Steering power of perfluoroalkyl substituents in crystal engineering: Tuning the π-π Distance while maintaining the lamellar packing motif for aromatics with various sizes of π-conjugation

Article abstract of DOI:10.1021/cg501894u

Previously, we reported that introducing perfluoroalkyl substituents onto aromatics promotes the formation of lamellar π-π stacked crystalline materials with short interplanar distances. In this report, we developed a new synthetic route that effectively prepares perfluoroalkylated N-containing aromatics by eliminating a side perfluoroalkylation reaction occurring on nonsubstituted C_sp2-H sites of the corresponding bromoaromatics without regioselectivity. This results in a significant improvement of the yield of target perfluoroalkylated aromatics and facilitates the purification and scale-up processes. X-ray single crystal structural analyses show that lamellar π-π stacked structures with tunable interplanar distances are achieved with fused N-containing aromatics with varying sizes of π-conjugation. Both crystal structures and theoretical calculations demonstrated that the interplanar distance can be fine-tuned with the size of π-conjugation, with larger π-conjugation favoring shorter interplanar distances while still maintaining a lamellar π-π stacked packing motif. Compared to our previous results, we find that simply changing the perfluoroalkyl substituent positions and patterns can change molecular topology to exclusively form lamellar π-π stacked packing motifs through prioritization of specific steric effects. Electrochemical results and absorption spectra confirm that the band gap is reduced due to increasing π-conjugation, and the first reduction potential exhibits a significant positive shift due to both increasing π-conjugation and perfluoroalkylation.

Full text of DOI:10.1021/cg501894u

Article
pubs.acs.org/crystal
Steering Power of Perfluoroalkyl Substituents in Crystal Engineering:
Tuning the π−π Distance While Maintaining the Lamellar Packing
Motif for Aromatics with Various Sizes of π‑Conjugation
Mohammad O. BaniKhaled, Jeffery D. Mottishaw, and Haoran Sun*
Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069, United States
S
* Supporting Information
ABSTRACT: Previously, we reported that introducing
perfluoroalkyl substituents onto aromatics promotes the
formation of lamellar π−π stacked crystalline materials with
short interplanar distances. In this report, we developed a new
synthetic route that effectively prepares perfluoroalkylated N-
containing aromatics by eliminating a side perfluoroalkylation
reaction occurring on nonsubstituted Csp2−H sites of the
corresponding bromoaromatics without regioselectivity. This
results in a significant improvement of the yield of target
perfluoroalkylated aromatics and facilitates the purification and
scale-up processes. X-ray single crystal structural analyses show
that lamellar π−π stacked structures with tunable interplanar
distances are achieved with fused N-containing aromatics with
varying sizes of π-conjugation. Both crystal structures and theoretical calculations demonstrated that the interplanar distance can
be fine-tuned with the size of π-conjugation, with larger π-conjugation favoring shorter interplanar distances while still
maintaining a lamellar π−π stacked packing motif. Compared to our previous results, we find that simply changing the
perfluoroalkyl substituent positions and patterns can change molecular topology to exclusively form lamellar π−π stacked packing
motifs through prioritization of specific steric effects. Electrochemical results and absorption spectra confirm that the band gap is
reduced due to increasing π-conjugation, and the first reduction potential exhibits a significant positive shift due to both
increasing π-conjugation and perfluoroalkylation.
dipoles directed towards the open space, favoring the formation
of Csp2−H dipole−π interactions.28 To solve these challenges,
we propose to install perfluoroalkyl substituents onto the other
side of the benzo[a,c]phenazine molecules, and also to separate
them. We expect this approach to eliminate the T-shaped
molecular packing motif in the solid state through addressing
both steric and electronic issues that were discussed above.
An obstacle to this, however, is that its brominated
benzo[a,c]phenazine precursor possesses four different active
Csp2−H positions on the phenathrane ring that are susceptible
to radical perfluoroalkylation reactions.29,30 This side reaction
significantly lowers the yield of target product perfluoroalky-
lated benzo[a,c]phenazine to less than 20% and complicates
final target product purification.27 Repetitive column chroma-
tography, followed by recrystallization, is needed to give an
analytically pure final target product. The number of side
reaction products increases as the aromatic ring size increases
and more accessible Csp2−H positions are available to the
radical perfluoroalkylation. This places a great barrier for
utilizing the perfluoroalkylation strategy on large aromatic
1. INTRODUCTION
The functionalities of many bulk organic materials, ranging
from liquid crystals,1,2 organic semiconductor materials,3−7
nonlinear optics,8−10 optoelectronics,5,11,12 electro-optics, and
spintronics,13−15 strongly depend on the secondary and tertiary
structures of molecular assemblies in condensed phase that are
largely organized by weak noncovalent intermolecular inter-
actions.1,12,16−20 Although modern organic chemistry has
advanced in the formation of covalent bonds, the rational
design and control of solid state structures of many functional
organic materials remains a significant challenge.17,21−26 Our
recent work shows that addition of perfluoroalkyl groups onto
benzo[a,c]phenazine and dithiophenphenazine leads to lamellar
π−π stacked solid state structures with tunable interplanar
distances and specific packing motifs.27,28 Though ortho-
bisperfluorobutyl dithiophenphenazine shows a perfect lamellar
π−π stacked solid state structure, the ortho-bisperfluorobutyl
benzo[a,c]phenazine (compound 4 in Chart 1) exhibits a
twisted lamellar π−π stacked structure with an unfavorable
interplanar distance of 3.416 Å.27 One possible reason for this
may be that the two perfluorobutyl groups adjacent to each
other possess large steric hindrance that prevents a π−π stacked
structure with short interplanar distances. Another reason is
possibly that the molecule itself contains at least four Csp2−H
Received: December 31, 2014
Revised: March 9, 2015
© XXXX American Chemical Society
A
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Chart 1. List of Compounds Studied
Scheme 1. Synthesis of Compound 1
Scheme 2. Synthesis of Key Intermediate 10 and Compounds 1, 2, and 3
systems for practical organic semiconductor applications in
which perfluoroalkylation can significantly increase the electron
affinity (EA) to provide potential operational air-stability of
organic semiconductor devices.31,32 In this report, we develop a
new synthetic pathway that eliminates the late stage radical
perfluoroalkylation side reaction by using a key perfluoroalky-
lated 1,2-diketone intermediate (compound 10, Scheme 2).
This reaction sequence is suitable for various sizes of aromatic
cores, resulting in easier purification of the final target products
and significantly increases the overall yield of the target
products.
excellent purity. The only limitation here for making different
sized perfluoroalkylated N-containing aromatics is the avail-
ability of the corresponding diamine. X-ray single crystal
analyses show that these compounds possess perfect lamellar
π−π stacked structures with tunable interplanar distances. The
interplanar distance decreases as a result of increased π-
conjugation and reduced steric hindrance of the perfluoroalkyl
groups by placing them away from each other (compounds 2 vs
4).
2. RESULTS AND DISCUSSION
By utilizing this key reaction intermediate 10, we successfully
demonstrated that various perfluoroalkylated aromatics (com-
pounds 1, 2, and 3 in Chart 1) can be made in good yield with
2.1. Synthesis and Characterization. Compound 1 was
first synthesized with our previously reported method from a
corresponding brominated compound 7 that was made from
B
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Table 1. Summary of Crystallographic and Refinement Data for Compounds 1, 2, 3, 9, and 10
1
2
3
9
10
formula
C24H8F18N2
666.32
C28H10F18N2
716.38
100
C32H12F18N2
766.44
100
C26H14F18O4
732.37
100
C22H6F18O2
644.27
100
formula wt
temp, K
100
cryst setting
space group
Hall symbol
international tables no.
a, Å
triclinic
monoclinic
C2/c
monoclinic
C2/c
monoclinic
P21/c
monoclinic
P21/c
P1
̅
−P 1
2
−C 2yc
15
−C 2yc
15
−P 2ybc
14
−P 2ybc
14
5.277
26.226
13.519
7.2866
90
25.107
16.153
7.0392
90
18.5776
10.848
13.0816
90
13.6468
11.232
15.260
90
b, Å
11.642
19.253
97.891
93.345
97.865
2
c, Å
α, deg
β, deg
91.280
90
91.569
90
90.747
90
103.574
90
γ, deg
Z
4
4
4
4
cell vol (Å3)
density (diffrn), g cm−3
absorb coeff, mm−1
F(000)
1157.1(9)
1.913
2582.7
1.842
2853.6
1.784
2636.3
1.845
2273.9
1.882
0.216
0.200
0.188
0.207
0.220
656
1416
1520
1456
1264
data range (θmin−θmax), deg
index ranges
measured reflns
independent reflns
reflns with I > 2σ(I)
max/min trans
restraints/params
GOF
1.071−25.382
6, 13, 23
3462
1.553−25.516
1.623−25.402
1.096−25.419
22, 13, 15
9965
1.535−25.357
16, 13, 18
9949
31, 16,
8
30, 19,
8
4991
3795
4201
2407
2629
4857
4180
3092
1995
2015
4382
3944
0.900/0.813
0/397
1.012
0.990/0.923
0/217
1.033
0.991/0.963
0/235
1.046
0.990/0.969
0/433
0.916/0.803
0/388
1.054
1.144
R [F2 > 2σ(F2)]
wR (F2)
0.0515
0.1549
0.0323
0.0914
0.0351
0.0941
0.0280
0.0995
0.0699
0.1816
bromination and followed by condensation of 9,10-phenan-
threnedione (Scheme 1). After the ethylene diamine condensed
with bromodiketone 6, a deprotonation reaction occurs on the
ethylene sp3 carbon under the condensation conditions.33,34
Because of gaining aromaticity, this reaction proceeded
smoothly. For this particular reaction, the perfluoroalkylation
reaction gives an excellent yield of target compound 1.
However, as we discussed earlier, the same reaction condition
is not suitable for perfluoroalkylation of larger aromatic
compounds.27,29
To overcome this synthetic challenge, we introduce an early
stage perfluoroalkylation strategy to make a key intermediate
10, then perform the final condensation reaction with diamine
compounds to form the final desired target compounds
(compounds 1, 2, and 3) in good yield (Scheme 2).
Bromination of 9,10-phenanthrenedione proceeded efficiently
with a reasonably good yield, as reported in the literature.35
Our attempt on direct perfluoroalkylation of compound 6 gives
a complex mixture with no or very little target compound 10
observed. This may be due to the diketone precursor 6 reacting
with Cu or CuI under the perfluoroalkylation condition, leading
to a copper complex.36 Thus, we first protected the carbonyl
groups on compound 6 by reacting it with ethylene glycol at 80
°C in the presence of camphorsulfonic acid to give compound
8 in 80% yield.37 The perfluoroalkylation reaction of 8 with 1-
iodoperfluorobutane in the presence of copper in dry DMSO
and HFE-7200 at 135 °C afforded compound 9 in 62% yield.
Deprotection of the carbonyl groups with 70% perchloric acid
in CH2Cl2 at room temperature gave compound 10 as a stable
yellow solid in 81% yield.38 These reactions can be easily scaled
up to gram scale with our modified deprotection method. The
key intermediate 3,6-bis(perfluorobutyl)-9,10-phenanthrene-
dione (10) was obtained in a substantial amount in four
steps starting from building block 9,10-phenanthrenedione.
This compound 10 is a key intermediate toward the
preparation of various new perfluoroalkylated N-containing
heteroaromatics. Compounds 1, 2, and 3 were synthesized by
condensation reactions of compound 10 with corresponding
diamine derivatives in good to excellent isolated yield. Because
of the possibility of large scale preparation of compound 10,
this synthetic route can be used to prepare a variety of useful
perfluoroalkylated N-containing aromatics possibly serving as
potential n-type organic semiconductor materials. Such a
synthetic effort is currently underway in our laboratory, with
the goal to build a library of perfluoroalkylated organic
semiconductor materials.
All the compounds including the reaction intermediates were
characterized by 1H NMR, 19F NMR, MS, and elemental
analysis (see the Supporting Information for details). The
crystal structures of intermediate compounds 9 and 10 give
1
consistent results with their H and 19F NMR spectra. The
crystal structure of compound 9 (Supporting Information, CIF
file) also indirectly assigned the structure of compound 8 that
previously could not be solved by NMR spectra.39 Final target
compounds were all characterized by X-ray single crystal
structure analysis (via infra). Target compounds 1, 2, and 3
were also characterized by UV−visible absorption spectra and
electrochemistry to access their band gap and molecular orbital
information.
2.2. Crystal Structural Characterization. Single crystals
suitable for X-ray diffraction were obtained from CHCl3 by
slowly evaporating the solvent in 5 mm outer diameter glass
C
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tubes (30−60 cm long) over a few weeks to a couple of
months. The glass tube was capped with a plastic cap (for 5
mm NMR tube) that provides a physical barrier that is not
perfectly airtight, allowing the solvents to slowly evaporate over
a long period of time through the gap between the glass tube
and the plastic cap. The tube was then placed in a dark place
and left undisturbed for single crystal growth. If the single
crystal obtained was too small or was of poor quality, we
redissolved most of the small crystals, but left a few of them as
seed crystals. We found that the purity of the compounds and
cleanliness of the solution (filtered with 0.2 um or 0.45 um
PTFE syringe filter) were key to growing high quality single
crystals that are suitable for XRD data collection. Single crystal
XRD data were collected at 100 K using Mo−Kα radiation (λ =
0.71073 Å) on a Bruker CCD APEXII diffractometer.
Structures were solved by the direct method using SHELXL-
97 in conjunction with standard differential Fourier techniques
and subsequently refined by full-matrix least-squares analyses.
All hydrogen atoms were placed in their ideal positions, and all
non-hydrogen atoms were refined anisotropically. Detailed
crystal structure refinement data are listed in Table 1, and the
corresponding CIF file is given in the Supporting Information.
Comparing to the previously reported crystal structure of
compound 4, its constitutional isomer compound 2 shows a
perfect π−π stacked lamellar crystal packing motif and shorter
interplanar distance (3.400 Å) between adjacent aromatic rings
(Figure 1) than that of compound 4 (3.416 Å). It is not
perfluorobutyl group nearby. In contrast, there are fewer of
Csp2−H dipoles in compound 2 that could form Csp2−H
dipole−π interactions due to the steric hindrance of the nearby
perfluorobutyl groups. The simple space-filling models of these
two compounds clearly demonstrate the different possibilities
to form Csp2−H dipole−π interactions between these two
compounds (Figure 1). Thus, compound 2 possesses much less
probability to form Csp2−H dipole−π interactions that would
favor a T-shaped molecular packing motif. Furthermore,
compound 2 possesses a different topology from compound
4, which may also play a role that affects the molecular packing
in the solid state.
This interesting result promoted us to further investigate
different aromatic systems with similar substituents and
topology. Compounds 1 and 3 were synthesized by the same
type of condensation reaction, except for compound 1, which
involved a deprotonation step after the condensation reaction.
Because of the formation of a new aromatic system after
deprotonation, the formation of compound 1 is favorable even
under ambient conditions. Compound 3 shows a perfect π−π
stacked lamellar structure with an interplanar distance of 3.277
Åonly 0.03 Å longer than the previously reported interplanar
distance for dithiophenephenazine.27 Compounds 2 and 3 pack
in a very similar packing motif. The increased π-conjugation
assists to significantly reduce the interplanar distance from
3.400 to 3.277 Å, with a total 0.123 Å reduction of interplanar
distance without alternating the solid state structure. This result
demonstrates the steering power of the perfluoroalkyl
substituents in crystal engineering. A comparison of crystal
structures of compounds 2 and 3 shows a clear trend of how π-
conjugation can change the interplanar distances between
adjacent molecules in the solid state (Figure 2).
Figure 2. Interplanar distances and π−π overlapping mode (top, top
view; bottom, side view) of compounds 2 and 3.
Figure 1. Crystal structures (top), space-filling models (middle), and
overall molecular packing motifs (bottom) of compound 2 (left) and
its constitutional isomer, previously reported compound 427 (right).
Compound 1 also shows an overall perfect π−π stacked
lamellar packing motif; however, it involves different packing
motifs in the crystal structure compared to compounds 2 and 3.
In addition, it shows a different local molecular packing motif
between adjacent molecules. Multiple interplanar distances of
3.150, 3.307, and 3.389 Å were observed between different pairs
of adjacent molecules. The topology of the compound 1
provides a perfect equilateral triangle shape on the aromatic
region; such a topological shape provides an opportunity for
molecules tightly packing together in the same layer in an
surprising to see such a reduction of interplanar distance as a
result of the reduced steric hindrance of perfluorobutyl groups
on compound 2 compared to that on compound 4. The
structure of compound 4 shows a slightly twisted overall crystal
packing motif, though a locally π−π stacked structure was
observed in the crystal structure.27 Such a twisted packing motif
involves some level of Csp2−H dipole−π interactions from the
edge of the aromatic core where there is no bulky
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Figure 3. Topological view of a layer of compound 1 in the crystal structure (top) and π−π overlapping mode (middle, top view of two layers of
compound 1; bottom, side view of the same stack). Top layer aromatics are coded in pink color, the bottom layer aromatics are coded in blue color.
antiparallel fashion (Figure 3). The highest density of
compound 1 among these five compounds further confirms
the tight molecular packing in the solid state (Table 1).
Compound 10 displays a cross-shaped packing motif (Figure
4) in the solid state. When compound 10 is compared with
compounds 1, 2, and 3, compound 10 possesses two CO
double bonds that have a much larger dipole moment than the
Csp2−H dipole moment of compounds 1−3 due to the high
electronegativity of oxygen. Because of the strong electron-
withdrawing power of the two carbonyl and two perfluorobutyl
substituents on the ring, the acidity of the protons on the
aromatic ring increases significantly. This results in increased
hydrogen bonding strength between oxygen and hydrogen on
the aromatic ring. The shortest O···H distance of 2.466 Å is
0.254 Å shorter than that of the sum of the van de Waals radii
of the oxygen and hydrogen atoms. These multiple strong
hydrogen bonds potentially interrupt the π−π stacking
controlled by relatively weaker quadrupole−quadrupole and
dispersion interactions. Though two perfluorobutyl substituents
block the possibility of formation of Csp2−H dipole−π
interaction in this molecule, the two CO dipoles are largely
exposed. Furthermore, the topological structure of this
molecule gives a perfect opportunity to form hydrogen bonding
when the molecules pack in a head-to-tail fashion.
The crystal structure of reaction intermediate compound 9
indirectly assigned the structure of compound 8 that previously
could not be confirmed by the NMR structure analysis.39 The
large steric hindrance of ethylene groups prevents the potential
formation of π−π stacked interactions, though C−H dipole−π
interactions are also clearly observed. One thing in common
with other perfluoroalkylated and fluorinated compounds
reported previously is that many F···F interactions were
observed in the solid state.27,28,40−47 Space-filling models
(Supporting Information, Figure S-1) of these crystal structures
clearly demonstrate again the phase separation in the solid
state; i.e., the fluorous phase separates itself from the
hydrocarbon phase due to the generally weaker Csp3−F···H−
Figure 4. Crystal structure of compound 10 showing CO···H−Csp2
hydrogen bonding.
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Csp2/sp3 interactions compared to other possible Csp3−F···F−
Csp3 interactions and Csp2/sp3−H···H−Csp2/sp3 interactions.
Examining the intermolecular interactions through the
analysis of short contacts in the crystal structures provides
additional insights into why certain compounds pack in
different packing motifs. These short contacts include (1)
Csp3−F···H−Csp2 contacts between the perfluoroalkyl chain and
hydrogen on the aromatic ring, (2) Csp3−F···F−Csp3 contacts
between perfluoroalkyl chains of adjacent molecules, (3) C···C
contacts between aromatic rings or C···N contacts between
aromatic rings, (4) CO···H−Csp2 contacts between adjacent
molecules, and (5) Csp2−H···π contacts. For these five
compounds reported here, the bond angles of Csp3−F···F
within Csp3−F···F−Csp3 contacts are in the range of 112−165°,
similar to the bond angles of typical fluorine-containing halogen
bonds.43−46 The bond angles of Csp3−F···H and F···H−Csp2
within Csp3−F···H−Csp2 contacts are in the similar range, while
Csp3−F···H bond angles are typically smaller than those of F···
H−Csp2 in these compounds. This observation is in line with
recent results from Chopra and co-workers45 and our previous
reports on crystal structures of perfluoroalkylated aro-
matics.27,28 Similar to what we reported previously, we also
observed short contacts involving three fluorine atoms,
(Csp3)F···(Csp3)F···(Csp3)F, from three different molecules
with F···F···F bond angles in the range of 76−87°.
The difference, Δd = [dab − d(ra+rb)], between the short
contact distance dab and the sum of van de Waals radii of two
contact atoms d(ra+rb) is likely one of the useful parameters to
potentially link crystal structures to intermolecular interaction
energies. It is interesting to observe that the largest Δd values
observed in compounds 1, 2, 3, and 4 are the short contact
between Csp3−F···F−Csp3 between perfluoroalkyl chains or C···
C between aromatic rings or C···N between aromatic rings
(Supporting Information, Table S-1). The Δd values observed
for any existing Csp3−F···H−Csp2/sp3 and Csp2−H···π contacts in
these compounds are far smaller than these short contacts. In
contrast, the largest Δd value observed in compound 10 is the
short contact between carbonyl groups and Csp2−H on the
aromatic ring (CO···H−Csp2). The Δd values observed for
the Csp3−F···F−Csp3 short contact between perfluoroalkyl
chains are much smaller than that for CO···H−Csp2. This
observation indicates that the relatively stronger Csp3−F···F−
Csp3 interaction among all the intermolecular interactions is
beneficial for the formation of π−π stacked lamellar packing
motifs. The relatively stronger dipole−π interactions will
potentially destroy the lamellar π−π stacked packing motif
and could favor the herringbone packing in the solid state. This
again demonstrates that the relative strength of different type
interactions plays a key role in the molecular packing in organic
solid state materials. The relative stronger hydrogen bonding,
however, might lead to either the lamellar π−π stacked packing
motif as demonstrated in the literature48,49 or the herringbone
packing motif shown in Figure 4. This observation implies that
the orientation of the hydrogen bonding likely also plays a role
in forming a specific solid state structure.
potential shifts to positive as a result of introduction of
perfluorobutyl substituents and the increase of π-conjugation in
the aromatic ring (Figure 5). The first reduction potential for
Figure 5. Cyclic voltammograms of compounds 2, 3, and 5 in 1,2-
difluorobenzene (DFB) with 0.2 M TBAPF6 as supporting electrolyte.
Potential sweep rate: 100 mV/s.
compound 2 is 220 mV more positive than its nonsubstituted
aromatic core (compound 5). Because of the strong electron-
withdrawing effect of the perfluorobutyl substituents, the
oxidation of compounds 1 and 2 was not observed within the
electrochemical window of the 1,2-DFB/0.2 M TBAPF6
solution.50,51 By further increasing the π-conjugation, com-
pound 3 shows a one-electron reversible reduction at E1/2
=
−1.410 V (vs Fc+/Fc), which is 247 mV more positive than that
of compound 2, and a one-electron irreversible oxidation at Ep/2
= 1.142 V (vs Fc+/Fc) (Table 2). The redox potential
difference between its first reduction and first oxidation is 2.552
V, consistent with observed optical band gap of 2.408 eV (from
onset absorption) measured by UV−visible absorption spectra
in CH2Cl2 (Figure 6). This changing trend of redox potentials
when comparing compound 2 to compound 3 indicates that
addition of another fused phenyl ring to compound 3 could
lead to the first reduction potential shifting further to the
positive direction by another couple hundred mV and the first
oxidation potential shifting further to the negative direction by
at least the same amount. Thus, one could expect to narrow the
band gap by roughly a half eV after adding another fused
phenyl ring onto compound 3. Such synthetic efforts to make
materials with a similar structure, yet possessing larger aromatic
π-conjugation, are currently being undertaken in this lab.
The electronic structure of these compounds in the solid
state was further studied by dispersion corrected density
functional theory (DFT-D).27,28,52−56 Compounds 2 and 3
have very similar packing motifs, showing that the total
intermolecular interaction energies of these compounds
strongly depend on the size of the π-conjugation. The highly
electron-withdrawing nature of the perfluorobutyl substituents
reduces π-cloud density through inductive effects, minimizing
repulsion that could prevent close π−π stacking distances.
Figure 7 shows a comparison of HOMO and LUMO maps of
dimers 2 and 3. Compound 3 shows clear LUMO overlapping
between adjacent molecules on the same π−π stacked column.
The increase in orbital overlap provides excellent opportunities
for increasing charge mobility along the same π-stacked column
in the solid state.57,58
2.3. Physicochemical Properties. The basic physico-
chemistry properties of these compounds were accessed
through cyclic voltammetry and absorption spectra measure-
ments. A reversible one-electron transfer reduction reaction is
observed for all the compounds in Chart 1 except for
compound 1, in which the first reduction potential is too
negative, that is, out of the usable electrochemical window of
the 1,2-DFB/0.2 M TBAPF6 electrolyte solution. The redox
3. CONCLUSION
We developed a new synthetic route that prepares perfluor-
oalkylated N-containing aromatics in good to excellent yield
and is suitable for various sizes of π-conjugation with the ability
to be scaled up. Lamellar π−π stacked structures with tunable
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Table 2. Calculated Intermolecular Dimerization Energies, Measured Interplanar Distances, Electrochemical Properties, and
Optical Band Gaps of Compounds 1−5
c
c
c
cmpd
Edim/kcal/mol
packing mode
dπ−π/Å
E1/2red/V
Ep/2ox/V
EgapEC/eV
Egapopt/eV
refs
a
1
π−π
3.150
3.307
3.389
3.400
3.277
3.416
nd
nd
nd
3.379
a
a
2
3
4
5
−26.00
−31.47
−23.23
π−π
π−π
π−π
T-shaped
−1.657
−1.410
−1.655
−1.877
nd
nd
3.024
2.408
2.884
2.988
1.142
1.496
1.401
2.552
3.151
3.278
ab
,
ab
,
a
b
c
This work. Reference 27. nd = not detected.
the band gap is greatly reduced and the first reduction potential
shifts to the positive direction.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures, NMR and mass spectra,
additional structure information, CIF file, and additional
references. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Haoran.Sun@usd.edu (H.S.).
Figure 6. UV−visible absorption spectra of compounds 1, 2, and 3 in
dichloromethane.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
The authors are thankful for the financial support from the NSF
CAREER program (grant number CHE1355677). J.D.M
thanks the NSF IGERT program (Award No. DGE-0903685)
for support of his graduate study. The authors also acknowl-
edge NSF-EPSCoR (EPS-0554609) for the purchase of a
Bruker SMART APEX II CCD diffractometer, NSF-MRI grant
(CHE-1229035) for the purchase of a Bruker 400 MHz NMR
spectrometer, and the University of South Dakota high
performance computing facility. The authors thank Dr.
Anjaneyulu Putta for the initial helpful discussion and Mr.
Madhubabu Alaparthi for assistance in solving the crystal
structure of compound 3.
Figure 7. Comparison of HOMO and LUMO of compounds 2 (left)
and 3 (right) in the dimer state (took directly from the crystal
structure for the single-point energy calculation).
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