Trifluoromethyl-substituted conjugated oligoelectrolytes

Article abstract of DOI:10.1002/chem.201000885

Conjugated oligoelectrolytes (COEs) are being introduced into a variety of optical and electronic technologies, yet the dependence of their properties as a function of molecular structure remains poorly understood. In response, we designed, synthesized, and examined a new tetracationic COE, namely, 1,4-bis{9′,9′-bis[6″-(N,N,N-trimethylammonium)hexyl] -2′-fluorenyl)-2,5-bis(trifluoromethyl)benzene tetrabromide (FPF-F6), which contains bulky electron-withdrawing trifluoromethyl groups, and compared its properties with the unsubstituted counterpart 1,4-bis{9′,9′- bis[6″-(N,N,N-trimethylammonium)hexyl]-2′-fluorenyl}benzene tetrabromide (FPF). The groundstate geometry of FPF-F6 is primarily twisted with little electronic communication between the aromatic units, as confirmed by single-crystal X-ray diffraction studies of the neutral precursor. However, absorption and photoluminescence spectroscopies reveal that the excited state of FPF-F6 displays strong intramolecular charge-transfer characteristics. Solution AFM in aqueous media shows that introduction of trifluoromethyl groups changes the size and aspect ratio of supramolecular aggregates that are brought together as a result of hydrophobic interactions. Furthermore, addition of ssDNA to FPFF6 leads to interoligoelectrolyte complexes wherein the backbone is more planar; the environment the chromophore experiences under these conditions is also considerably less polar. These findings provide considerable insight into the complex photophysics of electronically conjugated materials in aqueous media.

Full text of DOI:10.1002/chem.201000885

DOI: 10.1002/chem.201000885
Trifluoromethyl-Substituted Conjugated Oligoelectrolytes
[
a, b]
[a]
Yan Ren
and Guillermo C. Bazan*
Abstract: Conjugated oligoelectrolytes
COEs) are being introduced into a va-
thylammonium)hexyl]-2’-fluorenyl}ben-
zene tetrabromide (FPF). The ground-
state geometry of FPF-F6 is primarily
twisted with little electronic communi-
cation between the aromatic units, as
confirmed by single-crystal X-ray dif-
fraction studies of the neutral precur-
sor. However, absorption and photolu-
minescence spectroscopies reveal that
the excited state of FPF-F6 displays
strong intramolecular charge-transfer
characteristics. Solution AFM in aque-
ous media shows that introduction of
trifluoromethyl groups changes the size
and aspect ratio of supramolecular ag-
gregates that are brought together as a
result of hydrophobic interactions. Fur-
thermore, addition of ssDNA to FPF-
F6 leads to interoligoelectrolyte com-
plexes wherein the backbone is more
planar; the environment the chromo-
phore experiences under these condi-
tions is also considerably less polar.
These findings provide considerable in-
sight into the complex photophysics of
electronically conjugated materials in
aqueous media.
(
riety of optical and electronic technolo-
gies, yet the dependence of their prop-
erties as a function of molecular struc-
ture remains poorly understood. In re-
sponse, we designed, synthesized, and
examined a new tetracationic COE,
namely, 1,4-bis{9’,9’-bis
methylammonium)hexyl]-2’-fluorenyl}-
,5-bis(trifluoromethyl)benzene tetra-
ACHTUNGTRENNUN[G 6’’-(N,N,N-tri-
2
bromide (FPF-F6), which contains
bulky electron-withdrawing trifluoro-
methyl groups, and compared its prop-
erties with the unsubstituted counter-
Keywords: biosensors
transfer · conjugation · oligoelectro-
lytes · photoluminescence
·
charge
AHCTUNGTRENNUNG
part
1,4-bis{9’,9’-bis AHCTUNETGRNNUN[G 6’’-(N,N,N-trime-
Introduction
ities or mediation of charge-carrier injection/transport. For
example, cationic COEs can be complexed with (anionic)
dye-labeled single-stranded DNA (ssDNA) in aqueous
Conjugated oligoelectrolytes (COEs) contain a well-defined
number of p-delocalized repeat units with pendant function-
al groups capable of being ionized in high dielectric media.
These molecules are becoming incorporated into a range of
innovative technologies that require light-harvesting capabil-
[2]
media. The resulting electrostatic aggregates contain struc-
tural features that allow for differential interactions with
various proteins and optical properties that change in re-
sponse to these structural/local site modifications. As a
result, it is possible by using various ssDNA sequences to
create an array of aggregates that can identify various pro-
teins without relying on lock-and-key principles. A thin
COE layer can also be introduced adjacent to a metallic
electrode to lower the barrier to electron injection into an
[1]
[
a] Dr. Y. Ren, Prof. G. C. Bazan
[1,3]
Departments of Materials and Chemistry & Biochemistry
Center for Polymers and Organic Solids
University of California, Santa Barbara, California 93106 (USA)
Fax : (+1)805-893-4270
organic semiconductor.
This process requires preferential
alignment of dipoles and involves surface reconstruction
[4]
processes in the bulk that remain poorly understood.
Despite the progress in applying COEs to various technol-
ogies, there have been only a few studies on how their mo-
lecular structure influences intrinsic optical properties under
various conditions and in different media. Conjugated poly-
electrolytes, that is, COE analogues with a larger number of
repeat units along the backbone, have been examined more
E-mail: bazan@chem.ucsb.edu
[
b] Dr. Y. Ren
State Key Laboratory of Crystal Materials
Shandong University, Jinan 250100 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000885. It contains crystal
data, structure refinement, and full analysis of intermolecular interac-
tions in lattice for p-FPF and p-FPF-F6. UV absorption and photolu-
minescence (PL) spectra of FPF-F6 in water, PL spectrum of FPF/
DNA complex.
[5]
broadly. These studies have raised awareness for processes
that are strongly influenced by interchain contacts and by
[6]
different chemical functionalities. Such features are a spe-
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Chem. Eur. J. 2010, 16, 11028 – 11036

FULL PAPER
cial concern in water, where the hydrophobic aromatic p-de-
localized backbone provides the driving force for spontane-
ous aggregation, even in the absence of an oppositely
Results and Discussion
Syntheses and characterization: Compound FPF was synthe-
sized according to a procedure reported previously. The
synthesis of FPF-F6 is shown in Scheme 1. Treatment of 2-
[7]
[9]
charged polyelectrolyte. In response to the absence of
basic information on COE photophysics, supramolecular
[8]
structures,
tendencies, we examine in this
contribution 1,4-bis{9’,9’-bis[6’’-
N,N,N-trimethylammonium)-
and aggregation
AHCTUNGTRENNUNG
(
hexyl]-2’-fluorenyl}benzene tet-
rabromide (FPF) and 1,4-bis-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
{9’,9’-bis
ammonium)hexyl]-2’-fluorenyl}-
,5-bis(trifluoromethyl)benzene
ACHTUNGTRENNUNG[ 6’’-(N,N,N-trimethyl-
2
tetrabromide (FPF-F6), with
the goal of understanding the
influence of attaching a bulky Scheme 1. Synthesis of FPF-F6: i) 2,5-dibromo-1,4-bis(trifluoromethyl)benzene (2), [Pd
ACHTUNGTRENN(NGU PPh ) ], K CO , tolu-
3 4 2 3
electron withdrawing group to ene, 958C; ii) NMe , THF, methanol, À788C.
3
the conjugated segment.
[10]
bromo-9,9-bis(6’-bromohexyl)fluorene
to)diboron under air-free conditions provided 2-(4,4,5,5-
tetramethyl[1,3,2]dioxaborolane)-9,9-bis(6’-bromohexyl)-
with bis(pinacola-
AHCTUNGTRENNUNG
fluorene (1). Bromination of 1,4-bis(trifluoromethyl)ben-
zene was achieved by using trifluoroacetic acid (TFA) as the
solvent in the presence of catalytic H SO to give 2,5-dibro-
2
4
mo-1,4-bis(trifluoromethyl)benzene (2) in 90% yield. The
following step was a Suzuki cross-coupling reaction between
compounds 1 and 2 to provide the neutral precursor oligo-
mer 1,4-bis(9’, 9’-bis(6’’-bromohexyl)-2’-fluorenyl)-2,5-bis(tri-
fluoromethyl)benzene (p-FPF-F6, in Scheme 1). Characteri-
zation of p-FPF-F6 was accomplished by mass spectrometry
1
13
by using FAB/NBA as well as H and C NMR spectrosco-
py and elemental analysis. The m/z signals observed in the
range from 1190 to 1198 are consistent with the correct iso-
topic distribution due to four bromide atoms. In the last
step, compound p-FPF-F6 was quaternized by using NMe3
in a THF/MeOH solvent mixture to yield FPF-F6, which ex-
hibits good solubility in aqueous media and highly polar or-
ganic solvents.
The composition of FPF-F6 was confirmed by NMR spec-
1
troscopy and ESI-MS. Compared with the H NMR spec-
trum of the neutral p-FPF-F6, a singlet appears at d=
3
.06 ppm for FPF-F6, with an integrated area that coincides
Herein, we describe the synthesis of FPF-F6 and subse-
quently show spectral features associated with structural dis-
tortions of the backbone and intramolecular charge-transfer
characteristics that stem from the electron-withdrawing tri-
fluoromethyl groups. Single-crystal X-ray diffraction experi-
ments of precursor materials are used to support evidence
of the conformational differences between FPF and FPF-F6.
Finally, we report the differences in the aggregation of these
COEs, both spontaneously as a report of hydrophobic inter-
actions, and upon electrostatic binding to an oppositely
charged macromolecule.
with the anticipated number of protons from four trimethyl-
1
3
ammonium groups. In the C NMR spectrum of FPF-F6,
the occurrence of a peak at d=56.2 ppm with highest inten-
sity among all the alkyl resonances is consistent with quater-
nization by trimethylamine. At the same time, the chemical
shift of the terminal carbon in the pendant alkyl group
moves to higher field (d=59.1 ppm) owing to the action of
the adjacent nitrogen-based cation. ESI-MS supports the
proposed structure: the quasimolecular ion peak m/z is
+
277.67 [MÀ4Br] . Thermogravimetric analysis (TGA) re-
[11]
vealed that FPF-F6 is highly hydroscopic. After standing
in air for an extended period of time one finds up to seven
adsorbed water molecules per conjugated segment. TGA
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11029

G. C. Bazan and Y. Ren
also showed that the content of the water absorbed in the
powder is variable based on the duration of storage.
Absorption and photoluminescence spectroscopy: Figure 1
compares the UV/Vis and photoluminescence (PL) spectra
of FPF and FPF-F6 in phosphate buffer. This aqueous
Figure 2. Absorption (line) and PL (square) spectra of 1 (black), p-FPF-
F6 (red), and FPF-F6 (blue) in DMF.
F6 in the same solvent (DMF). We surmise that neither the
solvent nor the presence of adjacent charged groups influ-
ence the absorption spectrum of the integral p component
in FPF-F6. Furthermore, despite the apparently more ex-
tended conjugation length, the absorption of FPF-F6 be-
haves similar to that of a single fluorene fragment. These
data suggest ineffective delocalization in the ground state
and would be consistent with the aromatic rings in the fluo-
rene–phenylene–fluorene framework being severely twisted
relative to each other.
Figure 1. Absorption and PL spectra of FPF (solid) and FPF-F6 (dashed)
in phosphate buffer.
medium was of specific interest to us because of its rele-
vance to conditions usually found in biosensory schemes.
However, spectral features observed in buffer are essentially
identical to those observed in water (see the Supporting In-
formation). We begin by noting that the absorption maxi-
mum (lmax) of FPF, which lacks the two trifluoromethyl
groups, is observed at 340 nm. Using an excitation wave-
Solvatochromic effects on the PL properties of the chro-
mophores were explored by taking advantage of the neutral
precursors. According to previous studies on neutral and
charged distyrylbenzene derivatives, ionic groups on the pe-
riphery of the molecular structure do not greatly perturb the
[12]
length (l ) of 340 nm leads to a vibrationally defined emis-
excited-state energies. Rather, their function is to increase
solubility in polar media. As a result, the PL properties of
neutral and charged chromophores are very close to each
other when in the same solvent. This effect has already been
demonstrated in Figure 2. Hexane and DMSO were used for
the solvatochromic studies described herein, since the pre-
cursor to FPF, 1,4-bis[9’, 9’-bis(6’’-bromohexyl)-2’-fluorenyl]-
benzene (p-FPF) and p-FPF-F6 possess acceptable solubility
in these solvents.
ex
sion band with a maximum (l ) at 380 nm, corresponding
em
to a Stokes shift of 40 nm (0.39 eV). Examination of the ab-
sorption spectrum of FPF-F6 reveals that lmax =310 nm,
which is considerably blueshifted relative to that of FPF.
The spectral characteristics of the FPF-F6 absorption are
also considerably sharper and with more fine structure. The
PL spectrum of FPF-F6 is broad with lem =413 nm, corre-
sponding to a Stokes shift of 103 nm (1.0 eV). The PL quan-
tum yields of FPF and FPF-F6 in phosphate buffer with qui-
nine sulfate as the standard are 0.85 and 0.72, respectively.
Understanding the differences in spectral properties, partic-
ularly the absorbance between FPF and FPF-F6, guides the
choice of subsequent experiments.
We next focused on possible perturbations as a result of
the ionic functionalities. Figure 2 shows that the absorption
spectra of FPF-F6 and the precursor oligomer p-FPF-F6 in
DMF are nearly indistinguishable. DMF was chosen because
sufficient quantities of the neutral and charged versions of
the chromophores can be dissolved in this solvent. It is help-
ful to note here that the absorption spectra of FPF-F6 in
DMF and aqueous media are very similar to each other.
Figure 2 also shows that the absorption spectra of FPF-F6
and p-FPF-F6 resemble that of compound 1 (Scheme 1),
which corresponds to a well-defined fluorene chromophore.
However, the PL of 1 is blueshifted relative to that of FPF-
As shown in Figure 3, the PL maxima of p-FPF-F6 red-
shifts from lem =376 nm in hexane to lem =405 nm in
DMSO, corresponding to a solvatochromic shift of 29 nm
A( ꢀ0.24 eV). In the case of p-FPF the shift is much less pro-
H
U
G
R
N
N
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Conjugated Oligoelectrolytes
FULL PAPER
Figure 3. Solvent effects on the PL spectra of p-FPF in hexane (black)
and DMSO (red); p-FPF-F6 in hexane (green), and DMSO (blue), [p-
FPF]=[p-FPF-F6]=5 mm, lex (p-FPF)=340 nm, lex (p-FPF-F6)=310 nm.
Figure 4. PL spectra of FPF in ethylene glycol at 293 (solid line) and
77 K (dashed line).
nounced, in the order of 9 nm (ꢀ0.08 eV). Larger solvato-
chromic shifts and broader spectra are observed for mole-
[13]
cules with more polar excited states.
This observation
arises due to the ability of the solvent to lower the chromo-
phore energy through Coulomb-type interactions between
the solvent dipole and the electrostatic distribution in the
[14]
excited state.
The larger solvatochromic effect observed
with p-FPF-F6 than p-FPF indicates that introduction of the
trifluoromethyl groups leads to a more polar excited state.
This observation, in conjunction with the more redshifted
emission, leads us to propose that p-FPF-F6, and by infer-
ence that of FPF-F6, has a more planar geometry in the ex-
cited than in the ground state. Under these conditions, the
influence of the electron-rich terminal rings is more pro-
nounced and leads to an excited state with intermolecular
charge-transfer (ICT) characteristics. In the case of FPF, re-
gardless of the preferred conformation, there is a much
weaker driving force for ICT, the excitation is primarily de-
scribed by a p–p* transition.
Figure 5. PL spectra of FPF-F6 in ethylene glycol at 293 (solid line) and
77 K (dashed line).
temperature (Figure 3), where solvent stabilization by
dipole interactions is anticipated to be minimal. The PL
spectrum at low temperature is about 3–6 nm blueshifted
from that at room temperature in hexane. Although not
quantitative in nature, the collective set of observations de-
scribed above indicates that the viscous environment of eth-
ylene glycol at 77 K not only slows down the rate of solvent
reorganization to stabilize the more polar excited state, but
also partially decreases the rate of inter-ring rotation, lead-
ing to less effective electronic delocalization.
Low-temperature PL measurements were performed to
obtain additional insight into the role of the solvent. One
particular interesting challenge is to untangle how intramo-
lecular planarization and solvent reorganization processes
contribute to differences in spectral characteristics. To de-
crease the solvent relaxation as much as possible at low tem-
perature, we measured the PL of FPF and FPF-F6 in the
polar solvent ethylene glycol, the viscosity of which increas-
es gradually with decreasing temperature and does not crys-
[14,15]
tallize.
The PL spectra obtained with FPF and FPF-F6
Single-crystal X-ray diffraction studies: Single crystals of
FPF and FPF-F6 suitable for X-ray diffraction studies
proved difficult to obtain. Both species form highly viscous
saturated solutions in polar solvents such as methanol. How-
ever, the situation is different for the precursor molecules.
Single crystals of p-FPF and p-FPF-F6 were grown by slow
evaporation of solutions in hexane at room temperature.
Figure 6 shows the molecular structures determined by X-
ray diffraction (complete details concerning crystal data and
structural refinements are provided in the Supporting Infor-
mation). Both p-FPF and p-FPF-F6 crystallize in a triclinic
in ethylene glycol at 77 and 293 K are shown in Figures 4
and 5, respectively. From Figure 4, one observes little differ-
ence between the PL maxima of FPF at 293 and 77 K (5 nm
blueshift upon decreasing the temperature, 0.04 eV). In con-
trast, there is 20 nm (0.17 eV) blueshift for FPF-F6 at 77 K
(
Figure 5). Furthermore, note that the PL spectrum of FPF-
F6 at low temperature (lem =370 nm) is blueshifted from
that of FPF at room temperature (lem =380 nm), which is
opposite to the case at room temperature (see Figure 1). It
is also informative to compare the PL spectrum of FPF-F6
in ethylene glycol at 77 K with p-FPF-F6 in hexane at room
¯
lattice with the space group P1. The two fluorene rings in
Chem. Eur. J. 2010, 16, 11028 – 11036
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G. C. Bazan and Y. Ren
the hydrophobic aromatic framework is known to play an
important role in determining the optical performance of
[16]
optically amplified fluorescent biosensory assays.
This
body of work led us to investigate, by using liquid phase
[17]
AFM, to what degree differences in the molecular struc-
tures of FPF-F6 and FPF might change the sizes and aspect
ratios of the multichromophore aggregates. Compared with
traditional AFM, liquid-phase AFM can produce high-reso-
lution topographic images of supramolecular assemblies in
solution upon adsorption onto a negatively charge mica sur-
face.
A typical liquid AFM experiment follows the deposition
of 50 mL of COE solution at a concentration of 1 mm on
freshly cleaved V1 grade mica surface for 30 min in a closed
humid chamber. After deposition, images of samples were
recorded under phosphate buffer inside an AFM liquid
chamber. The relatively high concentration of FPF and FPF-
F6 was chosen to encourage their self-assembly. Figure 7
shows the resulting images. In the case of FPF, one observes
fiberlike structures that densely adsorb onto the mica sur-
face. FPF-F6 instead forms spherically shaped nanoparticles,
and the surface coverage is less pronounced. The size and
aspect ratio of the aggregates reflect that the optimal inter-
molecular packing geometries for minimizing the p-compo-
nent contact with water for the two molecular systems are
different. We recognize that binding to the negatively
charged mica may perturb the general physical characteris-
tics of the supramolecular structures. For example, aggre-
gates on mica may favor a curvature lower than the corre-
sponding free species. Mica may also allow closer packing of
oppositely charged terminal groups at the surface than is
found in free micelles, since the surface acts as a highly
Figure 6. Molecular structures of p-FPF and p-FPF-F6. The hydrogen
atoms are omitted for improving the clarity of the molecular features.
(
blue: C, red: Br, green: F)
[18]
charged laterally extended counterion. Despite these un-
certainties, the data in Figure 7 highlight that the final struc-
tures of the aggregates and ability of conjugated oligoelec-
trolytes to bind to oppositely charged macromolecules or
surfaces is heavily influenced by the internal molecular fea-
tures of the conjugated framework; such considerations are
of particular importance for situations in which selective
binding to charged surfaces is an important part of the sen-
the molecules are parallel to each other. Their pendant
groups situate perpendicular to the fluorene planes. Howev-
er, p-FPF and p-FPF-F6 exhibit different torsion angles be-
tween the internal ring and the outer fluorene planes. For
the two independent molecules in the unit cell of p-FPF, the
torsion angles are similar to each other with values of À26.5
(
8) and À27.5(8)8. In contrast, for p-FPF-F6, the torsion
[19]
angles of the trifluoromethyl-substituted benzene plane to
the fluorene ring are different in the two independent mole-
cules: 72.7(10) and À58.2(11)8, which are much larger than
the torsion angles in p-FPF. These details are fully in agree-
ment with the assignment of the more blueshifted absorp-
tion in FPF-F6 (Figure 1) due to less effective electronic de-
localization in the ground state.
Examination of the lattice diagrams (see the Supporting
Information) shows close intermolecular contacts in the case
of p-FPF. For example, face-to-face contact between sym-
metric molecules along the p-stacking direction is 3.27 ꢁ.
Introduction of the trifluoromethyl groups leads to a consid-
erable weakening of the interaction between aromatic units.
A full analysis can be found in the Supporting Information.
sory process, for example, in DNA chips.
Photophysics of electrostatic aggregates: Electrostatic bind-
ing of cationic COEs to negatively charged macromolecules
induces electrostatic complexation and the formation of
[2]
inter-oligoelectrolyte aggregates. While the spectral prop-
erties of such nanoheterogeneous systems provide the basis
[20]
for transduction,
for example, in protein identification
[2]
assays, the effect of local structure on the basic optical fea-
tures of the COEs remains unexplored. Conversely, intro-
ducing molecules within these higher-order supramolecular
systems can be used to gain insight into the basic character-
istics of optical reporters. For the studies reported herein,
we examined how FPF and FPF-F6 interact with negatively
charged ssDNA in dilute phosphate buffer. The randomly
chosen ssDNA sequence corresponds to 5’-TGA CCA ACC
ACA CCA ACC-3’. The COE/ssDNA stoichiometry was
Liquid-phase AFM images of aggregates: Aggregation of
COEs and related molecules in aqueous media as a result of
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Conjugated Oligoelectrolytes
FULL PAPER
Figure 8. Absorption spectra of a) FPF/ssDNA and b) FPF-F6/ssDNA in
phosphate buffer with the ratio of positive to negative charge R+/À =0.26
(
dashed lines). The solid line is the spectrum without ssDNA shown for
ease of comparison. The concentration of the oligomers is 5 mm.
Figure 7. Imaging of the aggregates of a) fiberlike FPF and b) spherically
shaped nanoparticles of FPF-F6 by using solution AFM. All images were
collected in the scale of 1.2ꢂ1.2 mm, the images represent height channel
(
the z scale corresponds to 10 nm).
calculated based on the ratio of positive and the negative
charges.
Figure 9. PL spectra of FPF-F6/ssDNA (dashed line) in phosphate buffer
Figure 8 compares the absorption spectra of FPF and
FPF-F6 in the absence and presence of ssDNA. The absorp-
tion maxima of FPF remain relatively unperturbed. Howev-
er, in the case of FPF-F6 one observes the appearance of a
redshifted shoulder when ssDNA is present. Figure 9 shows
that the PL spectra of FPF-F6 blueshifts in the presence of
the ssDNA. No such changes occur for FPF (see the Sup-
porting Information). Therefore, at least in the case of FPF-
F6, complexation to ssDNA yields a redshift in absorption
and a blueshift in emission. The former we attribute to
structural modification of the p system when incorporated
into the aggregate, for example, a more coplanar arrange-
ment of the aromatic units, which would increase the effec-
with the positive to negative charge ratio R+ =0.26. Shown in a solid
black line is the spectrum without ssDNA for comparison.
/À
tive electronic delocalization. The emission at higher ener-
gies is attributed to increased hydrophobic contacts within
the inter-oligoelectrolyte aggregates, which decreases the
ability of water to stabilize the charge-transfer excited state.
Conclusion
We report on the synthesis and characterization of FPF-F6,
a new COE that contains bulky trifluoromethyl substituents
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11033

G. C. Bazan and Y. Ren
on a central phenylene unit flanked by two fluorene frag-
ments. The optical properties of this molecule reveal fea-
tures as a result of steric constraints that disfavor a coplanar
arrangement of the aromatic units in the ground state and
therefore minimize p delocalization. However, the strong
solvatochromic dependence of the PL emission indicates an
excited state with substantial intramolecular charge-transfer
character. We propose that excitation leads to a planariza-
tion of the molecule and that this geometry favors intramo-
lecular charge transfer, wherein the outer fluorene units
take up the role of donor units and the internal, electron-
poor unit behaves as the acceptor site. The overall process is
summarized in Scheme 2.
fer characteristics allows one to establish that the chromo-
phores in the ssDNA oligoelectrolyte aggregates find them-
selves in a much less polar environment, relative to the sit-
uation when they are dissolved in buffer. While the internal
structure of these electrostatic aggregates is unknown at this
point, the data indicate that not only is charge pairing signif-
icant, but also interactions between the hydrophobic compo-
nents, which ultimately are capable of effectively shielding
the optical units from contact with water.
Findings described herein highlight that significant com-
plexity can be found even in the most basic optical proper-
ties and supramolecular structures of COEs. A better under-
standing of the myriad processes involved in determining
the final optical response provides a better rationale for in-
corporation of these emerging materials within technologies
associated with biosensors and optoelectronic devices.
Structural characterization of the precursor molecules p-
FPF and p-FPF-F6 confirm the higher torsional angle be-
tween aromatic segments. Furthermore, the lattice arrange-
ment shows much weaker interactions for the fluorinated
species. It is tempting to assume that the tighter p–p con-
tacts in the precursor p-FPF molecule are also present in
the hydrophobic aggregates of FPF, and that this feature is
responsible for the fiberlike aggregates observed by solution
AFM. The difference in intramolecular arrangements and
steric substitution not only influences the shape of these ag-
gregates, but also their ability to bind to oppositely charged
species. This latter point is demonstrated by the lower densi-
ty of FPF-F6 aggregates imaged on mica; see Figure 7a and
b. It is also worth noting the relevance to biosensory assays
that depend on the electrostatic binding of reporter COEs
upon specific biomolecular recognition events, as for in-
stance hybridization of complementary ssDNA to neutral
Experimental Section
General: Unless otherwise indicated, all reagents were obtained from
commercial suppliers (Sigma–Aldrich, Acros), of which N-bromosuccini-
mide (NBS) was used after recrystallization in hot water, whereas other
1
13
chemicals were used without further purification. H and C NMR spec-
tra were collected on a Varian UNITY INOVA 400 MHz NMR spec-
trometer. Mass spectroscopy and elemental analysis were performed in
the UC Santa Barbara Mass Spectrometry Lab and Elemental Analysis
Center. TGA measurements were finished in Materials Research Labora-
tory, UC Santa Barbara, by using a Mettler TGA/sDTA851e thermogra-
vimetric analyzer. UV spectra were recorded on a Shimadzu UV-2401
PC diode array spectrometer. Fluorescence spectra were recorded using
a Spex Fluorolog 2 spectrometer equipped with a xenon lamp light
source and photomultiplier tube detector.
[19]
PNA-containing surfaces.
Monitoring the absorbance of FPF-F6 to ssDNA reveals
that the absorption is redshifted, which we take to indicate
that the association between oppositely charged groups has
an influence on the conformation of the p-conjugated seg-
ment. These conditions appear to lead to a less twisted inter-
nal conformation. Furthermore, that the PL spectra of FPF-
F6 displays a solvatochromic effect due to the charge-trans-
Liquid-phase AFM images: The AFM images were recorded in the am-
plitude-modulated tapping mode by using a MultiMode microscope
equipped with an E scanner controlled by Nanoscope IIIA (Veeco, Santa
Barbara, CA). For experiments performed under liquid, a new type of sil-
icon nitride probes (MSNL-Au coated) with a resonance frequency of ap-
À1
proximately 8 kHz and a spring constant of kꢀ0.03 Nm (Veeco Probes,
Santa Barbara, CA) were used. Images were processed and analyzed by
using NanoScope software (Veeco, Santa Barbara, CA) and leveled by a
first-order plane fit to correct for the sample tilt.
Scheme 2. Schematic illustration of the geometry transition of FPF-F6 from a twisted ground state (gs) to a planar excited state (*).
11034
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Chem. Eur. J. 2010, 16, 11028 – 11036

Conjugated Oligoelectrolytes
FULL PAPER
Single-crystal structural analysis: The crystal was mounted on a glass
fiber and transferred to a Bruker CCD platform diffractometer. The
SMART program was used to determine the unit cell parameters and
data collection (25s per frame, 0.38 per frame for a sphere of diffraction
[4] C. V. Hoven, A. Garcia, G. C. Bazan, T. Q. Nguyen, Adv. Mater.
2008, 20, 3793–3810.
[5] a) H. Jiang, P. Taranekar, J. R. Reynolds, K. S. Schanze, Angew.
Chem. 2009, 121, 4364–4381; Angew. Chem. Int. Ed. 2009, 48, 4300–
4316; b) B. S. Gaylord, A. J. Heeger, G. C. Bazan, Proc. Natl. Acad.
Sci. USA 2002, 99, 10954–10957; c) D. L. Wang, X. Gong, P. S.
Heeger, F. Rininsland, G. C. Bazan, A. J. Heeger, Proc. Natl. Acad.
Sci. USA 2002, 99, 49–53; d) J. Tian, C. C. Wu, M. E. Thompson,
J. C. Sturm, R. A. Register, Chem. Mater. 1995, 7, 2190–2198;
e) A. F. Thꢃnemann, Adv. Mater. 1999, 11, 127–130; f) T. Kawai, T.
Yamaue, K. Tada, M. Onoda, S. H. Jin, S. K. Choi, K. Yoshino, Jpn.
J. Appl. Phys. Part 2 1996, 35, L741–L744; g) V. Cimrovꢄ, W.
Schmidt, R. Rulkens, M. Schulze, W. Meyer, D. Neher, Adv. Mater.
1996, 8, 585–588.
[
21]
data). The data were collected at 220 K by using an Oxford nitrogen
gas cryostream system. The raw frame data were processed by using the
[
22]
SAINT program.
The empirical absorption correction was applied
based on psi-scan. Subsequent calculations were carried out by using the
[
23]
SHELXTL program.
1
Compound 1: H NMR (400 MHz, CDCl
3
): d=0.55–0.60 (m, 4H), 1.03–
1
2
3
.09 (m, 4H), 1.13–1.21 (m, 4H), 1.40 (s, 12H), 1.6–1.67 (m, 4H), 1.96–
.02 (m, 4H), 3.25–3.29 (t, J=7.0, 4H), 7.33–7.36 (m, 3H), 7.70–7.75 (m,
H), 7.80–7.83 ppm (d, J=7.6, 1H); elemental analysis calcd (%) for
C
31
H
43BBr
2
O
2
: C 60.36, H 6.92; found: C 60.68, H 6.93.
1
[6] a) Self-Assembling Complexes for Gene Delivery. From Laboratory
to Clinical Trial (Eds.: A. V. Kabanov, P. Felgner, L. W. Seymour),
Wiley, New York, 1998; b) M. Stork, B. S. Gaylord, A. J. Heeger,
G. C. Bazan, Adv. Mater. 2002, 14, 361–366; c) J. W. Hong, W. L.
Henme, G. E. Keller, M. T. Rinke, G. C. Bazan, Adv. Mater. 2006,
Compound 2: H NMR (400 MHz, CDCl
3
): d=8.01 ppm (s, 2H).
Compound p-FPF-F6: Compounds
.38 mmol), and [Pd(PPh
ene (50 mL) and 2m K
stirred at 958C for 24 h under an argon atmosphere. The mixture was
cooled to room temperature. CH Cl was used to extract the product,
which was then washed with brine and water, and dried with anhydrous
MgSO . The crude product was obtained by removing the solvent under
vacuum; this was then purified by chromatography on a silica gel column
using 3:1 (v/v) hexane/CH Cl as the eluent to give p-FPF-F6 as colorless
crystal (2.9 g, 45%). H NMR (400 MHz, CDCl ): d=0.66–0.67 (m, 8H),
.08–1.14 (m, 8H), 1.18–1.26 (m, 8H), 1.64–1.71 (m, 8H), 1.99–2.04 (t,
J=8.2, 8H), 3.28–3.31 (t, J=7.0, 8H), 7.36–7.39 (m, 10H), 7.76–7.81 (m,
1
(7.096 g, 11.29 mmol),
2 (2 g,
5
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
3
)
4
] (130 mg) were added to a solution of tolu-
2
CO (26.9 mL),. The mixture was vigorously
3
1
8, 878–882; d) C. Chi, A. Mikhailovsky, G. C. Bazan, J. Am. Chem.
2
2
Soc. 2007, 129, 11134–11145; e) J. G. Mꢃller, E. Atas, C. Tan, K. S.
Schanze, V. D. Kleiman, J. Am. Chem. Soc. 2006, 128, 4007–4016;
f) R. Q. Yang, H. B. Wu, Y. Cao, G. C. Bazan, J. Am. Chem. Soc.
4
2
006, 128, 14422–14423.
[7] a) C. Y. Tan, M. R. Pinto, K. S. Schanze, Chem. Commun. 2002,
46–447; b) H. Schnablegger, M. Antonietti, C. Goltner, J. Hart-
2
2
1
3
4
1
mann, H. Colfen, P. Samori, J. P. Rabe, H. Hager, W. Heitz, J. Col-
loid Interface Sci. 1999, 212, 24–32; c) K. Y. Pu, L. P. Cai, B. Liu,
Macromolecules 2009, 42, 5933–5940; d) P. Bjçrk, D. Thomsson, O.
Mirzov, J. Wigenius, O. Inganꢅs, I. G. Scheblykin, Small 2009, 5, 96–
1
3
4
3
1
3
H), 7.86 ppm (s, 2H); C NMR (400 MHz, CDCl ): d=23.7, 28.0, 29.2,
2.8, 34.2, 40.5, 55.4, 111.5, 119.7, 120.3, 122.3, 123.1, 123.8, 125.1, 127.3,
27.8, 127.9, 130.3, 131.3, 131.6, 137.1, 140.7, 141.3, 141.6 ppm; MS (FAB/
1
03; e) C. Y. Tan, E. Atas, J. G. Mꢃller, M. R. Pinto, V. D. Kleiman,
NBA): m/z: 1190–1198; elemental analysis calcd (%) for C58
8.48, H 5.38; found: C 58.43, H 5.36.
Compound FPF-F6: p-FPF-F6 (119 mg, 0.1 mmol) was dissolved in THF
15 mL). Condensed trimethylamine (2 mL) was added dropwise under a
slight vacuum to the solution at À788C. After that, the mixture was grad-
ually warmed to room temperature with stirring overnight. A colorless
precipitate was observed. After methanol (30 mL) was added to dissolve
this precipitate, additional trimethylamine (1.5 mL) was added at À788C.
The mixture was vigorously stirred for another 24 h. The solvent was re-
moved under vacuum. The residue was washed with hexane and acetone
sequentially, and dried in vacuo. FPF-F6 was obtained as white solid
4 6
H64Br F : C
K. S. Schanze, J. Am. Chem. Soc. 2004, 126, 13685–13694; f) H.
Jiang, X. Y. Zhao, K. S. Schanze, Langmuir 2007, 23, 9481–9486;
g) J. Kim, T. M. Swager, Nature 2001, 411, 1030–1034; h) M. R.
Pinto, B. M. Kristal, K. S. Schanze, Langmuir 2003, 19, 6523–6533;
i) C. Y. Tan, M. R. Pinto, M. Erkan Kose, I. Ghiviriga, K. S. Schanze,
Adv. Mater. 2004, 16, 1208–1212.
5
(
[
[
8] F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. Schenning,
Chem. Rev. 2005, 105, 1491–1546.
9] B. Liu, B. S. Gaylord, S. Wang, G. C. Bazan, J. Am. Chem. Soc. 2003,
1
25, 6705–6714.
10] S. Wang, B. Liu, B. S. Gaylord, G. C. Bazan, Adv. Funct. Mater. 2003,
3, 463–467.
[
[
[
(
126 mg, 89%). TGA showed that one mole of molecule co-crystallized
1
1
with seven moles of water. H NMR (400 MHz, CD
3
OD): d=0.60–0.73
m, 8H), 1.13–1.21 (m, 8H), 1.56–1.64 (m, 8H), 2.08–2.14 (m, 8H), 3.07
s, 36H), 3.20–3.25 (m, 8H), 7.38–7.41(m, 6H), 7.46–7.49(m, 4H), 7.83–
.90 (m, 6H), 7.90 ppm (s, 2H); C NMR (400 MHz, CD OD): d=26.4,
11] J. H. Ortony, R. Yang, J. Z. Brzezinski, L. Edman, T.-Q. Nguyen,
G. C. Bazan, Adv. Mater. 2008, 20, 298–302.
12] a) J. W. Hong, H. Y. Woo, B. Liu, G. C. Bazan, J. Am. Chem. Soc.
(
(
7
A
H
U
G
R
N
N
ACHTUNGTRENNUNG
1
3
3
2
005, 127, 7435–7443; b) S. J. K. Pond, O. Tsutsumi, M. Rumi, O.
2
7.5, 29.7, 33.1, 43.9, 56.2, 59.1, 70.4, 123.4, 123.9, 126.4, 126.8, 127.5,
29.1, 131.1, 133.9, 134.9, 135.2, 140.9, 144.7, 145.5, 145.8, 154.1, 154.6;
MS (ESI-MS): m/z: 277 [MÀ4Br]
Kwon, E. Zojer, J.-L. Brꢆdas, S. R. Marder, J. W. Perry, J. Am.
Chem. Soc. 2004, 126, 9291–9306.
1
4
+
.
[
[
13] J. N. Wilson, M. Josowicz, Y. Q. Wang, Uwe H. F. Bunz, Chem.
Commun. 2003, 2962–2963.
14] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer,
Singapore, 2006.
Acknowledgements
[15] a) P. S. Sardar, S. S. Maity, L. Das, S. Ghosh, Biochemistry 2007, 46,
4544–14556; b) H. Inoue, T. Mori, T. Sakurai, Chem. Lett. 1980,
1
The authors are grateful to the DOE and the ICB for financial support,
to Dr. Huaping Li for providing the compounds p-FPF and FPF, to Dr.
Arkadiusz Chworos for liquid-phase AFM measurements, to Dr. Gang
Wu for assistance with the crystallographic studies, and to Alexander Mi-
khailovsky for help on spectra measurements. Y.R. thanks the financial
supports of the State National Natural Science Foundation of China
683–686.
[16] a) S. Wang, B. S. Gaylord, G. C. Bazan, Adv. Mater. 2004, 16, 2127–
2132; b) O. K. Nag, M. Kang, S. Hwang, H. Suh, H. Y. Woo, J. Phys.
Chem. B 2009, 113, 5788–5793; c) Y. Jin, R. Yang, H. Suh, H. Y.
Woo, Macromol. Rapid Commun. 2008, 29, 1398–1402; d) G. C.
Bazan, J. Org. Chem. 2007, 72, 8615–8635; e) B. Liu, G. C. Bazan, J.
Am. Chem. Soc. 2006, 128, 1188–1196.
(
grant no. 50603011) and China Scholarship Council.
[
17] a) J. K. H. Horber, M. J. Miles, Science 2003, 302, 1002–1005;
b) H. G. Hansma, J. Vesenka, C. Siegerist, G. Kelderman, H. Mor-
rett, R. L. Sinsheimer, V. Elings, C. Bustamante, P. K. Hansma, Sci-
ence 1992, 256, 1180–1184; c) A. Chworos, I. Severcan, A. Y. Koyf-
man, P. Weinkam, E. Oroudjev, H. G. Hansma, L. Jaeger, Science
2004, 306, 2068–2072; d) B. Drake, C. B. Prater, A. L. Weisenhorn,
S. A. Gould, T. R. Albrecht, C. F. Quate, D. S. Cannell, H. G.
[
1] R. Q. Yang, Y. H. Xu, X. D. Dang, T. Q. Nguyen, Y. Cao, G. C.
Bazan, J. Am. Chem. Soc. 2008, 130, 3282–3283.
[
[
2] H. P. Li, G. C. Bazan, Adv. Mater. 2009, 21, 964–967.
3] Y. H. Xu, R. Q. Yang, J. B. Peng, A. Mikhailovsky, Y. Cao, T. Q.
Nguyen, G. C. Bazan, Adv. Mater. 2009, 21, 584–588.
Chem. Eur. J. 2010, 16, 11028 – 11036
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11035

G. C. Bazan and Y. Ren
Hansma, P. K. Hansma, Science 1989, 243, 1586–1589; e) H. G.
Hansma, L. Pietrasanta, Curr. Opin. Chem. Biol. 1998, 2, 579–584;
f) S. Manne, J. P. Cleveland, H. E. Gaub, G. D. Stucky, P. K.
Hansma, Langmuir 1994, 10, 4409–4413.
d) K. P. R. Nilsson, P. Hammarstrçm, Adv. Mater. 2008, 20, 2639–
2645; e) K. P. R. Nilsson, A. Herland, P. Hammarstrçm, O. Inganꢅs,
Biochemistry 2005, 44, 3718–3724.
[21] SMART Software Users Guide, Version 5.1, Bruker AXS, Inc.,
Madison, Wisconsin, 1999.
[22] SAINT Software Users Guide, Version 5.1, Bruker AXS, Inc., Madi-
son, Wisconsin (USA), 1999.
[23] SHELXTL, Version 6.12, G. M. Sheldrick, Bruker AXS, Inc., Madi-
son, Wisconsin (USA), 2001.
[
18] S. Manne, T. E. Schꢅffer, Q. Huo, P. K. Hansma, D. E. Morse, G. D.
Stucky, I. A. Aksay, Langmuir 1997, 13, 6382–6387.
[
[
19] B. Liu, G. C. Bazan, Proc. Natl. Acad. Sci. USA 2005, 102, 589–593.
20] a) X. L. Feng, X. R. Duan, L. B. Liu, F. D. Feng, S. Wang, Y. L. Li,
D. B. Zhu, Angew. Chem. 2009, 121, 5420–5425; Angew. Chem. Int.
Ed. 2009, 48, 5316–5321; b) F. Pu, Z. Huang, D. Hu, J. Ren, S.
Wang, X. Qu, Chem. Commun. 2009, 7357–7359; c) F. Feng, F. He,
L. An, S. Wang, Y. Li, D. B. Zhu, Adv. Mater. 2008, 20, 2959–2964;
Received: April 8, 2010
Published online: August 16, 2010
11036
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Chem. Eur. J. 2010, 16, 11028 – 11036