Design of Long-Lived Ru(II) Terpyridine MLCT States. Tricyano Terpyridine Complexes

The photophysics of Ru(tpy)(CN)(3)(-) and Ru(ttpy)(CN)(3)(-) (where tpy = 2,2':6',2"-terpyridine and ttpy = 4'-(p-tolyl)-2,2':6,2"-terpyridine) has been studied in detail. The complexes exhibit strongly solvatochromic behavior. Emission energies correlate linearly with solvent acceptor number, consistent with the metal-to-ligand charge transfer (MLCT) nature of the emission and the occurrence of second-sphere donor-acceptor interactions at the cyanide ligands. On the other hand, the correlation of emission lifetimes is clearly biphasic, with a sharp maximum found for solvents of intermediate acceptor number. Such a behavior is explained in terms of competition between a direct deactivation channel and an indirect, thermally activated decay pathway, with relative efficiencies depending strongly on MLCT state energy. A gain of 2 orders of magnitude in lifetime is obtained using the tricyano complexes (Ru(tpy)(CN)(3)(-), tau = 48 ns in DMSO; Ru(ttpy)(CN)(3)(-), tau = 40 ns in CH(3)CN) instead of the corresponding bis-terpyridine species (Ru(tpy)(2)(2+), tau = 250 ps in CH(3)CN; Ru(ttpy)(2)(2+), tau = 860 ps in CH(3)CN). This shows how an appropriate choice of ancillary ligands can be used to improve the properties of photosensitizers containing the Ru(II)-terpyridine chromophoric unit.