Intraligand Lowest Excited States in Tricarbonylhalobis(styrylpyridine)rhenium(I) Complexes

The complexes XRe(CO)₃L₂ (X = Cl, Br; L = trans-3-styrylpyridine, rrons-4-styrylpyridine) have been synthesized and characterized. Electronic absorption spectral studies reveal that the lowest excited state is an intraligand excited state. Irradiation of the styrylpyridine complexes in the lowest absorption band (313 or 366 nm) results in trans → cis isomerization of the coordinated ligand with a quantum efficiency in the range 0.49-0.64 and stationary states which are no less than 84% cis. Triplet sensitization gives stationary states and quantum yields for the coordinated styrylpyridines which are near those found for the free ligands themselves. Little photosubstitution (Φ < 0.01) of the coordinated styrylpyridine occurs. Thermal substitution of the styrylpyridines at 56° provides a pathway for photoassistance activity of the XRe(Co)₃L₂ complexes.