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 (Φt →c) on the irradiation wavelength. Φt →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 Φt →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.