Solvent effect on thermally activated delayed fluorescence by 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene.

Thermally activated delayed fluorescence (TADF) is fluorescence arising from a reverse intersystem crossing (RISC) from the lowest triplet (T1) to the singlet excited state (S1), where these states are separated by a small energy gap (ΔEst), followed by a radiative transition to the ground state (S0). Rate constants relating TADF processes in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) were determined at four different solvent polarities (toluene, dichloromethane, ethanol, and acetonitrile). We revealed that the rate constant of RISC, kRISC, which is the most important factor for TADF, was significantly enhanced by a reduced ΔEst in more polar solvents. The smaller ΔEst was mainly attributable to a stabilization of the S1 state. This stabilization also induced a Stokes shift in fluorescence through a relatively large change of the dipole moment between S1 and S0 states (17 D). Despite of this factor, we observed a negative correlation between ΔEst and efficiency of the delayed fluorescence (φd). This was ascribed to a lower intersystem crossing rate, kISC, and increased nonradiative decay from S1, k(s)nrs, in polar solvents.