Structure–Photoreactivity Relationship of 3-Hydroxyflavone-Based CO-Releasing Molecules

Carbon monoxide (CO) is an endogenous signaling molecule that regulates diverse physiological processes. The therapeutic potential of CO is hampered by its intrinsic toxicity, and its administration poses a significant challenge. Photoactivatable CO-releasing molecules (photoCORMs) are an excellent tool to overcome the side effects of untargeted CO administration and provide precise spatial and temporal control over its release. Here, we studied the CO release mechanism of a small library of derivatives based on 3-hydroxy-2-phenyl-4H-benzo[g]chromen-4-one (flavonol), previously developed as an efficient photoCORM, by steady-state and femto/nanosecond transient absorption spectroscopies. The main objectives of the work were to explore in detail how to enhance the efficiency of CO photorelease from flavonols, bathochromically shift their absorption bands, control their acid–base properties and solubilities in aqueous solutions, and minimize primary or secondary photochemical side-reactions, such as self-photooxygenation. The best photoCORM performance was achieved by combining substituents, which simultaneously bathochromically shift the chromophore absorption spectrum, enhance the formation of the productive triplet state, and suppress the singlet oxygen production by shortening flavonol triplet-state lifetimes. In addition, the cell toxicity of selected flavonol compounds was analyzed using in vitro hepatic HepG2 cells.