Synthesis and photochemical properties of porphyrin–azobenzene triad

Porphyrin–azobenzene triad E-2, having ester spacers between the chromophores, was synthesized and its photochemical and thermal isomerization properties were investigated. Triad E-2 showed little electronic communication among the chromophores according to a comparison of the steady-state absorption and fluorescence spectra of E-2 and their model compounds. E-2 showed photochemical E–Z isomerization and subsequent thermal Z-to-E isomerization. The porphyrin chromophore in Z-2, obtained by photoisomerization, did not strongly affect the transient state of thermal Z-to-E isomerization of the azobenzene unit. The high E-isomer composition at the photostationary state indicates the occurrence of triplet energy transfer between porphyrin and azobenzene. E-2 forms a 1:1 complex with 4,4′-bipyridyl (Bipy). The stability constant for E-2 with Bipy was determined (log K = 4.20 mol−1 dm3) by iterative least-squares fitting to a 1:1 binding model.