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Single-component azobenzene thiol (Me-SH) and two-component azobenzene dodecyl disulfide (Me-SS) have been used to understand how “space” necessary for trans ↔ cis photoisomerization, “intermolecular interactions” and “phase separation” affects the repeated photoswitching ability of azobenzene units on flat gold surfaces. Me-SH self-assembled monolayers (SAMs) with available space and π–π stacking interactions between photoswitchable azobenzene units showed ∼51% of the first trans-to-cis photoisomerization and good photoswitching performance in response to alternating irradiation with UV and visible light. On the other hand, our absorption spectral data and AFM observations suggest that each chemical component of Me-SS did not move continuously on a flat gold surface and was almost fixed after partially phase-separated SAM formation. The first trans-to-cis photoconversion of the Me-SS SAMs with additional free space arising from phase separation reached as high as 75 ± 3%. Repeated irradiation of the Me-SS SAMs with UV and visible light seems to cause (i) out-of-plane (homeotropic and/or oblique) reorientation and/or (ii) random arrangements of mixed trans- and cis-azobenzenes. Consequently, the initial excellent photoswitching ability declined gradually as the number of repeated irradiations increased. The observations indicate that a balance between space, intermolecular interactions, and phase separation plays an important role in forming repeatedly photoswitchable monolayers.

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.

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 ( e ) in comparison with both meta-alkylated azobenzenes (4m and 5m) and 7p lacking the meta and ortho substituents. For ortho -alkylated 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.

We designed azobenzene alkyl disulfide to visualize molecular photoswitches in a monolayer system. Repeated changes in the surface nanostructure between a stripe-shaped structure and a smoother structure as a result of the reversible trans ↔ cisphotoisomerization of azobenzene units in phase-separated nanodomains were obviously observed using atomic force microscopy and UV-vis absorption spectroscopy.