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 ReI → phen, bpe metal-to-ligand charge transfer (MLCT) excitation of fac-[ReI(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-[ReI(phen)(CO)3(cis-bpe)]+. Time-resolved absorption and infrared (TRIR) measurements reveal the existence of an intermediate with a lifetime of ∼28 ns (k = 3.6 × 107 s-1). According to the results of the transient IR measurements, the intermediate is ligand-based, presumably the olefin-localized twisted triplet 3p. TRIR measurements in the ν(CC) region point to a significant twisting around the CC bond in 3p.