An electron spin resonance study of spiropyran-merocyanine Quasi-Liquid Crystal

We report on an ESR study of the spin-probe-doped "quasi-liquid crystal" (QLC) films made of spiropyran-merocyanine molecular complexes, susceptible to macroscopic alignment by an external electrostatic field. The orientation-dependent ESR spectra suggest that the free radicals partition between a macroscopically isotropic solvation site (site I) and a locally ordered environment (site II), the latter associated with an order parameter of approximately one-third. The principal values of the magnetic g and hyperfine tensors are, by virtue of their uncommon magnitude, suggestive of rather strong solute-solvent interactions. The ESR results are interpreted by assuming that the spiropyran molecules assemble into asymmetric micellar aggregates that experience long-range orientational order. This liquid-crystal-like mesophase offers two solvation sites: an intermicellar site, associated with an anisotropic spectral response, and an intramicellar site, associated with an isotropic spectral component. Previously examined dopants such as fluorescent dyes and merocyanine molecules reside, presumably, at the former site only whereas the nitroxide free radicals exhibit comparable solubilities within and in between the micelles.