Photostability and molecular structure

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Abstract
Photostability, defined as the inverse of the quantum yield of a photochemical reaction, is assumed to be proportional to the rate of internal conversion. The rate of internal conversion is inversely proportional to the energy difference between the first excited state and the ground state. In intramolecularly hydrogen‐bonded π systems, the photoexcited enol form rearranges itself to an excited keto form. SCF LCAO MO calculations indicate that the energy difference between the first excited state and the ground state is smaller in the keto form than in the enol form. Experiments are reported in support of these theoretical considerations. Weak, strongly red‐shifted fluorescence is observed at 77°K for several photostable quinazolines, pyrimidines, quinolines, pyrazines, benzotriazoles, and benzophenones with an o‐hydroxyphenyl group in ortho position to a ring nitrogen or a carbonyl group. For some compounds a moderately red‐shifted fluorescence is observed as well. No phosphorescence is detected. None of the compounds fluoresce at room temperature. When intramolecular hydrogen bonding is destroyed by, e.g., methylation, the compound shows phosphorescence and moderately red‐shifted fluorescence. With data from the literature, the increase in photostability due to enol‐keto tautomerism in the excited state is estimated. The increase agrees well with the experimental results.