Photoluminescent iridium(III), platinum(II) and gold(I) complexes containing bis-N-heterocyclic carbene and/or 2-acetylbenzo[b]thiophen-3-olate ligands and their applications in photo-catalysis and materials science

Abstract

The synthesis, structures, photo-physical and spectroscopic properties of four classes of luminescent iridium(III), platinum(II) and gold(I) complexes containing bis-N-heterocyclic carbene (bis-NHC) and/or 2-acetylbenzo[b]thiophen-3-olate (bt) ligands are described. And their applications as photo-catalysts, dual emission emitters and white or red-light emitters are studied. A new class of cyclometalated Ir(III) complexes supported by bidentate C-deprotonated cyclometalated C^N and bis-NHC ligands have been synthesized. Among these Ir(III) complexes, complex 3.4Meb containing C-deprotonated fluorenyl-substituted C^N ligand shows good photo-stability and is a highly efficient photo-catalyst for reductive C–C bond formation from C(sp3/sp2)–Br bonds. In the presence of this complex and an additional cobalt catalyst, visible-light-driven CO2 reduction with excellent turnover numbers (> 2400) and selectivity (CO over H2 in gas phase: > 95%) was observed upon irradiation with visible-light (λ > 440 nm). A series of [(bis-NHC)Pt(bt)]PF6 complexes containing bis-NHC and 2-acetylbenzo[b]thiophen-3-olate (bt) ligands have been prepared. Ultra-fast time-resolved spectroscopic measurments on [(bis-NHC)Pt(bt)]PF6 (4.1b) and [(bis-NHC)Pt(nt)]PF6 (4.1c, nt = 2-acetylnaphtho[2,3-b]thiophen-3-olate) reveal a moderate intersystem crossing rate (ISC-1) and an ultrafast rate (ISC-2) for 4.1b (S1/T1: 220 ps; Sn/T1 (n>1): < 100 fs) and 4.1c (S1/T1: 840 ps; Sn/T1 (n>1): < 100 fs) respectively. TD-DFT calculations show that the origin of dual emission comes from an inappreciable participation of metal orbital in S1 state and a significant metal-to-ligand-charge-transfer (MLCT) character in Sn state, which leads to moderate ISC-1 rate and ultrafast ISC-2 rate. Light-emitting electrochemical cells (LECs) have been fabricated using 4.1c, which emits fluorescent green light and phosphorescent red light simultaneously leading to nearly white light with CIE color coordinates of (0.563, 0.372). A series of dinuclear Au(I) complexes containing a newly designed bis-NHC ligand [deprotonated (12Z,52Z)-11H,51H-1(1,3),5(3,1)-dibenzo[d]imidazol-3-iuma-3,7(1,2)-dibenzenacyclooctaphane-13,53-diium] has been synthesized and structurally characterized. Similar to [Au2(dcpm)2]2+ (e.g. dcpm = bis(dicyclohexylphosphanyl)-methane), the metal-metal bonded 3(5dσ*6pσ) state of [Au2(bis-NHC)2]2+ plays a pivotal role in luminescence properties and photo-catalysis. Ultra-fast spectroscopic studies reveal rapid binding processes between solvent molecules or anions present in the solution and triplet excited state of [Au2(bis-NHC)2]2+. These binding processes are susceptible toward micro-environments, which can be harnessed for applications. For example, the [Au2(bis-NHC)2]2+ has been shown to be a useful photo-catalyst for cleavage of C(sp2)-Br bonds, hydroxylation of arylboronic acid with aerated oxygen, and decarboxylation of amino acids. In cooperation with Br- ion, a complex (5.2c) is generated and can be doped into polyvinylcarbazole (PVK) to fabricate a white light emitting device with CIE color coordinates of (0.34, 0.39). A new class of thermally stable and strongly luminescent cyclometalated Ir(III) complexes containing bt ligand have been synthesized and used for the fabrication of red organic light-emitting diodes (OLEDs). These heteroleptic Ir(III) complexes with bt as ancillary ligand have a high decomposition temperature and low emission self-quenching constants compared to that of their corresponding acac counterparts. The performance of stable red emission OLEDs with CIE chromaticity coordinates of (0.67, 0.33) fabricated with 6.6 is comparable to that of the analogous Ir(III) complex that bears acac ligand (6.6a) fabricated under similar conditions.