Enhanced emission and its switching in fluorescent organic nanoparticles

Article abstract of DOI:10.1021/ja0269082

A new class of organic nanoparticles (CN-MBE nanoparticles) with a mean diameter of ca. 30-40 nm, which exhibit a strongly enhanced fluorescence emission, were prepared by a simple reprecipitation method. CN-MBE (1-cyano-trans-1,2-bis-(4′-methylbiphenyl)ethylene) is very weakly fluorescent in solution, but the intensity is increased by almost 700 times in the nanoparticles. Enhanced emission in CN-MBE nanoparticles is attributed to the synergetic effect of intramolecular planarization and J-type aggregate formation (restricted excimer formation) in nanopaticles. On/off fluorescence switching for organic vapor was demonstrated with CN-MBE nanoparticles.

Full text of DOI:10.1021/ja0269082

Published on Web 11/08/2002
Enhanced Emission and Its Switching in Fluorescent Organic
Nanoparticles
Byeong-Kwan An,† Soon-Ki Kwon,‡ Sang-Don Jung,§ and Soo Young Park*
Contribution from the School of Materials Science & Engineering, Seoul National UniVersity,
Seoul, 151-744, Korea, Department of Polymer Science & Engineering, Gyeongsang National
UniVersity, Chinju, 660-701, Korea, and Basic Research Laboratory, Electronics &
Telecommunications Research Institute, Daejon, 305-350, Korea
Received May 15, 2002
Abstract: A new class of organic nanoparticles (CN-MBE nanoparticles) with a mean diameter of ca. 30-
40 nm, which exhibit a strongly enhanced fluorescence emission, were prepared by a simple reprecipitation
method. CN-MBE (1-cyano-trans-1,2-bis-(4′-methylbiphenyl)ethylene) is very weakly fluorescent in solution,
but the intensity is increased by almost 700 times in the nanoparticles. Enhanced emission in CN-MBE
nanoparticles is attributed to the synergetic effect of intramolecular planarization and J-type aggregate
formation (restricted excimer formation) in nanopaticles. On/off fluorescence switching for organic vapor
was demonstrated with CN-MBE nanoparticles.
Introduction
dependent fluorescent properties from those of bulk samples.
Recently, Yao et al. reported similar results with pyrazoline
Fluorescent inorganic semiconductor or metal nanoparticles
have attracted considerable research interests due to their unique
properties originating from quantum-size effects.1 They have
been extensively investigated for various potential applications
including the fluorescent biological labels,2 photovoltaic cells,3
light-emitting diodes (LEDs),4 and optical sensors.5 As far as
the application is concerned, fluorescent organic nanoparticles
(FONs) are expected to hold the higher potentials because FONs
allow much more variability and flexibility in materials synthesis
and nanoparticle preparation. Investigations on the FONs,
however, are only at their very initial stages presently. System-
atic research works on FONs have been initiated by Nakanishi
and co-workers,6-9 who have demonstrated that perylene and
phthalocyanine nanoparticles show very different and size-
nanoparticles.10
Motivated by the attractive potentials of FONs particularly
for the nanosized optoelectronic devices applications, we have
been engaged in the synthesis of a new class of strongly
fluorescent organic molecules which spontaneously assemble
into a stable FON. 1-Cyano-trans-1,2-bis-(4′-methylbiphenyl)-
ethylene (CN-MBE) is a typical molecule of our work which
formed extremely uniform and stable FON with unusual
fluorescence behavior. It was observed that the fluorescence
emission from the CN-MBE nanoparticle is extremely strong,
although CN-MBE itself is nonfluorescent in dilute solution.
This fluorescence change is quite unusual considering that the
fluorescence efficiency of organic chromophores generally
decreases in the solid state, although they show high fluores-
cence efficiency in solution. This decreasing fluorescence
efficiency in the solid state is quite general and is mainly
attributed to the intermolecular vibronic interactions which
induce the nonradiative deactivation process, that is, fluorescence
quenching, such as excitonic coupling, excimer formations, and
excitation energy migration to the impurity traps.11 Very
recently, however, several exceptional cases are being reported
for a few organic fluorophores such as arylethynyl compounds,12
poly(phenyleneethynylene)s,13 poly(phenylenevinylene) deriva-
tive,14 pentaphenylsilole,15 and pseudoisocyanine derivatives,16
which all together show unique enhanced emission rather than
a fluorescence quenching in the solid state. Although the origins
* To whom correspondence should be addressed. E-mail: parksy@plaza.
snu.ac.kr.
† Seoul National University.
‡ Gyeongsang National University.
§ Electronics & Telecommunications Research Institute.
(1) (a) Brus, L. E. J. Chem. Phys. 1984, 80, 4403-4409. (b) Alivisatos, A. P.
Science 1996, 271, 933-937. (c) Empedocles, S. A.; Bawendi, M. G.
Science 1997, 278, 2114-2117.
(2) (a) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science
1998, 2013-2016. (b) Michalet, X.; Pinaud, F.; Lacoste, T. D.; Dahan,
M.; Bruchez, M. P.; Alivisatos, A. P.; Weiss, S. Single Mol. 2001, 2, 261-
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5842.
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Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18-52.
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Watanabe, A.; Ito, O.; Toyotama, H.; Nakanishi, H. Mol. Cryst. Liq. Cryst.
1997, 294, 173-176.
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H. Jpn. J. Appl. Phys. 1996, 34, L221-L223.
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A.; Ito, O.; Nakanishi, H. Chem. Lett. 1997, 1181-1182.
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(12) Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U. H. F.;
Garcia-Garibay, M. A. J. Am. Chem. Soc. 2001, 123, 4259-4265.
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9
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J. AM. CHEM. SOC. 2002, 124, 14410-14415
10.1021/ja0269082 CCC: $22.00 © 2002 American Chemical Society

Enhanced Emission in Fluorescent Nanoparticles
A R T I C L E S
of these enhanced emissions are still in debate, it is assumed
that this unique fluorescence change is more or less related to
the effects of intramolecular planarization or a specific aggrega-
tion (H- or J-aggregation) in the solid state. Enhanced fluores-
cence observed in CN-MBE FON seems to belong to this
category of unusual fluorescence, while it is importantly noted
as a unique observation in a nanoparticle medium.
In this paper, we report a synthesis and fluorescence behavior
of CN-MBE nanoparticles with specific discussions on the pos-
sible origin of enhanced emission. Application of CN-MBE
FON to the organic vapor sensor is also demonstrated in this
work.
Figure 1. SEM images of CN-MBE nanoparticles obtained from nano-
particles’ suspension containing 80% volume fractions of water in THF.
Inset shows the magnified SEM image.
Results and Discussion
CN-MBE nanoparticles were prepared by a simple precipita-
tion method without surfactants. By this method,17 water was
used as a nonsolvent for CN-MBE in THF. After 60% volume
fractions of water addition, CN-MBE (2 × 10-5 mol L-1) in
mixture solution started to aggregate into nanosize particles.
Nanoparticles’ suspensions were very transparent with no pre-
cipitates and were observed to be stable even after 3 months.
This long-term stability was also observed for several surfactant-
free organic nanoparticles in water.7,8,10 Recently, Abe et al.
have shown that particular hydrophobic compounds which have
polar CdO groups and high viscosity are stable in water for at
least a year after nanoparticles’ preparation without surfactants.18
This stability is ascribed to the formation of a particular structure
around the polar groups which induces the negative ú potential
in nanoparticles. Electrostatic repulsion due to the negative ú
potential presumably prevents flocculation/coalescence of nano-
particles in water. We have found from experiment that CN-
MBE nanoparticles which also possess polar cyano groups
showed a negative ú potential from -3 to -5 mV. Most
probably, the excellent stability of CN-MBE nanoparticles seems
to be related to this negative ú potential. The shape of nano-
particles was observed by field emission scanning electron
microscopy (FE-SEM). FE-SEM images in Figure 1 show that
CN-MBE nanoparticles obtained by 80% volume fractions of
water addition are very fine spheres with a mean diameter of
about 30-40 nm.
Figure 2. (a) The fluorescence emission of CN-MBE (2 × 10-5 mol L-1
)
in THF (left) and THF/water mixture (80% volume fractions of water)
(nanoparticles’ suspension) (right) under the UV light (365 nm). (b) Relative
quantum yields (Φf) of CN-MBE (2 × 10-5 mol L-1) depending on water
fractions in THF.
virtually no fluorescence at all (Φf ) 0.001), whereas the Φf
values increase drastically from 60% volume fractions of water
addition when CN-MBE starts to form nanoparticles. In the case
of 80% volume fractions of water addition, the Φf value of CN-
MBE nanoparticles’ suspension is almost 700 times higher than
that of CN-MBE solution in THF.
In recent years, a few cases of enhanced emission in the solid
state of specific organic molecules have been reported and
interpreted in terms of the intra- and intermolecular effects
exerted by fluorophore aggregation.12-16,20,21 Intramolecular
effects on fluorescence enhancement are simply explained by
the conformational changes of chromophores. It is supposed
that the twisted conformations of chromophores in solution tend
to suppress the radiative process, whereas planar ones of
chromophores induced in the solid state activate the radiation
process.22-24 Intermolecular effects by aggregation also influence
the fluorescence changes in π-conjugated organic chromophores.
Effects on fluorescence changes by intermolecular interactions
are correlated with the aggregation morphology such as H-type
or J-type aggregation. H-aggregates where molecules are aligned
parallel to each other with strong intermolecular interaction tend
to induce the nonradiative deactivation process.11 Formation of
H-aggregates is characterized by blue-shifted absorption bands
with respect to those of the isolated chromophore.25 In contrast,
CN-MBE showed a dramatic change of fluorescence intensity
from the nonfluorescent isolated single molecule in THF to the
strongly fluorescent nanoparticles’ suspension in THF/water
mixtures (Figure 2a). This enhanced emission from CN-MBE
nanoparticles is clearly depicted by the changes of relative
quantum yield (Φf) of CN-MBE (2 × 10-5 mol L-1) in various
fractions of THF/water mixtures (Figure 2b).19 Isolated CN-
MBE up to 50% volume fractions of water addition shows
(15) (a) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok,
H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740-
1741. (b) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.;
Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Eng. News
2001, 79 (41), 29.
(16) (a) Jelly, E. E. Nature 1936, 138, 1009-1010. (b) Scheibe, G. Angew.
Chem. 1936, 49, 563.
(17) For general preparation methods of organic nanoparticles, see: (a) Kasai,
H.; Nalwa, H. S.; Okada, S.; Oikawa, H.; Nakanish, H. Handbook of
Nanostructured Materials and Nanotechnology; Academic Press: New
York, 2000; Vol. 5, Chapter 8, 433-473. (b) Horn, D.; Rieger, J. Angew.
Chem., Int. Ed. 2001, 40, 4330-4361.
(20) Walters, K. A.; Ley, K. D.; Schanze, K. S. Langmuir 1999, 15, 5676-
5680.
(21) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F.
Macromolecules 2000, 33, 652-654.
(22) Oelkrug, D.; Tompert, A.; Egelhaaf, H.; Hanack, M.; Steinhuber, E.;
Hohloch, M.; Meier, H.; Stalmach, U. Synth. Met. 1996, 83, 231-237.
(23) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.; Hanack, M.;
Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902-1907.
(24) Souza, M. M.; Rumbles, G.; Gould, I. R.; Amer, H.; Samuel, I. D. W.;
Moratti, S. C.; Holmes, A. B. Synth. Met. 2000, 111-112, 539-543.
(25) Gruszecki, W. I. J. Biol. Phys. 1991, 18, 99-109.
(18) Kamogawa, K.; Akatsuka, H.; Matsumoto, M.; Yokoyama, S.; Sakai, T.;
Sakai, H.; Abe, M. Colloids Surf., A 2001, 180, 41-53.
(19) Fluorescence quantum yields (Φf) were calculated using 9,10-diphenylan-
thracence (DPA) in benzene as a standard reference: Berlman, I. B.
Handbook of Fluorescence Spectra of Aromatic Molecules; Academic
Press: New York, 1971.
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A R T I C L E S
An et al.
Scheme 1. Proposed Mechanism of Enhanced Emission in
CN-MBE Nanoparticles
Unlike the almost perfect planarization of biphenyl and stilbene
compound in a crystal,29,30 CN-MBE is speculated to be
somewhat twisted even after aggregation because there are
additional steric interactions caused by the cyano group in CN-
MBE. As a result of this conformation change, however, the
fluorescent property of CN-MBE seems to be altered as to
induce the enhanced fluorescence emission.
It is considered that the aggregation-induced planarization
extends the effective conjugation length and increases the
oscillator strength of CN-MBE. Semiempirical AM1 calculations
supported this postulation. The UV absorption maximum (λmax
)
and magnitude of oscillator strength (f) at λmax increase from
the twisted form (λmax ) 341 nm, f ) 1.41) to the planar form
(λmax ) 379 nm, f ) 1.53) of CN-MBE. Therefore, aggregation-
induced planarization is considered as one of the probable
mechanisms of enhanced emission for CN-MBE nanoparticles.
It is also expected that the aggregation-induced planarization
inevitably causes unique intermolecular interactions in nano-
particles. It is well known that the strong intermolecular
interactions are aroused in the solid state of planar π-conjugated
chromophores which induce the formation of excimer complex
leading to the fluorescence quenching.11 The bulky and polar
cyano group in CN-MBE, however, plays an important role of
restricting the parallel face-to-face intermolecular interactions
in the aggregated state.22,23 Preventing parallel orientation of
conjugated chromophores tends to favor J-aggregation with
enhanced fluorescence emission instead of H-aggregation in the
solid state. The J-aggregation mechanism for the enhanced
fluorescence was already observed in the pseudoisocyanine
(PIC) dye which was characterized by the twisted structure and
bulky groups preventing face-to-face intermolecular interac-
tion.31,32
J-aggregates where the molecules are arranged in head-to-tail
direction induce a relatively high fluorescence efficiency with
a bathochromic shift of UV absorption maximum.25
The enhanced fluorescence emission in CN-MBE nanopar-
ticles (Figure 2) is also explained in terms of the intra- and
intermolecular effects. CN-MBE consists of two biphenyl
molecules with a cyano-stilbene moiety as a bridging group.
The optimized geometry of CN-MBE in the gas phase was
calculated by using the PM3 parametrization in the HyperChem
5.0 program (Hypercube). It was obtained that isolated CN-
MBE molecules are twisted by the steric interactions in biphenyl
unit26 as well as the bulky cyano group attached into vinylene
moiety27(Scheme 1). It is well known that the conformation of
the neighboring phenyl rings in biphenyl molecules is deter-
mined by the opposing influences of the intramolecular repulsion
of ortho-hydrogen atoms and intermolecular packing forces.28
Conformational studies on biphenyl and PPP oligomers have
revealed that a more planar conformation is favored in a
crystalline environment, even though the twist conformation is
preferred in the isolated state.28,29 The observed planarity in the
crystalline state is mainly due to the fact that constraints within
the unit cell might be strong enough to overcome the ortho-
hydrogen repulsions.28,29 Similar conformational changes also
have been observed in the case of the trans-stilbene unit in poly-
(p-phenylene vinylene) (PPV) oilgomers30 and twisted aryl-
ethynyl compounds.12
Therefore, most probably, the enhanced emission of CN-MBE
is attributed to the combined effects of aggregation-induced
planarization and J-aggregate formation. This postulated mech-
anism of enhanced emission in CN-MBE nanoparticles is
depicted in Scheme 1.
Our speculation on the mechanism of enhanced emission is
supported by the experimental UV and photoluminescence (PL)
data of CN-MBE nanoparticles. From 60% volume fractions
of water addition, when the nanoparticles begin to form, the
maximum peaks in the absorption spectra of CN-MBE are red-
shifted, and a new shoulder band appears around 420 nm (Figure
3a). The inset of Figure 3a shows the resolved absorption bands
located at 366 and 420 nm, respectively. The 366 nm band is
associated with the π-π* transition of the CN-MBE molecule
which is red-shifted from that in dilute solution (342 nm). This
bathochromic effect indicates that effective conjugation length
of CN-MBE is extended from the isolated twisted molecule to
the planar one in nanoparticles. This result is also in accord
with the calculated absorption band shift from the twisted
conformer to the planar conformer (Figure 3a, bottom). On the
Therefore, the CN-MBE molecule consisting of the biphenyl
and substituted stilbene unit is likely to show similar confor-
mational planarization due to the aggregation in a nanoparticle.
(26) The dihedral torsional angle of 44.4° in biphenyl molecules has been widely
recognized as the reference value for theoretical calculations: Allmennin-
gen, A.; Bastiansen, O.; Fernholt, L.; Cyvin, B. N.; Cyvin, S. J.; Samdal,
S. J. Mol. Struct. 1985, 128, 59-76.
(27) It is well known that cyano groups attatched into vinylene linkage with
phenyl rings cause the deformation of coplanarity, see refs 22-24 and
Lange, F.; Hohnholz, D.; Leuze, M.; Ryu, H.; Hohloch, M.; Freudenmann,
R.; Hanack, M. Synth. Met. 1999, 101, 652-653.
(28) (a) Baker, K. N.; Fratini, A. V.; Resch, T.; Knachel, H. C.; Adams, W.
W.; Socci, E. P.; Farmer, B. L. Polymer 1993, 34, 1571-1587. (b) Corish,
J.; Morton-Blake, D. A.; O’Donoghue, F.; Baudour, J. L.; Be´nie`re, F.;
Toudic, B. J. Mol. Struct. 1995, 358, 29-38. (c) Guha, S.; Graupner, W.;
Resel, R.; Chandrasekhar, M.; Chandrasekhar, H. R.; Glaser, R.; Leising,
G. Phys. ReV. Lett. 1999, 82, 3625-3628.
(29) In the gaseous and solution state, biphenyl molecules show a twisted
conformation between two phenyl rings, with a dihedral angle estimated
from electron diffraction as 42-45°. At room temperature, however, the
biphenyl molecule in the crystal is practically planar. (a) Trotter, J. Acta
Crystallogr. 1961, 14, 1135-1140. (b) Hargreaves, A.; Rizvi, S. H. Acta
Crystallogr. 1962, 15, 365-373. (c) Ambrosch-Draxl, C.; Majewski, J.
A.; Vogl, P.; Leising, G. Phys. ReV. B 1995, 51, 9668-9676. (d) Bre´das,
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B 1984, 29, 6761-6773.
(30) The ab initio calculation and electron diffraction analysis suggested that
trans-stilbene is not planar (ca. 20-30°) in the isolated state, whereas the
structure was proved to be nearly planar (ca. 3.4-6.8°) in the crystalline
state by X-ray diffraction analysis. (a) Woo, H. S.; Lhost, O.; Graham, S.
C.; Bradley, D. D. C.; Friend, R. H.; Quattrocchi, C.; Bre´das, J. L.; Schenk,
R.; Mu¨llen, K. Synth. Met. 1993, 59, 13-28. (b) Traetteberg, M.; Frantsen,
E. B.; Mijlhoff, F. C.; Hoekstra, A. J. Mol. Struct. 1975, 26, 57-68. (c)
Tian, B.; Zerbi, G.; Mu¨llen, K. J. Chem. Phys. 1991, 95, 3198-3207. (d)
Lagowski, J. B. J. Mol. Struct. 2002, 589-590, 125-137.
(31) Yoshika, H.; Nakatsu, K. Chem. Phys. Lett. 1971, 11, 255-258.
(32) Bujda´k, J.; Iyi, N.; Hroba´rikova´, J.; Fujita, T. J. Colloid Interface Sci. 2002,
247, 494-503.
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Enhanced Emission in Fluorescent Nanoparticles
A R T I C L E S
Figure 3. (a) UV absorption spectra changes of CN-MBE (2 × 10-5 mol L-1) depending on the water fractions in THF. The calculated absorption maximum
peaks of twisted (dot line) and planar form (solid line) of CN-MBE are shown at the bottom. Inset shows peak separation of CN-MBE nanoparticles in the
case of 80% water addition. (b) PL spectra changes of CN-MBE (2 × 10-5 mol L-1) depending on the water fractions in THF. Inset shows the change in
the emission maximum peak of CN-MBE.
Figure 4. (a) UV absorption spectra changes of DPST (2 × 10-5 mol L-1) depending on the water fractions in THF. (b) PL spectra changes of DPST (2
× 10-5 mol L-1) depending on the water fractions in THF. Inset shows the changes in the emission maximum peak of DPST.
other hand, the band at 420 nm is assigned to the J-aggregates
of CN-MBE. In general, the J-aggregation band is distinctly
red-shifted and appears as an intense narrow absorption band
due to the motional narrowing,33 but that of CN-MBE nano-
particles in Figure 3a is relatively broad and weak. This result
suggests that the molecules in nanoparticles may be oriented in
a less optimal way of J-aggregation. Lattice disorder is
considered as one of the possible reasons of it.34
Extended conjugation and J-aggregation lead to the dramatic
increase of the fluorescence intensity in CN-MBE nanoparticles
as shown in Figure 3b. It is seen that the fluorescence intensity
of CN-MBE increases discontinuously from 60% volume
fractions of water addition as shown in Figure 3b. The inset of
Figure 3b shows that the peak position of fluorescence emission
is also red-shifted discontinuously above 60% volume fractions
of water.
It is supposed that the cyano group in CN-MBE is effective
in reducing the face-to-face intermolecular interaction in nano-
particles which consequently induces the formation of J-
aggregates as was observed in pseudoisocyanine (PIC) mol-
ecules. To investigate the role of the cyano group, the UV and
PL spectra of CN-MBE were compared with those of a reference
model compound, trans-4,4′-diphenylstilbene (DPST), which
lacks only the cyano group from CN-MBE. Both CN-MBE and
DPST show high energy tails in the visible region of the UV
absorption spectrum above 60% volume fractions of water
additions when nanoparticles are formed (Figures 3a and 4a,
respectively). These tails in the visible region are attributed to
Mie scattering caused by nanosized particles.35 The striking
difference of UV absorption spectra between CN-MBE and
DPST is the direction of spectral shift. As nanoparticles are
formed, absorption maximum peaks of CN-MBE are red-shifted,
while those of DPST are blue-shifted. The blue shift is explained
by H-aggregate formation resulting from the strong π-stacking
interactions, which is consistent with many other spectroscopic
findings.35,36 In contrast to the drastic increase of CN-MBE
emission, the emission intensity of DPST showed a drastic
decrease in nanoparticles (Figures 3b and 4b, respectively). New
red-shifted bands at around 450 and 480 nm of DPST are
attributed to the excimer formation by H-aggregation.
Application of CN-MBE nanoparticles to the on/off fluores-
cence switches was investigated by use of the different
fluorescent intensity in solution and nanoparticles. Under the
365 nm of UV light illumination at room temperature, CN-
MBE nanoparticles on the TLC plate show bright blue fluo-
rescence which switches off reversibly in the atmosphere of
dichloromethane vapor (Figure 5). This fluorescence switch-
ing behavior of CN-MBE nanoparticles provides a novel con-
cept of nanosized fluorescence switches which sense organic
vapors.
(33) (a) Rossi, U. D.; Daehne, S.; Reisfeld, R. Chem. Phys. Lett. 1996, 251,
259-267. (b) Tachibana, H.; Sato, F.; Terrettaz, S.; Azumi, R.; Nakamura,
T.; Sakai, H.; Abe, M.; Matsumoto, M. Thin Solid Films 1998, 327-329,
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Rieger, J.; Weiss, H. Angew. Chem., Int. Ed. 1999, 38, 2188-2191.
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A R T I C L E S
An et al.
Scheme 2. Synthetic Route of CN-MBE
Figure 5. On/off fluorescence switching of MBAN nanoparticles on the
TLC plate without vapor (left) and in vapor (dichloromethane) (right) under
UV light (365 nm) illumination at room temperature.
In conclusion, we have demonstrated a new class of fluo-
rescent organic nanoparticles (CN-MBE FONs) which are very
fine spheres with a mean diameter of about 30-40 nm. They
exhibit a strongly enhanced emission and on/off fluorescent
switching which senses organic vapor. Enhanced fluorescence
emission in CN-MBE nanoparticles was attributed to the
synergetic effect of planarization and J-aggregation (restricted
excimer formation).
Experimental Section
Synthesis. 4,4′-Dimethyl Biphenyl (1). 4-Chlorotoluene (79 mmol)
was added to the mixture of nickel chloride (3.9 mmol), 2,2′-bipyridine
(BPy) (3.9 mmol), triphenylphosphine (PPh3) (15.7 mmol), and zinc
(122.3 mmol) in N,N-dimethylformamide (DMF) solution (50 mL). The
mixture was stirred at 90 °C for 5 h. After being cooled, 1 N HCl
solution was poured into the reaction mixture. The precipitate was
filtered and washed with methanol (yield: 75%). 1H NMR (300 MHz,
CDCl3) δ [ppm]: 7.49 (d, 4H, Ar-H), 7.25 (d, 4H, Ar-H), 2.38 (s,
6H, -CH3). MS (EI) (calcd for C14H14, 182.26; found, 182) m/e: 182,
167, 152, 128, 115, 90, 76, 63, 51.
(d, 2H, Ar-H), 7.28 (d, 2H, Ar-H), 2.42 (s, 3H, -CH3). MS (EI)
(calcd for C14H12O, 196.24; found, 196) m/e: 196, 165, 152, 115, 97,
82, 63, 51.
1-Cyano-trans-1,2-bis-(4′-methylbiphenyl)ethylene (CN-MBE) (4).
The mixture of 2 (1 mmol) and 3 (1 mmol) in tert-butyl alcohol (7
mL) and THF (3 mL) was stirred at 50 °C for 1 h. Tetrabutylammonium
hydroxide (TBAH) (1 M solution in methanol) (0.1 mL) was slowly
dropped into the mixture and stirred for 1 h. The green precipitate was
collected by filtration and washed with methanol (94%) (Scheme 2).
1H NMR (300 MHz, CDCl3) δ [ppm]: 7.98 (d, 2H, Ar-H), 7.69 (m,
6H, Ar-H), 7.58 (s, 1H, vinyl), 7.52 (d, 4H, Ar-H), 7.25 (d, 4H, Ar-
H), 2.39 (s, 6H, -CH3). FT-IR (KBr, cm-1): 3040, 2950, 2220 (-CN
attached to vinylene), 1500, 1150, 800. MS (EI) (calcd for C29H23N,
385.5; found, 385) m/e: 385, 370, 294, 278, 252, 193, 165, 105, 91,
65, 57. Anal. Calcd for C29H23N: C, 90.35; H, 6.01; N, 3.63. Found:
C, 90.27; H, 5.94; N, 3.62.
(4′-Methyl-biphenyl-4-yl)-acetonitrile (2). N-Bromosuccinimide
(NBS) (13.3 mmol) was added to a solution of 1 (13.3 mmol) in benzene
(40 mL), and the mixture was refluxed for 12 h. The mixture was poured
into a 500 mL of water and stirred for 2 h. After the solvent was
removed under vacuum, the residue was added into a solution of NaCN
(51.2 mmol) in tetrahydrofuran (THF) (30 mL). The mixture was stirred
at 50 °C for 24 h. After being cooled to room temperature, the resulting
mixture was column chromatographed on silica gel, eluting with ethyl
acetate/hexane (1:3 by volume) solvent. After solvent was removed
trans-4,4′-Diphenylstilbene (DPST). DPST (Lancaster, 99%) was
purchased commercially and used as obtained without further purifica-
tion.
1
under vacuum, the solid product was obtained (45%). H NMR (300
MHz, CDCl3) δ [ppm]: 7.58 (d, 2H, Ar-H), 7.47 (d, 2H, Ar-H),
7.40 (d, 2H, Ar-H), 7.27 (d, 2H, Ar-H), 3.79 (s, 2H, -CH2CN), 2.40
(s, 3H, -CH3). MS (EI) (calcd for C15H13N, 207.27; found, 207) m/e:
207, 192, 165, 152, 139, 115, 89, 76, 63, 51.
Nanoparticles’ Preparation. Distilled water was regularly injected
into the chromophore/THF with vigorous stirring at room temperature,
using a syringe pump. Before distilled water injection, distilled water
and chromophore solution were filtered by membrane filter with 0.2
µm pore size. As volume fractions of injected distilled water increased,
those of THF solution gradually decreased. In all samples, the
concentration of chromophores (2 × 10-5 mol L-1) was constant after
distilled water injection.
Instrumentation. 1H NMR spectra were recorded on a JEOL JNM-
LA300 (300 MHz) in CDCl3 solutions with CHCl3 at 7.26 ppm as the
internal standard. IR spectra were measured on a Midac FT-IR
spectrophotometer using KBr pellets. Mass spectra were measured on
a JMS AX505WA by EI mode. UV-visible absorption and fluores-
cence emission spectra were recorded on a HP 8452-A and a Shimadzu
RF-500 spectrofluorophotometer, respectively. FE-SEM images were
4′-Methyl-biphenyl-4-carboxaldehyde (3). Bromination reaction of
1 (13.3 mmol) was carried out by the same procedure as above. After
bromination, the residue was added into a solution of hexamethylene-
tetramine (HMTA) (51.2 mmol) in chloroform (30 mL). The mixture
was refluxed for 12 h. After being cooled to room temperature, the
solvent was removed under vacuum. Next 50 mL of a glacial acetic
acid (AcOH) and water mixture (1:1 by volume) was poured into the
reaction mixture and refluxed for 2 h. The resulting mixture was column
chromatographed on silica gel, eluting with ethyl acetate/hexane (1:3
by volume) solvent. After solvent was removed under vacuum, the solid
1
product was obtained (25%). H NMR (300 MHz, CDCl3) δ [ppm]:
10.0 (s, 1H, -CHO), 7.95 (d, 2H, Ar-H), 7.75 (d, 2H, Ar-H), 7.55
9
14414 J. AM. CHEM. SOC. VOL. 124, NO. 48, 2002

Enhanced Emission in Fluorescent Nanoparticles
A R T I C L E S
acquired on a JSM-6330F (JEOL) by dropping nanoparticles’ suspen-
sion on carbon tapes. The fluorescence displays of CN-MBE nanopar-
ticles’ suspension and CN-MBE nanparticles on the TLC plate were
photographed on a Nikon-Coolpix 995 digital camera. CN-MBE
nanoparticles on the TLC plate were prepared by dropping nanopar-
ticles’ suspension using the same method for acquiring FE-SEM images
of CN-MBE nanoparticles on the carbon tapes. The ú potential of CN-
MBE nanoparticles was measured by an electrophoretic light-scat-
tering spectrophotometer (ELS-8000) (Otsuka Electronics, Japan) using
the standard sample of polystyrene latex particles.
Acknowledgment. This work was partly supported by ETRI,
and we are grateful for the instrumental support from the
equipment facility of CRM-KOSEF, Korea University. Helpful
discussion with Prof. B. Z. Tang is greatly appreciated.
JA0269082
9
J. AM. CHEM. SOC. VOL. 124, NO. 48, 2002 14415