Photochemical charge transfer excitation of trans-4-stilbazole at a silver electrode

When adsorbed at the surface of a polycrystalline silver electrode, trans-4-stilbazole photoreacts, in the presence of dissolved oxygen, to form trans-4′-hydroxy-4-stilbazole. The structure of the photoproduct is confirmed by (1) a comparison of the surface Raman results with the normal Raman of trans-4′-hydroxy-4-stilbazole, and by (2) by ex-situ analysis of the irradiated electrode surface using high-resolution mass spectrometry. The surface photochemistry appears to be a one-photon process whose threshold wavelength lies in the middle of the visible range though neat trans-4-stilbazole absorbs only in the ultraviolet. The surface Raman intensity of the photoproduct increases synchronously with the Faradaic current of oxygen reduction. Attendant incorporation of oxygen exclusively at the 4′ position of end-on trans-4-stilbazole evidences a photo-initiated electron transfer from the electrode to oxygen reducing at the other end of the molecule. Scattering from stilbazole adsorbed at two different surface sites is evident in the surface Raman spectrum: charged (Ag⁺) sites and more neutral surface sites. We assign excitation profile maxima for stilbazole at Ag⁺ sites and the photochemical initiation to a silver-to-stilbazole charge transfer at those sites. Ab-initio molecular orbital calculations on a model composed of a ground state Li⁰-trans-4-stilbazole and the anion radical excited state Li⁺-trans-4-stilbazole^? reveal a low-energy transition that supports assignment of the visible excitation to a silver-to-stilbazole electron transfer.