Structure-Photoreactivity Relationship Study of Substituted 3-Hydroxyflavones and 3-Hydroxyflavothiones for Improving Carbon Monoxide Photorelease

Carbon monoxide (CO) is notorious for its toxic effects but is also recognized as a gasotransmitter with considerable therapeutic potential. Due to the inherent challenges in its delivery, the utilization of organic CO photoreleasing molecules (photoCORMs) represents an interesting alternative to CO administration characterized by high spatial and temporal precision of release. This paper focused on the design, synthesis, and photophysical and photochemical studies of 20 3-hydroxyflavone (flavonol) and 3-hydroxyflavothione derivatives as photoCORMs. Newly synthesized compounds bearing various electron-donating and electron-withdrawing groups show bathochromically shifted absorption maxima and considerably enhanced CO release yields compared to the parent unsubstituted flavonol, exceeding 0.8 equiv of released CO in derivatives exhibiting excited states with a charge-transfer character. Until now, such outcomes have been limited to flavonol derivatives possessing a pi-extended aromatic system. In addition, thione analogs of flavonols, 3-hydroxyflavothiones, show substantial bathochromic shifts of their absorption maxima and enhanced photosensitivity but provide lower yields of CO formation. Our study elucidates in detail the mechanism of CO photorelease from flavonols and flavothiones, utilizing steady-state and time-resolved spectroscopies and photoproduct analyses, with a particular emphasis on unraveling the structure-photoreactivity relationship and understanding competing side processes.