THE PHOTOEXCITED TRIPLET-STATE AS A PROBE OF DYNAMICS AND PHASE MEMORY IN A MULTIPHASE LIQUID-CRYSTAL - TIME-RESOLVED ELECTRON-PARAMAGNETIC RESONANCE SPECTROSCOPY

Differences between mesophases and hysteresis effects can be examined by electron paramagnetic resonance (EPR) detection of light-induced transient triplets oriented in liquid crystals (LC). Time-resolved EPR spectroscopy (direct detection) has been applied to study the photoexcited triplet state dynamics of tetraphenylporphyrin (free-base) oriented in a multiphase LC, ZLI-1167, having a negative anisotropic susceptibility, DELTA-chi < 0. The feasibility of triplet detection over a temperature range of 100-370 K, enables one to monitor conspicuous changes of the triplet line shape associated with the different mesophases, i.e., cryst. <--> (287 K) smec. <--> (305 K) nem. <--> (356 K) isot. Line shape analysis shows that all phases consist of domains having their directors spanned in a plane normal to the external magnetic field, as expected for DELTA-chi < 0. The triplet spectra in the fluid smectic phase depend on sample rotation, indicating the known rigidity of this phase. On the other hand, the spectra in the nematic phase are independent of the sample rotation but strongly depend on the temperature and sample alignment in the magnetic field. Two mechanisms of triplet dynamics are proposed; namely an inter- and intradomain triplet energy transfer between the porphyrin chromophores. In addition, the results suggest that, in a specific alignment procedure, the smectic character is maintained deep in the nematic and, to some extent, in the isotropic phases. Thus the EPR line shapes and temporal behavior of the nematic spectra, are associated with a phase memory effect (hysteresis), and the smectic phase persistence may indicate the existence of cybotactic groups within the nematic and isotropic phases.