Electronic absorption and emission spectra of 4-pyrones, 4-thiopyrones and 4-pyridones

Abstract

The ultraviolet absorption and phosphorescence emission spectra of various methyl and phenyl substituted 4-pyrones, 4-thiopyrones and 4-pyridones have been measured. The lowest energy absorption of the unhindered 4-pyrones and 4-thiopyrones is the carbonyl n, π* transition, whereas 4-pyridones do not show the n, π* transition. The hypsochromic shifts and hypochromic effects observed for the hindered 4-pyrones, 4-thiopyrones and 4-pyridones are attributed to the steric effect of substituents. These compounds show little or no fluorescence at room temperature or at 77 K, while the lowest triplet states are mainly of π, π* orbital character. An intramolecular heavy-atom effect appears to play an important role in the phosphorescence spectra of 4-thiopyrones. The results of SCF LCAO MO calculations with configuration interaction are reported for each of these three classes of compounds.