Thiophene-centered azomethines: Structure, photophysical and electronic properties Zita Tok  arov  a a, * , Petra Maxianov  a a , Tom  a  sV  ary b , Vojtech N  ada  zdy c , Daniel V  egh d , Kamil Tok  ar e, f a Department of Chemistry, Faculty of Natural Sciences, University of Ss. Cyril and Methodius, N am. J. Herdu 2, 91701, Trnava, Slovakia b Institute of Nuclear and Physical Engineering, Faculty of Electrical Engineering and InformationTechnology, Slovak University of Technology, Ilkovi cova 3, 812 19, Bratislava, Slovakia c Centre for Advance Material Application, Slovak Academy of Sciences, Dúbravsk a Cesta 9, 84511, Bratislava, Slovakia d Institute of Organic Chemistry, Catalysis and Petrochemistry, Faculty of Chemical and Food Technology, Slovak University oTechnology, Radlinsk eho 9, 81237, Bratislava, Slovakia e Center for Computational Material Science, Institute of Physics, Slovak Academy of Sciences, Dúbravsk a Cesta 9, 84511, Bratislava, Slovakia f Advanced Technologies Research Institute ATRI, Faculty of Materials Science and Technology inTrnava, Slovak University of Technology, J ana Bottu 8857/ 25, 917 14, Trnava, Slovakia article info Article history: Received 2 September 2019 Received in revised form 16 November 2019 Accepted 27 November 2019 Available online 3 December 2019 Keywords: Azomethines Thiophene Frontier molecular orbitals Energy-resolved electrochemical impedance spectroscopy abstract We present the synthesis and structure characterization of three novel symmetrical azomethines 3a-c derived from 2,5-diamino-thiophene-3,4-dicarbonitrile together with the unexpected by-product 4 containing the saturated hemiaminal type bond. Photophysical properties (UVeVis and photo- luminiscence) are shown to be affected mainly by the central thiophene unit with no influence from the type of linkage e unsaturated imine (-HC]N-) bond in 3a-c vs. the saturated hydroxymethylamino circuit [eHNeC(OH)H-] in 4. For the first time the electronic properties, such as distribution of the frontier molecular orbitals and resulting energy gaps, for compounds 3b,c and 4 are calculated by means of density functional theory methods and confirmed by an immediate experimental method - the energy-resolved electrochemical impedance spectroscopy. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Among the various groups of organic compounds suitable for applications in functional materials [1e4], azomethines are explored as components in optoelectronic devices [5,6]. Both the single molecule azomethine-scaffold or polyazomethines have been successfully employed as active components in organic light emitting diodes (OLEDs) [7], hole-transporting units in solar cells [8] and donor blocks in bulk heterojunction solar cells (BHJ) [9]. The imine bond (-HC]N-) in azomethines is isoelectronic to the vinyl bond (-C]C-) [8e10] in their vinylene analogues, which are still recognized as the ultimate building blocks in organic semi- conductors (OSCs) and organic photovoltaic devices (OPVs) [1 ,3]. Most of these conjugated compounds are prepared by coupling aryl-aryl bond forming reactions. The advantage of the hetero- aromatic derivatives over their homoaryl counterparts is their straightforward preparation, in particular the reaction towards azomethines is performed according to the simple Schiff-type condensation between an amine and aldehyde [11]. Expensive catalysts are not required and water is the only by-product. Importantly, physico-chemical properties comparable to vinylenes are possible with azomethines containing the thiophene ring/s [7 , 12e14]. The thiophene core affect the energy of frontier molec- ular orbitals (FMO) and enhances the p-conjugation [14e16]. Consequently, the successful OSC and/or OPV device performance is strongly dependent on the molecular structure of these hetero- structures in regard to suitably placed highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) [17]. The determination of the density of states (DOS) in the entire energy range from HOMO to LUMO including the presence of de- fects in the band gap is a prerequisite to the succesfully design of optoelectronic devices. In these regards we have synthesised and * Corresponding author. E-mail address: zita.tokarova@ucm.sk (Z. Tok arov a). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc https://doi.org/10.1016/j.molstruc.2019.127492 0022-2860/© 2019 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1204 (2020) 127492