Solution-Processable n-Type Organic Semiconductors Based on Angular-Shaped 2-(12 H -Dibenzofluoren-12-ylidene)malononitrilediimide

Article abstract of DOI:10.1021/acs.orglett.5b01343

The angular-shaped n-type semiconductors 2-(12H-dibenzofluoren-12-ylidene)malononitrilediimide 2a and 2b were successfully designed, synthesized, and fully characterized by optical absorption and fluorescence, cyclic voltammetry, X-ray crystal structure a

Full text of DOI:10.1021/acs.orglett.5b01343

Letter
pubs.acs.org/OrgLett
Solution-Processable n‑Type Organic Semiconductors Based on
Angular-Shaped 2‑(12H‑Dibenzofluoren-12-
ylidene)malononitrilediimide
Debin Xia,† Tomasz Marszalek,† Mengmeng Li,† Xin Guo,† Martin Baumgarten,*,† Wojciech Pisula,†,‡
and Klaus Mullen*,†
̈
†Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
‡Department of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland
S
* Supporting Information
ABSTRACT: The angular-shaped n-type semiconductors 2-(12H-dibenzofluoren-12-ylidene)malononitrilediimide 2a and 2b
were successfully designed, synthesized, and fully characterized by optical absorption and fluorescence, cyclic voltammetry, X-ray
crystal structure analysis, XRD, and OFET device performance. The varying alkyl chain lengths of 2a and 2b caused different
molecular orientations with respect to the substrate. Thus, 2a presents an electron mobility of 0.01 cm2 V−1 s−1, whereas 2b
resulted in poor device performance with a much lower electron mobility of 5 × 10−4 cm2 V−1 s−1.
n recent years, organic field-effect transistors (OFETs) have
1
Iattracted considerable attention. Growing efforts have been
devoted to n-type organic semiconductors,2 but they largely lag
behind those of the p-type counterpart. In order to obtain high
performance materials for electron transport, there are three
key factors: (1) suitable lowest unoccupied molecular orbital
(LUMO) energy levels allowing efficient electron injection
from metal electrodes that can be achieved by introduction of
electron-withdrawing groups or atoms (benzothiadiazole,
dicyanovinylenes, imides, fluorines, etc.) into known p-type
molecules;3 (2) coplanarity favoring strong π−π intermolecular
interaction and thus improving the charge carrier transport in
the solid state;4 and (3) an edge-on molecular orientation with
respect to the substrate usually resulting in high-performance
OFET, due to minimized grain boundaries and charge traps.1d
As another factor, “shape” is rarely considered but is especially
important, as it influences packing in the solid state. Almost all
n-type organic semiconductors reported so far adopt linear
arrangements.1b,e,2b Therefore, exploration of angular-shaped n-
type molecules might expand the potential applications in
OFETs.
Figure 1. Structures of DNT, DNC, and 2a−b.
suitable molecular arrangements in the solid states.5 In another
study, dinaphthocarbazole (DNC) (Figure 1) with seven fused
rings presented good stability compared to the linear acene
counterpart, arising from stabilization of the highest occupied
molecular orbital (HOMO). Additionally, DNC exhibited a
hole mobility of 0.055 cm2 V−1 s−1 from solution-processed
OFETs.6 Considering both the high mobility of DNT and the
Very recently, V-shaped molecules with extended π-
conjugation have been revealed to be good candidates for p-
type OFETs. For example, OFETs based on solution-
crystallized dinaphthothiophene (DNT) (Figure 1) showed
extremely high hole mobility as 9.5 cm2 V−1 s−1 because of the
Received: May 8, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01343
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
stability of the DNC of such V-shaped molecules, electron-
withdrawing groups at the end points of the V-shaped coplanar
core have been combined in the current design. This also
enables the details of packing of these molecules between each
other and with respect to the surface in comparison to linear
fused acceptors to be studied.
Based on such a molecular design, 2-(12H-dibenzofluoren-
12-ylidene)malononitrilediimides (2a−b, Figure 1) are ex-
pected to be good candidates as solution-processable n-type
materials according to the following considerations: (1) the
electron-withdrawing dicarboximides and dicyanovinylene units
will dramatically lower the LUMO energy level for effective
electron injection; (2) alkyl chains can be easily attached at the
nitrogen atoms of the end-capped dicarboximides to induce
solubility and formation of ordered thin films; and (3) the rigid
and planar conjugated framework facilitates efficient intermo-
lecular charge-transfer interaction. Herein, we report the
synthesis, characterizations, physical properties, thermal behav-
ior, and semiconducting characteristics.
2,3,6,7-Tetramethyl-9H-fluoren-9-one (4) is the key building
block, as extension of π-conjugation can be achieved by the
functionalization of tetramethyl and carbonyl groups of 4.
Compound 4 was achieved in 75% yield through Pd-catalyzed
dehydrogenative cyclization (Scheme 1).7 Radical bromination
Scheme 1. Synthesis of 1a−b and 2a−b
Figure 2. (A) UV−vis absorption spectra and fluorescence spectra of
1a−b and 2a−b in dichloromethane. (B) Cyclic voltammetric profile
of 1a−b and 2a−b in dichloromethane with 0.1 M Bu4NPF6 as
supporting electrolyte. Potentials are reported versus the Fc+/Fc redox
couple as an external standard, scan rate = 100 mV/s.
and emission spectra of 2 in comparison with 1 can be
explained by the stronger electron-withdrawing malononitrile
group, which decreases the LUMO more than the HOMO
energies.10 The optical gaps estimated from the onset of the
absorption spectra are 2.60 and 2.25 eV for 1 and 2,
respectively (Table S1).
The redox properties of 1a−b and 2a−b were studied using
cyclic voltammetry. Cyclic voltammograms of all compounds in
DCM exhibited two reduction waves, with the E1/2 potential
around −1.39 V for 1 and −0.94 V for 2, but no oxidation
waves in the potential range available. The LUMO energies of 1
and 2 estimated through the equation ELUMO = −[Ered1/2
−
+
E(Fc /Fc) + 4.8] eV were around −3.41 and −3.86 eV,
respectively. The deeper LUMO level of 2 can be attributed
to the stronger electron-withdrawing character of the
dicyanovinylene group compared to that of the carbonyl
moiety. The low-lying LUMO of 2 indicates that they could be
well suited as n-type semiconductors with efficient electron
injection.
Thermogravimetric analysis (TGA) revealed that 1a, 1b, 2a,
and 2b exhibit good thermal stability, with a 5 wt % loss
occurring at 442, 411, 441, and 404 °C, respectively (Figure
S1). Differential scanning calorimetry (DSC) traces for 1a, 1b,
2a, and 2b showed melting points at 359, 235, 359, and 266 °C,
respectively (Figure S2).
was done with bromine in CCl4 upon irradiation with a 250 W
halogen lamp, which afforded 2,3,6,7-tetrakis(dibromomethyl)-
9H-fluoren-9-one 5 as a yellow solid in 81% yield. After
treatment of fluorenone 5 with NaI in DMF, a Diels−Alder
cycloaddition with maleimides containing different alkyl chains
afforded 1a−b in 67−72% yield. Finally, the target products
2a−b were obtained in 74−80% yield by Knoevenagel
condensation of 1a−b with the Lehnert reagent8 (TiCl4,
malononitrile, pyridine). The chemical structures of all new
compounds were fully supported by standard spectroscopic
characterizations and high resolution mass spectrometry
(Supporting Information).
The UV−vis absorption and photoluminescence spectra of
1a−b and 2a−b were recorded in dichloromethane (DCM)
solution (Figure 2A and Table S1). Compounds 1a (2a) and
1b (2b) displayed almost the same substructures in the
absorption spectra and identical emission wavelengths because
the alkyl chains do not influence the electronic properties in
dilute solution.9 The significant red shift of both the absorption
A single crystal of 2b was grown by slow evaporation of
DCM. The crystal structure determined by X-ray diffraction is
presented in Figure 3. The dicyanomethylene moiety is slightly
twisted with respect to the backbone of the core, which can be
caused by the close proximity of the cyano groups and the
hydrogen atoms. The crystal structure of 2b involves face-to-
face stacked dimers, which interact with each other in a one-
B
DOI: 10.1021/acs.orglett.5b01343
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
toward the alignment direction resulting in a reverse extinction
under POM compared to 2a.
The charge carrier transport of 2a and 2b was investigated by
fabricating OFET devices in the bottom-gate bottom-contact
configuration. Before drop-casting from 2 mg/mL chloroform
solution, the silicon wafer with 300 nm thick thermally grown
SiO2 was functionalized by hexamethyldisilazane (HMDS) to
minimize interfacial trapping sites. Au was evaporated as the
source and drain electrodes. The drop-cast films were annealed
at 120 °C for 30 min to remove the residual solvent before
OFET measurement. Figure 4 shows the transfer and output
characteristics of 2a and 2b, respectively.
Figure 3. Crystal packing of 2b. (A) Face-to-face π−π stacking with an
interplanar distance of 3.34 Å. (B) O···H− and N···H− hydrogen
bonding between stacks.
dimensional, slipped-stack arrangement as shown in Figure 3A.
Furthermore, O···H− (2.50 and 2.58 Å) and N···H−
(2.38 Å) hydrogen bonds are an additional driving force
contributing to the assembly of 2b. This crystal arrangement is
probably due to strong dipole−dipole interactions among the
imide and cyano groups.
To understand the influence of the alkyl substitution of the
V-shaped core on the surface self-organization of 2a and 2b, X-
ray diffraction (XRD) of both polycrystalline samples was
performed. Zone-casting, a solution processing method, was
utilized for the film deposition. This technique has been
reported to yield homogeneous and well-ordered polycrystal-
line films for various π-conjugated systems.11 Figure S3
presents XRD diffractograms obtained for zone-cast 2a and
2b. The interlayer distance of 2.60 nm for 2a is determined
from the main reflection (100 according to the Miller’s index)
observed at qz = 0.24 Å−1 (Figure S3A). Higher order
reflections (up to eighth) imply long-range organization of
the molecules in the out-of-plane direction of the film. This
interlayer distance is in agreement with the theoretical molecule
length of 2.65 nm calculated by Cerius2.12 This suggests that
the long axis of the V-shaped molecule is arranged
perpendicular to the silicon dioxide surface (Figure S4C). In
this edge-on organization the π-stacking direction is oriented
parallel to the surface which is beneficial for the charge carrier
transport in field-effect transistors.
For 2b the main reflection (100 according to Miller’s index,
Figure S3B) corresponds to a d-spacing of 2.00 nm which is in
agreement with the c parameter of the triclinic unit cell
obtained for the single crystal (Figure 3). It can be therefore
concluded that the c axis is arranged in the out-of plane
direction. In that case, 2b is organized in face-to-face stacked
dimers which are tilted with respect to the surface.
Indirect evidence for the tilting of 2b is provided by polarized
optical microscopy (POM) performed for the zone-cast layers
(Figure S4). All polycrystalline layers exhibit birefringence
between cross-polarizers. In the case of 2a, the POM image
remains dark when the crystal optical axis (or zone-casting
direction) is arranged parallel to one of the polarizers (0° and
90°). Maximum birefringence is observed for the angle of 45°.
In contrast, 2b shows no light transmission for the angle of 45°
indicating that axis b of the crystal is almost aligned parallel to
the zone-casting direction (Figure S5). In such a crystal
arrangement, the axis of the molecules is oriented around 45°
Figure 4. Transfer and output characteristics of OFET devices
fabricated by drop casting of 2a (A, B) and 2b (C, D). For B and D,
VDS = 80 V is applied.
In the positive drain mode of both compounds, the drain
current (IDS) first exhibits linear behavior with increasing gate
voltage (VGS) and then becomes saturated (Figure 4 A and C).
This transistor behavior is characteristic for an electron
transporting semiconductor which can be attributed to the
introduction of strong electron-withdrawing moieties. The
maximum saturated electron mobility of 2a is 0.01 cm2 V−1 s−1
and the on/off ratio is around 104. The threshold voltage of 12
V is low compared to 2b where the threshold voltage is
increased to 35 V. Such high contact resistance for 2b can only
be explained with the molecular organization in the drop-cast
film, also caused by disorder of the molecules close to the metal
semiconductor interface. In comparison to 2a, 2b shows a
relatively poor electron transport, with a maximum mobility of
5 × 10−4 cm2 V−1 s−1 and an on/off ratio of 103. Zone-casting
only slightly improved the device performance of 2b yielding
charge carrier mobilities of 1 × 10−3 cm2 V−1 s−1. The lower
performance of drop-cast 2b in comparison to 2a is ascribed to
the face-to-face organization of 2b and molecular tilting
induced by longer alkyl chains. Such organization of the
molecules causes the π-stacking interaction to not be in-plane
with the substrate.
In summary, two novel angular n-type molecules 2a and 2b
have been successfully synthesized based on the 2,3,6,7-
tetramethyl-9H-fluoren-9-one intermediate. While their elec-
tronic properties in diluted media are identical as proven by
cyclic voltammetry and optical spectra with a low lying LUMO
of −3.86 eV, the length of the alkyl chain plays a pivotal role in
the molecular orientation relative to the surface. For 2a, the
C
DOI: 10.1021/acs.orglett.5b01343
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(8) (a) Lehnert, W. Tetrahedron Lett. 1970, 11, 4723. (b) Lehnert, W.
Synthesis 1974, 1974, 667.
(9) Yin, J.; Qu, H.; Zhang, K.; Luo, J.; Zhang, X.; Chi, C.; Wu, J. Org.
molecules adopt an edge-on orientation with an OFET electron
mobility of 0.01 cm2 V−1 s−1, while 2b with longer branched
chains is tilted with respect to the substrate, thereby resulting in
poor device performance.
Lett. 2009, 11, 3028.
(10) (a) Kozaki, M.; Yonezawa, Y.; Okada, K. Org. Lett. 2002, 4,
4535. (b) Usta, H.; Facchetti, A.; Marks, T. J. Org. Lett. 2008, 10, 1385.
(c) Usta, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130,
8580. (d) Jacques, E.; Romain, M.; Yassin, A.; Bebiche, S.; Harnois,
M.; Mohammed-Brahim, T.; Rault-Berthelot, J.; Poriel, C. J. Mater.
Chem. C 2014, 2, 3292.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, synthesis, characterization, and theoretical
studies. The Supporting Information is available free of charge
on the ACS Publications website at DOI: 10.1021/
acs.orglett.5b01343.
(11) (a) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb,
U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mullen, K. Adv. Mater. 2005,
̈
17, 684. (b) Marszalek, T.; Nosal, A.; Pfattner, R.; Jung, J.; Kotarba, S.;
Mas-Torrent, M.; Krause, B.; Veciana, J.; Gazicki-Lipman, M.;
Crickert, C.; Schmidt, G.; Rovira, C.; Ulanski, J. Org. Electron. 2012,
13, 121.
AUTHOR INFORMATION
■
Corresponding Authors
(12) Cerius 2 calculation; Accelrys, Inc.: San Diego, CA, 1998.
*E-mail: baumgart@mpip-mainz.mpg.de.
*E-mail: muellen@mpip-mainz.mpg.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Transregio TR49
and ERC-Adv.-Grant 267160 (NANOGRAPH). We acknowl-
edge Dr. Dieter Schollmeyer at Johannes Gutenberg-University
for crystal analysis.
REFERENCES
■
(1) (a) Guo, X.; Baumgarten, M.; Mullen, K. Prog. Polym. Sci. 2013,
̈
38, 1832. (b) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. J. Am.
Chem. Soc. 2013, 135, 6724. (c) Di, C. A.; Zhang, F.; Zhu, D. Adv.
Mater. 2013, 25, 313. (d) Wang, C. L.; Dong, H. L.; Hu, W. P.; Liu, Y.
Q.; Zhu, D. B. Chem. Rev. 2012, 112, 2208. (e) Jiang, W.; Li, Y.; Wang,
Z. Chem. Soc. Rev. 2013, 42, 6113.
(2) (a) Li, Y. N.; Sonar, P.; Murphy, L.; Hong, W. Energy Environ. Sci.
2013, 6, 1684. (b) Liu, Z.; Zhang, G.; Cai, Z.; Chen, X.; Luo, H.; Li,
Y.; Wang, J.; Zhang, D. Adv. Mater. 2014, 26, 6965. (c) Durso, M.;
Gentili, D.; Bettini, C.; Zanelli, A.; Cavallini, M.; De Angelis, F.; Grazia
Lobello, M.; Biondo, V.; Muccini, M.; Capelli, R.; Melucci, M. Chem.
Commun. 2013, 49, 4298. (d) Yanai, N.; Mori, T.; Shinamura, S.;
Osaka, I.; Takimiya, K. Org. Lett. 2014, 16, 240. (e) Mori, T.; Yanai,
N.; Osaka, I.; Takimiya, K. Org. Lett. 2014, 16, 1334.
(3) (a) Wu, Q.; Ren, S.; Wang, M.; Qiao, X.; Li, H.; Gao, X.; Yang,
X.; Zhu, D. Adv. Funct. Mater. 2013, 23, 2277. (b) Li, H.; Kim, F. S.;
Ren, G.; Hollenbeck, E. C.; Subramaniyan, S.; Jenekhe, S. A. Angew.
Chem., Int. Ed. 2013, 52, 5513. (c) Shi, X.; Chang, J.; Chi, C. Chem.
Commun. 2013, 49, 7135. (d) Ie, Y.; Ueta, M.; Nitani, M.; Tohnai, N.;
Miyata, M.; Tada, H.; Aso, Y. Chem. Mater. 2012, 24, 3285. (e) Usta,
H.; Kim, C.; Wang, Z.; Lu, S.; Huang, H.; Facchetti, A.; Marks, T. J. J.
Mater. Chem. 2012, 22, 4459. (f) Guo, X.; Puniredd, S. R.;
Baumgarten, M.; Pisula, W.; Mullen, K. Adv. Mater. 2013, 25, 5467.
̈
(g) Kono, T.; Kumaki, D.; Nishida, J.-i.; Tokito, S.; Yamashita, Y.
Chem. Commun. 2010, 46, 3265.
(4) (a) Tian, H.; Deng, Y.; Pan, F.; Huang, L.; Yan, D.; Geng, Y.;
Wang, F. J. Mater. Chem. 2010, 20, 7998. (b) Gao, P.; Beckmann, D.;
Tsao, H. N.; Feng, X.; Enkelmann, V.; Baumgarten, M.; Pisula, W.;
Mullen, K. Adv. Mater. 2009, 21, 213. (c) Liang, Z.; Tang, Q.; Mao, R.;
̈
Liu, D.; Xu, J.; Miao, Q. Adv. Mater. 2011, 23, 5514.
(5) Okamoto, T.; Mitsui, C.; Yamagishi, M.; Nakahara, K.; Soeda, J.;
Hirose, Y.; Miwa, K.; Sato, H.; Yamano, A.; Matsushita, T.; Uemura,
T.; Takeya, J. Adv. Mater. 2013, 25, 6392.
(6) Pho, T. V.; Yuen, J. D.; Kurzman, J. A.; Smith, B. G.; Miao, M.;
Walker, W. T.; Seshadri, R.; Wudl, F. J. Am. Chem. Soc. 2012, 134,
18185.
(7) Li, H.; Zhu, R.-Y.; Shi, W.-J.; He, K.-H.; Shi, Z.-J. Org. Lett. 2012,
14, 4850.
D
DOI: 10.1021/acs.orglett.5b01343
Org. Lett. XXXX, XXX, XXX−XXX