Xiao-Chun Hang

Highly efficient blue-emitting cyclometalated platinum(II) complexes by judicious molecular design

Luminescent properties of cyclometalated Ir and Pt complexes have been the focus of considerable research, driven in large part by their potential use as emitters in organic lightemitting diodes (OLEDs). This class of phosphorescent emitters has demonstrated the ability to harvest both electrogenerated singlet and triplet excitons, resulting in a theoretical 100 % electron-to-photon conversion efficiency. Driven by the technological need for full-color displays and solid-state lighting applications, the development of stable and efficient Ir and Pt complexes that emit in the range of 400–460 nm (blue region) is vital. Thus far, the approach to achieve efficient blue-phosphorescent OLEDs has focused on Irbased complexes with either high triplet energy cyclometalated ligands, such as 4,6-difluorophenylpyridine, or electronwithdrawing ancillary ligands, such as picolinate and tetrakis(1-pyrazolyl)borate. There are comparatively few reports on deep blue phosphorescent emitters with fluorine-free cyclometalating ligands, despite potential for improved optoelectronic stability compared to fluorinated derivatives. An example of such a class of materials is metal complexes cyclometalated with the methyl-2-phenylimidazole (pmi) ligand and related analogues that are coordinated to the metal through a neutral carbene. Several Ir complexes have been reported to have efficient deep blue phosphorescent emission at room temperature, including mer-tris(Ndibenzofuranyl-N-methylimidazole) iridium(III) [Ir(dbfmi)], tris(1-cyanophenyl-3-methylimidazolin-2-ylidene-C,C2’) iridium(III) [Ir(cnpmic)], and mer-tris(phenyl-methyl-benzimidazolyl) iridium(III) [m-Ir(pmb)3]. [7c] However, these complexes suffer from either long luminescent decay or relatively low quantum efficiency compared to Ir complexes based on the cyclometalated 2phenylpyridine ligand that have quantum efficiency F of 0.8– 1 and a luminescent lifetime t of 1–5 ms. This difference can be attributed to the combined effects of a high non-radiative decay rate (knr) and low radiative decay rate (kr), which are dictated by the intrinsic properties of the selected metal complex system. Thus, it will be highly desirable to identify rational design motifs that can improve the luminescent properties of deep blue phosphorescent emitters. Compared to Ir analogues, there are relatively few reports on platinum complexes cyclometalated with phenylimidazole carbene ligands. However, one such compound, platinum(II) bis(methylimidazolyl)benzene chloride (Pt-16), has demonstrated impressive device performance with a maximum external quantum efficiency (EQE) of 15.7% and Commission Internationale de L clairage (CIE) coordinates of (0.16, 0.13). Moreover, Pt complexes can provide additional structural variation owing to the square-planar configuration allowing ligands to be designed that are bidentate, tridentate and tetradentate. These variations can significantly alter the ground and excited state properties of Pt complexes. Herein, we report (pmi)Pt-based complexes that demonstrate a higher luminescent quantum yield and faster radiative decay process than published Ir carbene analogues. A new class of Pt complexes with tetradentate ligands have been synthesized. The complexes have a conventional cyclometalated fragment bridged with oxygen to an LL chelating group, where LL is an ancillary chelate, such as, phenoxyl pyridine (POPy) or carbazolyl pyridine (CbPy). The structures of Pt[pmi-O-POPy], Pt[pmi-O-CbPy], and Pt[ppz-OCbPy] are shown in Scheme 1, and are denoted as PtOO7,

Ring-opening reactions of difluoro(methylene)cyclopropanes with halogens and amines.

The distal and proximal bond of difluoro(methylene)cyclopropanes (F2MCPs) could be cleaved, respectively, under different conditions to give the corresponding ring-opening products. The reaction mechanisms are discussed.

Tri- and di-fluoroethylation of alkenes by visible light photoredox catalysis

The tri- and di-fluoroethylation of alkenes with sulfonium salts, (Ph2S+CH2RF TfO−) (RF = CF3 or HCF2), by visible light photoredox catalysis to give tri-/di-fluoroethyl alkenes or methoxytri-/di-fluoroethylation products are described. It was found that varying the reaction solvent led to changes in the reaction path.