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 Huckel calculations indicate that the lowest e xcited state of these arylcarbyne complexes is based on a d(M) --> pi - (M=C-Ar) configuration. Consistent with this prediction, each of th e complexes displays a weak, mid-visible absorption band which is attr ibuted to the d --> pi transition. The tungsten complexes la, Ic, and Id also exhibit red luminescence with a lifetime in the 100-200 ns re gime. Nanosecond laser flash photolysis studies indicate that the aryl carbyne complexes display a strong transient absorption with a maximum in the 400-450 nm region. Close correspondence between decay lifetime s obtained by luminescence and transient absorption indicate that the transient absorption may be assigned to the d --> pi excited state.