ConspectusProton transfer is one of the most common processes in nature, and many chemical, material, and biological processes are sensitive to proton concentration, from acid-catalyzed reactions to the activities of many enzymes. Photoacids that reversibly undergo proton dissociation upon irradiation promise remote spatial and temporal control over proton-sensitive processes and could provide a way to convert photoenergy into other types of energy. The recently discovered metastable-state photoacids can produce a large proton concentration with high efficiency and good reversibility. A reversible pH change of over 2 units has been demonstrated using an aqueous solution of a metastable-state photoacid. Additionally, moderate-intensity visible light, for example, from LEDs and sunlight, can be used to activate this type of photoacid. This photocontrolled proton release occurs in aqueous and nonaqueous solutions and in polymeric materials. Therefore, this type of photoacid can be conveniently incorporated into different systems to control various proton transfer processes.Metastable-state photoacids are generally designed by linking an electron-accepting moiety and a weakly acidic nucleophilic moiety with a double bond. Photoinduced trans–cis isomerization of the double bond allows a nucleophilic cyclization reaction to occur between the two moieties. The tandem reaction generates a highly acidic metastable form, which releases a proton. In the dark, the metastable form relaxes to the original form and takes back the proton. Several electron-accepting and nucleophilic moieties have been used to construct different types of metastable-state photoacids for different applications. The advantages and disadvantages of these photoacids in terms of their photoacidity, dark acidity, reversibility, stability, etc. will be discussed in this Account.Metastable-state photoacids have been used to catalyze bond formation and bond-breaking reactions in which the reactions can be activated and stopped by turning on and off irradiation, respectively. They have been used to reversibly protonate molecules to affect the ionic and hydrogen bonding between molecules or between different moieties of a molecule. Protonation can also alter the electronic configuration of molecules to change their electronic and optical properties. Since a proton has a positive charge, photoacids have been used to control ion exchange processes. Applying metastable-state photoacids to control Fisher esterification, volume-changing hydrogels, the killing of bacteria, odorant release, the color of materials, the formation of nanoparticles, and polymer conductivity has been reported by our group. Metastable-state photoacids have also been utilized to control supramolecular assemblies, molecular switches, microbial fuel cells, cationic sensors, nanoparticle aggregation, and ring-opening polymerizations. The future prospects of this research area will be discussed at the end of this Account.

•The photochemical behavior of a merocyanine type photoacid in polymer films is studied.•The photoacid possesses high thermal and photostability in polymer.•Kinetics of the photoreaction is complicated due to slow proton transfer in polymer.•Addition of a salt that assists proton transfer enhanced the rate of the reverse reaction.Over the past years, merocyanine type photoacids have been used for the development of various visible-light responsive polymers and polymer sensors. However, the physicochemical behavior of this type of photoacid in polymers has not been studied previously. In this work, the photoactivity, reversibility, and stability of a merocyanine-type photoacid in polymer films are investigated. Polymer films doped with the photoacid were found to be stable under ambient conditions over half a year. Kinetics of the reaction is complicated for both the forward and backward process due to slow proton diffusion in the polymer. Improving proton transfer by addition of ammonium trifluoromethanesulfonate enhances the reverse reaction rate. The photoacid-doped polymer showed good photostability upon a test of 100 cycles of irradiation and recovery in the dark.

Carbon monoxide (CO) is a gasotransmitter that plays important roles in regulating cell functions and has shown therapeutic effects in clinic studies. CO releasing molecules (CORMs), which allow controlled release of CO in physiological conditions, have been intensively studied in the past decade. While most CORMs are metal complexes, several nonmetallic CORMs have also been developed and most of them were reported in recent years. The major advantages of nonmetallic CORMs are potentially low toxicity and easy modification for property tuning. Syntheses, CO-release mechanisms, biological behaviors, and physicochemical properties of these nonmetallic CORMs are reviewed here. The first part of this short review covers the nonmetallic CORMs that do not require irradiation to release CO, which includes methylene chloride, CORM-A1 and its derivatives, amine carboxyboranes, and bimolecular CORMs. The second part focuses on the CORMs that release CO under irradiation (PhotoCORMs) including unsaturated cyclic diketones, xanthene carboxylic acids, meso-carboxy BODIPYs, and hydroxyflavones. Future prospects are discussed at the end of this review.

Noncovalent crosslinking between polyvinyl pyridine and a copolymer of acrylic acid led to the formation of a polymer nanoparticle. In the presence of a metastable-state photoacid, reversible dissolution and formation of the nanoparticle can be controlled by visible light. Photo-induced proton transfer from the photoacid to the polymers broke the hydrogen bonding and ionic bonding and led to the dissolution of the nanoparticle. Cycles of dissolution and formation were controlled by turning on and off irradiation, and were demonstrated by the transmittance change.

A metastable-state photoacid that can reversibly release a proton in PBS buffer (pH = 7.4) under visible light is reported. The design is based on the dual acid–base property and tautomerization of indazole. The quantum yield was as high as 0.73, and moderate light intensity (102 μmol·m2·s–1) is sufficient for the photoreaction. Reversible pH change of 1.7 units was demonstrated using a 0.1 mM aqueous solution. This type of photoacid is promising for control of proton-transfer processes in physiological conditions and may find applications in biomedical areas.

A dicopper(I) complex, (pip = (2-picolyliminomethyl)pyrrole anion), was utilized to catalyze A3 coupling reactions, which led to the formation of propargylamines. Aldehydes, alkynes and amines with a variety of structures have been tested. A low catalyst loading of 0.4 mol% was sufficient to give good to excellent yields. The low catalyst loading, broad scope of substrate and easy preparation make this dicopper complex a useful catalyst for A3 coupling.

A novel dinuclear copper complex CuI2(pip)2 was used as a catalyst for alkyne–azide cycloaddition (CuAAC) reaction. High yields (95–99%) were obtained for various substrates at a low loading of 0.2 mol %. The unique structure, high stability of the dinuclear structure in solution, and easy preparation make this complex not only a high-efficiency catalyst but also a model for understanding the mechanism of the CuAAC reaction.

Novel organic photoCORMs based on micelle-encapsulated unsaturated cyclic α-diketones were designed and synthesized. These photoCORMs can be activated by visible light, have potentially low toxicity, allow the delivery of carbon monoxide to be monitored by fluorescence imaging techniques, and thus are useful tools for the study of the biological function of CO.

Novel organic photoCORMs based on micelle-encapsulated unsaturated cyclic α-diketones were designed and synthesized. These photoCORMs can be activated by visible light, have potentially low toxicity, allow the delivery of carbon monoxide to be monitored by fluorescence imaging techniques, and thus are useful tools for the study of the biological function of CO.