Photoluminescence and electroluminescence by gallium(III) complexes of N-salicylidene-o-aminophenol and its derivatives Adamantia Kagkelari a , Vlasoula Bekiari b , Elias Stathatos c , Giannis S. Papaefstathiou d , Catherine P. Raptopoulou e , Theodoros F. Zafiropoulos a , Panagiotis Lianos f,Ã a Department of Chemistry, University of Patras, 26500 Patras, Greece b Department of Aquaculture and Fisheries, Technological Educational Institute of Messolonghi, 30200 Messolonghi, Greece c Electrical Engineering Department, Technological Educational Institute of Patras, 26334 Patras, Greece d Laboratory of Inorganic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis,15771 Zografou, Greece e Institute of Materials Science, NCSR ‘‘Demokritos’’, 15310 Aghia Paraskevi Attikis, Greece f Engineering Science Department, University of Patras, 26500 Patras, Greece article info Article history: Received 16 May 2008 Received in revised form 15 December 2008 Accepted 18 December 2008 Available online 25 December 2008 Keywords: N-salicylidene-o-aminophenol Gallium complexes Photoluminescence Electroluminescence Organic LED abstract N-salicylidene-o-aminophenol and two of its derivatives bearing either an electron-donating methyl group or an electron-withdrawing Br group were used as ligands for the synthesis of three Ga(III) complexes. The complexes involved the participation of one acetylacetonate and one ethanol or methanol molecule. The geometry of the dianion of the Schiff base in the complexes is planar while photoluminescence data showed that photoluminescence intensity was extensively increased upon complex formation. Complexes emitted ligand-centered luminescence by ligand-centered excitation. Substituent groups broadly modified emission maximum. These substitutions can be exploited to tune light emission by the complex. All three complexes were used for the construction of electrolumines- cence devices and all emitted electroluminescence. Both photoluminescence and electroluminescence emission was enhanced in the case of substituted Schiff bases. & 2009 Elsevier B.V. All rights reserved. 1. Introduction Small molecule organic light-emitting diodes (SMOLED) [1–4] are very interesting from many points of view: they are relatively easy to fabricate, reasonably stable and their active luminescent component is simple and rather easy to synthesize. Best-known among SMOLEDs are those based on Alq 3 (q ¼ 8-hydroxyquino- line) [1,2,5]. Recent research on these metal-organic complexes, among other subjects, also focuses on the synthesis of Alq 3 variants. One possibility is to synthesize dinuclear complexes by adding a tridentate Schiff-base ligand: N-salicylidene-o-amino- phenol (saphH 2 , see Fig. 1 for chemical structure), thereby increasing glass-transition temperature and improving stability of the emitting layer [6]. Another variant is to modify the emission wavelength by attaching an electron withdrawing chemical group (EWG) or an electron donating chemical group (EDG) [1,7–9]. Finally, a third variant is, of course, to change the complexed metal itself [6,10,11]. In the present work we focus on some of these possibilities with emphasis on the effect of substituent groups, either EWG or EDG. For this purpose, we have synthesized chelates employing a single Schiff base, i.e. saphH 2 , thus simplifying the procedure. For the choice of metal we opted for Ga, since the obtained Ga–saph 2 chelates were strongly photoluminescent, i.e. good candidates for SMOLED fabrication. Saph 2 complexes with various metals, notably, Ga, Al and Be, are known for a long time and they have been used as fluorogenic reagents in various applications [8,9]. Fig. 2 shows the bonding mode of Ga to saph 2 . Bonds are formed with the two O atoms and N (tridentate ligand). Since the usual coordination number of Ga is 6, the complexes are formed by simultaneously complexing one mole- cule of the bidentate chelate acetylacetonate (acac  ) and one molecule of ethanol (EtOH) or methanol (MeOH). An EtOH or a MeOH molecule was inevitably involved in the formation of the complex since these materials were used as solvents to synthesize the complexes. These chelates emit ligand-centered luminescence with ligand-centered excitation. The presence of the metal simply serves to stabilize ligand geometry. Indeed, since the intensity of fluorescence increases by metal complexation, without changing the spectral structure of the fluorescence spectrum, it is assumed that bonding with the metal, which inhibits rotation of the aminophenol vs. the salicylidene group, favors planarity and thus limits loss of excitation energy by internal conversion [12]. In addition, the planar geometry makes a more stable complex. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.12.017 Ã Corresponding author. E-mail address: lianos@upatras.gr (P. Lianos). Journal of Luminescence 129 (2009) 578–583