Following Oxygen Consumption in Singlet Oxygen Reactions via Changes in Sensitizer Phosphorescence

Light-responsive and Protic Ruthenium Compounds Bearing Bathophenanthroline and Dihydroxybipyridine Ligands Achieve Nanomolar Toxicity towards Breast Cancer Cells

We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)2 Ru(n,n'-dhbp)]Cl2 with n = 6 and 4 in 1A and 2A , respectively). Full characterization data are reported for 1A and 2A and single crystal X-ray diffraction for 1A . Both 1A and 2A are diprotic acids. We have studied 1A , 1B , 2A , and 2B (B = deprotonated forms) by UV-vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy 3 MLCT states relative to the acidic forms. Complexes 1A and 2A produce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC50 light values as low as 0.50 μM with PI values as high as >200 vs. MCF7. Computational studies were used to predict the energies of the 3 MLCT and 3 MC states. An inaccessible 3 MC state for 2B suggests a rationale for why photodissociation does not occur with the 4,4'-dhbp ligand. Low dark toxicity combined with an accessible 3 MLCT state for 1 O2 generation explains the excellent photocytotoxicity of 2.

Singlet Oxygen Phosphorescence as a Probe for Triplet-State Dissolved Organic Matter Reactivity

Triplet-state chromophoric dissolved organic matter (³CDOM*) plays an important role in aquatic photochemistry, yet much remains unknown about the reactivity of these intermediates. To better understand the kinetic behavior and reactivity of ³CDOM*, we have developed an indirect observation method based on monitoring time-resolved singlet oxygen (¹O₂) phosphorescence kinetics. The underpinning principle of our approach relies on the fact that O₂ quenches almost all triplets with near diffusion limited rate constants, resulting in the formation of ¹O₂, which is kinetically linked to the precursors. A kinetic model relating ¹O₂ phosphorescence kinetics to triplet excited states produced from isolated humic substances and in whole natural-water samples (hereafter referred to as ³CDOM*) was developed and used to determine rate constants governing ³CDOM* natural lifetimes and quenching by oxygen and 2,4,6-trimethylphenol (TMP), a common triplet probe molecule. ³CDOM* was found to exhibit smaller O₂ and TMP quenching rate constants, ∼9 × 10⁸ and ∼8 × 10⁸ M^-1 s^-1, respectively, compared with model sensitizers, such as aromatic ketones. Findings from this report shed light on the fundamental photochemical properties of CDOM in organic matter isolates and whole waters and will help refine photochemical models to more accurately predict pollutant fate in the environment.

Photodriven Oxygen Removal via Chromophore-Mediated Singlet Oxygen Sensitization and Chemical Capture

We report a general, photochemical method for the rapid deoxygenation of organic solvents and aqueous solutions via visible light excitation of transition metal chromophores (TMCs) in the presence of singlet oxygen scavenging substrates. Either 2,5-dimethylfuran or an amino acid (histidine or tryptophan methyl ester) was used as the substrate in conjunction with an iridium or ruthenium TMC in toluene, acetonitrile, or water. This behavior is described for solutions with chromophore concentrations that are pertinent for both luminescence and transient absorption spectroscopies. These results consistently produce TMC lifetimes comparable to those measured using traditional inert gas sparging and freeze-pump-thaw techniques. This method has the added benefits of providing long-term stability (days to months); economical preparation due to use of inexpensive, commercially available oxygen scrubbing substrates; and negligible size and weight footprints compared to traditional methods. Furthermore, attainment of dissolved [O₂] < 50 μM makes this method relevant to any solution application requiring low dissolved oxygen concentration in solution, provided that the oxygenated substrate does not interfere with the intended chemical process.