Synthesis and characterization of electron-deficient and highly soluble (bis)indenofluorene building blocks for n-type semiconducting polymers

Article abstract of DOI:10.1021/ol8000574

(Chemical Equation Presented) New electron-deficient and soluble indenofluorene-based and bisindenofluorene-based ladder-type building blocks embedding carbonyl and dicyanovinylene functionalities were synthesized, and their optical and electrochemical pr

Full text of DOI:10.1021/ol8000574

ORGANIC
LETTERS
2008
Vol. 10, No. 7
1385-1388
Synthesis and Characterization of
Electron-Deficient and Highly Soluble
(Bis)Indenofluorene Building Blocks for
n-Type Semiconducting Polymers
Hakan Usta, Antonio Facchetti,* and Tobin J. Marks*
Department of Chemistry and the Materials Research Center, Northwestern UniVersity,
2145 Sheridan Road, EVanston, Illinois, 60208
a-facchetti@northwestern.edu; t-marks@northwestern.edu
Received January 9, 2008
ABSTRACT
New electron-deficient and soluble indenofluorene-based and bisindenofluorene-based ladder-type building blocks embedding carbonyl and
dicyanovinylene functionalities were synthesized, and their optical and electrochemical properties were characterized. These derivatives exhibit
optical band gaps of 1.83 to 2.44 eV and low LUMO energies of
n-type polymeric electronic materials.
−3.24 to −4.30 eV, representing a promising new building block class for
During the past several decades, π-conjugated polymers have
received major scientific and technological attention due to
their potential as electro-active materials in organic electron-
ics, particularly in printed thin-film transistors, photovoltaic
cells, and light-emitting displays.1 Among these applications,
organic thin-film transistors (OTFTs) are considered attrac-
tive alternatives to conventional inorganic transistors in
functions that require low-cost, low-temperature, solution-
phase device processing and mechanical flexibility.2 As one
important OTFT figure-of-merit, large charge carrier mobili-
ties exceeding 1 cm2 V-1 s-1 have been reported for
p-channel (hole transporting) and n-channel (electron trans-
porting) OTFT small-molecule semiconductors.3 However,
typical mobilities for polymeric semiconductors are far below
these optimized, single-crystal or vapor-deposited film
values.4 Among known polymeric semiconductors, soluble
n-channel polymers are rare, with the highest reported
mobility being 0.01-0.03 cm2 V-1 s-1.5 A critical goal then
is to realize polymers possessing high n-type mobility (>0.1
cm2 V-1 s-1) and air stability along with good OFET current
modulation (>105) characteristics. A polymer exhibiting
these characteristics would be an excellent candidate for
practical OTFT applications, reflecting the intrinsic techno-
logical attractions of polymers, such as facile film formation
and compatibility with low-cost manufacturing and direct-
write printing techniques on flexible plastic substrates.6 These
(4) (a) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Mac-
donald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang,
W. M.; Chabinyc, M. L.; Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat.
Mater. 2006, 5, 328. (b) Usta, H.; Lu, G.; Facchetti, A.; Marks, T. J. J.
Am. Chem. Soc. 2006, 128, 9034. (c) Ong, B. S.; Wu, Y. L.; Liu, P.; Gardner,
S. J. Am. Chem. Soc. 2004, 126, 3378.
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Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129,
7246. (b) A mobility of 0.1 cm2 V-1 s-1 has been reported for an n-type
polymer with an on/off ratio of ∼10. Babel, A.; Jenekhe, S. A. J. Am. Chem.
Soc. 2003, 125, 13656.
(1) (a) Barbarella, G.; Melucci, M.; Sotgiu, G. AdV. Mater. 2005, 17,
1581. (b) Katz, H. E. Chem. Mater. 2004, 16, 4748. (c) Sirringhaus, H.;
Tessler, N.; Friend, H. R. Science 1998, 280, 1741.
(2) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14,
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(3) (a) Facchetti, A. Mater. Today 2007, 10, 28. (b) Horowitz, G. AdV.
Mater. 1998, 10, 365.
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10.1021/ol8000574 CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/04/2008

considerations prompted us to seek new electron-depleted
(low LUMO) molecular building blocks as polymer precur-
sors based on ladder-type cores.7 Here we report the synthesis
and characterization of a new family of difunctionalized
ladder-type molecules (1-4) in which highly electron-
deficient carbonyl and dicyanovinylene groups are introduced
into the planar indenofluorene and bisindenofluorene struc-
tures along with solubilizing alkyl chains. The design
considerations for building blocks 1-4 are: (i) Highly
π-conjugated planar cores to facilitate efficient π-electron
delocalization and to favor good intermolecular π-π stack-
ing.8 (ii) Electron-withdrawing carbonyl and dicyanovinylene
functionalities to lower LUMO energies, which is crucial
for achieving electron transport.9 (iii) Alkyl side chains (n-
C12H25) to increase core solubility without disrupting back-
bone π-conjugation.10 (iv) Since Suzuki, Stille, and Yama-
moto polymerizations will be employed to synthesize the
corresponding polymers, all building blocks are designed to
accommodate aryl bromide functionalities at the molecular
termini.
The syntheses of indenofluorene-based compounds 1 and
2, and bisindenofluorene-based compounds 3 and 4, are
shown in Scheme 1.
Figure 1. (A) Normalized UV-vis absorption (solid lines) and
photoluminescence (dashed lines) spectra. (B) Cyclic voltammo-
grams of compounds 1, 2, 3, and 4 in THF.
Scheme 1. Synthesis of (Bis)Indenofluorene-Based Monomers
1-4
gives compound 7 as a colorless oil in 45% yield. The
moderate yield obtained for this Suzuki coupling is due to
the presence of the electron-withdrawing ester group in the
boronic ester reagent.11 Double intramolecular Friedel-Crafts
acylation is next achieved by treatment of the diester 7 with
concentrated sulfuric acid at 120 °C for 3 h. Indenofluorene-
dione product 8 is isolated as an orange solid in 93% yield.
Compound 8 is then regioselectively brominated at the 2 and
8 positions using FeCl3/Br2 with rigorous exclusion of light
to afford monomer 1 in 85% yield. Knoevenagel condensa-
tion of 1 to form dimalononitrile structure 2 is achieved in
50% yield using excess malononitrile along with pyridine
and TiCl4. Due to the substantial steric hindrance of the
carbonyl groups in compound 1, TiCl4 is used as the Lewis
acid for this reaction, but is not necessary for the synthesis
of 4 from less sterically hindered 3 (vide infra). Fluorene
boronic ester 9 is synthesized in 90% yield starting from
2,7-dibromo-9,9-didodecylfluorene which is double-lithiated
(8) (a) Woody, K. B.; Bullock, J. E.; Parkin, S. R.; Watson, M. D.
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Macromolecules 2003, 36, 8240. (d) Setayesh, S.; Marsitzky, D.; Mu¨llen,
K. Macromolecules 2000, 33, 2016.
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J. Am. Chem. Soc. 2006, 128, 12714. (b) Yoon, M.-H.; DiBenedetto, S.
A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 1348. (c)
Newman, C. R.; Frisbie, C. D.; Filho, D. A. da S.; Bre´das, J.-L.; Ewbank,
P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (d) Casado, J.; Pappenfus,
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R.; Hernandez, V.; Lopez Navarrete, J. T. J. Am. Chem. Soc. 2003, 125,
2524.
Kumada coupling of n-dodecylmagnesium bromide with
1,4-dichlorobenzene affords 1,4-di-n-dodecylbenzene (5) in
83% yield, which is then selectively brominated in CH2Cl2
under rigorous exclusion of light to afford 2,5-dibromo-1,4-
di-n-dodecylbenzene (6) in 75% yield. Suzuki coupling of
6 with 2-(ethoxycarbonyl)phenylboronic acid pinacol ester
(7) (a) Hadizad, T.; Zhang, J.; Wang, Z. Y.; Gorjanc, T. C.; Py, C. Org.
Lett. 2005, 7, 795. (b) Vak, D.; Lim, B.; Lee, S.-H.; Kim, D.-Y. Org. Lett.
2005, 7, 4229. (c) Jacob, J.; Sax, S.; Piok, T.; List, E. J. W.; Grimsdale, A.
C.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 6987. (d) Merlet, S.; Birau,
M.; Wang, Z. Y. Org. Lett. 2002, 4, 2157. (e) Frank, W.; Gompper, R.
Tetrahedron Lett. 1987, 28, 3083.
(10) Anthony, J. E. Chem. ReV. 2006, 106, 5028.
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1386
Org. Lett., Vol. 10, No. 7, 2008

with t-butyllithium and subsequently reacted with 2-iso-
propoxy- 4,4,5,5-tetramethyl[1,3,2]dioxaborolane. Compound
10 is prepared by Suzuki coupling of 9 with methyl 2-iodo-
5-bromobenzoate in high yield (96%). Treatment of 10 with
concentrated sulfuric acid at 165 °C for 3 h induces double
intramolecular acylation, yielding the target bisindeno-
fluorene diketone 3. The yield obtained for this ring closure
is 70%, with high selectivity at each reaction site (84%).
Knoevenagel condensation of 3 with malononitrile and
piperidine as the base affords 4 in 65% yield as a purple
solid. The new monomers are very soluble in common
organic solvents (CHCl3, CH2Cl2, THF, toluene), which
allows convenient purification by flash columnn chroma-
tography. Compounds 1-4 were characterized by 1H NMR,
13C NMR, elemental analysis, IR, and mass spectroscopy
(EI/ESI/MALDI-TOF).
nm for the bisindenofluorene core and is consistent with the
reported shifts for similar ladder structures.14,15 This behavior
is attributed to LUMO energetic stabilization due to the
stronger electron-withdrawing nature of the dicyanovinylene
compared to the carbonyl group, resulting in a band gap
contraction from 2.28 eV (1) to 1.83 eV (2), and from 2.44
eV (3) to 1.95 eV (4).15 These gaps are considerably smaller
than those of typical non-functionalized indenofluorene cores
(∼3.7 eV).7d
The fluorescence emission maxima of carbonyl-containing
cores 1 and 3 are located at 590 and 540 nm, respectively.
The large observed Stokes shifts are attributable to the
occurrence of internal energy transfers between two chro-
mophore units in these molecules: the indenofluorene/
bisindenofluorene core and the aryldiketone system.7c For
dicyanovinylene compounds 2 and 4, very broad, weak
emissions centered at 762 and 768 nm, respectively, are
observed. The fluorescence quenching in these highly
electron-deficient molecules may be due to the existence of
nonradiative internal energy/electron transfers before the
radiative emission process.16 Further detailed studies includ-
ing polarized absorption and emission measurements will be
required to fully define the nature of this emission quenching.
The UV-vis absorption and fluorescence spectra of
compounds 1-4 in THF are shown in Figure 1A, and optical
data are collected in Table 1. The absorption spectra of
Table 1. Electrochemical and Optical Absorption/Emission
Properties of Compounds 1, 2, 3, and 4, and Corresponding
Estimated Frontier Molecular Orbital Energies
To investigate the redox properties of these new ladder-
type molecules, cyclic voltammetry measurements were
performed in THF by using Pt as the working electrode, Ag
as the pseudo reference electrode, and ferrrocene (0.54 V
vs SCE) as the internal standard. The cyclic voltammograms
of 1-4 are shown in Figure 1B, and electrochemical data
are collected in Table 1. For all molecules, multiple reversible
reductions but no oxidations are observed, suggesting that
all building blocks are substantially electron-dopable.17
Analysis of the half-wave potentials reveals the importance
of carbonyl and dicyanovinylene functionalizations in modu-
lating the 1-4 frontier molecular orbitals.
c
red-1
e
λabs
(nm)
λem
(nm)
Eg
E1/2
(V)
EHOMOd/ELUMO
compd
(eV)
(eV)
1
2
3
4
368,484a
426,579a
365,455a
378,513a
590
2.28
1.83
2.44
1.95
-0.77
-0.14
-1.20
-0.53
-5.95/-3.67
-6.16/-4.30
-5.68/-3.24
-5.86/-3.91
762b
540
768b
a n-π* transition assigned to carbonyl groups. b Broad, weak emission
peak. c Band gap estimated from the low-energy band edge of the UV-vis
d
e
spectrum. EHOMO calculated from: Eg ) LUMO - HOMO.19 ELUMO
calculated as: -(E1/2
+ 4.44 eV).19
red-1
Indenofluorenedione 1 undergoes two reversible reductions
with the half-wave potentials of -0.77 and -1.31 V versus
SCE, which are assigned as reduction of the diketone to the
quinonoidal dianion.7c Ladder-type diketone structure 1 can
be reduced much more easily than non-ladder type ter-
phenylene and quaterphenylene compounds which show
reversible reductions at -2.40 V and -2.28 V (vs SCE),
respectively.18 Thus, it is evident that the reduction potential
is shifted to more positive values with increasing degree of
planarity and electron deficiency of the core. The reversibility
of both reductions demonstrates the redox stability of the
new diketone structure. The estimated LUMO energy is
-3.67 eV using the vacuum level energy of the standard
calomel electrode as 4.44 eV.19
diketone-functionalized monomers 1 and 3 exhibit three
maxima, two of them located below 400 nm, and the third
one at 450-485 nm. The higher energy maxima (368 nm
(1) and 365 nm (3)) correspond to the π-π* transitions of
the indenofluorene and bisindenofluorene backbones, re-
spectively, whereas the weaker absorptions at lower energies
(484 nm (1) and 455 nm (3)) are attributed to the symmetry-
forbidden carbonyl group n-π* transition.12 The absolute
absorption maxima of all compounds are shifted to longer
wavelengths by ∼100 nm compared to that of fluorenone
(258 nm in THF).13 This bathochromic shift is indicative of
enhanced π-conjugation of the molecular backbone and is
attributed to the highly planar, ladder-type structure of these
cores embedding two electron-withdrawing carbonyl groups.
When the carbonyl functionalities are replaced with the
dicyanovinylene groups, the absolute absorption maxima shift
to 426 nm for 2 and 378 nm for 4, and long-wavelength
absorptions are now observed at 579 and 513 nm for 2 and
4, respectively. The bathochromic shift upon dicyanovinylene
functionalization is ∼58 nm for the indenofluorene and ∼13
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Org. Lett., Vol. 10, No. 7, 2008
1387

Upon dicyanovinylene functionalization, compound 2
exhibits six reversible one-electron reductions with the first
reduction located at -0.14 V. The greater electron-accepting
capacity of 2 is attributable to the highly electron-withdraw-
ing character of the dicyanovinylene groups. The LUMO
energy is estimated as -4.30 eV. To our knowledge, this is
one of the lowest LUMO energies reported for an electron-
deficient, soluble polymer building block, and is comparable
to those of air-stable n-channel core-cyanated perylene and
naphthalene semiconductors.20 Bisindenofluorenedione 3
exhibits two reversible electron reductions at -1.20 and
-1.39 V. Upon dicyanovinylene functionalization, the reduc-
tion potentials exhibit anodic shifts, with the first reduction
now located at -0.53 V. Compound 4 exhibits five one-
electron reductions. It can be seen that dicyanovinylene
functionalization at either indenofluorene and bisindeno-
fluorene position decreases the voltage gap between the first
two reductions by 0.1-0.2 V, doubtless reflecting the more
electron-accepting nature of these derivatives, and rendering
the second reduction more facile. This similarity of the first
two potentials suggests that these reductions are largely
malononitrile-centered as previously observed for tricyano-
vinyl-capped oligothiophenes.21
electron mobility of 0.002 cm2 V-1 s-1 with a current on/
off ratio of ∼106 (Figure 2). Note that although soluble,
Figure 2. OFET plots of devices (n++-Si/SiO2-OTS (300 nm)/4
(50 nm)/Au contacts (50 nm)) fabricated with monomer 4. (A)
Transfer curve for VSD ) 100 V. (B) Output curve for various gate
voltages.
bromo/iodo functionalized monomeric building blocks are
generally poor OFET semiconductors, the corresponding
polymers can exhibit far greater performance. For example,
within the polythiophene family, although â-alkylsubstituted
oligothiophenes exhibit negligible/low carrier mobilities, the
corresponding polymers (e.g., PQT, P3HT) are among the
highest mobility organic semiconductors reported to date.3,4c
The observed electron mobility for 4 is therefore very
promising for the further optimization of these building
blocks as n-channel polymers.
In summary, we have successfully designed and synthe-
sized a new family of ladder-type organic semiconductor
building blocks. These structures exhibit high electron
affinities and solubilities, crucial requirements for n-channel
semiconducting polymers. The homo- and copolymerization
reactions of these monomers are currently under investigation
and will be reported in due course.
The anodic shift in the reduction potential from the
diketone to dicyanovinylene structure is ∼0.65 V for both
systems, which suggests that the dicyanovinylene substituent
electronically affects both cores in the same way. The
potential difference between successive reduction events
[∆E1/2 ) E1/2-2 - E1/2-1] for bisindenofluorene-based
compounds 3 and 4 is ∼0.19 V, whereas the difference is
almost three times that (∼0.52 V) in indenofluorene-based
structures 1 and 2. Here, the second reduction becomes more
difficult than the first due to less effective radical anion
delocalization and increased Coulombic repulsion within the
smaller indenofluorene π-system compared to the bisindeno-
fluorene π-system. A similar trend is observed for the R-nT
and phenylene (p-nP) series.18,22
Acknowledgment. We thank ONR (N00014-05-1-0766)
for support of this research, and the NSF-MRSEC program
through the Northwestern University Materials Research
Science and Engineering Center (DMR-0076097) for provid-
ing characterization facilities.
Top-contact OFETs were fabricated by vapor deposition
of compound 4 on OTS-treated n++-Si/SiO2(300 nm) sub-
strates. Preliminary measurements reveal an unoptimized
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39, 8712. (b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods-
Fundamentals and Applications; Wiley: New York, 1984.
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Chem. Mater. 2007, 19, 2703. (b) Jones, B. A.; Ahrens, M. J.; Yoon, M.
H.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. Angew. Chem., Intl.
Ed. 2004, 43, 6363.
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15, 616. (b) Pappenfus, T. D.; Burand, M. W.; Janzen, D. E.; Mann, K. R.
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Supporting Information Available: Detailed experi-
mental procedures, device fabrication /characterization de-
1
tails, spectroscopic characterizations; H NMR, 13C NMR,
IR, and EI/ESI/MALDI-MS spectra of 1, 2, 3, 4, and 8
(Figures S1-S16). This material is available free of charge
via the Internet at http://pubs.acs.org.
(22) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner,
M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13480.
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