RADIATIONLESS DEACTIVATION AND ANOMALOUS FLUORESCENCE OF SINGLET 1,12-BENZPERYLENE

Values of the rate constants for processes that deactivate the lowest excited singlet state of 1,12-benzperylene have been determined between -196 and 23° from measurements of the quantum yields of triplet formation, ΦT, fluorescence, ΦF, and the fluorescence lifetime, τF. The rate constants for intersystem crossing to the triplet state are nearly the same at -196 and 23°. The rate constant for direct radiationless deactivation to the lowest singlet is probably zero at -196° but increases to 6 × 105 sec-1 at 23°. This is similar to results obtained for other aromatic hydrocarbons. However, the fluorescence of 1,12-benzperylene is atypical. An increase in the rate constant of fluorescence, k1, from -196 to 23° is observed resulting from an increase in ΦF with temperature while τF decreases over the same temperature range. The increase in k1 with increasing temperature is attributed to a temperature-dependent emission from the second excited singlet state. Since the second excited singlet state of 1,12-benzperylene lies only 1275-1400 cm-1 above the 1Lb state, there is significant thermal population of the 1La state of 23°. The observation of a temperature-dependent anti-Stokes fluorescence supports this interpretation. The effect of solvent upon the energy gap between the 1La and 1Lb states and the fluorescence properties of 1,12-benzperylene are discussed.