Phosphorescence of cycloalkanones

The ³n, π¹ phosphorescence of a series of cycloalkanones has been studied in EPA at 77 K with the aim of deducing the importance of ring strain and substitution on radiationless decay processes. Results indicate that the contour of the phosphorescence spectrum (unstructured) remains the same (≈450 nm maximum) for the series cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, and cyclo-octanone, and that whereas α-methyl substitution decreases the phosphorescence intensity relative to the parent cycloalkanone, β-substitution results in increasing phosphorescence. The phosphorescence yield of cyclobutanone, cyclopentanone, cyclohexanone, cycloheptanone, cyclo-octanone, increase in the ratio 1 : 63 : 22 : 201 : 290. The low phosphorescence yield of cyclobutanone (ø_p=0.003) is considered to be an indication of a small triplet yield relative to the remaining molecules. The results suggest that the α-CH stretching mode is a factor in radiationless deactivation from the lowest triplet and that radiotionless deactivation processes may be different in the singlet and triplet states.