Department of Chemistry, University of Connecticut, 55 North Eagleville Road U-3060, Storrs, Connecticut 06269, USA.
J Org Chem. 2010 Jul 16;75(14):4817-27. doi: 10.1021/jo100866m.
Azobenzene undergoes reversible cis<-->trans photoisomerization upon irradiation. Substituents often change the isomerization behavior of azobenzene, but not always in a predictive manner. The synthesis and properties of three azobenzene derivatives, AzoAMP-1, -2, and -3, are reported. AzoAMP-1 (2,2'-bis[N-(2-pyridyl)methyl]diaminoazobenzene), which possesses two aminomethylpyridine groups ortho to the azo group, exhibits minimal trans-->cis photoisomerization and extremely rapid cis-->trans thermal recovery. AzoAMP-1 adopts a planar conformation in the solid state and is much more emissive (Phi(fl) = 0.003) than azobenzene when frozen in a matrix of 1:1 diethylether/ethanol at 77 K. Two strong intramolecular hydrogen bonds between anilino protons and pyridyl and azo nitrogen atoms are responsible for these unusual properties. Computational data predict AzoAMP-1 should not isomerize following S(2)<--S(0) excitation because of the presence of an energy barrier in the S(1) state. When potential energy curves are recalculated with methyl groups in place of anilino protons, the barrier to isomerization disappears. The dimethylated analogue AzoAMP-2 was independently synthesized, and the photoisomerization predicted by calculations was confirmed experimentally. AzoAMP-2, when irradiated at 460 nm, photoisomerizes with a quantum yield of 0.19 and has a much slower rate of thermal isomerization back to the trans form compared to that of AzoAMP-1. Its emission intensity at 77 K is comparable to that of azobenzene. Confirmation that the AzoAMP-1 and -2 retain excited state photochemistry analogous to azobenzene was provided by ultrafast transient absorption spectroscopy of both compounds in the visible spectral region. The isomerization of azobenzene occurs via a concerted inversion mechanism where both aryl rings must adopt a collinear arrangement prior to inversion. The hydrogen bonding in AzoAMP-1 prevents both aryl rings from adopting this conformation. To further probe the mechanism of isomerization, AzoAMP-3, which has only one anilinomethylpyridine substituent for hydrogen bonding, was prepared and characterized. AzoAMP-3 does not isomerize and exhibits emission (Phi(fl) = 0.0008) at 77 K. The hydrogen bonding motif in AzoAMP-1 and AzoAMP-3 provides the first example where inhibiting the concerted inversion pathway in an azobenzene prevents isomerization. These molecules provide important supporting evidence for the spectroscopic and computational studies aimed at elucidating the isomerization mechanism in azobenzene.
偶氮苯在光照下经历可逆的顺式-反式光异构化。取代基通常会改变偶氮苯的异构化行为，但并非总是可以预测。本文报道了三种偶氮苯衍生物 AzoAMP-1、-2 和-3 的合成和性质。AzoAMP-1（2,2'-双[N-(2-吡啶基)甲基]二氨基偶氮苯），具有两个邻位氨基甲基吡啶基团，表现出最小的反式-顺式光异构化和极快的顺式-反式热恢复。AzoAMP-1 在固态中采用平面构象，并且在 77 K 的 1:1 二乙醚/乙醇基质中冷冻时比偶氮苯更具发光性（Phi(fl) = 0.003）。两个强的分子内氢键，一个在苯胺质子和吡啶质子与偶氮氮原子之间，另一个在苯胺质子和偶氮氮原子之间，是这些不寻常性质的原因。计算数据预测，由于 S(1) 态中存在能垒，AzoAMP-1 不应在 S(2)<--S(0) 激发后发生异构化。当用甲基代替苯胺质子重新计算势能曲线时，异构化的障碍消失了。独立合成了二甲基化类似物 AzoAMP-2，并通过实验证实了计算预测的光异构化。当用 460nm 光照射 AzoAMP-2 时，其光异构化量子产率为 0.19，并且与 AzoAMP-1 相比，热异构化回到反式的速率要慢得多。它在 77 K 时的发光强度与偶氮苯相当。超快瞬态吸收光谱证实了 AzoAMP-1 和 -2 在可见光谱区域保留了类似于偶氮苯的激发态光化学性质。偶氮苯的异构化通过协同反转机制发生，其中两个芳环在反转之前都必须采用共线排列。AzoAMP-1 中的氢键阻止了两个芳环采用这种构象。为了进一步探究异构化的机制，合成并表征了只有一个苯胺甲基吡啶取代基用于氢键的 AzoAMP-3。AzoAMP-3 不会异构化，并且在 77 K 时发出荧光（Phi(fl) = 0.0008）。AzoAMP-1 和 AzoAMP-3 中的氢键模式提供了第一个抑制偶氮苯中协同反转途径以阻止异构化的例子。这些分子为旨在阐明偶氮苯异构化机制的光谱和计算研究提供了重要的支持证据。
