Photoswitchable Behavior Research Articles

Photoswitchable adsorption behavior on a silica surface: A comparison of the partially fluorinated surfactants with the hydrocarbon surfactants

The surfactant adsorption at different interfaces is of great importance in surface science. We studied the photoswitchable adsorption/desorption of the partially fluorinated Gemini surfactants (CFS-2, CFS-6) at water-air and quartz-water interfaces. The fluorinated Gemini surfactants exhibit higher surface activity and lower wetting performance for quartz, compared to the comparable hydrocarbon ones (CHS-2, CHS-6). After UV light irradiation, the surface activity of the Gemini surfactants is decreased, and the wetting performances are increased. The adsorption mechanisms of the fluorinated Gemini surfactants at quartz were analyzed using the wetting data. The adsorption of the fluorinated Gemini surfactants at the quartz surface was further investigated by quartz crystal microbalance with dissipation (QCM-D) and AFM. The fluorinated Gemini surfactants exhibit greater adsorption at quartz compared to the comparable hydrocarbon ones. The fluorinated surfactants form rigid films at quartz. Upon UV light irradiation, the fluorinated surfactants exhibit the decreased adsorption at quartz and appear to be desorbed from quartz. The kinetics of the fluorinated surfactant adsorption at quartz was studied using the real-time measurement of QCM-D. Taken together, the findings in the present work provide insight into the significant differences in the surface properties and the adsorption onto the quartz surface between the partially fluorinated surfactants and the hydrocarbon surfactants.

Photoswitchable Phase Separation and Oligonucleotide Trafficking in DNA Coacervate Microdroplets

AbstractCoacervate microdroplets produced by liquid–liquid phase separation have been used as synthetic protocells that mimic the dynamical organization of membrane‐free organelles in living systems. Achieving spatiotemporal control over droplet condensation and disassembly remains challenging. Herein, we describe the formation and photoswitchable behavior of light‐responsive coacervate droplets prepared from mixtures of double‐stranded DNA and an azobenzene cation. The droplets disassemble and reassemble under UV and blue light, respectively, due to azobenzene trans/cis photoisomerisation. Sequestration and release of captured oligonucleotides follow the dynamics of phase separation such that light‐activated transfer, mixing, hybridization, and trafficking of the oligonucleotides can be controlled in binary populations of the droplets. Our results open perspectives for the spatiotemporal control of DNA coacervates and provide a step towards the dynamic regulation of synthetic protocells.

Photoswitchable Phase Separation and Oligonucleotide Trafficking in DNA Coacervate Microdroplets.

Coacervate microdroplets produced by liquid-liquid phase separation have been used as synthetic protocells that mimic the dynamical organization of membrane-free organelles in living systems. Achieving spatiotemporal control over droplet condensation and disassembly remains challenging. Herein, we describe the formation and photoswitchable behavior of light-responsive coacervate droplets prepared from mixtures of double-stranded DNA and an azobenzene cation. The droplets disassemble and reassemble under UV and blue light, respectively, due to azobenzene trans/cis photoisomerisation. Sequestration and release of captured oligonucleotides follow the dynamics of phase separation such that light-activated transfer, mixing, hybridization, and trafficking of the oligonucleotides can be controlled in binary populations of the droplets. Our results open perspectives for the spatiotemporal control of DNA coacervates and provide a step towards the dynamic regulation of synthetic protocells.

Photoplastic Transformation Based on Dynamic Covalent Chemistry

The magical fantasy of decades-old transformer characters is becoming closer to scientific reality, as transformable materials can change their shapes in response to thermal, mechanical, electrical, and chemical stimuli. However, precise and prompt control of plastic shaping remains to be wanted. Photoresponsive materials provide a promising alternative for rapid optomechanical shaping with limited success. Here, we report a new class of photoplastic transformation based on dynamic covalently crosslinked polytriazole (PTA) networks, in which crosslinking points are comprised of photocleaveable hexaarylbiimidazole (HABI). Upon sub-500 nm light irradiation, HABI is dissociated into two triphenylimidazole radicals (TPIRs) followed by spontaneous recombination back to the initial state. This photoswitching effect is demonstrated to generate nonthermal shape change in the PTA-HABI gel network at will upon light stimulus. A unique photoalignment phenomenon has also been discovered which can form oriented nanoscale patterning in the PTA-HABI gel network upon laser irradiation. The solvent-free PTA-HABI elastomer exhibits photoenhanced automatic self-healing properties at temperatures ranging from 25 °C to freezing points, which is attributed to the dynamic equilibrium between TPIRs and HABI. A photoplastic spring is fabricated and exhibits photoswitchable plastic behavior, i.e., a reversible transformation between plastic strain and elastic strain upon light irradiation. HABI-based polymer networks, including solvated gel and solvent-free elastomer, are promising as smart materials for nonthermal photoactivated shape changing, transformation, and self-healing applications.

High strength dual-crosslinked hydrogels with photo-switchable color changing behavior

Herein, high strength dual-crosslinked hydrogels with photo-switchable color changing behavior were designed and fabricated. In the mixture solution of α-cyclodextrin (α-CD), α-cyclodextrin-spiropyran (α-CD-SP), and poly(ethylene glycol) diacrylate (PEGDA), the PEGDA chains first threaded into the cavity of α-CD/α-CD-SP and form inclusion complexes to generate the physical gel; then, the as-prepared physical gels were subjected to photopolymerization to form a covalently crosslinked network. With the contribution of both no-covalent and covalent bond, the dual-crosslinked hydrogels possess high tensile strength, which is about fifteen times that of the pure PEG hydrogel. And due to the sacrifice of no-covalent bond, hydrogels showed high energy dissipation ratio, up to 75.3% and high fatigue resistance during loading-unloading tests. While the ingenious introduction of SP moieties, enables the as-prepared gels photo-switchable spatiotemporal color changing behavior reversibly. Erasable patterns on hydrogels could be written using UV light and erased by Vis light for multiple cycles. This flexible hydrogel with good physical performance shows high potential in photochromic flexible wearable devices.

Fast responsive photo-switchable dual-color fluorescent cyclodextrin nanogels for cancer cell imaging

Photo-switchable dual-color fluorescent nanogels were fabricated via nanoprecipitation in aqueous solution. The spiropyran-modified β-cyclodextrin (β-CD-SP) and 4-amino-7-nitro-1,2,3-benzoxadiazole (NBDNH2) were encapsulated into nanogels in the presence of 1,6-hexamethylene diisocyanate (HMDI). In the nanogels, spiropyran moiety acted as fluorescence molecule acceptors under the irradiation of UV or visible light to quench or recover the fluorescence of the NBDNH2 to achieve reversible dual-color fluorescence. The feed ratio of the two fluorophores played vital role on the energy transfer efficiency (E) of the nanogels, and the results showed that high E (90.7%) can be achieved when the feed ratio of NBDNH2/β-CD-SP is 1:2 (mol/mol). In addition, the nanogels can maintain good photo-switchable fluorescent behavior as long as 5 weeks. With rapid photo-responsiveness (within 60 s), good reversibility, long-term stability, and excellent cytocompatibility, the as-prepared nanogels have been successfully applied to photo-switchable dual-color fluorescent imaging in cancer cell.

Improved nonlinear optical properties of 6-nitro-BIPS composited with graphene oxide or N719 and their photoswitchable behavior

Two kinds of novel photochromic composites—6-nitro-BIPS/graphene oxide and 6-nitro-BIPS/N719 were synthesized by esterification reaction. Spectroscopic characterization confirms 6-nitro-BIPS is covalently attached to graphene oxide and N719 through ester linkages. Compared with the individual components and the mechanical mixtures, 6-nitro-BIPS/graphene oxide and 6-nitro-BIPS/N719 show improved nonlinear optical properties at 532 nm in nanosecond region, resulting from the photoinduced electrons and/or energy transfer. Moreover, these two composites exibit photoswitchable nonlinear optical behavior, which are therefore promising candidates for use in optoelectronic and photonic devices.

Photoswitchable chromic behavior of conjugated polymer films for reversible patterning and construction of a logic gate

Photoswitchable fluorescent films were constructed using a conjugated polymer as a fluorescence-color changeable unit and a photochromic molecule as a phototriggered, selectively energy-accepting component.

Dynamic Control of Full‐Colored Emission and Quenching of Photoresponsive Conjugated Polymers by Photostimuli

A series of photoresponsive and full‐colored fluorescent conjugated copolymers is synthesized by combining phenylene‐ and thienylene‐based main chains with photochromic dithienylethene (DE) side chains. Solutions and cast films of the polymers exhibit various colored fluorescence in visible wavelengths of 400−700 nm corresponding to emissions of the conjugated main chain. The fluorescence is reversibly photoswitched between emission and quenching through DE photoisomerization using external stimuli from ultraviolet and visible light irradiation. The reprecipitation method with ultrasonication enables the polymers to form spherical aggregates with diameters of 20−70 nm in water. After investigating and comparing the optical properties, the resulting nanosphere solutions are assumed to exist in an intermediate state between an isolated state (i.e., in solution) and an aggregated state in cast film. The majority of the nanosphere solutions also exhibit the same photoswitchable fluorescence behavior as those in the solutions and the cast films. The results demonstrate that the visible fluorescence of the conjugated copolymers is reversibly switchable between emission and quenching using the photoisomerizing DE side chain regardless of the fluorescent colors and the polymer chain aggregation.

Energy transfer on demand: photoswitch-directed behavior of metal-porphyrin frameworks.

In this paper, a photochromic diarylethene-based derivative that is coordinatively immobilized within an extended porphyrin framework is shown to maintain its photoswitchable behavior and to direct the photophysical properties of the host. In particular, emission of a framework composed of bis(5-pyridyl-2-methyl-3-thienyl)cyclopentene (BPMTC) and tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) ligands anchored by Zn(2+) ions can be altered as a function of incident light. We attribute the observed cyclic fluorescence behavior of the synthesized porphyrin-BPMTC array to activation of energy transfer (ET) pathways through BPMTC photoisomerization. Time-resolved photoluminescence measurements show a decrease in average porphyrin emission lifetime upon BPMTC insertion, consistent with an ET-based mechanism. These studies portend the possible utilization of photochromic ligands to direct chromophore behavior in large light-harvesting ensembles.

From {Au(I)···Au(I)}-coupled cages to the cage-built 2-D {Au(I)···Au(I)} arrays: Au(I)···Au(I) bonding interaction driven self-assembly and their Ag(I) sensing and photo-switchable behavior.

Metal-metal bonding interactions have been used to generate a number of unique supramolecular assemblies with fascinating functions. We presented here a new class of gold(I)-containing metallosupramolecular cages and cage-built two-dimensional (2-D) arrays of {Au8L2}n (n = 1 or ∞, L = tetrakis-dithiocarbamato-calix[4]arene, TDCC), 1-3, which are constructed from the self-assembly of deep-cavitand calix[4]arene-based supramolecular cages consisting of octanuclear Au(I) motifs. Synchrotron radiation X-ray diffraction structural analyses of 1-3 revealed their quadruple-stranded helicate dimeric cage structure and the presence of 2-D arrays of cages linked together by inter- and intramolecular Au(I)···Au(I) interactions. Electronic absorption and emission studies of complexes 1-3 indicated the occurrence of a programmable self-assembly process in a concentration-dependent stepwise manner with the links built via aurophilic interactions. These novel gold(I) supramolecular cages exhibited green phosphorescence and have been shown to serve as highly selective proof-of-concept luminescent sensors toward Ag(I) cation among various competitive transition-metal ions.

Photoswitching of a thermally unswitchable molecular magnet Cu(hfac)₂L(i-Pr) evidenced by steady-state and time-resolved electron paramagnetic resonance.

Most photoswitchable molecular magnets exhibit thermally induced switching, as is typical of spin crossover (SCO), valence tautomerism and SCO-like phenomena. We report a rare case of a copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) that does not exhibit quantitative SCO-like behavior in the temperature range of its chemical stability (2-350 K); however, it can be switched to a metastable thermally inaccessible spin state via visible/near-IR light at cryogenic temperatures. By means of photogeneration, unique information on this otherwise unobservable spin state has been obtained using steady-state Q-band (34 GHz) and time-resolved W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In particular, we have found that the electronic structure and relaxation properties of the photoinduced state in Cu(hfac)2L(i-Pr) are very similar to those in its sister compound Cu(hfac)2L(n-Pr) that is thermally switchable and has been exhaustively characterized by many analytical methods, previously. The first observation of photoswitchable behavior in a thermally unswitchable copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) paves the way for photoswitching applications of this and similar compounds in the remarkably broad temperature range of 2-350 K.

Reversibly photoswitchable dual-color (blue ↔ green) phosphorescence from β-cyclodextrin inclusion complex materials

We report herein two new supramolecular materials based on β-cyclodextrin inclusion complex samples with partially included organic radicals, which emit blue phosphorescence (excitation at 365 nm), and exhibit higher phosphorescent quantum yields (9.5 and 20.3%). However, purely organic radicals emit green phosphorescence, and possess low quantum yields (1.8 and 3.3%). One inclusion complex material exhibits the reversible dual-color (blue ↔ green) switching behavior in the response to ultraviolet and visible light stimuli, respectively: upon excitation with ultraviolet light (365 nm), it emits blue phosphorescence with quantum yield of 9.5%; however, it emits green phosphorescence and possesses higher quantum yield of 29.0%, upon irradiation by visible light (425 nm). The reversibly photoswitchable behavior is probably ascribed to the light-driven guest molecular conformation conversions in the β-cyclodextrin cavities. This light-responsive dual-color phosphorescent supramolecule is unique in terms of molecular conformation conversions.

Innovative semi-transparent nanocomposite films presenting photo-switchable behavior and leading to a reduction of the risk of infection under sunlight

Novel sputtered polyethylene–TiO2 (PE–TiO2) thin films induce fast bacterial inactivation with concomitant photo-switchable hydrophobic to hydrophilic transition under light. RF-plasma pretreatments allowed an increased TiO2 loading on PE, favorably affecting the photocatalyst performance. ATR-FTIR spectroscopy shows that the increase in the cell lipid-layer fluidity leads to cell wall scission/bacterial inactivation.

Design of a Photoswitchable Hollow Microcapsular Drug Delivery System by Using a Supramolecular Drug-Loading Approach

In this study, photoswitchable microcapsules were fabricated based on host-guest interactions between α-cyclodextrin (α-CD) and azobenzene (Azo). Carboxymethyl dextran-graft-α-CD (CMD-g-α-CD) and poly(acrylic acid) N-aminododecane p-azobenzeneaminosuccinic acid (PAA-C(12)-Azo) were assembled layer by layer on CaCO(3) particles. α-CD-rhodamine B (α-CD-RhB), used as a model drug, was loaded on PAA-C(12)-Azo layers by host-guest interaction. After removal of CaCO(3) particles by ethylenediaminetetraacetic acid (EDTA), hollow microcapsules loaded with α-CD-RhB were obtained. Since the interactions between α-CD and Azo were photosensitive, the capsules could be dissociated with the irradiation of UV light, followed by the release of the model drug, α-CD-RhB. Compared with traditional drug-loading approaches such as chemical bonding and physical adsorption, our supramolecular drug-loading system has a facile loading process, ideal bonding strength, and photoswitchable behavior. These photosensitive microcapsules exhibit great potential in biomedical applications.

Optically induced valence tautomeric interconversion in cobalt dioxolene complexes

The discovery that a number of cobalt-dioxolene complexes undergo photoswitchable behavior was reported in the recent past. This phenomenon is always associated with valence tautomeric interconversion processes involving low-spin cobalt(III) and high-spin cobalt(II) species. Herein is stressed the strong formal correlation existing between these processes and the LIESST (Light-Induced Excited Spin State Trapping) effect shown by several iron(II) complexes undergoing spin crossover interconversion. The dynamics of the relaxation of the photoinduced metastable species to the ground state is discussed in terms of non-adiabatic processes within the Jortner theory of radiationless multiphonon relaxation.