Langmuir−Blodgett Films of Photochromic Polyglutamates. 9. Relation between Photochemical Modification and Thermotropic Properties

Azobenzene-Based Solar Thermal Fuels: A Review

The energy storage mechanism of azobenzene is based on the transformation of molecular cis and trans isomerization, while NBD/QC, DHA/VHF, and fulvalene dimetal complexes realize the energy storage function by changing the molecular structure. Acting as "molecular batteries," they can exhibit excellent charging and discharging behavior by converting between trans and cis isomers or changing molecular structure upon absorption of ultraviolet light. Key properties determining the performance of STFs are stored energy, energy density, half-life, and solar energy conversion efficiency. This review is aiming to provide a comprehensive and authoritative overview on the recent advancements of azobenzene molecular photoswitch system in STFs fields, including derivatives and carbon nano-templates, which is emphasized for its attractive performance. Although the energy storage performance of Azo-STFs has already reached the level of commercial lithium batteries, the cycling capability and controllable release of energy still need to be further explored. For this, some potential solutions to the cycle performance are proposed, and the methods of azobenzene controllable energy release are summarized. Moreover, energy stored by STFs can be released in the form of mechanical energy, which in turn can also promote the release of thermal energy from STFs, implying that there could be a relationship between mechanical and thermal energy in Azo-STFs, providing a potential direction for further research on Azo-STFs.

Polymer Nanocomposites for Photocatalytic Degradation and Photoinduced Utilizations of Azo-Dyes

Specially designed polymer nanocomposites can photo-catalytically degrade azo dyes in wastewater and textile effluents, among which TiO2-based nanocomposites are outstanding and extensively explored. Other nanocomposites based on natural polymers (i.e., chitosan and kaolin) and the oxides of Al, Au, B, Bi, Fe, Li, and Zr are commonly used. These nanocomposites have better photocatalytic efficiency than pure TiO2 through two considerations: (i) reducing the hole/electron recombination rate by stabilizing the excited electron in the conducting band, which can be achieved in TiO2-nanocomposites with graphene, graphene oxide, hexagonal boron nitride (h-BN), metal nanoparticles, or doping; (ii) decreasing the band energy of semiconductors by forming nanocomposites between TiO2 and other oxides or conducting polymers. Increasing the absorbance efficiency by forming special nanocomposites also increases photocatalytic performance. The photo-induced isomerization is exploited in biological systems, such as artificial muscles, and in technical fields such as memory storage and liquid crystal display. Heteroaryl azo dyes show remarkable shifts in photo-induced isomerization, which can be applied in biological and technical fields in place of azo dyes. The self-assembly methods can be employed to synthesize azo-dye polymer nanocomposites via three types of interactions: electrostatic interactions, London forces or dipole/dipole interactions between azo dyes, and photo alignments.

Random Planar Orientation in Liquid-Crystalline Block Copolymers with Azobenzene Side Chains by Surface Segregation

Rodlike liquid-crystalline (LC) mesogens preferentially adopt a homeotropic orientation by excluded volume effects at the free surface in side-chain LC (SCLC) polymer films. The homeotropic orientation is not advantageous for in-plane LC alignment processes. Surface segregation of polymers is the phenomenon in which one component with a low surface free energy covers the surface in a mixture of two or more polymers or a block copolymer film. In SCLC block copolymer films, the surface segregation structure induces a random planar orientation due to the formation of a microphase-separated interface parallel to the substrate via the covering of one of the segregated polymer blocks. This feature article focuses on the unique, random planar orientation induced by the surface segregation of SCLC block copolymer films with the photoresponsive azobenzene (Az) mesogenic group. A transition moment of the Az mesogens is parallel to the molecular long axis, and light irradiation is conducted perpendicular to the film surface in general photoreaction processes. Therefore, the homeotropic molecular orientation in the SCLC polymer systems with Az mesogenic units inhibits efficient photoreaction reorientations in thin films. The random planar orientations by the surface segregation of a coil block in SCLC block polymers provide efficient in-plane photoreorientation and photoswitching with LC hierarchical mesostructures, such as microphase-separated structures of SCLC block copolymers and laminated LC polymer films. On the other hand, surface-segregated SCLC blocks form a high-density polymer LC brush layer with a random planar orientation by self-assembly, which exhibits efficient angular selective photoreactions. These approaches using the surface segregation of SCLC block copolymers are expected to offer new concepts for the LC photoalignment process for LC polymer devices.

Controlling Heat Release from a Close-Packed Bisazobenzene-Reduced-Graphene-Oxide Assembly Film for High-Energy Solid-State Photothermal Fuels

A closed-cycle system for light-harvesting, storage, and heat release is important for utilizing and managing renewable energy. However, combining a high-energy, stable photochromic material with a controllable trigger for solid-state heat release remains a great challenge for developing photothermal fuels (PTFs). This paper presents a uniform PTF film fabricated by the assembly of close-packed bisazobenzene (bisAzo) grafted onto reduced graphene oxide (rGO). The assembled rGO-bisAzo template exhibited a high energy density of 131 Wh kg^-1 and a long half-life of 37 days owing to inter- or intramolecular H-bonding and steric hindrance. The rGO-bisAzo PTF film released and accumulated heat to realize a maximum temperature difference (DT) of 15 °C and a DT of over 10 °C for 30 min when the temperature difference of the environment was greater than100 °C. Controlling heat release in the solid-state assembly paves the way to develop highly efficient and high-energy PTFs for a multitude of applications.

Inducing Planar Orientation in Side-Chain Liquid-Crystalline Polymer Systems via Interfacial Control

For efficient photoresponses of liquid-crystal (LC) azobenzene (Az) polymer systems, planar LC orientation of the Az mesogenic group is required because the light irradiation process usually occurs with normal incidence to the film surface. However, LC molecules with a rodlike shape tend to orient perpendicularly to the film surface according to the excluded volume effect theory. This review introduces new approaches for inducing planar orientation in side-chain LC Az polymer films via interface and surface molecular designs. The planar orientation offers efficient in-plane photoalignment and photoswitching to hierarchical LC architectures from molecular LC mesogens and LC phases to mesoscopic microphase-separated structures. These approaches are expected to provide new concepts and possibilities in new LC polymer devices.

Monolayer formation of PBLG–PEO block copolymers at the air–water interface

Physicochemical properties of PBLG (poly(gamma-benzyl-l-glutamate))-PEO (poly(ethylene oxide)) diblock copolymers composed of PBLG as the hydrophobic rod component and PEO as the hydrophilic component were investigated at the air-water interface. Surface pressure-area isotherms obtained by the Wilhelmy plate method provide several variables such as molecular size, compressibility of PEO, and the free energy change of the PBLG-PEO block copolymer. GE-1 (M(w) of PBLG:PEO=103,700:12,000), with a relatively longer rod, has negative temperature effects and GE-3 (M(w) of PBLG:PEO=8400:12,000), with a relatively shorter rod, shows a positive temperature effect because of the large entropy loss. These competitions were based on the block size of PBLG and PEO and were affected by various microstructures of the PBLG-PEO diblock copolymer. Monolayer aggregations transferred onto mica from the air-water interface were analyzed with AFM. AFM images of GE-1 monolayers show cylindrical micelles, but the self-assembled structure has many large domains. The monolayer of GE-2 (M(w) of PBLG:PEO=39,800:12,000), which has a medium size rod, forms a spherical structure at the air-water interface. Monolayers of GE-3, with a short rod length, form bilayer structures. These results demonstrate that the microstructures of PBLG-PEO diblock copolymers are related to free energy changes between rod and coil blocks.

Photo-induced structural changes of azobenzene Langmuir-Blodgett films

Structural changes of the Langmuir-Blodgett (LB) films of azobenzene accompanied by photoisomerization are described. First, photoisomerization is explained in terms of 'free volume'. In the polyion complex monolayers of amphiphiles having two azobenzene units at the air-water interface, the area per molecule depends on the polycation species. The fraction of cis-azobenzene in the LB films at the photostationary state under the illumination with UV light increased with increasing area per molecule, which is consistent with the concept of free volume. Second, a counter example of the concept of free volume is presented. Three-dimensional cone-shaped structures developed with trans-to-cis photoisomerization in the polyion complex LB film of a water-soluble amphiphilic azobenzene. These structures appeared and disappeared reversibly by alternate illumination with UV and visible light. The results indicate that the two-dimensional LB film structure exerts significant modification by photoisomerization. This is against the concept of free volume because this concept does not consider the possibility that the two-dimensional LB film structures may change into three-dimensional ones. Finally, photo-induced J-aggregate formation of non-photochromic and photochromic dyes is described. Two cyanine dyes were each mixed with an amphiphilic azobenzene in the LB films. These cyanine dyes are known to form J-aggregates in single-component LB films. In the mixed LB films, the J-aggregate formation was suppressed to some extent. The alternate illumination of the films with UV and visible light caused the photoisomerization of azobenzene in the mixed LB films, which triggered the J-aggregate formation of the cyanine dyes. The J-aggregate formation was accompanied by the development of three-dimensional cone-shaped structures from the film surface. When an amphiphilic merocyanine was mixed with the azobenzene in the LB films, J-aggregate formation was also induced by the alternate illumination with UV and visible light. This J-aggregate formation was also accompanied by a large morphological change: circular domains changed into fractal-like ones. The J-aggregate formation of the dyes and the concomitant morphological change were irreversible. In these cases, the photoisomerization of azobenzene served as a trigger to induce self-organization of the dye molecules.

Self-Ordering within Thin Films of Poly(olefin sulfone)s

A study has been performed of the manner in which two structural features of poly(olefin sulfone)s, helical backbones and calamitic side chains, create order in films. For this purpose copolymers were prepared with one (polymer I) or two (polymer III) cyanobiphenyls per residue, and terpolymers were prepared with both such residues diluted to below the 5% level within an otherwise poly(eicosene sulfone) chain (respectively, polymers II and IV). The polymers all have ordered phases according to X-ray powder diffraction studies on samples cooled from the melt, a layer spacing of about 45 Å being detected in the films as in the bulk. Those polymers with mainly eicosene sulfone residues had crystalline phases with large domains, the layers deriving from the helical backbones alone, the smectic A phases of the parent poly(eicosene sulfone) being either suppressed or reduced in extent by the presence of the aromatic moieties, which were almost randomly orientated. Those with one or two cyanobiphenyls per residue were liquid crystalline. In the latter the layer spacing derives from both backbone and side chains and is reduced when the residues bear a second mesogen as a consequence of a constraining effect from the stiff backbone, as a novel model predicts. The spacers give rise to a glass transition and segregate the planes in which the stiff backbones are assembled from the regions in which the aromatic groups aggregate on account of the strong pi-pi interactions. Amorphous and optically isotropic spun cast films of these polymers became ordered on cooling from the melt or just on annealing, with the order, as determined by studies on the optical properties, being homeotropic for the aromatics and being planar for the backbones in a monodomain. For this arrangement we introduce the term homeo-planar smectic. Order parameters as high as 0.63 were measured for polymer I, from a clear film. The cyanobiphenyl chromophores formed H aggregates, with blue shifts in absorption and red shifts in fluoresence, and a little surprisingly these resulted in a circular dichroism, detectable when the films were inspected at an angle of 45 degrees to the normal.