Nonmesomorphic Solute−Mesomorphic Solvent Interactions Studies. 2. ESR Studies on the Different Phases of 5CB, 6CB, 7CB, and 8CB Liquid Crystals Using PD−Tempone and Tempo−Palmitate as Probes. Structural and Conformational Effects

An ESR study of interactions between the nonmesomorphic nitroxide spin probe solutes PD−tempone (PDT) and tempo−palmitate (TP) and homologues of the p-n-alkyl-p‘-cyanobiphenyl series of liquid-crystals has been carried out. The characteristics of the ESR spectra obtained show that the constraint for rotation about the x axis in normal liquids is reinforced for TP and relaxed for PDT in liquid crystals to the extent of making PDT switch to y axis rotation in liquid crystals. Evidence for such reinforcement/relaxation is provided by large and small hyperfine splitting, a, values for TP and PDT, respectively, in liquid-crystal solvents. These a values were rationalized on the basis of the nature of solute and solvent structures and interactions. This study also demonstrates that the g factor of the probe is sensitive to the structural and conformational characteristics of the liquid-crystal solvent molecules with g increasing as the degree of out of planarity of the benzene rings in their core increases and exhibiting an odd−even effect that is attributable to the polarizabilities of their alkyl tails. Finally, our results cast doubt on the generally held view that the greater the structural similarity between the probe molecule and the liquid-crystal solvent molecules the better is the probe at reflecting the liquid-crystalline properties of the solvent.