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.

High-pressure EPR studies of intermolecular interactions in solids

“…Both a decrease and increase in D-value (being of order of 0.1 cm -1) was found in different molecular crystals with a huge va¡ of D in acetophenone dibromobenzene [73]. It was discussed in terms of dipolar coupling, spin-orbit contributions and a vibronic coupling to higher energy triplet state [74].…”

Effect of Environment on Pressure-Induced Emission of Benzophenone, 4,4‘-Dichlorobenzophenone, and 4-(Dimethylamino)benzaldehyde in Solid Media

“…Since in aromatic carbonyls (specifically in ketones) the compression of medium may modify the twist of the phenyl rings relative to the carbonyl frame, 13 one would expect a substantial change in the character of the triplet state and so in phosphorescent properties. Indeed, our recent studies on a series of aromatic ketones and aldehydes as well as nitriles dissolved in rigid polymers show that these systems exhibit a strong dependence of their luminescence properties on external pressure.…”