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

Abstract 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, OCF 3 ) 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 × 10 4 to 5 × 10 5  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 (10 7 –10 8  M −1 ) binding site with characteristics of minor or major groove binding and a low‐affinity binding site (10 5  M −1 ) with characteristics of non‐specific binding to negatively charged DNA, with binding supported by hydrophobic interactions between complexes.