title: Photophysical properties of MM quadruply bonded complexes (M = Mo, W) supported by carboxylate ligands: charge delocalization and dynamics in S1 and T1 states.
abstract: Quadruply bonded dinuclear metal complexes of molybdenum and tungsten have the MM configuration σ(2)π(4)δ(2) and a considerable degree of attention has been devoted to studies of the δ → δ(*) transition. For compounds of the type M(2)(O(2)CR)(4), the CO(2) π(*) orbitals introduce a M(2) δ to ligand π(*) transition, a (1)MLCT absorption which may be lower in energy than the δ → δ(*) and is more intense, thus obscuring the observation of the latter. When the R group is a conjugated organic system such as an aryl group, the (1)MLCT shifts to even lower energy and emission is seen from this S(1) state in addition to phosphorescence from the T(1) state which may be either (3)MLCT or (3)MMδδ(*). The latter typically occurs around 1100 nm with a lifetime that ranges from ~1 μs (M = W) to 100 μs (M = Mo). The S(1)(1)MLCT states have lifetimes of ~1-20 ps, allowing for fs and ns studies of the charge distribution/localization with time in both the S(1) and T(1) states, which is quite rare for transition metal coordination complexes. Of particular interest and focus have been complexes of the type trans-M(2)L(2)L'(2) where L and L' are carboxylate or amidinate groups for which only one set of ligands allows for expansive Lπ-M(2)δ-Lπ conjugation and has a low energy (1)MLCT. Compounds of this type have excited states that may be considered as mixed valence (MV) ions [L-M(2)(+)-L(-)] ↔ [L(-)-M(2)(+)-L] where the hole resides on the M(2) unit and the electron is either localized on one ligand, a class I or II MV ion, or is fully delocalized over both ligands, a class III ion in the Robin and Day scheme. Examples of these systems will be described along with the newly prepared complexes trans-M(2)(T(i)PB)(2)(O(2)CC≡C-9-anthracene)(2), M = Mo, W, that have the IR-active reporter groups CO(2) and C≡C.
authors.affiliation: The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Brian G
authors.initials: BG
authors.lastname: Alberding
authors.forename: Samantha E
authors.initials: SE
authors.lastname: Brown-Xu
authors.forename: Malcolm H
authors.initials: MH
authors.lastname: Chisholm
authors.forename: Terry L
authors.initials: TL
authors.lastname: Gustafson
authors.forename: Carly R
authors.initials: CR
authors.lastname: Reed
authors.forename: Vesal
authors.initials: V
authors.lastname: Naseri