Unusual spectral and electrochemical properties of azobenzene-substituted porphyrins

A series of azobenzene-substituted porphyrin derivatives, MPAn (M = H2, Zn; n = 1, 2, 4), has been prepared. These compounds were characterized by spectral and electrochemical methods. The Soret and Q bands of ZnPAn were red-shifted along with the increase of Q-band intensity ratio (ɛα/ɛβ) as the substitution of azobenzene increased. Addition of imidazole towards ZnPAn caused a significant increase in Q band intensity ratio. A noteworthy characteristic is that the Q band intensity ratio was reversed (>1.0) by imidazole ligation for ZnPA4. Additionally, cyclic voltammetry of H2PAn exhibited anodic shift of the first oxidation potential but cathodic shift for the second oxidation potential for porphyrin free bases as azobenzene number increased. According to DFT calculation, the unusual features in UV–vis spectra and redox potentials arised mainly due to the intramolecular π-conjugation between porphyrin and azobenzene.