Lancaster Kelly, Odom Susan A, Jones Simon C, Thayumanavan S, Marder Seth R, Brédas Jean-Luc, Coropceanu Veaceslav, Barlow Stephen
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Am Chem Soc. 2009 Feb 11;131(5):1717-23. doi: 10.1021/ja808465c.
The electron spin resonance spectra of the radical cations of 4,4'-bis[di(4-methoxyphenyl)amino]tolane, E-4,4'-bis[di(4-methoxyphenyl)amino]stilbene, and E,E-1,4-bis{4-[di(4-methoxyphenyl)amino]styryl}benzene in dichloromethane exhibit five lines over a wide temperature range due to equivalent coupling to two 14N nuclei, indicating either delocalization between both nitrogen atoms or rapid intramolecular electron transfer on the electron spin resonance time scale. In contrast, those of the radical cations of 1,4-bis{4-[di(4-methoxyphenyl)amino]phenylethynyl}benzene and E,E-1,4-bis{4-[di(4-n-butoxyphenyl)amino]styryl}-2,5-dicyanobenzene exhibit line shapes that vary strongly with temperature, displaying five lines at room temperature and only three lines at ca. 190 K, indicative of slow electron transfer on the electron spin resonance time scale at low temperatures. The rates of intramolecular electron transfer in the latter compounds were obtained by simulation of the electron spin resonance spectra and display an Arrhenius temperature dependence. The activation barriers obtained from Arrhenius plots are significantly less than anticipated from Hush analyses of the intervalence bands when the diabatic electron-transfer distance, R, is equated to the N[symbol: see text]N distance. Comparison of optical and electron spin resonance data suggests that R is in fact only ca. 40% of the N[symbol: see text]N distance, while the Arrhenius prefactor indicates that the electron transfer falls in the adiabatic regime.
4,4'-双[二(4-甲氧基苯基)氨基]托烷、E-4,4'-双[二(4-甲氧基苯基)氨基]芪和E,E-1,4-双{4-[二(4-甲氧基苯基)氨基]苯乙烯基}苯在二氯甲烷中的自由基阳离子的电子自旋共振谱在很宽的温度范围内呈现五条谱线，这是由于与两个14N核的等效耦合，表明氮原子之间存在离域现象，或者在电子自旋共振时间尺度上发生了快速的分子内电子转移。相比之下，1,4-双{4-[二(4-甲氧基苯基)氨基]苯乙炔基}苯和E,E-1,4-双{4-[二(4-正丁氧基苯基)氨基]苯乙烯基}-2,5-二氰基苯的自由基阳离子的谱线形状随温度变化很大，在室温下显示五条谱线，而在约190 K时仅显示三条谱线，这表明在低温下电子自旋共振时间尺度上的电子转移较慢。通过对电子自旋共振谱的模拟获得了后一种化合物中分子内电子转移的速率，并显示出阿仑尼乌斯温度依赖性。当非绝热电子转移距离R等于N···N距离时，从阿仑尼乌斯图得到的活化能垒明显小于通过间隔能带的赫什分析预期的值。光学数据和电子自旋共振数据的比较表明，实际上R仅约为N···N距离的40%，而阿仑尼乌斯前因子表明电子转移处于绝热区域。
