Solution Phase and Surface Photoisomerization of a Hydrazone Switch with a Long Thermal Half-Life

Photoswitches can be employed for various purposes, with the half-life being a crucial parameter to optimize for the desired application. The switching of a photochromic hydrazone functionalized with a C6 alkyl thiolate spacer (C6 HAT) was characterized on a number of metal surfaces. C6 HAT exhibits a half-life of 789 years in solution. Tip-enhanced Raman spectroscopy (TERS) was used to study the photoisomerization of the C6 HAT self-assembled monolayers (SAMs) on Au, Ag, and Cu surfaces. The unique spectroscopic signature of the E isomer at 1580 and 1730 cm^-1 in TER spectra allowed for its discrimination from the Z isomer. It was found that C6 HAT switches on Au and Cu surfaces when irradiated with 415 nm; however, it cannot isomerize on Ag surfaces, unless higher energy light is used. Based on this finding, and supported by density functional theory calculations, we propose a substrate-mediated photoisomerization mechanism to explain the behavior of C6 HAT on these different metal surfaces. This insight into the hydrazone's switching mechanism on metal surfaces will contribute to the further exploitation of this new family photochromic compounds on metal surfaces. Finally, although we found that the thermal isomerization rate of C6 HAT drastically increases on metal surfaces, the thermal half-life is still 6.9 days on gold, which is longer than that of the majority of azobenzene-based systems.