Synthesis, electrochemical, thermal and photophysical characterization of quinoxaline-based π-extended electroluminescent heterocycles

Abstract

This work describes the synthesis of two sets of photoactive compounds with π-extended conjugation developed from a quinoxaline core condensed from an ortho-diamine and 1,2-diketone and followed by a Sonogashira coupling reaction. The electrochemical, photophysical and thermal properties of these compounds were evaluated. Thermogravimetric analysis (TGA) indicated high thermal stability with decomposition temperatures up to 370 °C for all dyes. The electrochemical behaviour of the alkoxy and tetrazole quinoxaline derivatives showed a reversible wave in the reduction region because of the formation of radical anions and two irreversible oxidation waves at more positive potentials. The alkoxy and tetrazole compounds showed absorption in the blue region (∼460 nm) and in the violet region (∼430 nm), respectively, with molar absorptivity coefficients ascribed to allowed π–π* electronic transitions. The band-gaps of the alkoxy and tetrazole compounds were calculated by onset peaks presenting values of approximately 2.35 eV and 2.55 eV, respectively. A relative broad emission located at approximately 550 nm was observed for the alkoxy derivatives with a positive solvatochromism of ∼20 nm. A less intense solvatochromism was observed for the tetrazole derivatives; however, the maxima were located in the cyan-green to blue regions. The large Stokes' shift for all compounds (∼100 nm) probably indicates a charge transfer mechanism (ICT) in the excited state. The fluorescence lifetimes of both sets of compounds were obtained by time-correlated single photon counting (TCSPC) in different solvents, rendering nearly solvent-independent behaviour to the alkoxy derivatives but a pronounced solvatochromic effect to the tetrazoles, with one of the compounds dimerizing in the excited state. Because of the high electronic density of the molecular cores and good fluorescence quantum yields, a organic light emitting diode (OLED) of great colour purity was fabricated with one of the alkoxy derivatives. The diode was prepared by solution-based protocols offering 103 cd m−2 at 9.8 V with Comission Internationale de l’Eclairage (CIE) coordinates of (0.50, 0.50).