Cisnetti Federico, Ballardini Roberto, Credi Alberto, Gandolfi Maria Teresa, Masiero Stefano, Negri Fabrizia, Pieraccini Silvia, Spada Gian Piero
Dipartimento di Chimica G. Ciamician, Università di Bologna, via Selmi 2, 40126 Bologna, Italy.
Chemistry. 2004 Apr 19;10(8):2011-21. doi: 10.1002/chem.200305590.
We have investigated the photophysical, photochemical and electrochemical properties of two bis(azo) derivatives, (E,E)-m-1 and (E,E)-p-1. The two compounds, which can be viewed as being composed of a pair of azobenzene units sharing one of their phenyl rings, differ only for the relative position of the two azo groups on the central phenyl ring-meta and para for m-1 and p-1, respectively. The UV-visible absorption spectra and photoisomerisation properties are noticeably different for the two structural isomers; (E,E)-m-1 behaves similarly to (E)-azobenzene, while (E,E)-p-1 exhibits a substantial red shift in the absorption bands and a decreased photoreactivity. The three geometric isomers of m-1, namely the E,E, E,Z and Z,Z isomers, cannot be resolved in a mixture by absorption spectroscopy, while the presence of three distinct species can be revealed by analysis of the absorption changes observed upon photoisomerisation of (E,E)-p-1. Quantum chemical ZINDO/1 calculations of vertical excitation energies nicely reproduce the observed absorption changes and support the idea that, while the absorption spectra of the geometrical isomers of m-1 are approximately given by the sum of the spectra of the constituting azobenzene units in their relevant isomeric form, this is not the case for p-1. From a detailed study on the E-->Z photoisomerisation reaction it was observed that the photoreactivity of an azo unit in m-1 is influenced by the isomeric state of the other one. Such observations indicate a different degree of electronic coupling and communication between the two azo units in m-1 and p-1, as confirmed by electrochemical experiments and quantum chemical calculations. The decreased photoisomerisation efficiency of (E,E)-p-1 compared to (E,E)-m-1 is rationalised by modelling the geometry relaxation of the lowest pi-pi* state. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, with predetermined photoreactivity.
我们研究了两种双（偶氮）衍生物（E,E）-m-1和（E,E）-p-1的光物理、光化学和电化学性质。这两种化合物可视为由一对共享一个苯环的偶氮苯单元组成，仅在中心苯环上两个偶氮基团的相对位置不同——m-1和p-1的偶氮基团分别位于间位和对位。两种结构异构体的紫外可见吸收光谱和光异构化性质明显不同；（E,E）-m-1的行为与（E）-偶氮苯相似，而（E,E）-p-1的吸收带出现显著红移且光反应性降低。m-1的三种几何异构体，即E,E、E,Z和Z,Z异构体，在混合物中无法通过吸收光谱分辨，而通过分析（E,E）-p-1光异构化时观察到的吸收变化可以揭示三种不同物种的存在。垂直激发能的量子化学ZINDO/1计算很好地再现了观察到的吸收变化，并支持了这样一种观点，即虽然m-1几何异构体的吸收光谱大致由其相关异构形式的组成偶氮苯单元光谱之和给出，但p-1并非如此。通过对E→Z光异构化反应的详细研究发现，m-1中一个偶氮单元的光反应性受另一个偶氮单元异构状态的影响。这些观察结果表明m-1和p-1中两个偶氮单元之间的电子耦合和通信程度不同，电化学实验和量子化学计算证实了这一点。通过对最低π-π*态几何弛豫的建模，解释了（E,E）-p-1与（E,E）-m-1相比光异构化效率降低的原因。这些结果预计对于基于偶氮苯单元设计具有预定光反应性的新型低聚物和聚合物具有重要意义。
