Structural characterization, absorption and photoluminescence study of symmetrical azomethines with long aliphatic chains Agnieszka Iwan a,⇑ , Ewa Schab-Balcerzak b , Marzena Grucela-Zajac b , Lukasz Skorka c a Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Poland b Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Poland c Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland highlights  We analyze the influence of diimine structure on their photoluminescence properties.  Imine emitted blue, violet or green light in solution.  The theoretical electrochemical energy band gap of diimines was observed at in the range of 3.65–4.08 eV.  Some of the diimines seems to be promising potential candidates for OLEDs. article info Article history: Received 30 July 2013 Received in revised form 4 October 2013 Accepted 29 October 2013 Available online 9 November 2013 Keywords: Photoluminescence Theoretical calculations Diimines Azomethines Liquid crystals abstract In this study, we investigated structural and optical properties of three symmetrical azomethines bis(4-(4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 11-heptadecafluoroundecyloxy)benzylidene)benzene-1,4-diamine (SAz1), bis(4-(4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 11-heptadecafluoroundecyloxy)benzylidene)biphenyl-4,4 0 - diamine (SAz2) and 4,4 0 -methylenebis(N-(4-(octadecyloxy)benzylidene)benzenamine (SAz3). Electronic properties, such as orbital energies and resulting energy gap of the three symmetrical azomethines were calculated theoretically by density functional theory (DFT). The photoluminescence (PL) and absorption UV–vis properties of the azomethines were investigated in chloroform solution. The effect of excitation wavelength and concentration on the PL properties was detected as well. Azomethines emitted violet, blue or green light. The highest PL intensity was found for SAz1. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The photoluminescence (PL) of organic compounds is essentially based on localized electronic systems within organic molecules. In order to produce an electronically excited state, a molecule must absorb energy equal to or greater than the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [1,2]. Incorporation of hetero atoms such as N, O, S within the p-conjugated systems usually causes the red (bathochromic) shift of absorption and emission peaks. Moreover, attachment of elec- tron donating (D) groups (e.g. –NH 2 , –OCH 3 ) or electron accepting (A) ones (e.g. –CN, –NO 2 ) causes red shifts of the UV–vis and PL spectra. Tuning the chemical structure, band gap of conjugated compounds (oligomer and polymers) and designing LEDs emitting selected colors remains a challenging issue. Mainly, three strategies are reported for tuning the emission color of organic compounds: (i) changing the main chain molecular structure, (ii) blending a light emitting compound with another nonemissive polymer e.g. poly(methylmethacrylate) (PMMA) and (iii) doping e.g. [3–20]. Additionally, the effects of excitation wavelength, con- centration, film thickness and kind of solvent on the photolumines- cence (PL) properties of the organic compounds are investigated. Among various group of organic compounds investigated as luminescence materials azomethines (imines, Schiff bases) are also analyzed e.g. [6,15,21–37]. Some papers are dedicated to investiga- tion optical, mainly PL properties of soluble, liquid crystalline (LC) azomethines e.g. [38–46]. For example, the Marin group [42–45] investigated photoluminescence of liquid crystalline azomethines with various chromophore units (pyrrole, indole, tiophene, fluorene and biphenyl). Investigated compounds emitted blue or green light [43]. Our previously work showed that liquid crystalline, unsymmet- rical azomethines obtained from 4-(4,4,5,5,6,6,7,7,8,8,9,9,10, 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.10.067 ⇑ Corresponding author. Tel.: +48 71 328 30 61. E-mail address: a.iwan@iel.wroc.pl (A. Iwan). Journal of Molecular Structure 1058 (2014) 130–135 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc