Article pubs.acs.org/JPCC
Structural, Spectroscopic, Electrochemical, and Electroluminescent Properties of Tetraalkoxydinaphthophenazines: New SolutionProcessable Nonlinear Azaacenes Kamil Kotwica,† Piotr Bujak,*,† Damian Wamil,† Adam Pieczonka,† Gabriela Wiosna-Salyga,‡ Piotr A. Gunka,† Tomasz Jaroch,§ Robert Nowakowski,§ Beata Luszczynska,‡ Ewelina Witkowska,‡ Ireneusz Glowacki,‡ Jacek Ulanski,‡ Malgorzata Zagorska,† and Adam Pron† †
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Department of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland § Institute of Physical Chemistry, Polish Academy of Science, Kasprzaka 44/52, 01-224 Warsaw, Poland ‡
S Supporting Information *
ABSTRACT: A series of solution-processable tetraalkoxy-substituted dinaphtho[2,3-a:2′,3′-h]phenazines were synthesized by reductive functionalization of indanthrone (6,15-dihydrodinaphtho[2,3-a:2′,3′-h]phenazine-5,9,14,18-tetraone), an old intractable dye. The melting point of these new compounds was found to decrease from 204 °C to 98 °C upon extension of the number of carbons from 4 to 12 in the alkoxy substituent. All derivatives show a strong tendency to self-organize in 2D as evidenced by STM investigations of monolayers deposited on HOPG. The 2D structure is less dense and shows different alkoxy group interdigitation pattern as compared to the 3D structure determined from the X-ray diffraction data obtained for the corresponding single crystals. Electrochemical, absorption, and emission properties of tetraalkoxy-substituted dinaphtho[2,3-a:2′,3′-h]phenazines, studied in solution, are essentially independent of the length of the alkoxy substituents. All derivatives exhibit high photoluminescence quantum yield, approaching 60%. When molecularly dispersed in a solid matrix consisting of poly(9-vinylcarbazole) (PVK) (60 wt %) and (2tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole) (PBD) (40 wt %) (so-called “guest/host configuration”), they show green electroluminescence due to an effective energy transfer from the matrix to the luminophore. The best light-emitting diodes were obtained for the butoxy derivative showing a luminance approaching 1500 cd/m2 and a luminous efficiency over 0.8 cd/A.
■
into an n-type conductor.9 In addition to linear azaacenes,9−13 nonlinear compounds of this group14 of zigzag15 and branched16,17 structures have been synthesized and their electronic and optical properties determined. Classical methods of the synthesis of azaacenes are based on the condensation of either o-quinones or o-dihydroarenes with o-diamines.18−26 Alternatively, Buchwald-type coupling between o-diaminoacenes and o-chloroacenes can be used.27−31 Unsubstituted acenes and azaacenes are very difficult to solution process, and their deposition in a form of thin layers has usually been carried out by evaporation. Thus, solubilization of these compounds still remains a challenge especially in view of a very limited number of solution-processable azaacenes, usually containing only one type of solubilizing substituents, namely silylethynyl groups.8
INTRODUCTION
Acenes constitute one of the most interesting groups of organic electroactive compounds, tested mainly as components of pchannel field effect transitors.1−3 High hole mobilities were reported for transistors made of pentacene,4 rubrene,5 or picene6,7 to name a few. In addition, their redox, electronic, and optical properties can be controllably modified by introduction of nitrogen atoms into the fused aromatic system. For these reasons, in recent years, significant research efforts have been put into the synthesis of new electroactive azaacenes.8 The most pronounced effect of the presence of nitrogen atoms in the conjugated acene-type core is an increase of the electron affinity which makes azaacenes either ambipolar or electron conductors. The exact effect depends on the number and the positions of nitrogen atoms in a given molecule. For example, the presence of two terminal pyrazine rings in a pentaring acene induces ambipolarity with equilibrated electron and hole mobilities. Introduction of four additional nitrogen atoms into the second and fourth ring transforms this azaacene © 2015 American Chemical Society
Received: February 15, 2015 Revised: April 15, 2015 Published: April 16, 2015 10700
DOI: 10.1021/acs.jpcc.5b01557 J. Phys. Chem. C 2015, 119, 10700−10708
Article
The Journal of Physical Chemistry C
Figure 1. Schematic cross-sections and dimensions of the fabricated type OLED.
BENFLEC, provided by Edinburgh Instruments. The contribution of indirect illumination on the sample emission was measured and taken into account in the QY determination. Cyclic Voltammetry. Cyclic voltammetry studies were performed in a three-electrode, one-compartment cell with a platinum working electrode (3 mm2), a platinum wire counter electrode and a Ag/0.1 M AgNO3/CH3CN reference electrode. A given indanthrone derivative was dissolved in 0.1 M Bu4NBF4 in dichloromethane to yield a 5 × 10−4 M solution. Differential Scanning Calorimetry (DSC). DSC curves were obtained with a TA Instruments DSC (Model Q-200) at a heating (cooling) rate of 10 °C/min using samples of ca. 5 mg mass. Scanning Tunneling Microscopy (STM). Monolayers deposited on HOPG graphite by drop-casting from a solution in hexane (∼2 mg/L) were imaged under ambient conditions, after evaporation of the solvent, using an STM system fabricated at the University of Bonn, Germany.39 X-ray Diffraction. In the case of DNP-C8 and DNP-C10, single crystals suitable for the X-ray studies were obtained. Diffraction data were recorded on an Agilent κ-CCD Gemini A Ultra diffractometer with graphite-monochromated Mo Kα radiation at 100(2) K for DNP-C8 and with mirrormonochromated Cu Kα radiation at 120(2) K for DNP-C10. Cell refinement and data collection as well as data reduction and analysis were performed with the CrysAlisPRO software.40 The structures were solved by direct methods and subsequent Fourier-difference synthesis with ShelXS2014 and refined by full-matrix least-squares against F2 with ShelXL2014 within the Olex2 program suite.41−43 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were introduced at calculated positions and refined as riding atoms with isotropic displacement parameters related to that of the parent atoms. Data analysis was carried out using Olex2, Mercury, and Platon.44−46 Crystal data and structure refinement parameters are given in Table S1 (Supporting Information). Electroluminescence Measurements and Light-Emitting Diode Fabrication. Electroluminescence spectra were registered using an optical system consisting of a custom-made optical collector, optical fiber, Micro HR Imaging Spectrograph, and CCD camera (Horiba Jobin−Yvon). The system has flat sensitivity response in the whole wavelength range 350−950 nm. Guest−host type electroluminescent diodes were fabricated by molecular dispersion of DNP-C4 (DNP-C8 and DNP-C12) (1 to 5 wt %) in a two-component matrix consisting of poly(9vinylcarbazole) (PVK) (70 wt %) and (2-tert-butylphenyl-5biphenyl-1,3,4-oxadiazole) (PBD) (30 wt %). This active layer of ca. 70 nm was deposited on top of an ITO electrode precoated with a layer of PEDOT:PSS (ca. 20 nm). In the subsequent step an ultrathin LiF layer was evaporated followed
Among new alternative routes to solution-processable electroactive materials, functionalization of well-known dyes became very popular in recent years. For example, Zhang et al.32 prepared a new organic semiconductor exploiting a synthetic procedure first published in 1949.33 Intensive research on electroactive materials derived from isoindigo ((E)1H,1H′[3,3′]indolylidene-2,2′-dione)34 or diketopyrrolopyrrole35 can be considered here as other instructive examples of this approach. Reductive condensation of diketopyrrolopyrrole dyes was also exploited as a new method of the preparation of azaacenes consisting of four fused five-membered rings.36 In our recent communication37 we proposed a new alternative route to solution-processable nonlinear azaacenes, based on functionalization of old intractable alizarin-type dyes such as indanthrone or flavanthrone. In particular we have demonstrated that indanthrone can be converted into its soluble derivative, namely tetraoctyloxydinaphtho[2,3-a:2′,3′h]phenazine via a simple one-pot preparation involving reduction of the carbonyl group with consecutive attachment of alkoxy-solubilizing substituents under phase transfer catalysis conditions. In this paper we present a systematic study of tetraalkoxysubstituted dinaphtho[2,3-a:2′,3′-h]phenazines, abbreviated as DNPs. In particular we focus on their redox behavior, absorption and emission spectroscopy, self-assembly capabilities, and electroluminescence.
■
EXPERIMENTAL METHODS Synthesis. The list of all chemicals together with detailed description of the synthesis of all investigated derivatives and the spectroscopic characterization data (1H NMR, 13C NMR) as well as elemental analyses can be found in Supporting Information. Spectroscopic Studies. Solution UV−vis-NIR spectra were recorded in chloroform on a Cary 5000 (Varian) spectrometer whereas the emission spectra were measured using an Edinburgh FS 900 CDT fluorometer (Edinburgh Analytical Intruments). Photoluminescence quantum yields were determined using quinine sulfate in 0.05 mol dm−3 H2SO4 (φfl = 0.51) as a standard.38 Fluorescence lifetimes were recorded using a home-built time-resolved single photon counting (SPC) device. The excitation was provided by IBH Nanoled emitting at 297 nm with pulse width
