Synthesis of 6,13-difluoropentacene

Article abstract of DOI:10.3762/bjoc.16.181

6,13-Difluoropentacene was synthesized from 1,4-difluorobenzene. Friedel-Crafts annulation of the latter with phthalic anhydride and subsequent reduction of the anthraquinone gave 1,4-difluoroanthracene. After ortho-lithiation and reaction with phthalic anhydride a carboxylic acid was obtained whose Friedel-Crafts acylation and subsequent reductive removal of the oxygen-functionalities resulted in the formation of the target compound. The HOMO-LUMO gap of 6,13-difluoropentacene was determined via UV-vis spectroscopy and compared to other fluorinated pentacenes.

Full text of DOI:10.3762/bjoc.16.181

Synthesis of 6,13-difluoropentacene
Matthias W. Tripp and Ulrich Koert*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 2136–2140.
Department of Chemistry, Philipps-Universität Marburg,
Hans-Meerwein-Straße 4, 35034 Marburg, Germany
https://doi.org/10.3762/bjoc.16.181
Received: 16 July 2020
Email:
Accepted: 25 August 2020
Published: 02 September 2020
Ulrich Koert* - koert@chemie.uni-marburg.de
* Corresponding author
This article is part of the thematic issue "Organo-fluorine chemistry V".
Guest Editor: D. O'Hagan
Keywords:
fluorinated acenes; Friedel–Crafts reaction; ortho-lithiation; synthesis
© 2020 Tripp and Koert; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
6,13-Difluoropentacene was synthesized from 1,4-difluorobenzene. Friedel–Crafts annulation of the latter with phthalic anhydride
and subsequent reduction of the anthraquinone gave 1,4-difluoroanthracene. After ortho-lithiation and reaction with phthalic an-
hydride a carboxylic acid was obtained whose Friedel–Crafts acylation and subsequent reductive removal of the oxygen-functional-
ities resulted in the formation of the target compound. The HOMO–LUMO gap of 6,13-difluoropentacene was determined via
UV–vis spectroscopy and compared to other fluorinated pentacenes.
Introduction
Pentacenes are a prototype in the field of organic semiconduc- semiconductor, the perfluorinated counterpart perfluoropen-
tors due to their expanded π-systems and low HOMO–LUMO tacene (2, PFP) showed an n-type behavior. In their synthesis
gaps with applications in different molecular-based organic and structural as well as electronic characterization of F4PEN 3,
electronics like OFETs, OLEDs or organic photovoltaics [1-3]. Bettinger et al. showed that the electronic properties of
An advantage of organic molecular electronics over inorganic pentacenes are not a simple linear function of the degree of
alternatives is the versatility in design of new molecular materi- fluorination [7,8]. They reported that the fluorination in the
als by derivatization of known compounds [4]. Since the perfor- 2,3,9,10-positions resulted in a larger HOMO–LUMO gap com-
mance of these devices is strongly dependent on the optoelec- pared to PEN, while PFP showed a smaller gap than PEN.
tronic solid-state properties of the pentacenes, predictable Another example for a partially fluorinated pentacene is the
control of the electronic structure is desirable. A common F6PEN 4 [9]. Its unilateral substitution pattern leads to a disct-
strategy to influence the electronic properties of pentacenes, inct electronic structure and a criss-cross packing motif, unlike
without significant effect on the molecular shape, is fluori- the herringbone structure of 1–3.
nation [5]. Suzuki et al. showed in 2004 the strong effect of
perfluorination on the electronic properties [6]. While unsubsti- Here, the synthesis and characterization of 6,13-difluoropen-
tuted pentacene (1, PEN, Figure 1) is known to be a p-type tacene (5, F2PEN) is reported. Since the central ring of
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Results and Discussion
6,13-Disubstituted pentacenes are often synthesized starting
from 6,13-pentacenedione [2,3,10-12]. Initial attempts to
prepare 6,13-difluoropentacene (5) along this route failed [13].
Therefore, an alternative route that commences with the central
ring already carrying the two fluorine substituents was investi-
gated. The retrosynthetic analysis for this strategy is shown in
Scheme 1. The formation of the C5,5a-bond colored in red
could be accomplished by an intramolecular Friedel–Crafts type
acylation with the acylium-cation intermediate 6. The corre-
sponding carboxylic acid precursor could be prepared by reac-
tion of the anthracenyllithium 7 with phthalic anhydride (8).
Intermediate 7 could be accessed by ortho-lithiation of
anthracene 9. The synthesis of 9 by two consecutive
Friedel–Crafts acylation reactions and reduction of the result-
ing anthraquinone could start from 8 and 1,4-difluorobenzene
(10).
Starting point of the synthesis was the conversion of 1,4-difluo-
robenzene (10) to 1,4-difluoroanthraquinone (11, Scheme 2)
[14]. The first Friedel-Crafts acylation of phthalic anhydride
was achieved using AlCl3 in boiling 1,4-difluorobenzene (10).
The second Friedel–Crafts acylation is then performed in
polyphosphoric acid at 140 °C, giving anthraquinone 11 in 53%
Figure 1: Structures of pentacene and fluorinated pentacenes.
pentacenes is known to be a weak spot (because of light-in- yield over two steps. Reduction of 11 to the anthracene 9
duced dimerization or oxidation) [2] the effects of fluorination proceeded smoothly using zinc powder in 1,4-dioxane and
in these positions are of interest.
aqueous ammonia under copper catalysis [15]. To achieve good
Scheme 1: Retrosynthetic analysis of F2PEN 5.
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Beilstein J. Org. Chem. 2020, 16, 2136–2140.
Scheme 2: Synthesis of F2PEN 5.
yields, it was crucial to perform the reaction in a sealed pres- The low yield for the formation of 13 seems to be an intrinsic
sure tube to prevent ammonia from degassing.
instability of its ortho-fluorobenzylic alcohol moiety. Moreover,
the compound quantitatively decomposes to 6,13-pentacene-
Different bases, temperatures and reaction times were screened quinone 15 in CD2Cl2 solution at room temperature within 2 h.
to optimize the ortho-lithiation of 9. The best results were A possible reaction pathway for this degradation in the pres-
achieved using LDA as a base at –78 °C for 15 min and then ence of moisture is an intermolecular attack of water, leading to
rapidly adding the lithiated species 7 to phthalic anhydride in a fluorohydrine 14 (Scheme 3). In absence of water, this reac-
THF precooled to –100 °C [16]. Using these conditions, the tion might take place in an intramolecular manner. The fluoro-
carboxylic acid 12 was isolated in 82% yield.
hydrine 14 then rapidly decomposes to 6,13-pentacenequinone
(15) after elimination of HF.
The subsequent Friedel–Crafts acylation proofed to be chal-
lenging. Using reagents like PCl5 resulted in a quantitative sub- The final aromatization of diol 13 to the target molecule 5
stitution of the fluorine substituents in the corresponding proceeded smoothly in 74% yield using SnCl2 in 1,4-dioxane
pentacenequinone by chloride. This side reaction could be and aqueous HCl [11,12]. F2PEN 5 can be stored under inertgas
avoided using polyphosphoric acid as Friedel–Crafts reagent. atmosphere at −20 °C for a month without noticeable decompo-
The pentacenequinone showed very low solubility, which sition. However, in solution under ambient atmosphere and
complicated a chromatographic isolation. However, the crude sunlight decomposition takes place quickly, which is indicated
1H NMR showed sufficiently clean conversion, hence the by decolorization of a purple solution in CH2Cl2 within 3 min.
subsequent reduction step using NaBH4 to diol 13 was per- 1H NMR analysis showed 6,13-pentacenequinone (15) as the
formed without further purification of the quinone.
degradation product. The rate of degradation in degassed C6D6
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Beilstein J. Org. Chem. 2020, 16, 2136–2140.
Scheme 3: Decomposition of diol 13 in solution.
was slow enough to obtain a clean 1H NMR spectrum (see While fluorine substituents in 2,3,9,10-positions (F4PEN 3)
Supporing Information File 1) [17].
result in a larger HOMO–LUMO gap compared to PEN [7,8],
substitution in the 6,13-positions leads to smaller band-gaps,
Next, the HOMO–LUMO gap of 5 was investigated via UV–vis similar to the unilaterally substituted F6PEN 4 [9].
spectroscopy in solution (Figure 2). The λmax was determined to
be at 597 nm, which corresponds to a HOMO–LUMO gap of Conclusion
ΔE = 2.08 eV. Comparison with the values of PEN Starting from 1,4-difluorobenzene a synthetic route to 6,13-
(ΔE = 2.13 eV) [9] and PFP (ΔE = 1.99 eV) [9] shows that the difluoropentacene was established (7% over seven steps). Key
electronic structure of F2PEN 5 lies in between these two. steps are Friedel–Crafts acylation reactions and an ortho-
directed lithiation of 1,4-difluoroanthracene. This strategy could
be applicable for the synthesis of differently substituted 6,13-
difluoropentacenes as well.
Supporting Information
Supporting Information File 1
Experimental details, spectroscopic and analytical data of
all new compounds.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-181-S1.pdf]
Funding
Financial support by the Deutsche Forschungsgemeinschaft
Figure 2: UV–vis spectrum of F2PEN 5 in CH2Cl2.
(SFB 1083, project A8) is gratefully acknowledged.
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Beilstein J. Org. Chem. 2020, 16, 2136–2140.
ORCID® iDs
Ulrich Koert - https://orcid.org/0000-0002-4776-8549
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
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