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MULTI‐SITE PHOSPHORESCENCE OF α, β‐ENONES:LEVEL INVERSION IN 6β, 19‐EPOXYCHOLEST‐4‐EN‐3‐ONE BY SOLVATION AND AGGREGATION
Author(s) -
BLOK PETER M. L.,
DEKKERS HARRY P. J. M.
Publication year - 1991
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1991.tb03652.x
Subject(s) - phosphorescence , methylcyclohexane , chemistry , emission spectrum , photochemistry , solvent , analytical chemistry (journal) , luminescence , spectral line , fluorescence , materials science , optics , organic chemistry , catalysis , physics , astronomy , optoelectronics
— Millisecond time‐resolved emission spectroscopy was used to probe the phosphorescence kinetics of the α‐β‐enone 6β, 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 3 n* state. Upon increasing the concentration a second emission grows which is characterized by a longer lifetime, a decreased fine structure and a hypsochromically shifted S 0 → 1 nπ* excitation spectrum. This phosphorescence is ascribed to 3 ππ* 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 3 nπ* and 3 ππ* 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 ππ* emission is shifted to the blue in the S 0 → 1 nπ* region and to the red in the S 0 → 1 ππ* 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 ππ* and 3 nπ* states are intermixed to a considerable extent.