Excimer or aggregate? Near infrared electro- and photoluminescence from multimolecular excited states of N^C^N-coordinated platinum(ii) complexes

Abstract

We present an experimental and theoretical study of aggregate excited states formed by complexes of the type Pt(N^C^N)Cl, which display near-infrared (NIR) photoluminescence in film and NIR electroluminescence in OLED devices. Here, N^C^N represents a tridentate cyclometallating ligand binding through a central benzene ring and two lateral N-heterocycles. A simple theoretical model for the bimolecular excited states of these complexes accounts for the observed behaviour in terms of metal–metal-to-ligand charge-transfer (MMLCT). Detailed photophysical study provides insight into the kinetic properties of Pt–Pt excimers in solution, in films processed from solution, and in vacuum-deposited films. A clear distinction between emission from dimers and higher oligomers (e.g., trimers and tetramers) is evident. We also demonstrate an alternative approach to the architecture of OLEDs based on aggregate states of Pt(II) complexes. An emissive layer (EML) as thin as 1 to 2 nm can be employed, without compromising the efficiency, while reducing the use of the precious-metal compound by >90% compared to typical devices using EMLs of ∼20–40 nm thickness. For example, OLEDs obtained with an EML of 2 nm thickness display an EQE of 1.2% with λ_EL = 817 nm and a maximum radiosity of 1.46 mW cm⁻². As for the photoluminescence of films, the longest-wavelength electroluminescence from devices employing pristine films of emitter is also attributed to trimers and tetramers.