FULL PAPER DOI: 10.1002/ejoc.201402241 Synthesis, Electrochemistry, Crystal Structures, and Optical Properties of Quinoline Derivatives with a 2,2'-Bithiophene Motif Aneta Slodek,* [a] Michal Filapek, [a] Grazyna Szafraniec, [a] Iwona Grudzka, [a] Wojciech Andrzej Pisarski, [a] Jan Grzegorz Malecki, [a] Lidia Zur, [a] Malgorzata Grela, [b] Witold Danikiewicz, [b] and Stanislaw Krompiec [a] Keywords: Donor–acceptor systems / Luminescence / Fluorescence / Electrochemistry / Density functional calculations New quinolines bearing a 2,2'-bithiophene motif have been prepared through Ru- and In-catalyzed reactions as well as by Suzuki cross-coupling reactions. The novel quinolines have been characterized by using EI-HRMS and NMR spec- troscopy. Additionally, the molecular structures of the novel quinolines were confirmed by X-ray crystallography. The photophysical and electrochemical properties of all quinoline derivatives were investigated by using absorption and lumi- nescence spectroscopy and cyclic voltammetry. The main in- tense absorption bands associated with the quinoline deriva- tives were located between 350 and 450 nm. Intense blue- Introduction Quinolines and their derivatives are very important het- erocyclic substances that occur in many natural products. [1] They play a crucial role as component compounds for the synthesis of antimalaria, antibacterial and many others drugs. [2] In addition, they have drawn attention for their application in bioanalytics as fluorescent sensors. [3,4] Be- sides their unquestionable medicinal properties, quinolines, and especially their substituted derivatives, are of major interest for their application in optoelectronic and organic electronics. [5–8] For the past few years, various quinoline de- rivatives have been used as ligands for transition-metal complexes, [9–11] which have found wide application in or- ganic light-emitting diodes (OLEDs) [7,9,12–15] and organic photovoltaics. [4,6] Quinolines are well-known electron-deficient heterocy- cles that, when connected with electron-rich molecules, form acceptor–donor (A–D) systems with efficient intramo- lecular charge transfer (ICT). Moreover, the electron A–D system can influence HOMO–LUMO energy levels and consequently modify the spectroscopic and electrochemical [a] Faculty of Mathematics, Physics and Chemistry, Institute of Chemistry, University of Silesia, Szkolna 9, 40-007 Katowice, Poland E-mail: a.slodek@wp.pl http://www.inorganic.us.edu.pl/ [b] Polish Academy of Science, Institute of Organic Chemistry, Kasprzaka 44/52, P. O. Box 58, 01-224 Warsaw 42, Poland Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201402241. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2014, 5256–5264 5256 green and orange-red fluorescence, moderate Stokes shifts, and microsecond lifetimes were observed. Quinolines con- taining one bithiophene moiety underwent oxidation leading to dimerization, whereas bis(bithiophenyl)-substituted quin- oline underwent polymerization. All the studied quinolines showed multistep reversible reduction, with 6-nitro-4-phen- ylquinoline being the easiest to reduce. The study of novel quinoline derivatives indicates a strong correlation between their electrochemical and spectroscopic properties and the character of the substituent (electron-donating or electron- withdrawing) in the quinoline ring. properties of compounds. [16–19] Thus, there has been con- siderable interest in developing a diverse range of small quinoline derivatives with enhanced optoelectronic, elec- tronic, and mechanical features for use as increasingly ef- ficient and stable OLEDs. [5,6,8] In addition, the substitution in the quinoline ring allows the possibility of improving the expected electrochemical and photochemical properties. In- spired by the excellent chemical and physical properties of quinoline derivatives, we synthesized quinolines derivatives containing the 2,2'-bithiophene motif. This motif was se- lected in the view of the fact that it is an electron-rich het- erocycle that can be used as the electron-donating part in an electron–donor–acceptor system for ICT. Recently, the synthesis of 2,4-disubstituted quinoline derivatives based on the exploitation of transition-metal-catalysts has been ex- tensively developed. [20–30] These approaches have been rec- ognized as providing a very straightforward and efficient strategy with which to obtain substituted quinolines. Synthesis of disubstituted quinolines using Rh I com- plexes, [21] ruthenium complexes, [22] [Ru 3 (CO) 12 ], [23] Zn II salts, [24] AuCl 3 /CuBr, [25] and FeCl 3 [26] as catalysts have been reported. The preparation of substituted quinolines under microwave irradiation has also been widely explored over the last several years. [27–30] The preparation of quinoline de- rivatives in the presence of Au I complex, [28] Zn(OTf) 2 , [29] and In(OTf) 3 [30] under microwave irradiation have been de- scribed. To our knowledge, although several quinolines with thiophene moieties have been synthesized, only a few with a 2,2'-bithiophen-5-yl moiety have been reported;