Design and synthesis of cyclometalated platinum(II) complexes : from photophysics to organic light−emitting devices and host−guest chemistry

Abstract

The present work reported the design and synthesis of various classes of luminescent cyclometalated platinum(II) complexes. Through judicious design of the molecular structures and the fine-tuning of the photophysical and electrochemical properties, these complexes can serve as promising candidates for the application in organic light-emitting devices (OLEDs) and host–guest assemblies. A series of dendritic carbazole-containing luminescent alkynylplatinum(II) complexes has been synthesized, characterized and applied as phosphorescent dopants in the fabrication of solution-processable OLEDs. Their thermal stability, electrochemistry, electronic absorption, luminescence and electroluminescence properties have been studied. These complexes exhibited high photoluminescence quantum yields of up to 80 % in spin-coated thin films. In addition, the incorporation of carbazole dendrons into the platinum(II) center could significantly suppress intermolecular interactions in solid-state thin films, giving rise to emission spectra that were similar to that found in solution and irrespective of dopant concentrations. High-performance solution-processable OLEDs have also been fabricated, with a maximum current efficiency of up to 37.6 cd A^–1 and external quantum efficiency of up to 10.4 % achieved. A series of highly luminescent bipolar alkynylplatinum(II) complexes has been synthesized, characterized and applied as phosphorescent dopants in the fabrication of solution-processable OLEDs. Through the incorporation of electron-donating carbazole or triphenylamine moieties and electron-accepting phenylbenzimidazole or oxadiazole moieties into the platinum(II) core, the platinum(II) complexes have been demonstrated to exhibit bipolar charge transport character with high photoluminescence quantum yields of up to 0.75 in thin films. The introduction of meta-linkages into the complexes further helped weaken the donor-acceptor interactions, facilitating better carrier-transporting abilities. More importantly, high-performance solution-processable green-emitting OLEDs with maximum current efficiencies of up to 57.4 cd A^–1 and external quantum efficiencies of up to 16.0 % have been realized. On the other hand, a series of deep red-/near infrared-emitting chloroplatinum(II) complexes has been synthesized, characterized and applied as phosphorescent dopants in the fabrication of solution-processable OLEDs. One of the chloroplatinum(II) complexes has been structurally characterized by X-ray crystallography. The complexes have been demonstrated to exhibit high photoluminescence quantum yields of up to 0.31 in thin films. Solution-processable deep-red emitting OLEDs with external quantum efficiencies of up to 1.4 %, corresponding to Commission Internationale de l'Éclairage (CIE) coordinates of (0.69, 0.31) have been realized. Apart from OLED application studies, a new class of cyclometalated dinuclear platinum(II) complexes has been designed and synthesized to act as the guest molecules for the construction of host–guest assemblies with the alkynylplatinum(II) terpyridine molecular tweezers. Drastic color changes accompanied with spectroscopic and luminescent changes were observed upon the addition of the guest to the host molecules, attributed to the formation of Pt(II)···Pt(II) and π–π stacking interactions. Their host–guest interactions have been studied by electron absorption and emission spectroscopies, high-resolution mass spectrometry, 1H NMR and 1H–1H ROESY NMR spectroscopy. They generally showed strong binding affinities towards the molecular tweezers with large binding constants of 10^4 to 10^6 M^–1. The binding behavior was found to be sensitive to choices of the spacer backbones for the dinuclear guest complexes.