Photochromic Dopant Research Articles

Predictions of molecular chirality and helical twisting powers: A theoretical study

A theoretical study of a number of chiral molecules has been undertaken using a molecular Monte Carlo simulation approach coupled with calculations of molecular chirality based on a chirality order parameter. Results for a variety of TADDOL (α,α,α′,α′-tetraaryl-1,3-dioxolan-4,5dimethanol) derivatives show good agreement with experimental findings for the sign, magnitude, and the temperature dependence of the helical twisting power (HTP). For a photochromic chiral dopant with variable HTP we are able to model the reduction in the HTP when photoisomerization occurs. Our studies on a liquid crystalline material with a single chiral center have reproduced a temperature dependent twist inversion in the material. We discuss the temperature and solvent dependence of the helical twisting power and argue that in all the systems studied here, preferential selection of certain molecular conformations at different temperatures and in different solvents are able to explain the observed experimental behavior of the HTP.

Light-responsive chiral photochromic liquid crystalline polymer systems

The authors reviewed their results obtained in recent years in the field of development of advanced optically controlled polymer systems based on photochromic cholesteric liquid crystalline (LC) polymers and their blends with each other and with low-molar-mass chiral and photochromic dopants. Recent advances in the research dealing with the design and study of photo-optical properties of photoresponsive LC polymers composed of nematogenic, chiral and one or two photochromic fragments incorporated in the same monomer units or entirely into the macromolecule as individual monomers are discussed. All polymer systems form cholesteric phase with a helical supramolecular structure. The light irradiation leads to the isomerization of one or two photochromic groups, which results in the variation in the helical twisting power of chiral fragments. This process is accompanied by dramatic changes in the supramolecular helical structure and optical properties of the LC polymer. This provides the possibility for controlling the pitch of helix, the rate of helix twisting and untwisting, spectral range, and the width of the selective light reflection. The photochromic LC systems offer new promising materials for reversible and irreversible colour data recording that can be used in optical memory systems (data storage), holography, colour projection techniques and give rise to a new generation of videodiscs, flat light guides and coatings with photocontrollable optical properties.

Cholesteric mixture containing chiral‐photochromic and diarylethene dopants as novel material with dual photochromism

AbstractNew cholesteric polymeric mixture containing nematic side‐chain homopolymer, 2 wt% of photochromic diarylethene dopant and 5 wt% of chiral‐photochromic dopant based on cinnamic acid and isosorbide was prepared. Upon UV irradiation of planarly‐oriented mixture films at room temperature, a transformation of open colorless form of diarylethene dopant into a closed colored form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region and decreasing selective light reflection intensity. This process is photo‐ and thermo‐reversible and many cycles ‘recording‐erasing’ can be realized. UV irradiation and subsequent annealing of the films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long‐wavelength region. This process is explained by the E‐Z isomerization of chiral photochromic groups of the dopant relative to a CC bond accompanied by a decrease in their twisting ability.It has been shown, that the use of the material prepared in this work provides an opportunity to record two images at the same time: one image is due to a change in the helical pitch and another (second) image is due to the photochromism of diarylethene dopant. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under study shows advantage as new photosensitive material for optics, optoelectronics and data recording. Copyright © 2002 John Wiley & Sons, Ltd.

Laser-induced concentric colour domains in a cholesteric liquid crystal mixture containing a nematic azobenzene dopant

The isothermal formation of concentric colour domains has been observed in a cholesteric mixture doped with 10 wt % of a photochromic nematic material. The host cholesteric mixture included the Merck materials BL131a and BL130, while the photochromic dopant was a mixture of 4-n-butyl-4′-n-alkoxyazobenzenes (BAAB). The helical pitch of the host cholesteric mixture was increased, as expected, on addition of the photochromic nematic material prior to irradiation with the molecules in the purely trans-configuration. On irradiation with low power (0.6 mW) argon ion laser light, cis-isomers formed within the interaction region and concentric colour domains appeared. Selective reflection from the colour domains occurred in the 400–560 nm spectral range. The coloured domains persisted in the time period following irradiation and extended beyond the interaction region because of diffusion of the cis-isomers. Using the diffusion equation, an expression has been obtained for the average concentration of cis-isomers in each of the coloured domains and the dependence of the reflection wavelength upon the concentration of cis-isomers has been determined.

Photoswitching of Helical Twisting Power by Chiral Diarylethene Dopants

Photochromic chiral compounds having two diarylethene units were synthesized in an attempt to use them as dopants for photoresponsive liquid crystals. Stable photoswitching of the photochromic dopants induced large pitch changes of chiral nematic liquid crystals composed of K-15 and a small amount of the chiral dopants.

Mixture of cholesteric copolymer with dithienylethene photochromic dopant: a new material combining optical properties of cholesterics with photochromism

The phase behaviour and optical properties of the mixture based on cholesteric copolymer with 6 wt% of a photochromic dithienylethene dopant was studied. It was shown that planarly-oriented films of the mixture possess a selective light reflection in the visible region of the spectrum (λmax ∼600 nm). UV irradiation of the mixture film leads to the appearance of an intense absorbance peak in the spectral range coinciding with the selective reflection band. This process is associated with the photocyclization of dopant and the “degeneration” of the selective light reflection. The photoinduced coloured form of dopant is thermally stable, but subsequent irradiation of the film by visible light (633 nm) induces a back decoloration process. For the first time the influence of temperature on the rates of forward and back photoprocesses in such polymeric liquid crystalline films was studied. The potential for the use of such a type of material for reversible data recording and storage was demonstrated.

Photoactive Cholesteric Polymeric Material With Dual Photochromism

ABSTRACTNew cholesteric polymeric material containing a liquid crystalline (LC) copolymer with side phenylbenzoate nematogenic and photochromic spiropyran groups and 5 mol % chiral photochromic dopant based on cinnamic acid and isosorbide was prepared. It was shown that UV irradiation and subsequent annealing of planarly-oriented mixture films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long-wavelength region. This process is explained by the E-Z isomerization of chiral photochromic groups of the dopant relative to a C=C bond accompanied by a decrease in their twisting ability. Upon UV irradiation of films at room temperature, a thermally reversible transformation of spiropyran groups into a merocyanine form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region. It was shown, that the use of the material prepared in this work provides an opportunity to record two images at a time: one image is due to a change in the helical pitch and another image (above) is due to the photochromism of spiropyran groups. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under examination shows promise as new photosensitive material for optics, optoelectronics, and data recording.

Effects of doped dialkylazobenzenes on helical pitch of cholesteric liquid crystal with medium molecular weight: utilisation for full-colour image recording

The effects of small amounts of achiral photochromic dopants, 4,4′-di-n-alkylazobenzenes, with various lengths of substituted alkyl chain (CnH2n + 1 ∶ n = 4, 7, 11, 12, 13, 14, 15, 16) on the helical pitch of a cholesteric liquid crystal (CLC) with ca. 1000 molecular weight were investigated in order to utilise the cholesteric reflected colour of these combinations for a rewritable colour image recording. The (E)-azobenzenes (~2 wt%) in the host CLC induced pitch changes dependent on temperature, the concentration of the azobenzenes and the length of substituted alkyl chain on the azobenzenes. In the low region of the cholesteric temperature (below 100 °C), some (E)-azobenzenes remarkably lengthened the pitch. Contrary to this, upon irradiation, E–Z photoisomerization of all the azobenzenes in the CLC phase induced consecutive pitch shortenings in response to the formation of the Z isomers. The changed pitch (reflected colour) could be fixed in cholesteric glassy solids. In the glassy solid state, the photo and thermal isomerization of the azobenzenes could not influence the cholesteric pitch any longer, therefore, the colour was stably stored. However, since the molecular arrangement of the CLC and the conformation of the doped azobenzenes completely returned to the initial state upon re-heating to isotropic temperatures, the stored colour could be easily erased. Based on these fundamental investigations, it is suggested that the mixed systems of the medium-molecular-weight CLCs and the 4,4′-dialkylazobenzenes will be effective materials for a rewritable full-colour image recording in a photon mode.

Phase Transition of a Liquid Crystal Induced by Chiral Photochromic Dopants

A chiral cyclohexane having two photochromic diarylethene unit was synthesized in an attempt to use it as a dopant for a photoresponsive liquid crystal. A stable photoswitching between nematic and cholesteric phases could be induced by the addition of a small amount of the chiral cyclohexane to a nematic liquid crystal 4-cyano-4′-penthylbiphenyl (K-15).

Design of photonic liquid crystal materials: synthesis and evaluation of new chiral thioindigo dopants designed to photomodulate the spontaneous polarization of ferroelectric liquid crystals

Irradiation of a ferroelectric S C * liquid crystal phase induced by the photochromic dopants (R,R)-6,6′-bis(2-octyloxy)-5,5′-dinitrothioindigo and (R,R)-5,5′-dichloro-6,6′-bis(2-octyloxy)thioindigo at λ=514 and 532 nm, respectively, causes an increase in spontaneous polarization (P S ) by a factor ranging from 1.4 to 4.0. This increase in P S is achieved without concomitant destabilization of the S C * phase, and is consistent with an increase in transverse dipole moment of the thioindigo core as a result of trans-cis photoisomerization. The contribution of the thioindigo core towards P S is achieved through stereo-polar coupling with the chiral side-chains, which is enhanced by the presence of the nitro and chloro substituents.

Relationship between reorientational motions of a photochromic dopant and local relaxation processes of a glassy polymer matrix

The reorientational relaxation process of the photochromic molecule 9-hydroxymethyl-10-[(naphthylmethoxy)-methyl]anthracene (HNMA) was observed in glassy poly(methyl methacrylate) (PMMA). For this purpose, the intramolecular photodimerization of HNMA incorporated in a PMMA matrix was selectively induced by irradiation with linearly polarized light. The optical anisotropy created in this way in the sample was allowed to relax to equilibrium by annealing at different temperatures. The reorientational relaxation of HNMA associated with this thermal treatment was subsequently observed by monitoring the time dependence of the two dichroic absorbance components of the anthracene moieties of HNMA. It was found that this relaxation process is well described by the sum of an exponential function of annealing time and a baseline which strongly depends on temperature. This fact implies that HNMA was not completely randomized in the glassy state of PMMA upon annealing. Furthermore, within the range of experimental temperatures, the activation energy of the reorientational relaxation rates of HNMA is close to that of the γ-process of the PMMA matrix, suggesting that the relaxation of the anthracene moieties is affected by the rotational motions of the α-methyl groups on the PMMA chain backbone. These preliminary results might provide some helpful guiding principles for controlling the stability of host/guest polymer materials using non-linear optical dyes.