Solution Processable Indoloquinoxaline Derivatives Containing Bulky Polyaromatic Hydrocarbons: Synthesis, Optical Spectra, and Electroluminescence

Abstract

New hybrid materials featuring the dipolar fragment 6H-indolo[2,3-b]quinoxaline attached to the bulkier polyaromatic hydrocarbons such as fluoranthene, triphenylene, or polyphenylated benzene have been synthesized by a two-step procedure involving Sonogashira and Diels-Alder reactions. They were characterized by absorption, emission, electrochemical, thermal, and theoretical investigations. The electronic properties of the compounds were dominated by the 6H-indolo[2,3-b]quinoxaline chromophore, and the incorporation of polyaromatic hydrocarbons reduces the chances of nonradiative deactivation processes associated with the excited state and improves the emission properties. The compounds displayed cyan emission with moderate quantum efficiency when excited at the absorption maximum. All of the compounds exhibited an irreversible reduction process corresponding to the addition of electron at the quinoxaline segment. They showed moderate thermal stability and glass transition temperature greater than 100 °C. The presence of rigid 6H-indolo[2,3-b]quinoxaline and bulkier polyaromatic hydrocarbon segments enhances the thermal stability and glass transition temperature significantly. Finally, the dyes were successfully applied as an electron-transporting and emitting layer in multilayered organic light-emitting diodes comprising a N,N'-bis(l-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine hole-transporting layer. The cyan emitting devices were characterized by moderate device performance parameters.