Organic reactions in liquid crystalline solvents. 5. : Intramolecular phenolic quenching of aromatic ketone triplets as a probe of solute conformational mobility in liquid crystals.

Abstract

The effects of liquid crystalline order on the confomational motions involved in end-to-end, intramolecular triplet quenching of aromatic ketones by a suitably situated phenolic moiety have been investigated. Triplet state quenching has been monitored using two independent probes: Norrish II fragmentation quantum yields of valeryl-substituted derivatives and direct lifetime measurements in methyl-substituted ketones, using the corresponding anisyl-substituted ketones as models for triplet state behaviour In the absence of phenolic quenching.Quantum yields for Norrish II fragmentation in the liquid crystalline solvents have been estimated using 4-methoxyvalerophenone (MVP) as the actinometer. The ability of liquid crystalline solvents to inhibit intramolecular phenolic quenching is dependent on both phase type and solute length. In one case, Intramolecular quenching Is completely suppressed In smectic phases, indicating that the conformational motions involved in achieving the quenching geometry are slowed by a factor of at least 10 3 relative to their rates in non-viscous, isotropic solvents. The Norrish II product ratios from photolysis of MVP in the liquid crystalline solvents are also affected dramatically by liquid crystalline order.