The single -, triplet absorption spectra of 16 photochemically active aromatic ketones and aldehydes have been investigated by the phosphorescence excitation method. Both n,r and r,r triplet states have been located in a number of molecules. The relation between our experimental findings and a recent theoretical interpretation of intersystem-crossing mechanisms in carbonyl compounds is discussed. The intensity of singlet + triplet transitions in carbonyl compounds is examined theoretically, and it is predicted that the S-T,,, transitions in aromatic compounds should be enhanced by the presence of the > G O substituent. This prediction is experimentally verified with several examples. Within the accuracy of our experiments, there was no external heavy atom effect on the intensity of the %Tu,, transitions, although the addition of ethyl iodide to the solvent did enhance the somewhat weaker S-rT,,, transitions by about a factor of 2. This difference in sensitivity to external heavy atoms provides a useful criterion for distinguishing ST,,, and S+T,,, transitions. The relation of the spectroscopic results to the interpretation of the photochemical properties of these molecules is discussed. nterest in the triplet-state properties of organic (15) S. K. Lower and M. A.

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Aus den Phosphoreszenzspektren der Ketone (I) und (II) bei 77 und 4°K geht hervor, daß die O‐O‐Bande des S0 ‐ S (n,1r )‐Überganges erlaubt und senkrecht zur Molekülebene polarisiert ist, was auf eine wesentliche Einbeziehung des einsamen Sauerstoffelektronenpaares in das aromatische Ringsystem schließen läßt. Ein großer Teil der Intensität (80‐90%) dieser Übergänge ist vibronisch induziert.