The Control of the Cholesteric Pitch by Some Azo Photochemical Chiral Switches

Light-driven reversible handedness inversion in self-organized helical superstructures

We report here a fast-photon-mode reversible handedness inversion of a self-organized helical superstructure (i.e., a cholesteric liquid crystal phase) using photoisomerizable chiral cyclic dopants. The two light-driven cyclic azobenzenophanes with axial chirality show photochemically reversible trans to cis isomerization in solution without undergoing thermal or photoinduced racemization. As chiral inducing agents, they exhibit good solubility, high helical twisting power, and a large change in helical twisting power due to photoisomerization in three commercially available, structurally different achiral liquid crystal hosts. Therefore, we were able to reversibly tune the reflection colors from blue to near-IR by light irradiation from the induced helical superstructure. More interestingly, the different switching states of the two chiral cyclic dopants were found to be able to induce a helical superstructure of opposite handedness. In order to unambiguously determine the helical switching, we employed a new method that allowed us to directly determine the handedness of the long-pitched self-organized cholesteric phase.

Light-driven nanoscale chiral molecular switch: reversible dynamic full range color phototuning

A light-driven nanoscale chiral molecular switch was found to impart its chirality to an achiral liquid crystal host to form a self-organized, optically tunable helical superstructure capable of fast and reversible phototuning of the structural reflection across the entire visible region.

Chiral Molecular Switches Based on Binaphthalene Molecules with Anthracene Moieties: CD Signal Due to Interchromophoric Exciton Coupling and Modulation of the CD Spectrum

By coupling the features of binaphthalene and anthracene, new binaphthalenes with two anthracene moieties were designed and synthesized, aiming at developing chiral molecular switches. A strong CD signal with negative sign due to the interchromophoric exciton coupling was observed for (S)-1 with -(CH2)2 as the linker. This new CD signal became weak and the sign reversed by changing the linker to -(CH2)3 in (S)-2 and -(CH2)6 in (S)-3. For (S)-4 with -(CH2)11 as the linker, no such CD signal was detected. Photodimerization of two anthracene moieties in these binaphthalene molecules can occur. The results show that the CD spectra of (S)-1, (S)-2, (S)-3, and (R)-1 can be reversibly modulated by alternating UV light irradiation and heating. Therefore, chiral molecular switches based on new binaphthalenes with two anthracene moieties are achieved.

Photoinduced Conformational Switch of Enantiopure Azobenzenes Controlled by a Sulfoxide

Two series of enantiopure azobenzenes with a p-tolylsulfoxide at the ortho or meta position with respect to the azo group, have been regioselectively synthesized. Both can act as enantiopure molecular switches showing different structural features owing to the presence of the stereogenic sulfur. The photoisomerization process, studied by UV-vis, circular dichroism (CD), NMR, and chiral HPLC evidenced a double role of the sulfoxide. A transfer of chirality from the sulfoxide to the azo system was observed by CD in both cis and trans-isomers of the meta sulfinyl derivatives 3, whereas this perturbation was evident for the ortho sulfinyl series 7 only in the cis isomer. The NMR study evidenced that the s-cis rigid conformation of the bisaromatic sulfoxide was fixing a different orientation of the overall system in each series both in the trans and cis isomers, by forcing a final U-shaped structure in cis-3 and an S-shaped structure in cis-7. Very different values of specific optical rotations were measured in both trans and cis isomers, also reflecting the existence of distinct chiral entities in the photostationary states. The easy and reversible changes occurring between different conformational states could find applications in the photocontrol of several molecular switches.

Rotational Reorganization of Doped Cholesteric Liquid Crystalline Films

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.

Effect of Strain on the Photoisomerization and Stability of a Congested Azobenzenophane: A Combined Experimental and Computational Study

The stability and trans-cis photoisomerization properties of a macrocycle constituted of two para-aminoazobenzene units connected by two methylene bridges have been investigated by a combination of experimental and computational techniques. Irradiation at 365 nm leads to a photostationary state in which only 50% of the azobenzene units have isomerized, in contrast with the behavior of para-aminoazobenzene, whose photoconversion is larger than 80%. In the case of the macrocycle, a faster cis --> trans thermal back-reaction is observed. To assist the interpretation of the experimental results, molecular mechanics and quantum chemical calculations have been carried out. Of the possible conformers, the most stable trans-trans geometric isomer has been identified along with the more plausible trans-cis and cis-cis isomers. Ground-state energy barriers along the NN torsional coordinates were also computed, along with excitation energies and intensities for the species that can contribute to the photostationary state. The calculations point to a sequential photoisomerization mechanism and support a predominance of the trans-cis photoproduct with minor contributions from the cis-cis species. The thermal and photochemical reactivity of the examined macrocycle is compared to that of previously investigated azobenzenophanes and explained in terms of strain and substituent effects both concurring to favor the thermal cis --> trans back-reaction.

Reversible Full-Range Color Control of a Cholesteric Liquid-Crystalline Film by using a Molecular Motor

By using a chiral molecular motor as a dopant in a cholesteric liquid-crystalline film, fully reversible control of the reflection color of this film across the entire visible spectrum is possible. The large difference in helical twisting power between the two isomeric forms of the motor allows efficient light- and heat-induced switching of the helicity of the cholesteric liquid-crystal superstructure.

Binaphthalene Molecules with Tetrathiafulvalene Units: CD Spectrum Modulation and New Chiral Molecular Switches by Reversible Oxidation and Reduction of Tetrathiafulvalene Units

By combining the features of binaphthalene and tetrathiafulvalene (TTF), compounds 1-4 were designed for studies of chiral molecular switches. Absorption and CD spectral studies clearly indicate that the CD spectra resulting from axial chiral binaphthalene units can be modulated through the redox reactions of TTF units, which means new chiral molecular switches can be established on the basis of binaphthalene molecules with TTF units. The reference compound 5, which has one TTF unit rather than two as in the case of compounds 1, 3, and 4, failed to show such property, hinting that the presence of two or more TTF units is required for the realization of CD spectrum modulation. In addition, the manner of the CD spectrum modulation has been found to be dependent on the way TTF units are linked to the binaphthalene skeleton, in terms of the linker length, the positions for substitution, and the number of TTF units.

Homochiral Helices of Oligonaphthalenes Inducing Opposite-Handed Cholesteric Phases

The helical structure of the chiral nematic phases (cholesterics) obtained by doping nematic solvents with chiral non-racemic compounds is a macroscopic proof of the solute chirality. Oligonaphthalene (tetra-, hexa-, octa-) derivatives linked at the 1,4-positions have been used as chiral dopants: When the chirality axes are configurationally homogeneous (that is, all-S), the molecular structures correspond to right-handed helices. Yet, we have found series of derivatives with the surprising property that the handedness of the induced cholesteric phase alternates from positive to negative and to positive again, on passing from tetra- to hexa- and to octanaphthalene. A comparison with oligonapthalene derivatives, which do not exhibit this twisting ability, points to the importance of the substitution pattern. Both the possibility of inducing oppositely-handed cholesteric phases by homochiral helices of different length, and the role played of substituents, are confirmed by calculations performed with the surface chirality model.

Amplification of chirality in liquid crystals

The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.

Dual Modulation of a Molecular Switch with Exceptional Chiroptical Properties

The ability to modulate the chiroptical properties of optically active molecules induced by external stimuli such as light, heat, and electrical fields allows for the design and development of molecular switches, memory devices, sensors, and photonic devices. A helical o-terphenyl compound functionalized with photoresponsive azobenzene and electroactive imide groups is designed as a dual-mode chiroptical molecular switch. Its exceptional optical activity (e.g., [alpha]436 = -9500) can be changed and modulated through photoisomerization of the azobenzene moiety using UV and visible light. Reversible modulation by electrochemical means was also achieved through the redox reaction occurring at the imide group. Large chiroptical read-out signals were observed during the redox cycles as indicated by the molar ellipticity values as high as 285,000 deg.cm2.dmol-1. Exceptionally high optical activity and large responses to both light and electrical bias make this chiral molecule suitable for the development of new molecular switches, sensors, and other optical devices.

Photoinduced molecular reorientation dynamics in confined domains of Langmuir monolayers

In a photoresponsive Langmuir monolayer comprised of smectic-C-like domains of mesogenic trans-azobenzene derivative embedded within an isotropic matrix of its cis isomer counterpart, several structurally differing circular droplets were irradiated with linearly polarized light. This report describes the structural rearrangements that occurred in these droplets upon illumination followed by Brewster angle microscopy analysis. Starting from initial well-characterized and symmetric states, final photoaligned situations were reached in which the azimuth angles of the rod-shaped elongated molecules were found to be perpendicular to the electric component of the excitation light. The dynamical aspects of the photoalignments, including their transient patterns, are captured by a theoretical model that couples a relaxational principle incorporating long-range elastic forces with a kinetic formalism presenting an anisotropic rate law.

Enantiopure Sulfinyl Azobenzenes as Chiroptical Switches

[reaction: see text] Photoswitchable enantiopure sulfinyl azo compounds have been synthesized. A remarkable perturbation of the azo system by the stereogenic sulfinyl moiety has been observed by CD in both the trans and the cis azobenzenes resulting by photoisomerization. After five irradiation cycles, the configurational integrity of these chiral switches remains unchanged.

Photoswitchable Orientational Patterns of Confined Domains in Monolayers

We report here a photoswitch process that involves collective molecular reorientation in a monolayer of an azobenzene derivative. Using polarized light we force the transition between two clearly distinguishable orientational mesoscopic configurations that can be monitored by reflection optical microscopy. A model that combines thermodynamic and kinetic arguments is proposed, and it is able to reproduce both the two states and the mechanism involved in the transition. We conclude that the phenomenon reported here is essentially different from the usual electric-field-induced molecular alignment often found in liquid crystalline materials and devices. Instead, it involves a photoexcitation concomitant with an H-aggregation process.