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 serie s of fulleropyrrolidine bisadducts has been examined. Photoexcited triplet states, populated by spin-orbit promoted intersystem crossing, have been us ed as spin probes to determine the preferential orientations of the guest m olecules in the anisotropic medium. Time-resolved electron paramagnetic res onance (TREPR) spectroscopy has been applied and spectra with the external magnetic field set parallel or perpendicular to the director n of the mesop hase 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 orie ntational distribution function obtained by spectral simulation. It has bee n found that the macroscopic order degree induced by the mesophase on the g uest molecules varies from one bisadduct to another. Bisadducts with D-2h s ymmetry have exhibited tripler spectra due to dimers besides those of monom ers, The measured zero field splitting parameters and magnetic level popula tions of the monomer and dimer triplet states have been analyzed, and a str ucture of the dimer has been postulated.