Mechanism of metal-to-ligand charge transfer sensitization of olefin trans-to-cis isomerization in the fac-[ReI(phen)(CO)3(1,2-bpe)]+ cation

Near-UV, largely Re-I --> phen, bpe metal-to-ligand charge transfer (MLCT) excitation of fac-[Re-I(phen)(CO)3(trans-bpe)](+) [where phen is 1,10-phenanthroline and bpe is 1,2-bis(4-pyridyl)ethylene] in CH3CN results in isomerization at the bpe ligand and formation of fac-[Re-I(phen)(CO)(3)(cis-bpe)](+). Time-resolved absorption and infrared (TRIR) measurements reveal the existence of an intermediate with a lifetime of similar to28 ns (k = 3.6 x 10(7) s(-1)). According to the results of the transient IR measurements, the intermediate is ligand-based, presumably the olefin-localized twisted triplet (3)p. TRIR measurements in the nu(C=C) region point to a significant twisting around the C=C bond in (3)p.