Fluorescence and radiationless transitions from the second excited states of azulene derivatives

Fluorescence yields of azulene derivatives with various kinds of substituents were determined in cyclohexane at room temperature. Most of them were found to emit fluorescence from the second excited state. In particular, 1,3‐dichloroazulene emits fluorescence which is twice as efficient as that of azulene. The quantum yield of “anomalous fluorescence” was found to be sufficiently well correlated with the energy gap δE between the first and the second excited states and to decrease with the decrease in δE. The rate constant of radiationless transition was calculated from the fluorescence yield and the calculated radiative lifetime. The rate constant was found to increase exponentially with decreasing δE. It was concluded that the internal conversion to the first excited state is the main route of the radiationless decay of the second excited state of azulene and its derivatives. Some discussions are given concerning the skeletal stretching modes as possible acceptors of the electronic energy upon the internal conversion S2∼→S1.