Thermodynamics and kinetics of protonated merocyanine photoacids in water

Harnessing Negative Photochromism in Styryl Cyanines for Light-Modulated Proton Transport

Efficient photoswitches capable of complete conversion to their metastable isomer are not so common, yet highly desirable for applications in smart materials and devices. Here, we report the photochromic behavior of a series of styryl cyanine photoswitches, all demonstrating high switching efficiency, with some achieving full conversion to the metastable form. Despite structural similarities to spiropyran photoswitches, we demonstrate that the photochemistry of styryl cyanines is fundamentally different. Unlike classical photoswitches that rely on double-bond rotation, these molecules undergo substantial geometric changes via the formation of a spiro carbon. This transformation disrupts conjugation, causing a desirable blue shift in absorbance ideal for creating responsive materials and devices. We further show that the switching kinetics can be finely tuned through electronic effects of various substituents or the choice of surrounding medium. These photoswitches exhibit excellent fatigue resistance and can be easily shifted into the visible region via their acidochromic properties. Taking advantage of their high switching efficiency and affinity for acidic polymers, we finally propose their use as smart dopants to develop light-responsive materials with tunable proton transport properties under visible light irradiation.

All-in-One Photoacid Generators with Green/Red-light Responsiveness and Cooperative Functionality

Photoacid generators (PAGs) are invaluable molecular tools that exhibited tremendous potential in emerging interdisciplinary researches of life-science, nanotechnology and smart materials. However, current PAGs are primarily mono-functional in terms of acid generation and rely on UV/deep-blue light excitation, posing a fundamental hurdle to their broader adoption. Developing cooperatively functioned PAGs with long-wavelength light responsiveness presents a formidable challenge due to the absence of suitable molecular scaffolds. Here, we introduce a newly-developed perylene bisimides PAG motif (PBI-PAG) that integrates desired multi-functionality and visible-light photo-reactivity. Taking advantages of characteristic opto-electronic properties of PBI scaffold, PBI-PAGs are capable of quantitative releasing (>99 %) a palette of acids upon green/red light (560-605 nm) excitation. Concurrently, a photo-generated counterpart is functioned as a photo-sensitizer that could perform cooperatively with acid as an anti-metastasis cancer therapy agent. These two processes constitute the first example of a cooperatively functioned PAG operated at substrate-adaptive wavelengths.

Responsive Surfactant-Driven Morphology Transformation of Block Copolymer Microparticles

Block copolymer (BCP) microparticles, which exhibit rapid change of morphology and physicochemical property in response to external stimuli, represent a promising avenue for the development of programmable smart materials. Among the methods available for generating BCP microparticles with adjustable morphologies, the confined assembly of BCPs within emulsions has emerged as a particularly facile and versatile approach. This review provides a comprehensive overview of the role of responsive surfactants in modulating interfacial interactions at the oil-water interface, which facilitates controlled BCP microparticle morphology. We elucidate how variations in the properties of responsive surfactants, activated by external stimuli, influence BCP chain arrangement and interfacial selectivity. Additionally, this review explores the applications of shape-switchable microparticles in advanced technologies such as smart display, fluorescence modulation, magnetic resonance imaging, drug delivery, and photonic crystal. Finally, the challenges and prospective future directions in this rapidly evolving field are discussed.

A novel metastable-state photoacid for reversible protonation of strong bases

Metastable-state photoacid (mPAH) has become a common tool for controlling and driving chemical processes with light. However, previously developed mPAHs could not be used for reversible protonation of strong bases, including many common amines and heterocycles. In this work, we developed a novel mPAH using benzimidazole as the structural moiety with the active proton. The very low dark acidity of the photoacid avoids the protonation of strong bases in the absence of irradiation. Absorption spectroscopy and NMR studies showed that the photoacid could be paired with NEt3 and DMAP and underwent reversible photoreaction and proton transfer. This new type of photoacid allows the photocontrol of functional materials with strong proton acceptors.

Regenerative Biomimetic Photosynthesis by Covalent-Organic Framework-Based Nanobiohybrids

Biomimetic photosynthesis, which leverages nanomaterials with light-responsive capabilities, represents an innovative approach for replicating natural photosynthetic processes for green and sustainable energy conversion. In this study, a covalent-organic framework (COF)-based artificial photosynthesis system is realized through the co-assembly of adenosine triphosphate (ATP) synthase and a light-responsive proton generator onto an imine-based COF, RT-COF-1. This system demonstrates an ATP production rate of 0.64 µmol ATP per mg protein within 90 s of light exposure and, for the first time, exhibites regenerative ATP production through multiple light on/off cycles. Furthermore, the ATP generated by the system facilitates the biocoupling of monosaccharides into disaccharides, confirming the hybrid system's capability to convert solar energy into chemical energy in the form of organic molecules. This approach shows significant potential for renewable bioenergy generation, offering precise and reliable control over biochemical processes through artificial photosynthesis.

Diazocines as Guests of Cucurbituril Macrocycles: Light-Responsive Binding and Supramolecular Catalysis of Thermal Isomerization

The photoswitching of supramolecular host-guest complexes is the basis of numerous molecularly controlled macroscopic functions, such as sol-gel transition, photopharmacology, the active transport of ions or molecules, light-powered molecular machines, and much more. The most commonly used systems employ photoactive azobenzene guests and synthetic host molecules, which bind as the stable E isomers and dissociate as the Z forms after exposure to UV light. We present a new, extraordinarily efficient cucurbit[7]uril (CB7)/diazocine host/guest complex with inverted stability that self-assembles under UV irradiation and dissociates in the dark. The association constants of the Z and E isomers in water differ by more than 104-fold. We also show that the thermally activated E → Z isomerization is significantly accelerated by CB7, which is a rare case of enzyme-like catalysis by transition state stabilization without product inhibition. In contrast to CB7, cucurbit[8]uril (CB8) binds both isomers with high affinity, showing good selectivity (∼1000-fold) toward the Z isomer. Notably, this isomer preferentially binds CB8 relative to CB7 by a factor greater than 1 × 106. We also use the system to introduce a supramolecular photoacid that builds on the increased basicity of a guest bound to CB7 and on the extremely high affinity of the E isomer, which is utilized to displace the acid from CB7, thereby switching the pH of the solution.

Amplified Light-Induced pKa Modulation with Diarylethene Photoswitches

The reversible modulation of acidity using molecular photoswitches enables the remote control of a variety of (bio)chemical processes with light. Herein we investigated the structural features that allow amplifying photoinduced pKa variation in phenol-diarylethene conjugates, which toggle between low- and high-acidity states by switching the conjugation between the ionizable moiety and electron-withdrawing groups upon photoisomerization. By tuning the structure of these conjugates, high pKa modulation amplitudes were accomplished that surpass those previously reported.

Spiropyran as Building Block in Peptide Synthesis and Modulation of Photochromic Properties

Light-controlled triggering of materials requires efficient embedding of molecular photoswitches into larger molecules. We herein present the synthesis of two new building blocks for the synthesis of photoswitchable peptides, embedding spiropyranes as a central unit into peptide-backbones via a novel, yet unreported approach. The synthesis presented here allows us to embed spiropyranes directly into solid-phase peptide synthesis (SPPS), further describing the resulting photophysical properties of the as-prepared photoswitchable peptides.

Exploring the molecular design principles for efficient diarylethene photoacid and photohydride generators based on the photochemical reaction mechanism

Photoacid generators (PAGs) and photohydride generators (PHGs) are specific photolabile protecting groups that release acid and hydride, respectively. Over the past decade, great efforts have been devoted to developing novel PAGs and PHGs with advanced efficiency, among which, two of the promising candidates are diarylethene (DAE)-based PAGs and PHGs, which release acids/hydrides during photochromic electrocyclization. The release quantum yield for PAGs is acceptable, while that of PHGs is only 4.2% even after molecular structure modification. In this work, time-resolved transient absorption spectroscopies with femtosecond and nanosecond resolutions along with DFT/TD-DFT calculations were utilized to unravel the detailed photochemical reaction mechanisms of DAE-based PAGs (1o) and PHGs (2o), respectively. The results show that the different photochemical mechanisms are the key that leads to distinctive release quantum yields between 1o and 2o. The factors affecting the release quantum yield are discussed in detail, and several key design principles are proposed to facilitate future rational design of DAE-based PAGs and PHGs.

Wiring proton gradients for energy conversion

Light-switchable buffer solutions based on merocyanine photoacids can be used as efficient photoenergy harvesting systems. Varying the solvation environment of merocyanine photoacids in water-methanol mixtures allows one to carefully tune their photoacidity, relaxation kinetics, and solubility, opening up the possibility to install persistent pH gradients of approximately 4 pH units under 500 nm light. When interfaced between two electrodes and exposed to asymmetric light irradiation, these solutions can be photoactivated precisely both in space and time, generating open circuit voltages as high as 240 mV that can last hours under steady-state irradiation - an outcome that is akin the peak performance of biological transmembrane proton pumps.

Switching from Molecules to Functional Materials: Breakthroughs in Photochromism With MOFs

Photochromic materials with properties that can be dynamically tailored as a function of external stimuli are a rapidly expanding field driven by applications in areas ranging from molecular computing, nanotechnology, or photopharmacology to programable heterogeneous catalysis. Challenges arise, however, when translating the rapid, solution-like response of stimuli-responsive moieties to solid-state materials due to the intermolecular interactions imposed through close molecular packing in bulk solids. As a result, the integration of photochromic compounds into synthetically programable porous matrices, such as metal-organic frameworks (MOFs), has come to the forefront as an emerging strategy for photochromic material development. This review highlights how the core principles of reticular chemistry (on the example of MOFs) play a critical role in the photochromic material performance, surpassing the limitations previously observed in solution or solid state. The symbiotic relationship between photoresponsive compounds and porous frameworks with a focus on how reticular synthesis creates avenues toward tailorable photoisomerization kinetics, directional energy and charge transfer, switchable gas sorption, and synergistic chromophore communication is discussed. This review not only focuses on the recent cutting-edge advancements in photochromic material development, but also highlights novel, vital-to-pursue pathways for multifaceted functional materials in the realms of energy, technology, and biomedicine.

Towards Energy-Efficient Direct Air Capture with Photochemically-Driven CO2 Release and Solvent Regeneration

The intensive energy demands associated with solvent regeneration and CO2 release in current direct air capture (DAC) technologies makes their deployment at the massive scales (GtCO2/year) required to positively impact the climate economically unfeasible. This challenge underscores the critical need to develop new DAC processes with significantly reduced energy costs. Recently, we developed a new approach to photochemically drive efficient release of CO2 through an intermolecular proton transfer reaction by exploiting the unique properties of an indazole metastable-state photoacid (mPAH), opening a new avenue towards energy efficient on-demand CO2 release and solvent regeneration using abundant solar energy instead of heat. In this Concept Article, we will describe the principle of our photochemically-driven CO2 release approach for solvent-based DAC systems, discuss the essential prerequisites and conditions to realize this cyclable CO2 release chemistry under ambient conditions. We outline the key findings of our approach, discuss the latest developments from other research laboratories, detail approaches used to monitor DAC systems in situ, and highlight experimental procedures for validating its feasibility. We conclude with a summary and outlook into the immediate challenges that must be addressed in order to fully exploit this novel photochemically-driven approach to DAC solvent regeneration.

Photoresponsive hydrogel friction

Photoresponsive hydrogels are an emerging class of stimuli-responsive materials that exhibit changes in physical or chemical properties in response to light. Previous investigations have leveraged photothermal mechanisms to achieve reversible changes in hydrogel friction, although few have focused on photochemical means. To date, the tribological properties of photoswitchable hydrogels (e.g., friction and lubrication) have remained underexplored. In this work, we incorporated photoresponsive methoxy-spiropyran-methacrylate monomers (methoxy-SP-MA) into a hydrogel network to form a copolymerized system of poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropane sulfonic acid-co-methoxy-spiropyran-methacrylate) (p(NIPAAm-co-AMPS-co-SP)). We demonstrated repeatable photoresponsive changes to swelling, friction, and stiffness over three light cycles. Our findings suggest that volume changes driven by the decreased hydrophilicity of the methoxy-SP-MA upon light irradiation are responsible for differences in the mechanical and tribological properties of our photoresponsive hydrogels. Our results could inform future designs of photoswitchable hydrogels for applications ranging from biomedical applications to soft robotics.

Photoinduced Carbon Dioxide Release via a Metastable Photoacid in a Nonaqueous Environment

Capturing carbon dioxide (CO2) from the atmosphere is a scientific and technological challenge. CO2 can be captured by forming carbamate bonds with amines, most notably monoethanolamine (MEA). Regenerating MEA by releasing captured CO2 requires that the carbamate solution be heated. Recently, photoacids were used to induce a pH change to release CO2 from aqueous carbonate solutions. We report a merocyanine photoacid that releases CO2 from nonaqueous carbamate solutions of MEA, which has a CO2 loading capacity that is higher than that of water. On the basis of the absorption spectra of the photoacid in the presence of acids and CO2, we show that the photoacid cycle and the CO2 capture of MEA are two separate equilibria coupled to each other via protons. We demonstrate that irradiating the sample with 405 nm light induces the release of CO2, which we detect using an in-line mass spectrometer. This work highlights an alternative path for optimizing a photoinduced CO2 capture and release system.

An ultrawide-range photochromic molecular fluorescence emitter

Photocontrollable luminescent molecular switches capable of changing emitting color have been regarded as the ideal integration between intelligent and luminescent materials. A remaining challenge is to combine good luminescence properties with wide range of wavelength transformation, especially when confined in a single molecular system that forms well-defined nanostructures. Here, we report a π-expanded photochromic molecular photoswitch, which allows for the comprehensive achievements including wide emission wavelength variation (240 nm wide, 400-640 nm), high photoisomerization extent (95%), and pure emission color (<100 nm of full width at half maximum). We take the advantageous mechanism of modulating self-assembly and intramolecular charge transfer in the synthesis and construction, and further realize the full color emission by simple photocontrol. Based on this, both photoactivated anti-counterfeiting function and self-erasing photowriting films are achieved of fluorescence. This work will provide insight into the design of intelligent optical materials.

Light-Responsive Shape- and Color-Changing Block Copolymer Particles with Fast Switching Speed

Polymer particles capable of dynamic shape changes in response to light have received substantial attention in the development of intelligent multifunctional materials. In this study, we develop a light-responsive block copolymer (BCP) particle system that exhibits fast and reversible shape and color transitions. The key molecular design is the integration of spiropyran photoacid (SPPA) molecules into the BCP particle system, which enables fast and dynamic transformations of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) particles in response to light. The SPPA photoisomerization, induced by 420 nm light irradiation, lowers the pH of the aqueous surroundings from 5.5 to 3.3. The protonated P4VP block substantially increases in domain size from 14 to 39 nm, resulting in significant elongation of the BCP particles (i.e., an increase in the aspect ratio (AR) of the particles from 1.8 to 3.4). Moreover, SPPA adsorbed onto the P4VP surface induces significant changes in the luminescent properties of the BCP particles via photoisomerization of SPPA. Notably, the BCP particles undergo fast, dynamic shape and color transitions within a period of 10 min, maintaining high reversibility over multiple light exposures. Functional dyes are selectively incorporated into different domains of the light-responsive BCP particles to achieve different ranges of color responses. Thus, this study showcases a light-responsive hydrogel display capable of reversible and multicolor photopatterning.

Solvation-Tuned Photoacid as a Stable Light-Driven pH Switch for CO2 Capture and Release

Photoacids are organic molecules that release protons under illumination, providing spatiotemporal control of the pH. Such light-driven pH switches offer the ability to cyclically alter the pH of the medium and are highly attractive for a wide variety of applications, including CO2 capture. Although photoacids such as protonated merocyanine can enable fully reversible pH cycling in water, they have a limited chemical stability against hydrolysis (<24 h). Moreover, these photoacids have low solubility, which limits the pH-switching ability in a buffered solution such as dissolved CO2. In this work, we introduce a simple pathway to dramatically increase stability and solubility of photoacids by tuning their solvation environment in binary solvent mixtures. We show that a preferential solvation of merocyanine by aprotic solvent molecules results in a 60% increase in pH modulation magnitude when compared to the behavior in pure water and can withstand stable cycling for >350 h. Our results suggest that a very high stability of merocyanine photoacids can be achieved in the right solvent mixtures, offering a way to bypass complex structural modifications of photoacid molecules and serving as the key milestone toward their application in a photodriven CO2 capture process.

Unravelling photoisomerization dynamics in a metastable-state photoacid

Direct access to trans-cis photoisomerization in a metastable state photoacid (mPAH) remains challenging owing to the presence of competing excited-state relaxation pathways and multiple transient isomers with overlapping spectra. Here, we reveal the photoisomerization dynamics in an indazole mPAH using time-resolved fluorescence (TRF) spectroscopy by exploiting a unique property of this mPAH having fluorescence only from the trans isomer. The combination of these experimental results with time-dependent density function theory (TDDFT) calculations enables us to gain mechanistic insight into this key dynamical process.

Reversible CO2 Capture and On-Demand Release by an Acidity-Matched Organic Photoswitch

Separation of carbon dioxide (CO2) from point sources or directly from the atmosphere can contribute crucially to climate change mitigation plans in the coming decades. A fundamental practical limitation for the current strategies is the considerable energy cost required to regenerate the sorbent and release the captured CO2 for storage or utilization. A directly photochemically driven system that demonstrates efficient passive capture and on-demand CO2 release triggered by sunlight as the sole external stimulus would provide an attractive alternative. However, little is known about the thermodynamic requirements for such a process or mechanisms for modulating the stability of CO2-derived dissolved species by using photoinduced metastable states. Here, we show that an organic photoswitchable molecule of precisely tuned effective acidity can repeatedly capture and release a near-stoichiometric quantity of CO2 according to dark-light cycles. The CO2-derived species rests as a solvent-separated ion pair, and key aspects of its excited-state dynamics that regulate the photorelease efficiency are characterized by transient absorption spectroscopy. The thermodynamic and kinetic concepts established herein will serve as guiding principles for the development of viable solar-powered negative emission technologies.

Breaking the photoswitch speed limit

The forthcoming generation of materials, including artificial muscles, recyclable and healable systems, photochromic heterogeneous catalysts, or tailorable supercapacitors, relies on the fundamental concept of rapid switching between two or more discrete forms in the solid state. Herein, we report a breakthrough in the "speed limit" of photochromic molecules on the example of sterically-demanding spiropyran derivatives through their integration within solvent-free confined space, allowing for engineering of the photoresponsive moiety environment and tailoring their photoisomerization rates. The presented conceptual approach realized through construction of the spiropyran environment results in ~1000 times switching enhancement even in the solid state compared to its behavior in solution, setting a record in the field of photochromic compounds. Moreover, integration of two distinct photochromic moieties in the same framework provided access to a dynamic range of rates as well as complementary switching in the material's optical profile, uncovering a previously inaccessible pathway for interstate rapid photoisomerization.

Photoacid for releasing carbon dioxide from sorbent

Carbon dioxide (CO2) is the most important greenhouse gas that causes global warming. Different sorbents, e.g. amines have been applied to capture CO2. Sorbent regeneration, which is currently conducted by thermal processes, is the most energy consuming step. In this work, we studied a photoacid, which can reversibly increase the acidity of a solution under light, and consequently lead to CO2 release. The photoactivity, acidity and solubility of the photoacid was tuned for CO2 release by structural design. The photoacid was mixed with morpholine, which is a well-studied amine for CO2 capture. Results showed that an aqueous solution containing the mixture can repeatedly capture and release CO2 under moderate irradiation of visible light. The CO2 released was nearly the same to that was captured, which indicates the high efficiency of this method.

Excited-State Proton Transfer in a Photoacid-Based Crystalline Coordination Compound: Reversible Photochromism, Near-Infrared Photothermal Conversion, and Conductivity

By harnessing the power of coordination self-assembly, crystalline materials can act as carriers for photoacids. Unlike their solution-based counterparts, these photoacids are capable of altering the properties of the crystalline material under light and can even generate proton transfer in a solid-state environment. Due to the photoinduced proton transfer and charge transfer processes within this functional material, this crystal exhibits powerful absorption spanning the visible to near-infrared spectrum upon light irradiation. This feature enables reproducible, significant chromatic variation, near-infrared photothermal conversion, and photocontrollable conductivity for this photoresponsive material. The findings suggest that the synthesis of pyranine photoacid-based crystalline materials via coordination self-assembly can not only enhance light-harvesting efficiency but also enable excited-state proton transfer processes within solid crystalline materials, thereby maintaining and even improving the properties of photoacids.

Spiropyran-based chromic hydrogels for CO2 absorption and detection

By enabling rapid, cost-effective, user-friendly and in situ detection of carbon dioxide, colorimetric CO₂ sensors are of relevance for a variety of fields. However, it still remains a challenge the development of optical chemosensors for CO₂ that combine high sensitivity, selectivity and reusability with facile integration into solid materials. Herein we pursued this goal by preparing hydrogels functionalized with spiropyrans, a well-known class of molecular switches that undergo different color changes upon application of light and acid stimuli. By varying the nature of the substituents of the spiropyran core, different acidochromic responses are obtained in aqueous media that allow discriminating CO₂ from other acid gases (e.g., HCl). Interestingly, this behavior can be transferred to functional solid materials by synthesizing polymerizable spiropyran derivatives, which are used to prepare hydrogels. These materials preserve the acidochromic properties of the incorporated spiropyrans, thus leading to selective, reversible and quantifiable color changes upon exposure to different CO₂ amounts. In addition, CO₂ desorption and, therefore, recovery of the initial state of the chemosensor is favored by irradiation with visible light. This makes spiropyran-based chromic hydrogels promising systems for the colorimetric monitorization of carbon dioxide in a diversity of applications.

A light-fueled dissipative aggregation-induced emission system for time-dependent information encryption

A light-fueled dissipative aggregation-induced emission (LDAIE) system is successfully fabricated based on reversible electrostatic interactions between cationic AIE luminogens (AIEgens) and anionic spiropyran (ASP) transformed from sulfonato-merocyanine photoacid (SMEH) upon 420 nm light irradiation. The novel LDAIE system can exhibit reversible and spontaneous AIE fluorescence on/off, showing potential in time-dependent information encryption with self-erasing ability. This work opens new opportunities to fabricate a unique fluorescent anti-counterfeiting platform with high-level security.

Maskless, Reusable Visible-Light Direct-Write Stamp for Microscale Surface Patterning

We report a straightforward method for creating large-area, microscale resolution patterns of functional amines on self-assembled monolayers by the photoinduced local acidification of a flat elastomeric stamp enriched with photoacid. The limited diffusivity of the photoactivated merocyanine acid in poly(dimethylsiloxane) (PDMS) enabled to confine efficient deprotection of N-tert-butyloxycarbonyl amino group (N-Boc) to line widths below 10 μm. The experimental setup is very simple and is built around the conventional HD-DVD optical pickup. The method allows cost-efficient, maskless, large-area chemical patterning while avoiding potentially cytotoxic photochemical reaction products. The activation of the embedded photoacid occurs within the stamp upon illumination with the laser beam and the process is fully reversible. Preliminary positive results highlight the possibility of repeatable use of the same stamp for the creation of different patterns.

Controlling DNA nanodevices with light-switchable buffers

Control over synthetic DNA-based nanodevices can be achieved with a variety of physical and chemical stimuli. Actuation with light, however, is as advantageous as difficult to implement without modifying DNA strands with photo-switchable groups. Herein, we show that DNA nanodevices can be controlled using visible light in photo-switchable aqueous buffer solutions in a reversible and highly programmable fashion. The strategy presented here is non-invasive and allows the remote control with visible light of complex operations of DNA-based nanodevices such as the reversible release/loading of cargo molecules.

Visible light responsive photoacids for subcellular pH and temperature correlated fluorescence sensing

Liao's photoacids (PAs) are a well-known type of visible light-responsive photoswitches. Here, taking advantage of the temperature-dependent thermal relaxation from the ring-closed to the ring-opened forms, PAs are proposed for the first time as a fluorescent temperature sensor in cells. The logarithmic lifetime (ln τ) of the ring-closed spiro-form exhibited an excellent linear response to the reciprocal of the temperature. In addition, the fluorescent ring-opened PAs were able to highlight lysosomes and responded to lysosomal pH changes. These properties made the PAs promising fluorescent probes in the sensing of subcellular pH and temperature.

Acidochromism of donor-acceptor Stenhouse adducts in organic solvent

First generation DASA derivatives can be reversibly isomerized from the coloured, open form to the colourless, closed isomer upon protonation, thus behaving as acidochromic compounds in halogenated organic solvent.

Photoenergy Harvesting by Photoacid Solution

Solar energy has seen 180 years of development since the discovery of the photovoltaic effect, having achieved the most successful commercialization in the energy-harvesting fields. Despite its long history, even the most state-of-the-art photovoltaics remain confined to solid-state devices, limiting spatial and light utilization efficiencies. Herein, a liquid-state photoenergy harvester based on a photoacid (PA), a chemical that releases protons upon light irradiation and recombines with them in the dark through a fully reversible reaction, is demonstrated. Asymmetric light exposure on a PA solution contained in a transparent tube generates a pH gradient (ΔpH = 2) along the exposed and dark regions, which charges the Nernst potential up to 0.7 V across the two electrodes embedded at each end, as if a capacitor. Owing to the reversibility of PAs, a PA-driven liquid-state photoenergy harvester (PLPH) generates capacitive currents up to 0.72 mA m^-2  on an irradiation. Notably, the transparent nature of the PLPH enables vertical stacking up to 25 units, which multiplies the light-harvesting efficiencies by over 1000%. This unique approach provides a new route to harness solar energy with a form-factor-free design that maximizes spatial and light-use efficiencies.

Basic-to-acidic reversible pH switching with a merocyanine photoacid

The application of merocyanine photoacids has previously been largely limited to neutral and acidic pH values. Here we introduce a new merocyanine photoacid with superior pH switching qualities. By increasing the pK_a in the dark (pKdarka) and the solubility we increased the reversible visible light induced pH jump to 3.5 units. Moreover, it is the first demonstration of a merocyanine photoacid able to generate a significant pH drop from a basic (pH 8.3) to an acidic (pH 5.2) environment.

Reversible photo control of proton chemistry

Spatial, temporal, and remote control of proton chemistry can be achieved by using photoacids, which are molecules that transform from weak to strong acids under light. Most of proton chemistry is driven by a high concentration of protons ([H⁺]), which is difficult to obtain using excited-state photoacids. Metastable-stable state photoacids (mPAHs) can reversibly generate a high [H⁺] under visible light with a moderate intensity. It has been widely applied in different fields, e.g. fueling dissipative assemblies, driving molecular machines, controlling organic reactions, powering nanoreactors, curing diseases, manipulating DNA and proteins, developing smart materials, capturing carbon dioxide in air etc. This article compares mPAH with excited-state photoacid as well as common acids e.g. HCl to explain its advantages. Recent studies on the thermal dynamics, kinetics, and photoreaction of mPAHs are reported. The advantages and disadvantages of the three types of mPAHs, i.e. merocyanine, indazole, and TCF mPAHs, are compared with regard to photo-induced [H⁺], switching rate, and other properties. The mechanisms of controlling or driving functional systems, which involve acid-base reactions, acid catalyzed reactions, ionic bonding, coordination bonding, hydrogen bonding, ion exchange, cation-π interaction, solubility, swellability, permeability, and pH change in biosystems, are described. Applications of mPAHs in the chemical, material, energy, biotechnology and biomedical fields published in the past 5 years are reviewed. Prospects in the development and application of mPAHs are discussed.

Light-Powered Dissipative Assembly of Diazocine Coordination Cages

Stimuli-responsive coordination cages allow reversible control over guest binding and release, relevant for adaptive receptors, carriers, catalysts, and complex systems. Light serves as an advantageous stimulus, as it can be applied with precise spatial and temporal resolution without producing chemical waste products. We report the first Pd-mediated coordination cage based on ligands embedding a diazocine photoswitch. While the thermodynamically more stable cis-photoisomer sloppily assembles to a mixture of species with general formula [Pd_ncis-L_2n], the less stable trans-isomer yields a defined [Pd₂trans-L₄] cage that reversibly converts back to the cis-system by irradiation at 530 nm or thermal relaxation. The [Pd_ncis-L_2n] species do not bind a given guest; however, [Pd₂trans-L₄] is able to encapsulate a bis-sulfonate as long as it is kept assembled, requiring continuous irradiation at 385 nm. In the absence of UV light, thermal relaxation results in back-switching and guest release. Assembly and properties of the system were characterized by a combination of NMR, ion mobility ESI-MS, single-crystal X-ray diffraction, and UV–vis absorption studies.

Spiropyran Photoisomerization Dynamics in Multiresponsive Hydrogels

Light-responsive, spiropyran-functionalized hydrogels have been used to create reversibly photoactuated structures for applications ranging from microfluidics to nonlinear optics. Tailoring a spiropyran-functionalized hydrogel system for a particular application requires an understanding of how co-monomer composition affects the switching dynamics of the spiropyran chromophore. Such gels are frequently designed to be responsive to different stimuli such as light, temperature, and pH. The coupling of these influences can significantly alter spiropyran behavior in ways not currently well understood. To better understand the influence of responsive co-monomers on the spiropyran isomerization dynamics, we use UV-vis spectroscopy and time-dependent fluorescence intensity measurements to study spiropyran-modified hydrogels polymerized from four common hydrogel precursors of different pH and temperature responsivity: acrylamide, acrylic acid, N-isopropylacrylamide, and 2-(dimethylamino)ethyl methacrylate. In acidic and neutral gels, we observe unusual nonmonotonic, triexponential fluorescence dynamics under 405 nm irradiation that cannot be explicated by either the established spiropyran-merocyanine interconversion model or hydrolysis. To explain these results, we introduce an analytical model of spiropyran interconversions that includes H-aggregated merocyanine and its light-triggered disaggregation under 405 nm irradiation. This model provides an excellent fit to the observed fluorescence dynamics and elucidates exactly how creating an acidic internal gel environment promotes the fast and complete conversion of the hydrophilic merocyanine speciesto the hydrophobic spiropyran form, which is desired in most light-sensitive hydrogel actuators. This can be achieved by incorporating acrylic acid monomers and by minimizing the aggregate concentration. Beyond spiropyran-functionalized gel actuators, these conclusions are particularly critical for nonlinear optical computing applications.

Dissipative control of the fluorescence of a 1,3-dipyrenyl calix[4]arene in the cone conformation

The temporal control (ON/OFF/ON) of the fluorescence of a dichloromethane/acetonitrile 1 : 1 solution of calixarene 3 decorated with two pyrenyl moieties at the upper rim is attained by the addition of CCl₃CO₂H used as a convenient chemical fuel.

Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

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.

Using photocaging for fast time-resolved structural biology studies

Careful selection of photocaging approaches is critical to achieve fast and well synchronized reaction initiation and perform successful time-resolved structural biology experiments. This review summarizes the best characterized and most relevant photocaging groups previously described in the literature. It also provides a walkthrough of the essential factors to consider in designing a suitable photocaged molecule to address specific biological questions, focusing on photocaging groups with well characterized spectroscopic properties. The relationships between decay rates (k in s-1), quantum yields (ϕ) and molar extinction coefficients (ϵmax in M-1 cm-1) are highlighted for different groups. The effects of the nature of the photocaged group on these properties is also discussed. Four main photocaging scaffolds are presented in detail, o-nitrobenzyls, p-hydroxyphenyls, coumarinyls and nitrodibenzofuranyls, along with three examples of the use of this technology. Furthermore, a subset of specialty photocages are highlighted: photoacids, molecular photoswitches and metal-containing photocages. These extend the range of photocaging approaches by, for example, controlling pH or generating conformationally locked molecules.

Light-Switchable Buffers

A visible light-switchable buffer system based on a merocyanine photoacid is presented. Para-substitution of the indolium side with a methoxy group affords a compound suitable for making hydrolytically stable aqueous buffers whose pH can be tuned between 7 and 4 using 500 nm light.

Spiropyran-Based Drug Delivery Systems

Nanocarriers are rapidly growing in popularity in the field of drug delivery. The ability of nanocarriers to encapsulate and distribute poorly soluble drugs while minimising their undesired effects is significantly advantageous over traditional drug delivery. Nanocarriers can also be decorated with imaging moieties and targeting agents, further incrementing their functionality. Of recent interest as potential nanocarriers are spiropyrans; a family of photochromic molecular switches. Due to their multi-responsiveness to endo- and exogenous stimuli, and their intrinsic biocompatibility, they have been utilised in various drug delivery systems (DDSs) to date. In this review, we provide an overview of the developments in spiropyran-based DDSs. The benefits and drawbacks of utilising spiropyrans in drug delivery are assessed and an outline of spiropyran-based drug delivery systems is presented.

Experimental and Theoretical Characterization of Ultrafast Water-Soluble Photochromic Photoacids

UV-visible transient absorption spectroscopy and quantum mechanical simulations are combined to elucidate the photochemical mechanism of two metastable merocyanine/spiropyran photoacids, 2-[(E)-2-(2-hydroxyphenyl)ethenyl]-3,3-dimethyl-1-(3-sulfopropyl)-3H-indol-1-ium (phenylhydroxy-MCH) and 2-[(E)-2-(1H-indazol-7-yl)ethenyl]-3-(3-sulfopropyl)-1,3-benzothiazol-3-ium (indazole-MCH). Transient absorption spectra demonstrate that trans-acid isomerization to the cis form results in deprotonation on a picosecond time scale. Ring closure to form spiropyran follows promptly from the appropriate conformation or follows at longer time delays (≫3.5 ns) following a barrier crossing for single-bond isomerization to the appropriate conformation. Consistent with the results of Berton et al. [ Chem. Sci. 2020, 11, 8457-8468] , we find that cis-phenylhydroxy-MCH is a stronger acid than trans-phenylhydroxy-MCH. The decrease in pK_a upon isomerization is further investigated to benchmark quantum chemical methods for their accuracy. Calculations were performed with nine levels of theory including continuum solvent models and explicit water. The calculations are not sufficient to describe the ΔpK_a following isomerization of these photoacids, and more work is necessary to properly evaluate the physical basis for the acidity of the cis photoacids.

Light-induced assembly and disassembly of polymers with Pd n L2n -type network junctions

Polymers containing Pd n L2n complexes as network junctions were obtained by reaction of poly(ethylene glycol)-linked N-donor ligands with Pd2+. The addition of a metastable state photoacid renders the networks light sensitive, and gel-sol transitions can be achieved by irradiation with light. The inverse process, a light-induced sol-gel transition, was realized by using a molecularly defined Pd complex as an acid-sensitive reservoir for Pd2+. Upon irradiation, Pd2+ ions are released, allowing the formation of an acid-resistant polymer network. Both the gel-sol and the sol-gel transitions are reversed in the dark.

Tailoring the optical and dynamic properties of iminothioindoxyl photoswitches through acidochromism

Multi-responsive functional molecules are key for obtaining user-defined control of the properties and functions of chemical and biological systems. In this respect, pH-responsive photochromes, whose switching can be directed with light and acid-base equilibria, have emerged as highly attractive molecular units. The challenge in their design comes from the need to accommodate application-defined boundary conditions for both light- and protonation-responsivity. Here we combine time-resolved spectroscopic studies, on time scales ranging from femtoseconds to seconds, with density functional theory (DFT) calculations to elucidate and apply the acidochromism of a recently designed iminothioindoxyl (ITI) photoswitch. We show that protonation of the thermally stable Z isomer leads to a strong batochromically-shifted absorption band, allowing for fast isomerization to the metastable E isomer with light in the 500-600 nm region. Theoretical studies of the reaction mechanism reveal the crucial role of the acid-base equilibrium which controls the populations of the protonated and neutral forms of the E isomer. Since the former is thermally stable, while the latter re-isomerizes on a millisecond time scale, we are able to modulate the half-life of ITIs over three orders of magnitude by shifting this equilibrium. Finally, stable bidirectional switching of protonated ITI with green and red light is demonstrated with a half-life in the range of tens of seconds. Altogether, we designed a new type of multi-responsive molecular switch in which protonation red-shifts the activation wavelength by over 100 nm and enables efficient tuning of the half-life in the millisecond-second range.

Synthesis of spiro(indoline-2,3′-hydropyridazine) via an “on-water” [4 + 2] annulation reaction

An on-water [4 + 2] annulation reaction between 2-methyl-3H-indolium salt and α-bromo N-acyl hydrazone has been developed. The environmentally friendly strategy provides the first facile access to spiro(indoline-2,3'-hydropyridazine) scaffolds.