Cruciform Electron Acceptors Based on Tetraindeno-Fused Spirofluorene

Article abstract of DOI:10.1021/acs.cgd.7b00272

Two cruciform tetraindenospirofluorene-based acceptors embedding carbonyl (Spiro-4O) and dicyanovinylene (Spiro-8CN) functionalities are synthesized in high yields. Single-crystal X-ray analysis reveals a one-dimensional π-π stacking arrangement for Spiro

Full text of DOI:10.1021/acs.cgd.7b00272

Article
pubs.acs.org/crystal
Cruciform Electron Acceptors Based on Tetraindeno-Fused
Spirofluorene
Debin Xia,†,‡,⊥ Xin Guo,§,⊥ Manfred Wagner,‡ Martin Baumgarten,*,‡ Dieter Schollmeyer,∥
and Klaus Mullen*,∥
̈
†MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical
Engineering, Harbin Institute of Technology, 150001 Harbin, P. R. China
‡Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
§Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 116023, Dalian, P. R. China
∥Johannes Gutenberg-University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
S
* Supporting Information
ABSTRACT: Two cruciform tetraindenospirofluorene-based
acceptors embedding carbonyl (Spiro-4O) and dicyano-
vinylene (Spiro-8CN) functionalities are synthesized in high
yields. Single-crystal X-ray analysis reveals a one-dimensional
π−π stacking arrangement for Spiro-4O, while Spiro-8CN
adopts a unique two-dimensional isotropic π-interaction.
Cyclic voltammetry suggests a high electron affinity of −3.76
eV for Spiro-8CN. Such a packing motif and low LUMO
energy for Spiro-8CN are important for bulk electron
transport.
thiophene, 3D-1, which showed a fascinating packing motif and
long-range inter-chain interactions.14 Wu et al. designed a
cruciform 6,6′-dipentacenyl, 3D-2, whose long pentacene units
overlapped with the pentacene units of neighboring molecules,
enabling improved macroscopic charge-carrier mobility in
organic field effect transistors (OFETs).15
1. INTRODUCTION
The solid-state morphology of conjugated small molecules has a
significant influence on the performance characteristics of
electronic devices, e.g., organic field-effect transistors
(OFETs)1−3 and organic photovoltaic cells (OPVs).4,5 Both
exciton migration and charge-carrier transport are strongly
modulated by the molecular packing.6 Among the reported
packing motifs, the commonly adopted arrangements with
strong intermolecular overlap are edge-to-face and face-to-face
modes, which hold promise to maximize electronic coupling
between adjacent molecules.7,8 Recently, several design strategies
have been developed for yielding materials with a face-to-face
packing motif. Holmes et al. reported that, by diminishing the
CH−π interactions, a face-to-face arrangement became acces-
sible in crystals.9 Chi et al. found that the attachment of fused
five-membered rings to two tetracenes could change the packing
structure from a herringbone arrangement to a face-to-face π-
stacking mode.10
For organic films, the charge transport between such π-
conjugated molecules is usually anisotropic, which is partially due
to the effect of disordered arrangements between source and
drain electrodes.11 Materials featuring π-stacking along two
directions can probably lead to isotropic charge transfer and thus
also diminish the effect of random molecular orientations in thin
films. Molecules which pack in the solid state along two π−π
stacking directions have thus attracted special interest. Cruciform
structures can provide a platform for yielding materials with
desired properties.12,13 As shown in Figure 1, Samuel et al.
developed a germanium spiro-center-based cruciform oligo-
These 2D isotropic materials are p-type semiconductors,
whereas materials featuring 2D isotropic electron transport have
not been documented, as far as we know. This shortcoming
prompted us to seek n-type 2D face-to-face π-stacking
semiconductors. Herein, we introduce tetraindeno-fused spiro-
fluorene derivatives Spiro-4O and Spiro-8CN (see Figure 1).
The design principles are as follow: (i) the π-conjugated
bisindenofluorene structure is planar, which facilitates good
intermolecular π−π stacking and favors intermolecular electronic
coupling;16−18 (ii) the spiro-carbon center gives rise to a
cruciform geometry, providing possibilities for 2D inter-
molecular interactions;19 (iii) the electron-withdrawing carbonyl
and dicyanovinylene groups reduce the energy of the lowest
unoccupied molecular orbital (LUMO), which is beneficial for
electron transport;1,20−22 and (iv) tert-butyl groups play a role in
increasing the solubility of the rigid core and favoring single-
crystal formation in comparison with long alkyl chains.7,23
Received: February 23, 2017
Revised: April 5, 2017
Published: April 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.7b00272
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Figure 1. Chemical structures of three-dimensional p-type (3D-1 and 3D-2) and n-type (Spiro-8CN) molecules with 2D face-to-face packing.
washed with brine, dried with Na2SO4, and filtered. The filtrate was
concentrated under reduced pressure. After purification by column
2. EXPERIMENTAL SECTION
General Methods. 1H NMR and 13C NMR spectra were recorded
chromatography with eluent hexane/ethyl acetate (10:1), the desired
in CD2Cl2 or C2D2Cl4 on Bruker DPX 250 or 700 MHz instruments.
High-resolution MALDI mass spectrometry measurements were
performed on a Solarix ESI-/MALDI-ICR (9.4 T) system (Bruker
Daltonics, Germany). Density functional theory (DFT) calculations
compound 1 was obtained as a white solid in a yield of 74%. 1H NMR
(250 MHz, C2D2Cl4) δ/ppm: 7.88 (d, J = 7.9 Hz, 4H), 7.56 (d, J = 8.0
Hz, 4H), 7.38 (dd, J = 7.8, 1.7 Hz, 4H), 7.30 (dd, J = 8.3, 1.8 Hz, 4H),
7.25 (d, J = 1.9 Hz, 4H), 6.65 (s, 4H), 3.06 (s, 12H), 1.25 (s, 36H). 13C
were carried out with the Gaussian 09 program at the B3LYP/6-311G
NMR (63 MHz, CD2Cl2) δ/ppm: 169.36, 155.13, 149.18, 141.90,
(d, p) level. X-ray crystallographic data were collected on a STOE IPDS
141.85, 141.03, 130.06, 128.70, 128.60, 127.98, 124.82, 124.60, 120.54,
2T diffractometer with Cu Kα IμS mirror system. The structures were
66.28, 51.97, 35.26, 31.31. HRMS (TOF MS ES+): m/z calcd for
resolved by direct methods (SIR-2004) and refined by SHELXL-2014
(full matrix): 1005 refined parameters for Spiro-4O and 814 refined
parameters for Spiro-8CN.
Synthetic Details. All reagents and solvents used for synthesis were
C73H73O8 1077.5305, found 1077.5299.
2,2′,2″,2‴-(9,9′-Spirobi[fluorene]-2,2′,7,7′-tetrayl)tetrakis(4-
(tert-butyl)benzoic Acid) (2). A solution of sodium hydroxide (12 g in
45 mL water) was added dropwise into a suspension of compound 1 (2.3
obtained from commercial suppliers and used without further
g, 2.14 mmol) in 300 mL of ethanol. The resulting solution was stirred
purification. Column chromatography was performed on silica gel 60
for 12 h at 90 °C. After cooling to room temperature, the solution was
(Macherey-Nagel, Si60). 2,2′,7,7′-Tetrakis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9′-spirobi[fluorene] was synthesized according to
a reported procedure.11 All reported yields are isolated yields.
concentrated under reduced pressure, and the residue was acidified with
concentrated hydrochloric acid. The white solid product was obtained
by filtration, followed by washing with water (1000 mL) and drying in
the vacuum oven. Finally, pure compound 2 was obtained in a yield of
99%. 1H NMR (250 MHz, CD2Cl2) δ/ppm: 7.84 (d, J = 7.7 Hz, 4H),
7.39 (dd, J = 12.9, 8.0 Hz, 8H), 7.29 (d, J = 8.5 Hz, 4H), 7.27 (s, 4H),
6.59 (s, 4H), 1.22 (s, 36H). 13C NMR (63 MHz, CD2Cl2) δ/ppm:
176.32, 154.99, 148.70, 141.90, 141.84, 141.13, 129.56, 128.37, 128.23,
127.45, 124.58, 124.53, 120.63, 66.35, 35.26, 31.21. HRMS (TOF MS
ES+): m/z calcd for C69H65O8 1021.4679, found 1021.4704.
3,3′,9,9′-Tetra-tert-butyl-6,6′-spirobi[cyclopenta[2,1-b:3,4-
b′]difluorene]-12,12′,15,15′-tetraone (Spiro-4O). Dry DMF (0.07
mL) was added dropwise to a solution of 2 (0.60 g, 0.59 mmol) and 2
mol/L oxalyl chloride (6 mL, 12 mmol) in dry DCM (120 mL). The
mixture was stirred for 10 h at room temperature. The solvent was
evaporated under reduced pressure. The residue was dissolved in 30 mL
of dry DCM and added to a suspension of anhydrous AlCl3 (848 mg, 4.8
mmol) in dry DCM (100 mL) at 0 °C. After 12 h later, the reaction
mixture was poured into ice water slowly, extracted with DCM (80 × 3
mL), dried with Na2SO4, and filtered. The solvent was removed, and the
crude product was purified by column chromatography, using eluent
ethyl acetate (20:1) to give Spiro-4O as a yellow solid in a yield of 82%
(0.46 g). 1H NMR (700 MHz, C2D2Cl4) δ/ppm: 8.19 (s, 4H), 7.55 (d, J
= 7.8 Hz, 4H), 7.27 (s, 4H), 7.24 (d, J = 7.8 Hz, 4H), 6.93 (s, 4H), 1.19
(s, 36H). 13C NMR (63 MHz, C2D2Cl4) δ/ppm: 192.54, 159.65, 153.64,
145.44, 143.83, 141.74, 135.94, 132.08, 126.64, 124.44, 117.94, 116.37,
116.12, 66.42, 35.55, 31.02. HRMS (TOF MS ES+): m/z calc. for
C69H57O4 949.4257, found 949.4249.
Methyl 2-Bromo-4-(tert-butyl)benzoate. NaOH (15.12 g, 0.38
mol) was dissolved in 140 mL of water, followed by the dropwise
addition of bromine (5.00 mL, 0.10 mol) in an ice bath. Next, a solution
of 1-(2-bromo-4-(tert-butyl)phenyl)ethan-1-one (6.20 g, 0.024 mol) in
120 mL of dioxane was slowly added. After 12 h, the solution was
acidified with concentrated hydrochloric acid and extracted with ethyl
acetate (150 × 3 mL). The combined organic layers were washed with
water (200 mL) and brine (200 mL) separately, and dried with Na2SO4.
The solvent was removed under reduced pressure to give crude product
2-bromo-4-(tert-butyl)benzoic acid, which was purified by column
chromatography with the eluent hexane/ethyl acetate (4:1). This
obtained pure precursor was added into a flask with 500 mL of methanol,
followed by the dropwise of concentrated sulfuric acid (2.0 mL). The
mixture was heated to reflux overnight. After cooling to room
temperature, the solution was concentrated, poured into water (300
mL), and extracted with DCM (200 × 3 mL). The combined organic
phases were washed with saturated NaHCO3 (50 × 2 mL) and dried
with Na2SO4. After filtration, the filtrate was concentrated to give the
1
pure methyl 2-bromo-4-(tert-butyl)benzoate in a 90% yield. H NMR
(250 MHz, CD2Cl2) δ/ppm: 7.78 (d, J = 8.2 Hz, 1H), 7.72 (d, J = 1.9 Hz,
1H), 7.44 (dd, J = 8.2, 1.9 Hz, 1H), 3.93 (s, 3H), 1.36 (s, 9H). 13C NMR
(62.5 MHz, CD2Cl2) δ/ppm: 166.66, 157.18, 131.92, 131.50, 129.44,
124.83, 121.87, 52.56, 35.28, 31.05. HRMS (TOF MS ES+): m/z calcd
for C12H15O2NaBr 293.0153, found 293.0164.
Tetramethyl 2,2′,2″,2‴-(9,9′-Spirobi[fluorene]-2,2′,7,7′-
tetrayl)tetrakis(4-(tert-butyl)benzoate) (1). A mixture of 2,2′,7,7′-
tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9′-spirobi-
[fluorene] (2.31 g, 2.89 mmol), methyl 2-bromo-4-(tert-butyl)benzoate
(3.6 g, 13.28 mmol), tetrakis(triphenylphosphine)palladium(0) (667
mg, 0.58 mmol), 2 M aqueous potassium carbonate solution (50 mL),
and toluene (200 mL) was stirred for 2 days at 110 °C under argon
atmosphere. After cooling to room temperature, the mixture was
extracted with DCM (80 × 3 mL). The obtained organic layers were
2,2′,2″,2‴-(3,3′,9,9′-Tetra-tert-butyl-6,6′-spirobi[cyclopenta-
[2,1-b:3,4-b′]difluorene]-12,12′,15,15′-tetraylidene)tetra-
malononitrile (Spiro-8CN). Spiro-4O (0.20 g, 0.21 mmol) and
malononitrile (1.39 g, 21 mmol) were dissolved in dry CHCl3 (80 mL)
under argon atmosphere. TiCl4 (2.3 mL, 21.0 mmol) and pyridine (2.49
mL) were added dropwise. The mixture was stirred overnight at 80 °C.
After the removal of solvent, the residue was subjected to column
chromatography, using pure DCM as eluent to afford product Spiro-
B
DOI: 10.1021/acs.cgd.7b00272
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Scheme 1. Syntheses of Spiro-4O and Spiro-8CN
Figure 2. Aromatic region of the 1H NMR spectra of Spiro-4O (top) and Spiro-8CN (bottom).
1
8CN as a grain solid in a yield of 90% (0.22 g). H NMR (250 MHz,
C2D2Cl4) δ/ppm: 8.98 (s, 4H), 8.21 (d, J = 8.4 Hz, 4H), 7.30 (s, 6H),
7.27 (s, 2H), 6.97 (s, 4H), 1.20 (s, 36H). 13C NMR (126 MHz,
C2D2Cl4) δ/ppm: 160.21, 159.95, 153.12, 144.02, 141.63, 141.57,
136.34, 132.35, 126.99, 126.97, 118.81, 118.09, 116.19, 113.54, 113.17,
76.30, 35.60, 30.84. HRMS (TOF MS ES+): m/z calcd for C81H56N8Na
1163.4526, found 1163.4541.
electrolyte, Ag as the reference electrode, a glassy carbon as the working
electrode, Pt wire as the counter electrode. The LUMO energy levels
were estimated from the onsets of the first reduction peak through the
equation ELUMO = −[Eredonset − E(Fc+/Fc) + 4.8] eV, using ferrocene as an
external standard.
3. RESULTS AND DISCUSSION
A facile synthetic route to the two rigid cruciform molecules is
illustrated in Scheme 1. Compound 1 was obtained in a yield of
74% via Suzuki−Miyaura reaction between 2-bromo-4-(tert-
Cyclic Voltammetry Measurements. Electrochemistry was
performed on a computer-controlled GSTAT12 in a three-electrode
cell in anhydrous CH2Cl2, containing 0.1 M Bu4NPF6 as the supporting
C
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Figure 3. X-ray diffraction structure of Spiro-4O. (Left) Molecular stacking pattern with a π−π stacking distance of 3.35 Å. (Right) Intermolecular
distances: (a) 3.32 Å (C···C); (b) 3.32 Å (C···C); (c: 3.16 Å (C···O). Hydrogen atoms are omitted for clarity.
Figure 4. Molecular stacking patterns of Spiro-8CN. Hydrogen atoms and solvent molecules (CH2Cl2) are omitted for clarity.
butyl)benzoate and 2,2′,7,7′-tetrakis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9′-spirobi[fluorene]. Hydrolysis of tetra-
ester 1, followed by acidification, gave tetraacid 2 as a colorless
solid in 99% yield. Tetra-fold intramolecular Friedel−Crafts
acylation was next achieved by treatment of compound 2 with
oxalyl chloride and DMF. The tetraketone Spiro-4O was isolated
as an orange solid in 82% yield. Finally, the target product Spiro-
8CN could be obtained in 90% yield by the Knoevenagel
condensation of Spiro-4O with the Lehnert reagent24,25 (TiCl4,
malononitrile, pyridine). The overall yield of Spiro-8CN is 54%.
Excellent thermal stabilities of Spiro-4O and Spiro-8CN were
demonstrated by thermogravimetric analysis (TGA) giving high
decomposition temperatures (5% weight loss) of 535 and 518
°C, respectively (Supporting Information, Figures S1 and S2).
The structures of Spiro-4O and Spiro-8CN are unambigu-
ously confirmed by NMR spectroscopy, high-resolution MS, and
single-crystal X-ray analysis. As shown in Figure 2, the
assignment of all aromatic protons is obtained by a combined
Spiro-8CN resonate at lower magnetic field than HC, HD, and
HE. Switching from carbonyl to dicyanovinylene units leads to an
impressive deshielding effect for HA and HB of Spiro-8CN with
chemical shifts of 8.98 and 8.22 ppm, respectively.
To investigate the solid-state packing of Spiro-4O and Spiro-
8CN, single crystals of both molecules were grown by slow
evaporation of the corresponding dichloromethane solutions. As
expected, two π-conjugated bisfluorenone units of Spiro-4O or
two dicyanovinylene-substituted bisindenofluorene units of
Spiro-8CN are connected via an sp3 carbon, which results in
the formation of cruciform structures. To the best of our
knowledge, Spiro-4O and Spiro-8CN are the largest cruciform
oligophenylenes to date that have been characterized by X-ray
diffraction. Figure 3 shows that a bisfluorenone unit of Spiro-4O
can form slipped face-to-face π-stacking with the same unit of
neighboring molecule, leading to a π−π distance as short as 3.35
Å. The second bisfluorenone unit interacts with two neighboring
molecules only via van der Waals forces. The right side of Figure
3 presents three types of intermolecular interactions in different
directions, namely, C···C (a), C···C (b), and C···O (c), with
distances of 3.32, 3.32, and 3.16 Å, respectively. For Spiro-8CN,
a 2D π−π stacking with two types of packing motifs arises due to
1
1
analysis of H NMR, HMBC, TOCSY, and NOESY. The H
NMR spectra reveal the symmetry of both molecules and the
relative strength of electron-withdrawing moieties. Due to the
stronger deshielding effect, protons HA and HB of Spiro-4O and
D
DOI: 10.1021/acs.cgd.7b00272
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the effective parallel alignment of the aromatic rings. As shown in
Figure 4, the interplanar distances for these two types of stacking
are 3.59 and 3.46 Å, respectively. Spiro-8CN is expected to show
2D isotropic charge transport, due to its unique packing structure
and the strong intermolecular interactions.
Spiro-4O and Spiro-8CN have good solubility in common
organic solvents such as dichloromethane and THF. The UV−vis
absorption spectra recorded in dichloromethane are shown in
Figure 5. These two molecules provide similar absorption bands
wave potentials at −1.36 V for Spiro-4O and −0.81 V for Spiro-
8CN, while no oxidation waves can be detected in the measured
potential range.4,17 The LUMO energy levels of Spiro-4O and
Spiro-8CN are −3.18 and −3.76 eV, respectively, based on the
first reduction potential onsets. These LUMO values are close to
that of the π-conjugated bisfluorenone unit and the dicyano-
vinylene-substituted bisindenofluorene, respectively,18 indicat-
ing that the spiro-carbon plays a role in interrupting the electron
conjugation. The stronger electron-withdrawing character of the
dicyanovinylene groups contributes to the deeper LUMO energy
level of Spiro-8CN. The HOMO energy of Spiro-4O and Spiro-
8CN are −5.51 and −5.62 eV, respectively, calculated from their
optical gaps according to the equation HOMO = LUMO − Eg.
Density functional theory (DFT) calculations are carried out at
the B3LYP/6-311G (d, p) level. As shown in Supporting
Information, Figure S3, upon going from carbonyl to dicyano-
vinylene groups the LUMO of Spiro-8CN is delocalized over the
whole molecule, together with deeper energy level, which is in
accordance with the CV results.
4. CONCLUSIONS
In summary, we have synthesized cruciform electron-deficient
molecules Spiro-4O and Spiro-8CN in high yields. The desired
2D isotropic face-to-face π-stacking arrangement is observed for
the single crystal of Spiro-8CN. The latter also possesses a strong
electron accepting capability, with a LUMO energy as low as
−3.76 eV. One expects that Spiro-8CN gives 2D electron
transport along the π-stacking axes. The corresponding device
property of OFETs is currently under investigation in our
laboratory.
Figure 5. UV−vis absorption spectra of Spiro-4O (black line) and
Spiro-8CN (red line) in CH2Cl2. The inset shows the UV−vis spectra in
the region 440−680 nm (n−π* transitions).
to their corresponding analogues BE1 and BE2 (see Supporting
Information, Scheme S1),18 which indicates that there is almost
no electronic coupling between the two perpendicular bis-
indenofluorene moieties in Spiro-4O and Spiro-8CN. The
π−π* transition of the bisindenofluorene units leads to the
absorption bands in the range of 280−400 nm, reflecting a
significant bathochromic shift compared to that of fluorenone.6
This is attributed to the enhanced π-conjugation of the molecular
backbone and the introduction of the electron-withdrawing
groups. From Spiro-4O to Spiro-8CN, the absorption band
redshifts significantly. This is mainly attributed to the decrease of
the LUMO levels arising from the stronger electron-withdrawing
nature of dicyanovinylene versus carbonyl substituents. The
optical gap (Eg) estimated from the absorption edge of the
solution spectra is 2.46 eV for Spiro-4O and 2.02 eV for Spiro-
8CN, respectively.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.7b00272.
Chemical structures of BE1 and BE2, TGA results, DFT
1
calculations, crystal data, and H NMR, 13C NMR, and
HRMS spectra (PDF)
Accession Codes
CCDC 1408336 and 1408347 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The electrochemical properties of Spiro-4O and Spiro-8CN
were investigated in CH2Cl2 by using cyclic voltammetry. As
shown in Figure 6, reduction waves are observed, with first half-
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: martin.baumgarten@mpip-mainz.mpg.de.
*E-mail: muellen@mpip-mainz.mpg.de.
ORCID
Debin Xia: 0000-0002-2658-2577
Martin Baumgarten: 0000-0002-9564-4559
Klaus Mullen: 0000-0001-6630-8786
̈
Author Contributions
⊥D.X. and X.G. contributed equally to this work.
Notes
Figure 6. Cyclic voltammograms of Spiro-4O and Spiro-8CN in
CH2Cl2.
The authors declare no competing financial interest.
E
DOI: 10.1021/acs.cgd.7b00272
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Article
ACKNOWLEDGMENTS
■
This work was financially supported by Transregio TR49,
Natural Science Foundation of China (Nos. 51603055,
81601542), the China Postdoctoral Science Foundation
(Grant No. 2016M601424), the Postdoctoral Foundation of
Heilongjiang Province, and Key Laboratory of Functional
Inorganic Material Chemistry (Heilongjiang University), Min-
istry of Education.
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DOI: 10.1021/acs.cgd.7b00272
Cryst. Growth Des. XXXX, XXX, XXX−XXX