| Abstract
| - 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), and2-pyrene (2-py); M = W, Ar = ph, 2-nap] complexes were investigated. The lowest energy absorption ofthe complexes is attributed to a metal-to-ligand charge transfer 1MLCT transition from the metal-based δHOMO to the π* O2C−Ar LUMO. The Mo2(O2C−Ar)4 complexes exhibit weak short-lived emission (<10ns) 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 solventdependence. Comparisons are made to the W2(O2C−Ar)4 complexes, which are easier to oxidize andexhibit greater spin−orbit coupling than the Mo2 systems. From the excited-state energy of the emissive1MLCT state and the electrochemical properties of the complexes, it is predicted that this excited stateshould be a powerful reducing agent. The crystal and molecular structure of Mo2(O2C−9-an)4 is also reportedtogether with electronic structure calculations employing density functional theory. To our knowledge, thisis the first observation of MLCT excited states in quadruply bonded complexes. In addition, the photophysicalproperties of the present systems parallel those of organic aromatic molecules and may be viewed asmetal-mediated organics. The introduction of the M2 δ* orbital in the complexes in conjugation with theorganic π-system of the ligands affords the opportunity to tune the excited-state energies and redoxpotentials.
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