Reversible photoisomerization of an azobenzene-functionalized self-assembled monolayer probed by sum-frequency generation vibrational spectroscopy

Abstract

Sum-frequency generation (SFG) vibrational spectroscopy is employed to investigate the reversible, photoinduced trans/cis isomerization of an azobenzene-functionalized self-assembled monolayer (SAM) on a gold substrate. A CN marker group at the outer phenyl ring is used as a direct measure of the switching state. The azobenzene unit is connected to the surface by a tripodal linker system with an adamantane core, which results in both a sufficient decoupling of the functional azobenzene unit from the metallic substrate and a free volume to prevent steric hindrance, thus allowing the isomerization process. Optical excitation at 405 nm induces the trans→cis isomerization, whereas light exposure at 470 nm leads to the back reaction. The effective cross sections for the reactions are σ_eff(cis) = 4 ± 1 × 10⁻¹⁸ cm² at 405 nm (trans→cis) and σ_eff(trans) = 2.5 ± 0.9 × 10⁻¹⁹ cm² at 470 nm (cis→trans). We propose that the photoisomerization is driven by a direct (intramolecular) electronic excitation of the azobenzene conjugate, analogous to the free molecules in solution.