Spin–Orbit Coupling and Radiationless Transitions in Aromatic Ketones

Abstract

The spin–orbital coupling mechanism for ${}^3{\mathrm{n\pi *\leftarrow }}^{1}A$ was investigated for several substituted benzophenones by use of polarized absorption spectra and high‐field Zeeman studies and was found to be consistent with the scheme already determined for benzophenone itself [S. Dym, R. M. Hochstrasser, and M. Schafer, J. Chem. Phys. 48, 646 (1968)]. Spin‐polarization experiments on anthrone in n‐heptane at 4.2 and 2°K and benzophenone in pyridine at 2°K showed that excitation into the singlet resulted in the population of the predominantly active spin state, ${\mathrm{|T}}_z〉$ . Examination of low‐temperature absorption spectra of a variety of acetophenones and benzophenones revealed band broadening within the triplet manifold, suggesting the presence of higher triplets in many cases. No such evidence for a higher triplet state below the singlet origin could be detected in the spectra of benzophenone and perdeuterobenzophenone. The difference in linewidths for the triplet and singlet origins of benzophenone was measured and interpreted as uncertainty broadening due to strong intersystem mixing; the rate of this process was calculated to be about 10¹¹ sec⁻¹. Additional broadening of the ${}^1\mathrm{n\pi *}$ , 0–1 carbonyl stretching band was observed and attributed to vibrational relaxation and an increased rate of intersystem crossing from this level. The use of $C_{\text{2υ}}$ symmetry labels as an appropriate basis for discussion of aromatic carbonyl transitions is discussed. Finally a mechanism for intersystem crossing in aromatic ketones is given involving direct coupling of modified ${}^1\mathrm{n\pi *}$ and ${}^3\mathrm{n\pi *}$ states.