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 σ²π⁴δ² and a considerable degree of attention has been devoted to studies of the δ → δ^* transition. For compounds of the type M₂(O₂CR)₄, the CO₂ π^* orbitals introduce a M₂ δ to ligand π^* transition, a ¹MLCT 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 ¹MLCT shifts to even lower energy and emission is seen from this S₁ state in addition to phosphorescence from the T₁ state which may be either ³MLCT or ³MMδδ^*. The latter typically occurs around 1100 nm with a lifetime that ranges from ∼1 μs (M = W) to 100 μs (M = Mo). The S₁¹MLCT states have lifetimes of ∼1–20 ps, allowing for fs and ns studies of the charge distribution/localization with time in both the S₁ and T₁ 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 Lπ–M₂δ–Lπ conjugation and has a low energy ¹MLCT. Compounds of this type have excited states that may be considered as mixed valence (MV) ions [L–M₂⁺–L⁻] ↔ [L⁻–M₂⁺–L] where the hole resides on the M₂ 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-M₂(TⁱPB)₂(O₂CCC-9-anthracene)₂, M = Mo, W, that have the IR-active reporter groups CO₂ and CC.