Photophysics of Tungsten and Molybdenum Arylcarbyne Complexes. Observation of the Lowest Excited State by Laser Flash Photolysis

The photophysics of the series of tungsten and molybdenum arylcarbyne complexes Cp(CO){P(OMe) 3 }M⋮C Ar ( 1a, M = W, Ar = phenyl; 1b, M = Mo, Ar = phenyl; 1c, M = W, Ar = o- tolyl; 1d, M = W, Ar = 2-naphthyl) has been examined by using absorption, emission, and transient absorption spectroscopy. Extended Hückel calculations indicate that the lowest excited state of these arylcarbyne complexes is based on a d(M) → π*(M⋮C Ar) configuration. Consistent with this prediction, each of the complexes displays a weak, mid-visible absorption band which is attributed to the d → π* transition. The tungsten complexes 1a, 1c, and 1d also exhibit red luminescence with a lifetime in the 100−200 ns regime. Nanosecond laser flash photolysis studies indicate that the arylcarbyne complexes display a strong transient absorption with a maximum in the 400−450 nm region. Close correspondence between decay lifetimes obtained by luminescence and transient absorption indicate that the transient absorption may be assigned to the d → π* excited state.