Photophysical Studies of Metal to Ligand Charge Transfer Involving Quadruply Bonded Complexes of Molybdenum and Tungsten

ConspectusPhotoinduced metal-to-ligand charge transfer transitions afford numerous applications in terms of photon energy harvesting. The majority of metal complexes studied to date involve diamagnetic systems of d6, d8, and d10 transition metals. These typically have very short-lived, ∼100 fs, singlet metal to ligand charge transfer (1MLCT) states that undergo intersystem crossing to triplet metal to ligand charge transfer (3MLCT) states that are longer lived and are responsible for much of the photophysical studies. In contrast, the metal–metal quadruply bonded complexes of molybdenum and tungsten supported by carboxylate, O2CR, and related amidinate ligands (RN)2C(R′) have relatively long-lived 1MLCT states arising from M2δ to Lπ* transitions. These have lifetimes in the range 1–20 ps prior to intersystem crossing to T1 states that may be 3MLCT or 3MMδδ* with lifetimes of 1–100 ns and 1–100 μs, respectively. The M2 quadruply bonded complexes take the form M2L4 or M2L4–nL′n where n = 1–3. Thus, in their...