Reversible Photochemical Reaction Research Articles

Proximal Photocleavage: Controlling Polymer Solubility through Pathway Dependent Wavelength‐Orthogonal Photoreactions

AbstractCurrent families of reversible photochemical reactions present challenges for light‐controlled polymers of either photostationary states, which are common in photoinduced cycloaddition/cycloreversion reactions, or exclusively intramolecular bond changes, which characterize most photochromic units. In response to these challenges, here the concept of “proximal photocleavage” is presented, which combines photochemical crosslinking with a photocleavable linker, enabling a one‐time bond formation/cleavage sequence. Proximal photocleavage methacrylate monomers comprising, in series along the pendant of the methacrylate, a coumarin unit for crosslinking and either a phenacyl or ortho‐nitrobenzyl photocleavable group for decrosslinking are reported. The photophysical properties of these monomers and their statistical copolymers with methyl methacrylate are described, and wavelength selective crosslinking and de‐crosslinking of thin polymer films are demonstrated.

Photochemically assisted patterning: An interfacial hydrodynamic model perspective

Photoresponsive organic liquids and polymers may undergo reversible photochemical reactions with accompanying change in chemical structure upon exposure to a suitable wavelength of light. Spatial variations in chemical structure can cause surface tension gradients along thin films of such materials, resulting in Marangoni flows when the material is in its fluid state. Consequently the fluid film can self-organize into topographical patterns. Here, we provide a hydrodynamic model perspective of photochemically assisted patterning in thin fluid films using principles of momentum and mass transfer. Photochemical reaction occuring simultaneously along with hydrodynamic flow is modelled. Dynamical variation of the photoproduct concentration, accounting for first order chemical reaction kinetics, is considered. Our simulations highlight the counteracting effects of reaction kinetics and Marangoni flow as the mechanism responsible for pattern evolution. We capture various pillar and hole array morphologies obtained by controlling the direction of Marangoni flow and reaction-induced mass transfer. The resolution and timescales of pattern formation are computed as function of experimental control parameters such as the reaction rate coefficient (K0), Marangoni number (M), and distance between the film and light source. A process map comparing feature sizes on a photomask (w) with those of the patterns evolved in the material is developed. It reveals optimum w − M and w − K0 combinations required for faithful reproduction of photomask feature sizes and deviation from ideally templated patterns.

Bistable Organic Materials in Optoelectrical Switches: Two-Electrode Devices vs Organic Field Effect Transistors

We present a short overview of research into properties of organic materials and structures that could be used in optoelectrical switches, i.e., switches in which changes in electrical properties are triggered by light of appropriate wavelengths. In particular, we describe the structures acting by virtue of reversible photochemical reactions occurring in photochromic molecular materials.

Reversible ON/OFF and OFF/ON, light-stimulated binding, or release processes of metal cations from isatin diarylhydrazone complexes in solution

In this paper, we studied the light-stimulated switching between states with a free metal cation and its bound form in a complex with diarylhydrazones I and II. Isatin diarylhydrazones with unique functionality have been prepared. The uniqueness of the functionality is because, with the help of hydrazone complexes, or hydrazones themselves in the presence of metals, it is possible to programmatically change (reversibly decrease or increase) the concentration of metals in a mixture of organic solvents and water or organic solvents by modulating light of different wavelengths. The E-isomers of the studied hydrazones are thermodynamically more stable than their Z-isomers. E-isomers form complexes with metals Mn+ (n = 1,2) e.g. Cu2+, Zn2+, Cd2+, Ni2+, Co2+, Fe2+, Sn2+, Hg2+, Pb2+. The E-isomers of the complexes do not thermally isomerize. After absorption of light (λ ≤ 405 nm), photochemical isomerization of the ligand occured, and the complex disappeared simultaneously. The product of this photochemical reaction was the Z-isomer of the ligand and the metal cation. By irradiating the reaction mixture thus formed with light (λ = 465 nm), a reversible photochemical reaction takes place, in which a complex with a ligand in the E configuration is formed. The Z-isomers do not form complexes with these metals.

Accelerated formation of persistent holographic gratings based on one-directional electron transfer in KCl-Ag/Ta2O5 nanocomposites

Permanent preservation of data is essential for massive information recording. Combination of semiconductor with plasmonic nanoparticles has been applied in multicolor display and high-density optical storage. However, bidirectional electron transfer occurs at the Schottky interface under UVA irradiation, resulting in reversible photochemical reaction, information erasure, low recording efficiency and writing rate. To address these issues, a novel Schottky heterostructure of Ag/Ta2O5 modified with alkali halide is developed to realize photoinduced one-directional electron transfer from metal to semiconductor. The recorded information in such a medium of KCl-Ag/Ta2O5 presents excellent holographic storage stability even under exposure of a strong UVA ray (360 nm, 385 mW/cm2). Meanwhile, grating growth rate and efficiency are significantly enhanced by optimizing Ag particle distance and Cl− anion loading amount. This work provides an important strategy for fast and persistent data storage.

Reversible Photochemical Switching via Plasmonically Enhanced Upconversion Photoluminescence

AbstractPhotochromic molecule‐incorporated optical devices offer desirable properties for photocontrollable optical systems, including advanced optical data storage and super‐resolution imaging. However, these molecules require multiple illumination sources, such as UV and visible light, for reversible photochemical reactions, which restricts their potential for advanced application. This study reports an effective strategy for modulating photoisomerization via a single near‐infrared light source assisted by plasmonically enhanced photoswitchable upconversion photoluminescence (UCPL). The proposed quasi‐periodic metal nanostructures to facilitate the resonance modes in the broadband region enable the substitution of the detrimental high‐energy light source (i.e., UV light) with near‐infrared stimuli, which is associated with UCPL enhancement of over two orders with spectrum orthogonality. To validate this concept, the accelerated reversible‐photoisomerization kinetics is experimentally confirmed by three‐ and tenfold amplification of the PL intensities of the photochromic disulfonyldiarylethene derivatives. Further validation of the proposed strategy is performed using photodynamic imaging, which reveals accelerated photoisomerization, high photocyclization stability, and high spatial resolution.

Rational design and development of a lit-active photoswitchable inhibitor targeting CENP-E.

In the emerging field of photopharmacology, synthetic photoswitches based on reversible photochemical reactions are fused to bioactive molecules. Azobenzene derivatives, which can undergo trans-cis photoisomerization, are typical photoswitches. Most azobenzene-based photochemical tools are active in the thermodynamically stable trans, but not cis, form. cis-Active photochemical tools would be ideal because they can be "initially inactive and active after light illumination" in a reversible mode only by light illumination. However, only a few rational strategies for constructing such "lit-active" photopharmacological tools has been developed. Herein, we report a rationally designed lit-active photoswitchable inhibitor targeting centromere-associated protein E (CENP-E). Using the lit-active inhibitor, we were able to photoregulate CENP-E-dependent mitotic chromosome location in cells. This study provides a framework to facilitate further progress in the development of photopharmacological tools.

Spiro-conjugated indenodiarylethenes: enabling steric-induced electronic tuning of photochromic and photoluminescent properties by spiro-conjugation

Light-operated excited-state photochemical reactions and the resulting photochromic materials have gained significant attention for their promising technological applications in molecular electronic and photonic devices. To obtain photochromic materials (e.g., diarylethenes) with desired properties, the way to tune energy processes of the excited state is essentially important, from both fundamental and practical viewpoints. The modified diarylethenes mostly focused on structural improvements to achieve efficient photochromism. We report here two new spiro-conjugated indenodiarylethenes (named, SFI-Th and SFI-Sul) with an orthogonally arranged spiro π-scaffold. The spiro fluorene sterically constrains the free volume of photoactive side aromatic rings, while the spiro-conjugation points toward modulation of the decaying channels of excited-state energy, thus allowing for tuning photochromic performances. These spirodiarylethenes SFI-Th and SFI-Sul are readily accessible with low cost and show distinct photochromic and photoluminescent properties. On one hand, the SFI-Th bearing bisthienyl side rings, is an excellent photochromic switch showing high thermal stability and high fatigue resistance in both solution and amorphous state, likely benefited from steric-induced excited-state energy transfer into reversible photochemical reactions, owing to higher π-orbital spiro-conjugation between the two molecular halves. On the other hand, the SFI-Sul with bissulfonyl side rings, most notably, displays quantitative photocyclization to yield SFI-Sulc upon excitation. However, the ring-closed SFI-Sulc has an extremely slow photocycloreversion, and meanwhile immediately switches on emissive state with a strong fluorescence (Φf=0.57 in amorphous powder state). We study the modulated roles of spiro π-scaffold in both cases computationally and experimentally, and attribute the facilitated photochemical reaction to the positively electronic effects of spiro-conjugation, and the photo-responsive radiative transition to the negatively electronic effects of spiro-conjugation, respectively. In all, this work demonstrates that the spiro-conjugation is a new and efficient strategy to deliver excitation energy transfer (EET) for featured photochromic properties, due to its advantage in steric-induced electronic modulation.

A Photoresponsive Hydrogel with Enhanced Photoefficiency and the Decoupled Process of Light Activation and Shape Changing for Precise Geometric Control.

Traditional shape-morphing hydrogels rely on structural implementation of inhomogeneity inside the material during fabrication to realize predetermined complex shape change upon activation. In recent years, several systems with reprogrammable shape-morphing capabilities have been developed. Among those, the photoresponsive hydrogels offer the best spatial and temporal control. However, for most photoresponsive hydrogels, upon light irradiation, they simultaneously deform, which requires the projection of the light pattern to be continuously adaptive to the deforming gel. It is impractical for complex 3D morphing. In this paper, by incorporating two photodissociable molecules that can form a reactive ion couple upon light activation into one hydrogel, the light irradiation process is decoupled with the morphing process, and the consumption of the reactive ion couple drives the reversible photochemical reaction forward. Consequently, the photochemical reaction efficiency is improved, and the photoresponsive molecules are locked in the activated state until a recovery stimulus is applied. Based on the proposed general scheme, a specific example is given by incorporating the triphenylmethane leucohydroxide and 2-nitrobenzaldehyde molecules into a polyacrylamide hydrogel. The swelling behavior is characterized, and the reprogrammable morphing with precisely controlled geometry is demonstrated.

Correlative dual-alternating-color photoswitching fluorescence imaging and AFM enable ultrastructural analyses of complex structures with nanoscale resolution.

There is a practical motivation for correlating different types of microscopy for revealing complementary information of ultrastructures with resolution beyond the diffraction limit. The correlative microscopy strategy based on the combination of super-resolution fluorescence imaging with atomic force microscopy (AFM) is expected to provide both the specificity and three-dimensional structural information of nanomaterials. Herein we synthesized a dual-alternating-color photoswitchable fluorescent probe based on a naphthalimide-spiropyran dyad (NI-SP) and explored the capability of such correlative microscopy for visualizing nanostructures with complex structural hierarchy. NI-SP underwent reversible photoswitching between green and red fluorescence based on a reversible photochemical reaction and such reaction-linked correlation between two distinct types of fluorescence signals intrinsically enabled mutual authentication in super-resolution fluorescence imaging. Additionally, such correlative microscopy also demonstrated mutual complementation between different pieces of structural information of the target acquired via fluorescence imaging and AFM, respectively, in which the former reveals spatial distribution of fluorescent dyes in the nanoscale polymer fibroid micelles while the latter maps the topographical structure of the target with complex structural hierarchy. The results obtained in this work proclaimed that the combination of such correlative microscopy with our NI-SP probe is an effective modality for ultrastructural analysis and has future applications in various complex systems such as tissue/organ imaging.

The study of the elementary acts of a reversible photochemical reaction generating quasistationary radio emission in a magnetic field

Constant coherent radio frequency radiation in the course of a reversible photochemical reaction of a 8-hydroxy-6-dibenzofuran—deuteromethanol system was detected and investigated. The observed effect is one of the few examples of the generation of stationary radio emission during a photoreaction and may have a practical application. The elementary stages and the mechanism of these photoreactions are presented.

Transient Species Mediating Energy Transfer to Spin-Forbidden Mn d States in II–VI Semiconductor Quantum Dots

The nature of the transient species leading to emission from the spin/orbital-forbidden Mn d–d transition in doped semiconductor quantum dots has intrigued scientists for a long time. This understanding is important in the quest for energy efficiency as the energy from the conduction band is transferred efficiently to Mn in the femtosecond time scale overcoming other nonradiative recombination pathways. In this work, we have shown the presence of the transient species using materials with band gaps in resonance with the energy of the Mn emission to understand the nature of the absorbing, transient, and emitting species. Detailed studies lead to the emergence of a transient Mn3+ state that is further corroborated with spin-dependent density functional theory calculations. This opens up a unique opportunity to realize a reversible photochemical reaction and high radiative efficiency in a semiconductor nanostructure by controlling the spin state of the magnetic ion by external illumination.

The keto→enol photoisomerization of N‐salicilydenemethylfurylamine: Nonadiabatic ab initio dynamics simulation

AbstractThe photoinduced isomerization of cis‐keto and trans‐keto isomers in N‐salicilydenemethylfurylamine has been studied using the surface‐hopping approach at the CASSCF level of theory. After the cis‐keto or trans‐keto isomer is excited to S1 state, the molecule initially moves to a excited‐state local minimum. The torsional motion around relative bonds in the chain drives the molecule to approach a keto‐form conical intersection and then nonadiabatic transition occurs. According to our full‐dimensional dynamics simulations, the trans‐keto and enol photoproducts are responsible for the photochromic effect of cis‐keto isomer excited to S1 state, while no enol isomer was obtained in the photoisomerization of trans keto on excitation. The cis keto to enol and cis keto to trans keto isomerizations are reversible photochemical reactions. It is confirmed that this aromatic Schiff base is a potential molecular switch. Furthermore, the torsion of CN bond occurs in the radiationless decay of trans‐keto isomer, while it is completely suppressed by an intramolecular hydrogen bonding interaction in the dynamics of cis‐keto form. Moreover, the excited‐state lifetime of cis keto is longer than that of trans‐keto form due to the O···HN hydrogen bond.

UV-resistant holographic data storage in noble-metal/semiconductor nanocomposite films with electron-acceptors

Metal/semiconductor nanocomposite systems with the ability of controllable holographic storage are fascinating for advancing information technology. Ag/TiO2 nanocomposite films present multicolor photochromism, which plays a key role in high-density optical memory. However, the film undergoes a reversible photo-redox reaction by the alternating action of visible and UV lights, which weakens the optical stability of stored information. To date, no effective method has been proposed to hinder the UV-erasure in the film. In this paper, the transferring behavior of electrons in a Schottky junction between Ag and TiO2 is inhibited effectively by introducing electron acceptors into the photochromic film. Plasmonic photo-dissolution is enhanced greatly, which is in accordance with the theoretical fitting based on the reversible photo-chemical reaction. Holograms can be written efficiently with high stability even under the destructive UV-irradiation, which are expected to be applied in an environmentally-stable photo-device.

Wavelength-Selective Coupling and Decoupling of Polymer Chains via Reversible [2 + 2] Photocycloaddition of Styrylpyrene for Construction of Cytocompatible Photodynamic Hydrogels.

Reversible photocycloaddition reactions have previously been employed in chemical cross-linking for the preparation of biomaterial scaffolds. However, the processes require activation by high-energy UV light, rendering them unsuitable for modification in biological environments. Here we demonstrate that the [2 + 2] photocycloaddition of styrylpyrene can be activated by visible light at λ = 400-500 nm, enabling rapid and effective conjugation and cross-linking of poly(ethylene glycol) (PEG) in water and under mild irradiation conditions (I = 20 mW cm-2). Notably, the reversion of the cycloaddition can be triggered by low-energy UV light at λ = 340 nm, which allows for efficient cleavage of the dimer adduct. Using this wavelength-gated reversible photochemical reaction we are able to prepare PEG hydrogels and modulate their mechanical properties in a bidirectional manner. We also demonstrate healing of the fractured hydrogel by external light triggers. Furthermore, we show that human mesenchymal stem cells can be encapsulated within the gels with high viability post encapsulation. This photochemical approach is therefore anticipated to be highly useful in studies of cell mechanotransduction, with relevance to disease progression and tissue regeneration.

Triplet Ground-State-Bridged Photochemical Process: Understanding the Photoinduced Chiral Inversion at the Metal Center of [Ru(phen)2(l-ser)]+ and Its Bipy Analogues.

One of the main concerns in the photochemistry and photophysics of ruthenium complexes is the de-excitation of the triplet metal centered ligand-field state 3MC. To understand the mechanism by which the 3MC states in some reversible photochemical reactions could avoid the fate of fast decay and ligand dissociations, the photoinduced chiral inversion at the metal center of the complexes [Ru(diimine)2(l-ser)]+ (diimine = 1,10-phenanthroline or 2,2'-bipyridine, l-ser = l-serine) has been analyzed at the first principle level of theory. The calculated equilibrium constants and ECD curves for the photoinduced equilibrium mixtures are in agreement with the observed ones. The results showed that the reversible photochemical process Δ(δS) ⇌ Λ(δS) on the potential surface of the lowest triplet excited state proceeds in three steps: 3CTΔ ↔ 3MCΔ, 3MCΔ ↔ 3MCΛ, 3MCΛ ↔ 3CTΛ, where the first and the third steps involve mainly the elongation and compression of the octahedral core of the reactant Δ(δS) and product Λ(δS), respectively. The chiral inversion Δ ↔ Λ takes place in the second step through a much distorted square-pyramid-like transition state, and actually proceeds on the triplet ground state 3MC due to the crossover of the triplet T1 and singlet S0 states. Inspecting the transient structures at the crossing points, we found that they become less distorted and their lowest or imaginary-frequency displacement vectors in triplet state still dominate the reaction path, which makes the reaction reversible without ligand release. Thus, the triplet ground-state-bridged photoinduced mechanism offers a new angle of view to understand the related reversible photochemical reactions.

Flexible interlocked porous frameworks allow quantitative photoisomerization in a crystalline solid

Photochromic molecules have shown much promise as molecular components of stimuli-responsive materials, but despite recent achievements in various photoresponsive materials, quantitative conversion in photochemical reactions in solids is hampered by the lack of intrinsic structural flexibility available to release stress and strain upon photochemical events. This issue remains one of the challenges in developing solid-state photoresponsive materials. Here, we report a strategy to realize photoresponsive crystalline materials showing quantitative reversible photochemical reactions upon ultraviolet and visible light irradiation by introducing structural flexibility into crystalline porous frameworks with a twofold interpenetration composed of a diarylethene-based ligand. The structural flexibility of the porous framework enables highly efficient photochemical electrocyclization in a single-crystal-to-single-crystal manner. CO2 sorption on the porous crystal at 195 K is reversibly modulated by light irradiation, and coincident X-ray powder diffraction/sorption measurements clearly demonstrate the flexible nature of the twofold interpenetrated frameworks.

Luminescence and photochemical properties of hydrazones of aromatic aldehydes

Luminescence and photochemical properties found by us earlier for several hydrazones of aromatic aldehydes have been shown to be characteristic also of other compounds of this class. The structure of the product of reversible photochemical reaction of hydrazones of aromatic aldehydes is explained by specific interactions between the lone electron pairs of the two vicinal nitrogen atoms in the molecule of hydrazine. Compounds possessing photochromic properties have been suggested and prerequisites of application of luminescent hydrazones as luminophors have been formulated.

Recent Progress in Photoswitchable Supramolecular Self‐Assembling Systems

The rise of supramolecular chemistry has drawn great attention to the designed self‐assembly of synthetic molecules. Supramolecular self‐assembly has become an efficient strategy in constructing well‐ordered nanostructures using a bottom‐up way in material science and has also shown its great potential in biological applications. Photochromic compounds are small organic molecules which can undergo reversible photochemical reactions between two chemical species with distinct properties. They have been incorporated into various materials for applications including optical devices, data recording and storage, smart polymers, and so forth. Introducing photochromic compounds into supramolecular self‐assembling systems endows the supramolecular nanostructures or materials with intriguing responsive behavior to light, which can be conveniently orthogonal to other stimuli. From another perspective, the well‐ordered supramolecular structures or materials with complexity and stimuli‐responsive properties have the capability to “amplify” the light‐controlled isomerization by transforming it into other signals, thus producing more sophisticated functions. This review mainly highlights the recent advances and emerging fields in fabricating artificial supramolecular self‐assembling systems with photochromic compounds as light‐responsive units. Important design principles, mechanisms and concepts, as well as challenges and perspectives are illustrated and discussed.

Antiphase dual-color correlation in a reactant-product pair imparts ultrasensitivity in reaction-linked double-photoswitching fluorescence imaging.

A pair of reversible photochemical reactions correlates their reactant and product specifically, and such a correlation uniquely distinguishes their correlated signal from others that are not linked by this reversible reaction. Here a nanoparticle-shielded fluorophore is photodriven to undergo structural dynamics, alternating between a green-fluorescence state and a red-fluorescence state. As time elapses, the fluorophore can be in either state but not both at the same time. Thus, the red fluorescence is maximized while the green fluorescence is minimized and vice versa. Such an antiphase dual-color (AD) corelationship between the red and green fluorescence maxima as well as between their minima can be exploited to greatly improve the signal-to-noise ratio, thus enhancing the ultimate detection limit. Potential benefits of this correlation include elimination of all interferences originating from single-color dyes and signal amplification of AD photoswitching molecules by orders of magnitude.