Hexasubstituted Benzenes with Ultrastrong Dipole Moments

Article abstract of DOI:10.1002/anie.201508249

Hexasubstituted benzenes have been synthesized with the highest known dipole moments, as determined by dielectric spectroscopy and DFT methods. Based on the preparation of 4,5-diamino-3,6-dibromophthalonitrile, combined with a novel method to synthesize d

Full text of DOI:10.1002/anie.201508249

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International Edition: DOI: 10.1002/anie.201508249
German Edition: DOI: 10.1002/ange.201508249
High Dipole Moments
Hexasubstituted Benzenes with Ultrastrong Dipole Moments
Jakob Wudarczyk, George Papamokos, Vasilis Margaritis, Dieter Schollmeyer, Felix Hinkel,
Martin Baumgarten, George Floudas, and Klaus Müllen*
Abstract: Hexasubstituted benzenes have been synthesized
with the highest known dipole moments, as determined by
dielectric spectroscopy and DFT methods. Based on the
preparation of 4,5-diamino-3,6-dibromophthalonitrile, com-
bined with a novel method to synthesize dihydrobenzimid-
azoles, these benzene derivatives have dipole moments in
excess of 10 debye. Such dipole moments are desirable in
ferroelectrics, nonlinear optics, and in organic photovoltaics.
Structure determination was achieved through single-crystal X-
ray crystallography, and the optical properties were determined
by UV/Vis absorption and fluorescence spectroscopy.
O
rganic molecules with large dipole moments have gar-
nered attention because of their potential applications in
ferroelectrics[1] and in nonlinear optics (NLO).[2] In addition,
the incorporation of strong dipole moments can lead to
improved charge separation at the interface of the donor and
the acceptor phases in bulk heterojunction solar cells.[3,4] To
generate dipolar benzenes, one can place electron-withdraw-
ing and electron-donating substituents on opposite sides: the
stronger the electron pull on the acceptor side and the
electron-pull on the donor side, the higher the expected total
dipole moment. Aminobenzonitriles with dipole moments in
the range 5.0–5.6 debye have already been the subject of NLO
and polarizability studies.[5] Ultrastrong dipole moments can
occur in polymethines having permanent ionic moieties or are
able to form zwitterionic species.[5–7] In contrast to these
charged polyenes, our approach based on fully functionalizing
benzene with the strongest polarizing substituents results in
an ultrastrong dipolar aromatic system, which is charge-free,
and so the presented molecules define a new chapter in
benzene chemistry.
Scheme 1. Synthesis of hexasubstituted benzene derivatives with ultra-
strong dipole moments. a) [Pd2dba3], Zn(CN)2, dppf, PMHS, DMAc, or
DMSO, 1008C, 12 h, 41–62%. dba=dibenzylideneacetone, dppf=1,1’-
bis(diphenylphosphanyl)ferrocene, PMHS=polymethylhydrosiloxane.
methods based on an electrophilic substitution were unsuc-
cessful, even under harsh conditions. In our approach, we
utilized in situ generated hypobromic acid[8] as a more
electrophilic species to attack the aromatic structure in 2.
We applied the method of Bedekar and co-workers[9] to afford
the target compound 1 in a yield of 59% under mild
conditions. This functionalization can further facilitate the
formation of the tetracyano derivative 5,6-diaminobenzene-
1,2,3,4-tetracarbonitrile (3). Moreover, we found that, in
For the first time, we were able to synthesize 4,5-diamino-
3,6-dibromophthalonitrile (1) by means of oxidative bromi-
nation (Scheme 1) of 4,5-diaminophthalonitrile (2); common
[*] J. Wudarczyk, Dr. F. Hinkel, Prof. Dr. M. Baumgarten,
Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: muellen@mpip-mainz.mpg.de
contrast to literature-known methods with dialkylketones[10]
,
Dr. G. Papamokos, V. Margaritis, Prof. Dr. G. Floudas
Department of Physics
University of Ioannina
1 can undergo a ring-closure reaction with dialkoxyalkanes at
room temperature to form dicarbonitrile 4 (yield: 72% in the
case of methyl, and 81% in the case of n-heptyl substituents).
Conversion of the bromo substituents of intermediate 1 into
cyano groups was not possible by a Rosenmund-von Braun
reaction. However, palladium-assisted cyanation[11] in dime-
thylacetamide (DMAc) furnished compound 3 in 60% yield,
with the pure product obtained by simple precipitation. The
tetracyanated dihydrobenzimidazoles 2,2-dimethyldihydro-
benzoimidazole 5a and its 2,2-diheptyl analogue 5b were
451 10 Ioannina (Greece)
Dr. D. Schollmeyer
Institute für Organische Chemie
Johannes-Gutenberg Universität Mainz
55128 Mainz (Germany)
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201508249.
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prepared by the same procedure in yields of 41% for 5a and
shift and induce fluorescence quenching.[6] The tetracyanated
62% for 5b. When the reaction was carried out in DMAc, the
solid tetracarbonitrile 3 always contained residual solvent, as
seen by solution NMR spectroscopy and the crystal structure
(Figure S3 in the Supporting Information). DMSO was also
chosen as the solvent for the cyanation reaction, since it
produced weaker complexes with compound 3 (see Fig-
ure S5).
The respective optical gaps were determined from the
UV/Vis spectra (see Figure 1 and Table 1). The introduction
of alkyl chains to dihydrobenzimidazole 4b facilitated the
compounds 3 and 5 exhibited strong blue fluorescence with
quantum yields of 48% (see Table S2), thus emphasizing that
there is no strong aggregation.
The geometries of molecules 1–5b (Table 1) were opti-
mized with tight optimization criteria at the DFT-B3LYP[12–15]
levels of theory with the aug-cc-pVTZ[16–18] (1–5a) basis set.
This combination has been shown recently to produce a good
correlation between theoretical and experimental results with
respect to dipole moments and polarizabilities.[19] The root
mean square deviation (RMSD) and the mean absolute error
(MAE) with respect to the experimental results for the
calculated dipole moments are 0.12 and 0.09, respectively,
while the RSMD and the MAE values are 0.30 and 0.18 for
the calculated polarizabilities. Moreover, the adopted level of
theory and basis has a comparable performance as MP2/aug-
cc-pVTZ and CCSD/aug-cc-pVTZ.
The experimental dipole moments were evaluated from
dielectric spectroscopic measurements performed at 258C
using a Novocontrol Alpha frequency analyzer (frequency
range from 10À2 to 107 Hz). Measurements of the complex
dielectric permittivity e* = e’Àie’’ (where e’ is the real and e’’ is
the imaginary part) were made with a BDS1308 liquid
parallel plate sample cell.[20] Dilute solutions in THF, and for
molecule 5b also in chloroform, were prepared and the
dielectric permittivity was measured as a function of solute
concentration (Figure 2). Following the formalism of
Bçttcher[21–23] and assuming ideal solutions of the two
components, the dipole moment of the solute could be
obtained from the derivative of e12 with respect to the
concentration at the limit of infinite dilution (Table 1 and see
the Supporting Information)
Figure 1. Comparison of the UV/Vis spectra of 1–5a in THF solutions
(c=10À5 molLÀ1).
processing of thin films on a glass substrate and allowed
measurement of the absorption in the bulk phase. A bath-
ochromic shift of the absorption by 45 nm in going from
solution (lmax = 387 nm) to the bulk (lmax = 423 nm, see
Figure S35) was found, while 5b showed only a slight bath-
ochromic shift of 7 nm under the same conditions (Fig-
ure S37). Typically, H-type aggregates are known to be
formed from cyanine dyes, which give a strong hypsochromic
The theoretical treatment in vacuo corroborated the steps
taken in the synthetic route in terms of the dipole moments as
well as the HOMO–LUMO energies. A general finding,
stemming both from the DFT calculations performed in va-
cuo and from the solution experiments, is that the dipole
Table 1: Molecular structures, calculated HOMO–LUMO (eV), experimental optical gap (eV), calculated and measured dipole moments (D), and
calculated polarizabilities of the molecules studied herein.[a]
1
2
3
4a
5a
5b
C8N4H4Br2
C8N4H6
C10N6H4
C11N4H8Br2
C11N6H8
C25N6H30
calcd LUMO [eV]
calcd HOMO [eV]
calcd DE(HOMO-LUMO) [eV]
exp. optical gap [eV]
À2.4
À6.8
4.4
À2.0
À6.6
4.6
À3.4
À7.4
4.0
À2.3
À6.3
4.0
À3.0
À6.8
3.8
À2.9
À6.6
3.7
3.4
3.5
3.0
3.0
3.0
2.9
calcd dipole moment [D]
calcd total mol. polarizability
9.6
29.2
10.0
21.9
9.6
27.9
11.5
35.3
12.3
34.2
12.7
57.9
[”10-40 C2 m2 JÀ1
]
exp. dipole moment [D]
12.3Æ0.4
10.5Æ0.2
14.1Æ0.7[b]
12Æ1
10.9Æ0.3
12.2Æ0.3
11.9Æ0.2[c]
[a] Level of theory: DFT-B3LYP, basis set: aug-cc-pVTZ for molecules 1–5a and 6-311+ +G(d,p) for molecule 5b. The arrows represent the direction of
the calculated dipole moment (not proportional to its magnitude). The experimental dipole moments were measured in THF. 5b was also measured in
chloroform solution. [b] The higher dipole moment reflects complex formation with DMAC. [c] Measured in chloroform solution.
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solvents. The experimental results show a slightly higher
dipole moment in the more polar solvent (THF) than in
chloroform. Likewise, DFTresults performed in vacuo and in
solution can vary significantly with respect to the electric
properties, such as the polarizability. Consequently, a similar
calculation was carried out for 2: Solvation effects were
simulated by the SMD[27] implicit solvation model at the DFT-
B3LYP level of theory and aug-cc-pVTZ basis set. The
calculated polarizability of 2 in THF was 31.0 ” 10À40 C2 m2 JÀ1,
namely, 42% higher than the value in vacuo, while the
computed dipole moment was 14.0 debye.
HOMO–LUMO energies (also given in Table 1 and the
Supporting Information) are indicative of the ability of
a molecule to donate or accept an electron, and are thus
relevant in the present context. The B3LYP functional is
widely employed to observe the trends of energy modulation
upon alternating the donor–acceptor groups, although the
experimental and theoretical determination of HOMO–
LUMO energies has stimulated discussions.[28]
Another important consequence of this study is that it
paves the way for strongly electron-deficient compounds to
be employed as building blocks in donor–acceptor polymers.
Benzothiadiazole has been widely used as an acceptor,[29–31]
whose incorporation into polymers would require bromine
atoms in positions 4 and 7. At the same time, one would want
to build in electron-withdrawing groups such as nitrile
functions at positions 5 and 6. The reaction of diaminoben-
zene 1 with thionyl chloride readily afforded 4,7-dibromo-
benzo-[c][1,2,5]thiadiazole-5,6-dicarbonitrile (6). The poten-
tial of the present method can be shown by its subsequent
transformation into the dithienyl derivative 7 (Figure 3). The
synthesis of 7 was reported recently through a different, less-
straightforward route.[32] Within this oligoaryl derivative, the
cyano groups further increase the electron-withdrawing
ability of the acceptor without experiencing significant steric
strain from the neighboring thienyl substituents. Our current
efforts focus on polymerizing the dibromo derivative of 7 with
donors, which will be published in the near future.
Figure 2. Dielectric permittivity as a function of concentration for 1–5b
in THF (red symbols) and for 5b in chloroform (blue symbols)
solutions. The highest concentrations refer to the solubility limit of
each compound. The lines represent linear fits to the e’(c) data. The
respective dipole moments were calculated from the slopes.
moments of all the benzene derivatives are above 9.3 debye
(DFT) and 10.5 debye (experiment; Table 1). The highest
calculated dipole moment was obtained for 5b, whereas the
highest measured dipole moment was for 3 (the compound
having the ability to form a complex with DMAc; see
Figure S3). The dihydrobenzimidazoles 4a, 5a, and 5b are
expected to show an increase in their dipole moments on alkyl
substitution and planarization of the molecule through ring
closure, which is supported by the calculated values. Notably,
these molecular dipole moments exceed those of disubsti-
tuted benzenes (4-nitro-N,N-dimethylaniline, m ꢀ 7 debye),[24]
of stilbene derivatives (m ꢁ 7 debye),[5] and even of tetrasub-
stituted benzenes/pyrazines with a variety of donor/acceptors
(m ꢁ 11 debye).[25] Since the smallest dipole moment is higher
than about 10 debye, hexasubstituted benzene derivatives
with a combination of amino and cyano groups can be
considered to be the smallest neutral molecular species with
the largest dipoles known today.
The highest values of the static isotropic molecular
polarizabilities (Table 1) are observed for molecules that
carry additional methyl groups and this is indicative of the
additivity of this molecular property.[26] The polarizability of
5b, upon substitution of methyl groups by alkyl chains,
increases significantly with respect to molecule 5a. Substitu-
tion of hydrogen atoms by selected groups at the o-positions
to the amino groups increases the molecular polarizability,
with bromine being the most effective substituent.
The experimental values of the dipole moments generally
exceed the ones calculated by DFT, which may result from
some remaining solvent contribution to the polarization. To
analyze the influence of different solvents, 5b was synthesized
with solubilizing alkyl chains to enable the use of less polar
Figure 3. Structure of 7, including numbering of the positions.
In conclusion, the synthesis of novel benzene derivatives
with dipole moments exceeding 10 debye by functionalization
of diaminophthalonitrile to its brominated and tetracyanated
derivatives is reported. Furthermore, we demonstrated a new
synthetic route towards dihydrobenzimidazoles through the
use of acetals instead of ketones under mild conditions.
Dielectric spectroscopy measurements performed in solution
together with DFT calculations performed in vacuo revealed
that the designed molecules have ultrastrong dipole moments.
Potential applications range from organic electronics such as
in ferroelectrics, to nonlinear optics, and to dopants for
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[10] R. C. Elderfield, V. B. Meyer, J. Am. Chem. Soc. 1954, 76, 1887 –
1891.
[11] a) Z. Iqbal, A. Lyubimtsev, M. Hanack, Synnlett 2008, 2287 –
2290; b) M. J. Ahrens, M. J. Fuller, M. R. Wasielewski, Chem.
Mater. 2003, 15, 2684 – 2686.
organic photovoltaics. To this end, blending organic semi-
conductors with molecules 1–5 that have ultrastrong dipole
moments will lead to an enhanced efficiency of charge
separation by decreasing the exciton binding energy.
[12] A. D. Becke, J. Chem. Phys. 1993, 98, 5648 – 5652.
[13] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785 – 789.
[14] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 1989,
157, 200 – 206.
Acknowledgements
[15] S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980, 58, 1200 –
1211.
[16] T. H. Dunning, Jr., J. Chem. Phys. 1989, 90, 1007.
[17] D. E. Woon, T. H. Dunning, Jr., J. Chem. Phys. 1993, 98, 1358 –
1371.
[18] R. A. Kendall, T. H. Dunning, Jr., R. J. Harrison, J. Chem. Phys.
1992, 96, 6796 – 6806.
[19] A. L. Hickey, C. N. Rowley, J. Phys. Chem. A 2014, 118, 3678 –
3687.
[20] G. Floudas in Polymer Science: A Comprehensive Reference,
Vol. 2.32 (Eds.: K. Matyjaszewski, M. Mçller), Elsevier BV,
2012.
[21] C. J. F. Bçttcher, Recl. Trav. Chim. Pays-Bas 1943, 62, 119 – 133.
[22] H. A. Rizk, I. M. Elanwar, Can. J. Chem. 1968, 46, 507 – 513.
[23] C. W. N. Cumper, P. G. Langley, Trans. Faraday Soc. 1971, 67,
35 – 43.
[24] J. J. Wolff, D. Längle, D. Hillenbrand, R. Wortmann, R.
Matschiner, C. Glania, P. Krämer, Adv. Mater. 1997, 9, 138 – 143.
[25] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem.
Phys. 1999, 1, 3519 – 3525.
[26] D. V. Nickel, A. J. Garza, G. E. Scuseria, D. M. Mittleman,
Chem. Phys. Lett. 2014, 592, 292 – 296.
We thank Jutta Schnee for the measurement of fluorescence
spectra, Thimon Schwaebel for the measurement of PLQY,
Patricia Schiel and Barbara Gräfen for assistance in the
synthesis and purification of the compounds. Dr. Manfred
Wagner is acknowledged for the measurement of NMR
spectra. Furthermore, we thank Dr. Yulian Zagraniarski and
Dr. Petar Petrov for fruitful discussions. BASF SE and the
International Research Training Group (IRTG 1404) are
graciously acknowledged for financial support. We thank
Harvard research computing facilities for the granting the
available time on the cluster “Odyssey”.
Keywords: aromatic substitution · dielectric spectroscopy ·
dihydrobenzimidazoles · dipole moments · donor–acceptor
systems
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3220–3223
Angew. Chem. 2016, 128, 3275–3278
[1] K. Asadi, M. Li, P. W. Blom, M. Kemerink, D. M. de Leeuw,
Mater. Today 2011, 14, 592.
[2] P. Wang, P. Zhu, W. Wu, H. Kang, C. Ye, Phys. Chem. Chem.
Phys. 1999, 1, 3519 – 3525.
[3] N. Camaioni, R. Po, J. Phys. Chem. Lett. 2013, 4, 1821 – 1828.
[4] S. Y. Leblebici, T. L. Chen, P. Olalde-Velasco, W. Yang, B. Ma,
ACS Appl. Mater. Interfaces 2013, 5, 10105 – 10110.
[5] L. T. Cheng, W. Tam, S. H. Stevenson, G. R. Meredith, G.
Rikken, S. R. Marder, J. Phys. Chem. 1991, 95, 10631 – 10643.
[6] F. Würthner, T. E. Kaiser, C. R. Saha-Mçller, Angew. Chem. Int.
Ed. 2011, 50, 3376 – 3410; Angew. Chem. 2011, 123, 3436 – 3473.
[7] F. Würthner, R. Wortmann, R. Matschiner, K. Lukaszuk, K.
Meerholz, Y. DeNardin, R. Bittner, C. Bräuchle, R. Sens, Angew.
Chem. Int. Ed. Engl. 1997, 36, 2765 – 2768; Angew. Chem. 1997,
109, 2933 – 2936.
[27] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B
2009, 113, 6378 – 6396.
[28] J. L. Bredas, Mater. Horiz. 2014, 1, 17 – 19.
[29] D. Mühlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller, R.
Gaudiana, C. Brabec, Adv. Mater. 2006, 18, 2884 – 2889.
[30] H. N. Tsao, D. M. Cho, I. Park, M. R. Hansen, A. Mavrinskiy,
D. Y. Yoon, R. Graf, W. Pisula, H. W. Spiess, K. Müllen, J. Am.
Chem. Soc. 2011, 133, 2605 – 2612.
[31] C. Luo, Kyaw, A. Ko Ko, L. A. Perez, S. Patel, M. Wang, B.
Grimm, G. C. Bazan, E. J. Kramer, A. J. Heeger, Nano Lett.
2014, 14, 2764 – 2771.
[32] A. Casey, Y. Han, Z. Fei, A. J. P. White, T. D. Anthopoulos, M.
Heeney, J. Mater. Chem. C 2015, 3, 265 – 275.
ˇ
[8] A. Podgorsek, M. Zupan, J. Iskra, Angew. Chem. Int. Ed. 2009,
48, 8424 – 8450; Angew. Chem. 2009, 121, 8576 – 8603.
[9] N. B. Barhate, A. S. Gajare, R. D. Wakharkar, A. V. Bedekar,
Tetrahedron Lett. 1998, 39, 6349 – 6350.
Received: September 3, 2015
Revised: November 26, 2015
Published online: February 2, 2016
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