Photoswitchable Molecules Research Articles

Mechanistic Insights into the Photoisomerization of N,N'-Disubstituted Indigos.

N,N′‐disubstituted indigos are photoswitchable molecules that have recently caught the attention due to their addressability by red‐light. When alkyl and aryl groups are utilized as the N‐substituents, the thermal half‐lives of Z isomers can be tuned independently while maintaining the advantageous red‐shifted absorption spectra. To utilize these molecules in real‐world applications, it is of immense importance to understand how their molecular structures as well as the environment influence their switching properties. To this end, we probed their photoisomerization mechanism by carrying out photophysical and computational studies in solvents of different polarities. The fluorescence and transient absorption experiments suggest for more polar excited and transition states, which explains the bathochromic shifts of absorption spectra and shorter thermal half‐lives. On the other hand, the quantum chemical calculations reveal that in contrast to N‐carbonyl groups, N‐alkyl and N‐aryl substituents are not strongly conjugated with the indigo chromophore and can thus serve as a tool for tuning the thermal stability of Z isomers. Both approaches are combined to provide in‐depth understandings of how indigos undergo photoswitching as well as how they are influenced by N‐substituent and the chemical surroundings. These mechanistic insights will serve as guiding principles for designing molecules eyeing broader applications.

Reversibly Photoswitching Upconversion Nanoparticles for Super‐Sensitive Photoacoustic Molecular Imaging

AbstractPhotoacoustic (PA) imaging uses light excitation to generate the acoustic signal for detection and improves tissue penetration depth and spatial resolution in the clinically relevant depth of living subjects. However, strong background signals from blood and pigments have significantly compromised the sensitivity of PA imaging with exogenous contrast agents. Here we report a nanoparticle‐based probe design that uses light to reversibly modulate the PA emission to enable photoacoustic photoswitching imaging (PAPSI) in living mice. Such a nanoprobe is built with upconverting nanocrystals and photoswitchable small molecules and can be switched on by NIR light through upconversion to UV energy. Reversibly photoswitching of the nanoprobe reliably removed strong tissue background, increased the contrast‐to‐noise ratio, and thus improved imaging sensitivity. We have shown that PAPSI can image 0.05 nM of the nanoprobe in hemoglobin solutions and 104 labeled cancer cells after implantation in living mice using a commercial PA imager.

Dynamic Wetting of Photoresponsive Arylazopyrazole Monolayers is Controlled by the Molecular Kinetics of the Monolayer.

Smart surfaces that can change their wettability on demand are interesting for applications such as self-cleaning surfaces or lab-on-a-chip devices. We have synthesized arylazopyrazole (AAP) phosphonic acids as a new class of photoswitchable molecules for functionalization of aluminum oxide surfaces. AAP monolayers were deposited on α-Al2O3(0001) and showed reversible E/Z photoswitching that can trigger contact angle changes of up to ∼10°. We monitored these changes on the macroscopic level by recording the contact angle while the monolayer was switched in situ. On the molecular level, time-dependent vibrational sum-frequency generation (SFG) spectroscopy provided information on the kinetic changes within the AAP monolayer and the characteristic times for E/Z switching. In addition, vibrational SFG at different relative humidity indicates that the thermal stability of the Z configuration is largely influenced by the presence of water which can stabilize the Z state and hinder E → Z switching of the AAP monolayer when it is wetted with H2O. Having established the switching times on the molecular scale, we additionally measured the dynamic contact angle and show that the time scales of the substrate and droplet dynamics can be extracted individually. For that, we report on a relaxation model that is solved analytically and is verified via a comparison with simulations of a Lennard-Jones system and with experimental data. The slower E to Z switching in the presence of the droplet as compared to the vapor phase is rationalized in terms of specific interactions of water with the exposed AAP moieties.

Reversibly Photoswitching Upconversion Nanoparticles for Super-Sensitive Photoacoustic Molecular Imaging.

Photoacoustic (PA) imaging uses light excitation to generate the acoustic signal for detection and improves tissue penetration depth and spatial resolution in the clinically relevant depth of living subjects. However, strong background signals from blood and pigments have significantly compromised the sensitivity of PA imaging with exogenous contrast agents. Here we report a nanoparticle-based probe design that uses light to reversibly modulate the PA emission to enable photoacoustic photoswitching imaging (PAPSI) in living mice. Such a nanoprobe is built with upconverting nanocrystals and photoswitchable small molecules and can be switched on by NIR light through upconversion to UV energy. Reversibly photoswitching of the nanoprobe reliably removed strong tissue background, increased the contrast-to-noise ratio, and thus improved imaging sensitivity. We have shown that PAPSI can image 0.05 nM of the nanoprobe in hemoglobin solutions and 104 labeled cancer cells after implantation in living mice using a commercial PA imager.

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.

Terphenylthiazole-based self-assembled monolayers on cobalt with high conductance photo-switching ratio for spintronics.

Two new photo-switchable terphenylthiazole molecules are synthesized and self-assembled as monolayers on Au and on ferromagnetic Co electrodes. The electron transport properties probed by conductive atomic force microscopy in ultra-high vacuum reveal a larger conductance of the light-induced closed (c) form than for the open (o) form. We report an unprecedented conductance ratio of up to 380 between the closed and open forms on Co for the molecule with the anchoring group (thiol) on the side of the two N atoms of the thiazole unit. This result is rationalized by Density Functional Theory (DFT) calculations coupled to the Non-Equilibrium Green's function (NEGF) formalism. These calculations show that the high conductance in the closed form is due to a strong electronic coupling between the terphenylthiazole molecules and the Co electrode that manifests by a resonant transmission peak at the Fermi energy of the Co electrode with a large broadening. This behavior is not observed for the same molecules self-assembled on gold electrodes. These high conductance ratios make these Co-based molecular junctions attractive candidates to develop and study switchable molecular spintronic devices.

Stability improvement of MAPbI3-based perovskite solar cells using a photoactive solid-solid phase change material

In this work, to increase the optical and thermal stability of perovskite solar cells, the composition of the perovskite layer is engineered by adding azobenzene (AZO) as a photoswitchable organic molecule. In this regard, solar cells with the FTO/b-TiO2/m-TiO2/CH3NH3PbI3/HTM/Au structure are fabricated using spiro-OMETAD hole transporting layer. Remarkably, an improvement of the optical, thermal, and structural stability of the devices comprising 5%, 10%, and 20% AZO is observed. Through the solid-solid phase-change mechanism of AZO, harmful UV radiation is absorbed and leads to photoisomerization between the trans and cis isomers, thus aiding in the management of thermal stresses on the device. Devices with pure perovskite absorber layer and perovskite absorber layer containing 10 wt% AZO retained 43% and 70% of their initial performances, respectively, after 70 min of exposure to sunlight. Furthermore, after 1440 h of storage in ambient conditions (25 ℃ and 42% relative humidity), the reference device maintains 35% of its initial performance while the device containing 10 wt% AZO retains 89% of its initial performance. In the case of thermal stability, the device containing 10% AZO shows superior thermal stability by keeping about 55% of its initial efficiency after exposure to a temperature of 85 ℃ and one sun illumination, simultaneously, for 60 min, compared to the reference device which retains only 35% of its performance under the same condition.

Preparation and application of multi-wavelength-regulated multi-state photoswitchable fluorescent polymer nanoparticles

The photoswitchable system with color and multi-state fluorescence changes is the key to the realization of advanced multi-level information storage, and its application potential has far exceeded the monochromatic fluorescent system. Herein, we introduced a novel multi-state photoswitchable fluorescent polymeric nanoparticles, which contains two energy-matched photoswitchable fluorescent molecules, i.e. diarylethene derivative (BH, donor) and spiropyran derivative (SPMA, acceptor), to construct a tunable fluorescence resonance energy transfer (FRET) system. By optimizing the feeding ratio of the two photochromic molecules to control the efficiency of energy transfer, the fluorescence of the nanoparticles can be reversibly switched among none, red, orange, and green by adjusting the wavelength of light stimuli. In addition, the nanoparticles display good water dispersibility, high brightness, fast responsiveness, excellent photoreversibility, and long-term fluorescence stability. We also verified that the system has great potential in optical data storage, multi-layer information encryption, and anti-counterfeiting.

Synthesis and Characterization of Thin Films Based on Azobenzene Derivative Anchored to CeO2 Nanoparticle for Photoswitching Applications

Azobenzene has attracted substantial attention as a photoswitchable molecule since its applications range from energy and data storage to biomedical applications. This work reports a new type of thin-film based on azobenzene derivative anchored to cerium oxide nanoparticles CeO2 NPs for photoswitching applications. The trans-cis isomerization and reverse isomerization occur by UV-light exposure and thermal relaxation process, respectively. The photoisomerization and reverse isomerization kinetics for CeO2 NPs-MR thin films are studied, investigated, and analyzed using UV-Vis absorbance spectra, FTIR spectroscopy, XRD, and scanning electron microscopy (SEM), in addition to differential scanning calorimetry (DSC) measurement to study the energy storage capacity. The results found that anchoring azobenzene to CeO2 NPs is successful in multisource storage of solar energy applications.

(Invited) Tailoring of Single-Walled Carbon Nanotube Luminescence as Photoswitchable Near-Infrared Emitters

Single-molecule localization microscopy (SMLM) has set a new paradigm in the field of optical imaging by delivering super-resolution images i.e. with resolution much better than the diffraction limit. We demonstrated over the last years that such approaches can be designed to understand basic excitonic processes in carbon nanotubes [1-2]. In the field of bioimaging, SMLM is currently limited to the visible or far-red range, missing the near-infrared region where biological tissues are however the most transparent. One reason for this is that single-molecule emitter optical properties still need to be tailored in the near-infrared, including photoswitching capabilities which is a basic ingredient to achieve single-molecule SMLM. To fill this gap, optical imaging of single-wall carbon nanotubes can first be greatly facilitated in brain tissues by covalent functionalization of luminescent defects, reducing the required excitation light intensity by one order of magnitude [3]. In addition, we have introduced a novel type of hybrid nanomaterials consisting of single-wall carbon nanotubes covalently functionalized with photo-switching molecules that are used to control the intrinsic luminescence of the single nanotubes in the near-infrared (beyond 1 µm) [4]. Through the control of photoswitching, we demonstrate super-localization imaging of nanotubes unresolved by diffraction limited microscopy opening the route toward SMLM in the near-infrared for biological applications. Photocontrol of individual near-infrared emitters will also be highly desirable for elementary optical molecular switches or information storage elements since most communication data transfer protocols are established in this spectral range.

Thioaurones: Recent Advances in Synthesis, Reactivity, and Biological Activity

Thioaurones are known for more than one hundred years; however, they are relatively less studied. In some way, due to their appealing as a candidate of photoswitchable molecules, they have led to generating considerable interest in the last years. In this literature survey, we report and discuss the recent advancements made in the synthesis, reactivity and applications of the thioaurone derivatives. To a great extent, the review summarizes published works from different databases since 2005.

"Photo-Rimonabant": Synthesis and Biological Evaluation of Novel Photoswitchable Molecules Derived from Rimonabant Lead to a Highly Selective and Nanomolar "Cis-On" CB1R Antagonist.

Human cannabinoid receptor type 1 (hCB1R) plays important roles in the regulation of appetite and development of addictive behaviors. Herein, we describe the design, synthesis, photocharacterization, molecular docking, and in vitro characterization of "photo-rimonabant", i.e., azo-derivatives of the selective hCB1R antagonist SR1411716A (rimonabant). By applying azo-extension strategies, we yielded compound 16a, which shows marked affinity for CB1R (Ki(cisform) = 29 nM), whose potency increases by illumination with ultraviolet light (CB1R Kitrans/cis ratio = 15.3). Through radioligand binding, calcium mobilization, and cell luminescence assays, we established that 16a is highly selective for hCB1R over hCB2R. These selective antagonists can be valuable molecular tools for optical modulation of CBRs and better understanding of disorders associated with the endocannabinoid system.

Molecular photoswitches in aqueous environments.

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Photoswitchable Metal-Organic Framework Thin Films: From Spectroscopy to Remote-Controllable Membrane Separation and Switchable Conduction.

The preparation of functional materials from photoswitchable molecules where the molecular changes multiply to macroscopic effects presents a great challenge in material science. An attractive approach is the incorporation of the photoswitches in nanoporous, crystalline metal-organic frameworks, MOFs, often showing remote-controllable chemical and physical properties. Because of the short light-penetration depth, thin MOF films are particularly interesting, allowing the entire illumination of the material. In the present progress report, we review and discuss the status of photoswitchable MOF films. These films may serve as model systems for quantifying the isomer switching yield by infrared and UV-vis spectroscopy as well as for uptake experiments exploring the switching effects on the host-guest interaction, especially on guest adsorption and diffusion. In addition, the straightforward device integration facilitates various experiments. In this way, unique features were demonstrated, such as photoswitchable membrane separation with continuously tunable selectivity, light-switchable proton conductivity of the guests in the pores, and remote-controllable electronic conduction.

Selective incorporation of one of the isomers of a photoswitchable molecule in wormlike micelles

Understanding the physical and chemical mechanisms underlying the interactions between giant micelles and photoswitchable molecules is essential because it is not evident why some photoswitchable chromophores with apparently good potential to modify micellar systems' macroscopic properties have a poor performance on UV-irradiation. We report a mechanism that describes the trans-isomer's expulsion embedded in the wormlike micelles when it is transformed into the cis-isomer on UV-irradiation. Consequently, a small change in the rheological properties of the micellar solution is observed on UV-irradiation. As a model system, 4-(phenylazo)benzoate ion as the photoresponsive chromophore and giant cylindrical micelles made of N-Tetradecyl-N,N-Dimethyl-3-ammonio-1-PropaneSulfonate, and Sodium Dodecyl Sulfate, in saline water, are used. The trans-isomer chromophore incorporates into the micelles modifying the solution viscosity and viscoelastic spectra, which follows the Maxwell model at low and intermediate frequencies. However, the effect of UV-irradiation on rheological properties is not significant. Micro-rheological experiments are performed with diffusive wave spectroscopy to obtain high-frequency viscoelastic spectra to estimate the characteristic length scales of the wormlike micellar network with the embedded chromophore to understand its rheological behavior better. Detailed interaction between the chromophore and the micelles is obtained with 1H-NMR and 1H DOSY NMR experiments to determine the chromophore conformation into the micelles and diffusivity of the isomers.

Visible light and temperature dual-responsive microgels by crosslinking of spiropyran modified prepolymers

HypothesisLight-responsive microgels are interesting colloidal systems with potential applications in the biotechnology and medicine. However, synthesis of light-responsive microgels with high loading of photoswitchable molecules is still very challenging. ExperimentsHerein we developed a new method to synthesize light and temperature dual-responsive spiropyran-modified poly(N-vinylcaprolactam) microgels. The novel and straightforward microgels synthesis route involved: a) synthesis of poly(N-vinylcaprolactam-co-vinylformamide) copolymers via RAFT polymerization followed by the hydrolysis to obtain primary amine groups, b) attachment of carboxyl-modified spiropyran molecules to polymer chains via coupling, and c) crosslinking of spiropyran-modified polymer chains in W/O miniemulsion to form microgels. FindingsVia this method, we successfully synthesized poly(N-vinylcaprolactam) microgels containing more than 10 mol% spiropyran. The reversible light responsiveness of the spiropyran-modified copolymers and microgels in aqueous solution, which originates from the spiropyran photoisomerization under irradiation with different wavelengths, was demonstrated by UV–Vis spectroscopy. Spiropyran-modified copolymers demonstrate shift of the lower critical solution temperature (LCST) due to the polarity change of spiropyran molecules under dark, UV and visible light. Surprisingly, dynamic light scattering (DLS) results show that the microgels based on the same copolymers are less affected by UV irradiation. Microgels are swollen in darkness when spiropyran molecules are in the polar, merocyanine form, and collapse after irradiation with visible light, due to the transformation of spiropyran to the relatively nonpolar, closed spirocyclic form. In addition, the spiropyran-modified microgels exhibit reversible temperature responsiveness by presenting a volume phase transition in water from a swollen state to a collapsed state with increasing temperature and the transition temperature decreased compared to the pristine microgels due to the hydrophobicity of spiropyran units.

Photochromic Polymers Based on Fluorophenyl Oxime Ester Photoinitiators as Photoswitchable Molecules

We designed and synthesized two novel fluorophenyl oxime ester photoinitiators (E-FBOXEs) with the ability to prepare photochromic polymers, and then photochromic polymers based on E-FBOXEs were pr...

Modulation of porosity in a solid material enabled by bulk photoisomerization of an overcrowded alkene.

The incorporation of photoswitchable molecules into solid-state materials holds promise for the fabrication of responsive materials, the properties of which can be controlled on-demand. However, the possible applications of these materials are limited due to the restrictions imposed by the solid-state environment on the incorporated photoswitches, which render the photoisomerization inefficient. Here we present responsive porous switchable framework materials based on a bistable chiroptical overcrowded alkene incorporated in the backbone of a rigid aromatic framework. As a consequence of the high intrinsic porosity, the resulting framework readily responds to a light stimulus, as demonstrated by solid-state Raman and reflectance spectroscopies. Solid-state 13C NMR spectroscopy highlights an efficient and quantitative bulk photoisomerization of the incorporated light-responsive overcrowded olefins in the solid material. Taking advantage of the quantitative photoisomerization, the porosity of the framework and the consequent gas adsorption can be reversibly modulated in response to light and heat.

Hexaarylbiimidazoles(HABI)-functionalized lyotropic liquid crystalline systems as visible light-responsive materials

Hexaarylbiimidazoles (HABIs) are a promising class of photoswitchable molecule that have received little attention in the literature. Among them, (2,2′-dimethoxydiphenylimidazole)-1,1′-binaphthyl (HABI1) displays unusual negative photochromism and is responsive to green light. This study investigates the potential of HABIs to serve as photo-responsive actuators controlling the structure of lyotropic liquid crystalline (LLC) materials. HABI1 with four methyl chains and HABI2 with four dodecyl chains were synthesized. Time resolved small angle X-ray scattering was used to characterize the potential disruptive effects of HABIs on the nanostructure of LLC systems. HABIs underwent rapid isomerization under irradiation, with a very slow reversion in the dark in toluene and in the LLC matrix, demonstrating excellent stability and photo-fatigue resistant. HABIs completely triggered phase transitions in the phytantriol-based materials, and HABI2 generated a greater disruption than HABI1 on the lipid packing due to the enhanced steric influence. Tuning the lipid composition yielded systems that transitioned from a “slow release” lamellar phase to a “burst release” bicontinuous cubic phase upon light irradiation. Such systems therefore may exhibit a triggered release behavior upon a short time of irradiation, showing great potential in “on demand” drug delivery.

Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes.

Operating photoswitchable molecules repetitively and reliably is crucial for most of their applications, in particular in (opto)electronic devices, and related to reversibility and fatigue resistance, which both critically depend on the photoisomerization mechanism defined by the substitution pattern. Two diarylethene photoswitches bearing biacetyl triplet sensitizers either at the periphery or at the core were investigated using both stationary as well as transient UV/Vis absorption spectroscopy ranging from the femtosecond to the microsecond time scale. The diarylethene with two biacetyl moieties at the periphery is switching predominantly from the triplet excited state, giving rise to an enhanced fatigue resistance. In contrast, the diarylethene bearing one diketone at the photoreactive inner carbon atom cyclizes from the singlet excited state and shows significantly higher quantum yields for both cyclization and cycloreversion.