Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, Japan.
J Phys Chem A. 2013 Jul 11;117(27):5607-12. doi: 10.1021/jp404120s. Epub 2013 Jun 26.
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.
热致延迟荧光（TADF）是指来自最低三重态（T1）到单重激发态（S1）的反向系间穿越（RISC）产生的荧光，其中这些态之间的能量差（ΔEst）较小，随后发生辐射跃迁回到基态（S0）。在 1,2,3,5-四（咔唑-9-基）-4,6-二氰基苯（4CzIPN）中，TADF 过程的速率常数在四种不同的溶剂极性（甲苯、二氯甲烷、乙醇和乙腈）下进行了测定。我们揭示了 RISC 的速率常数 kRISC，它是 TADF 的最重要因素，在更极性的溶剂中通过减小 ΔEst 而显著增强。较小的 ΔEst 主要归因于 S1 态的稳定化。这种稳定化还通过 S1 和 S0 态之间的偶极矩的相对较大变化（17 D）引起荧光的斯托克斯位移。尽管存在这个因素，我们观察到 ΔEst 和延迟荧光（φd）效率之间呈负相关。这归因于极性溶剂中较低的系间穿越速率 kISC 和来自 S1 的增加的非辐射衰减 k(s)nrs。
