Platinum(IV) Complexes with Tridentate, NNC-Coordinating Ligands: Synthesis, Structures, and Luminescence

Platinum(II) complexes of NNC-cyclometalating ligands based on 6-phenyl-2,2'-bipyridine (HL¹) have been widely investigated for their luminescence properties. We describe how PtL¹Cl and five analogues with differently substituted aryl rings, PtL^2-6Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(NNC-Lⁿ)Cl₃ (n = 1-6). The molecular structures of several of them have been determined by X-ray diffraction. These PtLⁿCl₃ compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt(NNC)(NC)Cl]⁺. Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL¹Cl₃ with HL¹ similarly generates [Pt(NNC-L¹)₂]²⁺, which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV-vis absorption spectra of all the PtLⁿCl₃ compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly ¹[π_Ar → π*_NN] character to the pertinent excited state. A similar trend is observed for the [Pt(NNC)(NC)Cl]⁺ complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL¹(ppy)Cl]⁺ and [Pt(L¹)₂]²⁺, and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily ³[π_Ar → π*_NN] character with relatively little participation of the metal.

Access to functionalized luminescent Pt(ii) complexes by photoredox-catalyzed Minisci alkylation of 6-aryl-2,2'-bipyridines

Photoredox-mediated C-H bond alkylation of 6-aryl-2,2'-bipyridines with N-(acyloxy)phthalimides is reported. The reaction exhibits excellent functional group tolerance, including chiral aliphatic groups. The influence of the incorporated C6'-alkyl group on the photophysical properties of the corresponding (N^N^C) cyclometalated Pt(ii) complexes is described, including chiroptical properties.

Recent advances in phosphorescent platinum complexes for organic light-emitting diodes

Phosphorescent organometallic compounds based on heavy transition metal complexes (TMCs) are an appealing research topic of enormous current interest. Amongst all different fields in which they found valuable application, development of emitting materials based on TMCs have become crucial for electroluminescent devices such as phosphorescent organic light-emitting diodes (PhOLEDs) and light-emitting electrochemical cells (LEECs). This interest is driven by the fact that luminescent TMCs with long-lived excited state lifetimes are able to efficiently harvest both singlet and triplet electro-generated excitons, thus opening the possibility to achieve theoretically 100% internal quantum efficiency in such devices. In the recent past, various classes of compounds have been reported, possessing a beautiful structural variety that allowed to nicely obtain efficient photo- and electroluminescence with high colour purity in the red, green and blue (RGB) portions of the visible spectrum. In addition, achievement of efficient emission beyond such range towards ultraviolet (UV) and near infrared (NIR) regions was also challenged. By employing TMCs as triplet emitters in OLEDs, remarkably high device performances were demonstrated, with square planar platinum(II) complexes bearing π-conjugated chromophoric ligands playing a key role in such respect. In this contribution, the most recent and promising trends in the field of phosphorescent platinum complexes will be reviewed and discussed. In particular, the importance of proper molecular design that underpins the successful achievement of improved photophysical features and enhanced device performances will be highlighted. Special emphasis will be devoted to those recent systems that have been employed as triplet emitters in efficient PhOLEDs.

Tetraphenylethylene- and fluorene-functionalized near-infrared aza-BODIPY dyes for living cell imaging

The new NIR absorbing aza-BODIPY dyes bearing tetraphenylethylene and fluorinyl substituents were synthesized and characterized. The application potential of these new dyes in living cell imaging was demonstrated.

Oxygen-insensitive phosphorescence in water from a Pt-doped supramolecular array

A supramolecular array consisting of polyvinylpyridine decorated with adamantyl-substituted Pt(ii) complexes is described. The phosphorescence in aqueous environments is insensitive to diffusional quenching by molecular oxygen, due to the dense packing of the assembly.

Platinum and Gold Complexes for OLEDs

Encouraging efforts on the design of high-performance organic materials and smart architecture during the past two decades have made organic light-emitting device (OLED) technology an important competitor for the existing liquid crystal displays. Particularly, the development of phosphorescent materials based on transition metals plays a crucial role for this success. Apart from the extensively studied iridium(III) complexes with d(6) electronic configuration and octahedral geometry, the coordination-unsaturated nature of d(8) transition metal complexes with square-planar structures has been found to provide intriguing spectroscopic and luminescence properties. This article briefly summarizes the development of d(8) platinum(II) and gold(III) complexes and their application studies in the fabrication of phosphorescent OLEDs. An in-depth understanding of the nature of the excited states has offered a great opportunity to fine-tune the emission colors covering the entire visible spectrum as well as to improve their photophysical properties. With good device engineering, high performance vacuum-deposited OLEDs with external quantum efficiencies (EQEs) of up to 30 % and solution-processable OLEDs with EQEs of up to 10 % have been realized by modifying the cyclometalated or pincer ligands of these metal complexes. These impressive demonstrations reveal that d(8) metal complexes are promising candidates as phosphorescent materials for OLED applications in displays as well as in solid-state lighting in the future.

Spotlight on the ligand: luminescent cyclometalated Pt(iv) complexes containing a fluorenyl moiety

A family of Pt(iv) complexes with 2-(9,9-dimethylfluoren-2-yl)pyridine is reported, including bis- and tris-cyclometalated derivatives, which exhibit intense ³LC phosphorescent emissions in fluid solution.

Efficient Red-Emitting Platinum Complex with Long Operational Stability

A tetradentate cyclometalated Pt(II) complex, PtN3N-ptb, was developed as an emissive dopant for stable and efficient red phosphorescent OLEDs. Devices employing PtN3N-ptb in electrochemically stable device architectures achieved long operational lifetimes with estimated LT97, of over 600 h at luminances of 1000 cd/m(2). Such long operational lifetimes were achieved utilizing only literature reported host, transporting and blocking materials with known molecular structures. Additionally, a thorough study of the effects of various host and transport materials on the efficiency, turn on voltage, and stability of the devices was carried out. Ultimately, maximum forward viewing EQEs as high as 21.5% were achieved, demonstrating that Pt(II) complexes can act as stable and efficient dopants with operational lifetimes comparable or superior to those of the best literature-reported Ir(III) complexes.

Yellow/orange emissive heavy-metal complexes as phosphors in monochromatic and white organic light-emitting devices

Owing to the electron spin-orbit coupling (SOC) and fast intersystem crossing (ISC), heavy-metal complexes (such as iridium(III), platinum(II) and osmium(II) complexes, etc.) are phosphorescent emitters at room temperature. Since 1998, heavy-metal complexes as phosphors have received considerable academic and industrial attention in the field of organic light-emitting diodes (OLEDs), because they can harvest both the singlet (25%) and triplet (75%) excitons for emission during the electro-generated processes. Among all the visible colors (blue, green, yellow, orange and red), the yellow/orange heavy-metal complexes play an important role for realizing full-color OLEDs as well as high-efficiency white OLEDs, and thus the development of highly efficient yellow/orange heavy-metal complexes is a pressing concern. In this article, we will review the progress on yellow/orange heavy-metal complexes as phosphors in OLEDs. The general principles and useful tactics for designing the yellow/orange heavy-metal complexes will be systematically summarized. The structure-property relationship and electrophosphorescence performance of the yellow/orange heavy-metal complexes in monochromatic phosphorescent OLEDs (PhOLEDs) and white OLEDs (WOLEDs) will be comprehensively surveyed and discussed.

Luminescent pincer platinum(II) complexes with emission quantum yields up to almost unity: photophysics, photoreductive C-C bond formation, and materials applications

Luminescent pincer-type Pt(II)  complexes supported by C-deprotonated π-extended tridentate RC^N^NR' ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time-resolved fluorescence measurements and time-dependent DFT calculations together reveal the dependence of excited-state structural distortions of [Pt(RC^N^NR')(CC-C6 F5 )] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R-C^N^NR')(CC-Ar)] are efficient photocatalysts for visible-light-induced reductive CC bond formation. The [Pt(R-C^N^NR')(CC-C6 F5 )] complexes perform strongly as phosphorescent dopants for green- and red-emitting organic light-emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two-photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).

Cyclometallated platinum(ii) complexes containing NHC ligands: synthesis, characterization, photophysics and their application as emitters in OLEDs

A series of novel luminescent NHC Pt(ii) complexes has been synthesized, characterized and used to prepare green OLEDs.

Recent advances in transition metal complexes and light-management engineering in organic optoelectronic devices

Two of the recent major research topics in optoelectronic devices are discussed: the development of new organic materials (both molecular and polymeric) for the active layer of organic optoelectronic devices (particularly organic light-emitting diodes (OLEDs)), and light management, including light extraction for OLEDs and light trapping for organic solar cells (OSCs). In the first section, recent developments of phosphorescent transition metal complexes for OLEDs in the past 3-4 years are reviewed. The discussion is focused on the development of metal complexes based on iridium, platinum, and a few other transition metals. In the second part, different light-management strategies in the design of OLEDs with improved light extraction, and of OSCs with improved light trapping is discussed.

Recent progress in metal–organic complexes for optoelectronic applications

This critical review reports recent advances in the development of metal–organic complexes for optoelectronic applications.

Luminescent palladium(II) complexes with π-extended cyclometalated [R-C^N^N-R'] and pentafluorophenylacetylide ligands: spectroscopic, photophysical, and photochemical properties

A series of cyclometalated Pd(II) complexes that contain π-extended R-C^N^N-R' (R-C^N^N-R'=3-(6'-aryl-2'-pyridinyl)isoquinoline) and chloride/pentafluorophenylacetylide ligands have been synthesized and their photophysical and photochemical properties examined. The complexes with the chloride ligand are emissive only in the solid state and in glassy solutions at 77 K, whereas the ones with the pentafluorophenylacetylide ligand show phosphorescence in the solid state (λmax =584-632 nm) and in solution (λmax =533-602 nm) at room temperature. Some of the complexes with the pentafluorophenylacetylide ligand show emission with λmax at 585-602 nm upon an increase in the complex concentration in solutions. These Pd(II) complexes can act as photosensitizers for the light-induced aerobic oxidation of amines. In the presence of 0.1 mol% Pd(II) complex, secondary amines can be oxidized to the corresponding imines with substrate conversions and product yields up to 100 and 99%, respectively. In the presence of 0.15 mol% Pd(II) complex, the oxidative cyanation of tertiary amines could be performed with product yields up to 91%. The Pd(II) complexes have also been used to sensitize photochemical hydrogen production with a three-component system that comprises the Pd(II) complex, [Co(dmgH)2 (py)Cl] (dmgH=dimethylglyoxime; py=pyridine), and triethanolamine, and a maximum turnover of hydrogen production of 175 in 4 h was achieved. The excited-state electron-transfer properties of the Pd(II) complexes have been examined.