Singlet–singlet electronic energy transfer: Dipole–dipole and exchange contributions

Singlet–singlet electronic energy transfer rates from p-difluorobenzene to acetone and its methyl-substituted derivatives have been measured in the gas phase. The results are compared to the data previously obtained with benzene as the donor, within the theoretical framework of the long range Coulombic and the short range exchange mechanisms. It is shown that the small energy transfer rates obtained with the highly substituted ketones can be attributed to the steric hindrance effect associated with the exchange mechanism; the steric hindrance factor in di-t-butyl ketone is ∼0.1 compared to acetone. No steric hindrance effect is observed for the dipole–dipole mechanism, as expected.