Luminescent organic 1D nanomaterials based on Bis(β-diketone)carbazole derivatives

Article abstract of DOI:10.1002/chem.201001858

A series of new triphenylamine-functionalized bis(β-diketone)s bridged by a carbazole (CnBDKC, n=1, 4, 8, 16) with twisted intramolecular charge-transfer emission in polar solvents has been synthesized. The length of the carbon chains has a significant effect on the self-assembling properties of the compounds. Well-defined 1D nanowires were easily generated from C1BDKC with a methyl group by a reprecipitation approach directed by π-stacking interaction, and the molecules packed into J-aggregates in the nanowires. In addition, 1D nanofibers based on C16BDKC bearing a long hexadecyl chain were prepared through the organogelation process, and H-aggregates were formed driven by the synergistic effect of π-stacking interaction and van der Waals force in the gel phase. C4BDKC and C8BDKC containing butyl and octyl side chains, respectively, cannot arrange into dispersed nanostructures, probably because π-π interaction between conjugated moieties might be disturbed by the interaction between the side chains, which is, however, not strong enough to dominate the self-assembling process. Notably, the nanowires based on C1BDKC and the gel nanofibers from C16BDKC can emit strong green light under irradiation, which suggests that these 1D nanomaterials may have potential applications in emitting materials as well as photonic devices. Given the green light: Well-defined 1D nanowires and nanofibers have been fabricated by reprecipitation and organogelation from triphenylamine-functionalized bis(β-diketone)s bridged by carbazole with alkyl chains of different length (CnBDKC, n=1, 16, respectively; see picture). The nanostructures give strong green fluorescent emission.

Full text of DOI:10.1002/chem.201001858

DOI: 10.1002/chem.201001858
Luminescent Organic 1D Nanomaterials Based on Bis(b-diketone)carbazole
Derivatives
Xingliang Liu,[a] Defang Xu,[a] Ran Lu,*[a] Bin Li,[b] Chong Qian,[a] Pengchong Xue,[a]
Xiaofei Zhang,[a] and Huipeng Zhou[a]
Abstract: A series of new triphenyla-
mine-functionalized bis(b-diketone)s
wires. In addition, 1D nanofibers based
on C16BDKC bearing a long hexadecyl
chain were prepared through the orga-
nogelation process, and H-aggregates
were formed driven by the synergistic
effect of p-stacking interaction and van
der Waals force in the gel phase.
C4BDKC and C8BDKC containing
butyl and octyl side chains, respective-
ly, cannot arrange into dispersed nano-
structures, probably because p–p inter-
action between conjugated moieties
might be disturbed by the interaction
between the side chains, which is, how-
ever, not strong enough to dominate
the self-assembling process. Notably,
the nanowires based on C1BDKC and
the gel nanofibers from C16BDKC can
emit strong green light under irradia-
tion, which suggests that these 1D
nanomaterials may have potential ap-
plications in emitting materials as well
as photonic devices.
bridged by a carbazole (CnBDKC, n=
1, 4, 8, 16) with twisted intramolecular
charge-transfer emission in polar sol-
vents has been synthesized. The length
of the carbon chains has a significant
effect on the self-assembling properties
of the compounds. Well-defined 1D
nanowires were easily generated from
C1BDKC with a methyl group by a
reprecipitation approach directed by p-
stacking interaction, and the molecules
packed into J-aggregates in the nano-
Keywords: ketones · luminescence ·
nanostructures
·
self-assembly
·
supramolecular chemistry
Introduction
creasing attention, as a result of the unique optical and op-
toelectronic properties superior to those of their bulk coun-
terparts.[1–3] Compared with zero-dimensional (0D) organic
nanoparticles,[4,5] one-dimensional (1D) nanomaterials[6,7]
are more promising building blocks for nanoscale devices,
such as organic field-effect transistors (OFETs),[8] solar
cells,[9] and nanoscale optical waveguides.[6b,10] Recently,
Zang et al. have fabricated 1D nanowires from perylene-
based n-type semiconductor molecules that possess highly
polarized, self-waveguided emission, which makes them
ideal candidates for application in nanolasers and other
angle-dependent optical nanodevices.[10e,11] Yaoꢀs group has
generated nanoribbon assemblies with multicolor emission
During the past few years, nanostructures based on p-conju-
gated organic molecules have become the subject of ever-in-
[a] X. Liu, D. Xu, Prof. R. Lu, C. Qian, Dr. P. Xue, X. Zhang, H. Zhou
Key Laboratory of Supramolecular Structure
and Materials College of Chemistry
Jilin University
Changchun (P. R. China)
Fax : (+86)431-88923907
E-mail: luran@mail.jlu.edu.cn
[b] Prof. B. Li
from
1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene
dye.[12]
Key Laboratory of Excited State Processes
Changchun Institute of Optics, Fine Mechanics and Physics
Chinese Academy of Sciences
Ajayaghosh and co-workers have prepared white-light-limit-
ing nanofibers through controlling the donor self-assembly
and modulation of excitation energy transfer.[13] Although
great efforts have been made to construct 1D organic nano-
materials, the understanding of the factors affecting self-as-
sembly, the final morphology of the fabricated nanomateri-
als, and the dependence of molecular arrangement on the
optoelectronic properties (e.g., emission, exciton migration,
and charge transport) of the assembled materials is still
challenging. Therefore, the design of new organic building
Changchun 130033 (P. R. China)
Supporting information (1H and 13C NMR spectra and MALDI-TOF
mass spectra for all new compounds; UV/Vis absorption and fluores-
cence spectra; fluorescence decay profiles; SEM, TEM, fluorescence
microscopy, and AFM images of the obtained nanomaterials; photo-
graphs of C16BDKC in different solvents under UV light; FTIR
spectra of the xerogel; and ground-state geometry of C16BDKC by
semiempirical calculations) for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001858.
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blocks and the development of self-assembly approaches for
the fabrication of 1D nanostructures with desired functional-
ity should be focused on.
well-defined 1D nanowires with high monodispersities in
both shape and size have been obtained from C1BDKC
with a methyl group, whereas 1D nanofibers, which are thin-
ner than C1BDKC-based nanowires, were generated from
C16BDKC with a hexadecyl group through organogelation.
The other compounds, C4BDKC and C8BDKC containing
butyl and octyl groups, respectively, produced ill-defined ag-
glomerates rather than 1D nanostructures. It should be
noted that the 1D nanostructures based on C1BDKC and
C16BDKC can give strong green fluorescent emission,
which may indicate potential applications in emitting mate-
rials, fluorescent chemosensors, and so on.
In addition, b-diketones have attracted current attention
in organic, inorganic, and materials chemistry on account of
their conventional chelation with cations, including metal or
boron ions, to yield complexes with high fluorescence emis-
sion,[14–16] and their high condensation reaction reactivity to
provide convenient synthetic pathways to different classes of
molecules.[17,18] In particular, the formation of the hydrogen-
bonded six-membered ring through O–H stretching modes
from tautomerization between the ketone and enol forms
would increase the molecular planarity, thus leading to the
inhibition of the nonradiative dissipation. Therefore, if p-
conjugated moieties are connected to b-diketones, the rigid
hydrogen-bonded six-membered ring would be helpful for
the molecular self-assembly directed by p-stacking interac-
tion along the 1D direction. To date, self-assembled nano-
structures based on b-diketone derivatives have not been ex-
tensively studied, although Yao and co-workers have fabri-
cated dibenzoylmethane-based nanotubes of different sizes
by using the immersion technique with a porous alumina
membrane as the template.[19] Therefore, the construction of
1D self-assemblies of b-diketone derivatives by a more
straightforward method remains challenging.
Herein, we report the facile fabrication of 1D organic
nanostructures with controllable morphology through the
self-assembly of new triphenylamine-functionalized bis(b-di-
ketone)s bridged by carbazole (CnBDKC, n=1, 4, 8, 16)
with alkyl chains of different length (Scheme 1) by a repreci-
pitation approach and organogelation, which are suggested
as simple and mild ways to generate 1D organic nanomateri-
als. It is found that the length of the side chains has an
effect on the self-assemblies of CnBDKC. For instance,
Results and Discussion
Synthesis and characterization: The synthetic routes for tri-
phenylamine-functionalized bis(b-diketone)s bridged by car-
bazole CnBDKC (n=1, 4, 8, 16) are shown in Scheme 2.
Firstly, the 3,6-diacetyl carbazoles 5–8 with different length
carbon chains were synthesized by Friedel–Crafts reactions
of N-alkyl-substituted carbazoles 1–4, respectively,[15e,20]
which were obtained by the alkylation of carbazole with
alkyl halide catalyzed by NaH in DMF in yields of over
90%.[21] The target molecules of C1BDKC, C4BDKC,
C8BDKC, and C16BDKC were prepared by Claisen con-
densation between methyl 4-(diphenylamino)benzoate and
the corresponding diacetyl-substituted carbazoles 5–8 in the
presence of sodium hydride in anhydrous THF, followed by
acidification with dilute HCl, to give yields of approximately
70%.[22] The intermediates and the target molecules were
1
characterized by H NMR, 13C NMR, and FTIR spectrosco-
py, MALDI-TOF mass spectrometry, and C, H, N elemental
analyses (see Supporting Information). CnBDKC com-
pounds are readily soluble in CHCl3, CH2Cl2, DMSO, THF,
ethyl acetate, and aromatic solvents (such as benzene, tolu-
ene, and xylene), whereas they have poor solubility in alco-
hols (such as methanol and ethanol) and aliphatic hydrocar-
bon solvents (such as n-hexane and n-heptane).
Photophysical properties of CnBDKC in solution: Com-
pounds C1BDKC, C4BDKC, C8BDKC, and C16BDKC
possess similar molecular structures except for the length of
the alkyl chains, so they give almost the same absorption
and emission characteristics in dilute solutions. Therefore,
C16BDKC was selected as an example to study their photo-
physical properties. As shown in Figure 1a, in THF
C16BDKC exhibits a broad absorption band at about
300 nm due to the carbazole- and triphenylamine-centered
transitions, and two other strong absorption bands at 412
and 432 nm (Table S1, Supporting Information) partly con-
tributed to the charge-transfer (CT) transitions, which can
be supported by the solvent-dependent absorption spectra.
In hexane, the absorption maximum of C16BDKC is located
at 421 nm, which redshifts gradually with increasing polarity
of the solvents and reaches 438 nm in acetonitrile (Figure 1,
and Figure S22 and Table S1 in the Supporting Information).
Scheme 1. Molecular structures of C1BDKC, C4BDKC, C8BDKC, and
C16BDKC.
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observation. When the polarity
of the solvent increases, the
fluorescence is structureless and
exhibits a solvatochromic red-
shift due to the dipole–dipole
interactions between the solute
and solvents.[25] For example,
the fluorescence emission maxi-
mum of C16BDKC shows a re-
markable redshift of 111 nm
from hexane (434 nm) to aceto-
nitrile (545 nm). The maximum
of the emission bands shifts sig-
nificantly to low energy, accom-
panied by obvious broadening
of the emission bands in polar
solvents, which indicates an in-
tramolecular
charge-transfer
(ICT) character for the excited
state.[26] The increase of Stokes
shifts with increasing polarity of
the solvent pointed to a large
solvent relaxation and stronger
stabilization of the excited state
in polar solvents.[24a,b] Therefore,
it can be expected that an excit-
Scheme 2. Synthetic routes for the compounds C1BDKC, C4BDKC, C8BDKC, and C16BDKC.
By comparison with the absorption spectral shapes of
C16BDKC in solvents of different polarity, broadened ab-
sorption bands appear in the range of 350–500 nm with a
long wavelength tail in more polar solvents, such as acetoni-
trile, and become structured accompanied by the disappear-
ance of the tail in nonpolar solvents, such as hexane and cy-
clohexane. These tails seem to originate from the ground-
state CT interaction between the donor (triphenylamine)
and acceptor (b-diketone) moieties.[23] On the other hand, it
should be noticed that an obvious absorption peak at 350–
380 nm can be detected in more polar solvents. We suggest
that it might originate from p–p* transition of C16BDKC,
which could be overlapped by the CT band in nonpolar sol-
vents. Upon increasing the polarity of the solvents, the CT
band redshifted obviously, which resulted in the emergence
of the p–p* transition band located at 350–380 nm.
Because the CT state is generally sensitive to the exoteric
microenvironment, it can be deduced that the fluorescence
emission of C16BDKC may be dependent on the solvent.
As shown in Figure S23 (Supporting Information), the emis-
sion color of C16BDKC in solution can be tuned from blue
to yellow with increasing polarity of the solvent. Figure 1b
shows the solvent-dependent photoluminescence (PL) spec-
tra of C16BDKC. It is clear that in nonpolar solvents, such
as hexane and cyclohexane, the fluorescence spectra of
C16BDKC exhibit vibrational structures, which indicates
two separated close-lying excited states, so it is deemed that
the excited-state contribution to the emission in hexane is
the locally excited (LE) one.[24] The weak solute–solvent in-
teractions in such nonpolar media that do not broaden the
vibronic transition too much should be responsible for this
ed state with a twisted geometry has emerged under excita-
tion of the p-conjugated donor–acceptor system of
C16BDKC. Thus, we suggest that C16BDKC gave twisted
intramolecular charge-transfer (TICT) emission in polar sol-
vents.
To demonstrate the conformational change of the excited-
state surface prior to the emission, we show in Figure 1c a
plot of the emission maximum energy as a function of the
Lippert solvent polarity.[27] In such “Lippert–Mataga”
plots,[24c,28] the slope can be used to evaluate the variation in
dipole moment upon excitation, and the break in the linear
relationship indicates the presence of two different excited
states. Accordingly, in the case of C16BDKC, the deviation
of the emission maximum energy in hexane and cyclohexane
from the linear relationship followed by those in other sol-
vents can further support the notion that the LE state is re-
sponsible for the emission in nonpolar solvents and the
TICT state contributes to the emission in polar solvents. On
the other hand, it is found that C16BDKC is strongly emis-
sive in solvents with medium polarity; for example, the fluo-
rescence quantum yields (Ff) of C16BDKC are 0.70 and
0.67 in chloroform and THF, respectively (Table S1, Sup-
porting Information). The lower Ff value in more polar sol-
vents (such as 0.17 in acetonitrile) can be attributed to the
strong ICT interaction.[29]
Well-defined nanowires based on C1BDKC: Generally, if p-
stacking interaction between the molecules with extended
p-conjugated units is predominant over the lateral associa-
tion caused by the hydrophobic interaction among the side
chains, the functional molecules would prefer to arrange
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Figure 2. a,b) SEM, c) TEM, d) fluorescence microscopy, and e) AFM
images of C1BDKC-based nanowires obtained by the reprecipitation ap-
proach; f) line-scan profile (marked in (e)). Note that due to the diffrac-
tion effect of the nanowires, their diameter in the optical microscopy
image appears larger than the real size as measured by SEM and TEM.
The inset in (c) depicts the electronic diffraction pattern. The excitation
wavelength for fluorescence microscopy measurements was 330–385 nm.
of 200–1000 nm. From the atomic force microscopy (AFM)
image as shown in Figure 2e, we can also find well-defined
nanowires, similar to those observed in the SEM and TEM
images. Furthermore, the line-scan profile suggests the
aspect ratio of the cross section (width/thickness) of the
nanowire is around 4:1 (Figure 2 f and Figure S24a,b in the
Supporting Information).[10e,11a] Notably, the appearance of
sharp diffraction spots in the electron diffraction pattern of
the nanowires (inset of Figure 2c) and the sharp diffraction
peaks in the wide-angle X-ray diffraction pattern (Figure 3a)
illustrate that C1BDKC molecules pack into highly ordered
arrays in the nanowires. For instance, the diffraction peak
with a d-spacing of 0.42 nm is close to a typical p–p stacking
distance (0.35 nm), suggesting that p–p interaction is the
main driving force for the formation of nanowires, which
can be further confirmed by the significant redshift of the
absorption of C1BDKC-based nanowires compared with
that in solution (see below). The other two sharp diffrac-
Figure 1. a) Normalized UV/Vis absorption and b) PL spectra of
C16BDKC in different solvents (2.0ꢂ10À6 m). c) Lippert–Mataga plot:
fluorescence emission maximum energy of C16BDKC as a function of
solvent polarity.
along the stacking axis into 1D nanostructures.[2a] In our
case, only a short alkyl chain (methyl) is connected to the 9-
position of carbazole in C1BDKC, which means that
C1BDKC would like to form 1D self-assemblies through
strong p–p stacking. As anticipated, 1D nanowires based on
C1BDKC were expediently fabricated by the reprecipitation
approach.[2,30] As shown in Figure 2a–c, the scanning elec-
tron microscopy (SEM) and transmission electron microsco-
py (TEM) images of the self-assemblies of C1BDKC reveal
well-defined 1D straight nanowires with high aspect ratio,
for example, a length of hundreds of micrometers and width
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tions correspond to the peaks with d-spacings of 1.32 and
0.45 nm, close to a ratio of 1:1/3. Therefore, we can deduce
a lamellar organization in the aggregates of C1BDKC with
an interlayer distance of 1.32 nm.[31] In addition, semiempiri-
cal (AM1) calculations have been performed to optimize
the ground-state geometry of C1BDKC to help us to under-
stand the molecular packing model in the nanowires. The
molecular width is estimated to be 1.32 nm (Figure 3b),
which is in accordance with the period of the lamellar struc-
ture in the crystal state based on XRD data.
Figure 4. Normalized UV/Vis absorption (c) and fluorescence (a,
lex =460 nm) spectra of C1BDKC-based nanowires deposited on a quartz
slide. The raised baseline for the absorption spectrum of the nanowire
film is primarily due to light scattering.
shown by the fluorescence microscopy image (Figure 2d and
Figure S24c in the Supporting Information). The fluores-
cence emission spectrum of C1BDKC-based nanowires is
given in Figure 4; the emission of the nanowires appears at
513 nm, which shows a redshift of approximately 89 nm rela-
tive to that in hexane. It further indicates that the aromatic
p stacking plays a key role in the self-assembly of C1BDKC.
In addition, from the fluorescence decay profiles of
C1BDKC in nanowires and in hexane solution (5.0ꢂ10À6 m),
we obtained lifetimes of 0.99 and 1.06 ns for the nanowires
and monomeric species, respectively (Figure S25). The short-
er fluorescence lifetime of C1BDKC in nanowires relative
to that in dilute solution further confirmed the formation of
J-aggregates in nanowires.[32] As a result, the C1BDKC-
based nanowires with strong emission may find potential ap-
plications in emitting materials as well as optical devices.
Organogel fibers based on C16BDKC: As discussed above,
C1BDKC with a short methyl chain prefers to self-assemble
into nanowires through strong p–p interaction. For compari-
son, we synthesized three other CnBDKC compounds with
butyl (C4BDKC), octyl (C8BDKC), and hexadecyl
(C16BDKC) groups. As for compounds C4BDKC and
C8BDKC, only ill-defined agglomerates, rather than 1D
nanostructures, are obtained by the reprecipitation ap-
proach. The reason might be that the p–p interaction is dis-
turbed by the interaction between the side chains, which is,
however, not strong enough to dominate the self-assembly
process. Nevertheless, when the side chain is lengthened to
hexadecyl, van der Waals interaction between the long alkyl
groups would cooperate with p–p interaction between the
conjugated units, and might lead to self-assembly behavior
of C16BDKC different from that of the other CnBDKC
compounds. Fortunately, 1D nanofibers based on C16BDKC
can be easily generated by organogelation in cyclohexane, 1-
hexanol, toluene/hexane (1:4, v/v), toluene/heptane (1:5, v/
v) etc. under ultrasound stimulation instead of a heating–
cooling process (Table S2, Supporting Information), because
the precipitates are obtained from the above solvents after
Figure 3. a) X-ray diffraction pattern of nanowires of C1BDKC deposited
on glass. b) Proposed molecular packing model of C1BDKC in nano-
wires.
On the other hand, as shown in Figure 4, the absorption
bands of the nanowires became broad and the maximum
redshifted significantly to 455 nm compared with that in
hexane (421 nm), which indicated the formation of J-aggre-
gates in the nanowires. Thus, we can deduce that C1BDKC
molecules are arranged in parallel along the growth direc-
tion of the wire and are further extended to form a 1D
structure. The molecular packing model is proposed in Fig-
ure 3b, in which the distance between two parallel bis(b-di-
ketone)carbazole moieties is 0.42 nm, which matched the
XRD result. Notably, the C1BDKC-based nanowires can
emit strong green light under irradiation at 330–385 nm, as
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cooling the hot solution of C16BDKC to room temperature
followed by aging for 1 h. The critical gelation concentration
(CGC) varied in the range of 1.8–2.2 mm depending on the
solvent. In addition, the obtained C16BDKC-based gels
were stable, and could not be destroyed after several
months at room temperature. To obtain the morphology of
the self-assemblies of C16BDKC in the gel state, SEM,
TEM, and AFM measurements were performed. The SEM
and TEM images of the xerogels obtained from cyclohexane
show lots of entangled 1D nanofibers around 100 nm in di-
ameter (Figure 5a–c and Figure S26a in the Supporting In-
To obtain information on the organization of chromo-
phores in the gel state, time-dependent UV/Vis spectra of
C16BDKC were measured in cyclohexane (2.0ꢂ10À3 m) by
ultrasound treatment followed by aging for a certain time
(Figure 6a). After
a brief sonication, the solution of
Figure 6. a) Time-dependent UV/Vis spectra of C16BDKC in cyclohex-
ane at 2.0ꢂ10À3 m with ultrasound treatment followed by aging for a cer-
tain time (the interval is 40 s). b) Normalized concentration-dependent
fluorescence spectra of C16BDKC in cyclohexane solution and in the gel
state at 258C (lex =415 nm). Inset: photographs of C16BDKC (2.0ꢂ
10À3 m) dissolved in cyclohexane at 708C (left vial) and the corresponding
organogel at 258C (right vial) under irradiation at 365 nm.
C16BDKC begins to transform into a gel. The absorption
band at 422 nm does not shift, but its intensity decreases
gradually during the gelation process. Additionally, a new
absorption peak located at 450 nm appears upon gel forma-
tion, and increases gradually with time. The findings indicate
that p–p interaction plays a key role in gel formation.[33]
Meanwhile, the fluorescence spectra of C16BDKC in cyclo-
hexane at different concentrations as well as in the gel sate
at 258C are given in Figure 6b. In dilute solution (1.0ꢂ10À7–
6.0ꢂ10À5 m), two emission bands at about 440 and 460 nm
were detected, and the ratio of the emission intensity of the
two peaks became smaller gradually with increasing concen-
tration. When the concentration reached 8.0ꢂ10À5 m, the
Figure 5. a,b) SEM, c) TEM, d) fluorescence microscopy, and e) AFM
images of the C16BDKC gel obtained from cyclohexane; f) z-height line-
scan profile over the fibers marked in (e). The excitation wavelength for
fluorescence microscopy measurements was 330–385 nm.
formation). Similarly, AFM images of the cyclohexane gel
shown in Figure 5e and f also illustrate that the intertwisted
fiber bundles are built up from thin fibrils around 100 nm in
width and several micrometers in length. The fluorescence
microscopy image (Figure 5d and Figure S26b) illustrates
that the C16BDKC-based nanofibers emit strong green light
in the gel phase.
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R. Lu et al.
former peak became a shoulder of the one at about 460 nm.
As the concentration was further increased, the emission
redshifted significantly to 478 nm at 1.0ꢂ10À3 m due to mo-
lecular aggregation. In the gel state, the emission peak of
C16BDKC emerged at 501 nm with a weak shoulder around
540 nm, which suggested the formation of p aggregates in
the gel phase. Moreover, we deem that H-aggregates are
formed from C16BDKC in the gel phase according to the
following evidence. Firstly, the fluorescence decay profile
for C16BDKC-based gel in cyclohexane (2.0ꢂ10À3 m, Fig-
ure S27) reveals that the fluorescence lifetime of C16BDKC
in the gel phase is 1.76 ns, which is longer than that in cyclo-
hexane solution at 1.0ꢂ10À5 m (1.09 ns). It is one piece of
evidence to testify the formation of H-aggregates.[34] Second-
ly, the excitation spectrum of C16BDKC-based gel (Fig-
ure S28) does not correspond in shape or position to its ab-
sorption spectrum: two bands at 342 and 465 nm appeared
in the excitation spectrum, while the absorption bands were
located at 422 and 450 nm. These results provide further
proof for the “trapping” or localization of excitation in H-
aggregates.[35] Finally, the decreased fluorescence emission
intensity of C16BDKC in the gel state relative to that in so-
lution (Figure S29) is also consistent with the formation of
H-aggregates.[36] Although the formed H-aggregates would
induce a decrease of the emission intensity, the gel can still
emit strong green light (inset of Figure 6b and Figure S26b
in the Supporting Information), which makes C16BDKC-
based nanofibers a good candidate for emitting materials.
The vibration frequencies for CH2, including antisymmet-
vents, so they can be used as TICT probes to sense the
changes of the external environment. It should be noted
that the length of the carbon chains plays a key role in the
self-assembling properties of CnBDKC. For example, well-
defined nanowires could be easily fabricated from C1BDKC
with a short methyl side chain by a reprecipitation approach,
and p-stacking interaction is confirmed as the main driving
force for the formation of nanowires, in which C1BDKC
packed into J-aggregates. Meanwhile, C16BDKC bearing a
long hexadecyl chain can self-assemble into an organogel
under ultrasound stimulation and lots of 1D nanofibers are
observed in the gel phase. However, H-aggregates are
formed through cooperation of p-stacking and van der
Waals interactions in the gel phase. In the cases of
C4BDKC and C8BDKC containing butyl and octyl groups,
respectively, only ill-defined agglomerates rather than 1D
nanostructures are obtained. The reason might be that p–p
interaction might be disturbed by the interaction between
the side chains, which is, however, not strong enough to
dominate the self-assembly process. Notably, the generated
1D nanowires based on C1BDKC and nanofibers from
C16BDKC can emit strong green light under irradiation,
which suggests that these 1D nanomaterials may have po-
tential applications in emitting materials as well as photonic
devices.
Experimental Section
ric nas
G
N
Measurement and characterization: 1H and 13C NMR spectra were re-
corded with a Mercury Plus instrument at 500 and 125 MHz by using
CDCl3 as the solvent in all cases. IR spectra were measured with a Nico-
let-360 FTIR spectrometer by incorporation of samples in KBr disks.
UV/Vis spectra were determined with a Shimadzu UV-1601PC spectro-
photometer. PL spectra were recorded with a Shimadzu RF-5301 lumi-
nescence spectrometer. Fluorescence lifetimes were measured by using
the time-correlated single photon counting technique with an FL920 fluo-
rescence lifetime spectrometer. The excitation source was an nF900
nanosecond flashlamp. Lifetimes were obtained by deconvolution of the
decay curves. Mass spectra were obtained with Agilent 1100 MS series
and AXIMA CFR MALDI-TOF (Compact) mass spectrometers. C, H,
and N elemental analyses were performed with a Perkin–Elmer 240C ele-
mental analyzer. SEM was performed on a JEOL JSM-6700F instrument
(operating at 5 kV). The sample was prepared by casting the nanowire
suspension in hexane or gel onto a clean silicon wafer followed by drying
in air. The dried sample was then annealed overnight in an oven at 458C,
followed by coating of gold. TEM experiments were performed by using
a JEM 3010 electron microscope (JEOL, Japan) with an acceleration
voltage of 300 kV. The samples for TEM measurement were prepared by
wiping a small amount of nanowires or gel onto a 300-mesh copper grid
followed by natural evaporation of the solvent. AFM measurement was
carried out with a commercial instrument (Digital Instruments, Dimen-
sion 3100, Santa Barbara, CA) running in tapping mode. Si cantilevers
(Nanosensors) with resonance frequencies of 250–350 kHz were used.
The samples were prepared by drop-casting the nanowire suspension in
hexane or gel onto a silicon wafer then evaporating the solvent slowly at
room temperature under vacuum. Fluorescence microscopy images were
taken on a fluorescence microscope (Olympus Reflected Fluorescence
System BX51, Olympus, Japan). The samples were prepared by casting
the nanowire suspension in hexane or gel on a glass slide and drying at
room temperature. XRD patterns were obtained on a Japan Rigaku D/
max-gA instrument equipped with graphite-monochromatized CuKa radi-
ation (l=1.5418 ꢃ), by employing a scanning rate of 0.028 sÀ1 in the 2q
could somewhat reflect the packing conformation of alkyl
chains,[7c,37] and therefore the FTIR spectrum of the xerogel
of C16BDKC is shown in Figure S30 (Supporting Informa-
tion). The nasACHTUNGTRENNUNG(CH2) and nsCAHTUNGTERN(NUGN CH2) bands appear at 2920 and
2850 cmÀ1, respectively, in a relatively low wavenumber
region, thus suggesting that the alkyl chains adopted an all-
trans conformation. Thus, the cooperation of van der Waals
and aromatic p-stacking interactions is critical for gel forma-
tion from C16BDKC. The XRD pattern of the xerogel of
C16BDKC obtained from cyclohexane shows a series of
sharp diffraction peaks (Figure S31). The strongest peak at
2q=6.808 corresponded to a d-spacing of 1.30 nm, which
was close to the width of the molecule (Figure S32). Al-
though it is difficult to propose the molecular packing
model in the self-assemblies of C16BDKC from XRD re-
sults, the observed diffraction pattern suggests the ordered
arrangement of C16BDKC in the gel state to some
extent.[38]
Conclusion
A series of new triphenylamine-functionalized bis(b-dike-
tone)s bridged by carbazole (CnBDKC, n=1, 4, 8, 16) with
alkyl chains of different length have been synthesized. Such
donor–acceptor-type p-conjugated molecules give CT transi-
tions in the ground state and TICT emission in polar sol-
1666
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1660 – 1669

Luminescent Organic 1D Nanomaterials
FULL PAPER
range from 0.7 to 108, and 0.058 sÀ1 in the 2q range from 10 to 308. The
samples were prepared by casting the nanowires or gels on glass slides
and drying at room temperature.
quickly to a solution of compound 5 (1.00 g, 3.77 mmol) and methyl 4-(di-
phenylamino)benzoate (2.80 g, 9.24 mmol) in THF (60 mL). The reaction
mixture was heated under an atmosphere of nitrogen at 608C for 24 h,
and then cooled to room temperature. The mixture was acidified with
dilute HCl and extracted with CH2Cl2. After solvent removal, the solid
residue was purified by column chromatography (silica gel; CH2Cl2) to
give a yellow solid (2.28 g). Yield 75%; m.p. 258.0–260.08C; 1H NMR
(500 MHz, TMS, CDCl3): d=8.75 (s, 2H), 8.12 (d, J=8.5 Hz, 2H), 7.88
(d, J=8.5 Hz, 4H), 7.41 (d, J=8.5 Hz, 2H), 7.32 (t, J=7.5, 8.0 Hz, 8H),
7.19–7.12 (m, 12H), 7.07 (d, J=8.5 Hz, 4H), 6.88 (s, 2H), 3.87 ppm (s,
3H); 13C NMR (125 MHz, CDCl3): d=185.8, 184.4, 152.0, 147.1, 144.3,
130.0, 128.9, 128.3, 128.1, 126.2, 124.9, 123.4, 120.9, 120.7, 109.2, 92.2,
30.0 ppm; IR (KBr): n˜ =669, 694, 752, 785, 1120, 1170, 1220, 1260, 1400,
1440, 1460, 1490, 1580, 1590, 1620, 2850, 2930 cmÀ1; MALDI-TOF MS:
m/z: calcd for C55H41N3O4: 807.9; found: 808.7; elemental analysis (%)
calcd for C55H41N3O4: C 81.76, H 5.11, N 5.20; found: C 81.65, H 5.21, N
5.11.
Fabrication of nanowires: C1BDKC-based nanowires were prepared by
the reprecipitation method. Typically, a concentrated chloroform solution
of C1BDKC (0.2 mL, 1.0 mm) was injected rapidly into hexane (10 mL)
with sufficient stirring followed by aging for 2 days at room temperature,
and then nanowires in the form of yellow aggregates were formed. The
aged aqueous suspension of the nanowires was transferred onto a quartz
slide followed by drying in air for spectroscopic measurements.
Preparation of C16BDKC-based gels: A clear solution of C16BDKC was
obtained by heating. The gel was formed after the hot solution was soni-
cated for 10 min followed by aging for 1 h at room temperature.
Synthesis: THF was freshly distilled from sodium and benzophenone.
CH2Cl2 was distilled from CaH2. The other chemicals and reagents were
used as received from commercial sources without further purification.
Compounds 1–6 and methyl 4-(diphenylamino)benzoate were synthe-
sized according to the literature.[20, 21,39–41]
3,6-Di(3-{1-[4-(diphenylamino)phenyl]-1,3-dioxopropyl})-9-(n-butyl)-car-
bazole (C4BDKC): By following the synthetic procedure for C1BDKC,
C4BDKC was synthesized with compound 6 (1.10 g, 3.58 mmol), methyl
4-(diphenylamino)benzoate (2.61 g, 8.60 mmol), and sodium hydride
(60%, 1.00 g, 25.00 mmol) as reagents. The crude product was purified by
column chromatography (silica gel; petroleum ether/CH2Cl2, 1:5 v/v) to
give a yellow solid (2.20 g). Yield 72%; m.p. 154.0–156.08C; 1H NMR
(500 MHz, TMS, CDCl3): d=8.77 (s, 2H), 8.11 (d, J=8.5 Hz, 2H), 7.89
(d, J=8.5 Hz, 4H), 7.43 (d, J=8.5 Hz, 2H), 7.32 (t, J=8.0, 7.5 Hz, 8H),
7.18–7.12 (m, 12H), 7.07 (d, J=9.0 Hz, 4H), 6.89 (s, 2H), 4.31 (t, J=7.0,
7.0 Hz, 2H), 1.90–1.84 (m, 2H), 1.44–1.36 (m, 2H), 0.96 ppm (t, J=7.5,
7.5 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=185.8, 184.4, 152.0, 147.1,
143.9, 130.0, 128.9, 128.3, 126.2, 126.0, 124.9, 123.5, 120.9, 120.7, 109.5,
92.2, 43.8, 31.5, 20.9, 14.2 ppm; IR (KBr): n˜ =621, 696, 752, 783, 1110,
1180, 1210, 1270, 1380, 1460, 1490, 1590, 2850, 2920 cmÀ1; MALDI-TOF
MS: m/z: calcd for C58H47N3O4: 850.0; found: 850.3; elemental analysis
(%) calcd for C58H47N3O4: C 81.95, H 5.57, N 4.94; found: C 81.83, H
5.68, N 4.72.
9-n-Hexadecylcarbazole (4): NaH (60%, 2.1 g, 52.0 mmol) and n-
C16H33Br (10.0 g, 32.8 mmol) were added to a solution of carbazole (5.0 g,
29.9 mmol) in DMF (60 mL). The mixture was stirred at room tempera-
ture until the disappearance of carbazole (monitored by TLC). The mix-
ture was poured into water (500 mL) and the precipitate was collected by
filtration. The solid was recrystallized from ethanol to give a white solid
(10.6 g). Yield 90%; m.p. 54.0–56.08C; 1H NMR (500 MHz, TMS,
CDCl3): d=8.09 (d, J=7.5 Hz, 2H), 7.45 (t, J=7.5, 7.5 Hz, 2H), 7.39 (d,
J=8.0 Hz, 2H), 7.21 (t, J=5.5, 9.0 Hz, 2H), 4.27 (t, J=7.5, 7.0 Hz, 2H),
1.88–1.82 (m, 2H), 1.38–1.23 (m, 26H), 0.88 ppm (t, J=6.5 Hz, 3H);
13C NMR (125 MHz, CDCl3): d=140.9, 126.0, 123.3, 120.8, 119.1, 109.1,
68.4, 43.5, 32.4, 30.1, 30.0, 29.9, 29.4, 27.8, 26.1, 23.2, 14.6 ppm; IR (KBr):
n˜ =719, 746, 1122, 1151, 1228, 1325, 1350, 1396, 1466, 1485, 1508, 1541,
1560, 1593, 1655, 1670, 1685, 1718, 2848, 2920 cmÀ1; MALDI-TOF MS:
m/z: calcd for C28H41N: 391.6; found: 391.5; elemental analysis (%) calcd
for C28H41N: C 85.87, H 10.55, N, 3.58; found: C 85.56, H 10.67, N, 3.65.
3,6-Diacetyl-9-(n-octyl)-carbazole (7): Acetyl chloride (7.2 g, 91.8 mmol)
3,6-Di(3-{1-[4-(diphenylamino)phenyl]-1,3-dioxopropyl})-9-(n-octyl)-car-
bazole (C8BDKC): By following the synthetic procedure for C1BDKC,
C8BDKC was synthesized with compound 7 (1.8 g, 4.95 mmol), methyl 4-
(diphenylamino)benzoate (3.76 g, 12.38 mmol), and sodium hydride
(60%, 1.41 g, 35.34 mmol) as reagents. The crude product was purified by
column chromatography (silica gel; petroleum ether/CH2Cl2, 2:3 v/v) to
give a yellow solid (3.14 g). Yield 70%; m.p. 128.0–130.08C; 1H NMR
(500 MHz, TMS, CDCl3): d=8.79 (s, 2H), 8.13 (d, J=8.5 Hz, 2H), 7.91
(d, J=9.0 Hz, 4H), 7.44 (d, J=8.5 Hz, 2H), 7.34 (t, J=8.0, 7.5 Hz, 8H),
7.21–7.14 (m, 12H), 7.10 (d, J=8.5 Hz, 4H), 6.91 (s, 2H), 4.31 (d, J=
5.5 Hz, 2H), 1.93–1.86 (m, 2H), 1.39–1.25 (m, 10H), 0.88 ppm (t, J=8.5,
7.5 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=185.9, 184.4, 152.0, 147.1,
143.9, 128.9, 128.3, 128.1, 126.2, 126.1, 124.9, 123.5, 120.9, 120.8, 109.5,
92.2, 44.0, 32.2, 29.7, 29.5, 29.4, 27.6, 23.0, 14.5 ppm; IR (KBr): n˜ =619,
698, 754, 783, 1120, 1180, 1220, 1270, 1380, 1460, 1490, 1590, 2850,
2920 cmÀ1; MALDI-TOF MS: m/z: calcd for C62H55N3O4: 906.1; found:
907.0; elemental analysis (%) calcd for C62H55N3O4: C 82.18, H 6.12, N
4.64; found: C 82.09, H 5.94, N 4.56.
was added slowly over 30 min to
a suspension of AlCl3 (12.2 g,
91.8 mmol) in 1,2-dichloroethane (100 mL) at 58C. Compound 3 (10.7 g,
38.3 mmol) was then added slowly, and the reaction mixture was stirred
for 2 h at room temperature and then for 2 h at 358C. After cooling, the
solvent was removed under reduced pressure. Dilute HCl (200 mL) was
added to the flask and a white solid appeared. The solid was collected by
filtration and washed with water, followed by recrystallization from etha-
1
nol to give a white solid (12.0 g). Yield 86%; m.p. 90.0–92.08C; H NMR
(500 MHz, TMS, CDCl3): d=8.75 (s, 2H), 8.16 (d, J=8.5 Hz, 2H), 7.42
(d, J=8.5 Hz, 2H), 4.31 (t, J=6.0, 7.0 Hz, 2H), 2.73 (s, 6H), 1.90–1.84
(m, 2H), 1.38–1.20 (m, 10H), 0.85 ppm (t, J=7.0, 7.0 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d=197.8, 144.3, 130.1, 127.4, 123.3, 122.4, 109.4, 44.0,
32.1, 29.7, 29.5, 29.3, 27.6, 27.0, 23.0, 14.4 ppm; IR (KBr): n˜ =621, 669,
721, 814, 901, 957, 1026, 1072, 1130, 1254, 1306, 1367, 1468, 1491, 1570,
1593, 1672, 2848, 2920 cmÀ1
; MALDI-TOF MS: m/z: calcd for
C24H29NO2: 363.5; found: 363.9; elemental analysis (%) calcd for
C24H29NO2: C 79.30, H 8.04, N 3.85; found: C 79.02, H 7.96, N 3.79.
3,6-Diacetyl-9-(n-hexadecyl)-carbazole (8): By following the synthetic
procedure for compound 7, compound 8 (11.0 g) was synthesized with
acetyl chloride (4.8 g, 61.2 mmol), AlCl3 (8.2 g, 61.2 mmol), compound 4
(10.0 g, 25.5 mmol), and 1,2-dichloroethane (100 mL) as reagents. Yield
91%; m.p. 111.0–113.08C; 1H NMR (500 MHz, TMS, CDCl3): d=8.75 (s,
2H), 8.15 (d, J=8.5 Hz, 2H), 7.42 (d, J=8.5 Hz, 2H), 4.29 (t, J=6.5,
6.5 Hz, 2H), 2.73 (s, 6H), 1.89–1.84 (m, 2H), 1.36–1.22 (m, 26H),
0.87 ppm (t, J=7.0, 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=197.7,
144.2, 130.1, 127.3, 123.2, 122.3, 109.3, 68.3, 44.0, 32.3, 30.1, 30.0, 29.9,
29.8, 29.7, 29.6, 29.3, 27.6, 27.4, 27.0, 26.8, 26.0, 23.1, 14.5 ppm; IR (KBr):
n˜ =621, 669, 721, 814, 903, 953, 1026, 1072, 1130, 1265, 1306, 1365, 1468,
1491, 1570, 1595, 1674, 2848, 2919 cmÀ1; MALDI-TOF MS: m/z: calcd
for C32H45NO2: 475.7; found: 476.4; elemental analysis (%) calcd for
C32H45NO2: C 80.79, H 9.53, N 2.94; found: C 80.68, H 9.42, N 2.70.
3,6-Di(3-{1-[4-(diphenylamino)phenyl]-1,3-dioxopropyl})-9-(n-hexadec-
yl)-carbazole (C16BDKC): By following the synthetic procedure for
C1BDKC, C16BDKC was synthesized with compound
8 (2.00 g,
4.20 mmol), methyl 4-(diphenylamino)benzoate (3.20 g, 10.55 mmol), and
sodium hydride (60%, 1.20 g, 30.00 mmol) as reagents. The crude product
was purified by column chromatography (silica gel; petroleum ether/
CH2Cl2, 2:3 v/v) to give a yellow solid (2.85 g). Yield 67%; m.p. 164.0–
1
166.08C; H NMR (500 MHz, TMS, CDCl3): d=8.72 (s, 2H), 8.08 (d, J=
8.0 Hz, 2H), 7.88 (d, J=8.5 Hz, 4H), 7.38 (d, J=8.0 Hz, 2H), 7.31 (t, J=
8.0, 7.5 Hz, 8H), 7.18–7.11 (m, 12H), 7.06 (d, J=9.0 Hz, 4H), 6.86 (s,
2H), 4.26 (s, 2H), 1.85 (s, 2H), 1.32–1.21 (m, 26H), 0.86 ppm (t, J=4.5,
6.5 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=185.8, 184.3, 152.0, 147.1,
143.8, 130.0, 128.9, 128.3, 128.0, 126.2, 126.0, 124.9, 123.5, 120.9, 120.7,
109.5, 92.2, 68.4, 51.0, 44.0, 32.4, 30.1, 30.0, 29.9, 29.8, 29.4, 27.7, 26.1,
23.1, 14.6 ppm; IR (KBr): n˜ =621, 692, 750, 783, 1120, 1180, 1220, 1270,
3,6-Di(3-{1-[4-(diphenylamino)phenyl]-1,3-dioxopropyl})-9-methylcarba-
zole (C1BDKC): Sodium hydride (60%, 1.00 g, 25.00 mmol) was added
Chem. Eur. J. 2011, 17, 1660 – 1669
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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R. Lu et al.
1380, 1460, 1490, 1590, 2850, 2920 cmÀ1; MALDI-TOF MS: m/z: calcd
for C70H71N3O4: 1018.3; found: 1018.7; elemental analysis (%) calcd for
C70H71N3O4: C 82.56, H 7.03, N 4.13; found: C 82.38, H 6.86, N 3.98.
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Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (NNSFC, No. 20874034), the 973 Program
(2009CB939701), and the Open Project of State Key Laboratory of
Supramolecular Structure and Materials (SKLSSM200901).
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