INTERSYSTEM CROSSING ENHANCEMENT IN PHENYL SUBSTITUTED METHANES

Abstract

The lowest singlet-singlet absorption in phenyl-substituted methanes becomes progressively stronger as the number of phenyl groups is increased. One would thus expect fluorescence to compete progressively more favorably with phosphorescence. However, it has been shown that $\phi_{p}/\phi p$ increases as the number of phenyl groups in the series is increased. Because of the near constancy in the measured phosphorescence lifetimes ($\tau_{p}$) the phosphorescence enhancement was explained in terms of an exciton splitting mechanism. We have estimated the quantum yield of triplet population ($\phi_{t}$) from fluorescence quantum yields ($\phi_{p}$) using the relation $\phi_{t}\sim(1-\phi_{p})$. The fluorescent quantum yields are nearly constant throughout the series. Therefore, the radiative phosphorescence lifetimes $\tau_{p}=(\phi_{t}/\phi_{p})\tau_{p}$ decrease with increasing molecular size. This seems more consistent with a spin orbit enhancement mechanism.