Photochemical and Electronic Properties of Conjugated Bis(azo) Compounds: An Experimental and Computational Study

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 π–π* state. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, with predetermined photoreactivity. Sono state studiate le proprietà fotofisiche, fotochimiche ed elettrochimiche di due composti bis(azo), (E,E)‐m‐1 and (E,E)‐p‐1. Tali composti, che possono essere considerati come due unità azobenzene che condividono uno dei loro anelli benzenici, differiscono solo per la posizione relativa dei due gruppi azo sull'anello benzenico centrale (ripettivamente meta e para per m‐1 e p‐1). Gli spettri di assorbimento UV‐visibile e le caratteristiche di fotoisomerizzazione dei due isomeri stru