Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.     

Photoinduced phase transitions

Employing actinic light to alter/stabilise a particular thermodynamic phase via the photo-isomerisation of the constituent molecules is an interesting tool to investigate soft matter from a new dimension. This article focuses on our recent results on several aspects of these non-equilibrium phase transitions, which are isothermal in nature. We specifically discuss (i) the influence of different parameters, such as confinement, applied electric field, pressure etc., on the dynamics associated with both the photochemical transition driving the equilibrium nematic to the non-equilibrium isotropic phase and the thermal back relaxation recovering the nematic phase, (ii) unique light-driven disorder–order transition in a reentrant system, (iii) dynamic self-assembly of the smectic A phase, which is stabilised only in the presence of actinic light, (iv) novel temperature-intensity phase diagrams and an example of primary and secondary photo-ferroelectric effects in an antiferroelectric smectic C system. These results highlight the fact that the actinic light can be used as a new tool to study phase transitions and the associated critical phenomena that could also bring about effects that are not seen in equilibrium situations.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Density studies in terephthalylidene-bis-p-n-alkylanilines

The temperature dependence of density in terephthalylidene-bis-p-n-alkylanilines (TBAA5 and 6) is studied to investigate the phase transitions, associated volume jumps, order of the transitions, estimated pressure dependence of transition temperatures, and pretransitional effects. The compounds exhibit nematic, smectic A, smectic C, smectic F, smectic G and smectic H phases with higher clearing temperatures. The smectic A to smectic C transition, which is a fluctuation induced first order transition in TBAA5, is found to be a second order transition in TBAA5 and 6. The results are discussed in the light of other experimental reports. The estimated pressure dependence of transition temperatures along with the reported experimental P-T data are discussed. The N-S_A transition is first order in TBAA5 and 6. The studies across other transitions are also discussed.     

Density studies in terephthalylidene-bis-p-n-alkylanilines

Abstract

The temperature dependence of density in terephthalylidene-bis-p-n-alkylanilines (TBAA5 and 6) is studied to investigate the phase transitions, associated volume jumps, order of the transitions, estimated pressure dependence of transition temperatures, and pretransitional effects. The compounds exhibit nematic, smectic A, smectic C, smectic F, smectic G and smectic H phases with higher clearing temperatures. The smectic A to smectic C transition, which is a fluctuation induced first order transition in TBAA5, is found to be a second order transition in TBAA5 and 6. The results are discussed in the light of other experimental reports. The estimated pressure dependence of transition temperatures along with the reported experimental P[sbnd]T data are discussed. The N[sbnd]S_A transition is first order in TBAA5 and 6. The studies across other transitions are also discussed.     

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase II_UV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase III_temp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase II_UV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase III_temp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase II_UV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light.     

Polar Order,Mirror Symmetry Breaking,and Photoswitching of Chirality and Polarity in Functional Bent-Core Mesogens

In recent years, liquid crystals (LCs) responding to light or electrical fields have gained significant importance as multifunctional materials. Herein, two new series of photoswitchable bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected to an azobenzene based wing and a phenyl benzoate wing are reported. The self-assembly of these molecules was characterized by differential scanning calorimetry (DSC), polarizing light microscopy (POM), electro-optical, dielectric, second harmonic generation (SHG) studies, and XRD. Depending on the direction of the COO group in the phenyl benzoate wing, core-fluorination, temperature, and the terminal alkyl chain length, cybotactic nematic and lamellar (smectic) LC phases were observed. The coherence length of the ferroelectric fluctuations increases continuously with decreasing temperature and adopts antipolar correlation upon the condensation into superparaelectric states of the paraelectric smectic phases. Finally, long-range polar order develops at distinct phase transitions; first leading to polarization modulated and then to nonmodulated antiferroelectric smectic phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror symmetry breaking. Fine-tuning of the molecular structure leads to photoresponsive bent-core (BC)LCs exhibiting a fast and reversible photoinduced change of the mode of the switching between ferroelectric- and antiferroelectric-like as well as a light-induced switching between an achiral and a spontaneous mirror-symmetry-broken LC phase.     

Polymer nanostructure development of fluorinated and aliphatic monoacrylates in smectic liquid crystals via photopolymerization

To develop viable polymer stabilized liquid crystal systems, it is crucial to understand the factors that affect polymer nanostructure evolution. This work examines the influence of the photopolymerization of aliphatic and fluorinated monoacrylate monomer within a room temperature smectic liquid crystal (LC). Additionally, the effect of LC order on polymerization kinetics, monomer and polymer organization, and the effect of the polymer on LC properties have been examined. Through this work, insight has been gained regarding the impact that the introduction of a fluorinated monoacrylate monomer has on polymerization kinetics, LC organization, and monomer/polymer segregation and organization within a polymer/LC system. Fluorinated moieties lower the surface energy of the monomer to enhance segregation between the smectic layers of the LC as compared with an analogous aliphatic monomer. Additionally, the enhanced segregation significantly increases the polymerization rate in the smectic phase and drives the continued segregation of the fluorinated polymer during and after polymerization. Fluorination also leads to the formation of an ordered polymer nanostructure if polymerized in ordered LC phases. This ordering is particularly evident when the fluorinated monomer is polymerized in the smectic phase in which the monomer is organized between the smectic layers of the LC. In addition, the ordered polymer structure found with the fluorinated monomer in the smectic phase leads to continued birefringence above the clearing point of the LC due to surface interactions between the LC and the ordered fluorinated polymer. The continued birefringence offers an exceptional opportunity to examine how factors such as polymer molecular mass and UV light intensity affect the overall polymer morphology of these polymer/LC systems. As the initiator concentration and UV light intensity are decreased, longer polymer chains form lattice-type morphologies; whereas, shorter polymer chains form smoother morphologies that more closely mirror the texture of the LC smectic phase.     

Polymer nanostructure development of fluorinated and aliphatic monoacrylates in smectic liquid crystals via photopolymerization

To develop viable polymer stabilized liquid crystal systems, it is crucial to understand the factors that affect polymer nanostructure evolution. This work examines the influence of the photopolymerization of aliphatic and fluorinated monoacrylate monomer within a room temperature smectic liquid crystal (LC). Additionally, the effect of LC order on polymerization kinetics, monomer and polymer organization, and the effect of the polymer on LC properties have been examined. Through this work, insight has been gained regarding the impact that the introduction of a fluorinated monoacrylate monomer has on polymerization kinetics, LC organization, and monomer/polymer segregation and organization within a polymer/LC system. Fluorinated moieties lower the surface energy of the monomer to enhance segregation between the smectic layers of the LC as compared with an analogous aliphatic monomer. Additionally, the enhanced segregation significantly increases the polymerization rate in the smectic phase and drives the continued segregation of the fluorinated polymer during and after polymerization. Fluorination also leads to the formation of an ordered polymer nanostructure if polymerized in ordered LC phases. This ordering is particularly evident when the fluorinated monomer is polymerized in the smectic phase in which the monomer is organized between the smectic layers of the LC. In addition, the ordered polymer structure found with the fluorinated monomer in the smectic phase leads to continued birefringence above the clearing point of the LC due to surface interactions between the LC and the ordered fluorinated polymer. The continued birefringence offers an exceptional opportunity to examine how factors such as polymer molecular mass and UV light intensity affect the overall polymer morphology of these polymer/LC systems. As the initiator concentration and UV light intensity are decreased, longer polymer chains form lattice-type morphologies; whereas, shorter polymer chains form smoother morphologies that more closely mirror the texture of the LC smectic phase.     

Photocontrolled Phase Transitions and Reflection Behaviors of Smectic Liquid Crystals by a Chiral Azobenzene

The photocontrolled phase transitions and reflection behaviors of a smectic liquid crystal, 4‐octyl‐4′‐cyanobiphenyl (8CB), tuned by a chiral azobenzene, are systematically investigated. For the smectic 8CB doped with the chiral azobenzene (1R)‐(?)‐4‐n‐hexyl‐4′‐menthylazobenzene (ABE), the initial smectic phase can be switched to cholesteric and then to isotropic upon UV irradiation due to the trans‐to‐cis photoisomerization of ABE; however, no reflection band is observed. For the smectic 8CB doped with ABE and the chiral agent (S)‐(?)‐1,1′‐binaphthyl‐2,2′‐diol (BN), a reflection band located in the short‐wavelength infrared region is observed, which disappears after further UV irradiation. For the smectic 8CB doped with ABE and a chiral agent with higher helical twisting power, (S)‐2,2′‐methylendioxy‐1,1′‐binaphthalene (DBN), a phototunable system with cholesteric pitch short enough to reflect visible light is demonstrated. With a given concentration of the chiral dopant DBN, a reversible reflection color transition is realized tuned by the isomerization of azobenzene. The reverse phase transition from isotropic to cholesteric and then to smectic can be recovered upon visible irradiation. The photocontrolled phase transitions in smectic liquid crystals and the corresponding changes in reflection band switched by photoisomerization of azobenzene may provide impetus for their practical application in optical memories, displays, and switches.     

Photosalient Effect of Diarylethene Crystals of Thiazoyl and Thienyl Derivatives

The photoresponse of diarylethene crystals is found to depend on the intensity of UV light, that is, photoinduced bending is switched to photosalient phenomena by increasing the light intensity. The change in the size of the crystal unit cell upon UV irradiation is larger for asymmetric diarylethenes with thiazole and thiophene rings than that for the corresponding symmetric diarylethenes. As a result, the crystals of an asymmetric diarylethene show much more drastic photosalient effects than those of the corresponding symmetric diarylethene crystals upon UV irradiation. It is also found that the crystals of diarylethene, which have not previously been reported to exhibit a photosalient effect, show photosalient phenomena upon irradiation with strong UV light. Furthermore, the dependence of photosalient phenomena on the size and shape of the crystals is reported.     

A new image processing method for enhancing the detection sensitivity of smooth transitions in liquid crystals

A study of textural changes is presented concerning the nematic phase of 4- n-alkyloxybenzoic acids (in particular, 4-n-heptyl- and 4-n-octyl-oxybenzoic acids), by means of a statistical approach to the image data observed by polarized light microscopy (orthoscopic mode). A new image processing method is developed in order to detect with high sensitivity any structural change in the image frame. To do this, a set of parameters is introduced, charaterizing the observed textures. Such a set is a vector, working like a pathfinder strongly increasing the human eye or in general the sensor skilfulness to appreciate any change of the optical texture, both in space and in time. This is suitable for revealing smooth transitions, such as phase transitions between smectic and nematic phases (or between different smectic phases), or order transitions, like alignment transitions in poorly oriented nematic layers. In fact, by using this method for detecting the order transition between two nematic 'subphases' of 4-n-alkyloxybenzoic acids, the sensitivity turns out to be enhanced by a factor higher than 10 with respect to that for standard techniques. The new method allows us to define a characteristic size of the image texture: this concept is applied to analyse several image data for estimating the mean size of the domains appearing in the smectic and in the nematic phase of the compounds under study.     

Azobenzene‐containing LC polymethacrylates highly photosensitive in broad spectral range

Two photosensitive chiral liquid crystalline azobenzene‐containing polymethacrylates having different length of flexible spacer connecting chromophores with backbone were synthesized and their phase behavior and photo‐optical properties were studied. Both polymers consist of lateral methyl substituents in ortho‐position of azobenzene chromophores providing high photosensitivity even in red spectral region as well as high thermal stability of photoinduced Z‐form of azobenzene chromophores. It is shown, that smectic phase (SmA*) formation in films of polymer with longer spacer predetermines its quite unusual spectral response to UV and subsequent visible light actions. The SmA* phase promotes spontaneous homeotropic alignment of azobenzene chromophores in polymer films. UV‐irradiation induces not only E‐Z isomerization but also results in disruption of homeotropic alignment, whereas subsequent visible light action enables to obtain films with the low degree of chromophores orientation. The photo‐orientation phenomena under the action of polarized light of different wavelength on polymer films were studied. The possibility of using red polarized light of moderate intensity for optical photorecording on polymer films is demonstrated. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2962–2970     

A high resolution temperature scanning technique for optical studies of liquid crystal phase transitions

A temperature scanning attachment for the polarizing microscope is described that offers a simple but powerful technique with which to detect even subtle paramorphotic phase transitions in liquid crystals with high temperature resolution. This technique was applied to CE8, a well known reference compound exhibiting rich mesomorphism. All smectic transitions in CE8 were clearly detected by distinct singularities in the temperature dependence of the transmitted light intensity. Repeated temperature scans across the second order ferroelectric smectic A*-C* transition with various orientations of polarizer and analyser provide high resolution data of optical birefringence and optical tilt angle. These data confirm a mean-field to tricritical crossover in the power law exponent of the optical tilt.     

Anticlinic smectic C phase in new and novel five-ring hockey stick-shaped compounds

The synthesis of a new homologous series of novel five-ring compounds composed of hockey stick-shaped molecules derived from 4-hydroxybiphenyl-3-carboxylic acid which exhibit an interesting sequence of phase transitions is presented. Uniaxial nematic and smectic A phases, anticlinic smectic C phase and an unidentified smectic phase at a lower temperature than the latter have been observed. This is perhaps the first example of hockey stick-shaped compounds exhibiting anticlinic smectic C phase directly from the isotropic phase. Polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction measurements and electro-optical switching studies were carried out to identify the mesophases.     

Photomechanically induced phase transitions in ferroelectric liquid crystals

We demonstrate that the addition of small amounts of a novel azo-dye to a ferroelectric liquid crystal and illumination with low intensity (∼ 0.8 mW cm^-2) UV light can result in reversible, isothermal phase transitions and dramatic changes in the properties of the system. In particular we examine light induced transitions between the S*₁, and S*_C phases and the consequent photomechanical regulation of spontaneous polarization.     

Effect of terminal polar substituents on the nature of smectic A liquid crystals of a series of 4-(4'-octyloxybenzoyloxy)benzylidene-4'-substituted anilines

The nature of smectic A liquid crystals and the phase transitions from smectic A to nematic phases are studied in the homologous series of 4-(4'-octyloxy-benzoyloxy)benzylidene-4'-substituted anilines, in which the substituents are H, F, Cl, Br, I, NO₂ and CN. Measurements have been performed for the dipole moment of the molecule, the smectic A lattice period, the temperatures and the entropies of phase transitions, and the temperature dependence of the orientational order parameter, all of which provide information on the intermolecular pair potentials. It is shown that the introduction of a terminal polar substituent brings about counteracting contributions to the stability of the smectic A state. It is suggested that the reentrant nematic phase transition can appear in the strongly polar mesogens if the extent of antiparallel association of the molecules increases on lowering the temperature.     

Direct smectic A-smectic C*A phase transition

The anticlinic smectic CA phase belongs to the class of tilted smectic phases with an azimuthal angle alternating from one direction (theta=0) to the other (theta=pi) in successive layers. It occurs in general at lower temperature than the uniformly tilted smectic C phase, but may be obtained directly from the untilted smectic A phase. We use the chiral nCTBB9* series synthesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperature behaviour of the birefringence and of the optical rotatory power across the transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C*A soft modes. The main conclusion is that the only order parameter governing the critica Al behaviour of the phase transition is the tilt angle theta, as we get common divergence of both soft modes at the same temperature. This confirms previous high resolution calorimetric studies by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructuring transitions between the tilted subphases.     

Direct smectic A-smectic C*A phase transition

The anticlinic smectic CA phase belongs to the class of tilted smectic phases with an azimuthal angle alternating from one direction (theta=0) to the other (theta=pi) in successive layers. It occurs in general at lower temperature than the uniformly tilted smectic C phase, but may be obtained directly from the untilted smectic A phase. We use the chiral nCTBB9* series synthesized in this laboratory, in order to obtain a phase transition as close as possible to second order, as revealed by DSC. We measure the temperature behaviour of the birefringence and of the optical rotatory power across the transition in order to characterize the tilt angle. We finally study the optical response to a periodic electric field which excites separately the smectic C* and C*A soft modes. The main conclusion is that the only order parameter governing the critica Al behaviour of the phase transition is the tilt angle theta, as we get common divergence of both soft modes at the same temperature. This confirms previous high resolution calorimetric studies by Ema et al. that saw in MHPOBC an initial mean-field second order phase transition when the tilt appears, followed by sharp first order restructuring transitions between the tilted subphases.     

Luminescence behavior as a probe for phase transitions and excimer formation in liquid crystals: dodecylcyanobiphenyl

The luminescence behavior of a liquid crystal, dodecylcyanobiphenyl, as a function of temperature, is reported. The fluorescence intensity exhibits discontinuities at temperatures which correspond to the phase transitions: crystalline — smectic — isotropic. The results are interpreted in terms of singlet monomer and excimer formation.