Bis-oxazoline derivatives as ancillary ligands for bis-cyclometalated iridium complexes.

Organometallic iridium complexes with two cyclometalated ligands (CN) and one bis-oxazoline derived ancillary ligand (L^X), i.e. (CN)2Ir(L^X), are reported. The CN ligands are 1-phenylpyrazoline (ppz), 2-(4,6-difluorophenyl)pyridine (F2ppy), 2-phenylpyridine (ppy), 1-phenylisoquinoline (piq). The box ligand is (4S)-(+)-phenyl-α-[(4S)-phenyloxazolidin-2-ylidene]-2-oxazoline-2-acetonitrile. The emission of these complexes span across the visible and into the near-ultraviolet region of the electromagnetic spectrum with moderate to high photoluminescence quantum yields (ΦPL = 0.45-1.0). These complexes were found to emit from a metal-ligand to ligand charge transfer (ML'LCT) state and have lifetimes (1.3-2.1 µs), radiative rates (105 s-1), and nonradiative rates (104-105 s-1) comparable to state-of-the-art iridium emitters. The (ppy)2Ir(BOX-CN) complexes were resolved into the Δ- and Λ- diastereomers using differences in their solubility and additionally characterized by x-ray crystallography, stability, and chiroptic studies. The high ΦPL of these isomers results in the best to date brightness for circularly polarized luminescence (CPL) from iridium complexes (7.0 M-1 cm-1), with dissymmetry factors of -0.57 × 10-3 and +1.9 × 10-3 for 3Δ and 3Λ, respectively. The significant difference in CPL magnitude between 3Δ and 3Λ likely arises from interligand interactions (edge-to-face arrangement versus strong π-π interaction) for the pendant phenyl ring of the BOX-CN ligand which differ for the two isomers.

Impaired odontogenic differentiation of senescent dental mesenchymal stem cells is associated with loss of Bmi-1 expression.

INTRODUCTION: Dental mesenchymal stem cells (dMSCs) might differentiate into odontoblast-like cells and form mineralized nodules. In the current study, we investigated the effects of senescence on odontogenic differentiation of dMSCs. METHODS: dMSCs were serially subcultured until senescence. Telomere lengths and telomerase activities were determined by quantitative polymerase chain reaction. Expression of genes involved in cell proliferation and differentiation, eg, Bmi-1, p16(INK4A), osteocalcin (OC), dentin sialoprotein (DSP), bone sialoprotein (BSP), and dentin matrix protein-1 (DMP-1) were assayed by Western blotting and quantitative reverse transcription polymerase chain reaction. Exogenous Bmi-1 was expressed in dMSCs by using retroviral vectors. Odontogenic differentiation was assayed by alkaline phosphatase activity. RESULTS: Subculture-induced replicative senescence of dMSCs led to reduced expression of Bmi-1, OC, DSP, and BSP compared with rapidly proliferating cells, whereas p16(INK4A) level increased. The cells exhibited progressive loss of telomeric DNA during subculture, presumably as a result of lack of telomerase activity. Bmi-1 transduction did not affect proliferation of cells but enhanced the expression of OC and DSP in the late passage cultures. Bmi-1-transduced cells also demonstrated enhanced alkaline phosphatase activity and mineralized nodule formation. CONCLUSIONS: These results indicate that dMSCs lose their odontogenic differentiation potential during senescence, in part by reduced Bmi-1 expression.