Some new 2,8-disubstituted-1,7-dicyano-3,9-diazaperylenes

Concise synthetic protocols for a range of new 2,8-disubstituted-1,7-dicyano-3,9-diazaperylenes, starting from commercially available 1,5-diaminoanthraquinone


Introduction
It is widely accepted that fossil fuels give rise to serious environmental problems and there is a pressing need to develop renewable energy sources.The development of photovoltaic devices (PVs), which transform practically inexhaustible solar energy into electricity, is one of the most promising long-term solutions for clean, renewable energy.Organic photovoltaic devices (OPVs) using soluble small molecules have emerged as promising candidates for OPV applications.][3][4] A majority of OPV research has been focused on the development of electron donor, or so called "p-type", semiconductors; much less attention has been devoted to electron acceptor, or "n-type", materials. 3,5ullerenes, among the commonly used acceptor materials, suffer from high cost and other shortcomings, which has encouraged efforts towards the discovery and development of new, high-performance n-type small molecules for OPVs.New materials would assist the development of structure-property relationships and add versatility to the selection of donor/acceptor pairs to maximize, for example, absorption across the solar spectrum, or allow the use of materials with energy levels not suited to fullerenes. 6 feature generally regarded as desirable for stable, electron-accepting (n-type) small molecules is electron deficiency, to preferentially lower the energy of the LUMO.This can be achieved by strategies such as the substitution of the -system with -acceptors such as cyano and carbonyl groups, which also increases the conjugation length, or the substitution of atoms in an extended -system with more electronegative atoms, e.g.replacing carbocyclic rings with certain nitrogen heterocycles. 5o enable fabrication of OPV devices via solution processing, the incorporation of long alkyl side chains for increased solubility in organic solvents has been a frequently applied strategy.However, the nature of the side chains also affects the intermolecular interaction of the molecules and charge transport properties.In particular, the side chains affect the morphology of films and therefore the performance of OPV devices.An increased content of insulating alkyl side chains relative to the conjugated core of the molecule may result in deterioration in charge transport, so a balanced choice of suitable solubilizing groups at appropriate sites is crucial for fine-tuning the structure-properties relationship. 1lasses of non-fullerene n-type small molecules which exemplify the successful application of the above strategies include the rylene diimides, dicyanoimidazoles, diketopyrrolopyrroles bearing electron withdrawing groups, and fluoro-and cyanopentacenes.Ample discussion of these classes of molecules can be found in several reviews. 1,5,6he use of industrial dyes and pigments as starting points for discovery of organic semiconductors is attractive in several ways.These types of molecules possess attributes such as strong light absorption and photochemical stability, and are usually sourced from robust, large-scale synthetic protocols. 4Indeed, some of the classes of n-type small molecules listed above were first used industrially as dyes and pigments; most notably the rylene diimides and diketopyrrolopyrroles.
In light of all of the above, we noted two isolated reports by Tatke and Seshadri from the mid-1980s describing the preparation of 2,8-di(morpholin-4-yl)-1,7-dicyano-3,9-diazaperylene 3a (from the dye precursor 1,5-diaminoanthraquinone 1) 7 and its substitution with 4-chloroaniline 8 and more recent reports from Yi Liu and co-workers which described synthesis of 2,8-dialkyl-, 2,8-dialkoxy-, and 2,8-dialkylthio-1,7-diaryl-3,9-diazaperylenes and their conversion to corresponding diazacoronenes for incorporation into conjugated polymers for organic photovoltaic device studies 9 and as potential colorimetric and fluorescence proton sensors. 10Schneider and Perepichka 11 described synthesis of simpler 3,9-diazaperylenes, accompanied by isomeric 1,8-diazabenzo[e]pyrene side products, and Bock and co-workers 12 prepared extended heteroarenes containing the 3,9-diazaperylene system with additional ring fusions; both of these studies were scouting for potential n-type -functional materials.Further, derivatives of the related 1,7-diazaperylenes have been claimed as organic semiconductor materials for thin film transistors. 13n our view, the rare 3,9-diazaperylene ring system, particularly with electron withdrawing 1,7-dicyano substitution, appeared to satisfy many of the requirements for a n-type organic semiconductor material.In addition, scope existed for variation of the substituents at the 2-and 8-positions which could be used for tuning both solubility and electronic properties.
This paper describes synthetic methods established during an exploratory chemistry effort towards discovery of new n-type materials that led to several new 2,8-disubstituted-1,7-dicyano-3,9-diazaperylenes.

Results and Discussion
The synthetic route to the 2,8-bis(dialkylamino) compounds 3 was essentially that of Tatke and Seshadri 7 (Scheme 1).5][16][17] Interestingly, in our hands, the use of microwave heating afforded quicker and cleaner reactions than conventional heating.

Scheme 1
Reactions of 1,5-diaminoanthraquinone 1 with the Vilsmeier reagents prepared from cyanoacetamides 2b-d and phosphoryl chloride in dioxane at 80 °C afforded modest yields of the desired 1,7-dicyano-3,9-diazaperylenes 3b-d as intensely orange-red solids.In each case, the pyridine side-product 4, formed from the known 14 self-condensation of cyanoacetamides under the influence of phosphorus oxychloride, was also isolated during chromatographic purification.
The crystal structure of bis-diethylamino derivative 3c is shown in Figures 1-3.][20][21] Considering potential future device fabrication, it was observed that the solubility profile of compound 3c was appropriate for production of good quality films on UV-ozone-cleaned glass by spin coating, particularly from chloroform or 1,2-dichloroethane solutions (see Supplementary Material).
Thermal Gravimetric Analysis (TGA) of compounds 3b and 3c suggested thermal stability to above 200 °C (see Supplementary Material, also for Differential Scanning Calorimetry (DSC) curves).

Scheme 2
The structure of bis-adduct 5 was supported by X-ray crystallography (Figure 4), which confirmed new substitution at the C4 and C10 positions.The C4-substituted structure of mono-adduct 6 was assumed by analogy.The crystal structure of bis-anilino adduct 5 showed a different form of - stacking, compared to that of compound 3c.In crystals of adduct 5, the central polycyclic core is stacked between peripheral chlorophenyl rings of two adjacent molecules (see Figures 5 & 6), rather than the pentacyclic cores of adjacent molecules, as in compound 3c (see Figure 2).The plane separation between the chlorophenyl moiety and the core of an adjacent molecule is about 3.7 Å; perhaps slightly long (possibly due to steric repulsion from the Cl) for efficient electron transfer.The two peripheral chlorophenyl substituents on the diazaperylene core each interact with a further two core molecules forming a chain, thus giving a 2D network (Figure 6).Although there is a connection between the molecules in two dimensions, such an arrangement is not a continuous - stack in the usual sense.The type of solid-state packing of compound 5 (Figures 5 & 6) does not augur well for likely high charge mobility in a thin film of this compound, which would be required for use as an OPV material.
The bis-dialkylamino substituents of compounds 3, as well as the anilino moieties of compounds 5 and 6, can be considered somewhat electron-donating and therefore not a desirable feature in electron-accepting (n-type) small molecules for potential organic electronics applications.The dialkylamino groups also represent a "redox liability"; cyclic voltammetry experiments showed that compounds 3c (Figure 7) and 3d (Supplementary Material) underwent two reversible reductions, but an irreversible oxidation.Consequently, we sought to establish chemistry which would enable access to 3,9-diazaperylene compounds with a broader range of substituents at C2 and C8.
An effective synthetic protocol was eventually established and is outlined in Scheme 3. Reaction of 1,5-diaminoanthraquinone 1 with cyanoacetic acid in the presence of diphenyl phosphite and pyridine gave the bis-cyanoacetamide intermediate 7 which was cyclized by heating with DBU in acetonitrile to afford 1,7dicyano-3,9-diazaperylene-2,8-diol bis DBU salt 8. Treatment of the salt 8 with triflic anhydride in the presence of pyridine gave the very poorly soluble bis-triflate 9 (Scheme 3), which we envisaged could be a suitable substrate for palladium-catalyzed coupling reactions (e.g., Suzuki, Stille, and Sonogashira variants) to introduce conjugated groups at C2 and C8 of the diazaperylene core.Prior to synthesis of the bis-triflate 9, the preparation of the bis-chloro analogue was investigated.Heating the bis DBU salt 8 with either phosphoryl chloride/triethylamine hydrochloride, 25 phosphoryl chloride/DMF, 26 or thionyl chloride/DMF, 27 afforded, after aqueous workup, extremely insoluble solids, which were exceedingly difficult to characterize.
Bis-ethynyl compounds 11a-d were isolated as stable yellow solids, while the bis-thienyl compound 12 was a stable orange solid.All five compounds 11a-d are new derivatives of rare 3,9-diazaperylenes.

Scheme 4
The UV-vis spectra of compounds 3b, 3c, 3d, 5, 6, 11a-d, and 12 were recorded (Figure 8).Only the blue-colored anilino compounds 5 and 6 showed any significant absorption in the 600-700 nm region, an energy rich part of the visible spectrum over which the commonly studied n-type fullerene acceptors have low absorption, and a targeted region for absorption by candidate electron accepting n-type compounds.The red-colored compounds 3 absorbed up to wavelengths of ~600 nm, but compounds 11 and 12 (yellow to orange in color) only absorbed at lower wavelengths, typically <500 nm, which was indicative of an inadequate absorption profile for potential organic photovoltaic application.

Conclusions
Concise synthetic methodology for a range of new 2,8-disubstituted-1,7-dicyano-3,9-diazaperylenes has been developed.These synthetic protocols start from the cheap, readily available 1,5-diaminoanthraquinone, and enable the introduction of various tertiary-amino, substituted-ethynyl, and aryl groups at the C2 and C8 positions, as well as incorporation of aryl-amino groups at the C4 and C10 positions, of the rare 3,9diazaperylene core structure.This work provides synthetic pathways to a variety of highly substituted diazaperylenes, enabling the tuning of physical and optoelectronic properties.Such methodology, perhaps in combination with the related, complementary methodology developed by Yi Liu and co-workers, 10 may find use in the discovery of new materials for organic photovoltaic devices or other organic electronic applications.

Experimental Section
Methods and Materials.Melting points were determined using a Reichert hot stage microscope or a Gallenkamp MPD350.BM2.5 apparatus and are uncorrected.Analytical thin layer chromatography (TLC) was carried out on Merck Kieselgel 60 F254 silica aluminium backed sheets.Developed plates were visualized with either UV light (254 nm) or a solution of 10% (w/v) phosphomolybdic acid in EtOH followed by heating.Radial chromatography was performed on a Harrison Research Chromatotron (Model 7924T) using 1, 2, or 4 mm thick silica plates (Merck silica gel 60 PF254 containing gypsum).Light petroleum refers to a fraction boiling between 40 and 60 °C. 1 H NMR spectra were recorded on a Bruker AC-200 or a Bruker Av400 spectrometer at 200 and 400 MHz, respectively. 13C NMR spectra were recorded at room temperature on a Bruker AC-200 at 50 MHz, a Varian Gemini 300 at 75 MHz, a Bruker Av400 at 100.6 MHz or a Bruker DRX500 at 125.75 MHz.Electron impact (EI) mass spectra were recorded on a ThermoQuest MAT95XL mass spectrometer using ionization energy of 70 eV.Only the major fragments are given with their relative abundances shown in parentheses.Accurate mass measurements were obtained with a resolution of 5000-10000 using perfluorokerosene as the internal reference.Electrospray ionization (ESI) mass spectrometric analyses were performed on a Thermo Scientific Q Exactive mass spectrometer fitted with a high-resolution HESI-II ion source.Positive and negative ions were recorded in an appropriate mass range at 140,000 mass resolution.The probe was used with 0.3 mL/min flow of solvent (usually MeOH), and a solution of Reserpine was also introduced into the probe during the experiments to serve as a lock mass in both positive and negative ion modes.The nitrogen nebulizing/desolvation gas used for vaporization was heated to 350 °C in these experiments.The sheath gas flow rate was set to 35 and the auxiliary gas flow rate to 25 (both arbitrary units).The spray voltage was 3.0 kV and the capillary temperature was 300 °C.Some high-resolution mass spectrometric analyses were performed on a Thermo Scientific Q Exactive mass spectrometer (Thermo Scientific, Waltham, MA, USA) fitted with an ASAP ion source (M&M Mass Spec consulting). 36The design and method of ionization have been described previously. 37,38Samples were dissolved in acetonitrile.Positive and negative ions were recorded in an appropriate mass range at 140,000 mass resolution.The APCI probe was used without flow of solvent.The nitrogen nebulizing/desolvation gas used for vaporization was heated to 450 °C.The sheath gas flow rate was set to 25, the auxiliary gas flow rate to 5 and the sweep gas flow rate to 2 (all arbitrary units).The discharge current was 2 mA and the capillary temperature was 320 °C.UV-vis spectra were recorded in chloroform solution on a Perkin Elmer Lambda 1050 UV-vis Spectrophotometer.Thermal Gravimetric Analysis (TGA) was carried out on a Mettler Toledo TGA/SDTA851 and Differential Scanning Calorimetry (DSC) was performed on a Mettler Toledo DSC821.The electrochemistry (cyclic voltammetry) measurements were carried out using a Powerlab ML160 potentiostat interfaced via a Powerlab 4/20 controller to a PC running Echem for windows Ver.1.5.2.The measurements were run in nitrogen-purged DCM with tetrabutylammonium hexafluorophosphate (0.2 M) as the supporting electrolyte.The voltammograms were recorded using a standard 3 electrode configuration with a glassy carbon (2 mm diameter) working electrode, a platinum wire counter electrode and a silver wire pseudo reference electrode.The silver wire was cleaned in concentrated nitric acid and then in concentrated hydrochloric acid to generate the Ag/Ag+ reference.Voltammograms were recorded with a sweep rate of 50-200 mVs -1 .The sample concentration was 1 mM.All potentials were referenced to the E1/2 of the ferrocene/ferrocenium couple.Microanalyses were performed at The Campbell Microanalytical Laboratory, University of Otago, Dunedin, New Zealand.

X-Ray crystallography
Representative crystals were covered in a viscous oil and mounted a cryoloop and cooled to 173 K (3c) or 123 K (5).Datasets were collected using a Nonius KAPPA (3c) or Bruker APEXII (5) CCD diffractometer with MoKa radiation ( 0.71073 Å).Data collection and processing, including multiscan absorption corrections utilized proprietary software COLLECT/DENZO 39 or Apex2. 40The structures were solved and refined by conventional methods using the SHELX-2018 software suite. 41Non-hydrogen atoms were refined with anisotropic thermal parameters and hydrogen atoms attached to carbon were placed in calculated positions.Crystallographic data CCDC 2101284 (3c) and CCDC 2101285 (5) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.CCDC.cam.ac.uk/data_request/cif.

Figure 1 .
Figure 1.Crystal structure of the bis-diethylamino derivative 3c with 30% ellipsoids and hydrogen atoms as spheres of arbitrary size.

Figure 2 .
Figure 2. Packing of successive molecules of bis-diethylamino derivative 3c due to - stacking of arenes.

Figure 3 .
Figure 3. Space filling representations of molecular packing of molecules of bis-diethylamino derivative 3c.

Figure 4 .
Figure 4. Crystal structure of bis-anilino adduct 5 with 50% thermal ellipsoids and hydrogen atoms as spheres of arbitrary size.The molecule lies on a crystallographic inversion center relating the two halves of the molecule.Only the unique atoms have been labelled.Solvent CHCl3 molecule is not shown.

Figure 5 .
Figure 5. Ball and stick representation showing --stacking of the central polycyclic core of compound 5 between two Cl-Ph rings from adjacent molecules.

Figure 6 .
Figure 6.Cell contents of compound 5 as viewed down the b axis, showing the layered, supramolecular, 2-D sheet structure.Solvent CHCl3 molecules are shown.

Figure 7 .
Figure 7. Cyclic voltammogram of compound 3c showing two reversible reductions and an irreversible oxidation.