Photophysical and Photobiological Properties of Dinuclear Iridium(III) Bis-tridentate Complexes

摘要

A series of cationic dinuclear iridium(III) complexes (>Ir1 - >Ir5) bearing terpyridine-capped fluorenyl bridging ligand and different polypyridyl or cyclometalating terminal tridentate ligands were synthesized, characterized, and evaluated for their photophysical and photobiological activities. The influence of the bridging and terminal ligands on the photophysical properties of the complexes was investigated by UV-vis absorption, emission, and transient absorption spectroscopy, and simulated by TDDFT calculations. All of the complexes displayed strong bridging-ligand localized visible ¹π,π* absorption and red- or near-infrared (NIR) phosphorescence as well as broad triplet excited-state absorption across both visible and NIR wavelengths. These triplet states were assigned as predominantly ³π,π* for >Ir1 (τ = 3.1 μs) and >Ir4 (τ = 48 μs), and predominantly ³CT (charge transfer) for >Ir2, >Ir3 and >Ir5 (τ = 1.7–2.7 μs). Complexes >Ir1 – >Ir5 acted as in vitro photodynamic therapy (PDT) agents toward human SK-MEL-28 melanoma cells when activated with visible light, with submicromolar photocytotoxicity and phototherapeutic indices (PIs) ranging from 20 to almost 300. The in vitro PDT effects with visible light did not correlate with singlet oxygen (¹O2) quantum yields or DNA photocleaving capacity probed under cell-free conditions. All of the Ir(III) complexes phosphoresced brightly when associated with compromised cells (with or without a light treatment) and exhibited photoactivated cellular uptake, highlighting the theranostic potential of this new class of Ir(III) complex photosensitizers.