The n-π* Absorption and Emission of Optically Active trans-β-Hydrindanone and trans-β-Thiohydrindanone

The $\mathrm{n‐\pi *}$ transitions (absorption, circular dichroism, luminescence, linear and circular polarization of luminescence) of trans‐β‐hydrindanone (I) and trans‐β‐thiohydrindanone (II) are discussed. The $C_2$ symmetry of I and II is taken into account with the aid of expressions that are derived for nonrigid molecules having a twofold axis of symmetry. This allows the absorption of I to be split into a part due to transitions polarized along the twofold axis ($z$ axis) and a part due to x, y‐polarized vibration‐induced transitions. Accurate values for the electric and magnetic transition moments then are obtained. The vibrational structure of absorption and fluorescence and the latter's low degree of circular polarization are related to nonplanarity of the carbonyl group in the ${}^1\mathrm{(n\hspace{0.17em}\to \hspace{0.17em}\pi *)}$ state. A larger nonplanarity probably is present in the triplet state of I. Circular polarization in the fluorescence was also observed when racemic I was excited with circularly polarized light. For the absorption of II, an upper limit is found for the electric transition moment and a lower limit for the magnetic moment. These give no indication that $\text{3d}$ orbits play an important part in the optical activity of thioketones. The luminescence at − 196°C and the longest wavelength part of the visible absorption band do not show circular polarization. They probably are singlet–triplet transitions. There is some evidence that the thiocarbonyl group is planar in the triplet and in the excited singlet state.