ENVIRONMENTAL INTERCOMBINATIONAL $PERTURBATIONS^{*}$
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Date
1954
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Ohio State University
Abstract
A general study of enhancement of singlet-triplet absorption bands of aromatic molecules in heavy-atom-containing solvents has been made. It has been found that enhancement of inter-combinational absorption bands occurs with any of the following solvents: ethyl iodide $>$ ethylene bromide $>$ carbon disulfide = carbon tetrachloride, with effectiveness as perturbing agents as indicated. The first general conclusion resulting from the present study is that the environmental perturbation of spin-orbit coupling is a general phenomenon. Thus, enhancements were found in $\alpha$-chloronaphthalene, anthracene, 9,10-dichloro- and 9,10-dibromo-anthracene, thiophene, and chlorobenzene. In N-heterocyclics, e.g., pyridine, it was found that the observed increase in absorption with ethyl iodide is due to reaction, rather than to simple physical enhancement. The second general conclusion which can be made is that the solvent perturbation technique is the most unambiguous, if not most powerful, experimental method available for the location of intercombinational absorption bands. It was shown that singlet-singlet absorption bands in the molecules studied above were quite intensity insensitive to the presence of heavy atom solvents (there being only a small wavelength shift). Thus, although phosphorescence ($T\rightarrow$S) emission and singlet-triplet absorption studies may occasionally yield uncertain results due to the presence of impurities, the solvent-enhancement of weak bands can be shown to be impurity independent. Several arguments against molecular complex formation between absorbing molecule and solvent will be presented, including absence of charge-transfer bands in all cases studied.
Description
$^{*}$Work done under a contract between the Office of Scientific Research, Air Research and Development Command, and the Florida State University.
Author Institution: Department of Chemistry, Florida State University
Author Institution: Department of Chemistry, Florida State University