RADIATIONLESS DECAY OF THE S2 STATES OF AZULENE AND RELATED-COMPOUNDS - SOLVENT DEPENDENCE AND THE ENERGY-GAP LAW

The UV-visible absorption spectra, S2 lifetimes, and S2 --> S0 fluorescence quantum yields of azulene, azulene-d8, 1,3-di-chloroazulene, 1,3-dibromoazulene, 4,6,8-trimethylazulene, and 1,4-dimethyl-7-isopropylazulene (guaiazulene) have each been measured accurately in six solvents. The S2-S, electronic energy spacings of each solute vary by ca. 500 cm-1 in these solvents. The variations in the S2 nonradiative relaxation rates with electronic energy spacing are interpreted within the framework of the energy gap law of radiationless transition theory. S2 --> S1 internal conversion dominates the nonradiative decay in azulene and azulene-d8, but intersystem crossing (likely S2 --> T1) is important in the halogenated derivatives. The alkyl-substituted compounds exhibit anomalous behavior and demonstrate that factors other than the electronic energy spacing are involved in determining the rates of their radiationless relaxation. Previous energy gap law correlations based on data from a series of structurally different compounds must be reinterpreted.