The photoexcited triplet state as a probe of dynamics and phase memory in a multiphase liquid crystal: Time-resolved electron paramagnetic resonance spectroscopy

Abstract

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, Δχ<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 Δχ<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.

Triplet line shape simulation in continuous wave electron paramagnetic resonance experiments

“…5[a]). The emission/absorption pattern obtained in this case is typical to D Ͼ 0, with the population of the triplet X and Y levels considered here (14). Note also that the spectrum is not completely antisymmetric, due to the net emissive polarization of the triplet (as expected for this selective population scheme (24)).…”

“…[31]) is derived by first describing the distribution function of the molecules in the LCs frame of reference and then translating it to the distribution in the laboratory frame of reference. Figure 6 shows a line shape pattern typical for anisotropic distribution of planar triplet molecules in LC environment (14,33). As discussed above, molecular rotation can be accounted in the simulation by considering two or more exchanging species.…”

“…For example, in the case of isotropic solvents, P(,). In the case of anisotropic solvents (e.g., liquid crystals, LCs), the distribution function is usually more complicated (4,14).…”

“…Two main approaches are discussed: the "rotational diffusion" method (12,13) and the "multiple exchange/discrete jump" method (14,15). The first method is involved and thus is presented only in terms of its basic principles and primary results.…”

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

“…The orientation of the LC director L with respect to the magnetic field B is determined by the sign of the anisotropy of the diamagnetic susceptibility A Z (Az = ZH --2"1) [36]. Since for E7 A Z is positive, the director aligns parallel to the magnetic field, i.e., L li B.…”