title: Photophysical properties of MM quadruply bonded complexes supported by carboxylate ligands, MM = Mo2, MoW, or W2.
abstract: While chemists have extensively studied the photophysical properties of d(6), d(8), and d(10) transition metal complexes, their early transition metal counterparts have received less attention. Quadruply bonded complexes of molybdenum and tungsten supported by carboxylate ligands have intense metal-to-ligand charge transfer (MLCT) absorptions that arise from the electronic coupling of the metal-metal (MM) δ orbital with the CO(2) π-system. This coupling may in turn be linked to an extended π-conjugated organic functional group. The major interaction is akin to the so-called back-bonding in metal carbonyl complexes. By the appropriate selection of MM, its attendant ligands, and the organic group, this absorption can be tuned to span the visible and near IR range, from 400 to 1000 nm. Consequently, these complexes offer potential as photon harvesters for photovoltaic devices and photocatalysis.  In this Account, we describe recent studies of dinuclear M(II) containing complexes, where M = Mo or W, and show that there are both parallels and disparities to the monomeric transition metal complexes. These early transition metal complexes have relatively long lived excited state singlets when compared to other transition metal complexes. They also often show unusual dual emission (fluorescence and phosphorescence), with singlet (S(1)) lifetimes that range from 1 to 20 ps, and triplet (T(1)) lifetimes from 3 ns to 200 μs. The fluorescent S(1) states are typically (1)MLCT for both M = Mo and W. These extended singlet lifetimes are uncommon for mononuclear transition metal complexes, which typically have very short lived (1)MLCT states due to rapid femto-second intersystem crossing rates. However, the T(1) states differ. This phosphorescence is MLCT in nature when M = W, while this emission comes from the δδ* state for M = Mo. Through time-resolved femtosecond infrared spectroscopy, we can detect the asymmetric stretch of the CO(2) ligand in both the singlet and triplet δδ* states. Through these analytical methods, we can study how the charge distribution in the singlet and triplet excited states changes over time. In addition, we can detect delocalized or localized examples of MLCT states, which represent class III and I excited state mixed valence in the Robin and Day scheme.
authors.affiliation: Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210-1185, United States. chisholm@chemistry.ohio-state.edu
authors.forename: Malcolm H
authors.initials: MH
authors.lastname: Chisholm
authors.forename: Terry L
authors.initials: TL
authors.lastname: Gustafson
authors.forename: Claudia
authors.initials: C
authors.lastname: Turro