Photoisomerization Behavior Research Articles

Photoresponsive conductive polymer network based on azobenzene bridging crosslinked polycarbazole for boosting solar thermal storage

Azobenzene is one of the most extensively researched multifunctional chromophores and azobenzene including materials has a wide variety of applications due to their photoisomerization behavior. In this study, electroactive and light-harvesting carbazole and photoresponsive azobenzene units have been combined with a special macromolecular design. In this design the azo groups can be effectively isomerized in solid state, and free-standing films can be obtained by the electrochemical method. Thermal characterizations of both monomer and polymer have been performed and isomerization kinetics and solar-thermal properties have been investigated. The half-life at 60 °C and the gravimetric energy storage density of polymer was calculated as 103 min and 179.9 j g−1, respectively. Cross-linked polycarbazole structure causes dramatically increased solar thermal storage and half-life compared to respective monomer and brought unexpected mechanical and solvatochromic properties.

2-(1,1-Dicyanomethylene)rhodanine-Functionalized Oligothiophenes: A Structure-Property Investigation of Z/E Photoisomerization Behavior.

A series of oligothiophenes singly and doubly functionalized with dicyanorhodanine (RCN) units have been investigated to understand their Z/E photoisomerization behavior upon structural modulation. Monotopic RCN target molecules (1-Z-9-Z) were designed to observe the consequences of π-conjugation, solubilizing group substitution, and formylation of the thiophene units. In all cases, the Z isomer is obtained from synthesis as the thermodynamically stable isomer, whereas the E isomer is achieved through selective irradiation (including red light, λirr = 628 nm) as a Z/E mixture in solution. For the quarterthiophene entries, photoisomerization is inhibited, with photoirradiation resulting only in degradation. The result comports with concentration-dependent studies, which show that increasing π-conjugation results in greater aggregation and muted Z/E photoisomerization. Ditopic RCN targets (10-ZZ-12-ZZ), mimicking acceptor-donor-acceptor (A-D-A) oligomers relevant to OPV materials, also show evidence of photoisomerization in solution, with formation of Z,Z/Z,E mixtures at the photostationary state (PSS). Complementary ground- and excited-state DFT calculations show excellent agreement with the experimental findings. This comprehensive structure-property analysis is expected to both guide and caution the functional materials community with respect to the usage of photoisomerizable RCN-oligothiophenes for optoelectronic applications.

Control of Photoisomerization Kinetics via Multistage Switching in Bimetallic Ru(II)-Terpyridine Complexes.

In this study, we carried out detailed experimental and theoretical investigation on photophysical, electrochemical, and photoisomerization behaviors of a new array of luminescent binuclear Ru(II) complexes derived from a phenylene-vinylene-substituted terpyridyl ligand possessing RT lifetimes within 60.3-410.5 ns. The complexes experienced trans-to-cis isomerization in MeCN on irradiation with visible light, accompanied by significant changes in their absorption and emission spectral profiles. The reverse cis-to-trans process is also possible with the use of ultraviolet (UV) light. On conversion from trans to cis isomers, the emission intensity increases substantially, while for the reverse process, luminescence quenching occurs. Thus, "off-on" and "on-off" emission switching is facilitated upon treatment with visible and UV light alternatively. By the use of chemical oxidants (ceric ammonium nitrate and potassium permanganate) and reductants (metallic sodium) as well as light of appropriate wavelengths, multistate switching phenomena involving reversible oxidation-reduction and trans-cis isomerization have been achieved. Interestingly, the rate of this multistate photoswitching process becomes much faster compared to only two-state trans-cis isomerization of these complexes. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations are also performed to obtain a clear picture of the electronic environment of the complexes and also for the appropriate assignment of absorption and emission spectral bands.

Azobenzene-Imidazolium ionic liquid crystals: Phase properties and photoisomerization in solution state

This is the first report of phase properties and photoisomerization behaviour of a series of amphiphilic azobenzene-imidazolium derivatives. The cationic imidazolium moiety was functionalised with varying alkyl chain length (C10-C18) and connected to an azobenzene via a flexible ether spacer. Their phase transitional properties and photoisomerization in solution were modulated by the alkyl chains in the imidazolium moiety. C14 was the shortest chain length to induce the formation of a liquid crystal phase, wherein compounds having C14-C18 exhibited stable smectic A phase. All compounds photo switched from trans-to-cis isomers in solution when illuminated with UV radiation. DFT calculations revealed that the trans isomers could adopt two geometries, however, folded geometry (global minima) was thermodynamically more stable than the open chain structure. The long conjugate (C16) had a higher trans-cis isomerization barrier due to the multilayer folding geometry that reduced its structural flexibility as compared to the C10 homologue. The photo conversion efficiency (CE) was alkyl chain length dependent. The C10 salt had a larger CE than C16 homologue due to the transition gap between S1 excited state and ground surface is smaller in the former (0.211 eV) than that in the latter (0.278 eV), thus resulting in efficient photoisomerization in the former.

Self-healing recyclable polymers based on azobenzenes with thermoset like behaviour

Self-healing materials are considered intelligent materials capable of recovering their original shapes from damaged or distorted forms after exposure to specific stimuli, such as chemicals, heat, moisture, or light. The utilization of thermoreversible crosslinks in polymer networks has garnered increasing interest as a method to create such materials. In these polymers, damage can be repaired simply by heating them above a gel-liquid transition temperature. Upon cooling, the material regains its previous cross-linked properties. However, finding alternative external stimuli other than heat presents an intriguing challenge. Among the various possibilities, light is the preferred choice due to its potential for remote and spatially-controlled activation. Additionally, the process can be easily paused and resumed by turning the excitation light on and off.Furthermore, the photo-switching of the glass transition temperature (Tg) in azobenzene-containing polymers induces reversible transitions from a solid to a liquid state. These reversible trans-to-cis transitions are triggered by UV and visible light irradiation. This phenomenon offers a novel approach to designing healable polymers and allows for precise control over the mechanical properties of polymers with spatio-temporal resolution.The main objective of this study is to investigate the feasibility of using light as a self-healing trigger in a recyclable polymer based on physically cross-linked epoxy/amine formulations. To achieve this goal, the matrix was modified with 4-phenylazophenol (PAP), which forms homogeneous blends with the polymer through hydrogen bond interactions. Films were prepared and characterized using various experimental techniques. Polymerization kinetics of samples with and without PAP was determined using FTIR, which was also employed for characterizing the final materials. Tg of the resulting products was measured using DSC, revealing a decrease in its value for PAP-modified samples. The transition from a solid to a liquid state during UV light irradiation was observed using rheometric analysis. The reversible photoisomerization behaviour of epoxy/amine/PAP was investigated using UV–visible spectroscopy in films prepared by spin-coating THF solutions.The ability of the material to self-heal at room temperature using light as a remote source was confirmed through TOM (Transmission Optical Microscopy) observations and quantified by profilometric measurements. In addition, surface mechanical properties were evaluated in order to determine the effect of the incorporation of the PAP, and the ability of the repaired material to recover its properties. Promising findings indicate that this recyclable polymer holds significant potential for multiple applications where its self-healing ability will be of great importance.

Synthesis of bent-shaped azobenzene main-chain polymers for photo-switching properties

This work presents the synthesis of the new bent-core polymers with siloxane units connected to the one side of azobenzene units. The structure of siloxane-based azobenzene bent-core polymers, 7a–c, was elucidated by spectral analysis (nuclear magnetic resonance and Fourier-transform infrared spectroscopy). The results of gel permeation chromatography suggested that all polymers (7a–c) showed polydisperse (polydispersity index >1). Besides, the extent of polymerization in the following order: 7a > 7b > 7c, where the degree of polymerization values were 7, 8 and 11, respectively. Polarizing optical microscopy revealed that the bent-core liquid crystal (BCLC) monomers, 6a and 6b, displayed the smectic A phase, whereas BCLC monomer 6c and all siloxane-based main-chain polymers (MCPs) (7a–c) were crystalline in nature. The result of ultraviolet-visible spectroscopy demonstrated that all MCPs (7a–c) exhibited strong photoisomerization behavior in solution. All polymers (7a–c) showed trans to cis isomerization in about 200 s, whereas the reverse process required much longer times ranging from 400 to 520 min in solution. The photo-switching study on azobenzene containing polymers stated that the effect of alkyl chain length and type of central core units on trans to cis isomerization were negligible. In contrast, both parameters influence the cis to trans process in which the photo-switching behavior of these materials may be primarily suitably exploited in the field of photo-induced phenomenon.

Controllable pH- and light-responsive chitosan-based multicomponent interpenetrating networks

Intelligent chitosan-based multivariate interpenetrating network (ICMIPN) was facilely tailor-made via modulating its components. ICMIPN was generated through common radical cross-linking reaction among macromonomers, which were derived from chitosan, starch and poly(vinyl alcohol) respectively, and acrylic acid (AA) as well as 4-acryloyoxyazobenzene (AHAB). ICMIPN exhibited a reversible trans-cis-trans photoisomerization behavior when it was subjected to alternative UV and visible light irradiation or dark storage. ICMIPN also displayed pH-sensitive swelling in buffer salines. By adjusting the initial amount of AHAB or AA, the responsive property of ICMIPN was varied accordingly. In addition, the thermal stability of ICMIPN was changed with altering its composition.

Photoswitchable Azoheteroarene-Based Chelating Ligands: Light Modulation of Properties, Aqueous Solubility and Catalysis.

We report the design and synthesis of eight photoswitchable phenylazopyridine- and phenylazopyrazole-based molecular systems as chelation-type light-controllable ligands. Besides the studies on fundamental photoisomerization behaviour, the ligands were also screened for light-tuneable properties such as photochromism, phase transition, and solubility, especially in the aqueous medium. This investigation demonstrates how the modulation of aqueous solubility can be achieved through photoisomerization and can further be utilized towards controlling the amount of catalytically active Cu(I) species in the aqueous conditions. Through this approach, light control over the catalytic activity was achieved for Cu-catalyzed azide-alkyne cycloaddition (CuAAC) reactions, along with a partial recovery of the catalytically active species.

Synthesis, Characterization and Evaluating Photochemical Properties of Porphyrin-substituted Azobenzene Structure.

Herein a new azobenzene-substituted porphyrin molecule was synthesized, characterized and its optoelectronic properties were investigated by combining the high optoelectronic properties of porphyrin with the photosensitive properties of azobenzene. The carboxylic acid of azobenzene was covalently connected to -OH group of the porphyrin ring by using Steglich esterification. Molecular structure of the obtained azobenzene-porphyrin (8), was elucidated, by FTIR, 1H and 13C NMR and HRMS. After structural characterization absorption and emission, characteristics were determined in solvents that have different. And also, optical and fluorescence behaviors in the range of different acid pH with trans-cis photoisomerization behaviors were investigated in aqueous-THF solution in acid media.

Humidity-Controlled Molecular Assembly and Photoisomerization Behavior with a Bubble-Assisted Patterning Approach.

Precise control of molecular assembly is of great significance in the application of functional molecules. This work has systematically investigated the humidity effect in bubble-assisted molecular assembly. This work finds humidity is critical in the evolution of the soft confined space, leading to the formation of microscale liquid confined space under high humidity, and nanoscale liquid confined space under low humidity. It is also revealed that the differences in surface wettability and adhesion play the key role. Consequently, a flat pattern with thermodynamically favorable ordered structure and a sharp pattern with dynamically favorable disordered structure are achieved, which show different solid-state photoisomerization behaviors and photoresponsiveness. Interestingly, conductivity of sharp pattern with disordered structure is higher than that of flat pattern with layered ordered structure due to electronic transport mechanism of different spatial dimensions. This work opens a new way for manipulating the molecular self-assembly to control the morphology and function of molecular patterns.

Facile Synthesis of Light-Switchable Polymers with Diazocine Units in the Main Chain

Unlike azobenzene, the photoisomerization behavior of its ethylene-bridged derivative, diazocine, has hardly been explored in synthetic polymers. In this communication, linear photoresponsive poly(thioether)s containing diazocine moieties in the polymer backbone with different spacer lengths are reported. They were synthesized in thiol-ene polyadditions between a diazocine diacrylate and 1,6-hexanedithiol. The diazocine units could be reversibly photoswitched between the (Z)- and (E)-configurations with light at 405 nm and 525 nm, respectively. Based on the chemical structure of the diazocine diacrylates, the resulting polymer chains differed in their thermal relaxation kinetics and molecular weights (7.4 vs. 43 kDa) but maintained a clearly visible photoswitchability in the solid state. Gel permeation chromatography (GPC) measurements indicated a hydrodynamic size expansion of the individual polymer coils as a result of the Z→E pincer-like diazocine switching motion on a molecular scale. Our work establishes diazocine as an elongating actuator that can be used in macromolecular systems and smart materials.

The effect of position of (-)-menthyl on the mesomorphic and photoisomerization behavior in azobenzene chiral liquid crystals

As the mesomorphic type of liquid crystals could be largely altered by the molecular chirality, we herein explored the effect of position of chiral (-)-menthyl on the mesomorphic and photoisomerization behavior in azobenzene liquid crystals. By modifying the position of chiral (-)-menthyl in the menthol based liquid crystal compounds through gradually increasing the alkyl chain length of the dicarboxylic spacer, seven new chiral liquid crystal compounds 4-(4-menthyloxy-n-oxoalkanoyloxy)-4′-(4-ethoxybenzoyloxy) azobenzene MnAZO2B (n = 2–8) were prepared. A combination of analysis methods was conducted to systematically investigate their structures, mesophase behaviors and optical properties. The length and parity of the flexible spacers has a dramatic influence on the mesomorphic behaviors, and an odd-even effect is observed for these chiral liquid crystal compounds.

Ultrafast photoisomerization mechanism of azaborine revealed by nonadiabatic molecular dynamics simulations.

1,2-Dihydro-1,2-azaborine is an isoelectronic analog of benzene with a B-N substitution, and its unique photoisomerization behavior, which is distinct from that of benzene, has drawn significant attention. To understand the detailed mechanism of azaborine photochemistry considering the dynamical effect and gain a comprehensive understanding of photochemical reactions, we investigated the photoisomerization dynamics of azaborine using nonadiabatic molecular dynamics simulations with Tully's surface hopping algorithm. Herein, the structural and energetic analyses of the trajectories revealed three different paths: direct relaxation (path 1), relaxation via a prefulvene-like intermediate (path 2), and formation of the Dewar isomer as a photoproduct (path 3). Our results confirmed that the photoisomerization of azaborine follows the energetically favored pathway predicted by the previous minimum energy path (MEP) calculations, exclusively forming the Dewar isomer, which is consistent with the experimental observations. Additionally, despite the low quantum yield found in our simulations, the high-level excitation energy calculations support the complete conversion observed in the experiments.

Photoisomerization, assembling and fluorescence photoswitching behaviors of a water-soluble stiff-stilbene with cucurbit[7]uril

Photoresponsive supramolecular systems have merited extensive attention for their applications in materials science and life science. Here, we synthesized a water-soluble stiff-stilbene molecular photoswitch, exhibiting outstanding photoisomerization reaction between its (E)- and (Z)-configurations upon irradiation at distinct light. The photoswitch can assemble with cucurbit[7]uril (CB[7]) to form a superior fluorescent supramolecular complex (compared to the only guest) with excellent water solubility, which manifested dramatic photoluminescence enhancement caused by restriction of intramolecular rotation and remained good photochromic characteristics. Furthermore, introduction of CB[7] influence photoreaction quantum yield (Φ) of the stiff-stilbene, leading to reduction of ΦE→Z and increase of ΦZ→E of the stiff-stilbene. Importantly, the photoadjustable supramolecular assembly can act as a fluorescence switch, and the phototunable guest further selectively modulate G-quadruplex structure of Tel22 upon light irradiation or with addition of CB[7]. The study provides a new simple way for accurately regulating photochromic properties of molecular switches and developing smart materials with potential applications in controlled modulation of G-quadruplex, targeted biological imaging and so on.

Delicate and Independent Manipulation of Dynamic Fluorescence Behavior of Polymer Nanoparticles Based on a Core-Shell Strategy.

Fluorescent polymer nanomaterials with dynamic fluorescence properties hold great potential in many advanced applications, including but not limited to information encryption, adaptive camouflage, and biosensors. The key to improving the application value of materials is to establish an accurate control strategy for dynamic fluorescence behavior. Herein, we develop a core-shell engineering strategy to precisely and independently manipulate the dynamic fluorescence behavior through the shell polymeric matrix. The core-shell fluorescent polymer nanoparticles (CS-FPNPs) are constructed through a sequential process of miniemulsion polymerization and seeded emulsion polymerization. Taking advantage of the core-shell structure, the rigid core matrix ensures the strong initial emission of AIE units, while the photoisomerization behavior of spiropyrane (SP) units is delicately and independently regulated by the rigidness of the shell matrix. Thereby, CS-FPNPs exhibit bright time-dependent reversible dynamic fluorescence behavior under alternating UV/vis irradiation. Benefited from the excellent processability and film formation ability, we have successfully applied CS-FPNPs to dynamic decorative painting, warning labels, and dynamic QR code security. Impressively, the fluorescence manipulation strategy based on core-shell engineering allows the independent regulation of specific luminescent units in complicated emission systems to accurately embody designed emission behavior.

Central condensed ring changes for manipulating the self-assembly and photophysical behaviors of cyanostilbene-based hexacatenars

Two series of condensed ring hexacatenars consisting of a central naphthalene/anthracene and two cyanostilbene units as the π-conjugated rigid core were reported. The thermodynamics behaviors and liquid crystalline structures of these mesogens were investigated by the combination of POM, DSC and XRD. It is indicated that only higher naphthalene-based homologues exhibit hexagonal columnar phases, the other lower naphthalene-based homologues and all the anthracene-based homologues are non-mesogens. The results demonstrated that the steric effect provided by the larger central aromatic group hinders the ordered molecular stacking. The temperature-dependent fluorescence demonstrated that the fluorescence stability of these hexacatenars increased from central phenyl via naphthyl to anthryl due to the increased degree of molecular distortions induced by the central larger aromatic group. The solvatochromism and AIEE behaviors were also studied by UV–vis absorption, fluorescence emission and density functional theory (DFT) demonstrating that the anthracene-based compounds exhibit more distinct solvatochromism, higher fluorescence quantum yield than those of naphthalene-based compounds and more distinct ICT effect, and both series of compounds exhibited strong fluorescence in both solutions and aggregated state due to ICT effect and the long π-conjugated aromatic core with twisted conformations. Additionally, photoisomerization behaviors demonstrated both cis and trans conformations showed high stability in condensed and solution states.

Inclusion-Activated Reversible E/Z Isomerization of a Cyanostilbene Derivative Based on Cucurbit[8]uril under 365 nm Ultraviolet Irradiation.

The photoisomerization behavior of cyanostilbene molecules is a hotspot in supramolecular configuration transformation research. Here, we reported a cyanostilbene derivative that converted from the Z,Z-isomer to the E,E-isomer under UV light irradiation at 365 nm. This process can be reversibly converted only in the presence of cucurbit[8]uril under the same light source, accompanied by the reversible conversion of fluorescence from green to yellow. No effective configuration transformation occurred with guest molecules only or upon the addition of cucurbit[7]uril. The photoisomerization was fully characterized by UV-vis and fluorescence spectroscopy, NMR, high-resolution mass spectrometry, and transmission electron microscopy. This work provides a new method for the supramolecular macrocyclic-activated configuration transformation.

Cyanostilbene‐Based AIEgen Smart Film: Optical Switching by Engineering Molecular Packing Structure and Molecular Conformation Through Thermal and Photoinduced Monotropic Phase Transition

AbstractThe optical properties of newly synthesized cyanostilbene‐based AIEgen (D4CS) exhibiting a unique monotropic phase transition are modulated by engineering molecular packing structure and molecular conformation through temperature change and light irradiation. D4CS exhibits only a low‐ordered smectic liquid crystal (SmA) phase over a wide temperature range during cooling and subsequent heating processes, while D4CS crystallizes after a significantly long annealing time at room temperature. Crystallized D4CS exhibits a complex phase transition (SmCr–SmB–SmA–Iso) in the subsequent heating process. In addition, the photoisomerization behavior of D4CS from trans to cis conformer can induce a phase transition to the isotropic (Iso) state, and the reverse isomerization can occur by high‐temperature heat treatment. This unique thermal and photoinduced phase transition of D4CS is accompanied by changes in optical properties including transmittance and luminescence. Based on the optical switching behavior of D4CS, photopatternable temperature and time‐dependent optical films are demonstrated. The newly developed cyanostilbene‐based AIEgen is a smart optical material with great potential for advanced optical sensors, anti‐counterfeiting ink and smart display system.

Synthesis of Bridged Indigos and Their Thermoisomerization and Photoisomerization Behaviors.

Bridged indigos were synthesized by bridging the two nitrogen atoms in the indigo structure with a carbon chain, and their properties were carefully examined. These bridged indigos have intrinsic planar chirality, and the enantiomers were separated using chiral high-performance liquid chromatography. When the chiral bridged indigos were subjected to thermo- and photoisomerization, the corresponding (Z)-indigo was not observed at all, and racemization was observed. This phenomenon is caused by the low activation energy of inversion due to the 1.5 bond order of the double bond of the indigo skeleton and the large energy difference between the ground states of (E)-indigo and (Z)-indigo.

Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study.

The photoisomerization behavior of styryl 9M, a common dye used in material sciences, is investigated using tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy. Styryl 9M has two alkene linkages, potentially allowing for four geometric isomers. IMS measurements demonstrate that at least three geometric isomers are generated using electrospray ionization with the most abundant forms assigned to a combination of EE (major) and ZE (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections. Two additional but minor isomers are generated by collisional excitation of the electrosprayed styryl 9M ions and are assigned to the EZ and ZZ geometric isomers, with the latter predicted to have a π-stacked configuration. The isomer assignments are supported through calculations of equilibrium structures, collision cross sections, and statistical isomerization rates. Photoexcitation of selected isomers using an IMS-photo-IMS strategy shows that each geometric isomer photoisomerizes following absorption of near-infrared and visible light, with the EE isomer possessing a S1 ← S0 electronic transition with a band maximum near 680 nm and shorter wavelength S2 ← S0 electronic transition with a band maximum near 430 nm. The study demonstrates the utility of the IMS-photo-IMS strategy for providing fundamental gas-phase photochemical information on molecular systems with multiple isomerizable bonds.