The photophysics and excited-state dynamics of two dyads consisting of either a free-base or a zinc-tetraphenylporphyrin linked through a rigid bridge to a core-substituted naphthalenediimide (NDI) have been investigated by femtosecond-resolved spectroscopy. The absorption and fluorescence spectra differ substantially from those of the individual units, pointing to a substantial coupling and to a delocalisation of the excitation over the whole molecule, as confirmed by quantum chemistry calculations. A strong dependence of their excited-state dynamics on the solvent polarity has been observed. In toluene, the fluorescence quantum yield of the dyads is of the order of a few percent and the main decay channel of the emitting state is proposed as intersystem-crossing to the triplet state. However, in a medium polarity solvent like dichloromethane, the emitting state undergoes charge separation from the porphyrin to the NDI unit within 1–3 ps, and the ensuing charge-separated state recombines in about 10–20 ps. This solvent dependence can be explained by the weak driving force for charge separation in polar solvents and the large electronic coupling between the porphyrin and NDI moieties, making charge separation a solvent-controlled adiabatic process.
飞秒分辨光谱学研究了由自由碱或
锌-
四苯基卟啉通过刚性桥与核心取代的
萘二
亚胺(
NDI)连接而成的两个二元体的光物理和激发态动力学。吸收光谱和荧光光谱与单个单元的光谱有很大不同,这表明整个分子中存在大量的耦合和激发扩散,量子
化学计算也证实了这一点。据观察,它们的激发态动力学与溶剂的极性有很大关系。在
甲苯中,二元化合物的荧光量子产率仅为百分之几,而发射态的主要衰变通道被认为是向三重态的系统间交叉。然而,在
二氯甲烷等中等极性溶剂中,发射态会在 1-3 ps 内发生从
卟啉到
NDI 单元的电荷分离,随后的电荷分离态会在大约 10-20 ps 内重新结合。这种溶剂依赖性可解释为极性溶剂中电荷分离的驱动力较弱,以及
卟啉和
NDI 分子之间的电子耦合较大,使得电荷分离成为一个溶剂控制的绝热过程。