PRIMARY PHOTOPHYSICAL PROCESSES OF DISCRETE PSEUDOTETRAHEDRAL OXOVANADIUM CENTERS DISPERSED IN A SILICA XEROGEL MATRIX

The photophysical properties of discrete, catalytically relevant, oxov anadium (V=O) sites dispersed in a transparent silica xerogel matrix h ave been measured by time-resolved techniques. The lowest excited stat e of this chromophore is a long-lived, emissive (3)E state with phosph orescence lifetimes of similar to 4 and similar to 13 ms at room tempe rature and 14 K, respectively. The temperature dependence of the nonra diative decay rate from this state shows two well defined deactivation regions. A high frequency region occurring at T > 200 K with an activ ation energy of 2029 +/- 171 cm(-1) and a low frequency region at T < 200 K with an activation energy of 43 +/- 17 cm(-1). It is suggested t hat the low frequency deactivation process is due to coupling between the oxovanadium chromophore and phonon modes of the silica matrix. The quantum yield for intersystem crossing from the high energy (1)A(1) s inglet state to the triplet emissive state was also determined. This p rocess proved to be very efficient with values of the intersystem cros sing quantum yield approaching unity at 14 K and showing a weak temper ature dependence over the range measured.