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.

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.

Over the past years, protonated merocyanines (MEHs) have been used as photoacids to control various chemical, material, and biological processes using visible light. For the applications under aqueous conditions, stability of this type of photoacid has been a concern. While hydrolysis of merocyanines is well known, this work showed that deprotonation of MEH to form merocyanine is not necessary for the hydrolysis of MEH. The decomposition products were identified by ultraviolet‐visible spectroscopy and liquid chromatography–mass spectrometry. Comparing the behaviors of different MEHs under different conditions indicates that the hydrolysis is catalyzed by OH− and thus MEHs are more stable at a lower pH. Modifying an MEH with an electron‐donating group conjugated to the double bond significantly improved its stability. Photostability of an MEH was tested by conducting 100 irradiating/recovering cycles, and the photoacid showed good photostability.

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.20201184578468] , 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.

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.