New platinum complexes exhibiting host dependent photoluminescence as single dopants in double emitting layer, voltage independent hybrid white electroluminescence devices†
Four heteroleptic platinum complexes (FPtXNDs) bearing 4-hydroxy-1,5-naphthyridine (XND) derivatives functionalized with phenyl (X = Ph), dimethylamino (X = dma), morpholino (X = mor), or phenoxazino (X = pxz) units as one ligand and 2-(2,4-difluorophenyl)pyridine (F) as the other common ligand were newly synthesized and characterized. Each platinum complex has absorption and photoluminescence (PL) spectroscopic characteristics. As demonstrated by X-ray crystallography (of FPtdmaND and FPtpxzND), electrochemical and theoretical studies, the unusual spectroscopic features of FPtpxzND (such as its weak absorption band around 450–600 nm, its significant concentration dependent solution PL wavelength but low host dependent thin film PL wavelength (which is relatively long) and its particularly low solution PL quantum yield (ΦPL)) the relatively strong absorption of FPtdmaND and FPtmorND, and the relatively long PL wavelength of FPtPhND have been reasonably explained. After each platinum complex had been examined for monochromatic organic light-emitting diodes (OLEDs), having a relatively high ΦPL as well as an appropriate greenish yellow PL in the monomeric form and an orange to red PL in the aggregate/excimeric form, FPtdmaND and FPtmorND have been chosen as dopant materials in the construction of single or double emitting layer (EML) hybrid white organic light-emitting diodes (OLEDs). A high performance warm white electroluminescence (EL) (CIEx,y (0.42–0.43, 0.43–0.44)) was obtained where the same platinum complex dopant FPtdmaND was doped (5 or 8 wt%) in two EMLs, 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) harvesting monomeric emission and the highly fluorescent blue N,N′-di-1-naphthalenyl-N,N′-diphenyl-[1,1′:4′,1′′:4′′,1′′′-quaterphenyl]-4,4′′′-diamine (4P-NPD) harvesting excimer/aggregate emission. This warm white EL showed a high colour rendering index (CRI) of 86–88, and a reasonably good EL efficiency of 9.7%, 22.5 cd A−1, or 10.7 lm W−1. EL with improved white chromaticity (CIEx,y (0.37–0.39, 0.39–0.41)) can be obtained using FPtmorND as the single dopant, although it has a somewhat inferior CRI (84–86) and EL efficiency (8.3%, 20.1 cd A−1, or 9.5 lm W−1). Nevertheless, both double EML hybrid white OLEDs exhibit a rather constant CRI and CIEx,y under a driving voltage between 7 and 10 V.