Bromination Improves the Electron Mobility of Tetraazapentacene

Article abstract of DOI:10.1002/anie.201805728

A cyclocondensation of TIPS-ethynyl-substituted diaminoarenes with in situ obtained 4,5-dibromocyclohexa-3,5-diene-1,2-dione has led to the synthesis of tetrabromotetraazapentacene (BrTAP). BrTAP is easily reduced to its air-stable radical anion and elect

Full text of DOI:10.1002/anie.201805728

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Bromination Improves Tetraazapentacene´s Electron Mobilities
Authors: Hilmar Reiss, Lei Ji, Jie Han, Silke Koser, Olena Tverskoy,
Jan Freudenberg, Michael Moos, Alexandra Friedrich, Ivo
Krummenacher, Christoph Lambert, Holger Braunschweig,
andreas Dreuw, Todd B. Marder, and Uwe Heiko Bunz
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805728
Angew. Chem. 10.1002/ange.201805728
Link to VoR: http://dx.doi.org/10.1002/anie.201805728
http://dx.doi.org/10.1002/ange.201805728

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Bromination Improves Tetraazapentacene´s Electron Mobilities
Hilmar Reiss,[a] Lei Ji,[b] Jie Han,[e] Silke Koser,[a] Olena Tverskoy,[a] Jan Freudenberg,[a,c] Felix Hinkel,[a,d]
Michael Moos,[f] Alexandra Friedrich,[b] Ivo Krummenacher,[b] Christoph Lambert,[f] Holger Braunschweig,[b]
Andreas Dreuw,[e] Todd B. Marder*[b] and Uwe H. F. Bunz*[a,d]
Abstract: A cyclocondensation-based preparation of tetrabromotetra-
azapentacene (BrTAP) is described using TIPS-ethynyl-substituted
diaminoarenes with in-situ obtained 4,5-dibromocyclohexa-3,5-diene-
1,2-dione. BrTAP is easily reduced to its air stable radical anion; elec-
tron mobilities > 0.56 cm2 V-1 s-1 are achieved in thin film transistors.
Azaacenes[1,2,3] are electron-poor counterparts of acenes.[4] Appli-
cations of azaacenes and azaarenes range from emitters in organic
light-emitting diodes (OLEDs),[5] to electron-transporters in solar cells
6
, 7 ]
(SC)[
and n-type semiconductors in field-effect transistors
(FETs).[8,9] Among azaarenes, 6,13-bis(triisopropylsilylethynyl)-5,7,-
12,14-tetraaza-pentacene (TIPSTAP)[10] showed an electron mobility
of over 11 cm2 V-1s-1 in organic field-effect transistors (FETs).[11] TIPS-
TAP is isostructural with 6,13-bis(triisopropylsilylethynyl)pentacene
(TIPSPen), featuring a brick-wall packing motif and nearly identical
cell parameters.[12] Introduction of nitrogen atoms into the π-system
changes the electronic structure within the pentacene frame.
Peripheral substitution of the aromatic system alters the electronic
properties and the morphology further. Herein we describe an
alternative access to 1 (BrTAP, Figure 1).[13] The electronegative
bromine atoms lead to a higher electron affinity and increase the
electron mobility in FETs. Our synthetic concept exploits thiadiazolo-
[3,4-b]phenazine 2.
Scheme 1. Synthesis of brominated thiadiazolo[3,4-b]phenazine 2. Reductive
ring opening and subsequent condensation with 4,5-dibromo-benzene-1,2-diol
leads to TIPSTAP derivative 1.
To test this hypothesis, we prepared a solution of the dianion of 1 in
THF by reduction with [K(18-crown-6)(THF)2] anthracenide. Remar-
kably, this dianion is stable under nitrogen; it does not lose bromide
ions or form the organo potassium compound under these conditions.
Upon bubbling air through the solution of the dianion, according to
UV-vis spectroscopy, it is oxidized not to the neutral species but to the
radical anion, which is stable under ambient conditions in air for weeks
in diethyl ether solution, as the first electrochemical reduction potential
of oxygen is around -0.7 V. This is supported by the computed relative
stabilities of the neutral, anion and dianion of 1 in THF. While only
about 3.36 eV are required to remove an electron from 12- to arrive at
1●-, approximately 4.33 eV are needed to oxidize the anion back to the
neutral BrTAP.
Figure 1. TIPSTAP, BrTAP 1 and thiadiazolo[3,4-b]phenazine 2.
Our previous synthesis of 1 gave the material in only 10 % yield and
lacked reproducibility.[13] Herein, 1 is synthesized from TIPS-DBT
(Scheme 1). Cyclocondensation reaction with in-situ oxidized 4,5-
dibromocatechol furnishes 2. Subsequently, a reductive ring opening
with Zn in acetic acid and a second condensation reaction with 4,5-
dibromocatechol in the presence of periodate yielded 1 in 71 %, a
significant improvement upon the previously reported route.
The electron affinities were calculated at the level of DFT/CAM-
B3LYP to be 4.03 eV for the parent TIPSTAP and 4.33 eV for 1,
making 1 attractive for n-channel semiconductors; the first reduction
potential of 1 is remarkable (-0.7 V vs. ferrocene), suggesting that the
radical anion might be air stable.[13]
250000
1-
1
15000
10000
5000
0
1-
12-
200000
150000
100000
50000
0

-5000
-10000
1000
1200
 / nm
1400
1600

[a] Prof. U. H. F. Bunz, H. Reiss,O. Tverskoy, K. Broedner, Dr. J. Freudenberg, Organisch-
Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld
(INF) 270, 69120 Heidelberg, FRG
[b] Dr. L. Ji, Dr. A Friedrich, Dr. I. Krummenacher; Prof. H. Braunschweig, Prof. T. B.
Marder, Institut fꢀr Anorganische Chemie, Julius-Maximilians-Universitꢁt Wꢀrzburg, Am
Hubland, 97074 Wꢀrzburg, FRG
400
600
800
1000 1200 1400 1600
 / nm
[c] Dr. J. Freudenberg, InnovationLab, Speyerer Str. 4, 69115 Heidelberg, FRG
[d] Prof. U. H. F. Bunz, CAM, Ruprecht-Karls-Universität Heidelberg, INF 225, 69120
Heidelberg, FRG.
Figure 2. Absorption spectra of 1 in its neutral (black), mono- (green) and
dianionic form (red) determined by spectroelectrochemistry in THF/TBA-PF6.
[e] Prof. A. Dreuw, J. Han, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen
und Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität INF 205, 69120
Heidelberg, FRG
Figure 2 shows the absorption spectra of the neutral compound, the
radical anion and the dianion of 1. As in the case of TIPSTAP, the
spectrum of the radical anion is most red-shifted, followed by that of
[f]
M. Moose, Prof. C. Lambert, Institut fꢀr Organische Chemie, Julius-Maximilians-
Universitꢁt Wꢀrzburg, Am Hubland, 97074 Wꢀrzburg, FRG
*
uwe.bunz@oci.uni-heidelberg.de, todd.marder@uni-wuerzburg.de
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10.1002/anie.201805728
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neutral 1.[ 14 ] The dianion has the most blue-shifted absorption
spectrum and its solution in Et2O is somewhat fluorescent (quantum
yield 3.5%), despite the presence of the four bromine atoms, which
would be expected to facilitate intersystem crossing due to the heavy-
atom effect. The small Stokes shift reveals the -* nature of the S0-
S1 transition as well as the fluorescence nature of the emission, which
is supported by the emissive lifetimes ( 1 < 1 ns (26%), 2 = 2.0 ns
Addition of the bromine atoms does indeed lead to a shift of negative
charge towards the outer rings. This effect is more pronounced for the
anionic and dianionic species than for the neutral 1. This observation
will be important for the discussion of the device results. Quantum
chemical calculations confirmed that 12- is a singlet, as the triplet state
lies 1.159 eV above that of the singlet.
(22%),3 = 4.5 ns (52%)). The lowest computed vertical electronic
transitions of 1, its monoanion and dianion at the level of
TDDFT/CAM-B3LYP follow the trend of the experimental data (Table
1). The optical properties and excited electronic states are, in general,
very similar to that of TIPSTAP and its reduced species.
Table 1. Computed relative energies (E) and vertical excited states
of 1, its anion 1●- and dianion 12- at the DFT(TDDFT)/CAM-B3LYP/6-
311++G** level using Q-Chem 5.0.
1
1●-
12-
E[a]
0
-4.33
-7.69
1.91 (0.27)
[650 nm]
3.09 (1.03)
[401 nm]
0.97 (0.10)
[1280 nm]
1.87 (0.43)
[662 nm]
2.39 (1.36)
[519 nm]
[b]
BS1
BS2
[b]
[a] Relative energies in eV. The energy of neutral 1 is set to zero. [b] Vertical excitation
energies of bright states (BS) in eV, oscillator strengths in parentheses, absorption
wavelengths in brackets.
Figure 3. Simulated and experimental EPR spectrum of 1●-.
The EPR of 1●- is shown in Figure 3. The simulated spectrum gives
giso = 2.005, a(14N) = 3.7 MHz, a(1H) = 3.7 MHz, a(79,81Br) = 1.8 MHz.
The hyperfine coupling of nitrogen in 1●- is much smaller than that in
the TIPSTAP radical anion (a(14N) = 5.32 MHz), while the hyperfine
coupling constants of the protons in TIPSTAP●- are smaller than those
in 1●-. This indicates that for 1●- the unpaired electron resides more on
the outer rings than on the inner rings, which is the case for
TIPSTAP●-, which is an indication of the inductive effect of the bromine
substituents. The computed natural charges at DFT/CAM-B3LYP
support this finding (Supporting Information). The EPR spectrum of
1●- seems to be affected by anisotropic effects but changing the
solvents or concentration did not significantly improve its symmetry.
Figure 4. Crystal structure of 1K1. Molecular structure (top) and two views of
the packing of the molecules (middle, bottom).
We grew single crystals of 1●- suitable for X-ray-structure analysis
(Figures
4 and 5). As in former reported radical anions of
azaacenes,[14] the potassium cation does not show any interaction
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10.1002/anie.201805728
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with the negatively charged azaacene. As the charge is distributed
over the large -system, the radical anion lacks hard Lewis-basic
properties, needed to coordinate to the potassium cation in the
presence of THF. The packing of 1●- K+ is shown in the middle and
bottom parts of Figure 4. The (18-crown-6)(THF)2-coordinated
potassium ions are located between the large -systems, and the
cations and anions do not interact strongly. Figure 5 shows the minute
changes in bond lengths when going from 1 to its radical anion, which
are mostly less than 0.02 Ǻ. In the eastern and western rings of 1●-
the bond length alternation, distinct in the structure of 1, is diminished
upon reduction.
electrodes were thermally evaporated on to glass substrates and
chemical vapor deposited parylene C was used as the dielectric.
Compound 1 was dip coated from DCM (5 mg/mL – 5 mm/min) by
dewetting via spinning. The devices were finished with thermally
evaporated gold electrodes and measured under nitrogen. Both
BrTAP and TIPSTAP showed n-channel FET behavior (Figure S5).
Figure 6 displays the transfer (top) and output characteristics (bottom)
of 1. The on/off ratio is 105 and the average electron mobility is
0.26 cm2 V-1 s-1 with top mobilities of 0.56 cm2 V-1 s-1 for 1.
In comparison to similar devices fabricated from TIPSTAP (dip
coated; average 3.9 x 10-3 cm2 V-1 s-1 and top 1.3 x 10-2 cm2 V-1 s-1),
compound 1 thus showed 43 times higher electron mobilities
(calculated from top mobilities; 66 times higher calculated from
average mobilities), see Table 2.
In summary, an improved, high-yielding synthesis of 1 was developed
and its electronic and device properties were carefully investigated.
Several important trends emerge. 1) Addition of four bromine
substituents increases the electron affinity as it drastically stabilizes
the anionic and dianionic species. As a consequence, the radical
anion of 1 is now air-stable for at least 14 days. 2) The bromine
substituents influence the distribution of the negative charges in the
radical anion such that it has a higher probability of residing on the
outer rings. This seems to lead to an increased transfer integral in the
solid state of 1 in the presence of negative charges. 3) Electron
mobilities of 1 of up to 0.56 cm2 V-1 s-1 were measured; this is over 50
times higher than that of TIPSTAP under identical processing
conditions. Over all, bromination is a superb approach by which to
improve the semiconducting properties of TIPSTAP.
Figure 5. Bond length changes between 1 (top) and its radical anion (bottom).
Table 2. Summarized results from the optoelectronic charac-
terization of the compounds.
λmax,abs
[nm] a
680
Eaffinity,CV
[eV] b
3.99
µe.,top
µe.,average
[cm2 V-1 s-1]
3.9 x 10-3
Ion
/
[cm2 V-1 s-1] c
1.3 x 10-2
5.6 x 10-1
Ioff
TIPSTAP
1
104
105
718
4.19
2.64 x 10-1
a
b
Red-shifted maxima from the UV-vis spectra in hexane.
Extracted from the cyclic
voltammetry measurements. [Eaffinity CV = -4.80 eV – E(0/-1)] measured in CH2Cl2 with Bu4NPF6
d
as the electrolyte against Fc/Fc+ as an internal standard at 0.2 mV/s. Bottom-gate/top-
contact FETs. Structured silver electrodes were thermally evaporated on to glass substrates
and chemical vapor deposited parylene C was used as the dielectric. Semiconductors were
dip coated from DCM (5 mg/mL – 5 mm/min). The devices were finished with thermally
evaporated gold electrodes and measured in a nitrogen atmosphere.
EXPERIMENTAL SECTION
–
[K·18-crown-6·(THF)2]+1● (1K1): In an argon-filled glove box, 1 (9.5
mg, 10 mol) and [K·18-crown-6·(THF)2] anthracenide (6.9 mg, 11
mol) were dissolved in 2 mL of dry THF in a 20 mL vial and stirred
for 10 min to generate a yellow solution. Orange crystals formed by
slow diffusion of dry pentane into the reaction mixture at room
temperature.
[K·18-crown-6·THF]212–(1K2): In an argon-filled glovebox, 1 (6.2 mg,
6.5 mol) and [K·18-crown-6·(THF)2] anthracenide (9.9 mg, 15 mol)
were dissolved in 2 mL of dry THF in a 20 mL vial and stirred for 10
min to generate a red solution. Dark red crystalline solids formed by
slow diffusion of dry pentane into the reaction mixture.
Figure 6. Transfer (top) and output characteristic (bottom) for 1; device
dimensions: W = 8 mm, L = 38.62 μm.
Keywords: n-channel semiconductor; azaacene; bromination; thin film
transistor; radical anion
To assess charge carrier mobilities of BrTAP 1, we fabricated bottom-
gate/top-contact field effect transistors (FET). Structured silver
ASSOCIATED CONTENT
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10.1002/anie.201805728
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[5] a) P. Biegger, S. Stolz, S. N. Intorp, Y. Zhang, J. U. Engelhart, F. Rominger,
K. I. Hardcastle, U. Lemmer, X. Qian, M. Hamburger, U. H. F. Bunz, J. Org.
Chem. 2015, 80, 582-589. b) B. D. Lindner, Y. Zhang, S. Höfle, N. Berger, C.
Teusch, M. Jesper, K. I. Hardcastle, X. Qian, U. Lemmer, A. Colsmann, U. H.
F. Bunz, M. Hamburger, J. Mater. Chem. C 2013, 1, 5718-5724. c) M.
Ganschow, S. Koser, S. Hahn, F. Rominger, J. Freudenberg, U. H. F. Bunz,
Chem. Eur. J. 2017, 23, 4415-4421.
Supporting Information: Synthetic protocols, analytical data, NMR
spectra, optical spectra, electrochemical data as well as
computational studies and crystal structures can be found in the
supporting information.
Notes
[6]
E. H. Menke, D. Leibold, V. Lami, Y. J. Hofstetter, M. Mastalerz, Y. Vaynzof,
The authors declare no competing financial interest.
Org. Electron. 2017, 47, 211-219.
[7] a) D. Leibold, V. Lami, Y. J. Hofstetter, D. Becker-Koch, A. Weu, P. Biegger,
F. Paulus, U. H. F. Bunz, P. E. Hopkinson, A. A. Bakulin, Y. Vaynzof, Org.
Electron. 2018, 57, 285-291. b) V. Lami, D. Leibold, P. Fassl, Y. Hofstetter, D.
Becker-Koch, P. Biegger, F. Paulus, P. Hopkinson, M. Adams, U. H. F. Bunz,
S. Huettner, I. Howard, A. Bakulin, Y. Vaynzof, RRL Solar 2017, 1, 1700053.
ACKNOWLEDGMENT
UHFB and AD thank the Deutsche Forschungsgemeinschaft (SFB
1249) for generous support. T.B.M. thanks the Julius-Maximilians-
Universitꢁt Würzburg for support. J.H. thanks the Chinese
Scholarship Council for the Ph.D. scholarship (201406170060). T.B.M.
and C.L. thank the Bavarian State Ministry of Science, Research, and
the Arts (Collaborative Research Network “Solar Technologies Go
Hybrid”) and Deutsche Forschungsgemeinschaft (Grant Number:
GRK 2112).
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F. Paulus, J. Engelhart, P. Hopkinson, C. Schimpf, A. Leineweber, H.
Sirringhaus, Y. Vaynzof, U. H. F. Bunz, J. Mater. Chem. C 2016, 4, 1194-1200.
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TOC
H. Reiss, L. Ji, J. Han, S. Koser, O.
Tverskoy, J.Freudenberg, M. Moos, A.
Friedrich, I. Krummenacher, C. Lambert,
H. Braunschweig, A. Dreuw, T. B.
Marder* and U. H. F. Bunz*
Page No. – Page No.
Bromination Improves
Tetraazapentacene´s Electron
Mobilities
A fully air stable radical anion results for a bis-TIPS-ethynyl-substituted tetraazapen-
tacene if four bromine substituents are attached in the 2,3 and 9,10-positions. This material
is – due to its deep LUMO (i.e. high electron affinity) a promising n-channel semiconductor,
as the bromine sugstituents not only stabilize charge but also lead to a charge distribution
that favors the outer rings and therefore allows better charge transport in the solid,
crystalline state.
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[2] a) G.
H
Z
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