Pressure dependence of zero-field splittings in organic triplets. II. Carbonyls

Abstract

We have conducted optically detected magnetic resonance (ODMR) experiments at pressure up to 40 kbar for neat biactyl (BA), neat benzil (BZ), and acetophenone (AP) doped in dibromobenzene (DBB). The pressure dependences of their zero-field splitting (ZFS) parameters D and E are reported. For BA and BZ systems, the ‖D‖ value decreases greatly with increasing pressure. This behavior is in contrast with that of benzophenone (BP), whose ‖D‖ value increases sigmoidally 13% over the same pressure range. These results may be rationalized in a qualitative theory based on pressure modulation of the spin-orbit coupling (SOC) contribution to the ZFS. ln aromatic ketones, lattice compression modifies the twist angle of the phenyl ring(s) relative to the carbonyl frame, thus changing the energy of the 3ππ* state relative to that of the 3nπ* state. This variation of the energy denominator in a second order perturbation enhances the SOC contribution to the ZFS. In comparison, the increase of spin–spin (SS) dipolar interaction by isotropic compression is relatively unimportant. Consistent with this picture, the very small 3ππ*–3nπ* energy gap produces an enormous pressure sensitivity of D and E in AP/DBB. The behavior of the ZFS in this case may be interpreted as a consequence of pressure tuning of the 3ππ* state through an anticrossing region. In addition, a new set of high frequency ODMR signals appears under pressure. This is attributed to a new site of AP having the 3nπ* as the phosphorescent triplet state. The pressure dependence of ZFS for benzil shows complicated fine structure. This is a testimony to the flexible nature of benzil in both the dihedral angle of the dicarbonyl fragment and the phenyl twist angle.