Pancur T, Renth F, Temps F, Harbaum B, Krüger A, Herges R, Näther Chr
Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Ludewig-Meyn-Str. 8, D-24098 Kiel, Germany.
Phys Chem Chem Phys. 2005 May 7;7(9):1985-9. doi: 10.1039/b419236b.
Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the photoinduced isomerization reactions of a rotation-restricted trans-azobenzene (trans-AB) derivative capped by a crown ether (1), a chemically similar open derivative (2), and unsubstituted trans-AB (3) after excitation to the S1 (npi*) state at lamda=475 nm in dioxane solution. The observed biexponential temporal fluorescence profiles for 1 and 2 were almost indistinguishable within experimental error. The fitted fast fluorescence decay times (+/-2sigma) for the two compounds were tau1 (1) = (0.79 +/- 0.20) and tau1 (2) = (1.05 +/- 0.20) ps, compared to tau1 (3) = (0.37 +/- 0.06) ps. The second decay components could be described with tau2 (1) = (20.3 +/- 9.5) resp. tau2 (2) = (19.0 +/- 6.0) ps, vs. tau2 (3) = (3.26 +/- 0.85) ps. The very similar lifetimes strongly suggest that trans-cis isomerization of 1 and 2 after S1 excitation is governed by the same mechanism. Since 1 cannot isomerize by a simple large-amplitude rotation of one of the phenyl rings about the central NN bond, the isomerization dynamics of both ABs should be better described as "inversion" at the N atom(s) rather than large-amplitude "rotation".
飞秒时间分辨荧光上转换光谱已被用于研究在二氧六环溶液中，λ = 475 nm激发至S1（nπ*）态后，由冠醚封端的旋转受限反式偶氮苯（反式-AB）衍生物（1）、化学结构相似的开环衍生物（2）和未取代的反式-AB（3）的光致异构化反应。在实验误差范围内，观察到的1和2的双指数时间荧光曲线几乎无法区分。两种化合物拟合得到的快速荧光衰减时间（±2σ）分别为τ1（1）=（0.79±0.20）ps和τ1（2）=（1.05±0.20）ps，而τ1（3）=（0.37±0.06）ps。第二个衰减成分可以用τ2（1）=（20.3±9.5）ps和τ2（2）=（19.0±6.0）ps来描述，相比之下τ2（3）=（3.26±0.85）ps。非常相似的寿命强烈表明，S1激发后1和2的反式-顺式异构化受相同机制支配。由于1不能通过苯环之一围绕中心NN键的简单大幅度旋转进行异构化，两种AB的异构化动力学应更好地描述为N原子处的“反转”而非大幅度“旋转”。
