Correlation between the molecular structure and trans ↔ cis isomerization characteristics of azobenzenes

Photochemical and thermal isomerization of various azobenzenes was systematically investigated to understand the correlation between the molecular structure and trans ↔ cis isomerization characteristics of azobenzenes. A blue shift in π-π* absorption band of ortho-alkylated azobenzenes (1o and 2o) was observed together with a reduction in molar extinction coefficient ( ɛ?) in comparison with both meta-alkylated azobenzenes (4m and 5m) and 7p lacking the meta and ortho substituents. For orthoalkylated azobenzene, photochemical trans-to-cis isomerization and thermal back cis-to-trans isomerization in solution occurred slowly when compared with 4m, 5m and 7p. The half-life time of the cis form of 2o was found to be 380 h, which is about 8-50 times longer than those of comparable 4m, 5m (43-13 h) and 7p (7 h). Furthermore, comparison of the molecular structure and isomerization characteristics of azobenzene thiol (2o and 5m) self-assembled monolayers on flat gold surfaces indicates that the trans-to-cis photoconversion in monolayer systems is influenced by steric hindrance and strong intermolecular interaction between azobenzene units.