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

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journal contribution
posted on 14.12.2005, 00:00 by Matthew J. Byrnes, Malcolm H. Chisholm, Judith A. Gallucci, Yao Liu, Ramkrishna Ramnauth, Claudia Turro
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.

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