Primary photophysical processes of discrete pseudotetrahedral oxovanadium centers dispersed in a silica xerogel matrix

Abstract

The photophysical properties of discrete, catalytically relevant, oxovanadium (VO) sites dispersed in a transparent silica xerogel matrix have been measured by time-resolved techniques. The lowest excited state of this chromophore is a long-lived, emissive 3E state with phosphorescence lifetimes of ∼4 and ∼13 ms at room temperature and 14 K, respectively. The temperature dependence of the nonradiative decay rate from this state shows two well defined deactivation regions. A high frequency region occurring at T > 200 K with an activation energy of 2029 ± 171 cm −1 and a low frequency region at T < 200K with an activation energy of 43 ± 17 cm −1. It is suggested that 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 1A 1 singlet state to the triplet emissive state was also determined. This process proved to be very efficient with values of the intersystem crossing quantum yield approaching unity at 14 K and showing a weak temperature dependence over the range measured.