MULTISITE PHOSPHORESCENCE OF ALPHA,BETA-ENONES - 3N-PI-STAR-3-PI-PI-STAR LEVEL INVERSION IN 6-BETA, 19-EPOXYCHOLEST-4-EN-3-ONE BY SOLVATION AND AGGREGATION

Millisecond time-resolved emission spectroscopy was used to probe the phosphorescence kinetics of the alpha-beta-enone 6-beta,19-epoxycholest-4-en-3-one (1) as a function of concentration in several paraffinic and hydroxylic glasses at 77 K. Only in methylcyclohexane/methylcyclopentane glass at low concentration (10(-4) M) does the phosphorescence decay exponentially. It is interpreted as emission from the 3n-pi* state. Upon increasing the concentration a second emission grows which is characterized by a longer lifetime, a decreased fine structure and a hypsochromically shifted S0 --> 1n-pi* excitation spectrum. This phosphorescence is ascribed to 3-pi-pi* emission of aggregates of 1. In hydroxylic glasses the phosphorescence decay is multiexponential, even at 10(-4) M concentration; from emission band shapes and lifetimes it follows that both 3n-pi* and 3-pi-pi* type emissions are present, the latter increasing with the alcohol concentration in the solvent. The two types of phosphorescence have different excitation spectra: that of the structureless and long-lived 3-pi-pi* emission is shifted to the blue in the S0 --> 1n-pi* region and to the red in the S0 --> 1-pi-pi* region. This emission is ascribed to complexes of 1 with the alcoholic solvent. The results of time-resolved measurements of the circular polarization of the luminescence are consistent with the assignments given above and indicate that in the H-bonded and possibly also in the free species 3-pi-pi* and 3n-pi* states are intermixed to a considerable extent.