LOCAL ELECTRIC FIELD EFFECT ON THE CHLOROPHYLL FLUORESCENCE

Abstract. Polarized absorption and fluorescence of chlorophyll a and bacteriochlorophyll solutions in nematic liquid crystal mixtures have been studied in the presence of an external electric field.
The electric field caused a reorientation of the pigment molecules as concluded from changes in the polarized absorption. However, no correlation was found between the change in the polarized absorption and the change in the polarized fluorescence as a function of the field strength. The field influence was much stronger than expected only from the molecular orientation. The fluorescence polarized parallel to the direction of the liquid crystal was found to increase, whereas the similarly polarized absorption decreases. As a whole, the fluorescence yield significantly increased. It is suggested that the additional electric field is reinforced by a local field, most probably due to a protonation of the liquid crystal molecules.
Charged solvent molecules are reoriented in an external electric field which changes the local electric field acting on chlorophylls. Similar changes in CHI fluorescence yield due to local electric field can be created in vivo by the shift of charged molecules present in surrounding pigments. Such effects can be at least partially responsible for slow induction of fluorescence phenomena.     

Photodegradation of azobenzene-based self-assembled monolayers characterized by in-plane birefringence

Azobenzene-based self-assembled monolayers (azo-SAMs) are photoactive and become orientationally ordered when illuminated with linearly polarized light (LPL), making them attractive as dynamic alignment layers in liquid crystal cells. Azo-SAMs, however, are chemically unstable when exposed to both air and light. We have characterized the photodegradation of a methyl red-based SAM by measuring with a high-sensitivity polarimeter the optical anisotropy induced by illumination with linearly polarized actinic light after the sample is irradiated with circularly polarized light (CPL) in air. The number of unbleached, photoactive molecules in the SAM decays exponentially with CPL exposure time, lowering the reorientation rate during photowriting with LPL. Azo-SAMs in an argon atmosphere, in contrast, are chemically stable and remain photoactive even after exposure to CPL.     

Molecular alignment and field-induced reorientation in a twisted nematic liquid crystal pi-cell

A twisted nematic pi-cell has been studied by optical transmission measurement, polarized Fourier-transform infrared (pFTIR) absorption spectroscopy, and Raman spectroscopy. Our pFTIR results suggest that the LC molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the optical response were found to be 6 ms and 1.6 ms, respectively. The switching dynamics of the twisted pi-cell was also studied using time-resolved Raman spectroscopy. A normal mode associated with the C-H out-of-plane wag on the LC core was found to be enhanced after the electric field was switched off. Our data show that LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field-induced reorientation process. The methods employed in this study have yielded valuable information about LC alignment and field-induced reorientation with respect to functional group specificity.     

Molecular alignment and field-induced reorientation in a twisted nematic liquid crystal pi-cell

A twisted nematic pi-cell has been studied by optical transmission measurement, polarized Fourier-transform infrared (pFTIR) absorption spectroscopy, and Raman spectroscopy. Our pFTIR results suggest that the LC molecules undergo a restricted rotation about the molecular long axis. The rise and decay times of the optical response were found to be 6 ms and 1.6 ms, respectively. The switching dynamics of the twisted pi-cell was also studied using time-resolved Raman spectroscopy. A normal mode associated with the C-H out-of-plane wag on the LC core was found to be enhanced after the electric field was switched off. Our data show that LC molecules in the twisted pi-cell do not rotate like a rigid molecule during the field-induced reorientation process. The methods employed in this study have yielded valuable information about LC alignment and field-induced reorientation with respect to functional group specificity.     

Dynamics of liquid crystal azimuthal anchoring at a poly(vinyl cinnamate) interface measured in situ during polarized UV light irradiation

An automated in situ technique for the measurement of the director reorientation at a nematic-aligning photosensitive polymer interface during polarized UV light irradiation has been developed. Using this technique, the photoinduced azimuthal anchoring energy (~20 merg cm -2 ) and the adsorption part of the latter (~7 merg cm -2 ) have been evaluated for the E7-poly(vinyl cinnamate) system. The kinetics of the director reorientation have been shown to be a very slow process (~1 h) and probably controlled by adsorption-desorption of liquid crystal molecules at the interface.     

Dynamics of liquid crystal azimuthal anchoring at a poly(vinyl cinnamate) interface measured in situ during polarized UV light irradiation

An automated in situ technique for the measurement of the director reorientation at a nematic-aligning photosensitive polymer interface during polarized UV light irradiation has been developed. Using this technique, the photoinduced azimuthal anchoring energy (≈20 merg cm^-2) and the adsorption part of the latter (≈7 merg cm^-2) have been evaluated for the E7-poly(vinyl cinnamate) system. The kinetics of the director reorientation have been shown to be a very slow process (≈1 h) and probably controlled by adsorption-desorption of liquid crystal molecules at the interface.     

Synthesis, spectroscopic characterization and alignment of novel azobenzene-containing monomers

A series of novel bifunctionalized photochromic monomers were synthesized, focusing on those with polymerizable acrylic/methacrylic groups attached to both ends of an azobenzene core via flexible spacers. The phase behaviour of the monomers was investigated using DSC, polarizing optical microscopy and X-ray diffraction. The change in UV-vis absorbance of the monomers under illumination with non-polarized/polarized UV light was studied for both solutions and thin films; also studied was its relaxation in the dark. On illumination with LPUV light, in-plane reorientation of the molecules normal to the polarization of the exciting UV light, and aggregation of the molecules in the films, were found.     

Orientational optical nonlinearity of nematic liquid crystals induced by high-molecular-mass azo-containing compounds

Processes of light-induced reorientation of nematic liquid-crystalline molecules induced by the addition of low concentrations (0.1–2.0 wt %) of comb-shaped polymers and carbosilane dendrimers containing azobenzene fragments are studied. When the molecular structure of the above compounds becomes more complicated, the induced orientational nonlinearity increases. The introduction of 2G and 3G dendrimers into a nematic has for the first time made it possible to visualize and study a purely optical first-order Freedericksz transition in the field of a linearly polarized wave.     

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.     

Photoaddressable polymers for liquid crystal alignment

We demonstrate reversible photoinduced in situ reorientation of low molecular mass liquid crystals (LCs) by means of photoaddressable polymers (PAPs). These polymers contain mesogenic azobenzene side chains optimized to reorient cooperatively and effectively upon illumination with polarized light. Various low molecular mass LCs were introduced between two PAP layers and these sandwich devices were tested with respect to stability and reversibility of photoinduced orientation. Dissolution of the PAP layer by the low molecular mass LC was observed for several material combinations and systematically investigated. Different anisotropic dyes were added as fluorescence markers in order to monitor the photoinduced LC orientation. With an optimized material combination, more than 10 reversible reorientation processes could be realized with polarized light of either 514 or 405 nm wavelength, without any reduction in alignment quality. Further, microscopic polarized fluorescence patterns could be produced and erased within short exposure times.     

Photoinduced reorientation processes in thin films of photochromic LC polymers on substrates with a photocontrollable command surface

Orientation and reorientation processes that occur in nematic and cholesteric LC polymer systems under irradiation with plane-polarized light are studied. A copolyacrylate containing phenyl benzoate and azobenzene side groups is synthesized as a nematic polymer; the cholesteric mixture is prepared via doping of the nematic copolymer with the chiral dopant, the derivative of D-isosorbide. Thin layers of the azobenzene-containing photoorientant SD-1 are first used as orienting substrates for polymer liquid crystals. Thin layers of the copolymer and of the mixture are spin-coated on the substrate after irradiation of the photoorientant layer with polarized light. It is shown that after annealing phenyl benzoate and azobenzene side groups of the nematic copolymer orient strictly along the direction of orientation of surface molecules, whereas in the case of the cholesteric mixture, a partial formation of the helical structure is observed. It is demonstrated that all the systems under examination can experience the repeated cyclic reorientation of the cooperative type under irradiation and subsequent annealing of the films.     

Polarized luminescence in a cubic liquid crystal

In a liquid-crystalline cubic phase the kinetics of the luminescence induced by pulsed excitation of a luminescing dopant has been studied. The influence of the molecular reorientation on the decay kinetics of the polarized components of the light has been investigated. The correlation times of the molecular reorientation and the magnitude of the local order parameter have been determined.     

Comparative Analysis of Hydration Layer Reorientation Dynamics of Antifreeze Protein and Protein Cytochrome P450

Antifreeze proteins (AFPs) inhibit ice recrystallization by a mechanism remaining largely elusive. Dynamics of AFPs' hydration water and its involvement in the antifreeze activity have not been identified conclusively. We herein, by simulation and theory, examined the water reorientation dynamics in the first hydration layer of an AFP from the spruce budworm, Choristoneura fumiferana, compared with a protein cytochrome P450 (CYP). The increase of potential acceptor water molecules around donor water molecules leads to the acceleration of hydrogen bond exchange between water molecules. Therefore, the jump reorientation of water molecules around the AFP active region is accelerated. Due to the mutual coupling and excitation of hydrogen bond exchange, with the acceleration of hydrogen bond exchange, the rearrangement of the hydrogen bond network and the frame reorientation of water are accelerated. Therefore, the water reorientation dynamics of AFP is faster than that of CYP. The results of this study provide a new physical image of antifreeze protein and a new understanding of the antifreeze mechanism of antifreeze proteins.     

Correlation between Ferromagnetic Layer Easy Axis and the Tilt Angle of Self Assembled Chiral Molecules

The spin–spin interactions between chiral molecules and ferromagnetic metals were found to be strongly affected by the chiral induced spin selectivity effect. Previous works unraveled two complementary phenomena: magnetization reorientation of ferromagnetic thin film upon adsorption of chiral molecules and different interaction rate of opposite enantiomers with a magnetic substrate. These phenomena were all observed when the easy axis of the ferromagnet was out of plane. In this work, the effects of the ferromagnetic easy axis direction, on both the chiral molecular monolayer tilt angle and the magnetization reorientation of the magnetic substrate, are studied using magnetic force microscopy. We have also studied the effect of an applied external magnetic field during the adsorption process. Our results show a clear correlation between the ferromagnetic layer easy axis direction and the tilt angle of the bonded molecules. This tilt angle was found to be larger for an in plane easy axis as compared to an out of plane easy axis. Adsorption under external magnetic field shows that magnetization reorientation occurs also after the adsorption event. These findings show that the interaction between chiral molecules and ferromagnetic layers stabilizes the magnetic reorientation, even after the adsorption, and strongly depends on the anisotropy of the magnetic substrate. This unique behavior is important for developing enantiomer separation techniques using magnetic substrates.     

Polarized luminescence in a cubic liquid crystal

Abstract

In a liquid-crystalline cubic phase the kinetics of the luminescence induced by pulsed excitation of a luminescing dopant has been studied. The influence of the molecular reorientation on the decay kinetics of the polarized components of the light has been investigated. The correlation times of the molecular reorientation and the magnitude of the local order parameter have been determined.     

Brute Force Orientation of Matrix‐Isolated Molecules: Reversible Reorientation of Formaldehyde in an Argon Matrix toward Perfect Alignment

Brute force orientation by an electric field is a promising way of controlling the orientation of polar molecules in the gas phase, but its application to condensed‐phase molecules has been very limited. We studied the reorientation of formaldehyde molecules in a solid Ar matrix under the influence of a strong electric field using reflection absorption infrared spectroscopy. Asymptotically perfect alignment of the formaldehyde molecules along the field was achieved at field strengths exceeding 1×10⁸ V m⁻¹. The vibrational bands of the aligned molecules exhibited a unidirectional Stark shift proportional to the field strength. The reorientation of the molecules was reversible despite the cryogenic solid environment of the system.     

Dynamic light scattering in liquid and supercooled diphenylmethane

We use a dynamic light scattering technique to measure both polarized (VV) and depolarized (VH) spectra of liquid diphenylmethane (DPM) between 288 and 362 K, covering both normal and supercooled liquid ranges. Our results allow extracting information on structural relaxation processes, rotational motions, rotation-translation couplings, and molecular reorientation phenomena in liquid DPM. The VV spectra are modeled according to the microscopic theory of Wang, which assumes that a structural relaxation process dominates the spectrum. We find that the relaxation time of the structural relaxation in DPM follows an Arrhenius behavior. The Rayleigh dip was observed in the VH spectra, which are described using the Andersen-Pecora theory. Our results are discussed in terms of the rotation-translation coupling parameter, which we find independent of temperature over the experimental range. The collective reorientation time also follows an Arrhenius behavior with temperature. Finally, we calculate the hydrodynamic volumes for the reorientation process from geometric molecular models in two hydrodynamic limits: slip and stick boundary conditions. Our results suggest that the DMP molecule reorientates in quasi-slipping conditions in the bulk liquid.     

Field dependence of the optical activity of a chiral liquid crystal near the isotropic–smectic A transition

Abstract

Chiral liquid crystals exhibit molecular optical activity in the isotropic phase. We have studied the evolution of the optical activity as a function of an applied electric field on a 76·2 μm film of the chiral liquid crystal W7, which exhibits an isotropic–smectic A transition at approximately 40°C. We measured the optical activity by recording the rotation of the plane of polarization of an incident linearly polarized ray of light, provided by a He–Ne laser. The applied biasing electric field is parallel to the direction of the incident beam. We find that at 41·0°C, the plane of polarization shifts from ?1·1° for an applied voltage of 30 V to a maximum of ?4·0° at 70 V. The absolute value of the signal decreases beyond this voltage. These shifts are in the direction of the smectic A phase and are in general larger than those observed as a function of temperature. Close to the isotropic–smectic A phase transition, molecules inside the liquid coalesce to form dynamic coherent groups, which have smectic nature. These groups are randomly oriented with respect to each other in the absence of an electric field. The application of an electric field causes the molecules within these groups to align along the direction of the field and to contribute coherently to the optical activity of the system. The way the molecules align with the field depends on the relative values of the polarizability α, which contributes to the alignment of the long axis of the molecule, and the dipole moment p, which contributes to the alignment of the short axis of the molecule. Our preliminary results and calculations suggest that for small fields, the electric field couples with the dipole moment p, whereas for fields in excess of 70 V, the field couples with the polarizability of the long axis of the molecule, causing a rotational reorientation of the molecules in the isotropic phase. The value of the field at which this reorientation occurs may be controlled by temperature.     

Photoinduced optical anisotropy in films of photochromic liquid crystalline polymers

The light-induced modification of the optical properties of photochromic liquid crystalline (side-group) polymers (LCPs) containing azobenzene moieties was studied. Films of such polymers were irradiated with unpolarized and linearly polarized light. Unpolarized irradiation results in a modification of the order parameter, whereas the director orientation remains constant. The light-induced disturbance of the supramolecular order is strongly dependent on the structure of the polymer. A correlation with the enthalpic stability of the liquid crystalline phases is given. Linearly polarized irradiation causes a modification of the order parameter and a reorientation of the side-groups towards a direction perpendicular to the electric vector of the actinic light. This reorientation process is caused by an angular-dependent photoselection within the steady state of the photoisomerization of the azobenzene units. The amount and kinetic of the reorientation differ considerably as a function of certain structural features of the polymers, such as the content of photochromic moieties and the lengths of the spacer chains between the polymeric backbone and the rod-like moieties. It is shown that under certain circumstances the angular-dependent photoselection process may cause a reorientation of the non-photochromic moieties by a cooperative process. The result is a light-induced rotation of the optical axis of the LCP and thus an efficient modification of the birefringent and dichroic properties. This effect can be used in optical data storage.