Photophysical properties of MM quadruply bonded complexes (M = Mo, W) supported by carboxylate ligands: charge delocalization and dynamics in S1 and T1 states

Quadruply bonded dinuclear metal complexes of molybdenum and tungsten have the MM configuration sigma(2)pi(4)delta(2) and a considerable degree of attention has been devoted to studies of the delta -> delta* transition. For compounds of the type M-2(O2CR)(4), the CO2 pi* orbitals introduce a M-2 delta to ligand pi* transition, a (MLCT)-M-1 absorption which may be lower in energy than the delta -> delta* 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 (MLCT)-M-1 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 (MLCT)-M-3 or (MM)-M-3 delta delta*. The latter typically occurs around 1100 nm with a lifetime that ranges from similar to 1 mu s (M = W) to 100 mu s (M = Mo). The S-1 (MLCT)-M-1 states have lifetimes of similar to 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-M2L2L'(2) where L and L' are carboxylate or amidinate groups for which only one set of ligands allows for expansive L pi-M-2 delta-L pi conjugation and has a low energy (MLCT)-M-1. 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((TPB)-P-i)(2)(O2CC C-9-anthracene)(2), M = Mo, W, that have the IR-active reporter groups CO2 and C C.