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 (T ₁) to the singlet excited state (S₁), where these states are separated by a small energy gap (ΔE_st), 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, k_RISC, which is the most important factor for TADF, was significantly enhanced by a reduced ΔE_st in more polar solvents. The smaller ΔE_st 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 ΔE_st and efficiency of the delayed fluorescence (p_d). This was ascribed to a lower intersystem crossing rate, k_ISC, and increased nonradiative decay from S₁, knr^s, in polar solvents.