Photoactive amorphous molecular materials based on bisquinoline diamines and their synthesis by Friedländer condensation reaction
Graphical abstract
Introduction
Nitrogen-heterocycles including their oligomers and polymers have attracted attention for the preparation of light-emitting and electron-transporting layers (ETLs) for organic light-emitting diode (OLED) applications [1]. They contain an electron deficient moiety such as pyridine or quinoline in their main chains or side chains and, thereby, facilitating the electron transportation [1], [2], [3]. Furthermore, the color tuning of their emission could be achieved by protonation of the nitrogen heterocycles [2b–d]. Polyquinolines [3a–c], polyquinoxalines [3b], and polyanthrazolines [3c] are representative polymers that are explored as ETLs. Despite their excellent good charge transporting properties, they have not found practical applications because of their low quantum yields, low solubility in common organic solvents, and inconvenience in processability [3]. Various molecular compounds have also been explored as potential materials based on distyrylarylenes [4a], anthracenes [4b], spirobifluorenes [4c], metal chelates [4d], and siloles [4e]. Despite many of these compounds of diverse chemical architectures seem to be promising for OLED applications; their crystalline states limit their use. Moreover, they need to be processed by chemical vapor deposition technique, which is an elaborate and expensive process. Several quinoline-based oligomers bis(biphenyl)-4-phenylquinolines [5a], bis(phenylquinoline)-including 1,4-phenylenes [5b], bis(pyrenyl)-4-octylquinoline) [5c] have been described as prospective n-type semiconductors. They exhibit good thermal stability, satisfactory blue commission Internationale de L’Eclairage (CIE) coordinates, and tunability of light emission via protonation [5]. However, their major disadvantage is that they are mostly crystalline that makes them inconvenient for preparing thin films. Additionally, the thin films of crystalline materials have usually low quantum efficiencies. Amorphous molecular materials are desirable for organic semiconductor applications for several reasons. They are able to produce thin transparent films much like polymers but are of uniform size, that is, monodisperse. Their properties are homogeneous and have higher quantum efficiencies unlike crystalline compounds that yield polycrystalline films unsuitable for device applications. To achieve sustained glassy state in small molecules, they must fulfill several key chemical structural requirements. They must be rigid and have bulky substituents and non-coplanar structures. Moreover, high glass transition temperatures (Tgs) and good thermal stabilities are also desirable since Joules heating in device operations would limit their lifespan [6]. To satisfy these conditions we targeted to prepare three novel bisquinoline diamines from the Friedländer condensation reaction of 2-amino5-nitrobenzophenone with the respective diketones as shown in Scheme 1. In this condensation reaction, the first route we utilized sulfuric acid/acetic method and the second route we utilized tosic acid in the melt, that is, solvent-free green synthetic approach. The latter method is highly relevant for the synthesis of novel materials on a large scale for potential applications in modern science and technology. Compounds 1b and 2b are completely conjugated and contain pendant large phenyl groups to ensure their glassy (amorphous) states. Compound 3b having twelve methylene units connecting the two quinoline moieties are chosen to assess the differences in properties between the fully conjugated compounds and flexible one. Additionally, they have two amino groups attached to them, which would enable one to study their thermal and optical properties with respect to this functional group.
Section snippets
Results and discussion
Friedländer condensation reaction is a facile synthetic method for the preparation of various quinoline derivatives, oligoquinolines [7], polyquinolines [8], and polyanthrazolines [3a,c]. It is also used as a key step in total synthesis of many natural products like streptoniring [9a], camptothecin [9b], irinotecan [9c], and nothapodytine B [9d]. To prepare bisquinoline diamines, we used acid-catalyzed both solution and in the melt (solvent-free) Friedländer condensation reaction followed by
General
All of the starting materials including solvents were purchased from either Sigma–Aldrich or TCI and used without any further purification. Spectral grade chloroform and TFA were obtained from Alfa Aesar. The 1H and 13C NMR spectra were recorded with Varian NMR 400 spectrometer equipped with three RF channels operating and 400 MHz and 100 MHz, respectively. Solutions of these compounds were prepared by dissolving ca. 25 mg of respective compounds per milliliter of either d6-DMSO or d1-TFA with TMS
Conclusions
In summary, we have designed and developed a new series of bisquinoline diamines prepared by Friedländer condensation reaction, followed by reduction, as a concept for the development of novel amorphous molecular materials. Particularly, their precursors were synthesized in the solvent-free reaction condition by using tosic acid monohydrate in the melt, which provided a green chemistry approach for the synthesis of this class of heterocyclic diamines. Their thermal and optical properties were
Supplementary information
1H and 13C NMR and the IR spectra of compounds 1a–3a and 1b–3b are provided as supplementary data.
Acknowledgments
P.K.B. acknowledges, the University of Nevada Las Vegas for an Applied Research Initiative, the donors of the Petroleum Research Fund (PRF# 35903-B7), administered by the American Chemical Society, and an award (CCSA# CC5589) from Research Corporation for the support of this research. P.K.B. and T.S.J. also acknowledge the Graduate College for providing them in the form of SPGRA for the academic year 2010–2013, and P.K.B. acknowledges the UNLV for providing him a sabbatical leave for the
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