Cancers treated by chemotherapy use reagents such as cisplatin. Cisplatin, cis-[PtII(NH3)2Cl2] is a Pt(II) metal complex that terminates cancer cells after undergoing thermal hydrolysis in a solution that activates covalent binding to DNA. Cisplatin is nonselective in differentiating healthy cells from cancer cells and targets them indiscriminately causing unwanted side effects such as ototoxicity and nephrotoxicity. Photodynamic therapy uses photochemical properties of chemical reagents to selectively target cancer cells using visible light. Most photodynamic therapeutics are oxygen-dependent and cannot effectively treat tumors in hypoxic environments. This research aims to take on this challenge by using Rh(III) polypyridyl complexes’ photochemical properties. When irradiated with UV light at 308 nm, cis-[RhIII(NN)2Cl2]+ complexes (where NN is a bidentate polypyridyl ligand) can photodissociate chloride and then covalently bind to DNA, but UV light damages tissue. To make the cis-[RhIII(NN)2Cl2]+ complexes more visible light active, a BODIPY fluorophore is coupled to the Rh(III) metal center. BODIPY is an emissive fluorophore that absorbs visible light strongly and can undergo excited-state electron transfer. This research will discuss the photophysical and electrochemical properties of a new visible light active Rh-BODIPY complex. The coupling of BODIPY to Rh(III) greatly expands the electronic absorbance into the visible region with an absorption maximum at 529 nm which has been assigned as the BODIPY π→π*. Cyclic voltammetry of the Rh-BODIPY complex suggests a BODIPY-based HOMO at 1.09 V and a Rh(III)-based LUMO at –0.80 V. The fluorescence quantum yield (Φfl) of free BODIPY is 0.65 and when coupled to Rh(III) the Φfl is 0.0029, suggesting fluorescence quenching of the BODIPY bound to Rh(III). Transient absorption studies were performed to understand photochemical processes of the BODIPY coupled to Rh(III) at the pico- and femtosecond timescale. Upon irradiation using 532 nm light (green light), it is observed that the BODIPY π→π* transition at 529 nm decreases. Using spectroelectrochemistry, a similar decrease is observed upon BODIPY oxidation, which supports photo-oxidation of BODIPY upon irradiation with visible light. Binding studies show the complex's unique interactions with DNA. It is hypothesized that an excited-state electron transfer occurs from the BODIPY-based HOMO to the Rh(III)-based LUMO to photodissociate chloride and then bind to DNA.

Photobinding to DNA and ultrafast transient absorption studies of a Rh(III)-BODIPY complex

ACS Spring 2022

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