6,12-Diarylindeno[1,2- b ]fluorenes: Syntheses, photophysics, and ambipolar OFETs

Article abstract of DOI:10.1021/ja303402p

Herein we report the synthesis and characterization of a series of 6,12-diarylindeno[1,2-b]fluorenes (IFs). Functionalization with electron donor and acceptor groups influences the ability of the IF scaffold to undergo two-electron oxidation and reduction to yield the corresponding 18- and 22-π-electron species, respectively. A single crystal of the pentafluorophenyl-substituted IF can serve as an active layer in an organic field-effect transistor (OFET). The important finding is that the single-crystal OFET yields an ambipolar device that is able to transport holes and electrons.

Full text of DOI:10.1021/ja303402p

Communication
pubs.acs.org/JACS
6,12-Diarylindeno[1,2-b]fluorenes: Syntheses, Photophysics, and
Ambipolar OFETs
Daniel T. Chase,†,§ Aaron G. Fix,† Seok Ju Kang,‡ Bradley D. Rose,† Christopher D. Weber,† Yu Zhong,‡
Lev N. Zakharov,† Mark C. Lonergan,† Colin Nuckolls,*,‡ and Michael M. Haley*,†
†Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253, United States
‡Department of Chemistry, Columbia University, New York, New York 10027, United States
S
* Supporting Information
ABSTRACT: Herein we report the synthesis and
characterization of a series of 6,12-diarylindeno[1,2-
b]fluorenes (IFs). Functionalization with electron donor
and acceptor groups influences the ability of the IF scaffold
to undergo two-electron oxidation and reduction to yield
the corresponding 18- and 22-π-electron species, respec-
tively. A single crystal of the pentafluorophenyl-substituted
IF can serve as an active layer in an organic field-effect
transistor (OFET). The important finding is that the
single-crystal OFET yields an ambipolar device that is able
to transport holes and electrons.
onjugated polycyclic hydrocarbons have been the subject
Cof intense research over the last two decades for their
potential use in a variety of materials applications such as organic
light-emitting diodes, thin-film transistors, and solar cells.1 Of
particular interest is the indeno[1,2-b]fluorene (IF) skeleton
(e.g., 1 and 2, Figure 1), a 6−5−6−5−6 fused ring system that in
its fully conjugated state closely resembles pentacene.2 As IFs
Figure 2. (left) HOMO and (right) LUMO plots for IFs 4b (top) and 4j
(bottom).
possess two fewer carbon atoms and hence two fewer π electrons
than pentacenes, they could be considered as antiaromatic
molecules. This definition remains a formality, however, as the
We recently reported the synthesis and properties of a series of
internal core exhibits bond lengths and bond alternation
2,8-disubstituted derivatives of 2.5 These compounds possess
resembling a dibenzo-fused s-indacene.3 Furthermore, 1 and 2
UV−vis profiles similar to that of 1 with slightly larger HOMO/
are stable upon heating for prolonged periods (80 °C for 1, 150
LUMO gaps. As with our reported diynyl-IF-diones,6 2 and
°C for 2) without noticeable decomposition, a trait not typically
associated with antiaromatic compounds. These observations of
derivatives could accept two electrons in a quasi-reversible
fashion, exhibiting LUMO energy levels comparable to or further
stability parallel those found with structurally similar dibenzo-
from the vacuum level than that of the ubiquitous electron
pentalene derivatives.4
acceptor PCBM.7 Furthermore, the solid-state structures of two
derivatives showed one-dimensional π stacks with intermolecular
distances as short as 3.40 Å, an improvement over the
herringbone motif found for 1. Functionalization at the 2- and
8-positions, however, had only a modest effect on the HOMO
and LUMO energy levels, as calculations showed minimal orbital
density at those positions. In addition to our studies, Tobe and
Shimizu recently disclosed the synthesis of 11,12-
dimesitylindeno[2,1-a]fluorene (3),8 an isomeric IF species
possessing physical characteristics similar to those of 1 and 2.
Received: April 9, 2012
Published: June 14, 2012
Figure 1. Fully conjugated indenofluorenes 1−3.
© 2012 American Chemical Society
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Communication
Scheme 1. Synthesis of 6,12-Diarylindeno[1,2-b]fluorenes
Figure 3. Electronic absorption spectra for IFs 2, 4a−c, and 4j; all
spectra were recorded in CHCl3 at 15−25 μM.
using SnCl2 in toluene at 65 °C afforded a magenta solution from
which the parent 4a was obtained in 55% yield over two steps.
This reaction scheme was then extended to the series of 6,12-
diaryl-IFs 4b−j, which were isolated in good to modest yields
after recrystallization. The reduction was sluggish for strongly
electron-deficient arenes such as 6h−j, and the addition of a small
amount of trifluoroacetic acid (similarly used for the synthesis of
3)8 was required for the reduction to occur smoothly.
To expand further the versatility of the IF scaffold, we
considered substitution motifs beyond the previously used
triisopropylsilyl (TIPS)-substituted ethynyl moiety. Arylation at
the 6- and 12-positions represents a logical next step, as arenes
functionalized with either donor or acceptor groups might impart
a more diverse range of IF optoelectronic properties. This
supposition is supported by the calculated HOMO/LUMO plots
in Figure 2, which show considerable orbital density at those
positions. The synthesis of arylated IFs also fulfills a historic
purpose in that one of the original IFs exhibiting full conjugation,
6,12-diphenyl-IF 4a, was only briefly described by Scherf.9
Herein we disclose the syntheses of IFs 4a−j from readily
available 6,12-indenofluorenedione (5),10 along with the
respective optical, electrochemical, and computational data.
Importantly, we show that pentafluorophenyl derivative 4j can be
used to construct an organic field-effect transistor (OFET) that
exhibits ambipolar behavior.
Table 1 contains the UV−vis absorption data for all of the
diaryl-IFs; individual spectra of 2, 4a−c, and 4j are shown in
Figure 3 [see the Supporting Information (SI) for the spectra of
4d−i]. Like 2, 4a exhibits three low-energy absorptions (λmax
=
544 nm), but these are hypsochromically shifted by ca. 50 and 25
nm with respect to 1 and 2. This can be attributed to the
exchange of electron-withdrawing acetylenes with aryl groups, a
consequence also observed with substituted pentacenes.11
Variation of the aryl substituents at the 6- and 12-positions has
a notable impact on the absorption profiles: methoxyphenyl-
substituted 4b exhibits a λmax of 558 nm, while pentafluor-
ophenyl-substituted 4j has a λmax of 533 nm. Interestingly, the
mesityl derivative 4c exhibits a λmax of 516 nm, which is attributed
to the nearly orthogonal orientation of the mesityl and IF
The same strategy for synthesizing 2 via dione 510 can be
readily applied to give 4a (Scheme 1). Addition of lithiated
bromobenzene gave crude diol 6a, and subsequent reduction
Table 1. Computational, Electrochemical, and Optical Data for Indeno[1,2-b]fluorenes 2 and 4a−j
a
b
c
computational
electrochemical
optical
d
compd
EHOMO
ELUMO
Egap
E(A+/A), E(A2+/A+), E(A3+/A2+)
E(A/A−), E(A−/A2−
−0.69, −1.20
)
EHOMO
ELUMO
Egap
λmax
Egap
2
−5.51
−5.24
−5.00
−5.34
−5.13
−5.37
−5.42
−5.43
−5.65
−5.89
−5.92
−3.47
−3.07
−2.90
−2.92
−2.98
−3.20
−3.27
−3.28
−3.49
−3.73
−3.71
2.04
2.17
2.10
2.42
2.15
2.17
2.15
2.15
2.16
2.16
2.21
1.20
−5.88
−5.68
−5.50
−5.78
−5.61
−5.71
−5.74
−5.73
−5.91
−6.05
−6.17
−4.00
−3.72
−3.68
−3.56
−3.69
−3.76
−3.81
−3.80
−3.85
−3.97
−4.00
1.88
1.96
1.82
2.22
1.92
1.95
1.93
1.93
2.06
2.08
2.17
568
544
558
516
550
543
548
550
545
543
533
2.12
2.17
2.05
2.29
2.12
2.16
2.14
2.12
2.16
2.16
2.20
e
e
e
4a
4b
4c
4d
4e
4f
1.00, 1.31, 1.51
−0.96, −1.49
−1.01, −1.53
−1.12, −1.73
−0.99, −1.50
−0.93, −1.41
−0.88, −1.34
−0.89, −1.35
e
e
0.81, 1.07
e
e
1.10, 1.59
e
e
e
e
e
e
0.93, 1.21, 1.48
e
e
1.03, 1.24, 1.52
e
e
1.05, 1.30, 1.53
e
e
4g
4h
4i
1.04, 1.38, 1.51
e
e
1.22, 1.41, , 1.54
−0.84, −1.12
−0.73, −1.07
−0.68, −1.17
e
e
1.35, 1.67
e
e
4j
1.49, 1.66
a
b
Calculations were performed at the B3LYP/6-311+G** level of theory; energies are in eV. CVs were recorded using 1−5 mM of analyte in 0.1 M
Bu4NOTf/CH2Cl2 at a scan rate of 50 mV/s with a glassy carbon working electrode, a Pt coil counter electrode, and a Ag wire pseudo-reference
electrode. E is the half-wave potential (E1/2) for reversible processes and the potential of the peak anodic or cathodic current (Ep) for irreversible
processes. Potential values are reported in V vs SCE using the Fc+/Fc couple (0.46 V) as an internal standard. HOMO and LUMO energy levels in
eV were approximated using SCE = −4.68 eV vs vacuum (see ref 13) and E1/2 values for reversible processes or Ep values for irreversible processes.
c
d
Spectra were obtained in CHCl3; wavelengths are in nm. The optical HOMO/LUMO gap was determined as the intersection of the x axis and a
e
tangent line passing through the inflection point of the lowest-energy absorption; energies are in eV. Irreversible wave.
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Figure 7. (a) Schematic of an OFET with an active channel consisting of
a 4j single crystal. (b) Top-view OM of an OFET (top) and energy
diagram for Au/4j crystal/Au (bottom). (c, d) I−V transfer character-
istics of OFETs with (c) negative and (d) positive drain voltages.
Figure 4. X-ray crystal structure of 4c; H atoms have been omitted for
clarity, and ellipsoids are drawn at the 30% probability level.
the effects of the aryl substituents, as noted above. As observed
previously for 1 and 2, IFs 4a−j are nonemissive.
Cyclic voltammograms (CVs) for 4b, 4c, and 4j are displayed
in Figure 5 (see the SI for the CVs for 4a, 4d−i). All of the
voltammetry data are compiled in Table 1, which shows the half-
wave potentials (E1/2) for reversible waves or the potential of
peak current (Ep) for irreversible waves. E1/2 is generally very
close to the standard reduction potential. In solution, the neutral
diaryl-IF scaffold exhibits quasi-reversible behavior in accepting
up to two electrons in separate waves (A/A− and A−/A2−). The
E
1/2(A/A−) values for 4a−j range from −0.68 to −1.12 V vs SCE,
which in general are more negative than that observed for 2.
Electron-withdrawing groups shift E1/2 to more positive values
while electron-donating groups shift it to more negative values, as
such groups work respectively to stabilize and destabilize the
resulting anionic species. The wave for the second reduction
(A−/A2−) was irreversible in most cases, although the Ep showed
trends similar to those for E1/2(A/A−). With the exception of 4i
and 4j, the diaryl-IFs could be reversibly oxidized from their
neutral states, an unanticipated trait on the basis of our previous
observations for 1 and 2. The E1/2(A+/A) and Ep(A+/A) values
for 4a−j range from 0.81 to 1.49 V. Electron-withdrawing groups
destabilize the resulting cationic species, while electron-donating
groups provide the opposite effect. A second and sometimes
third anodic peak (4a and 4d−h) were observed for each
compound at potentials more positive than E1/2(A+/A), but
these processes were irreversible. The trend noted for E1/2(A+/
A) was also observed for Ep(A2+/A+) and for Ep(A3+/A2+) when it
was present. As a whole, the HOMO and LUMO energy levels of
4a−j span 0.7 and 0.4 eV ranges,13 respectively, which is far
beyond the 0.15 eV range demonstrated by 2 and derivatives.5 p-
Anisyl-IF 4b displays E1/2 values for the first reduction (−1.01 V)
and oxidation (0.81 V) of its neutral form, which closely mirror
those found for 6,13-bis(TIPS-ethynyl)pentacene, which are
−1.10 and 0.74 V, respectively.11,14
Figure 5. CVs for IFs 4b, 4c, and 4j.
Figure 6. (a) OM of single crystals of 4j prepared by a solvent-exchange
method. The inset shows an OM with crossed polarizers. (b) SAED
pattern and (inset) bright-field TEM image of a single crystal of 4j.
enforced by the o-methyl groups. The X-ray structure of 4c
(Figure 4) corroborates this hypothesis, as the dihedral angle
between the mesityl and indenofluorene rings is 72° (80.6°
calculated dihedral). Thus, little electronic communication
between the π systems exists in this instance, which effectively
reduces the conjugation path length in 4c, resulting in the
hypsochromic shift. All of the other arylated IFs are predicted to
exhibit more typical biphenyl dihedral angles12 (calculated 43−
45° for 4a−i, 50.6° for 4j), and their spectra more strongly reflect
Micron-scale single crystals of 4j could be prepared using a
solvent-exchange method.15 Figure 6 displays an optical
micrograph (OM) of these microcrystals on a silicon wafer
with 200 nm of SiO2 on its surface. The strong and uniform
birefringence of 4j along with the reflections observed in the
selected-area electron diffraction (SAED) pattern and the bright-
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Journal of the American Chemical Society
Communication
field TEM image (Figure 6b) indicate that these microcrystals
have a single-crystal orientation.
Division of Chemical Sciences, Geosciences, and Biosciences,
Office of Basic Energy Sciences, U.S. Department of Energy
through Grant DE-FG02-07ER15907. B.D.R. and C.D.W.
acknowledge the NSF for GK-12 (DGE-0742540) and IGERT
(DGE-0549503) Fellowships, respectively. C.N. thanks the DOE
and S.J.K. thanks NSF-MIRT (DMR-1122594).
After growth of the crystal on the silicon wafer, OFETs with a
microcrystal of 4j as the active channel in the device were
constructed.16 Figure 7a is a schematic of such a device, and
Figure 7b is an OM of a device.17 To construct the device, the
surface of the SiO2 interface was passivated with a monolayer of
octadecyltrichlorosilane. This treatment has been shown
previously to reduce the trap sites at the dielectric−semi-
conductor interface.18 The Au source and drain electrodes were
thermally deposited through a TEM grid to form a top-contact
bottom-gate transistor. The work function of Au is ca. 5.0 eV,
meaning that both holes and electrons can be injected from Au
into 4j single crystals through the Schottky barrier at the metal−
semiconductor contact.19
REFERENCES
■
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regime. The field-effect mobility can be calculated using IDS
=
(W/2L)Ciμ(VG − VT)2, where W and L are the width and length
of the channel and Ci (=17.8 nF cm−2), μ, and VT correspond to
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saturation regime, respectively.20 The HOMO and LUMO levels
of diaryl-IF 4j create well-balanced ambipolar OFETs with Au
electrodes because the work function of the electrodes is in the
middle of the gap. Interestingly, there are very few ambipolar
OFETs made from single crystals of organic semiconductors.21
In conclusion, we have prepared a family of 6,12-diarylindeno-
[1,2-b]fluorenes and shown that aryl substitution can signifi-
cantly affect the redox properties of the molecules. In cyclic
voltammetry experiments, the diaryl-IFs can either accept or
donate two electrons. Pentafluorophenyl-substituted IF 4j was
utilized as the active element in a single-crystal OFET that
exhibited ambipolar behavior with Au source/drain contacts.
Future work will focus on additional indenofluorene topologies
and molecule functionalization as well as further device studies.
(12) Almenningen, A.; Bastiansen, O.; Fernholt, L.; Cyvin, B. N.;
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ASSOCIATED CONTENT
* Supporting Information
Experimental details, NMR and absorption spectra, CVs, and
computational data for 4a−j; CIF file for 4c; OFET fabrication
details for 4j. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
(13) Reiss, H.; Heller, A. J. Phys. Chem. 1985, 89, 4207.
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L. J. Am. Chem. Soc. 2005, 127, 10496.
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(17) Thin films of 4j were inactive in OFETs, likely because of the
morphology of the films and the dielectric interface.
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S
AUTHOR INFORMATION
Corresponding Author
haley@uoregon.edu; cn37@columbia.edu
■
Present Address
§Department of Chemistry and Biochemistry, The University of
Texas, Austin, Texas 78712.
(20) We tested only 4j because it readily grew crystals, but the
HOMO/LUMO levels of the other diaryl-IFs indicate that these too will
be active in OFETs, research on which is ongoing.
Notes
The authors declare no competing financial interest.
(21) (a) de Boer, R. W. I.; Stassen, A. F.; Craciun, M. F.; Mulder, C. L.;
Molinari, A.; Rogge, S.; Morpurgo, A. F. Appl. Phys. Lett. 2005, 86,
No. 262109. (b) Takahashi, T.; Takenobu, T.; Takeya, J.; Iwasa, Y. Appl.
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ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-1013032) for
support of this research and for instrumentation grant support
(CHE-0923589). The electrochemical work was funded by the
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