Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
Phys Chem Chem Phys. 2019 Feb 13;21(7):3894-3902. doi: 10.1039/c8cp06459h.
Recently, a type of synthetic highly efficient OLED molecule based on a hybridized local excitation and charge transfer (HLCT) character has received much attention as a potential high-efficiency fluorescent OLED material. In this article, we report the relaxation dynamics of the excited states of cyano-substituted oligo α-phenylenevinylene-1,4-bis(R-cyano-4-diphenylaminostyryl)-2,5-diphenylbenzene (CNDPASDB) with HLCT character using steady-state and time-resolved spectroscopy as well as quantum chemical calculations. The dramatic dependence of the fluorescence quantum yield, radiative and non-radiative rate, as well as the excited state relaxation pathways on solvent polarity reveals that the solvation process controls the energy levels of two closely spaced electronic excited states. By employing femtosecond transient absorption spectra, the gradual transition from the LE state to the intramolecular CT state with an increase in solvent polarity is clearly resolved. In low-polarity solvents the fluorescence of CNDPASDB is mainly emission from the LE state, whereas in high-polarity solvents non-radiative decay from the CT state dominates. And in medium-polarity solvents, because of the relatively weaker solvation-induced stabilization of the CT state, its energy could be equal to or slightly lower than that of the LE state, leading to a smaller driving force for LE → CT interconversion; therefore complete LE → CT interconversion cannot take place. In this situation, LE ↔ CT intercrossed equilibration is established and the equilibrium constant is further estimated to be about 4 according to the obtained kinetics, and the equilibrium population of the CT state is about 80%. DFT/TDDFT analysis also confirmed an efficient intercrossing of LE and CT states with an increase in solvent polarity. It is found that upon increasing the solvent polarity, the hole and electron on a molecule are entirely separated, suggesting a complete CT character. These results provide guidance for understanding the relationship between solvent polarity and the HLCT process, as well as for designing and synthesizing advanced OLED materials.
最近，一种基于杂化局域激发和电荷转移（HLCT）特性的合成高效 OLED 分子引起了人们的广泛关注，被认为是一种有潜力的高效荧光 OLED 材料。在本文中，我们通过稳态和时间分辨光谱以及量子化学计算报告了具有 HLCT 特性的氰基取代寡聚 α-苯乙炔-1,4-双（R-氰基-4-二苯基氨基苯乙烯基）-2,5-二苯基苯（CNDPASDB）的激发态弛豫动力学。荧光量子产率、辐射和非辐射速率以及激发态弛豫途径对溶剂极性的剧烈依赖性表明，溶剂化过程控制了两个紧密间隔的电子激发态的能级。通过飞秒瞬态吸收光谱，可以清楚地分辨出随着溶剂极性的增加，从 LE 态到分子内 CT 态的逐渐转变。在低极性溶剂中，CNDPASDB 的荧光主要来自 LE 态，而在高极性溶剂中非辐射衰减则来自 CT 态。在中等极性溶剂中，由于 CT 态的溶剂化诱导稳定作用相对较弱，其能量可能等于或略低于 LE 态，导致 LE → CT 互变异构的驱动力较小；因此，不能完全发生 LE → CT 互变异构。在这种情况下，建立了 LE → CT 交叉平衡，根据获得的动力学进一步估计平衡常数约为 4，CT 态的平衡态占据约为 80%。DFT/TDDFT 分析也证实了随着溶剂极性的增加，LE 和 CT 态之间的交叉效率很高。结果表明，随着溶剂极性的增加，分子上的空穴和电子完全分离，表明完全的 CT 特性。这些结果为理解溶剂极性与 HLCT 过程之间的关系以及设计和合成先进的 OLED 材料提供了指导。
