Photo-release of acetonitrile in ruthenium(II) complexes with various substituted terpyridine ligands

The photo-release of acetonitrile is investigated in a series of ruthenium(II) complexes of the general formula [Ru(R-phtpy)(acac)(MeCN)](PF₆) (phtpy stands for 4'-phenyl-2,2':6',2''-terpyridine, and R = Et₂N, Me₂N, MeO, Me, H, NO₂). The experimental quantum yields of photo-release (ϕ_MeCN = MeCN released/photons absorbed) increases with the donating capability of R, with values ranging from ϕ_MeCN = 0 (NO₂) to ϕ_MeCN = 0.05 (Et₂N). The origin of this effect is investigated computationally using the density functional theory and compared to those reported recently by our group on related [Ru(R-phtpy)(acac)(NO)](PF₆) species capable of causing photo-release of NO. In the present case, the capability for MeCN release appears related to the relative energies of the metal-centered (³MC) vs. metal-ligand-charge-transfer (³MLCT) triplet states. The ³MC state, in which the Ru-NC distance is elongated to 4.2 Å, is expected to be responsible for the release. Additionally, four crystal structures are reported for the compounds in which R = Et₂N, MeO, H, and NO₂.