Diindenopyrazines: Electron-Deficient Arenes

Article abstract of DOI:10.1002/chem.202100372

The syntheses, properties and application of the air-stable electron acceptors, diindenopyrazines 4 a–g are reported demonstrating the introduction of functional aryl groups in the 6- and 12-positions. The targets are accessible on the hundred milligram to gram scale. The structure of the aryl groups in 4 a–g modulates their solubility, redox potentials and optical properties. The introduction of electron-poor aryl groups to the electron-poor diindenopyrazine backbone reduces the electron affinity to ?4 eV, making the compounds attractive as n-semiconductors. A simple organic field-effect transistor of 4 e –without optimization– shows electron transport with a mobility of up to 0.037 cm² V^?1 s^?1.

Full text of DOI:10.1002/chem.202100372

Accepted Article
Accepted Article
Title: Diindenopyrazines: Electron-Deficient Arenes
Authors: Victor Brosius, Svenja Weigold, Nikolai Hippchen, Frank
Rominger, Jan Freudenberg, and Uwe Heiko Bunz
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1/2020

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Diindenopyrazines: Electron-Deficient Arenes
[a]
[a]
[a]
[a]
[a]
Victor Brosius, Svenja Weigold, Nikolai Hippchen, Frank Rominger, Jan Freudenberg,* and
Uwe H. F. Bunz*[a]
So far, diradicaloid[12] and closed shell indenofluorene-type
compounds[13,15] were applied in organic field effect transistors as
ambipolar charge transport systems.[
1, 3 and 4d were prepared from 5a,b or 6 (Figure 1c) by double
addition of an aryl Grignard followed by reductive deoxygenation
Abstract: We report the syntheses, properties and application of air
stable, electron acceptors, the diindenopyrazines 4a-g. We
demonstrate introduction of functional aryl groups in 6,12-position.
The targets are accessible on the hundred milligrams to the gram
scale. The structure of the aryl groups in 4a-g modulate the solubility,
redox potentials and optical properties. Introduction of electron poor
aryl groups to the electron poor diindenopyrazine backbone reduces
the electron affinity to -4 eV, attractive as n-semiconductors. A simple
organic field-effect transistor - without optimization - of 4e shows
16]
2
of the intermediate biscarbinol with SnCl . While this is a time-
tested method, the yields are variable and for the synthesis of 4d
did not exceed 20%. We are interested in structures of the type 4
and present three different routes that lead to 4 featuring seven
different phenyl-substituents. Reaction of 7 with an excess of
lithium bis(trimethylsilyl)amide and reaction of the dianion with
fluorobenzene, hexafluorobenzene or 4-flurobenzonitrile gave the
2
electron transport with a mobility of up to 0.037 cm /Vs.
Diindenoacenes[1-3] are quinoidal systems
[4,5]
with the possibility
[6-8]
of diradical (-oid) character.
Such diradicals display attractive
magnetic,[ optical and theoretical properties. The diradical
character is prominent, when the central core is an anthracene.[12]
Indenofluorene 1 is a quinoidal closed shell system, as the
aromatization energy of benzene – around 21 kcal/mol – is not
sufficient to enforce the diradical character,[13] while
diindenoanthracene 2 displays a diradical ground state (Figure
9]
[10]
[11]
[
12]
1
a). N-Heterocyclic indenofluorenes are sparsely investigated
and only recently, Wang et al. described compounds of the type
and 4d.[ Their electron affinity is increased in comparison to
14]
3
that of 1. 3 is a triplet diradical while 4d is a closed-shell singlet,
easily understood, as 3 gains two Clar sextet upon “radicalization”,
while 4d, analogously to indenofluorene will only gain one of them
(
Figure 1b).
Scheme 1. Synthesis of diindenopyrazines 4a-g. i) LHMDS, F-Ara,e,g in THF,
r.t., 16 h. ii) one pot synthesis: LHMDS, F-Ara,e,g, chloranil in THF, r.t., 16 h. iii)
Pd
2
(dba)
3
, DavePhos, Br-Ara,b,c,d, Cs
2
CO
3
in DMAc, 80°C, 16 h. iv) PPA, 120 °C,
t
4
h. v) DDQ in PhMe, 100°C, monitored by thin layer chromatography or KO Bu,
chloranil in THF, r.t. 1 h.
compounds 9a,e,g after aqueous workup. Instead of quenching
the in situ generated dianions of 9a,e,g with water, addition of
chloranil forms 4a,e,g in a one pot synthesis in in 30-45% yield as
stable crystalline materials without diradical character. For e and
[
12,13]
Figure 1. a) Previous work from Haley et. al.
b)Previous work from Pei et
[
14]
al. c) Reactants for the synthesis of 1 (from 5a), 3 (from 5b), and 4d (from 6).
N
g the S Ar-mechanism is probably addition-elimination i.e.
associative. For fluorobenzene, an elimination-addition-mecha-
nism cannot be excluded. b,c and f gave inseparable mixtures of
compounds, probably due to the presence of an elimination-
addition reaction, which gives mixtures of stereoisomers,[
whereas d did not react at all.
17]
[
a]
MSc. V. Brosius, MSc. S. Weigold, MSc. N. Hippchen, Dr. F.
Rominger, Dr. J. Freudenberg, Prof. Dr. U.H. F. Bunz
Organisch-Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg,
Direct arylation via C-H activation of the diindenopyrazine works
a-d
0
well for Br-Ar in the presence of a Pd source and the biphenyl-
(
Germany)
based ligand DavePhos giving 9a-d in yields between 25% and
E-mail: uwe.bunz@oci.uni-heidelberg.de , freudenberg@oci.uni-
heidelberg.de
65%. The reaction does not work well with electron deficient
arylbromides, probably due to competing C-H activation of the aryl
bromide. Oxidation of 9a-g with DDQ in toluene at reflux gave
Supporting Information for this article is given via a link at the end of
the document.
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moderate yields of 4a-g. The dianions of 9a-g, generated in situ
by the addition of KOtBu, were oxidized by DDQ in yields > 85%.
The third method employs the 2,5-dialdehyde of para-
diphenylpyrazine. Addition of an aryl Grignard reagent followed
by dehydration of the carbinol by polyphosphoric acid gives 9 in
phenyl rings with respect to the diindenopyrazine. The other
phenyl-substituents display more planar geometries, i.e. enlarge
the -system. The pyrazine ring allows for a larger degree of
planarization, as the peri-hydrogens of the indenofluorenes are
absent. The observed red-shift – when compared to the
indenofluorenes 1 – is a result of the decreased torsion angle but
also due to an electronic effect of the pyrazine. Compared to 1,
HOMO and LUMO levels of 4 are stabilized by the electron poor
pyrazine core (Figure 3).
N
40-70% yield. The. S Ar and C-H activation allow the introduction
of aryl substituents that do not survive Grignard formation,
necessary to transform 5 or 6 into 1,3 or 4.
On a scale above 50 mg 4 and 9 were purified by crystallization
(
exception 4c, 9c); but we also note that 5 the starting material
used by Pei et.al. is synthesized by oxidation of 7, which we use
directly.[
14]
We obtained single crystalline specimen from 4c and 4d
o
(
Figure 4); 4c displays a torsion angle of only 28 , while mesityl
o
substituted 4d displays an angle of 63 . The larger twist compared
to 4c is caused by steric pressure of the ortho-methyl groups and
weakens the conjugation of the backbone to the aryl groups (see
above). This leads to the already mentioned blue-shift of 4d, as
1.0
0.8
0.6
0.4
0.2
0.0
4
4
4c
a
b
4
4
d
e
A
well as larger HOMO-LUMO / IP-E gaps (Table 1). The crystal
[14]
structure of 4d has been reported.
4f
4g
400
450
500
550
600
650
700
Wavelength [nm]
Figure 2. Normalized UV-Vis spectra of 4a-g in DCM.
Figure 4. Single crystal structure a,c) and packing b,d) of 4c and 4d,
respectively.
Figure 3. DFT- Calculated FMO levels (B3LYP/def2TZVP) of the compounds
a-4g. Dashed lines represent the FMOs of the indenofluorenes 1a,b,d-f, taken
4
from [12].
Both the single crystal structures but also the quantum chemical
calculations confirm the quinoidal and non-diradical character of
derivatives of 4. The molecules of 4c pack in one-dimensional
oblique stacks with -contacts between parallel diindenopyrazine
units, the attached phenyl rings however are oriented
perpendicular to those of the adjacent molecules, thus showing
CH--contacts among each other. In contrast, the two
independent centrosymmetric molecules of 4d show no parallel
Non-fluorescent compounds 4a-g absorb visible light up to
7
5
00 nm, with their absorption maxima λmax ranging between
37 nm (opt. gap 2.31 eV) for d and 586 nm (opt. gap 1.92 eV)
for b, c. (Figure 2, Table 1). The mesityl and the perfluorophenyl-
substituted diindenopyrazines display the most blue-shifted
features in their absorption spectra, due to the twisting of the
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stacking motif in the crystal lattice. In 4c the distance between the
-systems is 3.2 and 3.5 Å, i.e. below the van der Waals radius.
Figure 6. Synthesis, UV-Vis and single crystal structure of 4dH 2
as counterion.
+
with SbCl -
6
2
Figure 5. a) drop-casted film of 4e from chloroform (0.38 mg/mL), 50x darkfield
microscopy; b) schematic device architecture c) transfer characteristics of
bottom-gate/top-contact FET (Vds = 50 V) d) output characteristics.
We investigated the reduction of 4e with potassium anthracenide.
The radical anion formed easily; its EPR spectrum is in good
agreement with the simulated spectrum (Supporting Information)
and the UV-Vis spectrum displays an absorption at 1103 nm
(Figure 7). In air, the signal disappears after 90 minutes.
We built a transistor with a bottom gate top contact geometry of
4e, due to its sufficient solubility and thus good film forming
properties. We used a silicon substrate with successive layers of
1
1
00 nm thermally grown silica, solution processed alumina and
2-cyclohexyldodecylphosphonic acid (CDPA) as self-
a
[18]
assembled monolayer to modify the dielectric. A solution of 4e
was drop-cast on the substrate from chloroform (0.38 mg/mL)
resulting in a crystalline thin-film (Figure 5a). 40 nm silver was
evaporated through
a shadow mask to form the contact
electrodes. Figure 5b) shows the schematic device architecture.
4
e displays low hysteresis in the transfer curves and is without
-
hysteresis in the output curves, with a top mobility of µmax = 0.037
cm /Vs (Figure 5c,d). The average mobility is 0.022 cm /Vs (6
2
2
channels, 2 substrates). Compared to the literature known 1e,
+
-4
which displays ambipolar behaviour with µMax = 7x10 cm²/Vs,
-
-3
µ
Max = 3x10 cm²/Vs as single crystal transistor the mobility of 4e
[
13]
is an order of magnitude higher.
Reaction of 4d with SbCl
acetonitrile quantitatively gave the protonated species 4dH
identified by X-ray single crystal structure and UV-Vis spectrum
5 6
or NOSbF in dichloromethane/
2
+
2
,
2
+
(
Figure 6). 4dH
2
4
was obtained by reaction of 4d with HBF .
2
+
Addition of water to 4dH
2
re-forms 4d quantitatively. Oxidation
into the radical cation or the dication of 4d did not occur, probably
due to the instability of fluorenyl cation in addition to the
destabilizing pyrazine-core and lack of conjugation into the
mesityl substituents. For
derivative, Haley et. al. suggested
a
quinoidal diindenonaphthalene
higher degree of
a
delocalization of the positive charge compared to that of the
negative charge, localized at the fluorenyl-position.[19]
Figure 7. Reduction of 4e with potassium anthracenide in THF (top) and UV-Vis
-
spectrum of 4e∙ (bottom).
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In conclusion, we present three efficient routes towards
diindenopyrazines using S Ar, C-H activation and ring closure
N
reactions. Compounds 4, lacking diradical character, yet display
attractive properties. Particularly 4e is an n-channel
semiconductor, which even in un-optimized proof of concept
2
transistors display mobilities of up to 0.037 cm /Vs.
Table 1. Experimental and calculated properties of 4a-g.
Ox
E1
Red
[c]
[c]
[d]
[d]
compound
λ
max, abs
Opt.gap
[eV]
IP [eV]
E
A
[eV]
HOMO [eV]
LUMO [eV]
HOMO-LUMO
gap [eV]
E1/2
/2
[c]
[
b]
[
nm][
[c]
[
V]
[V]
a]
(DCM)
a
b
c
d
e
f
576
1.98
1.92
1.92
2.31
2.12
1.98
1.94
0.76
0.81
0.85
0.93
-1.12
-1.18
-1.18
-1.30
-0.88
-0.87
-0.90
-5.56
-5.61
-5.65
-5.73
-3.68
-3.62
-3.62
-3.50
-3.92
-3.93
-3.90
-5.37
-5.28
*
-3.43
-3.30
*
1.94
1.98
*
584
586
537
550
577
586
-5.53
-6.23
-6.05
-3.32
-3.98
-4.10
-4.02
2.21
2.25
1.95
1.97
[
[
[
e]
e]
e]
[e]
-
-
-
-
[e]
-
[e]
g
-
-5.99
Ox
Red
[
a] Measurements were performed in DCM. [b] Calculated from λonset, abs. [c] Oxidation E1/2and reduction E1/2 half-wave potentials measured
−1
4 6 A
by cyclic voltammetry (CV) in DCM with Bu NPF
as the electrolyte against Fc/Fc+ as the internal standard (−4.80 eV) at 0.2 Vꢀs . IP and E
estimated from cyclovoltammetric (CV) measurements [IP/EA CV= −4.80 eV – EOx/Red
] [d] Obtained from quantum-chemical calculations with
.
DFT/B3LYP/def2-TZVP. [ Not in the accessible stability window of the solvent. * It is not expected, that the longer alkyl group of 4c compared
e]
to 4b has a significant influence on the geometry or energy levels. Therefore, no separate calculation was performed.
(
m, 2H) ppm. 13C NMR (Tetrachloroethane-d
2
, 125 MHz): δ =
65.1, 142.2, 141.8, 138.0, 133.2, 129.7, 129.5, 128.5, 126.3,
23.3, 122.7, 120.2 ppm. IR: ν = 2960, 2920, 2851, 1659, 1632,
459, 1439, 1425, 1259, 1089, 1060, 1017, 850, 798, 778, 744,
1
1
1
7
5
5
̃
Experimental Section
-1
36, 719, 704, 683, 669, 661 cm . λmax, abs = 550 nm, λonset, abs
=
+
+:
86 nm. HR-MS (DART+): m/z: [M+H] : calcd. for C30
87.0528; found 587.0596, correct isotope distribution.
8 10 2
H F N
Gram scale synthesis of 4e:
6,12-dihydrodiindeno[1,2-b:1',2'-e]pyrazine
(7)
(1.50 g,
5
.85 mmol, 1.00 eq.) was dissolved in dry THF (500 mL) and
Crystallographic data
Deposition Numbers CCDC 2058078 (for 4c), CCDC 2058079
LHMDS (1 M in THF, 35.1 mL, 35.1 mmol, 6.00 eq.) and
hexafluorobenzene (4.36 g, 23.4 mmol, 4.00 eq.) was added
slowly. The reaction was stirred for 16 h at room temperature and
afterwards chloranil (8.63 g, 35.1 mmol, 6.00 eq.) was added.
Recrystallization from chlorobenzene gave 4e as a dark purple
2+) </url> contain the
(
for 4d), and CCDC 2058080 (for 4dH
2
supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge
Crystallographic Data Centre and Fachinformationszentrum
1
solid. Yield 1.10 g, 1.88 mmol, 32%. H NMR (Tetrachloroethane-
2
d , 400 MHz): δ = 7.59 (m, 2H), 7.23 (m, 2H), 7.18 (m, 2H), 6.96
Karlsruhe
Access
Structures
service
<url
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Acknowledgements
Victor Brosius thanks the ‘Studienstiftung des Deutschen
Volkes‘ for a scholarship. We thank the DFG (SFB 1249) for
generous support.
Conflict of Interests
The authors declare no conflict of interests.
Keywords: electron acceptors, organic field effect transistors,
quinoidal heteroaromatics, semiconductor, synthetic methods
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Electron deficient diindenopyrazines
with different aryl substituents were
synthesized and characterized.
Introduction of electron poor aryl
groups reduce the electron affinity to
Victor Brosius,[a] Svenja Weigold,
[a]
[a]
[a]
Nikolai Hippchen, Frank Rominger,
[
a]
Jan Freudenberg,* and Uwe H. F.
Bunz*[
a]
Page No. – Page No.
-
4 eV, making them attractive as n-
semiconductors. 4e was tested in a
Diindenopyrazines: Electron-
Deficient Arenes
OFET showing electron mobilities up
2
to 0.037 cm /Vs.
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