Diazapentacenes from Quinacridones

Article abstract of DOI:10.1002/chem.202004761

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 substit

Full text of DOI:10.1002/chem.202004761

Accepted Article
Accepted Article
Title: Diazapentacenes from Quinacridones
Authors: Thomas Wiesner, Lukas Ahrens, Frank Rominger, Jan
Freudenberg, and Uwe Heiko Bunz
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Diazapentacenes from Quinacridones
[
a]
[a]
[a]
[a]
[a]
Thomas Wiesner, Lukas Ahrens, Frank Rominger, Jan Freudenberg,* Uwe H. F. Bunz*
Bis(silylethynylated) 5,7- and 5,12-diazapentacenes were synthe-
sized 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.
elimination to furnish triisopropylsilyl- and triethylsilylethynylated
diazapentacenes 1a,b and 2a,b as intensely colored blueish-
green solids. Note that concentrated solutions of TES-substituted
2
b decomposed during workup at ambient temperature even
under protective atmosphere (vide infra, see Figure 4 for one of
the decomposition Diels-Alder adducts); TIPS-substituted 2a is
stable. Yet, performing workup, extraction, chromatography and
solvent removal at temperatures below 0 °C allows isolation of
pure 2b which is stable as an amorphous solid at -20 °C. All of the
other compounds are stable at 5 °C for several months.
Here 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
(
[
3]
[4]
find use in organic electronics. Stabilization and solubilization
of larger azaacenes (e.g. ≥5 annulated rings) is achieved by
triisopropylsilylethynyl substituents (e.g. in TIPS-Pen, TIPS-
TAP ) 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 s , both n- and p-
type,[ but critically depends on nature and position of the
substituents.[9] Silylethynyl groups retard oxidation[10] and
decrease rates of [4+4] cycloaddition to acene dimers.[11]
symmetric peri-substituted (aza)acenes are almost unknown.
[
5]
[
6]
-
1
-1
8]
D
2h
12]
-
[
Grimsdale et al. disclosed two only moderately stable 5,7-
diazapentacenes (1; R = hexyl or phenyl[13]) obtained via a Cu(I)-
catalyzed
reaction
of
diphenyliodonium
triflate
with
aminophenones - an approach to substituted (hetero)acenes;
these are commonly synthesized via nucleophilic addition to
[
14]
quinone precursors followed by deoxygenation.
This
established procedure allows facile introduction of different
substituents.
Scheme 1. Synthetic route towards diazapentacenes 1a,b. Synthesis of 2a,b is
carried out accordingly (Scheme S1, SI).
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 1a and 2a (hexane,
see SI Figure S2, S4 for spectra of 1b and 2b). The p- and α-
bands of weakly emissive 1a are distinctive with λmax,abs = 611 nm
Figure 1. Structures of diazaacenes (quinacridines) 1 and 2 and regioisomeric
diazapentacenes 3 and 3’.
(λmax,em = 630 nm, see SI Figure S5 for an emission spectrum).
Spectral features of non-emissive 2a are broader and slightly red-
shifted to λmax,abs = 631 nm. The absorbance spectra (Figure S6,
SI) are concentration independent. DOSY studies (Figure S7, SI)
result in similar diffusion coefficients. Thus, UV-vis peak
broadening of 2 is not attributable to aggregation in solution. 1 and
We added lithiated silylacetylenes to linear cis-[15] (S6) and trans-
quinacridone (5) after protection of the amine functions with Boc
anhydride (Scheme 1, see SI Scheme S1 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
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, SI).
[
a]
M. Sc. T. Wiesner, M.Sc. L. Ahrens, Dr. F. Rominger, Dr. J.
Freudenberg, Prof. U. H. F. Bunz
+
+
1
a (-1.24 V vs Fc/Fc , CV) and 2a (-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’ > 1a-b > 2a-b). The electron affinity is
independent of the silyl substituent (Table 1, Table S1, S2, SI).
1a,b and 2a,b are stable in dilute solution (Figure S1-S4,
c = 10-6 M) under ambient atmosphere in the dark. When exposed
freudenberg@oci.uni-heidelberg.de, uwe.bunz@oci.uni-
heidelberg.de
Organisch Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, FRG
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
to light under ambient atmosphere, solutions of 1a,b react within
the hour (Figure 2b) forming colorless products. Single crystals of
these products were unstable under X-ray conditions. HRMS
indicates peroxide formation (Figure S12, SI) and/or follow-up
degradation products.[16] Cis-quinacridines are more stable: 2a,b,
are stable for up to 3 days. Similarly, Grimsdale et al. reported
discoloration of their phenyl substituted derivative during “several
days”. 2b is persistent in the dark under ambient conditions for
The π-π-stacking distances (3.47 Å for 1a, 3.48 Å for 1b) are tight.
2a packs in one-dimensional dimeric staircase without π-π-
overlap between the dimers. In contrast to 1a,b the TIPS-groups
are an integral part of the one-dimensional stacks and do not
separate adjacent layers from one another. 2b 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 1a-b or 2a and might be an
attractive organic semiconductor. The IP and EA of 2b do not
match with standard electrode materials (Au or Ag), their
application is being evaluated at the moment.
‑
4
-1
4
d at concentrations of up to 5•10 M (0.28 mgmL , Figure S4,
SI). Concentrating 2b at room temperature results in the dimer 9
Figure 4). Trans-quinacridines 1a,b pack in a one-dimensional
staircase barely influenced by the size of the silyl group (Figure 3).
(
Figure 2. a) Normalized absorption spectra of 1a, 2a and 3 in n-hexane. b) Stability of 1b in DCM under ambient light and atmosphere. c) Stability of 2b in DCM
under ambient light and atmosphere.
Figure 3. Solid state packing of 1a,b and 2a,b.
Table 1. Optical, electrochemical and quantum-chemical data.
λ
[
max,abs
nm][a]
λ
onset, abs
[nm][a]
Opt. gap
[eV]
Compound
[b]
E(0/-) [V][c]
1
1
2
2
a
b
a
b
611
610
631
637
697
661
622
623
659
662
709
681
1.99
1.99
1.88
1.87
1.74
1.82
-1.24
-1.18
-1.39
-1.30
-1.05
-1.16
3[17]
3
’
[
a]
Absorption measurements were performed in n-hexane. λmax,abs denotes the
local absorption maximum at the longest wavelength. [b]Calculated from
[c]
λ
onset abs
, . First reduction potentials measured by cyclic voltammetry (CV) in
+
dichloromethane with Bu
4
NPF
6
as the electrolyte against Fc/Fc as an internal
−1
standard (−5.10 eV) [ at 0.2 mV s
18]
.
Figure 4. At high concentrations 2b via thermally allowed [4+2] cycloaddition
forming the dimer 9.
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Acknowledgements
In conclusion, four silylethynylated 5,7- and 5,12-
diazapentacenes (1a,b and 2a,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 2b susceptible to dimerization via Diels-Alder reactions,
although 2b can be handled at concentrations relevant for device
fabrication. To increase stability and lower the frontier orbital
energies i.e. 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.
We thank the DFG (SFB 1249) for generous support. L. A. thanks
the ‘Studienstiftung des deutschen Volkes’ for a scholarship.
Conflict of Interests
The authors declare no conflict of interests.
Keywords: azaacenes • quinacridines • solid-state packing •
photostability • semiconductors
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Entry for the Table of Contents
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T. Wiesner, L. Ahrens, F. Rominger, J.
Freudenberg,* U. H. F. Bunz*
Page No. – Page No.
Diazapentacenes from Quinacridones
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.
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