Knowledge Bank

experiments performed on single vibronic lines in the phosphorescence emission spectrum of p-benzoquinone-$h4$ (L = z($B1u$); M = y($B2u$)) as a trap in p-benzoquinone-$d4$ confirm the suggested $B1g$ symmetry of the lowest $n\pi \ast$ triplet state In this molecule. Indeed the phosphorescence spectrum (Z-polarized) at $2\circ K$ is dominated by a variety of $b1u$, $b2u$ and $b3u$ molecular modes arising from independently decaying z, y and x spin sublevels. However, none of these u-modes forms progressions in the spectrum, indicating that the lowest excited state is not so severely distorted as was Inferred previously from Stark modulation experiments on the absorption spectrum. To get a better insight into the connection between the excited state geometry of this lower state and Its absorption and emission spectra, we have startled EPR and ENDOR experiments on p-benzoquinone and sons of its isotopes. Preliminary measurements show indeed a drastic effect on the spin density distribution in this state after isotopic substitution. ENDOR experiments show that the triplet spin density in this state is mainly localized on the carbonyl functions. An interesting point of ab initio SCF calculations on p-benzoquinone is that the $\pi \ast$ excitations in this diketone are best described as localized excitations. This means that p-benzoquinone for the lower $n\pi \ast$ electronic states can be regarded as an excitonic molecule. This result may have wider applicability for the description of electronic states of other topologically related molecules.