Low-dimensional nanostructures fabricated from bis(dioxaborine)carbazole derivatives as fluorescent chemosensors for detecting organic amine vapors

Article abstract of DOI:10.1039/c0jm04274a

New triphenylamine functionalized bis(dioxaborine)carbazole derivatives (BDOBCn, n = 4, 8, 16) have been synthesized, and low-dimensional nanostructures with different morphologies have been fabricated from BDOBCn with carbon chains of different lengths.

Full text of DOI:10.1039/c0jm04274a

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PAPER
Low-dimensional nanostructures fabricated from bis(dioxaborine)carbazole
derivatives as fluorescent chemosensors for detecting organic amine vapors†
*
Xingliang Liu, Xiaofei Zhang, Ran Lu, Pengchong Xue, Defang Xu and Huipeng Zhou
Received 8th December 2010, Accepted 4th April 2011
DOI: 10.1039/c0jm04274a
New triphenylamine functionalized bis(dioxaborine)carbazole derivatives (BDOBCn, n ¼ 4, 8, 16) have
been synthesized, and low-dimensional nanostructures with different morphologies have been
fabricated from BDOBCn with carbon chains of different lengths. For example, 1D single-crystalline
nanowires are generated from BDOBC4 via a reprecipitation approach, 1D nanofibers based on
BDOBC16 are prepared through an organogelation process, and amorphous nanoparticles are
obtained from BDOBC8. It is interesting that the films formed from the obtained nanostructures can
give strong fluorescence emission under irradiation and can act as fluorescent chemosensors for probing
aniline vapors, with different response rates and different sensitivity controlled by their morphologies.
The mesh-like film obtained from the BDOBC16-based gel exhibits faster fluorescent response in
milliseconds and higher sensitivity than the nanoparticle- and nanowire-based films, because the
supramolecular objects primarily formed in the gelling process percolate and fill the entire volume at
a low-over all-volume fraction, and the entangled piling of the fibers leads to increased porosity. Thus,
it not only provides a large surface for enhanced adsorption and accumulation of gaseous molecules but
also enables expedient cross-film diffusion of gaseous species. Notably, the organogel fibers of
BDOBC16 allow high selectivity for probing aniline vapor, and the detection limit reaches 8.6 ppm.
self-assembling process of conjugated organic molecules, and the
Introduction
prediction of the microstructures in organic nanomaterials, is still
a great challenge.13 Among various functionalities of organic
nanomaterials, the sensing of important molecules is critical for
medical, environmental and security-checking purposes, because
the high surface-to-volume ratios associated with nanoscale che-
mosensors make their electrical and optical properties extremely
sensitive to the analytes adsorbed on the surfaces. In particular,
more effort has been made in developing various types of sensors
for detection of amines,5b–d,14–16 which is of importance in envi-
ronmental and industrial monitoring,17 quality control of food,18
and medical diagnosis of certain types of disease.19 Although the
detection of amines in solution has been studied extensively,14,15
the sensing of gaseous amines has been rarely reported to date on
account of the limitation of sensory materials that enable vapor
detection with high sensitivity, selectivity and reversibility.5c–d,16 It
is known that fluorescent sensing based on organic sensory
materials is proven to be a simple, expedient way for chemical
detection, but the fluorescence of p-conjugated organic molecules
is generally quenched in the solid state on account of molecular
aggregation. Therefore, it is of importance to design building-
blocks with strong emission, which are sensitive to analytes, in the
solid state. On the other hand, one group of typical fluorescent
dyes, 2,2-difluoro-1,3,2(2H)-dioxaborines (DOBs) has received
much attention recently due to their high fluorescence quantum
yields, large molar extinction coefficients, large two-photon
Organic nanostructures based on p-conjugated systems have
attracted increasing attention in recent years due to unique
properties superior to those of their bulk counterparts, leading to
promising applications in the fields of optoelectronics, nonlinear
optics, photonics and chemosensors as well as in nanoscale
devices.1–7 It is noted that the size and morphology of the nano-
structures play key roles in the optical, electronic, magnetic and
mechanical properties of the assemblies, and more strategies have
been developed to fabricate organic nanostructures with different
morphologies, crystallinities, and functionalities.2,8–11 For
example, Xia and coworkers have prepared many 1D organic
nanostructures, which make them ideal candidates for application
in n-type semiconductors in organic field-effect transistors
(OFETs).12 Yao et al. have fabricated nanomaterials from pyr-
azoline derivatives giving tunable emission.3d–g,11c However,
a good understanding of the factors, which have effects on the
State Key Laboratory of Supramolecular Structure and Materials, College
of Chemistry, Jilin University, Changchun, 130012, P. R. China. E-mail:
luran@mail.jlu.edu.cn; Fax: +86 431 88499179; Tel: +86 431 88499179
† Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra and mass spectrometry data for all new compounds; 11B
NMR spectra of BDOBC16; UV-vis absorption and fluorescence
spectra of BDOBCn; SEM, TEM, Fluorescence Microscopy, XRD
patterns of the obtained nanomaterials. See DOI: 10.1039/c0jm04274a
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absorption cross sections, high electron affinities and electron
mobilities, and sensitivity to the surrounding medium. Therefore,
they possess applications in photosensitizers, lasers, and electron
transport materials of organic light emitting diodes and organic
field effect transistors.20–23 In boron diketonate complexes, the
formation of a stable six-membered boron-chelating ring can
inhibit the nonradiative dissipation through O–H stretching
modes from tautomerization between ketone and enol forms, and
may extend the p-conjugation of the molecules, which would
favor molecular assembly via p–p interactions. However, to the
best of our knowledge, reports on the construction of low-
dimensional self-assemblies based on DOB derivatives are rare.24
Herein, we report the facile fabrication of organic nanostructures
from three triphenylamine functionalized bis(dioxaborine)
carbazole derivatives (BDOBCn, n ¼ 4, 8, 16, Chart 1) with alkyl
chains of different lengths. It is interesting that the length of the
side chain plays a key role in the self-assembly process and leads to
fluorescent nanomaterials based on bis(dioxaborine)carbazole
derivatives with different morphologies. For instance, the orga-
nogelation of BDOBC16 affords many nanofibers, 100–200 nm in
diameter, with strong red emission, and the xerogel-based film of
BDOBC16 exhibits high sensitivity and fast response to gaseous
amines. Meanwhile, using a facile reprecipitation approach, we
generate well-defined 1D single-crystalline nanowires from
BDOBC4, which can emit strong orange-red light and show poor
sensitivity to amine vapors. Although no 1D nanostructures can
be fabricated, nanoparticles based on BDOBC8 also emit strong
red light and can act as efficient fluorescent probes for organic
amine vapors, with slightly lower sensitivity and lower response
speed than BDOBC16-based nanofibers. Therefore, the
morphologies of the organic nanomaterials can be controlled
expediently via tuning the length of the side chains, which may
balance the strength of the intermolecular interactions in the self-
assembly process. Notably, the sensing properties of the fluores-
cent organic nanostructures can be determined by their shapes.
This provides a strategy to manipulate the morphology of organic
nanostructures with desired functionality.
Compounds 1, 2 and 3 (Scheme 1) were synthesized according to
the literature.25–29
4,40-(9-(n-Butyl)carbazole-3,6-diyl)di(6-(4-(diphenylamino)
phenyl)-2,2-difluoro-1,3,2(2H)-dioxaborine)
(BDOBC4).
Compound 1 (2.00 g, 2.35 mmol) was dissolved in dry CH2Cl2
(60 mL) to give a yellow solution. Then boron trifluoride-diethyl
etherate (2.50 mL, 19.34 mmol) was added under an atmosphere
of nitrogen via syringe. The reaction mixture was heated to reflux
for 12 h. After removal of the solvent, the crude product was
purified by column chromatography (silica gel; petroleum ether/
1
CH2Cl2, v/v ¼ 1/4) to give a red solid (2.10 g), yield 94%. H
NMR (500 MHz, DMSO-d6) d (ppm): 9.36 (s, 2H), 8.47 (d, J ¼
9.0 Hz, 2H), 8.24 (d, J ¼ 9.0 Hz, 4H), 7.94 (d, J ¼ 9.0 Hz, 2H),
7.80 (s, 2H), 7.48 (t, J ¼ 8.0 Hz, J ¼ 8.0 Hz, 8H), 7.33–7.29 (m,
12H), 6.92 (d, J ¼ 9.0 Hz, 4H), 4.59 (t, J ¼ 7.0 Hz, J ¼ 6.5 Hz,
2H), 1.85–1.79 (m, 2H), 1.37–1.30 (m, 2H), 0.90 (t, J ¼ 7.5 Hz,
J ¼ 7.5 Hz, 3H) (Fig. S1 and S2, ESI†). 13C NMR (125 MHz,
DMSO-d6) d (ppm): 180.1, 179.9, 154.7, 145.8, 132.3, 131.0,
128.6, 127.7, 127.1, 124.7, 124.0, 123.8, 122.8, 122.0, 118.4, 111.9,
93.4, 43.8, 31.6, 20.5, 14.6 (Fig. S3, ESI†). IR (KBr): n ¼ 698,
754, 796, 1030, 1090, 1130, 1190, 1230, 1340, 1400, 1470, 1510,
1560, 2850, 2920 cmꢀ1. Elemental analysis for C58H45B2F4N3O4.
Calcd: (%) C, 73.67; H, 4.80; N, 4.44; found: (%) C, 73.46; H,
4.68; N, 4.21. MALDI-TOF MS: m/z: calcd for 945.6; found:
945.2 (Fig. S4, ESI†).
4,40-(9-(n-Octyl)carbazole-3,6-diyl)di(6-(4-(diphenylamino)
phenyl)-2,2-difluoro-1,3,2(2H)-dioxaborine) (BDOBC8). By
following the synthetic procedure for compound BDOBC4,
BDOBC8 was synthesized by using compound 2 (3.00 g, 3.31
mmol) and boron trifluoride-diethyl etherate (9.00 mL, 26.48
mmol) as the reagents. The crude product was purified by
column chromatography (silica gel; petroleum ether/CH2Cl2, v/v
¼ 1/2) to give a red solid (2.98 g), yield 90%. 1H NMR (500 MHz,
CDCl3) d (ppm): 8.88 (s, 2H), 8.25 (d, J ¼ 9.0 Hz, 2H), 8.06 (d,
J ¼ 9.0 Hz, 4H), 7.49 (d, J ¼ 8.5 Hz, 2H), 7.39 (t, J ¼ 8.5 Hz, J ¼
7.5 Hz, 8H), 7.23 (d, J ¼ 8.0 Hz, 12H), 7.18 (s, 2H), 7.03 (d, J ¼
9.0 Hz, 4H), 4.36 (t, J ¼ 7.0 Hz, J ¼ 7.0 Hz, 2H), 1.92–1.88 (m,
2H), 1.38–1.24 (m, 10H), 0.85 (t, J ¼ 6.5 Hz, J ¼ 7.0 Hz, 3H)
(Fig. S5 and S6, ESI†). 13C NMR (125 MHz, CDCl3) d (ppm):
180.3, 179.8, 154.5, 145.9, 145.1, 131.3, 130.3, 127.7, 127.0, 126.2,
124.9, 123.6, 123.3, 122.5, 119.0, 110.2, 92.0, 44.3, 32.1, 29.6,
29.5, 29.4, 27.6, 23.0, 14.5 (Fig. S7, ESI†). IR (KBr): n ¼ 696,
754, 796, 1030, 1090, 1130, 1190, 1230, 1340, 1400, 1460, 1510,
Experimental
Materials
CH2Cl2 was distilled over CaH2. The other chemicals and
reagents were used as received without further purification.
1560, 1580, 2850, 2920 cmꢀ1
.
Elemental analysis for
C62H53B2F4N3O4. Calcd: (%) C, 74.34; H, 5.33; N, 4.19; found:
Scheme 1 Synthetic routes for compounds BDOBC4, BDOBC8 and
BDOBC16.
Chart 1 Molecular structures of BDOBC4, BDOBC8 and BDOBC16.
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(%) C, 73.55; H, 5.28; N, 4.10. MALDI-TOF MS: m/z: calcd for
microscopy images were taken on a fluorescence microscope
(Olympus Reflected Fluorescence System BX51, Olympus,
Japan). X-ray diffraction patterns were obtained on a Japan
Rigaku D/max-gA. XRD equipped with graphite mono-
1001.7; found: 1001.5 (Fig. S8, ESI†).
4,40-(9-(n-Hexadecyl)carbazole-3,6-diyl)di(6-(4-(diphenyla-
mino)phenyl)-2,2-difluoro-1,3,2(2H)-dioxaborine) (BDOBC16).
By following the synthetic procedure for compound BDOBC4,
BDOBC16 was synthesized by using compound 3 (2.80 g,
2.75 mmol) and boron trifluoride-diethyl etherate (7.48 mL,
22.00 mmol) as the reagents. The crude product was purified by
column chromatography (silica gel; petroleum ether/CH2Cl2,
ꢀ
chromatized Cu-Ka radiation (l ¼ 1.5418 A), employing
a scanning rate of 0.02ꢁ sꢀ1 in the 2q range from 0.7 to 10ꢁ, and
0.05ꢁ sꢀ1 in the 2q range from 10 to 30ꢁ. The samples for fluo-
rescence microscopy and XRD measurements were prepared by
casting a suspension of nanowires or nanoparticles in hexane, or
the gel in toluene/n-heptane (v/v ¼ 3/2) on a glass slide and drying
at room temperature. The thickness of the film was measured
using Veeco Dektak 150 Surface Profiler.
1
v/v ¼ 1/2) to give a red solid (2.75 g), yield 90%. H NMR (500
MHz, CDCl3) d (ppm): 8.45 (s, 2H), 8.03 (d, J ¼ 9.0 Hz, 2H), 7.98
(d, J ¼ 8.5 Hz, 4H), 7.37 (t, J ¼ 8.0 Hz, J ¼ 8.0 Hz, 8H), 7.32 (d,
J ¼ 9.0 Hz, 2H), 7.25–7.18 (m, 12H), 7.03 (s, 2H), 6.98 (d, J ¼ 9.0
Hz, 4H), 4.22 (t, J ¼ 6.5 Hz, J ¼ 7.0 Hz, 2H), 1.86–1.81 (m, 2H),
1.31–1.21 (m, 26H), 0.86 (t, J ¼ 6.5 Hz, J ¼ 7.0 Hz, 3H) (Fig. S9
and S10, ESI†). 13C NMR (125 MHz, CDCl3) d (ppm): 179.9,
179.5, 154.3, 145.9, 144.9, 131.3, 130.3, 127.6, 127.0, 126.2, 124.5,
123.2, 123.1, 122.0, 118.9, 110.1, 92.0, 68.4, 44.2, 32.3, 30.1, 30.0,
29.9, 29.8, 29.7, 29.4, 27.6, 26.0, 23.1, 14.5 (Fig. S11, ESI†). IR
(KBr): n ¼ 696, 756, 787, 1030, 1090, 1130, 1190, 1230, 1340,
1400, 1460, 1510, 1580, 2850, 2920 cmꢀ1. Elemental analysis for
C70H69B2F4N3O4. Calcd: (%) C, 75.48; H, 6.24; N, 3.77; found:
(%) C, 75.28; H, 6.08; N, 3.65. MALDI-TOF MS: m/z: calcd for
1113.9; found: 1113.6 (Fig. S12, ESI†).
Fabrication of nanowires based on BDOBC4
The nanowires based on BDOBC4 were generated using a rep-
recipitation method. Typically, 0.2 mL of the concentrated THF
solution of BDOBC4 (1 ꢂ 10ꢀ3 M) was injected rapidly into
10 mL hexane with sufficient stirring followed by aging for 3 days
at room temperature, and then the nanowires were formed. The
nanowires thus formed can be transferred onto silicon or glass
substrates by pipetting, and dried in air or vacuum for
measurements.
Fabrication of nanoparticles based on BDOBC8
The nanoparticles were prepared by injecting 0.1 mL of
BDOBC8 solution (1 ꢂ 10ꢀ3 M) in THF into 10 mL hexane with
sufficient stirring followed by aging for 1 day at room tempera-
ture. The aged aqueous suspension of nanoparticles is trans-
ferred onto silicon or glass substrates and dried in air or vacuum
for measurements.
Measurements and characterization
1H, 13C and 11B NMR spectra were recorded with a Mercury plus
instrument at 500, 125 and 160.4 MHz, using CDCl3 and
DMSO-d6 as the solvents. IR spectra were measured with
a Nicolet-360 FT-IR spectrometer by incorporation of samples
in KBr disks. UV/Vis spectra were determined with a Shimadzu
UV-1601PC spectrophotometer. Photoluminescence (PL)
spectra were carried out with a Shimadzu RF-5301 luminescence
spectrometer. Mass spectra were performed with Agilent
1100 MS series and AXIMA CFR MALDI-TOF (matrix-assis-
ted laser desorption ionization/time-of-flight) MS (COMPACT).
C, H and N elemental analyses were taken with a Perkin-Elmer
240C elemental analyzer. Scanning electron microscopy (SEM)
was performed on JEOL JSM-6700F (operating at 5 kV). The
samples were prepared by casting the suspension of nanowires or
nanoparticles in hexane, or the gel in toluene/n-heptane (v/v ¼
3/2) onto a clean silicon wafer followed by drying in air. The
Preparation of BDOBC16-based gels
A clear solution of BDOBC16 in the tested solvent was obtained
by heating. After the hot solution was subject to sonication for
5 min, followed by aging for 1 h at room temperature, the gel was
formed.
Fabrication of the nanostructure-based films for microscopy
characterization and sensory investigations
For the gel nanofiber-based film from BDOBC16: By diluting the
gel into a medium-quality solvent, such as the mixed solvent of
toluene/cyclohexane (v/v ¼ 1/1), well-dispersed nanofibers were
produced. Then the nanofibers were deposited onto the substrate
to give a mesh-like film. For the nanoparticle-based film of
BDOBC8 and the nanowire-based film from BDOBC4: The
suspension of the nanowires or nanoparticles in hexane were cast
on a glass slide and dried at room temperature. The thickness of
the films based on BDOBC16, BDOBC8 and BDOBC4 was ca.
150 nm, ca. 150 nm and ca. 250 nm on average, respectively.
The sensory investigations for the films based on the nano-
structures of BDOBCn were carried out at 25 ꢁC. Firstly, the
cotton gauze saturated with amine was placed in the bottom of
a quartz cell (10 mm in width) with a cap to prevent direct
contact of nanostructure-based films with amine and to maintain
a constant vapor pressure of the analyte. Then, the glass slide
coated with the prepared gel nanofibers, nanoparticles or
ꢁ
dried sample was then annealed overnight in an oven at 45 C,
followed by coating with gold. Transmission electron microscopy
(TEM) experiments were performed 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.
Atomic force microscopy (AFM) measurement was carried out
with a commercial AFM instrument (Digital Instruments,
Dimension 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 in toluene/n-heptane
(v/v ¼ 3/2) onto a silicon wafer through evaporating the solvent
slowly at room temperature under vacuum. Fluorescence
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nanowires was put in the cell quickly, and the time-dependent
fluorescence spectra and the changes of the emission intensity at
a certain wavelength (640 nm for nanofiber-based film, 635 nm
for nanoparticle-based film and 613 nm for nanowire-based film)
after exposure to amine were recorded. The amine vapor at
a certain concentration was afforded by diluting the saturated
vapor with nitrogen, and then injecting into the quartz cell.
Results and discussion
Synthesis and characterization
The synthetic routes for triphenylamine functionalized bisdiox-
aborines bridged by carbazole BDOBCn (n ¼ 4, 8, 16) were
shown in Scheme 1. 1,3-Diketones 1–3 were prepared according
to the procedures reported by our group.29 The target molecules
of BDOBC4, BDOBC8 and BDOBC16 could be easily prepared
through the complex reaction between 1,3-diketones 1, 2 and 3
and boron trifluoride-diethyl etherate in good yields of 90–
95%,21,22 and they were characterized by 1H and 13C NMR
spectroscopy, MALDI-TOF mass spectrometry, FT-IR spec-
troscopy and C, H, N elemental analyses (see ESI†). BDOBC8
and BDOBC16 are soluble in CHCl3, CH2Cl2, benzene, toluene,
xylene, DMSO and THF, but show poor solubility in alcohols
(such as methanol and ethanol), ethyl acetate and aliphatic
hydrocarbon solvents (such as cyclohexane and n-hexane) at
room temperature. BDOBC4 shows poor solubility in the above
solvents except for THF and CH2Cl2.
Photophysical properties of BDOBCn in solution
Since the molecular frameworks of BDOBC4, BDOBC8 and
BDOBC16 were similar, they gave quite similar absorption and
emission characteristics in solution. Therefore, the photophysical
properties of BDOBC16 were studied as a typical example. As
shown in Fig. 1a, a broad absorption band appeared in the range
of 290–350 nm in THF due to the carbazole and triphenylamine-
centered transitions, and the band at ca. 400 nm may originate
from a p–p* transition. The other two strong absorptions at 466
and 504 nm (3 ¼ 1.0 ꢂ 105 Mꢀ1 cmꢀ1) with high molar absorption
coefficients might partly be ascribed to charge-transfer (CT)
transitions, which could be supported by the solvent-dependent
absorption spectra. The maximal absorption band of BDOBC16
was located at 488 nm in cyclohexane, and red-shifted gradually
with increasing polarity of the solvent. It reached 513 nm in
DCM (Fig. 1a, Table S1, ESI†).
The CT state is generally sensitive to the exoteric microenvi-
ronment, so the fluorescent emission of BDOBC16 may be tuned
by the polarity of the solvent. From Fig. 1b it was clear that the
emission bands became broad and red-shifted significantly with
increasing polarity of the solvent.30,31 For example, the emission
maximum of BDOBC16 exhibited a remarkable red-shift of
114 nm from cyclohexane (511 nm) to DCM (625 nm). Such
a distinct red-shift and broadening of the emission in polar
solvents indicated an ICT (intramolecular charge transfer) char-
acter for the excited state.32 Meanwhile, a plot of the emission
maximum energy as a function of the Lippert solvent polarity was
given in Fig. 1c to illustrate the conformational change of the
excited state surface prior to the emission.33 A deviation of the
emission maximum energy could be found in cyclohexane and
Fig. 1 Normalized UV-vis absorption (a) and PL spectra (b) of
BDOBC16 in different solvents (1.0 ꢂ 10ꢀ6 M). (c) Lippert–Mataga plot:
fluorescence emission maximum energy of BDOBC16 as a function of
solvent polarity.
toluene from the linear relationship followed by those in other
solvents, suggesting a LE (locally excited)30b emission in nonpolar
solvents and a TICT emission in polar solvents.34
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Self-assembled nanowires based on BDOBC4
cross-section of the broken nanowires (Fig. 2c and S14b, ESI†).7e
In addition, the electron diffraction of a nanowire showed
a typical single-crystalline pattern with sharp diffraction spots
(inset of Fig. 2c), which demonstrated that BDOBC4 molecules
packed into single-crystalline 1D arrays.7e,12a,b
It is known that if p-stacking between the molecules with
extended p-conjugated units is predominant over the lateral
association caused by the hydrophobic interaction among the
side chains, the functional molecules would prefer to arrange
along the stacking axis into 1D nanostructures.2b In the case of
BDOBC4 with a short alkyl chain, it may prefer to form 1D self-
assemblies via strong p–p interactions. Unsurprisingly, well-
defined nanowires were easily generated from BDOBC4 via
a reprecipitation approach.2b,c From the scanning electron
microscopy (SEM) and transmission electron microscopy (TEM)
images as shown in Fig. 2a–c, S13 and S14, ESI,† we could find
1D nanowires with a high aspect ratio (200 to 500 nm in width
and several hundreds of micrometres in length). Atomic force
microscopy (AFM) images also exhibited nanowires (Fig. 2e),
and the line-scan profile over a single wire illustrated a flat
topography (Fig. 2f). The thickness of the wire as measured was
ca. 100 nm, and the aspect ratio of the cross-section (width/
thickness) was around 2 : 1, which could be seen from the
The XRD pattern of the BDOBC4-based nanowires can
provide information about the molecular self-assembly (Fig. 3a).
In the small-angle region, we could find two strong narrow
diffraction peaks corresponding to d-spacing of 1.52 nm and
0.82 nm, which were close to a ratio of 1 : 1/2. It indicated
a lamellar organization within the aggregates of BDOBC4 with an
interlayer distance of 1.52 nm.35 In addition, the ground-state
geometry of BDOBC4 was optimized by semiempirical (AM1)
calculations so as to reveal the molecular packing model (Fig. 3b).
The results showed that the molecular width was estimated to be
1.52 nm, which was in accordance with the period of the lamellar
structure in the crystal state. It should be noted that the absorp-
tion band of BDOBC4 in the nanowires became broad and red-
shifted to 544 nm compared with that in solution (504 nm,
Fig. S15, ESI†), indicating the formation of J-aggregates in the
nanowires. Accordingly, we deemed that BDOBC4 molecules
were arranged in parallel along the growth direction of the wires,
and further assembled into a 1D nanostructure. The proposed
molecular packing model was shown in Fig. 3b, in which
BDOBC4 molecules packed into a layered structure with an
interlayer distance of 1.52 nm in accordance with the XRD result.
It is known that the fluorescence of the p-conjugated molecules
is generally quenched in the solid state on account of molecular
aggregation, which would limit their applications in luminescent
materials. However, it is clear that the BDOBC4-based nanowires
can emit strong orange-red light under irradiation (due to the
formation of J-aggregates)10d from the fluorescence microscopy
image as shown in Fig. 2d, which suggests BDOBC4-based
nanowires have potential applications in fluorescent chemo-
sensors. The fluorescent emission band of the nanowires appeared
at 613 nm, which showed a red-shift of ca. 65 nm compared with
that in toluene (Fig. 1 and S15, ESI†). It further suggested that the
aromatic p-stacking interaction had a great effect on the forma-
tion of the nanowires based on BDOBC4.
Self-assembled nanofibers in BDOBC16-based organogel
The difference between BDOBC4 and BDOBC16 was that a long
carbon chain of hexadecyl was introduced in the latter one, so
Fig. 2 (a–b) SEM, (c) TEM, (d) Fluorescence microscopy (lex ¼ 510–
550 nm) and (e) AFM images of BDOBC4-based nanowires obtained by
a reprecipitation approach; (f) line-scan profile over the nanowires
marked in (e). The inset in (c) depicts the electronic diffraction pattern.
Note: The nanowire as observed by fluorescence microscopy looks larger
than the real size as measured by SEM and TEM due to the diffraction
effect in optical microscopy.
Fig. 3 (a) X-ray diffraction pattern of the nanowires based on BDOBC4
deposited on glass; (b) Schematic illustration of the layered structure with
a period of 1.52 nm in nanowires.
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van der Waals force would cooperate with p–p interactions
between the conjugated units in the self-assembly of BDOBC16.
It was interesting that BDOBC16 could form transparent orga-
nogels in the mixed solvents of benzene/n-hexane (v/v ¼ 1/1),
benzene/cyclohexane (v/v ¼ 1/1), toluene/n-hexane (v/v ¼ 2/1),
and toluene/cyclohexane (v/v ¼ 3/2), etc. under ultrasound
stimulation (Table S2, ESI†). The critical gelation concentrations
(CGC) were in the range of 0.6–1.0 mM in different solvents
(Table S2, ESI†). It meant that one gelator molecule of
BDOBC16 could entrap approximately 10 000 solvent molecules
in the gel phase, so BDOBC16 could be deemed as a super-
gelator.36 The obtained gels were stable for several months at
room temperature.
Fig. 5 (a) Time-dependent UV-vis spectra and (b) time-dependent
fluorescence spectra (lex ¼ 468 nm) of BDOBC16 upon aging the solution
in toluene/n-heptane (v/v ¼ 3/2, 8 ꢂ 10ꢀ4 M) after ultrasound stimulation
(the interval is 30 s). The arrows indicate the spectral changes from the
solution to gel. Inset: photographs of BDOBC16 (1 ꢂ 10ꢀ3 M) in solution
(left vial) and in gel state (right vial) in toluene/n-heptane (v/v ¼ 3/2) upon
irradiation at 365 nm.
The morphologies of the self-assemblies of BDOBC16 in the
gel state were studied by SEM, TEM and AFM measurements.
We could find 3D networks consisting of numerous intertwined
fibers with diameters of 100–200 nm (Fig. 4a and b) from SEM
and TEM images of the xerogel obtained from toluene/n-heptane
(v/v ¼ 3/2). The AFM image of the xerogel also showed many
fibrous aggregates (Fig. 4c).
On the other hand, in the xerogel-based film the maximal
absorption of BDOBC16 appeared at 478 nm (Fig. S16, ESI†).
Accordingly, Fig. 5b gave time-dependent emission spectra of
BDOBC16 upon aging the solution in toluene/n-heptane (v/v ¼
3/2, 8 ꢂ 10ꢀ4 M) after ultrasound stimulation. In solution,
BDOBC16 could emit strong orange light with an emission
centered at 594 nm (inset of Fig. 5b). Upon gel formation, the
emission red-shifted to 610 nm, with an accompanying decrease
of the fluorescent intensity. However, the organogel of
BDOBC16 still exhibited strong orange-red emission (inset of
Fig. 5b). The emission band may further red-shift to 640 nm in
the xerogel-based film (Fig. S16, ESI†), which could emit intense
red light as shown by fluorescence microscopy (Fig. 4d).
In order to obtain information on the organization of fluo-
rophores during the self-assembly process, time-dependent UV/
Vis spectra of BDOBC16 in toluene/n-heptane (v/v ¼ 3/2, 8 ꢂ
10ꢀ4 M) upon aging the solution, which was stimulated with
ultrasound for 2 min, were shown in Fig. 5a. We could find that
the absorption band at ca. 500 nm showed a gradual decrease
and blue-shift, and the shoulders at 410 nm and 550 nm increased
gradually during the gelation process. The absorption blue-
shifted to 468 nm in the gel state. It indicated the interactions
between the chromophores, and H-aggregates of BDOBC16
were formed in the gel state.9,10,37 The sol–gel transition was
reversible, and the absorption maximum could red-shift back to
500 nm when the gel was destroyed into the solution by heating.
Fig. 6a showed the XRD pattern of the xerogel of BDOBC16
obtained from toluene/n-heptane (v/v ¼ 3/2), and only one peak
at the d-spacing of 2.80 nm was detected. In addition, the semi-
empirical (AM1) calculations revealed that the length and width
of the BDOBC16 molecule are 2.91 and 2.80 nm, respectively.
Combined with the results of XRD, UV/Vis absorption and
semiempirical calculations, it was reasonable to suggest that the
molecules of BDOBC16 arranged into a lamellar structure using
the molecular width as a long period, in which the aromatic
moieties formed H-aggregates on account of the spatial
hindrance of hexadecyl groups (Fig. 6b). It should be noted that
the morphologies and the molecular packing model in the
assemblies of BDOBC4 and BDOBC16, whose molecular
structures are similar except the length of side chain, were quite
different, which might have an effect on their functionalities. For
example, the nanofiber-based film from BDOBC16 exhibited
Fig. 4 (a) SEM, (b) TEM, (c) AFM and (d) Fluorescence microscopy
(lex ¼ 510–550 nm) images of the xerogel based on BDOBC16 obtained
from toluene/n-heptane (v/v ¼ 3/2).
Fig. 6 (a) X-ray diffraction pattern of the xerogel of BDOBC16
deposited on glass; (b) Schematic illustration of the layered structures
with a period of 2.80 nm in the gel of BDOBC16.
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higher sensitivity and faster response rate in detecting amines
certain analytes. As shown in Fig. 8a, upon exposure to the
saturated vapor of aniline (880 ppm), the fluorescence emission
intensity of the film based on the nanofibers of BDOBC16 was
instantaneously quenched by about 90%. Fig. 8b shows the plot
of the emission intensity at 640 nm of the film vs. the exposure
time to the saturated vapor of aniline. By fitting the intensity
decay into single exponential kinetics, we can deduce the
response time of 0.67 s for the quenching process (defined as
decay lifetime), which is one of the fastest fluorescent responses
to aniline.5c,d We deduce that such high efficiency of the fluo-
rescence quenching of BDOBC16-based nanofibers in probing
aniline may originate from the interaction between the lone
electron pairs of the nitrogen atom in aniline and the orbital of
boron in BDOBC16, which can be proved by 11B NMR. The
chemical shift of boron in BDOBC16 appears at 4.39 ppm, and
shifts to 4.87 ppm after adding aniline (Fig. S21 and S22, ESI†).
The reason for the high sensitivity and fast response rate of the
chemosensor based on nanofibers is mainly due to the mesh-like
film obtained from BDOBC16-based gel that is primarily formed
by entangled piling of the fibers resulting in increased porosity
(Fig. 4). Thus, it not only provides large surface for enhanced
adsorption of gaseous molecules but also enables expedient
than BDOBC4-based nanowires, which is discussed below.
Self-assembled nanoparticles based on BDOBCn
As mentioned above, BDOBC4 with a short carbon chain could
arrange into nanowires via strong p–p interactions, while
BDOBC16 with a long side chain could self-assemble into
thinner nanofibers, which interwinded in 3D networks to prevent
the flow of the solvents, leading to the gel directed by the
cooperation of p–p interactions and van der Waals forces. Thus,
a question arose regarding the self-assembly of a similar molecule
with a carbon chain of moderate length. Therefore, BDOBC8
with an octyl group was synthesized. On the contrary, only
spherical aggregates with diameter of 100–200 nm can be
observed in the SEM image of the assemblies obtained from
BDOBC8 via a reprecipitation approach (Fig. 7 and S17,
ESI†).3d–g Although we could find a slight conglutination between
the adjacent nanoparticles, it was not strong enough to destroy
the monodispersity of the nanoparticles.
Fig. S18, ESI† showed the UV/vis absorption and fluorescence
spectra of BDOBC8 in dilute solution and in the self-assembled
nanoparticles. It was clear that the absorption and fluorescent
emission bands in the nanoparticles became broad and obviously
red-shifted compared to those of the isolated BDOBC8, sug-
gesting that p–p interactions had an effect on the formation of
the nanoparticles. It should be noted that the nanoparticles gave
strong red emission under excitation at 510–550 nm (Fig. S19,
ESI†), so it is possible that the particles could find application in
fluorescent sensors. In addition, no diffraction peak was detect-
able in the XRD pattern (Fig. S20, ESI†), implying that the
nanoparticles are amorphous. We deduced that the competition
of p–p interactions and van der Waals forces between octyl
groups makes it difficult to obtain long range regulation of the
self-assembly from BDOBC8.
Fluorescent chemosensors based on BDOBCn for detecting
amine vapors
Not only due to the strong fluorescent emission of the self-
assemblies (nanowires, nanofibers and nanoparticles) based on
BDOBCn, but also because of the intrinsic property of boron
diketonate complexes, which are sensitive to the surrounding
medium, it was possible that the obtained nanomaterials from
BDOBCn may become candidates as chemosensors to detect
Fig. 8 (a) Fluorescence spectra of the film based on nanofibers of
BDOBC16 before (dashed) and after (solid) exposure to the saturated
vapor of aniline (880 ppm) for 5 s. Inset of a: cycles of the PL quenching
and recovery by exposing the nanofibril film to the saturated vapor of
aniline and the air, respectively. The quenching was carried out by
exposing the nanofibers on glass wafers to the saturated vapor of aniline
(880 ppm) for 5 s, and the recovery was performed by exposing the
quenched nanofibers to the air for 10 min. The PL intensities at 640 nm
were normalized to the initial value before exposure to the saturated
vapor of aniline. (b) Time-course of fluorescence quenching of the film
based on nanofibers of BDOBC16 after addition into the cell filled with
the saturated vapor of aniline (880 ppm). The intensity was monitored at
640 nm. (c) The concentration-dependent fluorescence quenching effi-
ciency of the film based on nanofibers of BDOBC16 exposed to the vapor
of aniline for 5 s. (d) Fluorescence response of the nanofiber-based film
from BDOBC16 upon exposure to organic amine vapors at 880 ppm for
15 s: 1) ammonium hydroxide; 2) hydrazine; 3) cyclohexylamine; 4)
diethylamine; 5) triethylamine; 6) pyridine; 7) aniline. The monitoring
wavelength is 640 nm.
Fig. 7 SEM images of the nanoparticles based on BDOBC8 on silicon
wafer.
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diffusion of guest molecules across the film matrix. Interestingly,
the fluorescence of BDOBC16-based nanofibers was recovered
completely after the nonemissive film was exposed to the atmo-
sphere for ca. 10 min or blown with a hairdryer for 2 min. As
shown in the inset of Fig. 8a, we find that the reversibility of the
chemosensor of BDOBC16-based nanofibers in the detection of
aniline is excellent.
to facilitate the expedient cross-film diffusion of gaseous species,
and to enhance the collection and accumulation of the trace
vapor amines, while the nanoparticles usually prefer to accu-
mulate in a compact manner, resulting in less porosity within the
film. As for the nanowires based on BDOBC4, very slow fluo-
rescent response to amines was observed, and ca. 37% fluorescent
quenching of BDOBC4-based nanowires was observed upon
exposed to saturated aniline vapor for 200 s (Fig. S26 and S27,
ESI†) since BDOBC4 molecules are packed quite compactly in
the single-crystalline nanowires with a smooth surface, leading to
an absence of three-dimensional porous mesh and a poor sensing
ability.
In order to demonstrate the sensitivity of BDOBC16-based
nanofibers in sensing gaseous aniline, the concentration depen-
dent fluorescence quenching efficiency (1 ꢀ I/Io) at 25 ꢁC is
shown in Fig. 8c. It is clear that the quenching efficiencies can be
saturated under high concentration of aniline vapor above
360 ppm, while they are well-fitted to a linear relationship when
the concentration is below 88 ppm. Accordingly, the detection
limit of BDOBC16-based nanofibers can be estimated as 8.6 ppm
for the vapor of aniline. On the other hand, the sensing properties
of the nanofiber-based film for detecting vapors of other amines,
including primary, secondary and tertiary amines at the same
concentration (880 ppm) are compared with that of gaseous
aniline to illustrate the selectivity of the sensory material. It is
found that fluorescence quenching of greater than 90% was
observed for the nanofiber-based film upon exposure to the
saturated vapors of aniline for 15 s. As for the other amines
examined, only less than 20% fluorescence quenching was
observed (Fig. 8d). Therefore, the organogel fibers of BDOBC16
allow high selectivity for probing aniline vapor, which might
originate from a facile adsorption and diffusion of aniline into
BDOBC16-based gel matrix due to p–p interactions between
aniline and conjugated moieties in BDOBC16, which was absent
between aliphatic amines and BDOBC16. The effect of humidity
(water vapor) on the specificity of fluorescence response of the
film based on nanofibers to gaseous aniline was investigated. No
obvious changes in the fluorescence emission spectra of
BDOBC16 were observed upon exposure to saturated water
vapor at 25 ꢁC and 50 ꢁC for 10 s, and the fluorescence quenching
efficiencies of the film exposed to the mixtures of water vapor/
aniline were similar to those of the mixtures of N2/aniline
(Fig. S23, ESI†). As a result, the humidity cannot affect the
fluorescence response to the gaseous aniline. The film based on
the nanofibers of BDOBC16 showed robust stability in air, which
favors its application in sensing devices (Fig. S24, ESI†).
Conclusions
In summary, we have synthesized three new triphenylamine
functionalized bis(dioxaborine)carbazole derivatives (BDOBCn,
n ¼ 4, 8, 16) with different alkyl chains, and found that the length
of the carbon chain has a significant effect on their self-assem-
bling properties. BDOBC4 preferred to assemble into well-
defined 1D single-crystalline nanowires, in which J-aggregates
were formed directed by p–p interactions, via a reprecipitation
approach. 1D nanofibers based on BDOBC16 were facilely
fabricated through the organogelation process, and H-aggre-
gates were formed driven by the synergistic effect of p–p inter-
actions as well as van der Waals forces in the gel phase. However,
only amorphous nanoparticles instead of 1D nanostructures
could be generated from BDOBC8 containing an octyl group.
Notably, the single-crystalline nanowires of BDOBC4 can emit
strong orange-red light under irradiation, while the nanoparticles
from BDOBC8 and the nanofibers based on BDOBC16 can emit
strong red light, and they can act as fluorescent chemosensors for
detecting organic gaseous amines with different response rates
and different sensitivity. The nanofiber-based film formed from
BDOBC16 exhibited the lowest decay lifetime of 0.67 s in
detecting aniline vapor, while the response time is 2.46 s for the
nanoparticle-based film from BDOBC8. However, very slow
fluorescent response was observed for the nanowire-based film
containing BDOBC4. Moreover, 90% and 80% fluorescence
quenching of nanofibers and nanoparticles could be detected
upon exposure to the saturated aniline vapor for 15 s, but for
nanowires, only 37% fluorescence quenching could be detected
after exposed to the saturated aniline vapor for 200 s. Notably,
we also find that the organogel fibers of BDOBC16 allow high
selectivity for probing aniline vapor, and the detection limit
reaches 8.6 ppm for the vapor of aniline. The reason for the best
sensory ability for the nanofiber-based film from BDOBC16
among the three kinds of nanomaterials is that the supramolec-
ular objects primarily formed in the gelling process percolate and
fill the entire volume at a low-over all-volume fraction, and the
entangled piling of the fibers leads to increased porosity. Thus, it
not only provides a large surface for enhanced adsorption and
accumulation of gaseous molecules but also enables expedient
cross-film diffusion of gaseous species. Although the nano-
particles also exhibit a large surface area-to-volume ratio, the
diffusion of the guest molecules throughout the film became
a little difficult compared with that in the nanofiber-based film,
on account of the compact accumulation of the nanoparticles. In
the single-crystalline nanowires based on BDOBC4, the
In addition, investigations of the fluorescence sensing towards
amines were carried out on nanowires and nanoparticles fabri-
cated from BDOBC4 and BDOBC8, respectively. BDOBC8-
based nanoparticles deposited on a glass also gave a shorter
response time upon exposure to the saturated vapor of aniline,
and the decay lifetime was 2.46 s (Fig. S25, ESI†). Meanwhile, ca.
80% fluorescence quenching for the nanoparticle-based film was
detected upon exposure to the saturated vapors of aniline for 15 s
(Fig. S25 and S27, ESI†). On the one hand, the reason for the fast
fluorescence response with high sensitivity is that the amorphous
nanoparticles have large surface-to-volume ratios, which would
significantly enhance the contact with the vapors of the analyte.
On the other hand, it is found that the response speed as well as
sensitivity for BDOBC8-based nanoparticles is slightly lower
than that of the nanofiber-based film of BDOBC16, because the
primarily formed supramolecular objects in the gelling process
percolate and fill the entire volume at a low-over all-volume
fraction, leading to increased porosity in the trestle-like networks
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Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (NNSFC, No. 20874034 and
51073068), 973 Program (2009CB939701), and the Open Project
of State Key Laboratory of Supramolecular Structure and
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