Observation of ¹MLCT and ³MLCT Excited States in Quadruply Bonded Mo₂ and W₂ Complexes

The photophysical properties of the series of quadruply bonded M₂(O₂C−Ar)₄ [M = Mo, Ar = phenyl (ph), 1-naphthalene (1-nap), 2-naphthalene (2-nap), 9-anthracene (9-an), 1-pyrene (1-py), and 2-pyrene (2-py); M = W, Ar = ph, 2-nap] complexes were investigated. The lowest energy absorption of the complexes is attributed to a metal-to-ligand charge transfer ¹MLCT transition from the metal-based δ HOMO to the π* O₂C−Ar LUMO. The Mo₂(O₂C−Ar)₄ complexes exhibit weak short-lived emission (<10 ns) and a nonemissive, long-lived (40−76 μs) excited state detected by transient absorption spectroscopy. The short- and long-lived species are attributed to the ¹MLCT and ³MLCT excited states, respectively, based on the large Stokes shift, vibronic progression in the low-temperature emission spectrum, and solvent dependence. Comparisons are made to the W₂(O₂C−Ar)₄ complexes, which are easier to oxidize and exhibit greater spin−orbit coupling than the Mo₂ systems. From the excited-state energy of the emissive ¹MLCT state and the electrochemical properties of the complexes, it is predicted that this excited state should be a powerful reducing agent. The crystal and molecular structure of Mo₂(O₂C−9-an)₄ is also reported together with electronic structure calculations employing density functional theory. To our knowledge, this is the first observation of MLCT excited states in quadruply bonded complexes. In addition, the photophysical properties of the present systems parallel those of organic aromatic molecules and may be viewed as metal-mediated organics. The introduction of the M₂ δ* orbital in the complexes in conjugation with the organic π-system of the ligands affords the opportunity to tune the excited-state energies and redox potentials.