Synthesis, characterization, and application of cationic water-soluble oligofluorenes in DNA-hybridization detection

Article abstract of DOI:10.1002/chem.200701906

A simple and efficient approach was developed for the synthesis of a series of cationic water-soluble oligofluorenes up to a chain length of a heptamer. Bromoalkyl-substituted fluorenyl boronic esters as the key intermediates were synthesized by using a modified Miyaura reaction. With an increasing number of repeat units (trimer to hexamer), the size-specific oligomers have shown redshifts in both the absorption and emission maxima. The emission maximum reaches the limit for the hexamer in both water and buffer solution. The quantum yields of the oligomers decreased with increased oligomer size in water. Both fluorescence quenching of the oligomers by 9,10-anthraquinone-2,6-disulfonate and the fluorescence resonance energy transfer experiments with the oligomers as the donor and fluorescein (Fl)-labeled double-stranded DNA (dsDNA-Fl) as the acceptor revealed the chain-length-dependent behavior. The Stern-Volmer quenching constant increased with the molecular size, whereas the highest donor-sensitized Fl emission was observed for the hexamer. These size-specific oligomers also served as a model to study the structure-property relationships for cationic polyfluorenes.

Full text of DOI:10.1002/chem.200701906

DOI: 10.1002/chem.200701906
Synthesis, Characterization, and Application of Cationic Water-Soluble
Oligofluorenes in DNA-Hybridization Detection
Bin Liu* and Shudipto Konika Dishari[a]
Abstract: A simple and efficient ap-
proach was developed for the synthesis
of a series of cationic water-soluble oli-
gofluorenes up to a chain length of a
heptamer. Bromoalkyl-substituted fluo-
renyl boronic esters as the key inter-
mediates were synthesized by using a
modified Miyaura reaction. With an in-
creasing number of repeat units (trimer
to hexamer),the size-specific oligomers
have shown redshifts in both the ab-
sorption and emission maxima. The
emission maximum reaches the limit
for the hexamer in both water and
buffer solution. The quantum yields of
the oligomers decreased with increased
oligomer size in water. Both fluores-
cence quenching of the oligomers by
9,10-anthraquinone-2,6-disulfonate and
the fluorescence resonance energy
transfer experiments with the oligo-
mers as the donor and fluorescein (Fl)-
labeled
double-stranded
DNA
(dsDNA-Fl) as the acceptor revealed
the chain-length-dependent behavior.
The Stern–Volmer quenching constant
increased with the molecular size,
whereas the highest donor-sensitized Fl
emission was observed for the hexa-
mer. These size-specific oligomers also
served as a model to study the struc-
ture–property relationships for cationic
polyfluorenes.
Keywords: DNA
· fluorescein ·
fluorescence quenching · fluores-
cence resonance energy transfer
(FRET) · oligofluorenes
Introduction
FRET.[5] Our previous studies have shown that both CCP
backbone structure and the side chain length affect energy-
transfer efficiencies.[6] Improved signal amplification has
been demonstrated with shape-adaptable polymers in which
more conformational freedom and improved registry with
the analyte shape favor more-efficient FRET.[6a] The impor-
tance of matching energy levels between the donor–acceptor
pairs to minimize photo-induced charge-transfer processes
that compete with the desired FRET amplification was also
reported.[7] Recently,we also developed a nanoparticle-
based sensing strategy in which the energy-waste channel of
dye self-quenching upon CCP/DNA–dye complexation was
minimized. The collective response from polymers self-as-
sembled on the nanoparticle surface provided a signal am-
plification of the dye emission by over 110-fold.[8] Efforts
have also been made to tune the conjugated polymer emis-
sion to match different probe chromophores[6b] and to
change the solvent media for maximized signal amplifica-
tion.[9]
Water-soluble conjugated polymers (CPs) as optically sensi-
tive materials have been widely used in chemical and biolog-
ical sensors.[1] Cationic conjugated polymers (CCPs),in par-
ticular,have been proven useful for strand-specific DNA de-
tection by taking advantage of the light-harvesting proper-
ties of conjugated polymers and the fluorescence resonance
energy transfer (FRET) between the polymers and dye mol-
ecules attached to PNA,DNA,or peptide.
[2]
The same strat-
egy has also been used to detect RNA,protein,and other
small molecules in aqueous media and on a solid substrate.[3]
Efficient fluorescence resonance energy transfer from the
cationic conjugated polymer to the dye is a required condi-
tion for high sensitivity.[4] Maximized spectral overlap be-
tween the donor emission and the acceptor absorption,im-
proved orientation,and a shortened distance between the
donor and the acceptor should lead to more-efficient
Despite much success in demonstrating the sensing strat-
egy and improving the CCP-based sensor performance,fun-
damental information concerning important relationships
between structure and optical properties in cationic water-
soluble conjugated polymer solutions remain lacking.[10,6c]
Structural uncertainties in these polymers,such as the mo-
lecular-weight distribution,batch-to-batch variation in mo-
[a] Prof. B. Liu,S. K. Dishari
Department of Chemical and Biomolecular Engineering
National University of Singapore
Singapore 117576 (Singapore)
E-mail: cheliub@nus.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800216.
7366
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
Chem. Eur. J. 2008, 14,7366 – 7375

FULL PAPER
lecular weight,and defects prevent detailed study of the
structure–property relationships of CCPs and the chain-
length-dependent energy transfer/electron transfer between
donor molecules and dye-labeled biomolecules involved in
the FRET process. In the previous effort,water-soluble fluo-
rene-phenylene oligomers with one,two,or three fluorene
units on both sides of the phenylene ring were synthesized
to study the interactions between the oligomers and single-
stranded DNA (ssDNA) or double-stranded DNA
(dsDNA).[10b] Although the oligomer size was found to
affect the energy-transfer ratios of oligomer/ssDNA-fluores-
cein (Fl) to oligomer/dsDNA-Fl,the relationship between
the molecular size and the oligomer-sensitized Fl emission
intensity for dsDNA-Fl or ssDNA-Fl was not clear. In addi-
tion,the changes in the optical properties of the oligomers
and their response to DNA molecules could not be general-
ized to understand the polymer behavior as the oligomers
do not have the same repeat unit. Better understanding of
cationic conjugated polymers and the chain-length-depen-
dent electron- and energy-transfer processes is thus of high
importance to provide guidance for further improvement in
CCP-based sensor performance.
Herein,we describe the synthesis and characterization of
a series of cationic water-soluble conjugated oligofluorenes
with a chain length that varies from three units (trimer) to
seven units (heptamer). These molecules were used to ex-
amine the chain-length-dependent optical properties of
water-soluble polyfluorenes and served to illustrate how var-
iations in conjugation length could be used to optimize the
fluorescence-based sensory process. We started with the syn-
thesis and examination of the optical properties of the oligo-
mers. This was followed by the study of oligomer-quenching
behaviors in the presence of 9,10-anthraquinone-2,6-disulfo-
nate and the energy-transfer processes from the oligomers
to dsDNA-Fl. Poly
N
G
Scheme 1. Synthetic route to the intermediates for the oligomers. a) bis-
hexyl)fluorene) dibromide] was also studied to allow a com-
parison of the oligomers against the polymer structure.
(pinacolato)diboron,[PdCl
(PPh3)4],2 mK2CO3,toluene/H 2O,100 8C; c) Br2/I2,dichloromethane.
dppf=1,1’-bis(diphenylphosphino)ferrocene.
A
8C; b) [Pd-
AHCTREUNG
Results and Discussion
in 75% yield. Direct bromination of 3 and 7 at room tem-
perature by using liquid bromine in dichloromethane in the
presence of a trace amount of iodine afforded dibrominated
fluorenyl dimer 8 and trimer 9 in a yield of 60 and 45%,re-
spectively,after purification. For large bromide molecules,
such as 9,multiple column chromotography was necessary
for purification as the side products were difficult to remove
through recrystallization. Compounds 6 and 8 were trans-
formed to their corresponding boronic esters 10 and 11 in a
yield of 58 and 44%,respectively,by using the same
method as for 2 and 5. The availability of the alkyl bromide
containing arylboronates simplified the synthesis of precur-
sors for water-soluble conjugated molecules that rely on
Suzuki cross-coupling protocols. Furthermore,the new pro-
cedure bypassed the need to prepare trialkylamino-substi-
tuted oligomers or protected intermediates,which were
complicated or difficult to purify.[10] As compared with pre-
The intermediates for water-soluble oligomers were ob-
tained by the sequence of reactions shown in Scheme 1.
Starting from the mono- (1) and dibromide (4) of bromo-
hexyl-substituted fluorene,the corresponding mono- 2 and
diboronic esters 5 were synthesized by the modified Miyaura
reaction in the presence of bis(pinacolato)diboron and
KOAc by using dioxane as the solvent. Compounds 2 and 5
were obtained in a yield of 64 and 52%,respectively,after
purification by silica chromatography. Suzuki coupling be-
tween one equivalent of 1 and 2 with [Pd(PPh3)4] as the cat-
A
alyst in a mixture of toluene/2mK2CO3 aqueous solution for
24 h yielded the fluorenyl dimer 3 in 64% yield. Similarly,
reaction of 2 with 1.5 equivalents of 4 gave the monobromi-
nated fluorenyl dimer 6 in 65% yield,together with a small
fraction of trimer 7 in 19% yield. Compound 7 was also syn-
thesized from two equivalents of 2 and one equivalent of 4
Chem. Eur. J. 2008, 14,7366 – 7375
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
7367

B. Liu and S. K. Dishari
vious approaches for neutral monodispersed oligofluor-
enes,[11] which took advantage of the large reactivity differ-
ence between diazonium salts or trimethylsilyl groups and
aryl bromides in the cross-coupling reaction with aryl boro-
nates,[12] our strategy is much more straightforward and is
compatible with functional bromide groups at the end of the
side chain.
As shown in Scheme 2,the synthetic entry to the fluoren-
yl oligomers that range from the trimer to the heptamer in-
volved a palladium-mediated Suzuki cross-coupling reaction
spectroscopy and MALDI-TOF mass spectrometry. By
using the hexamer as an example,observed ionized mass
peaks correspond to 441.79 [Mꢁ7Br]7+,377.36 [ Mꢁ8Br]8+
,
254.06 [Mꢁ11Br]11+,and 224.54 [ Mꢁ12Br]12+. The polymer
has a number-average molecular weight of 33000 (ca. 50
repeat units) and a polydispersity of 1.8. Oligomers 16–18
are highly soluble in water (>20 mgmLꢁ1),whereas oligo-
mers 19 and 20 require ultrasonication to get a clear aque-
ous solution with a concentration of 20 mgmLꢁ1. The poly-
mer has a low solubility in water (<2 mgmLꢁ1).
The absorption spectra of the oligomers 16–20 and the po-
lymer in water with the same fluorene unit concentration of
3.0”10ꢁ6 m are shown in Figure 1 and the data are presented
Scheme 2. Synthetic route to the neutral and water-soluble oligomers.
a) [Pd(PPh3)4],2 mK2CO3,toluene/H O,100 8C; b) NMe3,THF/H O.
A
2
2
between the fluorenyl boronic esters and the brominated
fluorenes. The reactions proceeded in the presence of [Pd-
Figure 1. The absorption spectra of the fluorenyl oligomers 16–20 and the
polymer in water with [fluorene unit]=3.0”10ꢁ6 m.
(PPh3)4] in a mixture of 2m aqueous K2CO3 solution and
THF or toluene. The coupling between two equivalents of 2
and 4 or 8 afforded fluorenyl trimer 7 and tetramer 12 in a
yield of 75 and 52%,respectively. The fluorenyl pentamer
13 and hexamer 14 were obtained from a coupling reaction
between 5 or 11 and two equivalents of 6,both in a yield of
approximately 30%. Similarly,the heptamer 15 was ob-
tained in 27% yield by coupling 9 and 10. For fluorenyl olig-
omers,purification was done by using column chromatogra-
phy followed by recrystallization from hexane–dichlorome-
thane to afford the products as white or pale yellow solids.
in Table 1. The corresponding absorption spectra in metha-
nol are shown in Figure S1 in the Supporting Information.
The fluorenyl oligomers exhibit an unstructured absorption
band in both water and methanol. There is a progressive
redshift in the absorption maxima with increasing chain
length in both water and methanol. In water,the maximum
absorption wavelength increases from 353 nm for the trimer
to 385 nm for the heptamer and 395 nm for the polymer,
which is slightly redshifted as compared with that in metha-
nol. Previous studies have shown that methanol is a good
solvent for amphiphilic oligomers and polymers,which can
greatly reduce molecular aggregation in water.[13] In metha-
nol,the absorption maxima for the trimer to the heptamer
are 350,364,371,375,and 379 nm,respectively,which corre-
spond to the energies of 3.54,3.41,3.34,3.31,and 3.27 eV.
From the absorption maximum for each oligomer in metha-
nol,it is possible to determine the active conjugation length
of the polymer by examination of the plot of the absorption
energy versus the value of 1/n,where n represents the
number of fluorene units for each oligomer. As shown in
Figure 2,the absorption energy versus n gives a linear curve
(correlation coefficient=0.996) from which one can derive
Equation (1):
1
Results from H and 13C NMR spectroscopy,MALDI-TOF
mass spectrometry,and elemental analysis confirmed the
right molecular structures and high purity of the neutral
oligomers of 7 and 12–15. Quaternization of the pendant
bromide groups on the backbone,by addition of condensed
trimethylamine,provided the cationic water-soluble oligo-
mers 16–20 in a yield of 60–65% after precipitation from
acetone and subsequent drying. After quaternization,the
characteristic signals at d=3.40–3.20 ppm,which correspond
to the chemical shift of -CH2CH2Br for the neutral oligo-
mers,disappeared completely. A new signal at dꢀ3.60 ppm,
which is assigned to -CH2CH2N(CH3)3,appeared in the
G
1H NMR spectra of 16–20. The degree of quaternization is
thus nearly quantitative for the oligomers. The molecular
structures of 16–20 were also confirmed by using NMR
7368
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
Chem. Eur. J. 2008, 14,7366 – 7375

Detection of DNA Hybridization
FULL PAPER
the oligomers increase from 396 nm for the trimer to 422 nm
for the hexamer,and the emission maxima remain virtually
unchanged when the chain length is further increased. The
fact that the spectral absorption maxima continue to redshift
up to n=12,whereas the emission maximum does not shift
further to the red for n>6,suggests that the fluorene back-
bone geometry changes significantly in going from the
ground state (S0) to the vibronically relaxed excited state.[14]
Similar observation has also been reported previously in
which the emission maximum of neutral oligofluorenes in
tetrahydrofuran reaches the limit at n=6.[14] In methanol,
the emission maxima are slightly blueshifted for all the olig-
omers and the polymer,and the emission spectra are nar-
rower as compared with those in water. The narrowed emis-
sion spectra reflect a smaller conformational distribution of
excited species for 16–20 in methanol,as compared with
that in water. Both the oligomers and the polymer have
shown well-structured PL spectra in water and in methanol.
As shown in Figure 3,for each molecule,two well-resolved
emission bands that correspond to 0–0 and 0–1 intrachain
singlet transition are present. The ratio of the intensities for
the 0–0 transition to the 0–1 transition increases with oligo-
mer size. When normalized,the polymer shows the lowest
0–1 transition. This is due to the increased intrachain cou-
pling with increased chain length.[12a] The fluorescence quan-
tum efficiency (F) was determined against quinine sulfate in
0.1m H2SO4 (54%) as the standard,and the values are
shown in Table 1. The F values of the oligomers decrease
with increasing chain length in both water and methanol.
The F value varies from 94% for the trimer to 78% for the
polymer in methanol,and from 90% for the trimer to 45%
for the polymer in water,respectively.
Figure 2. Energy of the oligomer absorption maxima in methanol versus
the inverse ring number of fluorenyl oligomers.
EðeVÞ ¼ 3:06 þ 1:435=n
ð1Þ
From the absorption maximum of the polymer (390 nm,
which corresponds to 3.18 eV),one can estimate that the
cationic polyfluorene has an effective conjugation length of
about 11 to 12 repeat units. This is similar to the previous
report in which an effective conjugation length of 12 repeat
units was reported for poly(9,9-di-n-hexylfluorene-2,7-diyl),
which showed an absorption maximum of 390 nm in tetrahy-
drofuran.[14] The molar absorption coefficient (e) based on
the fluorene unit is nearly the same for all the oligomers
and the polymer in water,which has a value of approximate-
ly 2.9”104 Lmolꢁ1 cmꢁ1. In methanol,the e based on the flu-
orene unit is approximately 3.0”104 Lmolꢁ1 cmꢁ1,which is
slightly higher than that in water.
Understanding the optical properties of the oligomers and
the polymer in water with varying ionic strength is of high
importance for bioassay applications as buffer ions screen
The emission spectra of the oligomers and the polymer
were also studied in both water (Figure 3) and methanol
(Figure S1 in the Supporting Information),and the data are
summarized in Table 1. In water,the emission maxima of
[15]
negative charges on DNA,which facilitate hybridization.
The effect of ionic strength on the optical properties of the
oligomers and the polymer was studied by monitoring the
absorption and emission spectral changes of their aqueous
solutions upon addition of different amounts of NaCl. In
these experiments,the solution concentration based on the
fluorene unit was kept at 3”10ꢁ6 m,with [NaCl] varying
from 0 to 118 mm at an increase of 12 mm upon each NaCl
addition. The corresponding spectra are shown in Figure S2
and S3 in the Supporting Information. With the addition of
NaCl,there was a decrease in both absorption and fluores-
cence for all solutions. When the concentration of NaCl was
varied from 0 to 118 mm,the decrease in absorbance was
similar (around 40%) for all solutions and there was no ob-
vious precipitation observed. For the trimer and tetramer,a
slight redshift was also observed with increased NaCl con-
centration in solution. For the pentamer,hexamer,hepta-
mer,and the polymer,the absorption spectra also changed
their shapes even in the presence of 12 mm NaCl. A new
shoulder peak appeared at long wavelength and the peak in-
tensity increased with increased chain length. The most ob-
vious change was observed for the polymer in which the ab-
Figure 3. The emission spectra of the fluorenyl oligomers 16–20 and the
polymer in water at [fluorene unit]=3.0”10ꢁ6 m.
Chem. Eur. J. 2008, 14,7366 – 7375
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
7369

B. Liu and S. K. Dishari
sorption at 395 nm in water slightly redshifted to 397 nm in
the presence of NaCl and a more-intense new band ap-
peared at 411 nm. The changes in the absorption spectra of
16–20 and the polymer upon addition of NaCl reveals that
the conformational changes in the ground state is more ob-
vious for large-size molecules.
In general,when molecules aggregate and if the aggre-
gates are soluble,bathochromic and hypsochromic shifts are
observed for the absorption when the unaggregated form is
compared with the aggregated forms.[16] On the other hand,
when molecules aggregate and the aggregates are not very
soluble,aggregation or precipitation can result in a decrease
in the absorption band and an increase in the absorption at
the long wavelength side owing to particle scattering. Nei-
ther of these two effects are ob-
which changed from 45% in water in the absence of NaCl
to about 8% at [NaCl]=118 mm in solution. Quantitative
analysis of the changes in fluorescence quantum yield re-
quires lifetime measurement and calculation of radiative
and non-radiative decay rates,which is currently under
study. In the following studies,25 m m phosphate buffer solu-
tion (pH 7.4) was used as the medium,which was chosen to
maintain not only a relatively high fluorescence quantum
yield for all the donor molecules but also good stability of
double-stranded (ds)DNA in solution.[7,19] The absorption
and emission spectra of the oligomers and the polymer in
buffer solution are shown in Figure S4 in the Supporting In-
formation,and the corresponding data are summarized in
Table 1.
vious in Figure S2 in the Sup-
Table 1. Summary of the absorption and emission spectra for the fluorenyl oligomers and the polymer in
porting Information,which in-
dicates that the observed spec-
tral change is less likely to be
due to salt-induced aggregation.
The effect of NaCl on the effec-
tive diameters (EDs) of the
oligomers and the polymer in
solution was also studied by dy-
namic light-scattering tech-
niques.[17] At an equal [fluorene
unit]=1”10ꢁ5 m in water,the
methanol,water,and buffer soltion (25 m m phosphate buffer solution; pH 7.4).
Compounds
Methanol
lmax(em)
Water
lmax(em)
Buffer solution
lmax(abs)
F [%]
lmax(abs)
F [%]
lmax(abs)
lmax(em)
F [%]
trimer
350
364
371
375
380
390
392
404
410
412
413
414
94
90
88
85
80
78
353
367
374
380
385
395
396
408
415
422
422
422
90
89
82
70
52
45
354
372
379
383
386
411
398
412
423
426
426
428
82
72
67
57
46
17
tetramer
pentamer
hexamer
heptamer
polymer
EDs for solutions containing the trimer to the heptamer
were in the range of 350–450 nm,and the ED for the poly-
mer was around 600 nm. These values did not change within
24 h. Upon addition of 118 mm NaCl to each solution,the
change in ED was less than 5% when measured within
5 min. However,the EDs increased to approximately
1000 nm or more for all solutions when they were kept at
room temperature for 24 h. This indicates that the spectral
redshift and the decrease in absorbance in the presence of
NaCl (shown in Figure S2 in the Supporting Information)
are not attributed to aggregation formation. The new band
(>400 nm) in the presence of NaCl reflects the changes of
the backbone conformation of the oligomers/polymer from
a more disordered state to a less disordered state in solu-
tion.[18] The conformational change could favor molecular
agglomeration and aggregation,which eventually leads to
big aggregates and precipitation when the solutions are kept
for a long time.
Upon addition of NaCl to the oligomer/polymer solutions,
a redshift in emission was also observed for aqueous solu-
tions containing the pentamer,hexamer,heptamer,and the
polymer. The redshift also continued with increased NaCl
concentration. The rate of the decrease in fluorescence also
increased with the molecular size. When the NaCl concen-
tration was increased from 0 to 118 mm,a 40% decrease in
fluorescence intensity was observed for the trimer,whereas
a 90% decrease was observed for the polymer. The quan-
tum yield of the trimer did not change significantly with the
increased ionic strength in solution. The most obvious de-
crease in quantum yield was observed for the polymer,
To investigate the electronic communication and chain-
length dependent light-harvesting properties,it is useful to
examine the processes such as the fluorescence quenching
and energy transfer with different acceptor molecules. Fluo-
rescence quenching of the oligomers and the polymer was
examined by using the electron acceptor 9,10-anthraqui-
none-2,6-disulfonate (AQS2ꢁ) in 25 mm phosphate buffer so-
lution. Experiments were conducted at an equal concentra-
tion of the fluorene unit of 2”10ꢁ6 m. The quenching effi-
ciency is determined by using the Stern–Volmer equation:
F0/F=1 + Ksv[quencher],where F0 and F are fluorescence
G
intensities in the absence and presence of the quencher,re-
spectively. The Ksv plot of the oligomers quenched by
AQS2ꢁ in buffer solution is shown in Figure 4. The Stern–
Volmer constants (Ksv) obtained from the linear region of
the Stern–Volmer plot of F0/F versus [AQS2ꢁ] are 2.07”107,
7
7
7
2.35”107,3.25”10 7,3.57”10 ,3.89”10 ,and 6.20”10 ,for
the trimer to the heptamer and the polymer,respectively.
Such a high Ksv value is due to the static quenching by for-
mation of the ground-state complexes through charge pair-
ing.[20] Different from previous studies in which only Ksv be-
tween a small oligomer and a polymer was compared,our
study indeed provided more-detailed evidence to indicate
that there was a gradual increase in Ksv with the increased
chain length of donor molecules.[10a,21] This result suggests
that the ability of an oligomer/polymer to harvest light and
deliver excitons to the acceptors improves as the backbone
chain length increases. Higher Ksv values for larger-size mol-
ecules reflect more-efficient intra- and inter-chain electron-
transfer mechanisms with the increased chain length. This
7370
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
Chem. Eur. J. 2008, 14,7366 – 7375

Detection of DNA Hybridization
FULL PAPER
in Figure 5,there was an increase in Fl emission upon addi-
tion of all the donors to dsDNA-Fl ([dsDNA-Fl]=1”
10ꢁ8 m) when [fluorene unit] was in the range of 0–4”10ꢁ7 m.
This observed increase in Fl emission was due to the in-
creased number of fluorene units that were associated with
dsDNA–Fl,and were within a valid FRET distance. The Fl
emission intensity almost saturated at [fluorene unit]=4”
10ꢁ7 m,which corresponded to a charge ratio (positive charg-
es of donors to the negative charges of dsDNA) close to 1.
Within the tested donor concentration range,the most in-
tense donor-sensitized Fl emission was observed for the
hexamer,and the lowest was observed for the polymer. The
data in Figure 5 demonstrate that the intensity of donor-sen-
sitized Fl signals follows the following trend: hexamer>pen-
tamer>heptamer>tetramerꢀtrimer>polymer. By using
the spectral data (shown in Figure S4 in the Supporting In-
formation) and the donor quantum yields in the buffer solu-
tion (shown in Table 1),the Fçrster distance ( R0)[23] was cal-
culated to be 42.8,43.8,44.5,44.8,43.9,and 37.4 Š for the
trimer to heptamer and the polymer,respectively,assuming
an orientation factor (k2) of 2/3. The R0 for the polymer/flu-
orescein is similar to that of 37.2 Š,which was reported for
a polyfluorene derivative/fluorescein pair.[24] According to
Figure S4 in the Supporting Information,the overlap be-
tween the emission of the trimer and the absorption of fluo-
rescein is obviously smaller that that for the others,the high
quantum yield of the trimer in buffer solution compensates
for the small spectral overlap,which leads to an R0 value
that is only slightly smaller than other oligomers. Interest-
ingly,the hexamer-sensitized Fl emission was almost two
times higher than that from the trimer,despite a lower
quantum yield for the hexamer. As the number of fluorene
units was kept constant in all the experiments,the difference
in donor-sensitized Fl emission should be mainly attributed
to the improved spectral overlap between large oligomers
and Fl. Further increases in the molecular size from the
hexamer to the polymer do not contribute to the spectral
overlap as the emission maximum reaches the limit for the
hexamer in the buffer solution. In addition to the higher
donor-sensitized Fl emission,the hexamer also provides a
higher selectivity for dsDNA-Fl/ssDNA-Fl as compared
with that for the polymer. The results are shown in Figure
S5 in the Supporting Information.
Figure 4. Ksv plots of the oligomers and the polymer quenched by AQS2ꢁ
in 25 mm phosphate buffer solution. [fluorene unit]=2”10ꢁ6 m.
phenomena could be explained in terms of the “molecular
wire effect” in which a single quencher can effectively
quench many repeat units for the large-size molecules.[22]
FRET experiments between the oligomers/polymer
(donor) and a low-energy acceptor were also performed. Fl
was chosen as the acceptor because there was a spectral
overlap between the absorption of Fl and the emission of
the oligomers/polymer. Fl was attached to a dsDNA to
ensure the electrostatic interaction that could bring the
donor molecules and Fl into close proximity for energy
transfer. The dsDNA was obtained through hybridization of
a Fl-labeled single-streanded (ss)DNA (5’-Fl-ATC TTG
ACT ATG TGG GTG CT-3’) with its complementary strand
(5’-AGC ACC CAC ATA GTC AAG AT-3’). A comparison
of the Fl emission intensity by excitation of the donors at
their absorption maxima is shown in Figure 5. The donor
emission tail in the Fl emission region (500–700 nm) was
subtracted from the donor-sensitized Fl emission. As shown
Conclusion
In summary,we have presented an efficient approach to syn-
thesize water-soluble oligofluorenes up to the heptamer
length. Both the absorption and emission maxima of the
oligomers redshifted with increased chain length,and the
emission maximum reached the limit for the hexamer. The
fluorescence quantum yields of the oligomers decreased
with increased molecular size and were higher in water than
in buffer solution. Fluorescence-quenching experiments
demonstrated higher quenching rates with increased chain
length,whereas the FRET experiments showed that the
Figure 5. Fluorescence intensity of the donor-sensitized dsDNA-Fl emis-
sion with varying oligomer/polymer concentrations based on fluorene
units. [dsDNA]=2”10ꢁ8 m.
Chem. Eur. J. 2008, 14,7366 – 7375
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
7371

B. Liu and S. K. Dishari
9,9,9’,9’-Tetrakis(6’-bromohexyl)-2,2’-bifluorene (3): A flask charged with
compound 1 (0.61 g,0.99 mmol),compound 2 (0.58 g,0.99 mmol),and
potassium carbonate (0.68 g,4.93 mmol) in toluene (8 mL) and water
hexamer was the best donor for sensitized fluorescein emis-
sion. Comparison of the optical properties and sensing be-
haviors between the oligomers and the polymer revealed
the importance of molecular size in biosensing applications.
As compared with the oligomers,fluorescence quenching of
the polymer was more efficient,which is due to the “molec-
ular-wire effect” in which a single quencher could effectively
quench many repeat units for large molecules. For bioappli-
cations that take advantage of fluorescence quenching of
conjugated molecules,polymers could be more efficient
platforms than oligomers. On the other hand,less-efficient
signal amplification was demonstrated for the polymer,
which was due to the low quantum yield of the polymer in
buffer solution. Comparison between the polymer and the
hexamer also showed higher selectivity between ssDNA and
dsDNA for the hexamer. Designing new optical platforms
will require balancing of the properties of size,spectral over-
lap,and the quantum yield. Small molecules tend to have
higher quantum yields and are less sensitive to solvent
media (i.e. ionic strength),but they generally show less spec-
tral overlap than their larger counterparts. Large-size oligo-
mers have the advantage of high purity,high quantum
yields,little aggregation,and have shown similar spectral
overlap to acceptors as compared with polymers. The oligo-
mer approach could serve as a new strategy to provide
more-efficient donors for conjugated polymer-based bioas-
say applications.
(3 mL) was degassed for 15 min. [Pd(PPh3)4] (0.02 g) was added to the
N
flask and the mixture was degassed for another 15 min. The mixture was
refluxed at 1008C for 24 h and then cooled to room temperature. After
extraction with dichloromethane,the organic layer was washed with
water and brine and dried over magnesium sulfate. The solvent was re-
moved and the residue was purified by silica gel column chromatography
(dichloromethane/hexane=1:5),followed by recrystallization from
hexane to afford 3 (0.62 g,64%) as white crystals. 1H NMR (400 MHz,
CDCl3): d=7.80–7.62 (m,8H),7.38–7.26 (m,6H),3.30–3.26 (t,
J=
6.8 Hz,8H),2.07–2.04 (q,8H),1.69–1.65 (m,8H),1.27–1.19 (q,8H),
1.15–1.10 (q,8H),0.80–0.65 ppm (m,8H); 13C NMR (100 MHz,CDCl 3):
d=151.6,151.0,141.2,141.0,140.8,127.5,127.4,126.6,123.3,121.7,120.4,
120.2,55.5,40.6,34.1,33.0,29.4,28.1,24.0 ppm; MS (MALDI-TOF):
m/z(%): 982.11 [M].
2,7-Bis
N
N
AHCTREUNG
(2.72 g,10.72 mmol),and potassium acetate (2.64 g,26.8 mmol) were
placed in a 100-mL round bottom flask. Anhydrous dioxane (60 mL) and
[PdCl2(dppf)] (0.18 g,0.21 mmol) were added to the flask,and the reac-
R
tion vessel was degassed for 15 min. The mixture was stirred at 808C for
12 h under nitrogen. After the mixture had been cooled to room temper-
ature,dioxane was removed by rotary evaporation. The residue was ex-
tracted with dichloromethane,and the organic phase was washed with
water and brine,and dried over magnesium sulfate. The solvent was re-
moved and the residue was purified with silica gel column chromatogra-
phy (dichloromethane/hexane=1:2),followed by recrystallization from
hexane to afford 5 (1.05 g,52%) as white crystals. 1H NMR (400 MHz,
CDCl3): d=7.83–7.71 (m,6H),3.27–3.23 (t,
(m,4H),1.64–1.57 (q,J =7.0 Hz,4H),1.39
1.06–1.02 (q,4H),0.71–0.67 ppm (q,4H);
J=6.9 Hz,4H),2.03–1.99
AHCTREUNG
13C NMR (100 MHz,CDCl 3):
d=150.3,144.1,134.0,128.9,119.7,84.0,55.2,40.1,34.2,32.8,29.2,27.9,
23.6 ppm; MS (EI): m/z (%): 744 [M+].
7-Bromo-9,9,9’,9’-tetrakis(6’-bromohexyl)-2,2’-bifluorene (6): A flask con-
taining compound 2 (2.54 g,4.10 mmol),compound 4 (4.13 g,6.19 mmol),
and potassium carbonate (2.86 g,20.75 mmol) in toluene (30 mL) and
ExperimentalSection
Generalmethods : Chemicals were purchased from Aldrich and used
without further purification. 1H NMR and 13C NMR were recorded on
Bruker 400 Ultrashield.TM. Shimadzu UV-1700,UV-Vis spectrophotom-
eter was used to measure the absorption spectra. PL spectra were mea-
sured on Perkin Elmer LS55 luminescence spectrometer. The mass spec-
tra were obtained by using a Bruker Daltonics Autoflex II TOF system.
MALDI-TOF was performed by using 2,5-dihydroxybenzoic acid
(DHB) as the matrix under the reflector mode for data acquisition. THF
and 50% CH3OH (50% H2O) were used as solvents for neutral and
charged compounds,respectively. 2-Bromo-99,-bis(6 ’-bromohexyl)fluor-
ene (1) and 2,7-dibromo-9,9-bis(6’-bromohexyl)fluorene (4) were synthe-
sized according to the previous report.[6c,13a]
water (10 mL) was degassed for 15 min. [Pd(PPh3)4] (0.011 g) was added
N
to the flask and the mixture was degassed for another 15 min. The mix-
ture was kept at approximately 1008C for 24 h and cooled to room tem-
perature. After extraction with dichloromethane,the organic layer was
washed with water and brine,and then dried over magnesium sulfate.
The solvent was removed and the residue was purified by silica gel
column chromatography (dichloromethane/hexane=1:5) followed by re-
crystallization from hexane to afford 6 (2.83 g,65%) as a light-yellow
solid. 1H NMR (400 MHz,CDCl 3): d=7.86–7.79 (m,3H),7.74–7.64 (m,
5H),7.55 (s,2H),7.43–7.23 (m,3H),3.34–3.29 (t,
J=6.4 Hz,8H),2.12–
2.07 (m,8H),1.74–1.68 (m,8H),1.28–1.14 (m,16H),0.79–0.73 ppm (m,
8H); 13C NMR (100 MHz,CDCl 3): d=153.1,151.4,151.0,150.8,141.2,
140.9,140.8,140.4,140.0,139.5,130.4,129.3,128.5,127.5,127.2,126.7,
126.5,126.4,125.6,123.1,121.4,120.4,120.3,120.1,55.6,55.3,40.4,34.2,
32.8,29.2,27.9,23.8 ppm; MS (MALDI-TOF): m/z (%): 1061.61 [M].
2-[9,9-Bis(6-bromohexyl)fluorenyl]-4,4,5,5-tetramethyl
A
ACHTREUNG
7.95 mmol),bis(pinacolatodiboron) (3.02 g,11.93 mmol),and potassium
acetate (2.94 g,29.82 mmol) were placed in a 100-mL round bottom
9,9,9’,9’,9’’,9’’-Hexakis(6’-bromohexyl)-2,2’-7’,2’’-terfluorene (7): Com-
pound 7 was prepared according to the method for 6 by using compound
2 (2.54 g,4.10 mmol),compound 4 (1.37 g,2.05 mmol),potassium carbon-
flask. Anhydrous dioxane (80 mL) and [PdCl2(dppf)] (0.20 g,0.24 mmol)
R
were added to the flask and the reaction vessel was degassed. The mix-
ture was stirred at 808C for 12 h under nitrogen. After the mixture had
been cooled to room temperature,dioxane was removed by rotary evapo-
ration. The residue was extracted with dichloromethane,and the organic
phase was washed with water and brine,and dried over magnesium sul-
fate. The solvent was removed and the residue was purified by silica gel
column chromatography (dichloromethane/hexane=1:2) to afford
(3.13 g,64%) as white crystals. 1H NMR (400 MHz,CDCl 3): d=7.83–
7.70 (m,4H),7.35–7.32 (m,3H),3.28–3.25 (t, J=7.2 Hz,4H),2.03–1.95
ate (2.86 g,20.75 mmol),and [Pd (PPh3)4] (30 mg) in toluene (20 mL) and
N
water (10 mL). Column chromatography (hexane/dichloromethane=3:1)
over silica gel yielded 7 (2.26 g,75%) as an oily product. 1H NMR
(400 MHz,CDCl 3): d=7.80–7.63 (m,14H),7.41–7.30 (m,6H),3.30–3.27
(t, J=6.4 Hz,12H),2.18–2.00 (m,12H),1.75–1.63 (q,12H),1.30–1.05
2
(m,24H),0.85–0.65 ppm (m,12H);
13C NMR (100 MHz,CDCl 3): d=
152.1,151.8,151.1,141.1,141.0,140.5,127.7,126.5,123.3,121.8,120.5,
120.4,55.5,40.6,34.1,33.0,29.5,28.2,24.0 ppm; MS (MALDI-TOF):
m/z
(m,4H),1.67–1.58 (q,4H),1.40 (s,12H),1.18–1.16 (q,4H),1.05–1.03
(m,4H),0.63–0.55 ppm (m,4H); 13C NMR (100 MHz,CDCl ): d=151.1,
(%): 1472.62 [M]; elemental analysis calcd (%) for C75H92Br6: C 61.16,H
6.30; found: C 61.76,H 6.33.
149.7,144.3,141.1,134.0,128.9,127.8,127.0,123.0,120.4,119.3,83.9,
55.1,40.3,34.2,32.8,29.2,27.9,25.2,23.6 ppm; MS (EI):
m/z (%): 618.90
7,7’-Dibromo-9,9,9’,9’-tetrakis (6-bromohexyl)-2,2’-bifluorene (8): Com-
pound 3 (1.13 g,1.15 mmol) and dichloromethane (15 mL) were added
[M+].
7372
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
Chem. Eur. J. 2008, 14,7366 – 7375

Detection of DNA Hybridization
FULL PAPER
together in a 100-mL round bottom flask in an ice bath. Bromine (0.37 g,
2.30 mmol) in dichloromethane (10 mL) was added dropwise. The mix-
ture was stirred at room temperature for 20 h. A diluted potassium hy-
droxide solution (3%,ca. 25 mL) was added to quench the reaction. The
organic layer was separated and washed with water and brine and dried
over magnesium sulfate. The solvent was removed and the residue was
recrystallized in a mixture of dichloromethane and hexane to afford 8
(0.79 g,60%) as a light-yellow solid. 1H NMR (400 MHz,CDCl 3): d=
121.7,120.5,120.2,55.7–55.5,40.6,34.1,33.0,29.4,28.1,24.0 ppm; MS
(MALDI-TOF) 1963.44 [M]; elemental analysis calcd (%) for
C100H122Br8: C 61.18,H 6.26; found: C 61.22,H 6.30.
9,9,9’,9’,9’’,9’’,9’’’,9’’’,9’’’’,9’’’’-Decakis(6’-bromohexyl)-2,2’-7’,2’’-7’’,2’’’-7’’’,
2’’’’-pentafluorene (13): Compound 13 was synthesized according to the
same procedure used for 7 by using compound 6 (594 mg,0.56 mmol),
compound
5.84 mmol),and [PdCl
5
(190 mg,0.28 mmol),potassium carbonate (800 mg,
2(dppf)] (15 mg) in a mixture of tetrahydrofuran
AHCTREUNG
7.80–7.49 (m,12H),3.30–3.27 (t,
J=6.8 Hz,8H),2.10–1.98 (m,8H),
(10 mL) and water (4 mL). After reaction for 24 h at 858C,the mixture
was cooled down to room temperature. After evaporation of tetrahydro-
furan,dichloromethane (ca. 20 mL) was added to the reaction mixture,
and the organic layer was washed with water followed by brine,and then
dried over magnesium sulfate. The solvent was removed and the residue
was purified with silica gel column chromatography (dichloromethane/
hexane=1:3) to afford 13 (200 mg,27%) as a pale yellow solid. 1H NMR
(300 MHz,CDCl 3): d=7.95–7.60 (m,26H),7.45–7.30 (m,6H),3.31–3.27
(t, J=6.6 Hz,20H),2.20–2.00 (m,20H),1.80–1.60 (m,20H),1.35–1.10
1.72–1.64 (q,J =7.2 Hz,8H),1.30–1.06 (m,16H),0.75–0.65 ppm (m,
8H); 13C NMR (100 MHz,CDCl 3): d=153.3,151.3,141.2,140.2,139.9,
130.6,126.9,126.6,121.6,120.6,55.9,40.5,33.0,29.4,28.1,24.0; MS
(MALDI-TOF): m/z (%): 1139.89 [M].
7,7’’-Dibromo-9,9,9’,9’,9’’,9’’-hexakis(6-bromohexyl)-2,2’-7’,2’’ terfluorene
(9): Compound 9 was prepared according to the method for 8 by using
compound 7 (1.08 g,0.74 mmol) and bromine (0.37 g,2.3 mmol). The
product was purified using silica gel column chromatography (hexane/tol-
uene=3:1),which was followed by recrystallization from acetone at 4 8C
to afford 9 as a yellowish solid (0.54 g,45%). 1H NMR (300 MHz,
CDCl3): d=7.95–7.20 (m,18H),3.41–3.20 (m,12H),2.25–2.05 (m,12H),
1.80–1.60 (m,12H),1.35–1.20 (m,12H),1.25–1.15 (m,12H),0.80–
0.60 ppm (m,12H); 13C NMR (75 MHz,CDCl 3): d=152.9,151.5,150.8,
141.0,140.4,140.2,139.8,139.4,130.2,126.5,126.4,126.2,121.3,120.2,
(m,40H),0.90–0.65 ppm (m,20H);
13C NMR (75 MHz,CDCl 3): d=
151.4,151.1,150.6,140.7,140.4,140.1,127.1,126.9,122.8,121.3,121.2,
120.1,120.0,119.8,55.3–55.1,40.2,34.0,32.6,29.0,27.7,23.7 ppm; MS
(MALDI-TOF): m/z (%): 2453.93 [M]; elemental analysis calcd (%) for
C
125H152Br10: C 61.19,H 6.24; found: C 61.08,H 6.26.
9,9,9’,9’,9’’,9’’,9’’’,9’’’,9’’’’,9’’’’,9’’’’’,9’’’’’-Dodecakis(6-bromohexyl)-2,2’-7’,2’’-
7’’,2’’’-7’’’,2’’’’-7’’’’,2’’’’’-hexafluorene (14): Compound 14 was synthesized
according to the same procedure for 7 by using compound 11 (239 mg,
55.5,40.2,33.9,32.5,29.1,27.8,25.0,23.6 ppm; MS (MALDI-TOF):
m/z
(%): 1630.14 [M].
2-[9,9,9’,9’-Tetrakis(6’-bromohexyl)-7,2’-bifluorenyl-2-yl]-4,4,5,5-tetra-
0.19 mmol),compound
(820 mg,6.00 mmol),tetrabutylammonium bromide (0.05 g),and [PdCl
(dppf)] (15 mg) in a mixture of tetrahydrofuran (10 mL) and water
6 (411 mg,0.39 mmol),potassium carbonate
A
N
-
2
the procedure used for 2 by using compound 6 (1.28 g,1.20 mmol),bis(pi-
nacolato)diboron (0.46 g,1.81 mmol),potassium acetate (0.45 g,
AHCTREUNG
(4 mL). After reaction for 24 h at 858C,the mixture was cooled down to
room temperature. After evaporation of tetrahydrofuran,dichlorome-
thane (ca. 20 mL) was added to the reaction mixture,and the organic
layer was washed with water followed by brine,and then dried over mag-
4.52 mmol),and [PdCl 2(dppf)] (40 mg) in anhydrous dioxane (12 mL).
R
Purification with silica gel column chromatography (hexane/toluene=
3:1) yielded 10 (0.78 g,58%) as a white solid . 1H NMR (400 MHz,
CDCl3): d=7.73–7.60 (m,10H),7.37–7.30 (m,3H),3.29–3.24 (t,
J=
nesium sulfate. The solvent was removed and the residue was purified
with silica gel column chromatography (dichloromethane/hexane=1:3) to
afford 14 (170 mg,30%) as a pale yellow solid. 1H NMR (400 MHz,
CDCl3): d=7.89–7.63 (m,32H),7.36–7.31 (m,6H),3.31–3.27 (br,24H),
2.20–2.00 (br,24H),1.70–1.60 (br,24H),1.27–1.10 (br,48H),0.90–
0.65 ppm (br,24H); 13C NMR (100 MHz,CDCl 3): d=151.9,151.6,151.0,
141.2,141.0,140.5,127.5,127.4,126.8,126.6,123.2,121.8,120.5,120.2,
55.7–55.5,40.6,34.1,33.0,29.4,28.1,24.1–24.0 ppm; MS (MALDI-TOF):
m/z (%): 2943.44 [M]; elemental analysis calcd (%) for C150H182Br12: C
61.20,H 6.23; found: C 61.87,H 6.30.
6.4 Hz,8H),2.09–2.04 (m,8H),1.70–1.61 (m,8H),1.40 (s,12H),1.25–
1.05 (m,16H),0.75–0.60 ppm (m,8H); 13C NMR (100 MHz,CDCl 3) d=
152.2,151.5,151.0,150.2,144.3,141.5,141.2,141.0,140.5,134.5,129.3,
128.1,127.9,127.4,123.3,121.7,120.8,120.4,120.2,119.5,84.2,55.6–55.5,
40.6–40.5,34.1,33.0,29.5,28.1,25.4,24.0–23.9 ppm; MS (MALDI-TOF):
m/z (%): 1108.24 [M].
7,7’-Bis(4,4,5,5-tetramethyl[1.3.2]dioxaborolan-2-yl)-9,9,9’,9’-tetra(6’-bro-
R
mohexyl)-2,2’-bifluorene (11): Compound 11 was prepared according to
the procedure used for 5 by using compound 8 (2.6 g,2.28 mmol),bis(pi-
nacolato)diboron (1.73 g,6.84 mmol),potassium acetate (1.69 g,
9,9,9’,9’,9’’9’’9’’’,9’’’,9’’’’,9’’’’,9’’’’’,9’’’’’,9’’’’’’,9’’’’’’-Tetradecakis-(6-bromohex-
yl)-2,2’-7’,2’’-7’’,2’’’-7’’’,2’’’’-7’’’’,2’’’’’-7’’’’’,2’’’’’’-heptafluorene (15): Com-
pound 15 was synthesized by using compound 9 (0.23 g,0.14 mmol) and
compound 10 (0.3 g,0.28 mmol),according to the same procedure as for
17.12 mmol),and [PdCl 2(dppf)] (0.12 g) in anhydrous dioxane (40 mL).
N
Purification by using silica gel column chromatography (hexane/tolu-
ene=3:1) yielded 11 (1.24 g,44%) as a white solid. 1H NMR (500 MHz,
CDCl3): d=7.81–7.61 (m,12H),3.28–3.25 (t, J=7.0,8H),2.07–2.05 (m,
8H),1.68–1.62 (q, J=7.0,8H),1.41 (s,24H),1.22–1.19 (q,8H),1.10–
14,to yield
15 (0.13 g,27%) as
a
yellowish solid.
1H NMR
1.08 (q,8H),0.72–0.67 ppm (q, J=7.6,8H); 13C NMR (125 MHz,CDCl )
(300 MHz,CDCl3): d=7.85–7.35 (m,44H),3.30–3.20 (m,28H),2.20–2.05
(m,28H),1.70–1.67 (m,28H),1.28–1.18 (br,56H),0.85–0.70 ppm (m,
28H); 13C NMR (75 MHz,CDCl 3): d=151.9,151.5,151.0,141.0,140.5,
127.0,126.8,123.2,121.8,120.5,55.7,40.6,34.2,33.0,29.4,28.1,24.1 ppm;
MS (MALDI-TOF): m/z (%): 3434.46 [M]; elemental analysis calcd (%)
for C175H212Br14: C 61.20,H 6.23; found: C 61.76,H 6.38.
3
d=151.8,149.8,143.8,140.9,140.3,134.0,128.8,126.3,121.3,120.5,119.2,
83.8,55.2,40.1,33.9,32.6,29.0,27.7,25.0,23.5 ppm; MS (MALDI-TOF):
m/z (%): 1234.367 [M].
9,9,9’,9’,9’’,9’’,9’’’,9’’’-Octakis(6-bromohexyl)-2,2’-7’,2’’-7’’,2’’’-tetrafluorene
(12): Compound 12 was synthesized according to the same procedure for
9,9,9’,9’,9’’,9’’Hexakis(6’’-(N,N,N-trimethylammonium)hexyl)-2,2’-7’,2’’ter-
A
7 by using compound 8 (468 mg,0.41 mmol),compound
0.82 mmol),potassium carbonate (675 mg,4.93 mmol),and [PdCl
2 (506 mg,
2(dppf)]
fluorenyl hexabromide (16): Condensed trimethylamine (ca. 2 mL) was
added dropwise to a solution of compound 7 (1.0 g,0.68 mmol) in THF
(15 mL) at ꢁ788C. The mixture was allowed to warm up to room temper-
ature. The precipitate was re-dissolved by the addition of methanol
(10 mL). After the mixture was cooled down to ꢁ788C,extra trimethyl-
AHCTREUNG
(20 mg) in a mixture of toluene (8 mL) and water (3 mL). After reaction
for 24 h at 858C,the mixture was cooled down to room temperature.
After evaporation of toluene,dichloromethane (ca. 20 mL) was added to
the reaction mixture and the organic layer was washed with water fol-
lowed by brine and then dried over magnesium sulfate. The solvent was
removed and the residue was purified with silica gel column chromatog-
raphy (dichloromethane/hexane=1:4),followed by recrystallization from
hexane to afford 12 (418 mg,52%) as a pale yellow solid. 1H NMR
(400 MHz,CDCl 3): d=7.86–7.64 (m,20H),7.37–7.31 (m,6H),3.30–3.24
(t,J =6.8 Hz,16H),2.18–2.05 (m,16H),1.72–1.64 (m,16H),1.31–1.10
A
temperature. After the solvent had been removed,acetone was added to
precipitate 16 (0.75 g,60%) as light yellow powder.
1H NMR
a
(300 MHz,CD 3OD): d=8.46–8.32 (m,14H),8.00–7.88 (m,6H),3.83–
3.77 (br,12H),3.58 (s,54H),2.85–2.60 (br,12H),2.12 (br,12H),1.70
(br,12H),1.35–1.10 ppm (br,12H);
13C NMR (75 MHz,CD 3OD): d=
(m,32H),0.90–0.65 ppm (m,16H);
13C NMR (100 MHz,CDCl 3): d=
152.9,152.5,151.9,142.2,142.0,141.6,128.4,128.2,127.4,124.1,122.2,
121.4,120.9,67.6,56.7,53.6,41.3,30.3,26.9,25.0,23.8 ppm; MS
151.9,151.6,151.0,141.2,140.9,140.5,127.5,127.4,126.8,126.6,123.3,
Chem. Eur. J. 2008, 14,7366 – 7375
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
7373

B. Liu and S. K. Dishari
(MALDI-TOF): m/z (%): 529.30 [Mꢁ3Br]3+,377.01 [ Mꢁ4Br]4+,285.60
Acad. Sci. USA 2004, 101,7511–7515; c) H. A. Ho,M. Boissinot,
M. G. Bergeron,G. Corbeil,K. DorØ,D. Boudreau,M. Leclerc,
Angew. Chem. 2002, 114,1618–1621; Angew. Chem. Int. Ed. 2002,
41,1548–1551; d) F. Le Floch,H. A. Ho,L. P. Harding,M. Bedard,
P. R. Neagu,M. Leclerc, Adv. Mater. 2005, 17,1251–1254; e) I. B.
[Mꢁ5Br]5+
.
9,9,9’,9’,9’’,9’’,9’’’,9’’’-Octakis(6’’-(N,N,N-trimethylammonium)hexyl)-2,2’-
A
7’,2’’-7’’,2’’’-tetrafluorenyloctabromide (17): Compound 17 was synthe-
sized according to the procedure for 16 by allowing compound 12
(100 mg,0.05 mmol) to react with trimethylamine (2 mL) for 24 h. After
the solvent had been removed,acetone was added to precipitate 17
(76 mg,62%) as a white solid. 1H NMR (300 MHz,CD 3OD): d=8.47–
8.31 (m,20H),8.01–7.89 (m,6H),3.84–3.77 (br,16H),3.59 (s,72H),
2.80–2.69 (br,16H),2.15 (br,16H),1.73 (br,16H),1.32 ppm (br,16H);
13C NMR (75 MHz,CD 3OD): d=152.9,152.5,151.9,142.3,142.1,141.6,
128.4–128.3,127.4,124.1,122.2,121.3,120.9,67.7,56.7,53.6,41.2,30.4,
Kim,B. Erdogan,J. N. Wilson,U. H. F. Bunz,
Chem. Eur. J. 2004,
10,6247–6254; f) I. B. Kim,A. Dunkhorst,J. Gilbert,U. H. F. Bunz,
Macromolecules 2005, 38,4560–4562; g) M. D. Disney,J. Zheng,
T. M. Swager,P. H. Seeberger, J. Am. Chem. Soc. 2004, 126,13343–
13346; h) J. H. Wosnick,C. M. Mello,T. M. Swager, J. Am. Chem.
Soc. 2005, 127,3400–3405; i) K. P. R. Nilsson,O. Inganꢁs,
Nat.
Mater. 2003, 2,419–424; j) A. Herland,K. P. R. Nilsson,J. D. M.
26.9,25.0,23.8 ppm; MS (MALDI-TOF): m/z (%): 324.40 [Mꢁ6Br]6+
,
Olsson,P. Hammarstrom,P. Konradsson,O. Inganꢁs,
Soc. 2005, 127,2317–2323; k) M. R. Pinto,K. S. Schanze, Proc. Natl.
Acad. Sci. USA 2004, 101,7505–7510.
J. Am. Chem.
268.12 [Mꢁ7Br]7+,224.96 [ Mꢁ8Br]8+
.
9,9,9’,9’,9’’,9’’,9’’’,9’’’,9’’’’,9’’’’-Decakis-(6’’-(N,N,N-trimethylammonium)hex-
yl)-2,2’-7’,2’’-7’’,2’’’-7’’’,2’’’’-pentafluorenyl decabromide (18): Compound
18 was synthesized according to the procedure for 16 by allowing com-
pound 13 (100 mg,0.04 mmol) to react with trimethylamine (2 mL) for
24 h. After the solvent had been removed,acetone was added to precipi-
tate 18 (80.7 mg,65%) as a white solid. 1H NMR (300 MHz,CD 3OD):
d=8.47–8.31 (m,26H),7.99–7.89 (m,6H),3.80–3.60 (m,20H),3.59 (s,
90H),2.85–2.65 (br,20H),2.15 (br,20H),1.76 (br,20H),1.32 ppm (br,
20H); 13C NMR (75 MHz,CD 3OD): d=152.9,152.5,152.2,142.3,142.1,
141.7,142.3,142.1,141.7,128.4,127.4,124.1,122.2,121.4,120.9,67.7,
[2] a) B. S. Gaylord,A. J. Heeger,G. C. Bazan, Proc. Natl. Acad. Sci
USA 2002, 99,10954–10957; b) B. S. Gaylord,A. J. Heeger,G. C.
Bazan, J. Am. Chem. Soc. 2003, 125,896–900; c) B. Liu,; G. C.
Bazan, J. Am. Chem. Soc. 2004, 126,1942–1943 G. C. Bazan, Chem.
Mater. 2004, 16,4467–4476; d) S. Wang,G. C. Bazan, Adv. Mater.
2003, 15,1425–1428.
[3] a) B. Liu,S. Baudrey,J. Jaeger,G. C. Bazan,
2004, 126,4076–4077; b) M. B. AbØrem,A. Najari,H. A. Ho,J. F.
Gravel,P. Nobert,D. Boudreau,M. Leclerc, Adv. Mater. 2006, 18,
J. Am. Chem. Soc.
56.7,53.6,41.2,30.5,27.0,25.1,23.8; MS (MALDI-TOF):
m/z (%):
2703–2707; c) B. Liu,G. C. Bazan, Proc. Natl. Acad. Sci. USA,
2005, 102,589–593; d) K. Lee,J. M. Rouillard,T. Pham,E. Gulari,
J. Kim, Angew. Chem. 2007, 119,4751–4754; Angew. Chem. Int. Ed.
2007, 46,4667–4670; e) A. Najari,H. A. Ho,J.-F. Gravel,P. Nobert,
D. Boudreau,M. Leclerc, Anal. Chem. 2006, 78,7896–7899; f) F.
2968.09 [MꢁBr]+,306.09 [ Mꢁ8Br]8+,225.9 [ Mꢁ10Br]10+
.
9,9,9’,9’,9’’,9’’,9’’’,9’’’,9’’’’,9’’’’,9’’’’’,9’’’’’-Dodecakis-(6’’-(N,N,N-trimethylam-
monium)hexyl)-2,2’-7’,2’’-7’’,2’’’-7’’’,2’’’’-7’’’’,2’’’’’-hexafluorenyl dodecabro-
mide (19): Compound 19 was synthesized according to the procedure for
16 by allowing compound 14 (50 mg,0.016 mmol) to react with trimethyl-
amine (2 mL) for 24 h. After the solvent had been removed,acetone was
added to precipitate 19 (35 mg,60%) as a light yellow solid. 1H NMR
He,Y. Tang,S. Wang,Y. Li,D. Zhu,
12343–12346.
J. Am. Chem. Soc. 2005, 127,
[4] T. Fçrster, Ann. Phys. 1948, 2,55–75.
(300 MHz,CD OD): d=8.54–8.42 (br,32H),8.07–7.98 (br,6H),3.87 (br,
[5] J. R. Lakowicz in Principles of Fluorescence Spectroscopy,Kluwer
Academic/Plenum Publisher,New York, 1999.
[6] a) B. Liu,S. Wang,G. C. Bazan,A. Mikhailovsky, J. Am. Chem. Soc.
2003, 125,13306–13307; b) B. Liu,; G. C. Bazan, J. Am. Chem. Soc.
2004, 126,1942–1943; c) S. Wang,B. Liu,B. S. Gaylord,G. C.
Bazan, Adv. Func. Mater. 2003, 13,463–467.
[7] B. Liu,G. C. Bazan, J. Am. Chem. Soc. 2006, 128,1188–1196.
[8] Y. Wang,B. Liu, Chem. Commun. 2007,3553–3555.
[9] a) B. Liu,G. C. Bazan, Chem. Asian J. 2007, 2,499–504.
3
24H),3.67 (s,108H),3.00–2.85 (br,24H),2.2 (br,24H),1.81 (br,48H),
1.40 ppm (br,24H); 13C NMR (75 MHz,CD OD): d=152.1,151.7,151.1,
3
141.5,141.3,140.1,127.6–127.5,126.6,123.3,121.4,120.6,120.1,66.9,
55.9,52.7,40.5,29.6,26.1,24.3,22.9 ppm; MS (MALDI-TOF):
m/z (%):
441.79 [Mꢁ7Br]7+,377.36
[
Mꢁ8Br]8+,254.06
[
Mꢁ11Br]11+,224.54
[Mꢁ12Br]12+
.
9,9,9’,9’,9’’,9’’,9’’’,9’’’,9’’’’,9’’’’,9’’’’’,9’’’’’,9’’’’’’,9’’’’’’-Tetradecakis(6’’-(N,N,N-tri-
G
methylammonium)hexyl)-2,2’-7’,2’’-7’’,2’’’-7’’’,2’’’’-7’’’’,2’’’’’-7’’’’’,2’’’’’’-hepta-
fluorenyl tetradecabromide (20): Compound 20 was synthesized accord-
ing to the procedure for 16 by allowing compound 15 (20 mg,
0.006 mmol) to react with trimethylamine (2 mL) for 24 h. After the sol-
vent had been removed,acetone was added to precipitate 20 (16 mg,
65%) as a yellowish solid. 1H NMR (400 MHz,CD 3OD): d=8.53–8.37
(br,38H),8.05–7.96 (br,6H),3.93 (br,28H),3.65 (s,126H),3.00–2.89
[10] a) M. Stark,B. S. Gaylord,A. J. Heeger,G. C. Bazan,
Adv. Mater.
2002, 14,361–366.
[11] Electronic Materials: The Oligomer Approach (Eds: K. Mꢂllen,G.
Wegner),Wiley-VCH,Weinheim 1998.
[12] a) J. H. Jo,C. Y. Chi,S. Hçger,G. Wegner,D. Y. Yoon,
Chem. Eur.
J. 2004, 10,2681–2688; b) J. Li,M. Li,Z. S. Bo, Chem. Eur. J. 2005,
(br,28H),2.15 (br,28H),1.75 (br,56H),1.40 ppm (br,28H);
13C NMR
11,6930–6936; c) S. Sengupta,S. Bhattacharyya,
1997, 62,3405–3406.
J. Org. Chem.
(75 MHz,CD 3OD): d=152.0,151.5,140.5,128.7,126.6,123.3,121.2,
120.4,120.1,66.8,55.7,52.7,40.2,29.6,25.7,24.2,22.4 ppm; MS
[13] a) B. Liu,B. S. Gaylord,S. Wang,G. C. Bazan,
J. Am. Chem. Soc.
(MALDI-TOF): m/z (%): 1339.04 [Mꢁ3Br]3+,985.23 [ Mꢁ4Br]4+,772.28
2003, 125,6705–6714; b) B. Liu,W. L. Yu,Y. H. Lai,W. Huang,
Macromolecules 2002, 35,4975–4982; c) H. Jiang,X. Zhao,K. S.
Schanze, Langmuir 2006, 22,5541–5543.
[Mꢁ5Br]5+,629.82 [ Mꢁ6Br]6+
.
[14] G. Klaerner,R. D. Miller, Macromolecules 1998, 31,2007–2009.
[15] a) J. Israelachvili, Intermolecular & Surface Forces,Academic Press,
London 1992; b) Y. Zhang,V. T. Milam,D. J. Graves,D. A.
Hammer, Biophys. J. 2006, 90,4128–4136.
Acknowledgements
[16] U. H. F. Bunz,J. M. Imhof,R. K. Bly,C. G. Bangcuyo,L. Rozanski,
D. A. Vanden Bout, Macromolecules 2005, 38,5892–5896.
[17] R. Pecora, Dynamic Light Scattering: Application of Photo Correla-
tion Spectroscopy,Plenum Press,New York, 1985.
The authors are grateful to the NUS ARF 279–000–212–112/133,YIA
279–000–234–123,and MOE 279–000–255–122 for financial support. S. K.
D. thanks NUS for support through a research scholarship. We thank
Chew Chong Jieh Robert and Wong Xingrong Louis for the synthesis of
some monomers.
[18] F. B. Dias,M. Maiti,S. I. Hintschich,A. P. Monkman,
J. Chem.
Phys. 2005, 122,054904.
[19] a) S. J. Park,T. A. Taton,C. A. Mirkin,
Science 2002, 295,1503–
1506; b) J. Wang,A. N. Kawde,A. Erdem,M. Salazar, Analyst 2001,
126,2020–2024.
[20] S. Wang,B. S. Gaylord,G. C. Bazan, Adv. Mater. 2004, 16,2127–
[1] a) L. Chen,D. W. McBranch,H. Wang,R. Helgeson,F. Wudl,D.
Whitten, Proc. Natl. Acad. Sci. USA 1999, 96,12287–12292; b) S.
Kumaraswamy,T. Bergstedt,X. Shi,F. Rininsland,S. Kushon,W. S.
Xia,K. Ley,K. Achyuthan,D. McBranch,D. Whitten,
Proc. Natl.
2132.
7374
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
Chem. Eur. J. 2008, 14,7366 – 7375

Detection of DNA Hybridization
FULL PAPER
[21] a) B. S. Harrison,M. B. Ramey,J. R. Reynolds,K. S. Schanze, J. Am.
Chem. Soc. 2000, 122,8561–8562; b) C. Tan,M. R. Pinto,K. S.
Schanze, Chem. Commun. 2002,446–447; c) M. R. Pinto,B. M.
Kristal,K. S. Schanze, Langmuir 2003, 19,6523–6533.
[23] J. R. Lakowicz, Principle of Fluorescence Spectroscopy, 1999,
Plenum,New York.
[24] Q. H. Xu,B. S. Gaylord,S. Wang,G. C. Bazan,D. Moses,A. J.
Heeger, Proc. Natl. Acad. Sci. USA 2004, 101,11634–11639.
[22] a) D. T. Mcquade,A. E. Pullen,T. M. Swager, Chem. Rev. 2000, 100,
Received: December 4,2007
Revised: April 29,2008
Published online: July 11,2008
2537–2574; b) S. W. Thomas III,G. D. Joly,T. M. Swager,
Chem.
Rev. 2007, 107,1339–1386.
Chem. Eur. J. 2008, 14,7366 – 7375
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
7375