DNA-Binding Properties of Non-Intercalating Water-Soluble Organometallic Ir(III) Luminophores

A series of Ir(III) complexes, [Ir(C^N)₂(en)]⁺ (where C^N = 2-phenyl-benzo[d]thiazolyl cyclometalating ligand; en = ethylene diamine), is reported with structural variation via a substituent (H, Me, OMe, Cl, OCF₃) at the coordinated phenyl ring. The complexes were soluble in aqueous buffer, with solubility limits correlating inversely with the predicted logP. The complexes display efficient visible absorption at 400-500 nm (ε ∼ 5000 M^-1 cm^-1) due to charge-transfer transitions and are triplet emitters in aerated buffer (λ_em = 529-540 nm; lifetimes up to 0.763 µs; Φ_em ≤ 12%). Each complex was investigated, via computational and biophysical experiments, in the context of DNA binding. According to UV-visible titrations, the cationic complexes bind to DNA with apparent affinities ranging from 6 × 10⁴ to 5 × 10⁵ M^-1 with apparent binding site sizes between 0.4 and 1.0 base pairs. Isothermal titration calorimetry (ITC) showed that complexes [Ir(L1-3)₂(en)]Cl bind to DNA in two types of binding sites, viz., a high-affinity (10⁷-10⁸ M^-1) binding site with characteristics of minor or major groove binding and a low-affinity binding site (10⁵ M^-1) with characteristics of non-specific binding to negatively charged DNA, with binding supported by hydrophobic interactions between complexes.