Knowledge Bank

PMDR studies of the triplet state properties of a series of halobenzenes in non-heavy-atom hosts will be reported. The results include measurements of the total and relative radiative phosphorescence rates, the zero field splittings, and the halogen quadrupole and hyperfine interactions for para substituted chlorobenzenes $(-NH2,-OCH3,-CH3$, $-CI$ and $-Br)$, p-dibromobenzene, and m-dichlorobenzene. The fine structure parameter D remains constant to within 15% at ca. 4000 MHz for a variety of chlorobenzenes, whereas the magnitude of E varies considerably. D is significantly greater than 4000 MHz in the bromobenzenes, strongly suggesting an increased spin-orbit contribution to the zero field splittings. A simple method used for determining the absolute sign of D or the sign of the halogen quadrupole coupling constant in the triplet state from the zero field electron spin resonance spectrum will be outlined.