New Class of Homoleptic and Heteroleptic Bis(terpyridine) Iridium(III) Complexes with Strong Photodynamic Therapy Effects

Six homo- or heteroleptic tricationic Ir(R-1-tpy)(R-2-tpy)(3+) complexes (Ir1-Ir6, R-1/R-2 = Ph, 4'-N(CH3)(2)Ph, pyren-1-yl, or 4'-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}Ph, tpy = 2,2';6',2 ''-terpyridine) were synthesized and tested for photodynamic therapy (PDT) effects. The ground- and excited-state characteristics of these complexes were studied systematically via spectroscopic methods and quantum chemistry calculations. All complexes possessed intraligand charge transfer ((ILCT)-I-1)/metal-to-ligand charge transfer ( 1MLCT) dominated transition(s) in their low-energy absorption bands, which red- shifted with the increased electron-releasing strength of the R-1/R-2 substituent. Five of the complexes exhibited ligand- centered (3)pi,pi*/(ILCT)-I-3/(MLCT)-M-3 emission. With a stronger electron-releasing R-1/R-2 substituent, the degree of charge-transfer contribution increased, leading to a decrease of the emission quantum yield. When the 4'-N(CH3)(2)Ph substituent was introduced on both tpy ligands, the emission of Ir3 was completely quenched. Our study on the transient absorption of these complexes demonstrated that they all possessed broadband triplet excited-state absorption in the visible to the near-IR regions. Pyrenyl substitution of one or both tpy ligands, as in Ir4 and Ir5, increased the lifetimes of the lowest triplet excited state and the singlet oxygen (O-1(2)) production efficiencies. Ir1-Ir5 were nontoxic toward SK-MEL-28 cells, with photocytotoxicities that varied from 0.18 to 153 mu M (EC50 values). Among them, Ir4 had the highest O-1(2) quantum yield (0.81) in cell-free conditions, showing the largest photocytotoxicity against SK-MEL-28 cells for Ir(III) PSs to date, and was the most efficient generator of reactive oxygen species (ROS) in vitro. Ir4 possessed a very large phototherapeutic index (PI = dark EC50/light EC50) of >1657, the largest reported for an Ir(III) complex photosensitizer upon broadband visible light (400-700 nm) activation. Ir4 also exhibited a very strong PDT effect toward MCF-7 breast cancer cells and its xenograft tumor model. Upon 450 nm light activation, Ir4 dramatically inhibited the xenograft tumor growth and exhibited negligible side effects upon PDT treatment.