Spin Chemical Approach towards Long-Lived Charge-Separated States Generated by Photoinduced Intramolecular Electron Transfer in a Donor–Bridge–Acceptor System
作者:Yukie Mori、Yoshio Sakaguchi、Hisaharu Hayashi
DOI:10.1246/bcsj.74.293
日期:2001.2
Photo-induced intramolecular electron transfer (ET) and subsequent back ET have been studied for phenothiazine-bridge-pyromellitdiimide (D-B-A) and phenothiazine-bridge-pyromellitdiimide-nitroxide radical (D-B-A-R•) in various solvents by nano-second transient absorption spectroscopy. Upon excitation of the phenothiazine (D) moiety in these compounds, ET from 1D* to A took place at a rate of ca. 5 × 109 s-1 to give charge-separated (CS) states, 1[D•+-B-A•-] or 2[D•+-B-A•--R•], in high quantum yields. The decay dynamics of the CS states was determined by the interplay between the spin conversion and the back ET to the ground state. The efficiency of the interconversion between states with different spin multiplicities was strongly affected by the presence of R• and external magnetic fields. A faster spin conversion of [D•+-B-A•--R•] resulted in a slower initial decay of the CS state generated from the excited singlet state of D compared with [D•+-B-A•- ]. Such a spin effect manifested most markedly in dioxane. On the other hand, the back ET rate (kBET) varied by more than two orders of magnitude with the solvent polarity. In dimethyl sulfoxide, the back ET was predicted to be almost barrierless, kBET ≥ 109 s-1, while in benzene, this process fell in the inverted region, kBET = 6.5 × 106 s-1.
光诱导的分子内电子转移(ET)及其后续的反向电子转移(back ET)在不同溶剂中对苯噻嗪-桥接-吡罗梅利二酰亚胺(D-B-A)和苯噻嗪-桥接-吡罗梅利二酰亚胺-硝酸酯自由基(D-B-A-R•)进行了纳秒瞬态吸收光谱研究。当激发这些化合物中的苯噻嗪(D)部分时,1D*到A的电子转移以约5 × 10^9 s^-1的速率发生,生成带电分离(CS)态,1[D•+-B-A•-]或2[D•+-B-A•--R•],并且具有高量子产率。CS态的衰减动力学由自旋转化和回到基态的反向电子转移之间的相互作用决定。不同自旋多重态之间的相互转换效率受R•的存在和外部磁场的强烈影响。与[D•+-B-A•- ]相比,[D•+-B-A•--R•]的自旋转换速度更快,导致从D的激发单态生成的CS态初始衰减变慢。这种自旋效应在二噁烷中表现得尤为明显。另一方面,反向电子转移速率(kBET)随着溶剂极性的变化而变化超过两个数量级。在二甲基亚砜中,反向电子转移被预测几乎没有能量障碍,kBET ≥ 10^9 s^-1,而在苯中,该过程落入反转区,kBET = 6.5 × 10^6 s^-1。