Decrease of intermolecular interactions for less-doped efficient deep blue monomer light-emitting diodes

Intermolecular interactions arise from molecular aggregation and cause concentration quenching and excimer emission. Although aggregation or excimer induced emission are sometimes favored, for example for colour tuning, intermolecular interactions should usually be minimized because concentration quenching and excimer emission usually result in poor efficiency and colour instability. In this paper, two novel tert-butyl modified naphthyl derivates, 4,5'-di{N,N,N',N'- tetrakis-(4-tert- butyl-pheny)]phenyl-1''',4'''-diamine}-1,1'-binaph- thalenyl (BN5) and 1,5-di-{[N,N,N', N'-tetrakis-(4-tert-butyl-phenye]phenyl-1 '',4"'-diamine}-3,7-bi-tertbuty-lnaphthalenyl (BN6), have been synthesized and their intriguing photophysical properties and high-efficiency deep-blue electroluminescence (EL) in less-concentrated condensed phase were investigated. Electmluminescent devices based on BN5 in 50% matrix achieved a high external quantum efficiency (EQE) of 3.7% and the Commission Internationale de L' Eclairage (CIE) coordinates of (0.160, 0.081) which is very close to the National Television Standards Committee's (NTSC of USA) pure blue standard. Meanwhile, the non-doped device based on BN5 demonstrated a much higher EQE of 4.5%. Neat BN6-based device presented a pure monomer emission (lambda(peak )= 420 nm) with an extremely high EQE of 5.1% and a satisfactory CIE coordinate of (0.171, 0.076). Such values are the best ever reported for 420 nm EL emission with an un-doped layer. We attributed the enhanced performance of deep blue monomer light-emitting diodes to tert-butyl groups of BNs due to a reduction of intermolecular interaction, especially the tert-butyl substituents on the molecular backbone of BN6 play a major role in inhibiting dimer and crystallization-induced emission.