Multiple Azobenzene Research Articles

Multi‐azo Photoswitches for Improved Molecular Solar Thermal Energy Storage

AbstractMolecular solar thermal energy storage (MOST) based on photoisomerization represents a novel approach for the capture, conversion and storage of solar energy. Azo photoswitches can store energy by isomerization from their thermodynamically stable E isomers to higher energy metastable Z isomers. The energy density of azo photoswitches is crucial as it directly determines the storage capacity, but it is relatively low (160 J g−1). Therefore, enhancing the energy density through molecular structural design represents a central research focus in the field of MOST. A straightforward approach to enhance the energy density is to design multi‐azo photoswitches. This allows multiple azobenzene units to share a common framework while keeping the molecular weight as small as possible. In particular, when two azobenzene units are connected via a phenyl ring in a meta orientation, it facilitates efficient isomerization, thereby maximizing the energy density of the azo photoswitches (392 J g−1). This paper provides a brief overview of the development of multi‐azo photoswitches and highlights their outstanding performance as a MOST system. It also offers prospects for their future advancements in the field. We propose that, to further improve the energy density of multi‐azo photoswitches, one approach is to design wide spectrum of light photoisomerization of multi‐azo photoswitches. Additionally, introducing photo‐induced phase changes to multi‐azo photoswitches enables the simultaneous storage of both photon energy and ambient heat.

Deciphering Internal and External π-Conjugation in C3 -Symmetric Multiple Azobenzene Connected Systems in Self-Assembly.

Two tripodal C3 -symmetric photoswitchable molecular systems T1 and T2 are reported that have extended conjugation at external and internal positions using an acryl group. The influence of the extended π-bonds in their absorption properties, thermal relaxation of the photoisomers and their propensities in forming supramolecular self-assemblies have been explored through spectroscopy, and microscopic studies. In particular, the investigations on the self-assembly have been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), polarized optical microscopy (POM), X-ray diffraction studies (XRD) and atomic force microscopy (AFM). Remarkably, the position of the acryl group influences the behaviour of the two target molecules in supramolecular assembly, and also in the formation of photoresponsive organic hydrogels or microcrystals.

Stereoselective Excited-State Isomerization and Decay Paths in cis-Cyclobiazobenzene.

Herein, we have employed OM2/MRCI-based full-dimensional nonadiabatic dynamics simulations to explore the photoisomerization and subsequent excited-state decay of a macrocyclic cyclobiazobenzene molecule. Two S1/S0 conical intersection structures are found to be responsible for the excited-state decay. Related to these two conical intersections, we found two stereoselective photoisomerization and excited-state decay pathways, which correspond to the clockwise and counterclockwise rotation motions with respect to the N═N bond of the azo group. In both pathways, the excited-state isomerization is ultrafast and finishes within ca. 69 fs, but the clockwise isomerization channel is much more favorable than the counterclockwise one with a ratio of 74% versus 26%. Importantly, the present work demonstrates that stereoselective pathways exist not only in the photoisomerization of isolated azobenzene (AB)-like systems but also in macrocyclic systems with multiple ABs. This finding could provide useful insights for understanding and controlling the photodynamics of macrocyclic nanostructures with AB units as the main building units.

Sub-second dynamic phototuning of alignment in azodendrimer-doped nematic liquid crystal shells

The alignment of nematic liquid crystal 5CB in micron-thick shells, suspended in and containing aqueous liquid phases, can be rapidly switched between radial (homeotropic) and tangential (planar) director field by doping them with a photoresponsive dendrimer with multiple azobenzene moieties in the branches. The dendrimer spontaneously segregates to the inner as well as outer shell interfaces, folding into an amphiphilic conformation irrespective of the sign of interface curvature. The branches are directed into the liquid crystal, inducing a homeotropic ground state. Upon UV irradiation, the trans-cis isomerization of azobenzene triggers immediate switching to planar alignment. The very fast realignment and the simultaneous response throughout the shell leads to an initially random planar director field, with many topological defects of both positive and negative signs becoming visible within a second of irradiation. All but two +1 defects quickly annihilate, and the remaining defect pair moves up towards the thinnest part of the shell to form the planar steady state. By illuminating with visible light the homeotropic alignment is quickly recovered. By exchanging the solvent used for assisting the dendrimer dissolution, also dynamic phase separation phenomena can be studied in the shells, revealing that the dendrimer solubility in 5CB is greater in the UV-induced cis state than in the trans ground state.

A photo-responsive poly(amide–triazole) physical organogel bearing azobenzene residues in the main chain

The synthesis of a click poly(amide-triazole) bearing multiple azobenzene units along the main chain was reported. The polymer was found to form strong physical polymer organogels. This structural design facilitates a much more efficient photomechanical effect upon photochemical excitation, and a highly reversible sol-gel transformation can be smoothly triggered by light irradiation and thermal treatment.

Light or Heat? The Origin of Cargo Release from Nanoimpeller Particles Containing Upconversion Nanocrystals under IR Irradiation.

Nanoimpellers are mesoporous silica nanoparticles that contain azobenzene derivatives bonded inside the pores and rely on the continuous photoisomerization of multiple azobenzenes to release cargo under near UV irradiation. A recent study employs upconversion nanocrystal embedded particles to replace UV light with IR light to stimulate nanoimpellers. However, the photothermal effect of IR irradiation and its likely contribution to the observed release behavior are not examined. It is found that, in the absence of upconversion nanocrystals, the azobenzene co-condensed silica particles still respond to 980 nm illumination, which increases the nanoparticle temperature by 25 °C in 15 min, experimentally measured by an encapsulated nanothermometer. After suppressing the heating, the IR irradiation does not initiate the release, indicating that optical heating, not upconverted light, is responsible for the triggered cargo release. The results are explained by numerical analyses.

Light-induced contraction and extension of single macromolecules on a modified graphite surface.

Synthetic rigid-rod polymers incorporating multiple azobenzene photoswitches in the backbone were deposited from solution onto a monolayer of octadecylamine covering the basal plane of graphite. Large contractions and extensions of the single macromolecules on the surface were induced by irradiation with UV and visible light, respectively, as visualized by scanning force microscopy. Upon contraction, the single polymer chains form more compact nanostructures and also may move across the surface, resembling a crawling movement. We attribute the efficiency of these processes to the low mechanical and electronic coupling between the surface and polymers, the high density of azobenzenes in their backbones, and their rigidity, allowing for maximized photodeformations. The visualization of on-surface motions of single macromolecules directly induced by light, as reported herein, could help promote the development of optomechanical nanosystems.

Modification of nucleic acids by azobenzene derivatives and their applications in biotechnology and nanotechnology.

Azobenzene has been widely used as a photoregulator due to its reversible photoisomerization, large structural change between E and Z isomers, high photoisomerization yield, and high chemical stability. On the other hand, some azobenzene derivatives can be used as universal quenchers for many fluorophores. Nucleic acid is a good candidate to be modified because it is not only the template of gene expression but also widely used for building well-organized nanostructures and nanodevices. Because the size and polarity distribution of the azobenzene molecule is similar to a nucleobase pair, the introduction of azobenzene into nucleic acids has been shown to be an ingenious molecular design for constructing light-switching biosystems or light-driven nanomachines. Here we review recent advances in azobenzene-modified nucleic acids and their applications for artificial regulation of gene expression and enzymatic reactions, construction of photoresponsive nanostructures and nanodevices, molecular beacons, as well as obtaining structural information using the introduced azobenzene as an internal probe. In particular, nucleic acids bearing multiple azobenzenes can be used as a novel artificial nanomaterial with merits of high sequence specificity, regular duplex structure, and high photoregulation efficiency. The combination of functional groups with biomolecules may further advance the development of chemical biotechnology and biomolecular engineering.

Azobenzene photoisomerization-induced destabilization of B-DNA.

Molecular photoswitches provide a promising way for selective regulation of nanoscaled biological systems. It has been shown that conformational changes of azobenzene, one of the widely used photoswitches, can be used to reversibly control DNA duplex formation. Here, we investigate the conformational response of DNA upon azobenzene binding and isomerization, using a threoninol linker that has been experimentally investigated recently. To this end, nonequilibrium molecular dynamics simulations are carried out using a switching potential describing the photoinduced isomerization. Attachment of azobenzene leads to a distortion of the DNA helical conformation that is similar for the trans and cis forms. However, the trans form is stabilized by favorable stacking interactions whereas the cis form is found to remain flipped out of the basepair-stacked position. Multiple azobenzene attachment augments the distortion in DNA helical conformation. The distorted DNA retains nativelike pairing of bases at ambient temperatures, but shows weaker basepairing compared to native DNA at an elevated temperature.

Photoinduced Deformation of Rigid Azobenzene-Containing Polymer Networks

Photoresponsive poly(amide acid) (PAA) gels containing multiple azobenzene units in a rigid aromatic backbone were synthesized. A centimeter-long cantilever made up of the photoresponsive PAA gel exhibited reversible bending motions upon blue (442 nm) and visible light (>490 nm) irradiation. The network structure in the PAA gels during alternating photoirradiation of blue and visible light was characterized using in situ scanning microscopic dynamic light scattering (SMILS), which revealed reversible mesh-size changes synchronized with the photoisomerization of azobenzene moieties. The photomechanical responses of the PAA gel were likely due to photoinduced contracting and stretching motions of the polymer backbone. A numerical calculation of photon absorptions revealed that photoisomerization in a very thin layer of the surface (∼40 μm) generated large macroscopic motion and large strain in the gel cantilever. The photoresponsive capability is, however, reduced or eliminated when the PAA gels are transfo...

Electronic Decoupling Approach to Quantitative Photoswitching in Linear Multiazobenzene Architectures

A strategy to optimize the photoswitching efficiency of rigid, linear multiazobenzene constructs is presented. It consists of introducing large dihedral angles between azobenzene moieties linked via aryl-aryl connections in their para positions. Four bisazobenzenes exhibiting different dihedral angles as well as three single azobenzene reference compounds have been synthesized, and their switching behavior has been studied as well as experimentally and theoretically analyzed. As the dihedral angle between the two azobenzene units increases and consequently the electronic conjugation decreases, the photochromic characteristics improve, finally leading to individual azobenzene switches operating independently in the case of the perpendicular ortho,ortho,ortho',ortho'-tetramethyl biphenyl linker. The electronic decoupling leads to efficient separation of the absorption spectra of the involved switching states and hence by choosing the appropriate irradiation wavelength, an almost quantitative E → Z photoisomerization up to 97% overall Z-content can be achieved. In addition, thermal Z → E isomerization processes become independent of each other with increasing decoupling. The electronic decoupling could furthermore be proven by electrochemistry. The experimental data are supported by theory, and calculations additionally provide mechanistic insight into the preferred pathway for the thermal Z,Z → Z,E → E,E isomerization via inversion on the inner N-atoms. Our decoupling approach outlined herein provides the basis for constructing rigid rod architectures composed of multiple azobenzene photochromes, which display practically quantitative photoswitching properties, a necessary prerequisite to achieve highly efficient transduction of light energy directly into motion.

Photoregulated assembly/disassembly of DNA‐templated protein arrays using modified oligonucleotide carrying azobenzene side chains

DNA-templated self-assembly of nanomaterials provides great potential for applications including biosensors, nanoelectronics, and programmable and autonomous molecular machines. To switch or regulate the activities of those nanobiotechnological devices, non-invasive methods to assemble and disassemble specific nanoscale components are needed. Here, we describe photocontrol of assembly of DNA-templated protein arrays in solution, by using photo-controlled duplex formation of oligonucleotides carrying azobenzene. As a proof of concept prototype, we designed a one-dimensional protein array system that consists of a scaffold of DNA and two kinds of anchor DNA that were conjugated with fluorescent proteins (CFP and YFP, respectively). The scaffold DNA was modified to carry multiple azobenzene side chains so that the hybridization involving the scaffold DNA is regulated by photoirradiation through conformational changes of the azobenzene moieties. Melting temperatures of duplex made of the modified DNA scaffold and an anchor oligonucleotide were shifted significantly and reversibly by UV and visible photoirradiation (difference of T(m) was 34.8 degrees C in 150 mM potassium acetate). Measurements of Förster resonance energy transfer between CFP and YFP showed that the assembly of the protein array system was also changed by photoirradiation. Such non-invasive and reversible method to control assembly/disassembly of multiple, specific proteins in a DNA-templated protein array system would provide many functions for nanobiotechnological devices such as on/off switches and the ability to change the configuration reversibly.

Line up base pairs and intercalators one by one in a stable duplex

A stable double helix involving alternating base pairs and azobenzene moieties was constructed. In this supramolecule, base pairs and azobenzenes are lined up one by one to form an interstrand-wedged motif: each base pair is sandwiched with two azobenzenes, and each azobenzene intercalates between two base pairs. This motif was formed by the hybridization of two modified DNA tethering multiple azobenzene moieties at a frequency of one azobenzene for every two nucleotides. Furthermore, this structure could be simply dismantled by UV light irradiation and reformed with the irradiation of visible light. By using this new duplex motif, construction of a variety of photoresponsive nanostructures and nanodevices is highly expected.

Clear-cut photo-regulation of the formation and dissociation of the DNA duplex by modified oligonucleotide involving multiple azobenzenes

Multiple azobenzenes were introduced into the oligonucleotide on D-threoninol for the clear-cut photo-regulation of the formation and dissociation of DNA duplex. When azobenzenes took trans form, introduction of multiple azobenzenes (azobenzenes:nucleobases = 1:2) did not interfere with duplex formation at all compared with their native duplex. In contrast, melting temperature (Tm) uniformly decreased with the number of azobenzenes when they took cis-form. As a result, clear-cut photo-regulation of the duplex formation was attained under physiological conditions. Furthermore, nearest-neighbor parameters for these azobenzene-tethered oligonucleotides were obtained. It was found that Tms estimated from these parameters well coincided with those of measured ones.

Effective photo-regulation of transcription reaction by SP6 RNA polymerase with modified DNA tethering multiple azobenzenes.

Effective photo-regulation of transcription reaction by SP6 RNA polymerase (RNAP) was achieved with photo-responsive SP6 promoter tethering two azobenzenes. With one azobenzene in either TATA or RNAP binding region of the SP6 promoter, photo-regulation activity was very small. But when two azobenzenes were introduced into both regions, efficient photo-regulation of transcription was attained: transcription proceeded 3.5 fold faster under UV irradiation than under dark.

Photo-regulation of DNA function by azobenzene-tethered oligonucleotides.

Multiple azobenzene moieties were tethered to DNA on D-threoninol linker, and formation and dissociation of DNA duplex could be reversibly photo-regulated either by irradiating UV or visible light. With this photo-responsive DNA, RNase H reaction was also successfully photo-regulated.

Synthesis of Amorphous Molecular Materials with Multiple Azobenzene Units by Palladium-Catalyzed Amination and their Photochromism

New photochromic amorphous molecules containing core of triphenylamine and three branches of azobenzene were obtained in good yields by Pd-catalyzed amination. High glass temperature (Tg) above 100°C of these molecules was because of many triphenylamine units and high molecular weight. The stable amorphous films of these molecules could be easily prepared by spin-coated method. The photoinduced and thermal isomerization were observed in the amorphous films and the solutions. The size of the core had no clear effect on rates and activation energy values (Ea) of the cis→trans thermal isomerization in solution.