Yi Liao

Long-lived photoacid based upon a photochromic reaction.

A visible-light activatable photoacid has been studied, which upon irradiation, changes from a weak acid, with a pK(a) of 7.8, to a strong acid, which achieves nearly complete proton dissociation. This process is reversible and the half-life of the proton-dissociation state is ~70s. The long lifetime of the proton-dissociation state is due to a sequential intramolecular photochromic reaction. Using this photoacid, a pH change of 2.2 units has been achieved. In addition, we demonstrated that the photoinduced proton concentration can catalyze an esterification reaction, and greatly alter the volume of a pH-sensitive polymer. This work shows that acid-catalyzed and pH-sensitive processes can be photochemically controlled by using this type of photoacid.

A Reversible Photoacid Functioning in PBS Buffer under Visible Light.

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 (10(2) μmol·m(2)·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.

Controlled release of fragrant molecules with visible light.

Controlled release of odorous molecules is the key to digital scent technology which will add another dimension to electronics. Photorelease is a cold mechanism that promises better temporal and spatial control than thermal release. Herein we report a novel material composed of an acid-sensitive polymer carrying a fragrant aldehyde and a reversible metastable-state photoacid. It releases the fragrant molecule under visible light, and stops releasing it after the light is turned off. A metastable-state photoacid with a fast reverse-reaction rate was developed to quickly stop the release after irradiation. Both the carrier polymer and the photoacid can be reused after all the fragrant molecules have been released. The material combines the advantages of visible-light activity, fast on/off rate, easy preparation, and recyclability, and thus is promising for digital scent technology.

Physicochemical study of a metastable-state photoacid.

A photoacid that possesses a metastable acidic state induced by visible light is studied. Previous work showed that this photoacid can reversibly produce a large pH change capable of controlling chemical reactions, altering material properties, and killing bacteria. In this work, we studied the relaxation kinetics of the metastable acidic state in different solvents including water, ethanol, and DMSO. In all of these solvents, the kinetic data can be fitted well to a second-order rate equation, which indicates that protonation is involved in the rate-limiting step. The rate constants in water, ethanol, and DMSO are 73, 1.6, and 0.034 M(-1) s(-1), respectively. The slow relaxation in DMSO allowed us to fully characterize the structure of the metastable acidic state using proton NMR. We also measured the quantum yield of the photoreaction, which is as high as 0.37.

Visible-Light-Responsive Reversible Photoacid Based on a Metastable Carbanion

A new photoacid that reversibly changes from a weak to a strong acid under visible light was designed and synthesized. Irradiation generated a metastable state with high CH acidity due to high stability of a trifluoromethyl-phenyl-tricyano-furan (CF3 PhTCF) carbanion. This long-lived metastable state allows a large proton concentration to be reversibly produced with moderate light intensity. Reversible pH change of about one unit was demonstrated by using a 0.1 mM solution of the photoacid in 95 % ethanol. The quantum yield was calculated to be as high as 0.24. Kinetics of the reverse process can be fitted well to a second-order-rate equation with k=9.78×10(2)  M(-1)  s(-1) . Response to visible light, high quantum yield, good reversibility, large photoinduced proton concentration under moderate light intensity, and good compatibility with organic media make this photoacid a promising material for macroscopic control of proton-transfer processes in organic systems.

Design and Applications of Metastable-State Photoacids

Proton 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.

Incorporation of photo-carbon monoxide releasing materials into electrospun scaffolds for vascular tissue engineering.

Hyper-proliferation of smooth muscle cells (SMCs) and a reduction in endothelial cell function are reasons for poor patency rates of current tissue engineered small-diameter vascular grafts. The controlled delivery of carbon monoxide (CO), a gasotransmitter involved in cell signaling, could improve vascular cell function in these grafts. Current CO releasing molecules (CORMs) can improve endothelialization of injured vessels with appropriate doses, but they still have limitations. The goal of this project was to generate a novel tissue engineered scaffold that includes a non-toxic and photoactivatable CORM. This is the first use of a CORM for tissue engineering. The results demonstrated that CORM-loaded, electrospun poly(ɛ-caprolactone) scaffolds can be photo-activated and release CO. The fluorescence that develops after CO release can be used to non-destructively track the extent of reaction. Further, activation can occur when both dry and incubated in cell culture conditions. However, incubation in serum protein-containing media decreases the time frame for activation, demonstrating the importance of testing the release profile in culture conditions. Rat SMCs were able to attach, grow, and express contractile SMC markers on activated CORM-loaded meshes and controls. Overall, these findings demonstrate that CORM-loaded electrospun scaffolds provide a promising delivery system for vascular tissue engineering.

Photo retro-Diels-Alder reactions.

Photo-retro-Diels-Alder (PrDA) reactions of a variety of Diels-Alder (DA) adducts were studied. Experimental results showed that the photoreactivity (quantum yield) depends on the electron-donating ability of the diene component and the electron-withdrawing ability of the dienophile component. The mechanism was studied by trapping the reaction intermediate, O(2) quenching, time-resolved absorption, and fluorescence spectroscopy. All the results support a mechanism that involves a charge-separated intermediate generated from a singlet excited state. The PrDA reaction may find applications in photoresponsive materials, photolithography, drug delivery, and mechanistic research.

Photocontrolled proton transfer in solution and polymers using a novel photoacid with strong C–H acidity

Over the past few years, metastable-state photoacids (mPAHs) have been used to control various chemical, material and biological processes using visible light. In this work, a novel mPAH with strong C–H acidity was studied. The pKa of its photoacidic state was experimentally determined and compared to a commonly used mPAH. The photoacid showed low dark acidity and a large pKa change when irradiated by visible-light. The pKa for the photoinduced C–H acidity is 4.8 units lower than that for the O–H acidity in the dark. These features together with its easy preparation make it promising for the development of photoresponsive materials based on proton transfer. Proton transfer between the photoacid and proton acceptors with different pKas in solutions and polymer films was studied. The photoacid showed superior properties in polymer films compared to a commonly used mPAH. Photochromic films with various color changes were demonstrated as an example of potential application.

Facile Synthesis and Photoactivity of Merocyanine-Photoacid Polymers

In recent years, merocyanine photoacids have been utilized to control various chemical processes using visible light and have found applications in materials, energy, and biomedical areas. Molecular merocyanine photoacids are commonly used in the previous works. Covalently linking the photoacids to polymers improves their compatibility with different media, avoids leakage problems, and allows a localized proton concentration to be produced. However, the phenolic and indolinium moieties of the photoacids make them difficult to be polymerized with common methods. In this work, the monomer of a merocyanine photoacid is converted to a spiropyran in situ by adding trimethylamine to a dimethyl sulfoxide (DMSO) solution of the monomer. Free radical polymerization yields the polymers of the spiropyran, which is acidified to regenerate the photoacid. The photoacid polymers prepared show good solubility, photoacidity, and reversibility. Irradiating a thin film of a photoacid polymer doped with methyl orange through a mask copies the pattern of the mask to the film. The pattern can be erased by heating the film at 80 °C for 10 min, and a new pattern is created by irradiation through a different mask.