Modulating the Photoisomerization Mechanism of Semiconductor-Bound Azobenzene-Functionalized Compounds

The competing interplay of charge transfer and structural relaxation in the photoisomerization mechanism of azobenzene-functionalized semiconductor complexes is revealed by nonadiabatic excited-state molecular dynamics simulations. It is shown that fundamentally different structural dynamics occur in azo-compounds because of electronic charge transfer. If charge transfer occurs first, the photoinduced isomerization mechanism is quenched and the intramolecular relaxation occurs mainly through vibration excitations. To demonstrate the effect, azobenzene/π-bridge/TiO2 semiconductor structures were engineered to have very different interfacial electron transfer rates, albeit having similar molecular structures. This was accomplished with π-bridges constituted of biphenyl and 2,6,2′,6′-tetramethyl-biphenyl moieties, which exhibit very different electric conductances because of the twist angle of the comprising aromatic rings. It is demonstrated that ultrafast IET quenches the trans → cis photoisomerization in azo-compounds bound to semiconductor surfaces because of the dissociation of the electron–hole pair excitation. The present study aims at enlightening the role of the electronic dynamics in the photoisomerization mechanism of azo-compounds coupled to the environment, highlighting the importance of molecular tailoring for the design of photoresponsive materials.