Polarization of Aromatic Carbonyl Spectra

Abstract

The polarization of the phosphorescence and the polarized excitation spectra (using the 0–0 band of the polarized T→S emission) covering the region 4000–2500 Å have been examined for nine aromatic carbonyl compounds in EPA rigid media at 77°K. The principal conclusions are: (1) The S—T³U(n, π^*)→¹A transition is predominantly z or z′ polarized (coordinate orientation is given in Fig. 2) in accord with spin—orbit coupling predictions. (2) The principal source of intensity in the ¹U(n, π^*)←¹A bands is vibronic. The most important vibronic path involves a₂ vibrations in C₂v with analogs a in C₂ and a″ in C_s. These vibronic sources of intensification are more significant than the intrinsic allowedness engendered by formal destruction of C₂v symmetry in aromatic aldehydes and ketones. This conclusion requires the apparently allowed totally symmetric bands (possibly including the 0–0 band) in benzophenone to be predominantly vibronically induced. (3) The interaction of the rings in the double‐ring compounds, anthrone and benzophenone splits the localized ¹L_b(π, π^*) states, giving rise to the A state at a lower energy than the B state. The observed sequence and large energy separation (3000–4000 cm⁻¹) is not in accord with dipole—dipole interaction as the major source of splitting. (4) The 0–0 region of the ¹L_b(π,π^*)←¹A transition in the single‐ring carbonyl compounds is polarized predominantly perpendicular to the carbonyl‐ring axis in accord with previous measurements on other substituted benzenes, and with the carbonyl group acting approximately as a C₂v π‐electron perturbation on the ring. Weaker bands are predominantly z′ polarized requiring vibronic coupling with long‐axis states through b₁ analog vibrations. (5) The observed z (or z′) polarization of the 2500‐Å band in all the aromatic carbonyl compounds studied confirms Tanaka and Nagakura's hypothesis that this band is due to an intramolecular ring—carbonyl charge‐transfer transition.