Effects of Ancillary Ligands on Deep Red to Near-Infrared Cyclometalated Iridium Complexes

Ligand Dictated Photosensitization of Iridium(III) Dithiocarbamate Complexes for Photodynamic Therapy

Organelle-targeted photosensitizers (PSs) for photodynamic therapy (PDT) are considered as an effective therapeutic strategy for the development of next generation PSs with the least side effects and high therapeutic efficacy. However, multiorganelle targeted PSs eliciting PDT via both type I and type II mechanisms are scarce. Herein, a series of cyclometalated iridium(III) complexes were formulated [Ir(C∧N)2(S∧S)] (C∧N = 2-phenylpyridine (ppy) and 2-(thiophen-2-yl)pyridine (thpy); S∧S = diethyldithiocarbamate (DEDTC), morpholine-N-dithiocarbamate (MORDTC) and methoxycarbonodithioate (MEDTC)) and the newly designed complexes Ir2@DEDTC and Ir1@MEDTC were characterized by single crystal X-ray crystallography. Complexes containing thpy as C∧N ligand exhibit excellent photophysical properties such as red-shifted emission, high singlet oxygen quantum yield (ϕΔ) and longer photoluminescence lifetime when compared with complexes containing ppy ligands. Ir2@DEDTC exhibits the highest ϕΔ and photoluminescence lifetimes among the synthesized complexes. Therefore, Ir2@DEDTC was chosen to evaluate the photosensitizing ability to produce reactive oxygen species (ROS). Upon blue light irradiation (456 nm), it efficiently produces ROS, i.e., hydroxy radical (•OH) and singlet oxygen (1O2), which was confirmed by electron paramagnetic resonance (EPR) spectroscopy. In vitro photocytotoxicity toward HCT116, HeLa, and PC3 cell lines showed that out of all the synthesized complexes, Ir2@DEDTC has the highest photocytotoxic index (PI > 400) value. Ir2@DEDTC is efficiently taken up by the HCT116 cell line and accumulated mainly in the lysosome and mitochondria of the cells, and after PDT treatment, it elicits cell shrinkage, membrane blebbing, and DNA fragmentation. The phototherapeutic efficacy of Ir2@DEDTC has been investigated against 3D spheroids considering its ability to mimic some of the basic features of solid tumors. The morphology was drastically altered in the Ir2@DEDTC treated 3D spheroid after the light irradiation unleashed the potential of the Ir(III) dithiocarbamate complex as a superior PS for PDT. Hence, mitochondria and lysosome targeted photoactive cyclometalated Ir(III) dithiocarbamate complex exerting oxidative stress via both type I and type II PDT can be regarded as a dual-organelle targeted two-pronged approach for enhanced PDT.

2-(Thienyl)quinoxaline derivatives and their application in Ir(III) complexes yielding tuneable deep red emitters

The synthesis and characterisation of eleven different 2-(thienyl)quinoxaline species that incorporate different points of functionality, including at the thiophene or quinoxaline rings, are described. These species display variable fluorescence properties in the visible region (λem = 401-491 nm) depending upon the molecular structures and extent of conjugation. The series of 2-(thienyl)quinoxaline species were then investigated as cyclometalating agents for Ir(III) to yield [Ir(C^N)2(bipy)]PF6 (where C^N = the cyclometalated ligand; bipy = 2,2'-bipyridine). Eight complexes were successfully isolated and fully characterised by an array of spectroscopic and analytical techniques. Two Ir(III) examples were structurally characterised in the solid state using single crystal X-ray diffraction; both structures confirmed the proposed formulations and coordination spheres in each case showing that the thiophene coordinates via a Ir-C bond. The photophysical properties of the complexes revealed that each complex is luminescent under ambient conditions with a range of emission wavelengths observed (665-751 nm) indicating that electronic tuning can be achieved via both the thienyl and quinoxaline moieties.

Modular Imine Chelates with Variable Anionic Donors Promote Red Phosphorescence in Cyclometalated Iridium Complexes

The lack of red and deep-red emitting molecular phosphors with high photoluminescence quantum yields remains a significant fundamental challenge and has implications in optoelectronic technologies for color displays and other consumer products. In this work, we introduce a series of seven new red or deep-red emitting heteroleptic bis-cyclometalated iridium(III) complexes, supported by five different ancillary ligands (L^X) from the salicylaldimine and 2-picolinamide families. Previous work had shown that electron-rich anionic chelating "L^X" ligands can be effective in supporting efficient red phosphorescence, and the complementary approach described here, in addition to being synthetically simpler, offers two key advantages over the previous designs. First, the "L" and "X" functionalities can be independently tuned, providing excellent control over the electronic energy levels and excited-state dynamics. Second, these classes of L^X ligands can have beneficial impacts on the excited-state dynamics but do not significantly perturb the emission color profile. Cyclic voltammetry experiments show that the substituents on the L^X ligand impact the HOMO energy but have a minimal effect on the LUMO energy. Photoluminescence measurements reveal that all the compounds luminesce in the red or deep-red region as a function of the cyclometalating ligand and exhibit exceptionally high photoluminescence quantum yields (Φ_PL), comparable or superior to the best-performing red-emitting iridium complexes.

Tailoring the π-system of benzimidazole ligands towards stable light-harvesting cyclometalated iridium(III) complexes

The synthesis, structure, optical and redox properties as well as photovoltaic studies of iridium(III) complexes with cyclometalated 2-arylbenzimidazoles decorated with various polyaromatic fragments and an ancillary aromatic β-diketone are reported. Despite the strong preference of the iridium(III) ion to form bis- or tris-cyclometalated complexes in which the metal participates in five-membered metallacycles, the cyclometalation of the benzimidazole ligands containing rigid π-extended systems yields dimeric complexes containing strained five- or six-membered metallacycles and allows for generating an extremely rare monocyclometalated complex. X-ray crystallography shows that the steric strain observed in the dimers is retained in heteroleptic diketonate complexes which is also corroborated by gas-phase DFT calculations. While emission maxima and redox potentials of the heteroleptic complexes exhibit just a moderate variation upon the change of the cyclometalated ligands, the extension of the π-system of the benzimidazole ligands give the complexes remarkable light absorption in the visible spectral range, which meets the requirements for application in dye-sensitized solar cells. At the titania photoanodes, these iridium dyes retain their optical properties and exhibit power conversion efficiencies under standard AM 1.5 G conditions comparable to those of other iridium-based sensitizers. These results demonstrate that the size and position of the π-extended fragment in cyclometalated ligands can modulate not only the electronic structure of the corresponding iridium(III) complexes, but also affect their composition, structure and reactivity that may find implications in future design of emerging iridium dyes, emitters and catalysts.

Oxygen-Sensing Chemiluminescent Iridium(III) 1,2-Dioxetanes: Unusual Coordination and Activity

Next generation chemiluminescent iridium 1,2-dioxetane complexes have been developed which consist of the Schaap's 1,2-dioxetane scaffold directly attached to the metal center. This was achieved by synthetically modifying the scaffold precursor with a phenylpyridine moiety, which can act as a ligand. Reaction of this scaffold ligand with the iridium dimer [Ir(BTP)2(μ-Cl)]2 (BTP = 2-(benzo[b]thiophen-2-yl)pyridine) yielded isomers which depict ligation through either the cyclometalating carbon or, interestingly, the sulfur atom of one BTP ligand. Their corresponding 1,2-dioxetanes display chemiluminescent responses in buffered solutions, exhibiting a single, red-shifted peak at 600 nm. This triplet emission was effectively quenched by oxygen, yielding in vitro Stern-Volmer constants of 0.1 and 0.009 mbar-1 for the carbon-bound and sulfur compound, respectively. Lastly, the sulfur-bound dioxetane was further utilized for oxygen sensing in muscle tissue of living mice and xenograft models of tumor hypoxia, depicting the ability of the probe chemiluminescence to penetrate biological tissue (total flux ~ 106 p/s).

Enhanced deep red to near-infrared (DR-NIR) phosphorescence in cyclometalated iridium(iii) complexes

Bis-cyclometalated iridium complexes with highly conjugated cyclometalating ligands and electron-rich ancillary ligands have exceptional quantum yields for deep-red to near-infrared phosphorescence.