Dual-emissive 2-(2′-hydroxyphenyl)oxazoles for high performance organic electroluminescent devices: discovery of a new equilibrium of excited state intramolecular proton transfer with a reverse intersystem crossing process. Issue 5 (19th December 2017)

• Record Type: Journal Article
• Authors: Li, Bijin; Zhou, Linsen; Cheng, Hu; Huang, Quan; Lan, Jingbo; Zhou, Liang; You, Jingsong
• Is Part Of: Chemical science. Volume 9:Issue 5(2018)
• Page Start: 1213
• Page End: 1220
• Publication Date: 2017-12-19
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c7sc04464j
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Dual-emission of the enol-forms and keto-forms of 2-(2′-hydroxyphenyl)oxazoles and their application in OLEDs are investigated. Abstract : The photoluminescence (PL) and electroluminescence (EL) properties of two highly efficient excited state intramolecular proton transfer (ESIPT) molecules, 2-(2′-hydroxyphenyl)oxazoles containing one triphenylamine (TPA) (1 ) and two TPAs (2 ) respectively, are studied systematically. The enol-forms of both1 and2 possess highly hybridized local and charge transfer (HLCT) excited state character, while their excited-state keto-forms are not of obvious HLCT character. A1 -based device exhibits green-white electroluminescence with Commission Internationale d'Eclairage (CIE) coordinates of (0.25, 0.41) and a high external quantum efficiency (EQE) up to 5.3%, which is the highest EQE value recorded for single molecular white light-emitting materials. A2 -based device shows sky-blue emission with CIE coordinates of (0.18, 0.16) and an EQE of 8.0%, which is the highest EQE in the reported HLCT materials. The fluorescence intensities of the enol-forms of1 and2 in EL spectra are increased remarkably relative to their PL spectra. Experimental data and theoretical calculations reveal a new ESIPT equilibrium with a reverse intersystem crossing (RISC) process arising from the HLCT character. In EL, the RISC of the enol-form excitons from the triplet state to the singlet state triggers an increase in the number of enol-form singlet excitons, which further leads to a shift of the ESIPT equilibrium towards an enhanced enol-form emission. Thus, the difference between the ESIPT equilibria in PL and EL may be ascribed to the HLCT character of the enol-form excited state.