Luminescence properties of bipolar stylbeneamine–quinoxalines Gabriel Bernardo a , M.A. Esteves b , A.M. Guerreiro b , B. Gigante b , Jorge Morgado a,c, * a Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal b INETI-DTIQ, Estrada do Paço do Lumiar, 22, P-1649-038 Lisboa, Portugal c Departamento de Eng. Química e Biológica, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal article info Article history: Received 16 January 2008 Received in revised form 1 May 2008 Accepted 3 May 2008 Available online 18 June 2008 PACS: 73.61.Ph 74.25.Gz Keywords: Bipolar compounds Arylamines Quinoxalines Crystallization Luminescence abstract Novel electroluminescent materials combining three functionalities were used as active layers in light- emitting diodes. These functionalities are brought about by the presence of three moieties: diphenylstyl- beneamines, for hole transport, quinoxaline, for electron-transport, and a dehydroabietic acid methyl ester, to prevent crystallisation. The devices prepared with these materials and with magnesium cathodes show efficiencies up to 0.03 cd/A, which is about one order of magnitude higher than the efficiency obtained with the related diphenylstylbeneamines. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The optimisation of light-emitting diodes (LEDs) performance (efficiency, colour stability and lifetime), being these based on either polymers or low molecular weight molecules, depends not only on the device architecture but mostly on the active materials. Controlling their molecular structure has a direct impact on their properties. In particular, emission colour, photoluminescence (PL) efficiency, unipolar/bipolar charge transport properties, energetic position of the frontier levels and, up to some extent, supramolec- ular organisation in solid state, can be tuned by adequate design of the molecular structure. The combination of hole-transporting and electron-transporting materials, either in a blend form or as multilayer structures, has been successfully used to improve the charge balance inside the devices, aiming at maximizing exciton formation efficiency. Addi- tionally, moieties showing this complementary charge transport ability have been combined in single molecules [1–7]. Arylamines [7–9] and quinoxalines [7,10–12] are two typical molecular struc- tures evidencing good hole- and electron-transporting ability, respectively. Bipolar molecules combining these two moieties were previously prepared and used in two layer LEDs [13]. In this study [13], the bipolar compounds were used both as hole-trans- porting and emissive materials, in combination with a second elec- tron-transporting layer. Hole-blocking was the main role attributed to the quinoxaline segments. Here we report, on the solid state luminescence properties of solution processable low molecular weight materials combining three different moieties, which impart three different functional- ities: arylamines (or diphenylstylbeneamines), quinoxaline and a derivative of the dehydroabietic acid. While the first two moieties control charge transport (similarly to the study in [13]), the third moiety improves film stability towards crystallisation. We have previously reported on the properties of two stylbene-based tria- rylamines and on the role played by the dehydroabietic acid methyl ester moiety in the stabilisation of the amorphous nature of the films upon storage at room temperature [14]. We note that a quite successful route to prevent materials crys- tallisation has been the development of materials with high glass transition temperature, T g [9,15]. Still, for instance, for TPD, a widely used hole-transporting material, with T g = 60 °C, crystallisa- tion occurs upon storage. The chemical structure of these novel compounds is shown in Fig. 1. A preliminary report on their preparation, photophysics and electrochemical properties in solution was recently presented [16]. Here we address the luminescent and electroluminescent 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.05.001 * Corresponding author. Address: Instituto de Telecomunicações, Instituto Supe- rior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal. Tel.: +351 218418451. E-mail address: jorge.morgado@lx.it.pt (J. Morgado). Optical Materials 31 (2008) 320–327 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat