Diazapentacenes from Quinacridones
Graphical Abstract
Piecing together a diazaacene: Starting from linear cis- and trans-quinacridones, the synthesis, characteristics and stability of four diazapentacenes are reported leading to a design rule for 12,14-alkynylated heteroacenes for organic electronics (see figure).
Abstract
Bis(silylethynylated) 5,7- and 5,12-diazapentacenes were synthesized from cis- and trans-quinacridone using protection, alkynylation and deoxygenation. The solid-state packing of the targets is determined by choice and position of the silylethynyl substituents. The position of the substituents and nitrogen atoms influence the optical properties of the targets.
Herein, we disclose a strategy to transform industrially produced acridones into azaacenes. We convert cis- and trans-quinacridone,1, 2 into silylethynylated 5,7- (“cis”) and 5,12- (“trans”) diazapentacenes 1 and 2 and compare their properties with respect to solid-state packing and (photochemical) stability. (Aza-)acenes are structurally and fundamentally attractive; they find use in organic electronics.3 Stabilization and solubilization4 of larger azaacenes (e.g. ≥5 annulated rings) is achieved by triisopropylsilylethynyl substituents (e.g. in TIPS-Pen,5 TIPS-TAP6) or building blocks that provide additional Clar sextets to the aromatic backbone.7 Their solid-state packing (brickwall) allows charge carrier mobilities >1 cm V−1 s−1, both n- and p-type,8 but critically depends on the nature and position of the substituents.9 Silylethynyl groups retard oxidation10 and decrease rates of [4+4] cycloaddition to acene dimers.11 D2h-symmetric peri-substituted (aza)acenes are almost unknown.12 Grimsdale et al. disclosed two only moderately stable 5,7-diazapentacenes (1; R=hexyl or phenyl13) obtained via a CuI-catalyzed reaction of diphenyliodonium triflate with aminophenones—an approach to substituted (hetero)acenes; these are commonly synthesized via nucleophilic addition to quinone precursors followed by deoxygenation.14 This established procedure allows facile introduction of different substituents.
We added lithiated silylacetylenes to linear cis-1a (S6) and trans-quinacridone (5) after protection of the amine functions with Boc anhydride (Scheme 1, see Scheme S1 in the Supporting Information for the reaction sequence involving cis-quinacridone). Workup with MeI gives a mixture of 7 and 8. The same reactivity is also observed when starting from cis-quinacridone. Treatment of the mixture with TFA results in deprotection of the amine(s) with concomitant methanol elimination to furnish triisopropylsilyl- and triethylsilylethynylated diazapentacenes 1 a,b and 2 a,b as intensely colored blueish-green solids (Figure 1). Note that concentrated solutions of TES-substituted 2 b decomposed during workup at ambient temperature even under protective atmosphere (vide infra, see Figure 3 for one of the decomposition Diels–Alder adducts); TIPS-substituted 2 a is stable. Yet, performing workup, extraction, chromatography and solvent removal at temperatures below 0 °C allows isolation of pure 2 b which is stable as an amorphous solid at −20 °C. All of the other compounds are stable at 5 °C for several months.
Synthetic route towards diazapentacenes 1 a,b. Synthesis of 2 a,b is carried out accordingly (Scheme S1).
Structures of diazaacenes (quinacridines) 1 and 2 and regioisomeric diazapentacenes 3 and 3′.
The optoelectronic properties of cis- and trans- diazapentacenes 1 and 2 differ. The silyl group (TIPS vs. TES) has no effect. Figure 2 compares the absorption spectra of 1 a and 2 a (hexane, see Figures S2, S4 for spectra of 1 b and 2 b). The p- and α-bands of weakly emissive 1 a are distinctive with λmax,abs=611 nm (λmax,em=630 nm, see Figure S5 for an emission spectrum). Spectral features of non-emissive 2 a are broader and slightly red-shifted to λmax,abs=631 nm. The absorbance spectra (Figure S6) are concentration independent. DOSY studies (Figure S7) result in similar diffusion coefficients. Thus, UV-vis peak broadening of 2 is not attributable to aggregation in solution. 1 and 2 display blue-shifted absorption in comparison to 3 and 3′ (Figure 2; 3: λmax,abs=697 nm, 3′: λmax,abs=661 nm), the extent of which depends on the position of the nitrogen and to a lesser extent on the position of the alkyne groups as suggested by our computational study (Table S2).
a) Normalized absorption spectra of 1 a, 2 a and 3 in n-hexane. b) Stability of 1 b in DCM under ambient light and atmosphere. c) Stability of 2 b in DCM under ambient light and atmosphere.
Compound 1 a (−1.24 V vs. Fc/Fc+, CV) and 2 a (−1.39 V vs. Fc/Fc+, CV) are less easily reduced than 3 (−1.05 V vs. Fc/Fc+, CV). The trans-isomers are more electron-deficient than the syn-quinacridines (order of reduction potentials: 3>3′>1 a,b>2 a,b). The electron affinity is independent of the silyl substituent (Table 1, Tables S1, S2). 1 a,b and 2 a,b are stable in dilute solution (Figures S1–S4, c=10−6 m) under ambient atmosphere in the dark. When exposed to light under ambient atmosphere, solutions of 1 a,b react within the hour (Figure 2 b) forming colorless products. Single crystals of these products were unstable under X-ray conditions. HRMS indicates peroxide formation (Figure S12) and/or follow-up degradation products.15 cis-Quinacridines are more stable: 2 a,b, are stable for up to 3 days. Similarly, Grimsdale et al. reported discoloration of their phenyl substituted derivative during “several days”. 2 b is persistent in the dark under ambient conditions for 4 d at concentrations of up to 5×10−4 m (0.28 mgmL−1, Figure S4). Concentrating 2 b at room temperature results in the dimer 9 (Figure 3). Trans-quinacridines 1 a,b pack in a one-dimensional staircase barely influenced by the size of the silyl group (Figure 4). The π–π-stacking distances (3.47 Å for 1 a, 3.48 Å for 1 b) are tight. 2 a packs in one-dimensional dimeric staircase without π–π-overlap between the dimers. In contrast to 1 a,b the TIPS-groups are an integral part of the one-dimensional stacks and do not separate adjacent layers from one another. 2 b crystallizes in a dimeric herringbone arrangement with π–π-distances amounting to 3.52 Å between the dimer pairs and 3.33 Å within the dimers—the π–π-overlap is higher than for 1 a,b or 2 a and might be an attractive organic semiconductor. The IP and EA of 2 b do not match with standard electrode materials (Au or Ag), their application is being evaluated at the moment.
Compound |
λmax,abs [nm][a] |
λonset, abs [nm][a] |
Opt. gap [eV][b] |
E(0/−) [V][c] |
|---|---|---|---|---|
1 a |
611 |
622 |
1.99 |
−1.24 |
1 b |
610 |
623 |
1.99 |
−1.18 |
2 a |
631 |
659 |
1.88 |
−1.39 |
2 b |
637 |
662 |
1.87 |
−1.30 |
316 |
697 |
709 |
1.74 |
−1.05 |
3′ |
661 |
681 |
1.82 |
−1.16 |
- [a] Absorption measurements were performed in n-hexane. λmax,abs denotes the local absorption maximum at the longest wavelength. [b] Calculated from λonset,abs. [c] First reduction potentials measured by cyclic voltammetry (CV) in dichloromethane with Bu4NPF6 as the electrolyte against Fc/Fc+ as an internal standard (−5.10 eV)17 at 0.2 mV s−1.
At high concentrations 2 b reacts via thermally allowed [4+2] cycloaddition forming the dimer 9.
Solid state packing of 1 a,b and 2 a,b.
In conclusion, four silylethynylated 5,7- and 5,12-diazapentacenes (1 a,b and 2 a,b) were obtained from cis- and trans-quinacridones. Regioisomerism and steric demand of the substituents defines stability and decomposition pathways. cis-Diazapentacenes are more stable with respect to oxidation. While the smaller TES groups are beneficial for solid-state packing, they render 2 b susceptible to dimerization via Diels–Alder reactions, although 2 b can be handled at concentrations relevant for device fabrication. To increase stability and lower the frontier orbital energies, that is, the electron affinities, we currently work on the synthesis of tri- and tetraaza-derivatives of 1 and 2. A central pyrazinic ring should increase oxidative stability and suppress cycloaddition activity. Such targets might be attractive n-type semiconductors.
Experimental Section
Crystallographic data
Deposition Number(s) 2040 676 (1 a), 2040677 (1 b), 2040678 (2 a), 2040679 (2 b), and 2040679 (9) contain(s) the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.
Representative procedure towards 5,12-diaza-7,14-(triisopropylethynyl)pentacene (1 a)
In an oven-dried Schlenk tube under argon atmosphere, 2.5 m nBuLi (312 μL, 78.0 μmol, 4.00 equiv., in n-hexane) was slowly added to a solution of TIPS-acetylene (263 μL, 1.17 mmol, 6.00 equiv.) in anhydrous THF at −78 °C. After stirring for 1 h, Boc-protected quinacridone 6 (100 mg, 190 μmol, 1.00 equiv.) was added to the solution and the reaction was allowed to stir overnight while allowing the reaction to warm to room temperature. The reaction was subsequently quenched by addition of methyl iodide (120 μL, 1.90 mmol, 10.0 equiv.) and stirred for 8 h. A small amount of saturate aqueous ammonium chloride solution was added and the phases were separated. After concentrating of the organic layer in vacuo, the crude product was redissolved in DCM, washed with water, dried over magnesium sulfate and evaporated under reduced pressure. It is then dissolved in DCM (5 mL) and TFA (500 μL, 7.80 mmol, 40.0 equiv.). is added to the mixture. After stirring at 0 °C for 30 min an excess of sat. aqueous sodium bicarbonate solution is added and stirred until the bright green solution turned dark. The organic layer was separated, dried over Na2SO4 and evaporated under reduced pressure. As an eluent for purification via column chromatography a mixture of petrol ether and ethyl acetate (97:3) was chosen. The product was obtained as a dark solid and was recrystallized by overlaying a saturated dichloromethane solution with methanol to yield black crystalline needles with a metallic lustre (48.5 mg, 72.1 μmol, 36 % overall yield). NMR (400 MHz, CDCl3) δ=9.67 (s, 2 H), 8.52 (d, J=8.8 Hz, 2 H), 8.21 (d, J=8.9 Hz, 2 H), 7.75 (m, 2 H), 7.59–7.44 (m, 2 H), 1.47–1.36 (m, 6 H) 1.34 ppm (m, 36 H). 13C NMR (101 MHz, CDCl3) δ=151.3, 144.7, 131.1, 130.8, 128.4, 128.2, 127.9, 127.8, 127.2, 127.1, 111.6, 101.5, 77.4, 19.1, 11.7 ppm. IR (neat) 
=2943, 2859, 1526, 1456, 1429, 1396, 1060, 990, 879, 755, 662, 607, 455, 436 cm−1. HRMS (ESI+): m/z calcd (%) for C42H52N2Si2: 640.3669; found 641.3750 (M+H+). M.p.: 220–230 °C (dec.).
Acknowledgements
We thank the DFG (SFB 1249) for generous support. L.A. thanks the “Studienstiftung des deutschen Volkes” for a scholarship. Open access funding enabled and organized by Projekt DEAL.
Conflict of interest
The authors declare no conflict of interests.
