Investigation of S → T and S → S Transitions by Phosphorescence Excitation Spectroscopy VII. 1-Indanone and Other Aromatic Ketones

A detailed examination of the phosphorescence excitation spectra of three aromatic ketones, acetophenone, p‐bromoacetophenone and 1‐indanone, has been carried out at both 77 and 4°K. The studies of 1‐indanone demonstrate the following: (1) The 0–0 band of the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}S(n,\; \pi *)}$ transition is allowed and polarized perpendicular to the plane of the molecule, indicating substantial delocalization of the oxygen long‐pair orbital into the aromatic ring system. (2) Most of the intensity (80%–90%) in the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}S(n,\; \pi *)}$ transition is vibronically induced. (3) A strong carbonyl progression appears to be absent in the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}S(n,\; \pi *)}$ absorption spectrum, and this is attributed to mixing of the carbonyl stretching mode with other vibrational modes in the excited state. (4) The $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}T(n,\; \pi *)}$ transition has allowed character as a result of spin–orbit interaction between the $\mathrm{T(n,\; \pi *)}$ state and the ${}^1{L}_a\mathrm{(\pi ,\; \pi *)}$ state. (5) The intensification of the $S_0{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{L}_b$ transition in indanone relative to acetophenone is attributed to mixing of the ${}^1{L}_b$ state with the ${}^1{L}_a$ state induced by orthosubstitution. The introduction of ${}^1{L}_a$ character into the ${}^1{L}_b$ state is also responsible for the intensification of the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}T(n,\; \pi *)}$ transition in indanone relative to acetophenone. (6) The 0–0 band of the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}T(\pi ,\; \pi *)}$ transition is allowed by spin–orbit coupling with the allowed, out‐of‐plane polarized component of the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}S(\pi ,\; \pi *)}$ transition. (7) Vibronic coupling between the $\mathrm{T(\pi ,\; \pi *)}$ state and a nearby $\mathrm{T(n,\; \pi *)}$ state introduces significant in‐plane polarized intensity into the $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}T(\pi ,\; \pi *)}$ transition. Essentially similar results were obtained with acetophenone and p‐bromoacetophenone, with the exception that the strong $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}T(\pi ,\; \pi *)}$ transition in p‐bromoacetophenone was found to be in‐plane polarized parallel to the long‐axis polarized transition to the ${}^1{L}_a\mathrm{(\pi ,\; \pi *)}$ state. Diffuseness observed in the 4°K single‐crystal $S_0\mathrm{\hspace{0.17em}\to \hspace{0.17em}T}$ spectra of each of the three compounds is attributed to vibronic interaction of the $\mathrm{T(n,\; \pi *)}$ with nearly degenerate vibronic levels associated with a lower‐lying $\mathrm{T(\pi ,\; \pi *)}$ state.