Preferential orientation of fulleropyrrolidine bisadducts in E7 liquid crystal: A time-resolved electron paramagnetic resonance study

The effect of the orientating potential in the E7 liquid crystal on a series of fulleropyrrolidine bisadducts has been examined. Photoexcited triplet states, populated by spin-orbit promoted intersystem crossing, have been used as spin probes to determine the preferential orientations of the guest molecules in the anisotropic medium. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been applied and spectra with the external magnetic field set parallel or perpendicular to the director n of the mesophase have been measured. Spectrum analysis has enabled us to determine the preferred orientation of n with respect to the principal axis frame of the dipolar interaction and to calculate the order parameters by using the orientational distribution function obtained by spectral simulation. It has been found that the macroscopic order degree induced by the mesophase on the guest molecules varies from one bisadduct to another. Bisadducts with D-2h symmetry have exhibited triplet spectra due to dimers besides those of monomers, The measured zero field splitting parameters and magnetic level populations of the monomer and dimer triplet states have been analyzed, and a structure of the dimer has been postulated.