Investigation of photoluminescence properties of a DPA functionalized tris-cyclometalated iridium complex and its potential for metal sensing

Herein we report the synthesis and investigation of properties of a luminescent tris-cyclometalated complex [Ir(ppy)₂ppy-dpa] (1), derived from fac-[Ir(ppy)₃]. The complex carries a dipicolylamine (dpa) chelator on one of the 2-phenylpyridine (ppy) ligands. The dpa chelator exhibits minimal effect on the photophysical properties of the core iridium luminophore. The complex exhibits bright photoluminescence, which can be elicited with single photon and two-photon excitation. The two-photon excitation is achieved with light in the NIR region. Aggregation in aqueous solution has an effect on the photoluminescence properties of complex 1. The emission of complex 1 is quenched nearly completely in the presence of Cu²⁺ ions with a detection limit of 3.5 × 10⁻⁷ M. The luminescence of iridium complex 1 is unaffected by the presence of Co³⁺ and Zn²⁺. However, the emission of complex 1 is moderately quenched in the presence of Mn²⁺ and Fe²⁺ ions, and greatly quenched in the presence of Fe³⁺, Co²⁺, and Ni²⁺. However, Cu²⁺ causes its effect even when these competing cations are present. Complex 1 exhibits low cytotoxicity towards HeLa cells and can be imaged with single photon excitation at 405 nm or two-photon excitation at 800 nm.