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 tp y = 2,2':6',2 ''-terpyridine and ttpy = 4'-(p-tolyl)-2,2':6,2 ''-terpy ridine) has been studied in detail. The complexes exhibit strongly sol vatochromic behavior. Emission energies correlate linearly with solven t acceptor number, consistent with the metal-to-ligand charge transfer (MLCT) nature of the emission and the occurrence of second-sphere don or-acceptor interactions at the cyanide ligands. On the other hand, th e correlation of emission lifetimes is clearly biphasic, with a sharp maximum found for solvents of intermediate acceptor number. Such a beh avior is explained in terms of competition between a direct deactivati on channel and an indirect, thermally activated decay pathway, with re lative efficiencies depending strongly on MLCT state energy. A gain of 2 orders of magnitude in lifetime is obtained using the tricyano comp lexes (Ru(tpy)(CN)(3)(-), tau = 48 ns in DMSO; Ru(ttpy)(CN)(3)(-), tau = 40 ns in CH3CN) instead of the corresponding bis-terpyridine specie s (Ru(tpy)(2)(2+), tau = 250 ps in CH3CN; Ru(ttpy)(2)(2+), tau = 860 p s in CH3CN). 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.