Photochemistry of fac-[Re(CO)₃(dcbH₂)(trans-stpy)]⁺: New Insights on the Isomerization Mechanism of Coordinated Stilbene-like Ligands

In this work, a novel complex fac-[Re­(CO)₃(dcbH₂)­(trans-stpy)]⁺, (dcbH₂ = 4,4′-dicarboxylic acid-2,2′-bipyridine; trans-stpy = trans-4-styrylpyridine) was synthesized and characterized toward its spectroscopic, photochemical, and photophysical properties. The experimental data provide new insights on the mechanism of photochemical trans-to-cis isomerization of the stilbene-like ligand coordinated to Re­(I) polypyridyl complexes. The new complex exhibits an unusual and strong dependence of the isomerization quantum yield (Φ_t→c) on the irradiation wavelength. Φ_t→c was 0.81 ± 0.08 for irradiation at 365 nm and continuously decreased as the irradiation wavelength is shifted to the visible. At 405 nm irradiation Φ_t→c is almost 2 orders of magnitude lower (0.010 ± 0.005) than that observed at 365 nm excitation. This behavior can be explained by the low-lying triplet metal-to-ligand charge-transfer excited state (³MLCT) that hinders the triplet photoreaction mechanism under visible light absorption. Under UV irradiation, direct population of styrylpyridine-centered excited state (¹IL) leads to the occurrence of the photoisomerization via a singlet mechanism. Further experiments were performed with the complex immobilized on the surface of TiO₂ and Al₂O₃ films. The nonoccurrence of isomerization at the oxide surfaces even under UV excitation evidences the role of energy gap between the ¹IL/¹MLCT states on the photochemical/photophysical processes. The results establish important relationships between the molecular structure and the photoelectrochemical behavior, which can further contribute to the development of solid-state molecular switches based on Re­(I) polypyridyl complexes.