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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 (S_1), where these states ...are separated by a small energy gap (ΔE_<st>), followed by a radiative transition to the ground state (S_0). 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 S_1 state. This stabilization also induced a Stokes shift in fluorescence through a relatively large change of the dipole moment between S_1 and S_0 states (17 D). Despite of this factor, we observed a negative correlation between ΔE_<st> and efficiency of the delayed fluorescence (ϕ_d). This was ascribed to a lower intersystem crossing rate, k_<ISC>, and increased nonradiative decay from S_1, k_<nr>^s, in polar solvents.続きを見る
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