PHOTOCHEMICAL AND ELECTRONIC PROPERTIES OF CONJUGATED BIS(AZO) COMPOUNDS: AN EXPERIMENTAL AND COMPUTATIONAL STUDY

Molecular systems containing two or more azobenzene units are of interest for at least two reasons: (i) because of their multiphotochromic nature, such compounds can exist in a wealth of different states (up to 2n, where n is the number of photochromic units), a behaviour that may be useful for molecular-level information processing and storage; (ii) the cooperation of the different photoisomerizable units can produce an overall amplification of the geometrical changes related to the (E)-(Z) transformation, leading to new light-induced functions. In this work 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, that can be viewed as constituted 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 photoisomerization 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 (E,E), (E,Z) and (Z,Z) – 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 photoisomerization of (E,E)-p-1. Quantum-chemical ZINDO/1 calculations of vertical excitation energies reproduce nicely 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) photoisomerization 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 photoisomerization efficiency of (E,E)-p-1 compared to (E,E)-m-1 is rationalized by modelling the geometry relaxation of the lowest ππ* state at CIS/3-21G level of theory. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, having predetermined photoreactivity.