Molecular Photoswitch Research Articles

Macroscopic Actuation of Bisazo Hydrogels Driven by Molecular Photoisomerization

Living organisms use the amplification of molecular motions over orders of magnitude to produce deformation, motion, and function on the macroscale. The design of artificial molecular machines that can mimic their biological counterparts in the production of such macroscopic actuation is of great interest. Here, we designed a polymerizable bisazobenzene-based molecular photoswitch that displays a highly directional geometrical transformation upon photoisomerization. When this photoswitch is incorporated into polymer networks as a cross-linker, its directional contraction and expansion drive the movement and rearrangement of polymer chains and amplifies these motions to the macroscale, resulting into a direct volume change and deformation of the bulk polymeric hydrogels. These photoswitchable hydrogels therefore mimic macroscopic biological motions such as muscular contraction and light-driven bending similar to phototropism. Furthermore, the use of this photoswitch to tune the macroscopic properties of hydrogels is in principle transferable to a variety of polymeric systems. This work develops a clear connection between molecular motion and macroscopic actuation, providing a platform to investigate this relationship further.

Molecular Cage with Dual Outputs of Photochromism and Luminescence Both in Solution and the Solid State.

The rational design of stimuli-responsive materials requires a deep understanding of the structure-activity relationship. Herein, we proposed an intramolecular conformation-locking strategy─incorporating flexible tetraphenylethylene (TPE) luminogens into the rigid scaffold of a molecular cage─to produce a molecular photoswitch with dual outputs of luminescence and photochromism in solution and in the solid states at once. The molecular cage scaffold, which restricts the intramolecular rotations of the TPE moiety, not only helps to preserve the luminescence of TPE in a dilute solution but facilitates the reversible photochromism on account of the intramolecular cyclization/cycloreversion reactions. Furthermore, we demonstrate assorted applications of this multiresponsive molecular cage, e.g., photo-switchable patterning, anticounterfeiting, and selective vapochromism sensing.

Solar–Thermal Fuels and the Role of Carbon Nanomaterials: A Perspective with Emphasis on the Azobenzene System

Molecular switches have long been investigated aiming at developing sustainable and renewable energy-based technologies. Among molecular photoswitches, azobenzene (AB) has been the quintessential template for solar–thermal fuels (STFs), acting as “molecular batteries”, storing and discharging energy by converting between its trans- and cis-isomers upon absorption of UV light. Nevertheless, key properties for device performance such as stored energy, energy density, stability of the “charged” metastable isomer, or the control of energy release are highly influenced by a mediating interface. This has motivated the study of metal and carbon nanomaterials as templates in hybrid STFs. The latter have been shown to be especially promising owing to their unique electronic properties, topological tailoring, and abundant sourcing. This review discusses recent advances that explore enhancements on several molecular photoswitch systems, with a special emphasis on the AB system, associated with changes in the molecular substituents and the development of hybrid systems, involving mainly carbon nanotemplates, such as reduced graphene oxide, graphene, carbon nanotubes, and fullerenes. The energy density, τ1/2, grafting density, and charge transfer of the AB and the hybrid adsorbed AB system on carbon nanomaterials are discussed. Key facets of the design of hybrid STF devices are discussed, along with a perspective on the aspects that merit further investigation toward the development of efficient STFs.

Different competition mechanism between ESPT and TICT process regulated by protic and aprotic solvent in DHP

In this study, we adopt density functional theory and time-dependent density functional theory (DFT/TDDFT) methods to compare and analyze the disparate competing mechanisms of excited state proton transfer (ESPT) and twisted intramolecular charge transfer (TICT) process of 2-(4′-diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b] pyridine (DHP) in protic and aprotic solvent. The results show that the DHP molecule undergoes excited state intramolecular proton transfer (ESIPT) in aprotic solvent ether. Differently, ESIPT is effectively inhibited in protic solvent methanol, whereas the coupling process of excited state double proton transfer (ESDPT) and TICT occurs. Combined with the analysis of potential energy surfaces, we clarify that DHP molecule in ether is more inclined to the channel of TICT → ESIPT → reverse TICT process to implement the ESIPT process. In DHP-MeOH complex, we confirm that the significant redshift phenomenon of fluorescence spectra observed in the experiment is attributed to the ESDPT induced TICT process. Our work offers a new avenue for switching ESPT and TICT competing mechanisms through protic and aprotic solvent, and considerably help the synthesis and development of highly efficient molecular photoswitch.

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.

Thermal and (Thermo-Reversible) Photochemical Cycloisomerization of 1H-2-Benzo[c]oxocins: From Synthetic Applications to the Development of a New T-Type Molecular Photoswitch.

A novel T-type molecular photoswitch based on the reversible cyclization of 1H-2-benzo[c]oxocins to dihydro-4H-cyclobuta[c]isochromenes has been developed. The switching mechanism involves a light-triggered ring-contraction of 8-membered 1H-2-benzo[c]oxocins to 4,6-fused O-heterocyclic dihydro-4H-cyclobuta[c]isochromene ring systems, with reversion back to the 1H-2-benzo[c]oxocin state accessible through heating. Both processes are unidirectional and proceed with good efficiency, with switching properties─including reversibility and half-life time─easily adjusted via structural functionalization. Our new molecular-switching platform exhibits independence from solvent polarity, originating from its neutral-charge switching mechanism, a property highly sought-after for biological applications. The photoinduced ring-contraction involves a [2+2] conjugated-diene cyclization that obeys the Woodward-Hoffmann rules. In contrast, the reverse process initiates via a thermal ring-opening (T > 60 °C) to produce the original 8-membered 1H-2-benzo[c]oxocins, which is thermally forbidden according to the Woodward-Hoffmann rules. The thermal ring-opening is likely to proceed via an ortho-quinodimethane (o-QDM) intermediate, and the corresponding switching mechanisms are supported by experimental observations and density functional theory calculations. Other transformations of 1H-2-benzo[c]oxocins were found upon altering reaction conditions: prolonged heating of the 1H-2-benzo[c]oxocins at a significantly elevated temperature (72 h at 120 °C), with the resulting dihydronaphthalenes formed via the o-QDM intermediate. These reactions also proceed with good chemoselectivities, providing new synthetic protocols for motifs found in several bioactive molecules, but are otherwise difficult to access.

Peptide-Based Low Molecular Weight Photosensitive Supramolecular Gelators.

Over the last couple of decades, stimuli-responsive supramolecular gels comprising synthetic short peptides as building blocks have been explored for various biological and material applications. Though a wide range of stimuli has been tested depending on the structure of the peptides, light as a stimulus has attracted extensive attention due to its non-invasive, non-contaminant, and remotely controllable nature, precise spatial and temporal resolution, and wavelength tunability. The integration of molecular photo-switch and low-molecular-weight synthetic peptides may thus provide access to supramolecular self-assembled systems, notably supramolecular gels, which may be used to create dynamic, light-responsive “smart” materials with a variety of structures and functions. This short review summarizes the recent advancement in the area of light-sensitive peptide gelation. At first, a glimpse of commonly used molecular photo-switches is given, followed by a detailed description of their incorporation into peptide sequences to design light-responsive peptide gels and the mechanism of their action. Finally, the challenges and future perspectives for developing next-generation photo-responsive gels and materials are outlined.

The invalidity of intermolecular proton transfer triggered twisted intramolecular charge transfer in excited state for 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine

AbstractThe excited‐state dynamics of the excited‐state proton transfer and intramolecular twisted charge transfer (TICT) reactions of a molecular photoswitch 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine (DHP) in aprotic and alcoholic solvents have been theoretically investigated by using time‐dependent density functional theory. The excited‐state intramolecular proton transfer (ESIPT) reaction of DHP proceeding upon excitation in all the solvents has been confirmed, and the dual emission has been assigned to the enol and keto forms of DHP. However, for methanol and ethanol solvents within strong hydrogen‐bonded capacity, the intermolecular hydrogen bonds between DHP and methanol/ethanol would promote an excited‐state double proton transfer (ESDPT) along the hydrogen‐bonded bridge. Importantly, the previous proposed ESDPT‐triggered TICT mechanism of DHP in methanol and ethanol was not supported by our calculations. The twist motion would increase the total energy of the system for both the products of ESIPT and ESDPT. According to the calculations of the transition states, the ESDPT reaction occurs much easier in keto form generated by ESIPT. Therefore, a sequential ESIPT and ESDPT mechanism of DHP in methanol and ethanol has been reasonably proposed.

Remote-Controlled Exchange Rates by Photoswitchable Internal Catalysis of Dynamic Covalent Bonds.

The transesterification rate of boronate esters with diols is tunable over 14 orders of magnitude. Rate acceleration is achieved by internal base catalysis, which lowers the barrier for proton transfer. Here we report a photoswitchable internal catalyst that tunes the rate of boronic ester/diol exchange over 4 orders of magnitude. We employed an acylhydrazone molecular photoswitch, which forms a thermally stable but photoreversible intramolecular H-bond, to gate the activity of the internal base catalyst in 8-quinoline boronic ester. The photoswitch is bidirectional and can be cycled repeatedly. The intramolecular H-bond is found to be essential to the design of this photoswitchable internal catalyst, as protonating the quinoline with external sources of acid has little effect on the exchange rate.

Visible Light-Driven Alkyne-Grafted Ethylene-Bridged Azobenzene Chromophores for Photothermal Utilization.

Molecular photoswitches are considered to be important candidates in the field of solar energy storage due to their sensitive and reversible bidirectional optical response. Nevertheless, it is still a daunting challenge to design a molecular photoswitch to improve the low solar spectrum utilization and quantum yields while achieving charging and discharging of heat without solvent assistance. Herein, a series of visible-light-driven ethylene-bridged azobenzene (b-Azo) chromophores with different alkyne substituents which can undergo isomerization reactions promoted in both directions by visible light are reported. Their visible light responsiveness improves their solar spectrum utilization while also having high quantum yields. In addition, as the compounds are liquids, there is no need to dissolve the compounds in order to exploit this switching. The photoisomerization of b-Azo can be adjusted by alkyne-related substituents, and hexyne-substituted b-Azo is able to store and release photothermal energy with a high density of 106.1 J·g−1, and can achieve a temperature increase of 1.8 °C at a low temperature of −1 °C.

Near‐Infrared Light‐Driven Three‐Dimensional Soft Photonic Crystals Loaded with Upconversion Nanoparticles

AbstractEndowing advanced functional three‐dimensional (3D) nanostructures with remote and dynamic controllability is currently in the spotlight of soft‐matter photonics research. Herein, the near‐infrared (NIR) light‐driven reversible and dynamic photonic bandgap modulation of a soft photonic crystal, i.e., blue phase liquid crystals (BPLCs) with 3D nanostructures, is demonstrated through the integration of judiciously designed core‐shell upconversion nanoparticles (UCNPs) and a novel axially‐chiral azobenzene‐based molecular photoswitch. The UCNPs exhibit high UV/vis emissions upon irradiations with 808 nm NIR light, and the intensity ratio of UV/vis luminescence can be facilely modulated through changing the excitation power. With the UCNPs as NIR‐to‐UV/vis light nanotransducers, the chiral molecular photoswitch shows reversible photoisomerization upon modulating the 808 nm NIR light intensity. Furthermore, NIR light‐driven BPLCs loaded with the photoswitch and UCNPs are demonstrated, in which the photonic reflection can red‐shift toward a longer wavelength under 808 nm light irradiation with high‐power density and the blue‐shift took place at lower power density with the same wavelength of light. This work may provide new insights into the large‐scale fabrication of 3D photonic nanostructures with tailorable functionalities toward emerging applications of soft‐matter photonics and future integrated communication technologies.

Molecular engineering of an inverse hexacopper(II) coordination complex with a photoactive metallacyclophane centroligand as prototype of a magnetic photoswitch

A primary goal of molecular spintronics is detecting the response of the spins from a single magnetic molecule to an external stimulus and, eventually, being able to implement logic quantum memory capabilities for information data storage and processing applications in quantum computation. Having this in mind, a novel hexanuclear oxamatocopper(II) complex of formula {[Cu2(naba)2][Cu(pmdien)]4}(ClO4)4·8H2O (2) [naba = N,N'-1,5-naphthalenebis(oxamate) and pmdien = N,N,N’,N”,N”-pentamethyldiethylenetriamine] has been prepared by the reaction of stoichiometric amounts of (nBu4N)4[Cu2(naba)2] · 4H2O (1) (nBu4N+ = tetra-n-butylammonium cation) and [Cu(pmdien)](ClO4)2 (1:4 molar ratio) in water. 2 exhibits a ladder-like “dimer-of-trinuclears” structure featuring a photoactive dicopper(II) naphthalenophane core acting as inverse coordination centre (centroligand) towards four peripheral [Cu(pmdien)]2+ groups through the outer amidate/carboxylate oxygen atoms of the oxamato fragments. 2 undergoes a unique magnetic photoswitching ON/OFF behaviour upon UV light irradiation because of an intramolecular (“pseudo-bimolecular”) photocycloaddition of the two facing naphthalene spacers through the more reactive carbon atoms at the 1,4 and 1’,4’ positions of the benzene rings to afford the putative [4 + 4] cycloaddition product c-2. Hence, the two oxamato-bridged S = 1/2 CuII3 linear units antiferromagnetically coupled within the CuII6 precursor (2, ON state) become magnetically uncoupled in the corresponding [4 + 4] photocycloaddition product (c-2, OFF state) [jeff = –2.13 (2) and 0 cm−1 (c-2); H = – jeffSA.SB with SA = SB = 1/2]. This new example of inverse coordination complex constitutes thus a potential candidate to molecular magnetic photoswitch in the “bottom-up” approach to nanometer-scale spintronic and quantum computing devices.

Light-driven self-assembly of cyanostilbene derivative with reversible chirality in aqueous media

In summary, we presented a design strategy for a chiral, amphiphilic, and photoswitchable molecule, using cholesterol, sulfonate, and cyanostilbene. The molecule could self-assemble to the trans-isomer in aqueous solution, which is accompanied by strong CD signals. Both trans- and cis-isomers are thermally stable, and the high conversion photoswitching between the trans- and cis-isomers with good fatigue resistance and reversibility, can be achieved using different wavelengths of UV irradiation. Furthermore, the optical chirality-CD was also modulated by UV irradiation. According to the TEM images and dynamic light scattering data, the photoswitching process involves self-assembly/disassembly from approximately 150 nm spherical particles to 18 nm irregular particles. We proposed a rational strategy using light-driven self-assembly for a molecular photoswitch in which optical chirality can be modulated by light irradiation in aqueous media.

Photoswitchable molecular tweezers: isomerization to control substrate binding, and what about vice versa?

The linkage of two identical binding motifs by a molecular photoswitch has proven to be a straightforward and versatile strategy to control substrate binding affinity by light. Stimulus control of binding properties in artificial receptors is partly inspired by the dynamic behavior of proteins and is highly attractive as it could, for example, improve extraction processes and allow (de)activation of membrane transport on demand. This feature article summarizes the development and design principles of molecular tweezers containing a molecular photoswitch as the core unit. Besides the control of binding affinity by isomerization, the effect of substrate binding on the isomerization behavior is discussed where data is available. While the latter often receives less attention, it could be of benefit in the future creation of multi-stimuli-controlled molecular switching and machine-like systems.

Nanoparticle and surfactant controlled switching between proton transfer and charge transfer reaction coordinates.

The reaction coordinates of a molecular photo-switch 2-(4'-diethylamino-2'-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP) was tuned with a nanoparticle and surfactant. DHP undergoes excited state intramolecular proton transfer (ESIPT) and emits normal and tautomer emissions in N,N-dimethylformamide. Silver nanoparticles suppress the ESIPT and induce twisted intramolecular charge transfer (TICT). Further addition of surfactants alters the process. Interestingly, different surfactants cause different effects. Accordingly, the luminescence characteristics are altered. The anionic surfactant sodium dodecyl sulfate (SDS) restores the ESIPT process by completely detaching the molecule from the nanoparticle. The nonionic surfactant Triton X-100 (TX-100), at lower concentration, enhances the TICT emission and the ESIPT process is also observed due to the release of some fluorophore from the nanoparticle complex. But at higher concentration the fluorophores are released completely and the ESIPT process is restored. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB), at lower concentration, simply restores the ESIPT process by releasing the fluorophore. But at higher CTAB concentration, DHP enters the metalparticle-CTAB aggregate and shows enhanced ESIPT.

Photo-Responsive Self-Assembly of Plasmonic Magnetic Janus Nanoparticles.

Stimuli-responsive self-assembly of nanoparticles is a versatile approach for the bottom-up fabrication of adaptive and functional nanomaterials. For this purpose, anisotropic building blocks are of particular importance due to the unique shapes and structures that can be obtained upon self-assembly. Here, we demonstrate the photo-responsive self-assembly of plasmonic magnetic "dumbbell" Janus nanoparticles (Au-Fe3O4) via the host-guest interaction of the supramolecular host cyclodextrin and the molecular photoswitch arylazopyrazole. We developed efficient ligand exchange procedures that enable the introduction of functional ligands, respectively, to the surface of the gold or magnetite core of the dumbbell. Our results indicate that distinct nanoparticle superstructures arise in aqueous solutions if nanoparticle aggregation is crosslinker-induced or self-induced and that the reversible formation and fragmentation of the superstructures can be modulated with light.

Overtemperature-protection intelligent molecular chiroptical photoswitches

Stimuli-responsive intelligent molecular machines/devices are of current research interest due to their potential application in minimized devices. Constructing molecular machines/devices capable of accomplishing complex missions is challenging, demanding coalescence of various functions into one molecule. Here we report the construction of intelligent molecular chiroptical photoswitches based on azobenzene-fused bicyclic pillar[n]arene derivatives, which we defined as molecular universal joints (MUJs). The Z/E photoisomerization of the azobenzene moiety of MUJs induces rolling in/out conformational switching of the azobenzene-bearing side-ring and consequently leads to planar chirality switching of MUJs. Meanwhile, temperature variation was demonstrated to also cause conformational/chiroptical inversion due to the significant entropy change during the ring-flipping. As a result, photo-induced chiroptical switching could be prohibited when the temperature exceeded an upper limit, demonstrating an intelligent molecular photoswitch having over-temperature protection function, which is in stark contrast to the low-temperature-gating effect commonly encountered.

Photoisomerization Efficiency of a Solar Thermal Fuel in the Strong Coupling Regime

AbstractStrong exciton‐photon coupling is achieved when the interaction between molecules and an electromagnetic field is increased to a level where they cannot be treated as separate systems. This leads to the formation of polaritonic states and an effective rearrangement of the potential energy surfaces, which opens the possibility to modify photochemical reactions. This work investigates how the strong coupling regime is affecting the photoisomerization efficiency and thermal backconversion of a norbornadiene–quadricyclane molecular photoswitch. The quantum yield of photoisomerization shows both an excitation wavelength and exciton/photon constitution dependence. The polariton‐induced decay and energy transfer processes are discussed to be the reason for this finding. Furthermore, the thermal back conversion of the system is unperturbed and the lower polariton effectively shifts the absorption onset to lower energies. The reason for the unperturbed thermal backconversion is that it occurs on the ground state, which is unaffected. This work demonstrates how strong coupling can change material properties towards higher efficiencies in applications. Importantly, the experiments illustrate that strong coupling can be used to optimize the absorption onset of the molecular photoswitch norbonadiene without affecting the back reaction from the uncoupled quadricyclane.

Light-Driven Switching between Intramolecular Proton-Transfer and Charge-Transfer States.

A molecular photoswitch, 2-(4'-diethylamino-2'-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP), with mutually independent paths of excited-state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) was developed. Control over these processes was attained by switching the solvents. Depending on the solvent's hydrogen-bond capacity and polarity, either one of the photoprocesses (ESIPT or TICT) or both can be triggered. Accordingly, normal and tautomer emissions, normal and TICT emissions, or triple emission of normal, tautomer, and TICT were obtained from the molecule. The emissions were resolved by fluorescence lifetime. The conclusions were established by synthesizing and studying the methoxy derivative of the molecule.

Fluidic Platforms Incorporating Photo-Responsive Soft-Polymers Based on Spiropyran: From Green Synthesis to Precision Flow Control

In this paper, we describe how to create simple fluidic systems incorporating soft polymer actuator valves, that can provide highly precise control of flow rates in fluidic channels as an example of a 4D-materials based platform. The particular approach we describe employs photoresponsive gels that swell/contract via a light stimulus, enabling flow behavior to be controlled from outside the fluidic platform in a completely remote and non-contact manner. An improved synthesis of the spiropyran molecular photoswitch that delivers high yields (77%) using scalable green chemistry is described, along with details on how to build the valve structures in custom designed sites within the fluidic system. Fabrication of a demonstrator fluidic system incorporating up to four valves is described, along with electronics and in-house developed PID control software for achieving precise control of flow in the channels using LEDs. The resulting system demonstrates an innovative approach to microfluidics that offers scalability in terms of the number of polymer actuators along with wide variability of actuator form and function.