Heavy-Atom and Substituent Effects on S—T Transitions of Halogenated Carbonyl Compounds

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Abstract
The low‐lying S0→S and S0→T transitions in six substituted benzophenones and three carboxylic acid bromides have been investigated by direct absorption, phosphorescence excitation, and phosphorescence emission techniques in rigid media. In several bromine‐substituted benzophenones, the oscillator strength of the S0→Tn,π transition was found to be larger by a factor of 2 to 3 than in benzophenone. However, comparison with methyl‐ and chlorosubstituted benzophenones indicates that this enhancement is probably due to a substituent effect rather than a specific heavy‐atom effect. There was no intermolecular heavy‐atom enhancement of S0→Tn,π transitions. Similar results were obtained, with acetophenone The absorption and phosphorescence spectra of benzoyl bromide were measured, and it was concluded that the lowest triplet state of this compound is of (π, π) type. The weak (ε=1.8) 4300‐Å absorption band in oxalyl bromide has been reinvestigated. Studies of solvent effects on the absorption spectrum of oxalyl bromide and of the phosphorescence excitation spectrum of oxalyl bromide in rigid glass solutions containing naphthalene or phenanthrene confirm the assignment of the 4300‐Å band as an S0→Tn,π transition. The strength of the transition ε=1.7±0.1 is, however, anomalously high as compared to all other known S0→Tn,π or S→Tπ,π transitions in organic molecules containing bromine as the heaviest atom.