Observation of 1MLCT and 3MLCT excited states in quadruply bonded Mo2 and W2 complexes

The photophysical properties of the series of quadruply bonded M-2(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 (MLCT)-M-1 transition from the metal-based delta HOMO to the pi(star) O2C-Ar LUMO. The Mo-2(O2C-Ar)(4) complexes exhibit weak short-lived emission (<10 ns) and a nonemissive, long-lived (40-76 mu s) excited state detected by transient absorption spectroscopy. The short- and long-lived species are attributed to the (MLCT)-M-1 and (MLCT)-M-3 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 W-2(O2C-Ar)4 complexes, which are easier to oxidize and exhibit greater spin-orbit coupling than the Mo-2 systems. From the excited-state energy of the emissive (MLCT)-M-1 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 Mo-2(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 M-2 delta(star) orbital in the complexes in conjugation with the organic pi-system of the ligands affords the opportunity to tune the excited-state energies and redox potentials.