Fluorinated zinc and erbium complexes based on benzothiazole derived ligands for optoelectronic devices

Abstract

Three families of fluorinated ligands based on benzothiazole derivatives, including 2-(2-hydroxyphenyl)benzothiazoles, bis(benzothiazolyl)amines and N-benzothiazol- 2-yl-methanesulfonamides, have been synthesised with different locations and extents of fluorination. Zinc complexes of the fluorinated 2-(2-hydroxyphenyl)benzothiazoles and bis(benzothiazolyl)amines have been successfully prepared. Crystallographic characterisations of these zinc complexes have revealed various molecular packing behaviours in their crystals in terms of πH-πH, πH-πF and πF-πF stacking under the influence of partial and complete fluorination. DFT calculations and photophysical studies of these complexes have demonstrated that fluorination of these molecules decreases their HOMO and LUMO levels simultaneously by about the same amount, roughly 0.1 eV per substitution of hydrogen by a fluorine atom. An inverse correlation has been found between the dihedral angles of conjugating aromatic rings and the photoluminescence full width at half maximum (FWHM), which could be one of the reasons for the broadening of photoluminescence spectra upon fluorination. The applications of the zinc complex of 2-(2-hydroxy-3,4,5,6- tetrafluorophenyl)-4,5,6,7-tetrafluorobenzothizole (Zn(F-BTZ)2) in organic light emitting diode (OLEDs) have been investigated. The electroluminescence of Zn(F-BTZ)2 has shown remarkable phosphorescence in the red region, indicating enhanced spin mixing and intersystem crossing introduced by the substitution of the hydrogen atoms by the higher atomic number fluorine atoms. This ability of providing a large population of triplets, together with the lack of CH or OH oscillators of this perfluorinated Zn(F-BTZ)2 molecule, allowed its use as an efficient chromophore to sensitise the erbium ions in a long-lifetime erbium complex, erbium (III) tris (pentafluorophenyl)imidodiphosphinate (Er(FTPIP)3). By doping Er(FTPIP)3 into Zn(F-BTZ)2 in the OLED, we achieved significant and long-lifetime emission from erbium at the important telecommunication wavelength of 1.5 μm