A boron-containing carbazole dimer: Synthesis, photophysical properties and sensing properties

Article abstract of DOI:10.1039/c2ob00024e

A novel boron-containing π-conjugated compound BCCB has been synthesized by the introduction of electron-acceptors (dimesitylboron groups) at the 3,3′-positions of a carbazole dimer (electron-donor). The compound BCCB possesses excellent electrochemical properties and high fluorescence quantum yields. In addition, BCCB is a sensitive fluorescence sensor with remarkable colour changes and the results could be confirmed through theoretical calculations of the compounds BCCB and [ⁿBu₄N] ⁺₂[BCCB·(F)₂]^2-. Our studies indicate that BCCB could be used as an excellent optoelectronic material in OLED devices and a ratiometric fluorescent chemosensor.

Full text of DOI:10.1039/c2ob00024e

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PAPER
A boron-containing carbazole dimer: synthesis, photophysical properties and
sensing properties†
He-ping Shi,*a Jian-xin Dai,a Lei Xu,a Li-wen Shi,a Li Fang,a Shao-min Shuanga and Chuan Dong*b
Received 4th January 2012, Accepted 13th March 2012
DOI: 10.1039/c2ob00024e
A novel boron-containing π-conjugated compound BCCB has been synthesized by the introduction of
electron-acceptors (dimesitylboron groups) at the 3,3′-positions of a carbazole dimer (electron-donor).
The compound BCCB possesses excellent electrochemical properties and high fluorescence quantum
yields. In addition, BCCB is a sensitive fluorescence sensor with remarkable colour changes and the
results could be confirmed through theoretical calculations of the compounds BCCB and
[nBu4N]+ [BCCB·(F)2]2−. Our studies indicate that BCCB could be used as an excellent optoelectronic
2
material in OLED devices and a ratiometric fluorescent chemosensor.
carbazole) as electron-donors. Carbazole dimers have an elec-
Introduction
tron-rich structure, high thermal stability and unique electrical
and optical properties.11,12 Chen et al. have prepared a series of
In recent years, π-conjugated derivatives containing boron atoms
have attracted a great deal of attention because of their excellent
potential applications.1,2 Their outstanding photophysical proper-
ties, such as remarkable solvatochromism of emission spectra
and high fluorescence quantum yields, arise from the pπ–π con-
jugation between the vacant p-orbital on the boron centre with
the π-orbital of the π-conjugated molecular system.3 In addition,
three coordinated boron groups can be used as Lewis acids to
coordinate with Lewis bases, such as fluoride ions which has
attracted great attention because it is highly relevant to human
health and environmental issues.4 The complexation of the
boron groups with Lewis bases interrupts the pπ–π conjugation,
causing significant changes in photophysical properties.5 There-
fore, boron-containing π-conjugated derivatives have potential
applications as two-photon emission materials, nonlinear optical
materials and transporting and emissive materials in organic
light-emitting devices (OLEDs) as well as selective chemosen-
sors for the detection of fluoride ions.6–8 Therefore, organoboron
compounds have received worldwide interest and an increasing
number of studies have focused on utilizing donor–acceptor
structures to enhance the photophysical or sensing properties.9,10
In view of this situation, we became interested in boron-con-
taining π-conjugated compounds bearing boron groups as elec-
tron-acceptors and amine moieties (diphenylamine and
derivatives by incorporating diarylamines at the peripheries of a
carbazole dimer, which were fluorescent in the blue to yellow
region with moderate to good quantum yields.12 Although carba-
zole dimers have been widely studied, to the best of our knowl-
edge, investigations focused on functionalization of the
carbazole dimer via connecting electron-acceptors to the 3-,3′-
positions have never been reported.
For this reason, in this paper, we designed and synthesized a
novel A-π-D-π-A compound containing a large π-conjugated
system and two diaryl boron groups as side chains (electron-
acceptors) as well as a carbazole dimer as the core (electron-
donor), named (E)-1,2-bis(3-dimesitylboryl-9-ethylcarbazol-6-
yl)ethene (BCCB). The structure of compound BCCB was
characterized by 1H NMR, 13C NMR, mass spectrometry and
elemental analysis. In addition, its photophysical, electrochemi-
cal and sensing properties were investigated by experimental
methods and theoretical calculations. We anticipate that the
results will increase knowledge of the design and synthesis of
novel boron-containing π-conjugated compounds with excellent
photophysical and sensing properties.
Results and discussion
Synthesis of BCCB
aSchool of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, 030006, People’s Republic of China. E-mail: hepingshi@
sxu.edu.cn; Tel: +86 351 7018094
BCCB was synthesized in four steps as shown in Scheme 1. The
first step began with ethylation of carbazole followed by treating
the resulting 9-ethylcarbazole with POCl3 and DMF via Vilsme-
ier–Haack reaction to achieve 9-ethylcarbazole-3-carbaldehyde
(1) in 65% yield. Secondly, 6-bromo-9-ethylcarbazole-3-carbal-
dehyde (2) was prepared in high yield by the reaction of
bCenter of Environmental Science and Engineer Research, Shanxi
University, Taiyuan, 030006, People’s Republic of China
†Electronic supplementary information (ESI) available: 1H NMR, 13C
NMR, detailed titration spectra and additional data. See DOI: 10.1039/
c2ob00024e
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Scheme 1 The synthetic route to compound BCCB.
Fig. 1 HOMO and LUMO diagrams of compound BCCB
compound 1 with NBS. Thirdly, compound 2 was subjected to a
McMurry coupling reaction under nitrogen in the presence of
titanium(IV) chloride and Zn powder as a catalytic system to
furnish (E)-1,2-bis(3-bromo-9-ethylcarbazol-6-yl)ethene (3) in
75% yield. The final product BCCB was obtained in 56% yield
by treatment of compound 3 with t-BuLi followed by addition of
dimesitylboron fluoride. Further details are given in the Exper-
imental section.
value, the calculated HOMO level and HOMO–LUMO gap
(3.39 eV) are in good agreement with experimental data.
Photophysical properties
The UV-vis absorption spectra (Fig. 2) and fluorescence spectra
(Fig. 3) of compound BCCB were measured in various solvents
(1.0 × 10−5 M) of different polarity. As shown in Fig. 2, the
absorption spectra of BCCB display almost identical maxima in
the different solvents, indicating only slight solvent-dependence.
This indicates that the structural and electronic characteristics of
the ground and Franck–Condon (FC) excited states15 do not
differ much with a change in solvent polarity. Compound BCCB
exhibits an absorption band at 340–350 nm which is assigned to
the π–π* transition of the carbazole dimer core12 and a low-
energy broad band at 390–420 nm which is assigned to the over-
lapping ICT band from the electron-donor (carbazole dimer) to
the electron-acceptors (diaryl boron groups).
To further elucidate the absorption spectra of compound
BCCB observed experimentally, we computed a singlet–singlet
electronic transition based on the optimized geometry of the
ground state of BCCB, using time-dependent density functional
theory (TD-DFT) method at the B3LYP/6-31G (d, p) level. The
excitation energies, oscillator strengths and main configurations
for the most relevant absorption bands were listed in Table S1,
Theoretical calculations
Theoretical calculations can provide a reasonable qualitative
indication of the excitation and emission properties.13 Herein,
the frontier molecular orbitals of BCCB, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO), were investigated at the DFT/B3LYP/6-31G
(d, p) level. Molecular orbital calculations revealed that the
HOMO of compound BCCB is localized mainly on the carbazole
dimer, whereas the LUMO includes the two boron side chains
and the carbazole dimer (Fig. 1). Thus, the electronic transition
from the ground state to the first excited state mainly involves
the intramolecular charge transfer (ICT) from the carbazole
dimer to the peripheral boron groups.14 The calculated HOMO
and LUMO energy levels are −4.85 eV and −1.46 eV. Although
the calculated LUMO level is higher than the experimental
This journal is © The Royal Society of Chemistry 2012
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Table 1 The fluorescence quantum yields (Φ) and lifetimes (τ, ns) of
compound BCCB in different solvents
Solvents
Hex
THF
DCM
Acetone
DMSO
Quantum yields
Lifetimes (ns)
0.81
4.97
0.73
4.29
0.62
4.18
0.58
4.23
0.54(0.95a)
4.15(3.01a)
a Recorded in DMSO solution with saturated fluoride ions.
Fig. 2 UV-vis absorption spectra of compound BCCB in different
solvents.
Fig. 4 The fluorescence decay profile of compound BCCB in THF.
polar solvents at room temperature. The fluorescence quantum
yield was calculated from eqn (1).18
ꢀ ꢁ
2
Fs Ar nr
Φs ¼ Φr
ð1Þ
Fr As ns
where Φ is the fluorescence quantum yield, F is the integration
of the emission intensities, A is the absorbance at the excitation
wavelength, n is the refractive index of the solution and the sub-
scripts “r” and “s” denote reference and sample, respectively.
The fluorescence quantum yields (Φ) and fluorescence lifetimes
(τ, ns) are collected in Table 1. A typical fluorescence decay
curve of BCCB is shown in Fig. 4. As shown in Table 1, com-
pound BCCB exhibited high fluorescence quantum yields (Φmax
= 0.81), indicating that it could be used as an excellent optoelec-
tronic material in organic light-emitting diode devices.
Fig. 3 Fluorescence spectra of compound BCCB in different solvents.
Excited at 380 nm.
ESI.† Owing to the lowest-energy electronic transition, HOMO
→ LUMO, which mostly consists of the intramolecular charge
transfer, a weak absorption band occurs around 410 nm. Another
absorption band of the carbazole dimer occurs around 340 nm,
mainly consisting of a HOMO → LUMO + 3 vertical transition
(Fig. S1, ESI†).15,16 The calculated results are in good agreement
with the experimental absorption spectra.
In contrast to the slight solvatochromism in the UV-vis
absorption spectra, the differences in the corresponding fluor-
escence spectra are dependent on solvent polarity; compound
BCCB displayed remarkable solvatochromism. As shown in
Fig. 3, with increasing polarity of the solvents, a bathochromic
shift of 90 nm ranging from 405 nm (in hexane) to 495 nm (in
DMSO) was observed. Because polar solvents are propitious to
the ICT process,17 such a distinct solvatochromism indicated that
intramolecular charge transfer from the carbazole dimer to the
dimesitylboron groups takes place during the excitation process.
The fluorescence quantum yields and the fluorescence decay
behaviours of compound BCCB were measured in different
Electrochemical properties
The electrochemical properties of compound BCCB were
studied by cyclic voltammetry measurements (Fig. S4, ESI†).
The measurement was performed with 1.0 × 10−5 M MeCN sol-
ution of BCCB under argon using 1.0 × 10−3 M tetrabutylam-
monium perchlorate in anhydrous MeCN as the supporting
electrolyte. The HOMO energy level can be calculated by using
the empirical equation HOMO = −(Eox + 4.40) eV, where Eox
stands for the onset potentials for oxidation. Therefore, the
HOMO energy level of BCCB was estimated by using the onset
potentials for oxidation (0.93 V) and was found to be −5.33 eV.
From the absorption edges of the UV-vis spectra, the optical
band gap (Eg) was estimated as 3.05 eV. The LUMO energy
level, which was derived from the relationship Eg = HOMO −
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Fig. 5 (a) Fluorescence responses of BCCB upon addition of 20
equiv. of various anions in DMSO solution. (b) Fluorescence responses
of BCCB to F− (20 equiv.) containing 20 equiv. of various other anions.
LUMO, was calculated to be −2.28 eV. The low HOMO energy
level suggests that compound BCCB has high oxidative stability.
Fig. 6 (a) Fluorescence titration spectra of BCCB in DMSO upon
gradual addition of F−. Excited at 380 nm. (b) Fluorescence intensity
ratio changes (I420/I500) of BCCB upon gradual addition of F−. (c) Flu-
orescence intensity changes at 420 nm upon gradual addition of F−.
Sensing properties
BCCB·F22−). The fluorescence quantum yields (Φ) and lifetimes
(τ, ns) of compound BCCB·F2 in DMSO were also listed in
As a general adapted method, excess amounts (ca. 20 equiv.) of
various tetrabutylammonium anion salts were added to 1.0 ×
10−5 M DMSO solutions of BCCB. Compound BCCB shows
weak complexation with other halide anions (Br−, Cl−, I−),
AcO−, ClO4−, NO3−, H2PO4−, but not for F− and CN− (Fig. 5a).
In addition, competition experiments were also performed by
addition of 20 equiv. of F− to the DMSO solutions of compound
BCCB in the presence of 20 equiv. of miscellaneous anion salts
(Fig. 5b). The competitive anions had no obvious interference
with the detection of F−, indicating the system was hardly
affected by these coexistent anions.
2−
Table 1. Upon stepwise addition of F−, a clear isosbestic point at
460 nm could be observed; upon further addition of F−, the iso-
sbestic point shifted from 460 to 485 nm. Such a similar shift of
more than 15 nm has already been reported.20 The two different
isosbestic points of the emission spectra are obviously due to the
formation of the monoanion BCCB·F− and the dianion
BCCB·F22−, respectively. According to the 1 : 2 Benesi–Hilde-
brand equation (Fig. S5, ESI†), the plot of 1/(I − I0) against
1/[F−]2 shows a linear relationship (R2 = 0.9978), indicating that
BCCB indeed associates with F− in a 1 : 2 stoichiometry.21 The
association constant, K, is determined from the slope to be 3.98
× 108 M−2. When examining the emission changes at 410 nm
upon gradual addition of F−, a good linear working range from
0.25–3.25 equiv. was observed (Fig. 6c). The detection limit was
measured to be 3.81 × 10−7 M, which suggests that compound
BCCB is highly sensitive for the monitoring of fluoride ions.
In view of specific interaction between F− and the boron
centre, the response of compound BCCB (1.0 × 10−5 M) to F−
was investigated through absorption spectra (Fig. S2, ESI†) and
fluorescence spectra. As shown in Fig. 6a, with the stepwise
addition of F−, the green ICT emission band centered at 500 nm
underwent gradual “turn-off”, along with the blue emission band
of local excitation19 at 420 nm displaying a rapid “turn-on”, due
to the inhibition of the intramolecular charge transfer from the
carbazole dimer to the boron side chains. The changes in the flu-
orescence spectrum stopped and the ratio of the emission intensi-
ties at 420 nm and 500 nm (I420/I500) became constant when the
amount of F− added reached 4 equiv. (Fig. 6b); notably, the com-
plexation with fluoride ions caused an obvious increase in fluor-
escence efficiency (Φ = 0.54 for BCCB; Φ = 0.95 for
2−
The emission spectra of BCCB·F2 are quite close to those
of (E)-1,2-bis(9-ethyl-3,9′-bi(9H-carbazol)-6-yl)-ethene, which
does not contain the diaryl boron groups.12 It indicated that the
complexation of the boron groups with F− changed three-coordi-
nated boron to four-coordinated boron and interrupted the pπ–π
conjugation, just as the character of the boron centre is changed
from electron-acceptor to electron-donor. In order to gain a
This journal is © The Royal Society of Chemistry 2012
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Fig. 7 HOMO and LUMO diagrams of compound [nBu4N]+2[BCCB·(F)2]2−
.
detailed insight into the sensing properties of BCCB, we con-
ducted theoretical calculations of compound BCCB·F2 using
spectrometer (excited at 380 nm). Excitation and emission slit
widths were both set at 3 nm. Cyclic voltammetry experiments
were performed with a CHI-600C electrochemical analyzer, the
measurements were carried out with a conventional three-elec-
trode configuration consisting of a glassy carbon working elec-
trode, a platinum-disk auxiliary electrode and an Ag/AgCl
reference electrode. The scan speed is 50 mV s−1. The fluor-
escence decay curves were recorded with Time-Correlated Single
Photon Counting (TCSPC) technique using a commercially
available Edinburgh Instruments device. Fluorescence quantum
yields were determined using a standard actinometry method.
Quinine sulphate was used in the actinometer with a known flu-
orescence quantum yield of 0.55 in 0.1 M sulfuric acid, the
samples were excited at 350 nm. All measurements were per-
formed at room temperature. All calculations were carried out
using the Gaussian03 program package.22
2−
DFT/B3LYP/6-31G++ (d, p) method. As shown in Fig. 7, the
electron density of the HOMO and LUMO are both localized
primarily on the carbazole dimer. Compared with compound
BCCB, the frontier molecular orbitals of BCCB·F22− reveals the
inhibition of electron flow from the N centre (electron-donor) to
the B centre (electron-acceptor), causing the disappearance of
the ICT emission band around 500 nm. The result could lead to
the understanding of the changes in the fluorescence spectra.
Conclusions
In summary, a novel A-π-D-π-A compound BCCB was obtained
by the introduction of diaryl boron groups at the 3,3′-positions
of carbazole dimer. BCCB possesses outstanding oxidative stab-
ility and high fluorescence quantum yields, indicating its poten-
tial application as an excellent optoelectronic material in
OLEDs. The theoretical calculations are in good agreement with
the experimental results of the absorption spectra, and can
provide useful guidelines to understand the changes in the fluor-
escence spectra. Moreover, the competition and titration exper-
iments suggest that compound BCCB is a highly sensitive
ratiometric fluorescence sensory material with remarkable colour
changes. Compared with a blank sample, a saturated F− solution
increases the fluorescence efficiency about twofold. Our studies
will improve knowledge to facilitate the ongoing exploration in
optoelectronic applications and the design of novel boron-con-
taining π-conjugated derivatives with outstanding sensing
properties.
Synthesis of BCCB
The compounds 1,23, 224 and 312 were synthesized according to
literature procedures.
1
9-Ethylcarbazole-3-carbaldehyde (1). M.p.: 89.7–90.5 °C. H
NMR (600 MHz, CDCl3) δ (ppm): 10.09 (s, 1H), 8.61 (d, J =
1.5 Hz, 1H), 8.16 (d, J = 7.8 Hz, 1H), 8.01 (dd, J = 8.5, 1.6 Hz,
1H), 7.54 (ddd, J = 8.3, 7.2, 1.2 Hz, 1H), 7.47 (t, J = 7.9 Hz,
2H), 7.35–7.31 (m, 1H), 4.41 (q, J = 7.3 Hz, 2H), 1.47 (t, J =
7.3 Hz, 3H).
6-Bromo-9-ethylcarbazole-3-carbaldehyde (2). 9-Ethylcarba-
zole-3-carbaldehyde (1) (10 g, 44.8 mmol) was dissolved in
DMF (150 mL) and the solution was cooled in an ice bath. A
solution of NBS (8.77 g, 49.3 mmol) in DMF (50 mL) was
added dropwise. The reaction mixture was allowed to stir for
12 h at room temperature. Then, the mixture was poured into ice
water, and the white precipitate was collected by filtration to
afford 6-bromo-9-ethylcarbazole-3-carbaldehyde (2). Yield:
82%. M.p.: 131.4–132.2 °C. 1H NMR (600 MHz, CDCl3) δ
(ppm): 10.09 (s, 1H), 8.53 (d, J = 1.2 Hz, 1H), 8.25 (d, J = 1.9
Hz, 1H), 8.03 (dd, J = 8.5, 1.4 Hz, 1H), 7.61 (dd, J = 8.6, 1.9
Hz, 1H), 7.47 (d, J = 8.5 Hz, 1H), 7.33 (d, J = 8.6 Hz, 1H), 4.38
(q, J = 7.3 Hz, 2H), 1.46 (t, J = 7.3 Hz, 3H).
Experimental section
Materials and general methods
All reactions were carried out under a nitrogen atmosphere using
Schlenk techniques. Solvents were freshly distilled according to
standard procedures. All reagents were used as received from
commercial sources without further purification. Melting points
were determined on a WRS-1B melting point detector. All NMR
spectra were recorded on a Bruker Avance 600 MHz NMR spec-
trometer with CDCl3 as solvent. Absorption spectra were taken
on a Shimadzu UV-2450 spectrophotometer. Fluorescence
spectra were obtained on a Shimadzu RF-5301PC fluorescence
(E)-1,2-Bis(3-bromo-9-ethylcarbazol-6-yl)ethene(3). 6-Bromo-
9-ethylcarbazole-3-carbaldehyde (2) (10 g, 33.1 mmol) and Zn
3856 | Org. Biomol. Chem., 2012, 10, 3852–3858
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powder (8.656 g, 132.4 mmol) was suspended in anhydrous
THF (200 mL) under N2. Titanium(IV) chloride (7.28 mL,
66.2 mmol) was added dropwise to the reaction mixture at
−78 °C. Then, the cooling bath was removed, and the mixture
was refluxed for 10 h. After cooling to room temperature, ice
water (200 mL) was added and the reaction mixture was stirred
for a further 0.5 h. The mixture was extracted with CH2Cl2 and
the combined organic extracts were dried over anhydrous
MgSO4, filtered, and concentrated in vacuo. The residue was
purified by flash chromatography on silica gel (CH2Cl2–hexane)
to afford (E)-1,2-bis(3-bromo-9-ethylcarbazol-6-yl)ethene (3).
Yield: 75%. M.p.: 251.2–252.5 °C. 1H NMR (600 MHz, CDCl3)
δ (ppm): 8.24 (d, J = 1.9 Hz, 2H), 8.19 (s, 2H), 7.73 (dd, J =
8.5, 1.1 Hz, 2H), 7.54 (dd, J = 8.6, 1.8 Hz, 2H), 7.39 (d, J = 8.5
Hz, 2H), 7.32 (s, 2H), 7.27 (d, J = 8.6 Hz, 2H), 4.34 (q, J = 7.2
Hz, 4H), 1.44 (t, J = 7.3 Hz, 6H).
(E)-1,2-Bis(3-dimesitylboryl-9-ethylcarbazol-6-yl)ethene
(BCCB). Under a dry atmosphere of nitrogen, t-BuLi (1.6 M
solution) in hexane (2.4 mL, 3.84 mmol) was injected into a sol-
ution of 3 (1 g, 1.75 mmol) in anhydrous THF (40 mL) at
−78 °C, followed by warming to room temperature. After stir-
ring for 6 h, the reaction mixture was cooled to −78 °C again,
and dimesitylboron fluoride (1.08 g, 4 mmol) in THF (20 mL)
was added dropwise over 30 min. The temperature was allowed
to rise to room temperature and the mixture was stirred for a
further 12 h. After evaporation of the solvent, the residue was
dissolved in dichloromethane and the precipitate was filtered off.
The filtrate was concentrated under reduced pressure, and the
residue was purified by column chromatography on silica gel
(CH2Cl2–hexane) to give BCCB. Yield: 56%. M.p.:
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135.4–137.1 °C. H NMR (600 MHz, CDCl3) δ (ppm): 8.37 (s,
2H), 8.27 (s, 2H), 8.15 (d, J = 8.6 Hz, 2H), 7.67 (d, J = 8.2 Hz,
2H), 7.43–7.37 (m, 4H), 7.33 (s, 2H), 6.71 (s, 8H), 4.38 (q, J =
7.2 Hz, 4H), 2.25 (s, 12H), 2.00 (s, 24H), 1.47 (t, J = 7.1 Hz,
6H). 13C NMR (600 MHz, CDCl3) δ (ppm): 143.3, 141.0,
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119.0, 118.4, 108.9, 108.7, 108.6, 108.0, 37.7, 23.7, 22.3, 14.0.
MS (m/z): 910.5611 (M+). Anal. Calcd for C66H68B2N2: C,
87.03%; H, 7.52%; N, 3.08%. Found: C, 86.48%; H, 7.22%; N,
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Acknowledgements
This work was supported by the National Science Foundation of
China (No. 21175086) and Fund of Key Laboratory of Optoelec-
tronic Materials Chemistry and Physics, Chinese Academy of
Sciences (No 2011KL004). All the calculations have been per-
formed using the advanced computing facilities of the supercom-
puting centre of computer network information centre of
Chinese Academy of Sciences. All the authors express their deep
thanks.
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