Knowledge Bank

Singlet-triplet absorption and phosphorescence have been observed for single crystals of halogenated benzenes of $D2h,D3h$ and $D6h$ symmetry. The $T1\leftarrow S0$ absorption of p-dichloro-benzene (1) and p-dibromobenzene (2) are of the allowed type exhibiting strong origins which are coincident with the origins in emission. Vibrational analyses have been carried out for the $S-T1$ absorption in (1) and (2) as well as the $T1-S$ emission in (2). The polarized absorption leads directly to a $B2u$ assignment for the symmetry of the lowest triplet states of (1) and (2), which correlates with $B2u$ in $D6h$, in contradiction to theory. Preliminary results on the $T1\rightleftarrows S0$ spectra of 1, 3, 5-trichlorobenzene, 1, 3, 5-tribromobenzene, hexachlorobenzene and hexabromobenzene will also be presented.