Synthesis and characterization of fluorinated heterofluorenecontaining donor-acceptor systems

Article abstract of DOI:10.1021/jo902488w

Chemical Equation Presented A series of oligothiophene- perfluoro-9-heterofluorene donor-acceptor (DA) compounds was synthesized via a combination of nucleophilic aromatic substitution (S_NAr_F) and palladium coupling reactions. These compounds are of interest as possible building blocks for materials with useful electron transport properties, since they possess relatively low LUMO energy levels of-3.3 to -3.6 eV (as determined by differential pulse voltammetry). The HOMO-LUMO energy gaps, as determined by UV-vis spectroscopy, range between 2.4 and 2.5 eV, and photoluminescence emission spectra reveal λ_ems values in the range of 480-600 nm (corresponding to yellow-orange emission). Dilute solution-state photoluminescence quantum yields were significantly lower than those of the pure acceptor heterofluorenes (0.02-0.38 for the DA compounds vs ～1 for the pure acceptors), and notable solvatochromism in the fluorescence suggests emission from a charge-separated state. Theoretical calculations show that HOMO-level electron density is more localized on the thiophene fragment, while the LUMO level electron density is mostly associated with the electron-deficient portion of the molecule. Photovoltaic (PV) devices based on DA/poly-3-hexylthiophene (P3HT) blends exhibit improved performance over P3HT-only devices, suggesting the ability of these DA compounds to transport electrons in the solid state.

Full text of DOI:10.1021/jo902488w

pubs.acs.org/joc
Synthesis and Characterization of Fluorinated Heterofluorene-
Containing Donor-Acceptor Systems
Katharine Geramita,† Yuefei Tao,‡ Rachel A. Segalman,‡ and T. Don Tilley*,†
†Department of Chemistry and ‡Department of Chemical Engineering, University of California at Berkeley,
Berkeley California 94720
tdtilley@berkeley.edu
Received December 7, 2009
A series of oligothiophene-perfluoro-9-heterofluorene donor-acceptor (DA) compounds was
synthesized via a combination of nucleophilic aromatic substitution (SNArF) and palladium coupling
reactions. These compounds are of interest as possible building blocks for materials with useful
electron transport properties, since they possess relatively low LUMO energy levels of -3.3 to -3.6 eV
(as determined by differential pulse voltammetry). The HOMO-LUMO energy gaps, as determined
by UV-vis spectroscopy, range between 2.4 and 2.5 eV, and photoluminescence emission spectra
reveal λems values in the range of 480-600 nm (corresponding to yellow-orange emission). Dilute
solution-state photoluminescence quantum yields were significantly lower than those of the pure
acceptor heterofluorenes (0.02-0.38 for the DA compounds vs ∼1 for the pure acceptors), and
notable solvatochromism in the fluorescence suggests emission from a charge-separated state.
Theoretical calculations show that HOMO-level electron density is more localized on the thiophene
fragment, while the LUMO level electron density is mostly associated with the electron-deficient
portion of the molecule. Photovoltaic (PV) devices based on DA/poly-3-hexylthiophene (P3HT)
blends exhibit improved performance over P3HT-only devices, suggesting the ability of these DA
compounds to transport electrons in the solid state.
Introduction
photovoltaics (PVs), continues to attract considerable interest.1
Organic systems offer the possibility for cheap raw materials
and low processing costs and can be tailored to access a
wide range of physical, optical, and electrical properties.1-4
The use of organic semiconducting materials for applica-
tions in “plastic electronics”, such as field effect transistors
(FETs), organic light emitting diodes (OLEDs) and
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DOI: 10.1021/jo902488w
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Within this context, investigations have targeted conju-
gated oligomers and polymers that are electron-rich
(donor) or electron-poor (acceptor) in character. Fully
conjugated donor-acceptor systems, D-π-A, that incorpo-
rate both electron-rich and electron-poor segments, are
particularly attractive for their ambipolar charge injection
(electrons and holes) and transport properties. These
systems are of interest for a variety of electronic applica-
tions for which balancing the transport properties of both
holes and electrons is important (OLEDs, PVs, etc.).5
Conjugated D-π-A systems are promising new materials
for photonic applications (two-photon absorption, all opti-
cal computing, signal processing, etc.), since their inherent
dipole enhances the nonlinear optical response properties for
the molecule.6,7 Finally, the development of low band gap
materials (organic PVs and conductors) in which intramole-
cular charge transfer between donor and acceptor fragments
advantageously perturbs the HOMO and LUMO energy
levels has progressed as new D-π-A systems have become
available.8
Generally, conjugated organic π-systems that behave as
electron-donating, hole-transporting (p-type) materials
are readily available;9 however, organic compounds that
exhibit efficient electron-accepting, electron-conducting
(n-type) properties are much less developed.4,10 Useful
n-type organic materials possess a low barrier to electron
injection and effectively transport a negative charge.
While it is generally accepted that a low-energy LUMO
should facilitate charge injection, the attributes that con-
tribute to high electron mobility are not well understood
and include the nature and length of the conjugated
π-system, the strength of intermolecular interactions,
and various elements of device construction (including
surface passivation, assembly techniques and processing
conditions).5,11 Electron-deficient π-systems are thought
to be associated with high electron mobility, and pyri-
dines, oxadiazoles, perylene diimides, metalloles and
fluorinated aromatics have been identified as potential
n-type materials.12-17 Additionally, close cofacial intermole-
cular packing has been associated with increased charge
transport in a number of organic systems.9 Therefore, mole-
cular organic building blocks that contain an electron-
deficient, extended π-system, a stabilized LUMO, and close
cofacial stacking in the solid state would be of great interest
to the development of this field.
Metalloles (A), which are five-membered heterocycles
containing a heteroatom and a diene moiety, represent a
potentially large class of compounds of particular interest
to organic electronic materials research, since their struc-
tures should be readily modified to independently alter
electronic and physical properties. Metalloles are asso-
ciated with low energy LUMO levels, resulting from
stabilization of the diene π-system via interaction with
the heteroatom based σ* oribtals.18,19 The LUMO energy
level is generally unaffected by minor variations in the
R groups on the heteroatom,20,21 which can be tuned to
modify solubility and packing properties. Siloles (A, E =
Si) in particular have exhibited exceptionally good perfor-
mance in OLED devices, and relatively high electron mobi-
lities have been measured.14a,22 Crystal structures of many
silole-based materials have revealed poor intermolecular
cofacial interactions due to orientation of the aryl groups
that usually occupy the 3- and 4-positions (in many cases a
requirement for the silole synthesis), and this limits their
suitability for applications in which cofacial packing of
aromatic systems is important.14a,22
Main group compounds containing the dibenzometallole
(or metallofluorene) (B; E = Si, Ge, Sn, etc.) fragment are
interesting candidates for n-type materials, since they possess
a planar aromatic structure while maintaining the LUMO-
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CHART 1
away from the aromatic π-system, rendering it electron-
deficient and more suitable for electron accepting and trans-
porting applications.16,17,26 Additionally, highly fluorinated
aromatic systems tend to pack cofacially, rather than in the
herringbone motif observed for many nonfluorinated aro-
matic molecules.27,28 Perfluorometallofluorenes (C) are a
relatively unexplored class of molecules that combine these
two elements of molecular design and should produce a
series of molecules with low LUMO energies and electron-
deficient π-systems that exhibit cofacial π-stacking in the
solid state.
a variety of 2,7-diethynyl functionalized heterofluorenes
(eq 1).21
In addition to metalloles based on group 14 elements,
the analogous phosphorus-based systems (phospholes)
have shown great promise for a number of electronic appli-
cations.19,29 Phospholes possess several interesting char-
acteristics, including low aromatic character and σ*-π
conjugation (similar to that observed for metalloles),
which make them good candidates for organic electronic
applications.19,29 Phosphole-based systems have been
shown to function as hole-transporting materials; how-
ever, the related phosphole oxide derivatives possess a
more electron-deficient π-system. Thus the combination
of PR or P(O)R groups with the highly electron-deficient
perfluorofluorene framework should afford compounds
with high electron affinities.20,21
The development of efficient organic photonic and
electronic devices should benefit from the discovery of
new electron-deficient materials that possess the desired
physical and electronic characteristics. In pursuit of
such compounds, this laboratory has developed a
facile route to the 2,20-dibromo-4,40-diethynyl-3,30,5,50,6,
60-hexafluorobiphenyl moiety, which can easily be converted to
To date, poly- and oligothiophene derivatives are among
the most efficient organic hole transport materials for PV
and FET devices.30 This is a direct result of the electron-rich
nature of the thiophene ring and the ability of these rings to
stack in a cofacial manner. These characteristics make
oligothiophenes the ideal candidate for the donor fragment
in the initial studies on perfluoroheterofluorene-containing,
donor-acceptor (DA) compounds. In this report we describe
the synthesis and optical and electrochemical characteriza-
tion of a variety of thiophene-perfluoroheterofluorene DA
compounds (Chart 1). The synthesis and characterization of
these electron-deficient compounds provide a new series of
organic donor-acceptor materials for evaluation in various
applications.
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SCHEME 1
SCHEME 2
Results and Discussion
20-dibromooctafluorobiphenyl. The donor fragment, 2-ethynyl-
20-hexyl-5,50-bithiophene, was synthesized by a Pd-catalyzed
Sonagashira coupling reaction of 2-bromo-20-hexyl-5,50-bithio-
phene with ethynyltrimethylsilane, followed by trimethylsilyl
deprotection with K2CO3 in a mixture of MeOH/THF/H2O.
Synthesis of Donor-Acceptor Compounds. The synthesis
of heptafluoroheterofluorene-based DA compounds is de-
scribed in Schemes 1 and 2. The alkynyl functionality of 1
was installed via a nucleophilic aromatic substitution reac-
tion of triisoproylsilyl lithium acetylide with 2,20-dibromooc-
tafluorobiphenyl under conditions similar to those described
previously to prepare the dialkyne.21 The triisopropylsilyl
group was removed by addition of nBu4NF to a dilute
(0.0001 M) toluene/THF solution (∼2% THF) of the
4-(triisopropylsilylethynyl)-2,20-dibromo-3,30,40,5,50,6,60-hep-
tafluorobiphenyl intermediate, and the unprotected diyne
was then capped with C6F5 groups by an in situ Pd-catalyzed
Sonagashira coupling reaction with C6F5I at 45 °C to
give 1 in 43% yield based on the starting material 2,
n
Addition of 1 equiv of BuLi to a THF solution of the donor
alkyne produced the alkynyl lithium nucleophile in situ. The
alkynyl lithium solution was then added to a solution of 1 in
toluene at 0 °C to produce 2 (Br2-DA) in moderate conversion
(∼75% as determined by 19F NMR). Compound 2 was isolated
via column chromatography in 59% yield. Lithiation of 2 with
nBuLi in Et2O/toluene at -78 °C, followed by the addition of the
appropriate dichloroheteroatom reagent, produced the desired
DA products (Si-DA, Ge-DA, P-DA) in quantitative conversion,
leading to good isolated yields in all cases.
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SCHEME 3
Although the use of SNArF to link the donor and acceptor
fragments (as described above) is suitable for small DA
precursor molecules like 2, it is not sufficiently selective for
longer DA oligomer or polymer syntheses. To solve this
problem the synthetic pathway illustrated in Scheme 2, featuring
1-iodo-4-bromo-tetrafluorobenzene as a linker, was devel-
oped. This unsymmetrical linker was made in about 90%
conversion (as determined by 19F NMR) from 1,4-dibromo-
tetrafluorobenzene. The 1-iodo-4-bromotetrafluorobenzene
was selectively coupled with 2-ethynyl-20-hexyl-5,50-bithio-
phene at the iodo position via a Pd-catalyzed Sonagashira
coupling reaction at 45-50 °C (higher temperatures resulted
in competitive coupling at the bromo position) to form 3 in
75% isolated yield. In order to successfully couple 3 with
2,20-dibromo-4-ethynyl-3,30,40,5,50,6,60-heptafluorobiphenyl
(the acceptor monoalkyne fragment), the bromine of 3
must be exchanged for iodine. This was easily accomplished
by addition of the appropriate dichloroheteroatom reagent
(Scheme 3).
To investigate the effect of an alternate positioning of
donor and acceptor fragments within the molecule, a germa-
nium-based acceptor-donor-acceptor (ADA) molecule
(Ge-ADA) was synthesized as described in Scheme 4. The
starting compound 6 was obtained in nearly quantitative
yield by lithiation at the 5,5000 positions of 3,3000-dihexyl-
n
2,20,200,2000,50,500-quaterthiophene31 with BuLi, followed by
addition of I2. Coupling of 6 with an excess of ethynyltri-
methylsilane via a Pd-catalyzed Sonagashira coupling, fol-
lowed by trimethylsilyl deprotection with K2CO3 in a
mixture of MeOH/THF/H2O, produced the synthetic inter-
mediate 2,2000-diethynyl-4,4000-dihexyl-5,20,200,5000,50,500-qua-
terthiophene which must be quickly used and handled in
nonpolar solvents to avoid decomposition.
The 1-iodo-4-bromo-tetrafluorobenzene linker was coupled
with the “donor” diyne via a Pd-catalyzed Sonagashira
coupling reaction to form 7 in high conversion (>85% by
19F NMR spectroscopy, 42% isolated yield). Lithium-halogen
exchange of 7 with nBuLi at -78 °C, followed by the addition
of a solution of I2 in THF, resulted in quantitative conver-
sion to 8 (97% isolated yield). Coupling of 8 with 2 equiv of
the “acceptor” monoalkyne allowed high conversion to the
ADA precursor 9 (Br2-ADA, 95% by 19F NMR spectrosco-
py, 67% isolated yield). The final Ge-ADA compound was
formed by the lithiation of 9 with nBuLi in THF/toluene at
-78 °C, followed by addition of dichlorodiethylgermane.
Computational Studies. To further investigate the electronic
structures of the DA compounds, minimized structures of Si-
DA0 and 2T-C6F4 (Figure1) werecalculatedusingtheGaussian
98 suite of programs. Minimized structures and energies were
determined using the B3LYP/6-31G(d) level of theory. For ease
of calculation, the hexyl groups on the bithiophene fragment
n
by lithium-halogen exchange of 3 with BuLi at -78 °C
followed by addition of I2 in toluene, to produce 4 in 98%
isolated yield. Coupling of 4 with the acceptor monoalkyne
allowed for quantitative conversion to 2 (as determined
by 19F NMR spectroscopy; 56% isolated yield). The syn-
thetic pathway outlined in Scheme 2 is more amenable to
the production of DA-containing oligomers and polymers
(as compared to that in Scheme 1).
To explore the effect of molecular length and symmetry on
the properties of these DA systems, a series of donor-
acceptor-donor (DAD) compounds (Si-DAD, Ge-DAD,
P-DAD) were also synthesized (Scheme 3). The synthesis of
DAD precursor 5 (Br2-DAD) followed a procedure similar to
that for the E-DA precursor 2, except that the “acceptor”
diyne, 2,20-dibromo-4,40-diethynyl-3,30,5,50,6,60-hexafluoro-
biphenyl, was used in place of the monoyne and two equiv of
4 were required in the final coupling step. Pure 5 was isolated
via column chromatography in 90% yield. E-DAD com-
pounds Si-DAD, Ge-DAD, and P-DAD were formed by the
lithiation of 5 and nBuLi in Et2O/toluene at -78 °C, followed
(31) Kanato, H.; Takimiya, K.; Otsubo, T.; Aso, Y.; Nakamura, T.;
Araki, Y.; Ito, O. J. Org. Chem. 2004, 69, 7183.
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SCHEME 4
communication across the entire molecule. As illustrated in
Figure 2b,c the HOMO and LUMO orbitals of Si-DA0 have
significant contributions from the central 1,4-diethynyl-tetra-
fluorophenyl moiety. However, the HOMO orbital density is
shifted substantially toward the bithiophene substituent with
very little contribution from the heptafluorosilafluorene frag-
ment. The LUMO orbital density is delocalized across the
entire molecule; however, the largest contributions come from
the fluorinated fragments, with only minimal contribution
from the terminal thiophene ring. Similar results were obtained
with 2T-C6F4 (Figure 3).
FIGURE 1. Compounds used in computational studies.
were modeled as methyl groups. Of particular interest is the
distribution of the HOMO and LUMO orbital wave functions.
One extreme possibility would involve a somewhat even distribu-
tion of the orbital densities across the entire molecule, thus
resembling a more purely p- or n-type system.14a,32 Alternatively,
the HOMO and LUMO molecular orbitals could be concentra-
ted on the electron-rich and electron-poor regions, respectively,
producing characteristics more typical of push-pull systems.33
Results from the energy minimization of Si-DA0 (Figure 2a)
and 2T-C6F4 (Figure 3a) predict a highly planar overall
structure for the ground state, which should favor electronic
This predicted asymmetry in the HOMO and LUMO
orbital distributions indicate that these planar molecules
exhibit a break in conjugation. This orbital character could
(33) (a) Huang, J.; Fu, H.; Wu, Y.; Chen, S.; Shen, F.; Zhao, X.; Liu, Y.;
Yao, J. J. Phys. Chem. C. 2008, 112, 2689. (b) Rajkumar, G. A.; Sandanayaka,
A. S. D.; Ikeshita, K.-I.; Itou, M.; Araki, Y.; Furusho, Y.; Kihara, N.; Ito, O.;
Takata, T. J. Phys. Chem. A. 2005, 109, 2428. (c) Takahashi, S.; Nozaki, K.;
Kozaki, M.; Suzuki, S.; Keyaki, K.; Ichimura, A.; Matsushita, T.; Okada, K.
J. Phys. Chem. A. 2008, 112, 2533. (d) Peng, Q.; Park, K.; Lin, T.; Durstock,
M.; Dai, L. J. Phys. Chem. B. 2008, 112, 2801.
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TABLE 1. Selected Electrochemical Data for DA Compounds
LUMOa
(eV)
HOMOa
(eV)
Eg
(eV)
Eg
(eV)
EC
opt
Si-A
Ge-A
P-A
4
2 (Br2-DA)
Si-DA
Ge-DA
P-DA
5 (Br2-DAD)
Si-DAD
Ge-DAD
P-DAD
Ge-ADA
-3.2b
-3.2b
-3.4b
-2.6
-2.8
-3.3
-3.3
-3.5
-6.5b
-6.5b
-6.6b
-5.7
-5.7
-5.7
-5.7
-5.7
-5.7
-5.7
-5.7
-5.8
-5.5
3.3
3.3
3.2
3.1
2.7
2.7
2.7
2.6
2.7
2.6
2.7
2.6
2.4
3.1
2.9
2.4
2.4
2.2
-3.4
-3.4
-3.6
-3.3
2.4
2.4
2.2
2.2
aReferenced to Fc/Fcþ (HOMO energy level -4.8 eV below vacuum).34
bMeasured in MeCN with 0.1 M Bu4NPF6; calculated HOMO energy
opt
.
from HOMO energy = LUMO energy - Eg
Electrochemical Studies on Donor-Acceptor Compounds.
The solution-state redox behavior of all compounds was
investigated via differential pulse voltammetry (reduction
and oxidation potentials) and the relevant results are sum-
marized in Table 1. All measurements were performed in a
CH3CN/toluene electrolyte solution (0.1 M solution of
tetrabutylammonium hexafluorophosphate in CH3CN/
toluene) with a Ag wire reference electrode. HOMO and
LUMO energy levels were calculated with respect to the
appropriate ferrocene-ferrocenium (Fc/Fcþ) oxidation or
reduction (Fc HOMO energy of -4.8 eV relative to vacuum).34
Also included in Table 1 are results from the acceptor-only
compounds (E-A) (2,7-bis(pentafluorophenylethynyl)-9,9-
dimethyl-hexafluorosilafluorene (Si-A), 2,7-bis(pentafluor-
ophenylethynyl)-9,9-diethyl-hexafluorogermafluorene (Ge-A),
and 2,7-bis(pentafluorophenylethynyl)-9-ethyl-hexafluoropho-
sphafluorene oxide (P-A)).21
FIGURE 2. Gas-phase computational results for Si-DA0. (a) Opti-
mized ground-state geometry emphasizing the planarity of the
calculated structure. (b) Calculated HOMO wave function with
the majority of the orbital density localized on the bithiophene
portion of the molecule. (c) Calculated LUMO wave function.
The measured LUMO energy levels for the DA com-
pounds, -3.3 to -3.6 eV, are in a range established for
useful n-type conducting materials.4,16,35-37 These LUMO
energy levels are significantly lower than those of related
nonfluorinated heterofluorenes (-2.1 to -2.6 eV)35-37 and
only slightly lower than those observed for the related 2,7-
bis(pentafluorophenylethynyl)-hexafluoroheterofluorenes.21
This relatively small decrease in LUMO energy (e.g., Si-A vs
Si-DAD), despite a significant increase in conjugation
length with the addition of the donor fragment, suggests
that the thiophene moieties contribute only slightly to the
LUMO energy. Additionally, the similarity of the LUMO
energy levels of Ge-DA and Ge-ADA compounds suggests no
greater delocalization of the LUMO orbital in Ge-ADA
despite an effective increase of the conjugation length of
the molecule.
FIGURE 3. Gas-phase computational results for 2T-C6F4. (a) Op-
timized ground-state geometry emphasizing the planarity of the
calculated structure. (b) Calculated HOMO wave function with the
majority of the orbital density localized on the bithiophene. (c)
Calculated LUMO wave function.
produce significant charge reorganization in a photoexcited
state, perhaps even the generation of a charge-separated
excited state.33 Experimental evidence for this would be
interesting, as charge separation is required for the genera-
tion of unbound charge carriers in PV applications.33d
Additionally, significant differences in HOMO and LUMO
orbital wave functions can give rise to a large change in
dipole between the ground state and the excited state, which
has been shown to produce materials with interesting non-
linear optical properties.7 Finally, the noncontinuous nature
of the HOMO and LUMO orbital distributions suggests that
oligomers and polymers of these D-A units would not exhibit
extended conjugation lengths and a corresponding, reduced
band gap, as has been observed for many DA polymers.32
The HOMO energy levels of the DA compounds, -5.5 to
-5.8 eV, represent a significant increase over the calculated
HOMO energy of the acceptor-only molecules (Si-A, Ge-A,
and P-A) of around -6.5 eV and correlate quite well with
reported bithiophene and quaterthiophene HOMO energy
(34) Koepp, H. M.; Wendt, H.; Strehlow, H. Z. Elektrochem. 1960, 64,
483.
(35) Huang, T.-H.; Whang, W.-T.; Shen, J. Y.; Wen, Y.-S.; Lin, J. T.; Ke,
T.-H.; Chen, L.-Y.; Wu, C.-C. Adv. Funct. Mater. 2006, 16, 1449–1456.
(36) Mo, Y.; Tian, R.; Shi, W.; Cao, Y. Chem. Commun. 2005, 4925.
(37) Lee, B. L.; Yamamoto, T. Macromolecules 1999, 32, 1375–.
J. Org. Chem. Vol. 75, No. 6, 2010 1877

Geramita et al.
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Many donor-acceptor systems exhibit significant orbital
delocalization across both donor and acceptor fragments,
resulting in HOMO and LUMO energy levels that are greatly
influenced by both fragments.8a,20,39 However, there are also
numerous examples in which the LUMO energy level is
dependent solely on the electron-withdrawing fragment
and the HOMO energy level is governed predominantly by
the electron-donating fragment.38,40,41 The compounds in
this study demonstrate redox chemistry that is consistent
with the second type of DA system, as both the reduction and
oxidation potentials, as well as the reversibility behavior,
seem to correlate with the individual donor or acceptor
fragments. These types of donor-acceptor systems have
attracted considerable interest with respect to their potential
involvement in applications that require charge separation
for optimal device performance (e.g., PV).33d
Spectroscopic Characterization of Donor-Acceptor Com-
pounds. All UV-vis spectra were recorded in THF and toluene
solutions, and relevant data are summarized in Table 2. The
opt
corresponding optical HOMO-LUMO energy gaps (Eg
)
were calculated from the absorption onset wavelength,
which was taken to be the intersection of the leading edge
tangent with the x-axis in a standard absorption vs wave-
length plot. Also included in Table 2, for comparison, are
results from the pure acceptor compounds (E-A) (2,7-bis-
(pentafluorophenylethynyl)-9,9-dimethyl-hexafluorosilafluorene
(Si-A), 2,7-bis(pentafluorophenylethynyl)-9,9-diethyl-hexa-
fluorogermafluorene (Ge-A), and 2,7-bis(pentafluoro-
phenylethynyl)-9-ethyl-hexafluorophosphafluorene oxide
(P-A)), and the precursor compounds 2 (Br2-DA), 5 (Br2-
DAD), 9 (Br2-ADA), and 4,40-bis(ethynylpentafluoro-
phenyl)-2,20-dibromo-hexafluorobiphenyl (Br2-A).
The UV-vis spectra of all DA compounds exhibit at least
two intense peaks, as illustrated in Figure 5a for the germa-
fluorene-containing series Ge-DA, Ge-DAD, Ge-ADA.
(Note: the second peak for the Ge-DAD compound appears
as a high energy shoulder on the lower energy peak.) Molar
absorptivities, ε, were measured for a representative subset
of compounds and are reported in Table 2. The strong
intensities of the lowest energy transitions (ε = 37 000-
100 000 M-1 cm-1), coupled with the observation that ε
values for Br2-DAD (5) and Ge-DAD are twice those of Br2-
DA (2) and Si-DA (Table 2), respectively, allow the assign-
ment of the lowest energy peak to a π f π* transition
associated with the thiophene fragment. This conclusion is
further supported, in Figure 5b, by the similarity in shape
between the absorption spectra of 20-ethynyl-5,50-bithio-
phene (in toluene) and compound 4, (5-(1-ethynyl-4-iodo-
tetrafluorophenyl)-50-hexyl-2,20-bithiophene in THF), the
low-energy peaks of 2 (Br2-DA) and 5 (Br2-DAD), and the
molar absorptivity value for 4 (ε = 31 000, consistent with
a π f π* transition).
FIGURE 4. Cyclic voltammetry curves for the E-DA series. The
dotted lines represent subsequent irreversible reductions.
levels (-5.65 and -5.45 eV, respectively).38 Evaluation of the
redox behavior of 4 reveals the similarity of the HOMO
energy levels for compounds 4, 2 (Br2-DA), E-DA, and E-
DAD, which corroborates the theory that the donor frag-
ment is chiefly responsible for the oxidation behavior of
these compounds. An increase in the length of the donor
fragment, from bithiophene to quarterthiophene, results in a
notable destabilization of the HOMO energy (of 0.2 eV),
further supporting the participation of this fragment in the
HOMO orbital distribution. Finally, the similarity of the
HOMO energy levels of the E-DA and E-DAD compounds
suggests there is no electronic communication between the
two bithiophene units in the latter, disubstituted E-DAD
compounds.
The reversibility of the reduction behavior for compounds
Si-DA, Ge-DA, and P-DA was investigated via cyclic vol-
tammetry (CV) (Figure 4). As has been observed in previous
studies, the reversibility of the solution-state reduction of the
fluorinated heterofluorenes varies with changes in the het-
eroatom.20,21 P-DA exhibits two reversible reduction peaks,
and this behavior is consistent with previously studied
fluorinated phosphafluorene oxide molecules.21,22 Both Si-DA
and Ge-DA exhibit multiple reduction events, although only the
first of these can be considered either quasi-reversible (Si-DA)
or reversible (Ge-DA). This reversibility behavior is consistent
with reduction events dominated by the heterofluorene ring
system.20,21 Oxidation behavior, when observed, is always
irreversible. Poor solubility of the E-DAD and Ge-ADA com-
pounds in the electrolyte solution precluded the collection
of meaningful CV data; thus it is not possible to make
conclusions about the reversibility of redox events for these
compounds.
(39) (a) Aviram, A.; Ratner, M. Chem. Phys. Lett. 1974, 29, 277.
(b) Sonar, P.; Singh, S. P.; Sudhakar, S.; Dodabalapur, A.; Sellinger, A.
Chem. Mater. 2008, 20, 3184. (c) Sung, H.-H.; Lin, H.-C. Macromolecules
2004, 37, 7945.
(40) (a) Bauer, P.; Wietasch, H.; Lindner, S.; Thelakkat, M. Chem. Mater.
2007, 19, 88. (b) Wan, J.-H.; Feng, J.-C.; Wen, G.-A.; Wei, W.; Fan, Q.-L.;
Wang, C.-M.; Wang, H.-Y.; Zhu, R.; Yuan, X.-D.; Huang, C.-H.; Huang,
W. J. Org. Chem. 2006, 71, 2565. (c) Tonzola, C. J.; Alam, M. M.; Jenekhe, S.
A. Macromolecules 2005, 38, 9539.
(38) Hancock, J. M.; Gifford, A. P.; Champion, R. D.; Jenekhe, S. A.
Macromolecules 2008, 41, 3588.
(41) Strehmel, B.; Sarker, A. M.; Malpert, J. H.; Strehmel, V.; Seifert, H.;
Neckers, D. C. J. Am. Chem. Soc. 1999, 121, 1226.
1878 J. Org. Chem. Vol. 75, No. 6, 2010

Geramita et al.
JOCArticle
TABLE 2. Steady-State Solution Optical Characterization Data for DA Compounds
THF
toluene
Egopt (eV)a ems (nm)b
λ
abs (nm) (ε, M-1 cm-1
)
Egopt (eV)a
λ
ems (nm)b
ΦPL
λ
abs (nm)
349
ΦPL
c
c
λ
Br2-A
Si-A
306
349
3.8
3.3
3.3
3.2
2.7
2.7
2.7
2.6
2.7
2.6
2.7
2.6
2.4
2.4
378
376
391
∼1
∼1
∼1
Ge-A
P-A
347 (61 000)
355
3.3
378
∼1
2 (Br2-DA)
Si-DA
Ge-DA
P-DA
5 (Br2-DAD)
Si-DAD
Ge-DAD
P-DAD
9 (Br2-ADA)
Ge-ADA
405 (37 000)
408 (50 000)
406
412
406 (76 100)
418
413 (100 000)
417
444
544
538
579
556
574
558
532
0.12
0.13
0.03
409
408
409
2.7
2.7
2.6
481
482
502
0.30
0.30
0.28
0.04
0.08
0.02
421
414
418
2.6
2.7
2.6
488
485
512
0.33
0.33
0.38
440
592
0.07
447
2.4
550
0.28
aHOMO-LUMO energy gaps (Egopt) were calculated from the absorption onset wavelength, determined by the intersection of the leading edge
tangent with the x-axis. bEmission energies were measured with 350 nm excitation wavelength light. cPhotoluminescence quantum yields calculated with
respect to 9,10-diphenylanthracene in THF (ΦPL = 0.9) and are corrected for the solvent refractive index when using toluene.
The higher energy peaks (λb
∼310 nm) are associated
and electron-poor fragments, respectively.33,40 In this situa-
tion, an increase in conjugation length is achieved through
modification of the donor and acceptor fragments but not by
oligomerization/polymerization of the DA units.
max
with a π f π* transition localized on the electron-deficient
portion of the molecules. This assignment is supported by the
strong intensities of these peaks (ε = 28 000-70 000 M-1
cm-1) and by the red shift observed in the λbmax values upon
closure of the heterofluorene ring (cf. Br2-DAD, λbmax = 322 nm
and Ge-DAD, λbmax = 362 nm). This red shift in λbmax values for
the DA compounds is similar to the red shift observed upon
closure of the heterofluorene ring in the pure acceptor com-
pounds (cf. Br2-A, λmax = 306 nm and Ge-A, λmax = 347 nm,
Table2). (ForBr2-DAD and Ge-DAD, λbmax values appear to be
slightly red-shifted with respect to the corresponding values for
Br2-A and Ge-A, as a result of overlap with the bithiophene
absorption).
On the basis of the spectroscopic and electrochemical
data, it appears that the DA compounds in this study are
better described by the latter electronic structure, with
HOMO and LUMO orbital densities that are substantially
localized on the thiophene- and heterofluorene-containing
portions, respectively. This situation is generally observed
for DA systems that possess a definitive conjugation break
between donor and acceptor moieties, resulting from a
saturated organic linker or from a geometric constraint
which forces the donor and acceptor π-systems into a
mutually perpendicular alignment.33a-c However, as shown
by Huang and co-workers with a thiophene-oxadiazole
system40b and by Neckers and co-workers with partially
fluorinated oligophenylenevinylenes,41 there is some prece-
dence for “orbitally segregated” electronic structures in
planar π-systems.
opt
The observed Eg values for the donor-acceptor com-
pounds in the study are lower than those observed for the
pure acceptor compounds by 0.5-0.9 eV. This decrease in
opt
Eg
is expected, since addition of the donor fragment
should increase the conjugation length of the molecule.
Interestingly, there is very little difference in the absorption
behavior of the E-DA and E-DAD series of compounds
(Table 2, Figure 6), which suggests that the HOMO and
LUMO orbitals are not fully delocalized over the entire
molecule. These conclusions are supported by the similarity
Compounds with this orbital distribution often exhibit
spectroscopic evidence for some degree of charge separation
in the ground and/or excited states.33,40,41 This charge se-
paration can result in complete transfer of an electron from
the donor to the acceptor, i.e., DA f DþA-, or may involve
only a partial redistribution of charge (DδþAδ-, where δ < 1).
Spectroscopic evidence for the formation of a highly polarized,
charge-separated state may appear in the absorption behavior
opt
in Eg values for the Br2-DAD and E-DAD compounds
(Table 2), as the lack of a significant decrease in Egopt upon
closure of the heterofluorene ring implies a break in con-
jugation between the two bithiophene fragments, which is
consistent with the electrochemical behavior described pre-
viously.
opt
(including solvent polarity dependences of λmax and Eg
values and/or the emergence of a lower energy charge transfer
band) or in the emission behavior (including a large Stokes
shift between λmax,abs and λmax,ems values and a dependence of
In general, DA compounds may exhibit varying degrees of
orbital mixing between the donor and acceptor fragments, as
discussed in the pioneering work of Avrim and Ratner on
molecular rectifiers.39a If the HOMO and LUMO orbitals are
delocalized across significant portions of the molecule, the
corresponding energy levels are expected to be significantly
influenced by both electron-rich and electron-poor frag-
ments.39 For these molecules, extension of the π-system via
linking of DA units or by extension of the donor and/or
acceptor fragments will generally result in an increase in
λ
of the absorption behavior of the DA compounds in
max,ems and ΦPL values on solvent polarity).42 Examination
toluene (dielectric constant ε = 2.38) and THF (ε = 7.52)
(Table 2 and Figure 7) reveal a lack of solvatochromism
opt
in λmax,abs and Eg
values and no evidence for a direct
transition from the ground state to a charge-separated excited
state.43
conjugation lengthand a decrease inEg .
opt 39 Different proper-
ties result from DA systems in which the HOMO and LUMO
orbital densities are primarily localized on the electron-rich
(42) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic/Plenum Publishers: New York, 1999.
(43) Neuteboom, E. E.; Meskers, S. C. J.; Beckers, E. H. A.; Chopin, S.;
Janssen, R. A. J. J. Phys. Chem. A 2006, 110, 12363.
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Geramita et al.
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FIGURE 6. UV-vis spectra of the E-DA and E-DAD series in
THF, illustrating the similarity in Egopt values.
FIGURE 7. UV-vis spectra of the Ge-series of compounds in THF
and toluene, illustrating the lack of solvatochromism in the absorp-
tion behavior.
observed for 2,7-bis(ethynylpentafluorophenyl)-hexafluoro-
heterofluorenes (∼20 nm)20,21 and for oliogothiophenes
(∼70 nm).44 Additionally, the photoluminescence quantum
yields (ΦPL) for the DA compounds in THF are 1-2 orders of
magnitude lower than those observed for the acceptor-only
compounds (E-A, Table 2). This dramatic increase in Stokes
shifts and the decrease in ΦPL values suggest the possibility of
an intramolecular charge separation accompanied by molec-
ular reorganization in the excited state.42 With toluene as the
solvent, the emission maxima of the donor-acceptor com-
pounds are significantly blue-shifted, and the ΦPL values are
3-10 times higher than those observed in THF solvent. This is
consistent with less structural reorganization in the excited
state in a nonpolar solvent. A more extensive study of the
solvent-dependent emission behavior for Ge-DAD (Figure 8)
reveals a significant red shift in the emission λmax value with
increasing solvent polarity (dielectric constant for each solvent
is provided in the legend). This strong solvatochromism is also
FIGURE 5. Absorption spectra for DA compounds in THF. (a)
Changes in absorption for the Ge-series. (b) Changes in absorption
spectra with extension of the bithiophene fragment. (c) Comparison
of the absorption spectra of acceptor-only and DA compounds.
Photoluminescence spectra were recorded with an optical
density of <0.1 in THF and toluene, and relevant data are
summarized in Table 2. The emission λmax values for the DA
compounds in THF ranged from 530-590 nm and varied
with heteroatom, donor length, and acceptor length. These
emission wavelengths represent a Stokes shift of about
140-150 nm, which is significantly greater than those typically
(44) Sakamoto, Y.; Komatsu, S.; Suzuki, T. Synth. Met. 2003, 133-134,
361.
1880 J. Org. Chem. Vol. 75, No. 6, 2010

Geramita et al.
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FIGURE 9. Emission spectra for Ge-DAD in cylcohexane (298 K),
THF (298 K), and 1:4 methanol/ethanol (77 K), λex = 380 nm.
FIGURE 8. Normalized emission spectra for Ge-DAD illustrating
the red shift in emission λmax value with increasing solvent polarity.
The dielectric constant for each solvent is provided in the legend.
in agreement with the possible formation of a charge-separated
excited state. Additionally, the absence of vibronic structure in
solvents expected to better stabilize a polarized intermediate
(versus that observed in cyclohexane) is consistent with the
idea that emission originates from a charge-separated excited
state. The increase in noise observed for the high dielectric
constant solvents is due to the decreased emission intensity
with these samples, although precise ΦPL values were not
calculated (note that Figure 8 reports normalized emission).
Photoinduced intramolecular charge transfer processes,
which usually follow Marcus theory, are generally slower at
lower temperatures. Thus luminescence spectroscopy at 77K
was expected to reveal emission from local excited states only
(both singlet and/or triplet states) that are greatly enhanced
with respect to that from charge-separated states. The low-
temperature (77 K) emission spectrum of Ge-DAD (Figure 9)
reveals two major peaks at 475 and 635 nm. The high energy
emission (at 475 nm) correlates well with the room tempera-
ture emission of Ge-DAD in cyclohexane (in both vibronic
structure and energy) and thus can be assigned to emission
from the singlet local excited state. The structured lower
energy band (λ = 635 nm and λ = 590 nm) can be assigned to
emission from the triplet local excited state.42 The lack of
defined structure and the clear solvatochromism in the room
temperature emission behavior of Ge-DAD further supports
the occurrence of emission from a charge-separated state in
all cases except cyclohexane.
Fluorescence lifetime (τ) measurements for Si-DA, Ge-
DA, and P-DA in toluene, dichloromethane, and THF reveal
decay behavior that is best modeled by a single exponential
decay. Fluorescence lifetimes exhibit little dependence on
solvent (τ ≈ 650 ps for Si-DA, Ge-DA and τ ≈ 700-850 ps for
P-DA) and confirm that the room temperature emission is
from a singlet state and not from a triplet state, as this would
result in much longer fluorescence lifetimes. Transient ab-
sorption spectroscopy experiments, which could identify the
presence of a radical anion and cation, have proven difficult.
The required excitation wavelength of 466 nm produces
a bleach signal across the absorption region for the
FIGURE 10. Absorption and emission of Ge-A and Ge-DAD
illustrating the overlap in the emission of Ge-A and the absorption
of Ge-DAD.
bithiophene radical cation (495 nm),45 and attempts to determine
the absorption spectrum for the germafluorene-based radical
anion via spectroelectrochemistry experiments have not yet been
successful. Attempts to perform TDDFT calculations were
unsuccessful as a result of issues with nonconvergence.
While spectroscopic data suggests the formation of a charge-
separated state in these DA compounds, other energy transfer
mechanisms cannot be completely ruled out. For example,
overlap between the emission spectra of the heterofluorene
fragments and the absorption spectra for the extended bithio-
phene fragments (as illustrated for Ge-DAD and its component
molecules in Figure 10) suggests that a resonance energy transfer
from the acceptor to the donor might be possible.40a,42,46
(45) Huang, J.; Fu, H.; Wu, Y.; Chen, S.; Shen, F.; Zhao, X.; Liu, Y.; Yao,
J. J. Phys. Chem. C 2008, 112, 2689.
(46) Bauer, P.; Wietasch, H.; Lindner, S.; Thelakkat, M. Chem. Mater.
2007, 19, 88.
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Geramita et al.
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TABLE 3. Selected PV Device Performance Data for Ge- and P-Based
Compounds
Voc
(V)a
Isc
(mA)b
Jsc
(mA/cm2)c
fill
factord
η
(%)e
P3HT-alone
0.040
0.107
0.563
0.298
0.092
0.542
0.905
0.743
-1.3
-1.95
-4.9
-7.0
-4.6
-0.8
-6.3
-1.8
-0.044
-0.065
-0.162
-0.234
-0.154
-0.025
-0.211
-0.064
0.253
0.265
0.333
0.272
0.258
0.303
0.184
0.255
0.0004
0.002
0.031
0.019
0.003
0.004
0.035
0.012
Ge-DA/P3HT
Ge-DAD/P3HT
Ge-ADA/P3HT
P-DA/P3HT
P-DAD/P3HT
Ge-A/P3HT
P-A/P3HT
aOpen circuit voltages (Voc) were calculated from the x-intercept (zero
current value) of the current vs applied potential data. bShort circuit
currents (Isc) were calculated from the y-intercept (zero voltage value) of
the current vs applied potential data. cShort circuit current densities (Jsc)
were calculated by Jsc = Isc/(pixel area). dFill factor is calculated from
the maximum power IV/IscVoc. eEfficiency (η) is calculated from η = Voc
Jsc fill factor/intensity of incoming light.
3
FIGURE 11. Applied voltage-current density performance for
P3HT alone and germanium donor-acceptor/P3HT series: Ge-DA,
Ge-DAD, Ge-ADA.
Preliminary Device Studies. Previous work has demon-
strated that photovoltaic (PV) devices fabricated with 2,7-
bis(pentafluorophenylethynyl)-hexafluoroheterofluorene/
poly-3-hexylthiophene (P3HT) blends have higher overall
efficiencies than devices made with P3HT alone.21 Addition-
ally, the tendency for 2,7-substituted hexafluoroheterofluor-
enes to pack cofacially in the solid state20,21 makes these DA
compounds excellent candidates for use as charge-transport-
ing materials in solar cells. Although the DA molecule
described here should be able to internally separate charge,
the length scale is much smaller than an exciton diffusion
length, so here we describe devices that use the DA to
separate charge within a matrix of P3HT hole transporter.
Preliminary device studies included evaluation of germa-
fluorene (Ge-DA, Ge-DAD, Ge-ADA) and phosphafluorene
oxide containing compounds (P-DA, P-DAD) for use as
electron transport materials in PV devices. Unoptimized
PV cells were made from 1:1 wt % blends of the DA
compound and P3HT that were spin-coated onto an ITO
surface from chloroform solution (10 mg/mL, ∼80 nm thick
films), followed by vapor deposition of an aluminum cathode.
The devices were tested under an argon atmosphere with an
oriel xenon arc lamp with an AM 1.5G solar filter, and the
results are summarized in Table3. Alsoincluded in Table 3 are
results from similar devices made using 2,7-bis(pentafluo-
rophenylethynyl)-9,9-diethyl-hexafluorogermafluorene (Ge-A)
and 2,7-bis(pentafluorophenylethynyl)-9-ethyl-hexafluoro-
phosphafluorene oxide (P-A).21 Figure 11 illustrates the
current density-applied voltage performance of the germa-
nium-based series, and additional current-voltage data is
available in Supporting Information.
As illustrated in Figure 11 and Table 3, the PV devices
made with blends of donor-acceptor compounds/P3HT
clearly exhibit improved performance over the P3HT-only
devices. This can be attributed to a much higher open
circuit voltage (Voc) in most of the blend systems, which
results in greater charge separation than in the pure P3HT
system. Additionally, in most cases, the short circuit cur-
rent density (Jsc) of the blend devices is significantly larger
than that of the pure P3HT system, which suggests that
incorporation of the DA molecules improves overall charge
transport in the devices. The compounds with longer
acceptor fragments have higher open circuit voltages,
generally resulting in higher overall efficiencies. The
germanium-based materials exhibit slightly better perfor-
mances with respect to their phosphine oxide-based coun-
terparts. In these devices, the presence of the thiophene
donor fragment does not improve PV performance. This
may be due to the short nature of the donor fragment used
(2 or 4 rings) or the small overall size of the DA molecules.
The development of longer DA oligomers may lead to
improved interactions between P3HT and the donor por-
tion of the DA compounds, resulting in improved inter-
molecular charge transfer.
The efficiencies of these devices are rather low
(0.002-0.031%) compared to those obtained for optimized
devices based on P3HT/PCBM (4-5%, PCBM = [6,6]-
phenyl-C61-butyric acid methyl ester)47 and P3HT/perylene
diimide (0.5-1%),48 which may be due to the short length scale
of the DA molecules. Further modifications to the structures of
such fluorinated, heterofluorene-based compounds to increase
donor and acceptor fragment lengths may provide more
efficient charge transport. Alternatively the application of these
shorter DA molecules might function effectively as blend
compatibilizers (both mechanically and electronically) in do-
nor and acceptor homopolymer blend systems.
Concluding Remarks
The oligothiophene-fluorinated heterofluorene systems
described here exhibit interesting donor-acceptor (DA)
properties. Results from calculations and the optical and
electrochemical characterizations suggest that the HOMO
and LUMO orbitals do not extend across the full length of
the molecules and that the HOMO and LUMO orbital wave
functions are mostly localized onto the donor and acceptor
fragments, respectively. From these results it can be con-
cluded that further reduction in the HOMO-LUMO energy
gap and subsequent improvement in solar radiation spec-
trum overlap will require extension of the individual donor
and/or acceptor moieties. Also, the conjugation lengths of
ꢀ
(47) Backer, S. A.; Sivula, K.; Kavulak, D. F.; Frechet, J. F. J. Chem.
Mater. 2007, 19, 2927.
(48) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li,
Y.; Zhu, D.; Kippelen, B.; Marder, S. J. Am. Chem. Soc. 2007, 129, 7246.
1882 J. Org. Chem. Vol. 75, No. 6, 2010

Geramita et al.
JOCArticle
these molecules cannot be increased solely through oligo-
merization or polymerization. While there is no evidence of
an intermolecular charge transfer (ICT) in the ground state,
the distinct solvatochromism in the photoluminescence sug-
gests a possible charge transfer in the excited state. Clearly
the excited-state behavior of the compounds is complex and
warrants further investigation.
Preliminary photovoltaic (PV) studies with DA/poly-3-
hexylthiophene (P3HT) blends demonstrate improved effi-
ciency over P3HT-only devices, although overall efficiencies
are still quite low (<0.05%). Current research is focused on
modification of both the donor and acceptor portions of the
molecule to improve solar radiation overlap and on devel-
opment of improved charge separation and charge transport
behavior.
4-Bromo-1-iodotetrafluorobenzene. 1,4-Dibromopentafluor-
obenzene (2.0 g, 6.5 mmol) was dissolved in toluene/Et2O
(100 mL/ 20 mL), and the resulting solution was cooled to
-78 °C. To this was added butyllithium (4 mL, 1.6 M in
hexanes), and the reaction mixture was stirred for 20 min. Iodine
(2.0 g, 7.9 mmol) was dissolved in Et2O (20 mL), the resulting
solution was added to the 1-bromo-4-lithiotetrafluorobenzene,
and the resulting reaction mixture was stirred for 10 min at -78 °C.
Aqueous Na2S2O3 was added, and the reaction mixture was
allowed to warm to room temperature with stirring over 1 h. The
reaction mixture was washed with water (2 ꢀ 50 mL,) and the
organic components were dried over MgSO4 and then filtered
through Celite. Rotary evaporation of the solution provided white
crystals of the following composition: 92% 4-bromo-1-iodotetra-
fluorobenzene, 5% 1,4-dibromotetrafluorobenzene, and 3%, 4-
dibromotetrafluorobenzene, as determined by 19F NMR and
GC:MS. 4-Bromo-1-iodotetrafluorobenzene: 19F NMR (CDCl3,
376 MHz, 25 °C) δ -119.67 (1F, m), -132.14 (1F, m); MS
354 (Mþ). 1,4-Dibromotetrafluorobenzene: 19F NMR (CDCl3,
376 MHz, 25 °C) δ -132.54 (s); MS 308 (Mþ). 1,4-Dibromotetra-
fluorobenzene: 19F NMR (CDCl3, 376 MHz, 25 °C) δ -119.40 (s);
MS 402 (Mþ).
2,20-Dibromo-4,40-diethynyl-3,30,5,50,6,60-hexafluorobiphenyl.
To a solution of 2,20-dibromooctafluorobiphenyl (0.230 g,
0.5 mmol) in toluene (25 mL) at room temperature was added
a solution of lithium triisopropylsilylacetylide (0.210 g, 1.1 mmol)
in THF (8 mL). The resulting solution was stirred for 3-5 days. The
resulting yellow solution was washed with water (2 ꢀ 30 mL). The
organic components were dried over MgSO4 and filtered through
Celite. The resulting solution was then diluted with wet toluene
(600 mL) and distilled THF (45 mL). The solution was sparged with
nitrogen for 20 min, and then Bu4NF (1.1 mL, 1.0 M in THF 1%
H2O) was added. The resulting mixture was stirred for 5 min. The
light brown solution was washed with water (2 ꢀ 150 mL), and the
organic components were dried over MgSO4 and passed through a
silica plug. The resulting solution was concentrated via rotary
evaporation. Note: It is critical at this step to not combine concen-
trated solutions of unprotected diyne with THF or CH2Cl2. This will
result in decomposition of the compound. The resulting solution was
absorbed onto silica and the diyne was isolated via column
chromatography and used without further purification. 1H NMR
(CDCl3, 400 MHz, 25 °C) δ3.737 (s); 19FNMR(CDCl3, 376 MHz,
25 °C) δ -102.99 (1F, m), -130.68 (1F, m), -137.38 (1F, m); MS
462 (Mþ), 383, 302.
2,20-Dibromo-4-ethynyl-3,30,40,5,50,6,60-heptafluorobiphenyl.
To a solution of 2,20-dibromooctafluorobiphenyl (0.500 g,
1.1 mmol) in toluene (20 mL) at room temperature was added a
solution of lithium triisopropylsilylacetylide (0.244 g, 1.3 mmol) in
THF (5 mL), and the resulting solution was stirred for 3-5 days.
The resulting yellow solution was washed with water (2 ꢀ 30 mL)
and the organic components were dried over MgSO4 and filtered
through Celite. The resulting solution was then diluted with wet
toluene (800 mL) and distilled THF (40 mL) and was then sparged
with nitrogen for 20 min. Under a flow of nitrogen, Bu4NF
(1.1 mL, 1.0 M in THF 1% H2O) was added, and the resulting
solution was stirred for 5 min. The light brown solution was
washed with water (2 ꢀ 150 mL) and the organic components
were dried over MgSO4 and passed through a silica plug. The
resulting solution was concentrated via rotary evaporation. Note:
It is critical at this step not to combine concentrated solutions of
unprotected alkyne with THF or CH2Cl2. This will result in decom-
position of the compound. The resulting solution was absorbed onto
silica and the monoyne was isolated via column chromatography
and used without further purification. 1H NMR (CDCl3, 400
MHz, 25 °C) δ 3.737 (s); 19F NMR (CDCl3, 376 MHz, 25 °C) δ
-101.86 (1F, m), -128.47 (1F, m), -130.68 (1F, m), -135.34 (1F,
m), -137.30 (1F, m), -150.90 (1F, m), -155.00 (1F, m). MS: 462
(Mþ), 383, 302.
Experimental Section
General. All reactions were performed under an inert atmo-
sphere of nitrogen using standard Schlenk techniques unless
otherwise specified. Tetrahydrofuran (THF) was dried over Na
and stored under nitrogen. Hexanes, Et2O, and toluene were
dried on a VAC systems solvent purifier column and stored
under nitrogen. Dichlorothethylphosphine, hydrogen peroxide
(30% in water), butyllithium (1.6 M in hexanes), tetrabutylam-
monium hexafluorophosphate, dibromotetrafluorobenzene,
and dichlorodiethylgermane were used as received. Dichlorodi-
methylsilane was distilled prior to use. Fluorescence quantum
yields (ΦPL) were calculated with respect to freshly sublimed 9,9-
diphenylanthracene in THF (Φ = 0.90). Fluorescence lifetime
measurements were performed using the time correlated single
photon counting (TCSPC) method. The system was equipped
with a Triax 320 and a Single Photon Counting Controller and a
440 nm LED NanLED, maximum repetition rate of 1 MHz and
a pulse duration <200 ps. Unless otherwise stated, 1H, 31P, and
19F NMR spectra were measured in CDCl3 with a 400 MHz
spectrometer. All chemical shifts are reported in ppm units.
For 19F NMR spectra C6F6 was used as internal reference at
-163 ppm and 1H NMR chemical shifts were referenced to the
residual peak of the deuterated CDCl3 at 7.26 ppm. Melting
point determinations are uncorrected. Cyclic voltammetry po-
tentials were measured versus a Ag wire reference electrode, with
Pt disk (PTE) as working electrode and a Pt wire axial electrode,
in acetonitrile/toluene (8:2) containing 0.1 mol/L tetrabutylam-
moniumhexafluorophosphate, with ferrocene as external stan-
dard (HOMO = -4.8 eV) and potential sweep rate of 100 mV/s
unless otherwise stated.
Lithium triisopropylsilylacetylene was prepared from triiso-
n
propylsilylacetylene and BuLi in hexanes. 2-Bromo-20-hexyl-
5,50-bithiophene was prepared by bromination of (5-hexyl-2,20-
bithiophene)49 with NBS. 2,20-Dibromooctafluorobiphenyl,50
2-bromo-3-hexyl-thiophene,51 and 3,3000-dihexyl-2,20,200,2000,50,-
500-tetrathiophene52,53 were prepared as described in the
literature.
The syntheses of intermediate compounds 4-bromo-1-iodotetra-
fluorobenzene, 2,20-dibromo-4-ethynyl-3,30,40,5,50,6,60-heptafluor-
obiphenyl, and 2,20-dibromo-4,40-diethynyl-3,30,5,50,6,60-hexafluo-
robiphenyl are described below.
(49) Sotgiu, G.; Zambianchi, M.; Barbarella, G.; Botta, C. Tetrahedron
2002, 58, 2245.
(50) Cohen, S.; Fenton, D.; Tomlinson, A.; Massey, A. J. Organomet.
Chem. 1966, 6, 301.
(51) Takahashi, M.; Masui, K.; Sekiguchi, H.; Kobayashi, N.; Mori, A.;
Funahashi, M.; Tamaoki, N. J. Am. Chem. Soc. 2006, 128, 10930.
(52) Kanato, H.; Takimiya, K.; Otsubo, T.; Aso, Y.; Nakamura, T.;
Araki, Y.; Ito, O. J. Org. Chem. 2004, 69, 7183.
(53) Radke, K. R.; Ogawa, K.; Rasmussen, S. Org. Lett. 2005, 7, 5253.
J. Org. Chem. Vol. 75, No. 6, 2010 1883

Geramita et al.
JOCArticle
2,20-Dibromo-4-pentafluorophenylethynyl-heptafluorobiphenyl
(1). To a solution of 2,20-dibromooctafluorobiphenyl (0.500 g,
1.1 mmol) in toluene (20 mL) at room temperature, a solution of
lithium triisopropylsilylacetylide (0.244 g, 1.3 mmol) in THF
(5 mL) was added, and the resulting solution was stirred for 3-5
days. The resulting yellow solution was washed with water (2 ꢀ
30 mL), and the organic components were dried over MgSO4
and filtered through Celite. The resulting solution was then
diluted with wet toluene (800 mL) and distilled THF (40 mL)
and was then sparged with nitrogen for 20 min. Under a flow of
nitrogen, Bu4NF (1.1 mL, 1.0 M in THF 1% H2O) was added,
and the resulting solution was stirred for 5 min. The light brown
solution was washed with water (2 ꢀ 150 mL), and the organic
components were dried over MgSO4 and passed through a silica
plug. The resulting solution was concentrated via rotary eva-
poration. Note: It is critical at this step not to combine concen-
trated solutions of unprotected alkyne with THF or CH2Cl2. This
will result in decomposition of the compound. The resulting
solution was absorbed onto silica, and the monoyne was isolated
via column chromatography. 2,20-Dibromo-4-ethynyl-3,30,40,-
5,50,6,60-heptafluorobiphenyl and iodopentafluorobenzene
(0.7 mL, 2.4 mmol) were combined with NEt3 (10 mL), and
the resulting solution was sparged with nitrogen for 10 min. The
degassed solution was added to a Schlenk flask containing
PdCl2(PPh3)2 (0.05 g, 0.07 mmol), CuI (0.06 g, 0.3 mmol), and
PPh3 (0.06 g, 0.2 mmol), and the entire mixture was heated at
40 °C for 12 h, with vigorous stirring, under a flow of nitrogen.
Toluene (20 mL) was added to the solution, and the resulting
mixture was washed with aqueous NH4Cl (2 ꢀ 20 mL) and water
(2 ꢀ 20 mL). The combined organic components were dried with
MgSO4, filtered through Celite, and concentrated via rotary
evaporation. The monosubstituted product was isolated via
column chromatography (hexanes) to give 0.295 g (0.47 mmol,
43%) of 1 as a white powder. 19F NMR (CDCl3, 376 MHz,
25 °C) δ -101.73 (1F, m), -128.35 (1F, m), -129.61 (1F, m),
-135.30 (1F, m), -135.41 (2F, m), -136.81 (1F, m), -150.43
(1F, m), -150.68 (1F, m), -154.86 (1F, m), -161.90 (2F, m).
Anal. Calcd for C28Br2F16: C, 38.25. Found: C, 38.11. Mp
132-135 °C.
0.22 mmol) was dissolved in toluene/THF (40 mL/10 mL) and
cooled to 0 °C. To this solution of 1 was added the alkynyl
lithium, and the reaction mixture was allowed to warm to room
temperature with stirring over 24 h. The resulting yellow solution
was quenched with water (2 ꢀ 15 mL), and the organic compo-
nents were extracted into CH2Cl2 (2 ꢀ 20 mL). The combined
extracts were dried over MgSO4, filtered through Celite, and
concentrated via rotary evaporation. The resulting solid was
absorbed onto silica gel and purified by column chromatography
(hexanes/toluene 100:5) to give 0.120 g (59%, 0.13mmol) of 2 as a
light orange powder.
Method 2. 2,20-Dibromo-4-ethynyl-3,30,40,5,50,6,60-hepta-
fluorobiphenyl (0.6 g, 1.3 mmol) and 4 (0.200 g, 0.36 mmol)
were dissolved in NEt3 (30 mL), and the solution was sparged
with nitrogen for 10 min. The degassed solution was added to a
Schlenk flask containing Pd(PPh3)4 (0.050 g, 0.04 mmol), CuI
(0.040 g, 0.21 mmol), and PPh3 (0.040 g, 0.15 mmol), and the
entire mixture was heated at 40 °C for 12 h, with vigorous
stirring, under a flow of nitrogen. Toluene (20 mL) was added to
the solution, and the resulting mixture was washed with aqueous
NH4Cl (2 ꢀ 20 mL) and water (2 ꢀ 20 mL). The organic com-
ponents were dried over MgSO4, filtered through Celite, and
concentrated via rotary evaporation. The resulting solid was
absorbed onto silica gel and purified by column chromatogra-
phy (hexanes/toluene 100:5) to give 0.180 g (56%, 0.2 mmol) of 2
as a light orange powder. 1H NMR (CDCl3, 400 MHz, 25 °C) δ
0.888 (3H, t), 1.3-1.4 (6H, m), 1.667 (2H, q), 2.806 (2H, t), 6.714
(1H, d), 7.039 (1H, d), 7.063 (1H, d), 7.325 (1H, d); 19F NMR
(CDCl3, 376 MHz, 25 °C) δ -101.59 (1F, m), -128.38 (1F, m),
-129.50 (1F, m), -135.27 (1F, m), -136.85 (3F, m), -137.56
(2F, m), -150.71 (1F, m), -154.87 (1F, m). Anal. Calcd for
C36H17Br2F11S2: C, 49.00; H, 1.94; S, 7.27. Found: C, 48.81; H,
1.80; S, 7.17. Mp 144-146 °C.
5-(1-Ethynyl-4-bromo-tetrafluorophenyl)-50-hexyl-2,20-bithio-
phene (3). Trimethylsilylacetylene (0.2 mL, 5 mmol) was com-
bined with NEt3 (30 mL), and the entire solution was sparged
with nitrogen for 10 min. The degassed solution was added to a
Schlenk flask containing 2-bromo-20-hexyl-5,50bithiophene
(0.180 g, 0.55 mmol), PdCl2(PPh3)2 (0.030 g, 0.03 mmol), CuI
(0.020 g, 0.10 mmol), and PPh3 (0.020 g, 0.08 mmol), and the
entire mixture was heated at 40 °C for 12 h, with vigorous
stirring, under a flow of nitrogen. Toluene (20 mL) was added to
the solution, and the resulting mixture was washed with aqueous
NH4Cl (2 ꢀ 20 mL) and water (2 ꢀ 20 mL). The organic
components were dried over MgSO4, filtered through Celite,
and concentrated via rotary evaporation. The resulting solid
was redissolved in hexanes, passed through an alumina plug,
and concentrated via rotary evaporation. The resulting oil was
added to a mixture of THF/MeOH/H2O (15 mL/15 mL/3 mL),
and the resulting mixture was stirred with K2CO3 (0.120 g, 0.9
mmol) for 3 h. Hexanes (30 mL) were added to the solution, and
the mixture was washed with water (2 ꢀ 20 mL). The organic
components were dried over MgSO4, filtered through Celite,
and concentrated via rotary evaporation. The resulting solid
was dissolved in NEt3 (50 mL), and the solution was sparged
with nitrogen for 10 min. The degassed solution was added to a
Schlenk flask containing 4-bromo-1-iodo-terafluorobenzene
(0.320 g, 0.9 mmol), Pd(PPh3)4 (0.030 g, 0.03 mmol), CuI
(0.020 g, 0.10 mmol), and PPh3 (0.020 g, 0.08 mmol), and the
entire mixture was heated at 30-35 °C for 12 h, with vigorous
stirring, under a flow of nitrogen. Toluene (40 mL) was added to
the solution, and the resulting mixture was washed with aqueous
NH4Cl (2 ꢀ 20 mL) and water (2 ꢀ 20 mL). The organic
components were dried over MgSO4, filtered through Celite,
and concentrated via rotary evaporation. The resulting solid was
absorbed onto silica gel and purified by column chromatography
(hexanes) to give 0.160 g (58%, 0.32 mmol) of 3as a yellow powder.
1H NMR (CDCl3, 400 MHz, 25 °C) δ 0.894 (3H, t), 1.3-1.4
2,20-Dibromo-4-(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethyny-
ltetrafluorophenyl)-heptafluorobiphenyl (2). Method 1. Trimet-
hylsilylacetylene (0.05 mL, 1.2 mmol) was combined with NEt3
(30 mL), and the entire solution was sparged with nitrogen for
10 min. The degassed solution was added to a Schlenk flask
containing 2-bromo-20-hexyl-5,50-bithiophene (0.150 g, 0.47 mmol),
PdCl2(PPh3)2 (0.030 g, 0.03 mmol), CuI (0.020 g, 0.10 mmol),
and PPh3 (0.020 g, 0.08 mmol), and the entire mixture was
heated at 40 °C for 12 h, with vigorous stirring, under a flow of
nitrogen. Toluene (20 mL) was added to the solution, and the
resulting mixture was washed with aqueous NH4Cl (2 ꢀ 20 mL)
and water (2 ꢀ 20 mL). The organic components were dried over
MgSO4, filtered through Celite, and concentrated via rotary
evaporation. The resulting solid was redissolved in hexanes,
passed through an alumina plug, and concentrated via rotary
evaporation. The resulting 5-ethynyltrimethylsilyl-50-hexyl-2,20-
bithiophene was dissolved in a mixture of THF/MeOH/H2O
(5 mL/5 mL/2 mL) and stirred with K2CO3 (0.050 g, 0.36 mmol)
for 3 h. Hexanes (20 mL) were added to the solution, and the
mixture was washed with water (2 ꢀ 20 mL). The organic
components were dried over MgSO4, filtered through Celite,
and concentrated via rotary evaporation. The resulting viscous
oil was dissolved in hexanes (5 mL), the resulting solution was
transferred to a Schlenk flask, and the solvent was then removed
under vacuum. The resulting viscous oil was combined with
THF (2 mL), to this was added butyllithium (0.14 mL, 1.6 M in
hexanes), and the solution was stirred for 20 min at room
temperature. In a separate Schlenk flask, 2,20-dibromo-
4-(pentafluorophenylethynyl)-heptafluorobiphenyl (1) (0.140 g,
1884 J. Org. Chem. Vol. 75, No. 6, 2010

Geramita et al.
JOCArticle
(6H, m), 1.672 (2H, q), 2.80 (2H, t), 6.705 (1H, d), 7.019 (1H, d),
7.049 (1H, d), 7.293 (1H, d); 19F NMR (CDCl3, 376 MHz,
25 °C) δ -134.46 (1F, m), -136.51 (1F, m). Anal. Calcd for
C22H17BrF4S2: C, 52.55; H, 3.42; S, 12.79. Found: C, 52.55; H,
3.59; S, 13.16. Mp 80-82 °C.
7.315 (1H, d); 19FNMR(CDCl3, 376 MHz, 25 °C) δ-99.84 (1F, m),
45
-125.48 (1F, m), -125.69 (1F, m), -126.80 (1F, dm, JFF
=
45
162 Hz), -132.99 (1F, m, JFF = 162 Hz), -137.30 (2F, m),
-137.85 (2F, m), -151.12 (1F, m), -152.70 (1F, m). Anal.
Calcd for C40H27F11GeS2: C, 56.30; H, 3.19. Found: C, 56.20;
H, 3.22. Mp 295-296 °C.
5-(1-Ethynyl-4-iodo-tetrafluorophenyl)-50-hexyl-2,20-bithiop-
hene (4). Compound 3 (0.410 g, 0.82 mmol) was dissolved in
toluene/Et2O (50 mL/ 20 mL), and the solution was cooled to
-78 °C. To this was added butyllithium (0.52 mL, 1.6 M in
hexanes), and the solution was stirred for 20 min at -78 °C.
Iodine (0.210 g, 0.83 mmol) was dissolved in Et2O (15 mL), and
this solution was then added to the stirred reaction mixture.
Aqueous Na2S2O3 was added, and the reaction mixture was
allowed to warm to room temperature, with stirring, over 1 h.
The mixture was washed with water (2 ꢀ 50 mL), and the
organic components were dried over MgSO4 and filtered
through Celite, and the product was isolated via rotary evapora-
tion to give 0.440 g (98%, 0.81 mmol) of 4 as a yellow powder.
1H NMR (CDCl3, 400 MHz, 25 °C) δ 0.894 (3H, t), 1.3-1.4 (6H,
m), 1.663 (2H, q), 2.800 (2H, t), 6.705 (1H, d), 7.018 (1H, d),
7.047 (1H, d), 7.294 (1H, d); 19F NMR (CDCl3, 376 MHz, 25 °C)
2-(1-(5-Ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluoro-
phenyl)-9-ethyl-heptafluorophosphafluorene Oxide (P-DA). 2,20-
Dibromo-4-(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetra-
fluorophenyl)-heptafluorobiphenyl (2) (0.055 g, 0.06 mmol) was
dissolved in toluene/Et2O (10 mL/ 2 mL), and the mixture was
cooled to-78°C. Tothiswas addedbutyllithium(0.08mL, 1.6M
in hexanes), and the resulting solution was stirred for 20 min at
-78 °C, after which dichloroethylphosphine (0.05 mL) was
added, and the entire solution was allowed to warm to room
temperature over 12 h with stirring. The solvent and excess
dichloroethylphospine were removed under vacuum, and the
resulting orange solid was redissolved in toluene (20 mL) and
washed with water (2 ꢀ 15 mL). The organic components were
dried over MgSO4, filtered through Celite, and concentrated via
rotary evaporation. THF (20 mL) and H2O2 (0.2 mL, 30 wt %
solution in H2O) were added, and the resulting mixture was
stirred for 4 h. Toluene (20 mL) was added, the resulting
reaction mixture was washed with water (2 ꢀ 15 mL), and the
organic components were dried over MgSO4, filtered through
Celite, and concentrated via rotary evaporation. The product
was isolated via recrystallization from hot hexanes/acetone to
give 0.025 g (50%, 0.03 mmol) of P-DA as a light orange powder.
1H NMR (CDCl3, 400 MHz, 25 °C) δ 0.896 (3H, t), 1.152 (3H,
dt), 1.3-1.4 (6H, m), 1.675 (2H, q), 2.471 (2H, m), 2.805 (2H, t),
6.711 (1H, d), 7.037 (1H, d), 7.059 (1H, d), 7.320 (1H, d); 19F
NMR (CDCl3, 376 MHz, 25 °C) δ -105.43 (1F, m), -118.15
δ -121.59 (1F, m), -136.07 (1F, m). Anal. Calcd for C22H17
F4IS2: C, 48.18; H, 3.12; S, 11.69. Found: C, 48.03; H, 3.40;
S,11.66. Mp 89-90 °C.
-
2-(1-(5-Ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluoroph-
enyl)-9,9-dimethyl-heptafluorosilafluorene (Si-DA). 2,20-Dibromo-
4-(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluorophenyl)-
heptafluorobiphenyl (2) (0.100 g, 0.11 mmol) was dissolved in
toluene/Et2O (10 mL/ 2 mL), and the resulting mixture was cooled
to -78 °C. To this was added butyllithium (0.14 mL, 1.6 M in
hexanes), and the resulting solution was stirred for 20 min at -78 °C,
after which dichlorodimethylsilane (0.1 mL) in THF (2 mL) was
added, and the entire solution was allowed to warm to room
temperature, with stirring, over 12 h. The resulting yellow solution
was washed with water (2 ꢀ 15 mL), and the organic components
were dried over MgSO4, filtered through Celite, and concentrated
via rotary evaporation. The product was isolated via recrystalliza-
tion from hot hexanes to give 0.070 g (82%, 0.09 mmol) of Si-DA
as a light orange powder. 1H NMR (CDCl3, 400 MHz, 25 °C) δ
0.623 (6H, s), 0.896 (3H, t), 1.3-1.4 (6H, m), 1.686 (2H, q), 2.805
(2H, t), 6.716 (1H, d), 7.032 (1H, d), 7.059 (1H, d), 7.315 (1H, d);
19F NMR (CDCl3, 376 MHz, 25 °C) δ -102.38 (1F, m), -124.31
45
(1F, m), -127.11 (1F, dm, JFF = 132 Hz), -131.33 (1F, m),
-132.52 (1F, m, JFF45 = 132 Hz), -136.76 (2F, m), -137.45 (2F,
m), -143.96 (1F, m), -148.53 (1F, m). 31P NMR (CDCl3, 161.9
MHz, 25 °C) δ42.57 (s). MS: 799 (Mþ), 727, 699. HRMS: calcd for
C38H22F11OPS2 798.067408, found 789.067320. Mp > 250 °C.
2,20-Dibromo-4,40-bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-
ethynyltetrafluorophenyl)-hexafluorobiphenyl (5). 2,20-Dibromo-
4,40-ethynyl-3,30,5,50,6,60-hexafluorobiphenyl (0.129 g, 0.29 mmol)
and 4 (0.400 g, 0.73 mmol) were dissolved in NEt3 (30 mL), and
the solution was sparged with nitrogen for 10 min. The degassed
solution was added to a Schlenk flask containing Pd(PPh3)4
(0.040 g, 0.034 mmol), CuI (0.040 g, 0.21 mmol), and PPh3
(0.040 g, 0.15 mmol), and the entire mixture was heated at 40 °C
for 12 h, with vigorous stirring, under a flow of nitrogen. Toluene
was added to the solution (20 mL), and the resulting mixture was
washed with aqueous NH4Cl (2 ꢀ 20 mL) and water (2 ꢀ 20 mL).
The organic components were dried over MgSO4, filtered
through Celite, and concentrated via rotary evaporation. The
resulting solid was absorbed onto silica gel and purified by
column chromatography (hexanes/toluene 100:5) to give 0.340 g
(90%, 0.26 mmol) of 5 as a light orange powder. 1H NMR
(CDCl3, 400 MHz, 25 °C) δ 0.899 (3H, t), 1.3-1.4 (6H, m), 1.690
(2H, q), 2.809 (2H, t), 6.713 (1H, d), 7.042 (1H, d), 7.068 (1H, d),
7.330 (1H, d); 19F NMR (CDCl3, 376 MHz, 25 °C) δ -101.66
(1F, m), -129.39 (1F, m), -136.85 (3F, m), -137.56 (2F, m).
Anal. Calcd for C60H34Br2F14S4: C, 55.05; H, 2.62; S, 9.80.
Found: C, 54.77; H, 2.32; S, 10.15. Mp 186 °C dec.
45
(1F, m), -127.87 (1F, dm, JFF = 147 Hz), -128.50 (1F, m),
-133.96 (1F, m, JFF45 = 147 Hz), -137.32 (2F, m), -137.83 (2F,
m), -150.02 (1F, m), -152.82 (1F, m). Anal. Calcd for
C38H23F11S2Si: C, 58.45; H, 2.92; S, 8.21. Found: C, 58.20; H,
2.97; S, 7.95. Mp 296-297 °C.
2-(1-(5-Ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluoro-
phenyl)-9,9-diethyl-heptafluorogermafluorene (Ge-DA). 2,20-Di-
bromo-4-(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetra-
fluorophenyl)-heptafluorobiphenyl (2) (0.080 g, 0.09 mmol) was
dissolved in toluene/Et2O (10 mL/ 2 mL), and the mixture was
cooled to -78 °C. To this was added butyllithium (0.14 mL,
1.6 M in hexanes), and the resulting solution was stirred for
20 min at -78 °C. In a separate Schlenk flask, dichlorodiethyl-
germane (0.1 mL, 0.7 mmol) was added to dry THF (2 mL), and
this mixture was deoxygenated via two series of freeze/pump/
thaw cycles and then added to the cooled reaction mixture. The
resulting solution was warmed to room temperature, with
stirring, over 8 h. The resulting yellow solution was quenched
with water (2 ꢀ 15 mL), and the organic components were
extracted into CH2Cl2 (2 ꢀ 20 mL). The combined extracts were
dried over MgSO4, filtered through Celite, and concentrated via
rotary evaporation. The product was isolated via recrystalliza-
tion from hot hexanes to give 0.045 g (83%, 0.075 mmol) of Ge-
2,7-Bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluor-
ophenyl)-9,9-dimethyl-hexafluorosilafluorene (Si-DAD). 2,20-Di-
bromo-4,40-bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynylte-
trafluorophenyl)-hexafluorobiphenyl (5) (0.022 g, 0.019 mmol)
was dissolved in toluene/Et2O (5 mL/ 1 mL), and the mixture
was cooled to -78 °C. To this was added butyllithium (0.024 mL,
1.6 M in hexanes), and the resulting solution was stirred for
20 min at -78 °C, after which dichlorodimethylsilane (0.1 mL)
in THF (1 mL) was added, and the entire solution was allowed to
1
DA as a bright yellow powder. H NMR (CDCl3, 400 MHz,
25 °C) δ 0.896 (3H, t), 1.091 (6H, t), 1.3-1.4 (10H, m), 1.686
(2H, q), 2.805 (2H, t), 6.711 (1H, d), 7.035 (1H, d), 7.059 (1H, d),
J. Org. Chem. Vol. 75, No. 6, 2010 1885

Geramita et al.
JOCArticle
warm to room temperature over 12 h with stirring. The resulting
yellow solution was quenched with water (2 ꢀ 10 mL), and the
organic components were extracted into toluene (2 ꢀ 10 mL). The
combined extracts were dried over MgSO4, filtered through Celite,
and concentrated via rotary evaporation. The product was isolated
via slow evaporation of hot hexanes/CH2Cl2 to give 0.011 g (61%,
0.009 mmol) of Si-DAD as an orange powder. 1H NMR (CDCl3,
400 MHz, 25 °C) δ 0.637 (3H, s), 0.895 (3H, t), 1.3-1.4 (6H, m),
1.685 (2H, q), 2.803 (2H, t), 6.708 (1H, d), 7.036 (1H, d), 7.0560
(1H, d), 7.316 (1H, d); 19F NMR (CDCl3, 376 MHz, 25 °C) δ
-102.19 (1F, m), -124.28 (1F, m), -132.66 (1F, m), -137.22 (2F,
m), -137.80 (2F, m). LRMS: 1207 (Mþ). HRMS calcd for
C62H40F14S4Si 1206.1586, found 1206.1578. Mp 195 °C sub.
2,7-Bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluo-
rophenyl)-9,9-dimethyl-hexafluorogermafluorene (Ge-DAD). 2,20-
Dibromo-4,40-bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethyny-
ltetrafluorophenyl)-hexafluorobiphenyl (5) (0.080 g, 0.06 mmol)
was dissolved in toluene/Et2O (20 mL/ 5 mL), and the mixture
was cooled to -78 °C. To this was added butyllithium (0.08 mL,
1.6 M in hexanes), and the resulting solution was stirred for
20 min at -78 °C. In a separate Schlenk flask, dichlorodiethyl-
germane (0.1 mL, 0.7 mmol) was added to dry THF (2 mL), and
this mixture was deoxygenated via two series of freeze/pump/
thaw cycles. The dichlorodiethylgermane solution was added to
the dilithio solution at -78 °C, and the resulting reaction mixture
was warmed to room temperature with stirring over 8 h. The
solvent and excess dichlorodiethylgermane were removed via
vacuum transfer, and the resulting yellow/orange solid was redis-
solvedintoluene (30 mL) andwashedwith water (2ꢀ 20 mL). The
organic components were dried over MgSO4, filtered through
Celite, and concentrated via rotary evaporation. The product
was isolated via recrystallization via hot hexanes/acetone to give
0.050 g (64%, 0.04 mmol) of Ge-DAD as an orange powder.
1H NMR (CDCl3, 400 MHz, 25 °C) δ 0.896 (3H, t), 1.091 (3H, t),
1.3-1.4 (8H, m), 1.686 (2H, q), 2.805 (2H, t), 6.711 (1H, d), 7.035
(1H, d), 7.059 (1H, d), 7.315 (1H, d); 19FNMR (CDCl3, 376 MHz,
25 °C) δ -99.61 (1F, m), -125.67 (1F, m), -131.72 (1F, m),
-137.25 (2F, m), -137.82 (2F, m). Anal. Calcd for C64H44F14-
GeS4: C, 60.06; H, 3.47; S, 10.02. Found: C, 59.83; H, 3.27; S,
10.05. Mp 175 °C sub.
2,2000-Diiodo-4,4000-dihexyl-5,20,200,5000,50,500-quaterthiophene (6).
3,3000-Dihexyl-2,20,200,2000,50,500-quaterthiophene (0.360 g, 0.72 mmol)
was dissolved in THF (100 mL), to this was added butyllithium
(1.0 mL, 1.6 M in hexanes), and the mixture was stirred for 1 h at
room temperature. In a separate Schlenk flask iodine (0.450 g,
1.8 mmol) was dissolved in THF (20 mL), the resulting solution
was added to the quaterthiophene-containing mixture, and the
entire reaction mixture was stirred for 20 min. Toluene was
added to the solution (40 mL), and the mixture was washed with
aqueous Na2S2O3 (2 ꢀ 40 mL) and water (2 ꢀ 40 mL). The
combined organic components were dried over MgSO4, filtered
through Celite, and concentrated via rotary evaporation and to
give 0.547 g (100%, 0.72 mmol) of 6 as yellow crystals that were
used without further purification. 1H NMR (CDCl3, 400 MHz,
25 °C) δ 0.883 (3H, t), 1.3-1.4 (10H, m), 1.58 (2H, m), 2.872
(2H, t), 6.960 (1H, d), 7.077 (1H, s), 7.107 (1H, d).
2,2000-Bis(1-ethynyl-4-bromo-tetrafluorophenyl)-4,4000-dihexyl-
5,20,200,5000,50,500-quaterthiophene (7). Quaterthiophene 6 (0.547 g,
0.72 mmol) was dissolved in NEt3 (50 mL), and to this was added
trimethylsilylacetylene (0.3 mL, 2.1 mmol). The entire solution was
sparged with nitrogen for 10 min and was then added to a Schlenk
flask containing PdCl2(PPh3)2 (0.040 g, 0.06 mmol), CuI (0.040 g,
0.21 mmol), and PPh3 (0.040 g, 0.15 mmol), and the entire mixture
was heated at 40 °C for 12 h, with vigorous stirring, under a flow of
nitrogen. Toluene was added to the solution (30 mL), and the
resulting mixture was washed with aqueous NH4Cl (2 ꢀ 20 mL)
and water (2 ꢀ 20 mL). The organic components were dried over
MgSO4, filtered through Celite, and concentrated via rotary
evaporation. The resulting solid was dissolved in a mixture of
THF/MeOH/H2O (20 mL/20 mL/5 mL) and stirred with K2CO3
(0.184 g, 1.3 mmol) for 3 h. Toluene was added to the solution
(40 mL), and the mixture was washed with water (2 ꢀ 20 mL). The
organic components were dried over MgSO4, filtered through
Celite, and concentrated via rotary evaporation. The resulting
solid was dissolved in NEt3 (50 mL), and the resulting solution was
sparged with nitrogen for 10 min. The degassed solution was added
to a Schlenk flask containing 4-bromo-1-iodo-terafluorobenzene
(1.00 g, 2.80 mmol), PdCl2(PPh3)2 (0.040 g, 0.06 mmol), CuI (0.030 g,
0.16 mmol), and PPh3 (0.030 g, 0.11 mmol), and the entire mixture
was heated at 30-35 °C for 12 h, with vigorous stirring, under a
flow of nitrogen. Toluene was added to the solution (40 mL), and
the mixture was washed with aqueous NH4Cl (2 ꢀ 20 mL) and
water (2 ꢀ 20 mL). The organic components were dried over
MgSO4, filtered through Celite, and concentrated via rotary
evaporation. The resulting solid was absorbed onto silica gel and
purified by column chromatography (hexanes:toluene) to give
0.275 g (39%, 0.28 mmol) of 7 as a red powder. 1H NMR
(CDCl3, 400 MHz, 25 °C) δ 0.890 (3H, t), 1.3-1.4 (6H, m),
1.670 (2H, q), 2.77 (2H, t), 7.106 (1H, d), 7.172 (1H, d), 7.260
(1H, d); 19F NMR (CDCl3, 376 MHz, 25 °C) δ -134.48 (1F, m),
-136.43 (1F, m). Anal. Calcd for C44H32Br2F8S4: C, 52.81; H,
3.22; S, 12.82. Found: C, 52.86; H, 3.29; S, 12.97. Mp 180-181 °C.
2,2000-Bis(1-ethynyl-4-iodo-tetrafluorophenyl)-4,4000-dihexyl-5,-
20,200,5000,50,500-quaterthiophene (8). Compound 7 (0.066 g, 0.066
mmol) was dissolved in THF (50 mL), and the resulting solution
was cooled to -78 °C. To this was added butyllithium (0.9 mL,
1.6 M in hexanes), and the resulting solution was stirred for
20 min at -78 °C. Iodine (0.35 g, 0.14 mmol) was dissolved in
THF (10 mL), and the iodine solution was then added to the
stirred reaction mixture. Aqueous Na2S2O3 was added, and the
reaction mixture was allowed to warm to room temperature with
stirring over 1 h. Toluene was added to the solution (40 mL), and
the resulting mixture was washed with water (2 ꢀ 50 mL). The
organic components were dried over MgSO4 and filtered
through Celite, and the product was isolated via rotary evaporation
to give 0.072 g (94%, 0.065 mmol) of 8 as a red powder. 1H NMR
(CDCl3, 400 MHz, 25 °C) δ 0.890 (3H, t), 1.3-1.4 (6H, m), 1.670
(2H, q), 2.77 (2H, t), 7.106 (1H, d), 7.172 (1H, d), 7.260 (1H, d);
2,7-Bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyltetrafluoro-
phenyl)-9,9-dimethyl-hexafluorophosphafluorene Oxide (P-DAD).
2,20-Dibromo-4,40-bis(1-(5-ethynyl-50-hexyl-2,20-bithienyl)-4-ethynyl-
tetrafluorophenyl)-hexafluorobiphenyl (5) (0.077 g, 0.058 mmol)
was dissolved in toluene/Et2O (20 mL/ 5 mL), and the resulting
mixture was cooled to -78 °C. To this was added butyllithium
(0.073 mL, 1.6 M in hexanes), and the resulting solution was stirred
for 20 min at -78 °C, after which dichloroethylphosphine (0.05 mL)
was added, and the entire solution was allowed to warm to room
temperature over 12 h with stirring. The solvent and excess dic-
hloroethylphosphine were removed via vacuum transfer, and the
resulting yellow/orange solid was redissolved in toluene (30 mL) and
washed with water (2 ꢀ 20 mL). THF (10 mL) and H2O2 (2 mL,
30 wt % solution in H2O) were added, and the resulting mixture
was stirred for 4 h. The reaction was quenched by washing with
water (2 ꢀ 15 mL), and the organic components were dried over
MgSO4, filtered through Celite, and concentrated via rotary
evaporation. The product was isolated via recrystallization from
hot hexanes/acetone/CH2Cl2 to give 0.055 g (76%, 0.045 mmol)
of P-DAD as a light orange powder. 1H NMR (CDCl3, 400 MHz,
25 °C) δ 0.896 (6H, t), 1.170 (3H, dt), 1.3-1.4 (12H, m), 1.667 (4H,
q), 2.517 (2H, m), 2.805 (4H, t), 6.711 (2H, d), 7.035 (2H, d), 7.059
(2H, d), 7.315 (2H, d); 19F NMR (CDCl3, 376 MHz, 25 °C) δ
-105.33 (1F, m), -118.28 (1F, m), -131.47 (1F, m), -136.70 (2F,
m), -137.44 (2F, m); 31P NMR (CDCl3, 161.9 MHz, 25 °C) δ 41.68
(s).LRMS:1225(Mþ). Anal. Calcd for C62H39F14OPS4: C, 60.78; H,
3.21; S, 10.47. Found: C, 60.47; H, 3.05; S, 10.38. Mp 214 -217 °C.
1886 J. Org. Chem. Vol. 75, No. 6, 2010

Geramita et al.
JOCArticle
19F NMR (CDCl3, 376 MHz, 25 °C) δ -121.51 (1F, m), -135.99
(1F, m). Anal. Calcd for C44H32I2F8 I2S4: C, 48.27; H, 2.95; S,
11.72. Found: C, 48.26; H, 2.95; S, 11.78. Mp 188-189 °C.
2,2000-Bis(1-ethynyl-4-(2,20-dibromo-4-ethynyl-heptafluororbip-
henyl)-tetrafluorophenyl)-4,4000-dihexyl-5,20,200,5000,50,500-quater-
thiophene (9). 2,20-Dibromo-4-ethynyl-3,30,40,5,50,6,60-hepta-
fluorobiphenyl (0.060 g, 0.13 mmol) and 8 (0.035 g, 0.032 mmol)
were dissolved in NEt3 (50 mL), and the solution was sparged
with nitrogen for 10 min. The degassed solution was added to a
Schlenk flask containing Pd(PPh3)4 (0.005 g, 0.007 mmol), CuI
(0.005 g, 0.03 mmol), and PPh3 (0.010 g, 0.04 mmol), and the
entire mixture was heated at 40 °C for 12 h, with vigorous
stirring, under a flow of nitrogen. Toluene (40 mL) was added to
the solution, and the resulting mixture was washed with aqueous
NH4Cl (2 ꢀ 30 mL) and water (2 ꢀ 30 mL). The organic com-
ponents were dried over MgSO4, filtered through Celite, and
concentrated via rotary evaporation. The resulting solid was
absorbed onto silica gel and purified by column chromatography
(hexanes/toluene 5:1) to give 0.035 g (67%, 0.02 mmol) of 9 as a
brick red powder. 1H NMR (CDCl3, 400 MHz, 25 °C) δ 0.890
(3H, t), 1.3-1.4 (6H, m), 1.670 (2H, q), 2.77 (2H, t), 7.106 (1H,
d), 7.172 (1H, d), 7.260 (1H, d); 19F NMR (CDCl3, 376 MHz,
25 °C) δ -101.59 (1F, m), -128.40 (1F, m), -129.50 (1F, m),
-135.27 (1F, m), -136.85 (3F, m), -137.56 (2F, m), -150.71
(1F, m), -154.87 (1F, m). Anal. Calcd for C72H32Br4F22S4: C,
49.05; H, 1.83. Found: C, 49.00; H, 1.47. Mp 276-278 °C.
2,2000-Bis(1-ethynyl-4-(2-ethynyl-9,9-diethyl-heptafluorogerma-
fluorene)-tetrafluorophenyl)-4,4000-dihexyl-5,20,200,5000,50,500-qua-
terthiophene (Ge-ADA). 2,2000-Bis(1-ethynyl-4-(2,20-dibromo-4-
ethynyl-heptafluororbiphenyl)-quaterfluorophenyl)-4,4000-dihexyl-
5,20,200,5000,50,500-quaterthiophene (9) (0.045 g, 0.025 mmol) was
dissolved in toluene/THF (10 mL/10 mL), and the mixture was
cooledto-78°C. Tothiswas addedbutyllithium(0.06mL, 1.6M
in hexanes), and the resulting brown/yellow solution was stirred
for 20 min at -78 °C. In a separate Schlenk flask, dichloro-
diethylgermane (0.1 mL, 0.7 mmol) was added to dry THF (2 mL),
and this mixture was deoxygenated via two series of freeze/
pump/thaw cycles. The dichlorodiethylgermane solution was
added to the dilithio solution at -78 °C, and the resulting
solution was warmed to room temperature with stirring over 8 h.
The resulting red solution was quenched with water (2 ꢀ 15 mL),
and the organic components were extracted into toluene (2 ꢀ
20 mL). The combined extracts were dried over MgSO4, filtered
through Celite, and concentrated via rotary evaporation. The
product was isolated via recrystallization from hot CH2Cl2 to
give 0.030 g (72%, 0.018 mmol) of Ge-ADA as a bright red
Solar Cell Device Construction and Measurement. Sputtered
ITO-coated (150 nm, 20 Ω m-1) glass substrates were obtained
from Thin Film Devices, Inc. The substrates were subjected to
ultrasonication for 20 min in acetone, and then 2% Helmanex
soap in water for 20 min, followed by extensive rinsing with
deionized water and ultrasonication in deionized water and then
2-propanol. The substrates were then dried under a stream of
nitrogen before treatment for 10 min in an oxygen plasma at
300 mTorr (1 Torr = 133.28 Pa) at high power. PEDOT:PSS in
water (Baytron-P) was filtered through a 0.45 μm filter and spin-
coated onto the ITO surface at 4000 rpm for 60 s and baked for
1 h at 125 °C, affording a 30 nm layer. All procedures after this
point were performed in an inert-atmosphere glovebox. Solu-
tions of P3HT and DA compounds were prepared separately in
CHCl3 (10 mg mL-1). Immediately prior to deposition, the
solutions were passed though a 0.2 μm polytetrafluoroethylene
syringe filter. For a 1:1 wt % P3HT/DA compound device,
equal amounts of each solution were combined. The blend
solution was applied to the substrate and spun at 1000 rpm for
60 s. A small portion of the organic layer was removed with
tweezers to allow contact with the ITO. The substrates were then
placed in a resistive-heating evaporation chamber and held
under vacuum (10-7 torr) for 4 h before evaporating 100 nm
of Al through a shadow mask at a rate of 0.1-0.5 nms-1 while
rotating the substrates at approximately 1 Hz to ensure even
electrode deposition. The configuration of the shadow mask
afforded eight independent devices on each substrate and one
rectangular pad to connect the Al to the ITO substrate. Devices
were left to cool to room temperature before further processing.
Testing of the devices was performed under an argon atmo-
sphere with an oriel xenon arc lamp with an AM 1.5G solar
filter. Current-voltage behavior was measured with a Keithly
236 SMU instrument. Reported efficiencies are the average from
8 independent measurements made on one spin-coated device.
Acknowledgment. This work was supported by the National
Science Foundation under research grant CHE0314709 and
the DuPont Center for Collaborative Research and Educa-
tion. Photovoltaic device fabrication and characterization
was supported by the DOE-BES Plastic Electronics Program
at Lawrence Berkeley National Laboratories. Work at the
Molecular Foundry was supported by the Office of Science,
Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231. The
ꢀ
authors would like to thank A. D’Aleo for low-temperature
fluorescence data and numerous insightful discussions.
1
powder. H NMR (CDCl3, 400 MHz, 25 °C) δ 0.905 (3H, t),
1.094 (6H, t), 1.3-1.4 (10H, m), 1.680 (2H, q), 2.780 (2H, t),
7.117 (1H, d), 7.182 (1H, d), 7.281 (1H, d); 19F NMR (CDCl3,
376 MHz, 25 °C) δ -99.85 (1F, m), -125.48 (1F, m), -125.71
Supporting Information Available: NMR spectra for all new
compounds, tables of atom coordinates and absolute energies
for the theoretical calculations, and additional photovoltaic
device performance data. This material is available free of
charge via the Internet at http://pubs.acs.org.
45
(1F, m), -126.83 (1F, dm, JFF = 173 Hz), -133.02 (1F, m,
JFF45 = 173 Hz), -137.25 (2F, m), -137.85 (2F, m), -151.15
(1F, m), -152.70 (1F, m). Anal. Calcd for C80H52F22Ge2S4: C,
56.36; H, 3.07. Found: C, 55.48; H, 3.02. Mp >300 °C.
J. Org. Chem. Vol. 75, No. 6, 2010 1887