNQ is predicted to be 0.98 eV.