Effect of the alkyl linker length on the photoisomerization of hydrazone switches on metal surfaces

A series of bistable hydrazone switches containing alkyl thiolate linkers of various lengths (Cn HAT, n = 3, 8, 10, and 11) were synthesized. We explore the length effect of the carbon chain on the photoisomerization of hydrazone switches using UV-vis spectroscopy and tip-enhanced Raman spectroscopy (TERS). The conversion efficiency (photostationary state, PSS, after irradiation at 415 or 340 nm) of the isomerization of Cn HAT monolayers on Au rises with increasing length of the alkyl chain, with an optimum result for n-octyl (C8) thiolate or longer linkers. The low PSS415 of C3 HAT is attributed to strong quenching by the metal surface, as confirmed by density functional theory (DFT) calculations based on a one-dimensional double-well model. The partial trap of photo-induced hot carriers in the excited states of C3 HAT in the potential well of Au reduces the lifetime of its excited states. Such calculation results provide insight into the detailed mechanism of surface quenching effect. Furthermore, UV-vis results suggest that after irradiation at 415 nm, Cn HATs cannot isomerize when bound to Ag; higher photon energies are necessary in this case. These results validate the previously proposed substrate-mediated isomerization mechanism.