Solvent dependent photosensitized singlet oxygen production from an Ir(iii) complex: pointing to problems in studies of singlet-oxygen-mediated cell death

Abstract

A cationic cyclometallated Ir(III) complex with 1,10-phenanthroline and 2-phenylpyridine ligands photosensitizes the production of singlet oxygen, O2(a(1)Δ(g)), with yields that depend appreciably on the solvent. In water, the quantum yield of photosensitized O2(a(1)Δ(g)) production is small (ϕ(Δ) = 0.036 ± 0.008), whereas in less polar solvents, the quantum yield is much larger (ϕ(Δ) = 0.54 ± 0.05 in octan-1-ol). A solvent effect on ϕ(Δ) of this magnitude is rarely observed and, in this case, is attributed to charge-transfer-mediated processes of non-radiative excited state deactivation that are more pronounced in polar solvents and that kinetically compete with energy transfer to produce O2(a(1)Δ(g)). A key component of this non-radiative deactivation process, electronic-to-vibrational energy transfer, is also manifested in pronounced H2O/D2O isotope effects that indicate appreciable coupling between the Ir(III) complex and water. This Ir(III) complex is readily incorporated into HeLa cells and, upon irradiation, is cytotoxic as a consequence of the O2(a(1)Δ(g)) thus produced. The data reported herein point to a pervasive problem in mechanistic studies of photosensitized O2(a(1)Δ(g))-mediated cell death: care must be exercised when interpreting the effective cytotoxicity of O2(a(1)Δ(g)) photosensitizers whose photophysical properties depend strongly on the local environment. Specifically, the photophysics of the sensitizer in bulk solutions may not accurately reflect its intracellular behavior, and the control and quantification of the O2(a(1)Δ(g)) "dose" can be difficult in vivo.

Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers: Exploring the Domain of Resonant Energy Transfer at 1 eV

“…This could reflect an increase in CT character in the cluster due to the electron-withdrawing NO 3 ligands. Such CT character could facilitate channels for excited state deactivation, both within the cluster itself and within the sensitizer–O 2 (X 3 Σ g – ) collision complex. − Indeed, in support of the former, note that the triplet state lifetime of compound 6 in deoxygenated solution is shorter than that of compounds 1 – 5 (Table ). In support of the latter, despite large values of f T in compound 6 , ϕ Δ remains low.…”

“…Even then, features of the excited state sensitizer–O 2 (X 3 Σ g – ) collision complex can preclude energy transfer to produce O 2 (a 1 Δ g ). For example, if this collision complex has an appreciable amount of charge transfer (CT) character, then two other channels of sensitizer deactivation can effectively compete against the energy transfer channel: (a) electron transfer to form the superoxide radical anion and/or (b) CT-mediated nonradiative deactivation to re-form the ground state molecules. ,− However, through extensive studies, it has been demonstrated that the channel for energy transfer will be increasingly favored when the energy of the sensitizer triplet state decreases to approach a condition of resonance with the unique low-energy O 2 (X 3 Σ g – )–O 2 (a 1 Δ g ) transition. , …”

A ligand substituted tungsten iodide cluster: luminescence vs. singlet oxygen production

“…34c,d Furthermore, solvent-effects could potentially also tune the excited state properties of the clusters. 34a, 35 The functionalization of photophysically active clusters by means of deposition, incorporation, and encapsulation is a logical need for further applications. 10…”

Significantly Increased Stability of Donor–Acceptor Molecular Complexes under Heterogeneous Conditions: Synthesis, Characterization, and Photosensitizing Activity of a Nanostructured Porphyrin–Lewis Acid Adduct

“…This observation seems to be mainly due to the higher photooxidative stability of the catalyst under aqueous conditions. It should be noted that because of the short lifetime of singlet oxygen in water, the singlet oxygen quantum yield of photosensitizers is lower in water compared to that in organic solvents. − …”