Luminescent ruffled iridium(iii) porphyrin complexes containing N-heterocyclic carbene ligands: structures, spectroscopies and potent antitumor activities under dark and light irradiation conditions

Abstract

A panel of iridium(III) porphyrin complexes containing axial N-heterocyclic carbene (NHC) ligand(s) were synthesized and characterized. X-ray crystal structures of the bis-NHC complexes [IrIII(ttp)(IMe)2]+ (2a), [IrIII(oep)(BIMe)2]+ (2d), [IrIII(oep)(IiPr)2]+ (2e) and [IrIII(F20tpp)(IMe)2]+ (2f) display ruffled porphyrin rings with mesocarbon displacements of 0.483–0.594 Å and long Ir–CNHC bonds of 2.100–2.152 Å. Variable-temperature 1H NMR analysis of 2a reveals that the macrocycle porphyrin ring inversion takes place in solution with an activation barrier of 40 ± 1 kJ mol−1. The UV-vis absorption spectra of IrIII(por)–NHC complexes display split Soret bands. TD-DFT calculations and resonance Raman experiments show that the higher-energy Soret band is derived from the 1MLCT dπ(Ir) → π*(por) transition. The near-infrared phosphorescence of IrIII(por)–NHC complexes from the porphyrin-based 3(π, π*) state features broad emission bands at 701–754 nm with low emission quantum yields and short lifetimes (Φem < 0.01; τ < 4 μs). [IrIII(por)(IMe)2]+ complexes (por = ttp and oep) are efficient photosensitizers for 1O2 generation (Φso = 0.64 and 0.88) and are catalytically active in the light-induced aerobic oxidation of secondary amines and arylboronic acid. The bis-NHC complexes exhibit potent dark cytotoxicity towards a panel of cancer cells with IC50 values at submicromolar levels. The cytotoxicity of these complexes could be further enhanced upon light irradiation with IC50 values as low as nanomolar levels in association with the light-induced generation of reactive oxygen species (ROS). Bioimaging of [IrIII(oep)(IMe)2]+ (2c) treated cells indicates that this Ir complex mainly targets the endoplasmic reticulum. [IrIII(oep)(IMe)2]+ catalyzes the photoinduced generation of singlet oxygen and triggers protein oxidation, cell cycle arrest, apoptosis and the inhibition of angiogenesis. It also causes pronounced photoinduced inhibition of tumor growth in a mouse model of human cancer.