Photophysical processes in fluorinated acetones

Abstract

Absorption, fluorescence and fluorescence excitation spectra, and fluorescence quantum yields and decay times are presented for a variety of fluorinated acetones in the vapour phase. The effect of fluorine substitution upon derived kR values is shown to be compatible with that expected on the basis of frequency changes of the ground state promoting mode for the radiative transition provided the dependence is stronger than linear. The decrease in kNR upon substitution is attributed to a reduction in Franck–Condon factors for the S1→T1 intersystem crossing upon replacement of the hydrogen atoms on the α-carbons. For high excess energies, the rapid increase in rate constant for non-radiative decay is shown for 1,1,1-trifluoroacetone to be compatible with values expected if internal conversion to the ground state becomes of importance, rate constants for S1→S0 internal conversion and S1→T1 intersystem crossing becoming comparable in magnitude for an excess energy of ∼4 000 cm–1.