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Toward Long Wavelength Absorption: Dinuclear Photo-activated CO Releasing Moieties


Although carbon monoxide is well known as a poisonous gas causing several adverse effects including fatality, CO has also been found to play a significant role in anti-inflammation, anti-apoptosis, wound healing, and vasorelaxation in mammals through various mechanisms. However, gaseous CO has a very low solubility in water (~1.8 mM) and hemoglobin's high affinity for CO serves as a buffer against beneficial effects at the cellular level. Photochemical CO release from appropriate precursors (mostly metal carbonyls) can give spatial and temporal control. Those precursors, known as photoCORMs (photo-induced carbon monoxide releasing moieties), should have biocompatibility, aerobic and thermal stability in dark and decent solubility in biosystem for in vivo CO delivery. Some of photoCORMs meet the above requirements but require Uv light which has low penetration into biological system and causes detrimental effects on cells during irradiation. Although employment of UCNP (upconverting nanoparticles) is accessible, multiphoton upconversion efficiency is still a limiting factor. Developing long wavelength visible light or near infrared light activated photoCORMs remains a challenge in this area.

In this work, a new strategy has been proposed for the delivery of carbon monoxide to physiological targets: rhenium-manganese dinuclear metal carbonyl complexes were synthesized, of which metal-metal (M-M) bond is the most labile bond, and the homolytic cleavage of this bond occur through the photo-excitation of a σMMb to σMM* transition. With conjugated diimine ligands (L), σMMb to πL* charge transfer appears at even lower energy. The homolytic cleavage of the M-M bond generates the mononuclear metal radicals. Although the radicals themselves are unlikely to be particularly reactive toward CO release, they have proved to be reactive with dioxygen to form metal oxides, at the same time to form species that are much more labile toward CO release. In this context, we are studying photochemistry of these dinuclear carbonyl complexes and explored using the hydrophobic versions of these dinuclear complexes with biocompatible polymer PLGA (poly(lactic-co-glycolic acid)) matrices as their drug delivery systems in the form of nano/micro carriers.

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