Highly sensitive detection of low-level water content in organic solvents and cyanide in aqueous media using novel solvatochromic AIEE fluorophores

Article abstract of DOI:10.1039/c4ra15199b

A great deal of effort has been devoted to develop easy-to-use fluorescent probes for detecting analytes due to their advantages in the field of chemo- and bio-sensing. Herein, two novel 2,2′-biindenyl-based derivatives BDM and BDBM containing dicyanoviny

Full text of DOI:10.1039/c4ra15199b

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Highly sensitive detection of low-level water
content in organic solvents and cyanide in aqueous
media using novel solvatochromic AIEE
fluorophores†
Cite this: RSC Adv., 2015, 5, 12191
a
a
b
b
a
*
*
Wei Chen, Zhiyun Zhang, Xin Li, Hans A˚gren and Jianhua Su
A great deal of effort has been devoted to develop easy-to-use fluorescent probes for detecting analytes
due to their advantages in the field of chemo- and bio-sensing. Herein, two novel 2,20-biindenyl-based
derivatives BDM and BDBM containing dicyanovinyl groups have been designed and synthesized, and are
shown to possess the remarkable dual properties of solvatochromism and aggregation-induced emission
enhancement (AIEE). Importantly, both of them are found to serve as fluorescent indicators for the
qualitative and quantitative detection of low-level water in organic solvents. Meanwhile, both BDM and
BDBM emit yellowish orange and orange fluorescence, respectively, in their aggregated states.
Furthermore, with dicyanovinyl groups as the recognition sites, both compounds can act as
colourimetric and fluorescent sensors for highly sensitive and selective detection of cyanide in aqueous
media, and the apparent response signals can be observed by the naked eye even in the presence of
various interference anions, promising practical applications for detecting cyanide in drinking water.
Besides, optical spectroscopic techniques, NMR titration measurements, and density functional theory
calculations are conducted to rationalize the sensing mechanisms of the two probes.
Received 25th November 2014
Accepted 13th January 2015
DOI: 10.1039/c4ra15199b
www.rsc.org/advances
biindenyl-based AIE uorophores with tunable solid-state
emission, which cover the whole visible region.12 That work
Introduction
motivated us to pursue further design of functional materials
utilizing biindenyl-based AIE molecules. In this work it is our
intention to utilize the biindenyl functional group as a building
block for smart materials with applications in chemo- and
biosensing. We aim to modify the biindenyl-based chromo-
phores to form stimuli-responsive compounds with
pronounced solvatochromic effect, which might offer novel
chemo- or biosensors for detecting metal cations, toxic anions,
biomolecules, explosives, etc.
Qualitative and quantitative detection of low-level water as
impurities in organic solvents is also of great signicance in
several elds of chemistry and industry processes such as
pharmaceutical manufacturing, food processing, paper
production, biomedical and environmental monitoring.13
Although there exist extensive traditional methods for quanti-
cation of humidity in gas or liquid phases, which feature high
sensitivity and which might be applicable to a large variety of
samples, these methods also have some drawbacks: for
instance, time-consuming, costly and involving sophisticated
instruments. Recently, optical sensors for water sensing have
drawn considerable attention due to the exibility in readout,
the possibility of remote and in situ monitoring as well as the
ease and low-cost of fabrication.14,15 In particular, uorescence
methods, such as lifetime, intensity and wavelength ratiometric
Development of luminogens whose aggregates emit more effi-
ciently than in solution has triggered much interest in recent
years. Tang's group in 2001 rst reported this unusual
phenomenon, which is exactly opposite to the aggregation-
caused quenching (ACQ) effect, and coined it “aggregation-
induced emission” (AIE).1 In 2002, the group of Park also
observed the phenomenon of aggregation-induced emission
enhancement (AIEE).2 Since then, luminogens with AIE(E)
properties have been intensively studied, including varieties of
proposed mechanisms and potential applications. To date, a
large number of molecules with AIE(E) properties have been
developed and extensively used in organic light-emitting diodes
(OLEDs),3,4 supramolecular polymers,5 bio/chemosensors,6,7
drug delivery,8,9 two-photon absorption (2PA) materials,10,11 and
more. Previously, our group reported a series of novel 2,20-
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
Universtity of Science & Technology, Shanghai 200237, P. R. China. E-mail: bbsjh@
ecust.edu.cn; Fax: +86-21-64252288
bDivision of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal
Institute of Technology, SE-10691 Stockholm, Sweden. E-mail: lixin@theochem.kth.se
† Electronic supplementary information (ESI) available: Photophysical data,
1
13
crystal data, theoretical calculated data, H and C NMR spectra and ESI-TOF
mass spectra of BDM and BDBM. CCDC 1016979. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4ra15199b
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sensing approaches, have been widely investigated and applied
for the detection of water content in organic solvents.16–22
Cyanide has been continuously of concern all over the world
due to its high toxicity and widespread use in industrial
processes such as acrylic ber manufacturing, metallurgy,
herbicide production, and silver or gold extraction.23,24 Uptake
of toxic cyanide could occur through absorption by lungs,
exposure to skin, and also from contaminated food and
polluted drinking water.23–25 Trace amount of cyanide can
directly lead to the death of human beings in several minutes
because it strongly binds to the active site of cytochrome-c,
disrupting the mitochondrial electron-transport chain and
inhibiting cellular respiration in mammalian cells.26 The World
Health Organization (WHO) has set the maximum permissive
level of cyanide in drinking water as 1.9 mmol Lꢀ1 to prevent
poisoning.27 Therefore, it is of great signicance to nd conve-
nient and efficient detection methods at the environmental and
Scheme 1 Chemical structures and synthetic routes of compounds
biological levels. Among various approaches for the detection of BDM and BDBM.
cyanide such as electrochemical analysis, ion chromatography
and the involvement of intelligent instruments,28 optical
sensors show considerable advantage over others for their
simplicity, convenience and low-cost of implementation. The
sensing process is oen accompanied by changes in absorption
or uorescence spectra that can be precisely monitored and in
some cases detectable by naked eye. To minimize the interfer-
ence of other anions, cyanide chemodosimeters take advantage
of the unique nucleophilicity of cyanide to realize recognition
through the bond-formation reaction between the probing
molecule and cyanide anions.29 Nevertheless, since the presence
of water will weaken the nucleophilic addition reaction of
cyanide,30,31 most of the chemodosimeters need to be operated
either in pure organic solvents or a mixture of organic and
dioxane as well as the detection of cyanide in water (containing
1% DMSO) under the assistance of cetyltrimethylammonium
bromide (CTAB).49 These facts motivate research on multi-
functional biindenyl-based uorophores acting as indicators for
the detection of water content and cyanide anions. Further-
more, our sensing systems effectively avoid the negative effect of
water on specic cyanide reactions due to the presence of CTAB,
promising their practical applications for detecting cyanide in
drinking water.
Results and discussion
aqueous solvent, which largely restricts their practical applica- Synthesis
tions.32,33 To date, reports relating sensors that recognized
The synthetic routes of compounds BDM and BDBM are shown
cyanide in complete water have been scarce.34–39 We believe that
there is large room for improvement in terms of sensitivity and
applicability, and that it thereby is important to develop effec-
tive chemosensors for detection of cyanide in aqueous media.
Herein, two novel probes BDM and BDBM have been
designed and synthesized, as shown in Scheme 1. In the
molecular design, the dicyanovinyl group was introduced
because it can act not only as a strong withdrawing group to
induce an intramolecular charge transfer (ICT) transition but
also as a cyanide reactive unit.40–47 It is thus expected that these
two compounds possess intense ICT effects, which bestow the
derivatives with pronounced solvatochromism.48 In addition,
in Scheme 1, and the key compounds 2 and 3 were synthesized
according to the literature.12 Mono- or dibromination of 1 gave
the corresponding benzyl bromide, which was heated in dime-
thylsulfoxide (DMSO) to afford 2 or 3. Aer subsequent Knoe-
venagel reaction with excess malononitrile, the desired
products were obtained. Both target compounds were carefully
characterized by proton and carbon nuclear magnetic reso-
nance spectroscopy (1H NMR, 13C NMR) and high resolution
mass spectroscopy (HRMS) (see the ESI† for the details).
Solvatochromic effect
the nucleophilic attack of cyanide at the a-position of the The photophysics of the D–p–A conjugates in solution strongly
dicyanovinyl group would improve their water solubility and depends on the solvent polarity.50 Due to the presence of
lead to signicant changes in uorescence and absorption, electron-withdrawing dicyanovinyl groups, BDM and BDBM
enabling them as practical reaction-based cyanide sensors in possess typical D–p–A structures. Thus, we rstly studied their
aqueous media.33 Indeed, the two indicators inherit the AIE spectral properties in solution, and the results are summarized
characterization and nonplanar distortion from the mother in Table S1.† As shown in Fig. S1a,† the absorption spectra
compound
1,1,10,10-tetramethyl-3,30-diphenyl-2,20-biindanyl, of BDM display minor changes in different solvents. In
and exhibit highly efficient red emission in the solid state with contrast, as the solvent polarity increased (from n-hexane to
quantum yields of 24% and 22%, respectively. More impor- acetonitrile), the emission bands of BDM gradually red-shied
tantly, both compounds can serve as excellent uorescent from 537 nm to 637 nm because of the dipole–dipole interac-
indicators for the detection of low-level water content in THF or tions between the solute and solvents,51 exhibiting a remarkable
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bathochromic effect (Fig. 1a and Table S1†). Meanwhile, a direct transition from the ground state.54 The excited states are
notable reduction in the uorescence quantum yield (FF) was therefore better stabilized by the polar solvents compared with
observed. The maximum of the emission bands shied signif- their respective ground states, explaining the observed
icantly to lower energy region, accompanied by a broadening of substantial solvatochromism of both compounds. This provides
the emission bands in polar solvents, which indicates an ICT a good evidence for the two compounds to act as a suitable
character for the excited state.52 Results shown in the absorp- solvent polarity indicators.
tion and uorescence spectra imply that the combination of the
To elucidate the inuence of the geometric and electronic
electron-donating biindene and the electron-withdrawing structure of BDM and BDBM on solvatochromism, we con-
dicyanovinyl group offers ICT process in the molecule and ducted theoretical calculations. The electronic structures of
that the absorption bands at around 400 nm arise from ICT BDM and BDBM exhibit differences owing to the difference in
transition.
their chemical structures. As shown in Fig. S3,† the energy levels
When the second dicyanovinyl groups was introduced into of the HOMO and LUMO of BDM are higher than those of
biindene to give BDBM, the maximum of the absorption band BDBM, and BDM has a narrower HOMO–LUMO gap. Interest-
was blue-shied from 407 nm to 400 nm in cyclohexane ingly, the LUMO and LUMO + 1 of BDBM reside in similar
compared with BDM (Table S1†). The existence of two dicya- energy levels, while in BDM the energy levels of LUMO and
novinyl groups may be responsible for the hypochromatic shi LUMO + 1 differ from more than 1 eV. Contour plots of the
because dicyanovinyl group as a strong electron-withdrawing frontier molecular orbitals (Fig. 2) reveal that the HOMO is
group weakens the electron-donating ability of biindene. With located on the central indene rings in both BDM and BDBM;
further increase of solvent polarity, the absorption peaks were however, the LUMO and LUMO + 1 of these two compounds are
hardly changed, while the emission bands showed gradual red- quite different. In BDBM, both LUMO and LUMO + 1 are located
shiing, similar to that of BDM (Fig. S1b† and 1b). Under UV on the peripheral phenyl rings with dicyanovinyl substitutions,
irradiation, the emission can be nely tuned from green to red while in BDM the LUMO exclusively reside on the only
by changing the solvent from n-hexane to acetonitrile (Fig. 1d). dicyanovinyl-substituted phenyl branch. The LUMO + 1 of BDM
To evaluate the solvatochromism of the two compounds, the is located on the central indene rings instead of peripheral
relationship between the solvent orientation polarizability (Df) phenyl rings. This leads to the very different characteristics in
and the Stokes shi (Dn) was studied using the Lippert–Mataga the excited states of BDM and BDBM.
equation.53 From the Dn–Df plots (Fig. S2a†) it is found that the
Time-dependent density functional (TD-DFT) calculations
slopes of the tting lines for BDM and BDBM are as large as provide further insight into the excited states of the two
12 021.6 and 8613.3, respectively. In comparison with 9347.7 for compounds. As shown in Table S2,† the lowest excited states
a tetraphenylethene derivative (TPEBM),26 the slope of the (S1) of both BDM and BDBM correspond to HOMO / LUMO
tting line for BDM is much larger, implying BDM possesses transitions. Owing to the different composition of the frontier
strong solvatochromism. The slope of the tting line for BDM is molecular orbitals, the intramolecular charge transfer from the
much larger than BDBM, suggesting that the ICT effect of the central indene rings to the dicyanovinyl-substituted phenyl
former is more pronounced than the latter. In both compounds, branches is estimated as 0.649e for BDM and 0.613e for BDBM,
the excited state has a larger dipole moment than the ground respectively. Meanwhile, the estimated differences between me
state, owing to the substantial charge redistribution during the and mg are 13.2 Debye for BDM and 10.5 Debye for BDBM (Table
ICT-type excitation, which originates from the relaxation of the S3†), which are sufficiently large to cause signicant sol-
initially formed Frank–Condon excited state, instead of the vatochromism according to previous studies.55 The calculated
charge transfer and difference between the excited-state and
ground-state dipole moments suggest a stronger ICT effect for
BDM than that for BDBM, in accordance with the slopes of the
Fig. 1 Normalized fluorescence spectra of (a) BDM and (b) BDBM in
different solvents. Photographs of (c) BDM and (d) BDBM under UV
illumination in different solvents. Hex, hexane; Tol, toluene; Dio,
dioxane; THF, tetrahydrofuran; DCM, dichloromethane; BuOH, n-
butanol; MeCN, acetonitrile. Concentration: 10 mmol Lꢀ1; excitation
wavelength: 400 nm.
Fig. 2 Frontier molecular orbitals of (a) BDM and (b) BDBM.
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tting lines in the Dn–Df plots (Fig. S2a†). TD-DFT calculations
suggest that the oscillator strengths of the HOMO / LUMO
and the transition in BDM is twice as large as that of BDBM
(Table S2†); however, it is noteworthy that the second lowest
excited state (S2) of BDBM is also of ICT character and exhibits a
slightly larger excitation energy than the S1 state. The oscillator
strength of the S0 / S2 excitation for BDBM is ve-fold larger
than that of S0 / S1 excitation for BDBM and two-fold larger
than that of S0 / S1 excitation for BDM, explaining the
experimentally observed higher extinction coefficient of BDBM
at around 400 nm (Fig. S1†). TD-DFT calculations also predict
that the S3 and S4 states of BDBM contribute to the absorption
band at around 320 nm, while for BDM the S2 and S3 states
make non-negligible contribution. The calculated oscillator
strengths of these excited states agree with the higher extinction
coefficient of BDBM in the ultra-violate region (Fig. S1†).
Therefore, the absorption bands of both compounds at the
longer wavelengths (ꢁ400 nm) originate from ICT-type excita-
tions, e.g. HOMO / LUMO transition of BDM and HOMO /
LUMO and HOMO / LUMO + 1 transition of BDBM, while the
absorption bands at the shorter wavelengths (ꢁ320 nm) corre-
spond to the local excitations of the biindene chromophore. It is
also noteworthy that the second lowest excited state (S2) of
BDBM, which corresponds to HOMO / LUMO + 1 transition, is
also of ICT character and exhibits a slightly larger excitation
energy compared with the S1 state. Differently, in BDM the
LUMO + 1 orbital is located on the central indene rings and the
S2 state corresponding to HOMO / LUMO + 1 transition has a
much larger excitation energy in the ultraviolet region.
Fig. 3 Fluorescence spectra of (a) BDM and (b) BDBM in THF solution
in the presence of increasing amount of water. The emission changes
of (c) BDM and (d) BDBM in THF solution containing different amounts
of water. The emission changes of (e) BDM and (f) BDBM in dioxane
solution containing different amounts of water. Concentration:
10 mmol Lꢀ1; excitation wavelength: 400 nm.
comparison with reported solvatochromic dyes for the detection
of water in organic solvents,26,56 the detection limit of BDM was
moderate, demonstrating that BDM is sensitive to low-level
water content in THF. Compared with BDM, the uorescent
intensity of BDBM as a function of water was a bit less sensitive,
but it could also be used as an indicator of water (Fig. 3b).
Similar to THF, the emission spectra of BDM and BDBM in
dioxane solution also underwent signicant changes from pure
dioxane to dioxane containing 10% (v/v) water. In comparison
with THF, the emission maxima of both compounds in dioxane
solution were more sensitive to water, which displayed relatively
large red-shis (BDM: 18 nm; BDBM: 33 nm) (Fig. S6a and b†
and 3e and f). As shown in Fig. S5d,† the uorescence peak of
BDM also showed a good linear relationship below 1% (v/v), and
the detection limit of BDM for water was 0.019% (212 ppm) in
dioxane. Therefore, the distinct uorescence responses of BDM
and BDBM caused by water revealed that they can serve as
suitable water indicators to detect low-level water content in
THF or dioxane qualitatively. Particularly for BDM, it can be also
utilized to quantitatively detect low-level water content in
organic solvents.
Detection of low-level water
Clearly, the uorescence emissions of BDM and BDBM are
highly sensitive to solvent polarity according to the above
discussions. Therefore, we further investigated their emission
behaviour in non-hydrogen-bond donating water-miscible
solvents, i.e., THF and dioxane. As shown in Fig. S4,† the
absorption spectra of BDM and BDBM in THF solution under-
went slight changes from pure THF to THF containing 10% (v/v)
water, indicating that neither solvent polarity nor hydrogen
bonding has a signicant inuence on the absorption spectra.
In contrast, upon further addition of water in THF solution, the
emission band of BDM was effectively quenched and that of
BDBM was gradually diminished along with bathochromic shi
from 597 nm to 617 nm (Fig. 3a–d). These phenomena can be
attributed to the cooperative effect of the solvent polarity and
the ICT.56,57 Fig. S5a† shows in details how the measured uo-
AIEE behaviours and efficient solid emitter
rescent intensity changed as a function of water content in the To investigate the AIEE properties of BDM and BDBM, we
THF solution of BDM. As can be seen from the graph, the employed anhydrous DMSO as the good solvent and water as
uorescent intensity of BDM decreased dramatically when the the poor solvent. The absorption and emission spectra of both
water content was below 4.00% (v/v): the reduction in relative compounds in DMSO and DMSO/water mixture solutions were
uorescence intensity reached nearly 57%, whereas such effect recorded (Fig. 4 and S7†). Clearly, the uorescent intensity of
became moderate when the water content was higher than BDBM was rather weak in pure DMSO solution (Fig. 4b and d).
4.00% (v/v). More importantly, the uorescent intensity of BDM However, the uorescent intensity increased dramatically when
as a function of water content showed a good linear relationship the water content was above 40%. As the water content reached
below 1% (v/v), and the detection limit of BDM for water was 70%, the photoluminescence intensity was boosted to the
determined as 0.010% (113 ppm) in THF (Fig. S5b†). In maximum. With the increase of the water content from 70% to
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DMSO solution implied the existence of large-amplitude relax-
ation in the excited state, such as intramolecular charge transfer
(ICT).61 Nevertheless, aggregation forced the molecules to reside
in an apolar environment which is unfavourable for ICT
process, resulting in the blue shi of the emission maximum
and was also helpful for the light emission. In contrast to the
uorescence, the absorption spectra of BDBM was red-shied
and broadened upon addition of water, due to the nano-
aggregated structure of the molecules (Fig. S7b†). Meanwhile,
the level-off tails in the visible region of the absorption spectra
clearly indicated the formation of aggregates. The scanning
electron microscopy (SEM) images also displayed the formation
of well-dened nanoparticles in aqueous mixtures (Fig. 4f).
A similar phenomenon was observed for BDM. The pure
DMSO solution of BDM was almost nonemissive and the uo-
rescence spectra was almost parallel to the abscissa. However,
BDM exhibited a strong luminescence with a 56-fold increase in
the I/I0 ratio when the water concentration reached 90% (Fig. 4a
and c), demonstrating that BDM possessed the typical charac-
ters of AIEE. Besides, the emission maximum of the aggregates
of BDM blue-shied from 628 nm to 600 nm, and the emission
colour changed from red to orange.
The AIEE feature was also conrmed by the intense uo-
rescent images of the solid (Fig. 5). The solid was recrystallized
from acetone and the emission colours of BDM and BDBM were
bright yellowish orange and orange, respectively. To evaluate
Fig. 4 Fluorescence spectra of BDM (a) and BDBM (b) in DMSO–water
mixtures with different water contents. Plots of I/I0 values versus water
content of the solvent mixture for BDM (c) and BDBM (d). Inset: the
corresponding fluorescence images under UV illumination. SEM
images of BDBM nanoaggregates obtained from (e) 70% and (f) 90% the AIEE effect, the emission spectra and uorescence quantum
DMSO–H2O mixtures, respectively. Concentration: 10 mmol Lꢀ1
excitation wavelength: 400 nm.
;
yield of the two compounds were measured. Both BDM and
BDBM exhibited intense emission peaking at 626 nm and 635
nm, respectively. In contrast to the low quantum yields of <1%
and 1% in DMSO solution (Table S1†), the solid state uores-
cent quantum yields of BDM and BDBM were determined as
24% and 22%, respectively. Such high FF values endorse their
great prospects in OLED applications.62
The single crystal of BDBM was obtained for further veri-
cation of the mechanism of the AIEE system. As shown in
Fig. 6a, the interplane angle (q1) between the two indene
aromatic rings was 48.2ꢂ, which indicated that the core
90%, the emission spectra of BDBM showed a decrease in the
intensity. Eventually, the emission intensity of BDBM was
approximately 8 folds higher than its molecularly dispersed
species in DMSO. The apparent emission enhancement of
BDBM was thus induced by aggregation, verifying its AIEE
nature owing to the restricted intramolecular motions (RIM)
mechanism:58,59 large amount of water resulted in formation of
aggregated nanoparticles, which subsequently impeded the
intramolecular rotations of the aromatic rotors of BDBM and
endowed the aggregates with intense emission. The nano-
particles from the aqueous mixtures were obtained by solvent
evaporation. As shown in Fig. 4e and f, the nanoparticles
formed in the 70% aqueous mixture were larger than those
obtained from the 90% aqueous mixture. The decrease of the
uorescence intensity mentioned above was related to this
result, which was probably ascribed to the change in the
packing pattern of the dye molecules in the aggregates.60 In the
mixture with 70% water content, the dye molecules are expected
to steadily assemble in an ordered fashion to form more
emissive crystalline aggregates, while in the mixture with 90%
water content, the dye molecules may quickly agglomerate in a
random way to form less emissive amorphous particles.
Moreover, compared to the pure DMSO solution of BDBM,
the emission maximum of the aggregates blue-shied from 625
nm to 600 nm, and the emission colour changed from red to
orange (Fig. 4d insert). The weak and redder emission in pure
Fig. 5 Normalized fluorescence spectra of the two solids. Inset: the
corresponding fluorescence images of BDM and BDBM under illumi-
nation. Excitation wavelength: 400 nm.
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Detection of cyanide
Recently, the dicyanovinyl group as the reactive unit has been
widely employed to construct reaction-based cyanide
sensors.63,64 Thus BDM and BDBM may have great potential for
detection of cyanide in aqueous media based on their AIEE
properties. The sensor systems were prepared by mixing the
AIEE dyes in DMSO (50 mL, 2.0 mmol Lꢀ1) and CTAB in water
(5 mL, 2.0 mmol Lꢀ1).49 Both two compounds were readily
dispersed in water and the mixture solution emitted intense
uorescence. The quenching of uorescence was attributed to
two main aspects: (i) the nucleophilic addition of cyanide to the
dicyanovinyl group of BDM and BDBM led to the formation of
water-soluble compounds with negative charges, resulting in
disassembly of the nanoparticles and decrease of the emis-
sion;26 (ii) the nucleophilic attack of cyanide to the dicyanovinyl
group of the two compounds would interrupt the p-conjugation
and disrupt the ICT transition, leading to signicant decrease in
uorescence.46 It is certain that the ICT transition reduction is
the reason for uorescence quenching, because the absorbance
at excited wavelength markedly decreased aer CNꢀ addition.
Compound BDBM was utilized to investigate in detail the
interaction of both compounds with cyanide through absorp-
tion and uorescence spectra titration experiments. Probe
BDBM was light yellow and exhibited main absorption bands
centred at 333 and 408 nm, respectively (Fig. 7a). Upon addition
of 0–9.0 equiv. of cyanide, both bands gradually decreased and
Fig. 6 (a) Single-crystal structure of compound BDBM. (b) Optimized
geometries. From left to right: BDBM-s (stacked), BDBM-t (twisted),
BDM-s (stacked), BDM-t (twisted).
structure of 2,20-biindene was nonplanar. Furthermore, the
indene planes and the aryl substituents were not coplanar
either, where the dihedral angles (q2) were 45.0ꢂ. Owing to the
twisted structure, the conformation of BDBM strongly deviated
from a planar conformation and prevented the rings from
undergoing p–p stacking interactions, thus inducing intense
emission in the condensed state. Another notable feature of
BDBM was that the terminal dicyanovinyl groups were almost
coplanar with the attached benzene ring (dihedral angle q3 ¼
4.22ꢂ), suggesting the effective conjugation between the dicya-
novinyl groups and the 2,20-biindene main chain framework.
To gain insight into the structure–property relationship of
compounds BDM and BDBM, we employed density functional
(DFT) calculations to optimize their geometries, as shown in
Fig. 6b. Each compound has two possible conformations,
namely the stacked and the twisted conformation of the two
phenyl rings. Our calculations suggest that the stacked
conformation is more stable than the twisted conformation by
around 7 kJ molꢀ1 (Table S4†), in consistency with experimen-
tally observed crystal structure. In fact, the dispersion interac-
tion plays an important role in stabilizing the p–p stacked
conformations, which offers
a
stabilization energy of
29 kJ molꢀ1 for BDBM and 19 kJ molꢀ1 for BDM, respectively.
Fig. 7 (a) Absorption spectral changes of BDBM (20 mmol Lꢀ1) upon
We also measured the dihedral angles between different panels addition of 0–9.0 equiv. of cyanide in 2 mmol Lꢀ1 CTAB micellar
as shown in Fig. S8.† The dihedral angles between the two ve-
membered rings are around 44ꢂ in both compounds, while the
dihedral angles between the central unit and peripheral phenyl
rings range from 46ꢂ to 49ꢂ, in which q3 of BDM is the smallest,
solution. Insert: the images of BDBM with different amount of cyanide
(left: none; right: 9.0 equiv. cyanide). (b) Benesi–Hildebrand plot of
absorbance at 333 nm. (c) Fluorescence spectra of BDBM
(20 mmol Lꢀ1) upon addition of 0–9.0 equiv. of cyanide in 2 mmol Lꢀ1
CTAB micellar solution. Insert: the images of fluorescence with
most likely owing to the relatively weak steric effect of the different amount of cyanide (left: none; right: 9.0 equiv. of cyanide). (d)
Plot of fluorescence intensity versus cyanide concentration, lex ¼ 400
nm, lem ¼ 601 nm, R2 ¼ 0.9987, k ¼ 2.04 mMꢀ1, s ¼ 0.20. The standard
deviation (s) was obtained by fluorescence responses (10-times of
consecutive scanning on the Varian Cary Eclipse Fluorescence Spec-
trophotometer). The detection limit was calculated as 0.29 mM by the
formula (3s/|k|).
methyl group. The calculated dihedral angles for BDBM are
close to the corresponding dihedral angles in single-crystal
structure (43.6ꢂ/48.2ꢂ, 48.5ꢂ/45.0ꢂ). These calculations demon-
strate the nonplanar structures of two compounds, in agree-
ment with their AIEE properties.
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the one at 408 nm eventually vanished at the saturation point eventually disappeared upon addition of 2.0 equiv. of cyanide.
(9.0 equiv.). This change suggests that the large p-conjugation Concurrently, two new signals appeared at 4.52, 4.48 ppm (Hb,
was interrupted, accompanied by the formation of [CN–BDBM– Hb0), respectively. The upeld shi of aromatic proton signal
CN]2ꢀ due to the nucleophilic attack of cyanide toward the was induced by the broken conjugation between the dicyano-
dicyanovinyl groups of BDBM. The Benesi–Hildebrand plot of vinyl groups and the 2,20-biindene framework, which was
absorbance at 333 nm upon addition of cyanide is shown in caused by the nucleophilic addition of cyanide to the dicyano-
Fig. 7b. The linear correlation between 1/DA333 and 1/[CNꢀ] vinyl groups. Furthermore, no signal corresponding to the
implied the 1 : 1 stoichiometric reaction between BDBM and malononitrile moiety (Ha) was observed in the 1H NMR spectra,
cyanide. Moreover, the solvent faded and became almost col- implying the formation of a stabilized anionic species in the
ourless, which could be clearly observed by naked eye (Fig. 7a medium.41 The formed anionic adduct BDBM–CN was dissolved
inset).
in water, destructing the aggregated nanoparticles and
As expected, the uorescence emission of BDBM was quenching the emission. The 1H NMR titration of BDM by CNꢀ
quenched by the nucleophilic addition reaction between presented a similar pattern to that of BDBM (Fig. S10†). Upon
cyanide and dicyanovinyl group. Upon excitation at 400 nm, a the addition of 1.0 equiv. of cyanide anion, the vinylic proton
broadened emission band at around 601 nm was detected, (Ha) at 8.46 ppm gradually vanished. Meanwhile, a new signal
which gradually decreased upon addition of cyanide and appeared at 4.51 ppm (Hb), indicating that the cyanide reacted
became saturated with 9.0 equiv. of CNꢀ (Fig. 7c). Additionally, with the dicyanovinyl groups. It was further conrmed by MS
a linear relationship between the uorescent intensity at 601 (ESI) experiments (Fig. S11 and S12†).
nm and the concentration of CNꢀ in the low concentration
In addition, the rapid detection of cyanide in aqueous media
region was obtained (Fig. 7d). The detection limit was as low as is one of the most important factors for practical applications.
0.29 mM, which amounts to one sixth of the maximum In order to estimate the response rates of BDM and BDBM to
contaminant level (MCL) for CNꢀ in drinking water (1.9 mM) set cyanide, time-dependent uorescence intensity changes with
by the World Health Organization (WHO). In comparison with the addition of different concentrations of CNꢀ were measured
reported probes,26,35,37 the detection limits of BDBM is one of the (Fig. 9). It is noteworthy that the uorescence of BDM and
lowest uorescence detection limits to cyanide in water. The BDBM were quenched almost completely in less than 100
prompt response of BDBM to cyanide in CTAB micelles was seconds upon addition of 7.0 equiv. and 9.0 equiv. of CNꢀ,
attributed to the electrostatic interactions between anionic respectively. The fast response was attributed to two main
cyanide and cationic CTAB, which would attract the cyanide to aspects: (i) the electrostatic interaction between anionic cyanide
the micellar surface and thus facilitate the attack of cyanide to and cationic CTAB accelerated the rapid attack of cyanide to the
BDBM.65,66 Moreover, CTAB provided a hydrophobic environ- dicyanovinyl groups of two compounds; and (ii) the formation
ment which could effectively eliminate the negative effect of of the uniform and smaller nanoparticles of BDM and BDBM
water on specic cyanide reaction. These results indicate that
BDBM possesses a high sensitivity toward cyanide in nearly
pure water surroundings, endorsing its practical application for
detecting cyanide.
Similar to probe BDBM, the absorption spectra of probe
BDM showed two absorption bands centred at 326 and 398 nm,
respectively (Fig. S9a†). Upon addition of 0–7.0 equiv. of
cyanide, both absorption bands gradually decreased and the
band at 408 nm nally disappeared. The linear correlation
between 1/DA326 and 1/[CNꢀ] conrmed the 1 : 1 stoichiometric
reaction between BDM and CNꢀ with formation of [BDM–CN]ꢀ
(Fig. S9b†). Accordingly, the solution of probe BDM changed
from light yellow to colourless upon addition of CNꢀ, which can
be detected by naked eye (Fig. S9a† inset). Furthermore, upon
excitation at 400 nm, a broadened emission band at around 603
nm was detected, which gradually reduced with the addition of
cyanide and vanished with 7.0 equiv. of CNꢀ (Fig. S9c†). The
detection limit of probe BDM for cyanide was obtained using a
uorescence intensity plot against CNꢀ concentration ranging
from 0.4 to 1.1 equiv. (Fig. S9d†), which provided a good linear
correlation and an outstanding detection limit of 0.32 mM.
To investigate the reaction processes of the probes, we
monitored the 1H NMR spectral changes of probe BDM and
BDBM in DMSO-d6 upon addition of 0–1.0 and 0–2.0 equiv. of
cyanide in CH2Cl2, respectively. In the case of BDBM (Fig. 8), the
Fig. 8 1H NMR spectra of BDBM in DMSO-d6 upon addition of cyanide
two vinylic protons (Ha) at 8.41 ppm gradually decreased and in CH2Cl2.
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Fig. 9 Time-dependent changes of (a) BDM and (b) BDBM in the
fluorescence intensity with different amounts of cyanide. Concen-
tration: 20 mmol Lꢀ1; excitation wavelength: 400 nm.
molecules assisted by CTAB contributed to the short detection
process.26,49,67
Fig. 11 Frontier molecular orbitals of [CN–BDBM–CN]2ꢀ
.
Furthermore, high selectivity and anti-interference ability
are important features for chemodosimeters. To evaluate the
selectivity of the two probes for cyanide detection, the changes
of BDM and BDBM in absorption and uorescence spectra
there was no signicant change in spectra, solution colour and
emission colour when other anions were added. These results
demonstrated the high sensing selectivity of BDM and BDBM
toward cyanide over other anions in nearly pure water
surroundings. More importantly, the competitive experiments
further conrmed that both probes could selectively detect
cyanide even in the presence of higher concentration of the
above-tested anions (Fig. 10e and S14†). The excellent selectivity
of two compounds in water was ascribed to the specic nucle-
ophilic addition of cyanide to the dicyanovinyl group,
strengthening their great potentials in practical applications.
To gain further insight into the cyanide-sensing process of
BDM and BDBM, DFT calculations were carried out. Upon
addition of cyanide, the dicyanovinyl groups are subject to
nucleophilic reaction, forming the adducts [CN–BDBM–CN]2ꢀ
and [CN–BDM]ꢀ, whose optimized geometries are shown in
Fig. S15.† The electronic structures of the two compounds are in
turn signicantly altered. As shown in Fig. 11, the HOMO and
HOMO ꢀ 1 of [CN–BDBM–CN]2ꢀ are both located at the anionic
part, while HOMO ꢀ 2 receives contributions mainly from the
central indene rings. The lowest excited state of [CN–BDBM–
CN]2ꢀ corresponds to the HOMO ꢀ 2 / LUMO transition,
which is the local excitation of the indene units. Similarly, the
HOMO of [CN–BDM]ꢀ is located at the anionic part, and the
HOMO ꢀ 1 receives its main contribution from the central
indene units (Fig. S16†). The lowest excited state of [CN–BDM]ꢀ
corresponds to the HOMO ꢀ 1 / LUMO transition, namely the
local excitation of the indene units. The ICT effects in [CN–
BDBM–CN]2ꢀ and [CN–BDM]ꢀ thus vanished due to nucleo-
philic attack of the cyanide anions, and the maximum absorp-
tion wavelengths of both adducts are reduced to around 350 nm
(Table S5†). The uorescence at around 600 nm thus dis-
appeared, in agreement with experimental observations.
caused by CNꢀ (7.0 equiv., 9.0 equiv.) and miscellaneous
interference anions (Fꢀ, Clꢀ, Brꢀ, I, H2PO4ꢀ, AcOꢀ, HCO3
,
ꢀ
NO3ꢀ, NO2ꢀ, 45 equiv.) were recorded. As shown in Fig. 10a–d
and S13,† the addition of CNꢀ produced remarkable changes in
both absorption and uorescence spectra, with the solution
colour changing from light yellow to colourless and the emis-
sion colour from intense orange to colourless. Nevertheless,
Fig. 10 (a) Absorption spectra and (b) fluorescence of BDM in CTAB
micelles (2 mmol Lꢀ1) upon addition of various anions (CNꢀ: 7.0 equiv.,
other anions: 45.0 equiv.). The corresponding colour (c) and emission
(d) changes of BDM in CTAB micelles (2 mmol Lꢀ1) upon addition of
various anions. Concentration: 20 mM; excitation wavelength: 400 nm.
(e) The fluorescence intensity changes at 603 nm of BDM (20 mM)
upon addition of 7.0 equiv. of CNꢀ and 45.0 equiv. of various inter-
ference anions.
Conclusions
Novel biindenyl-based D–p–A derivatives, BDM and BDBM,
containing electron-withdrawing dicyanovinyl groups have
been successfully developed. Both compounds display intense
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ICT uorescence in different solvents and exhibit highly effi- where Dn is the Stokes shi, Df is the solvent orientation
cient emission in the solid state. Moreover, they could be polarizability, h is the Planck constant, c is the speed of light in
utilized as uorescent sensors for the qualitative detection of vacuum, a is the Onsager cavity radius, me and mg are the dipole
low-level water content in THF or dioxane. In particular, the low moments in the excited (e) and ground (g) states, respectively.
detection limit of BDM (113 ppm in THF) enables it a super-
sensitive water sensor for practical applications. Furthermore,
both compounds show prompt responses to cyanide under the
assistance of CTAB, thus giving two sensitive chemo-sensors
with highly selectivity and low detection limits. In particular,
a remarkable detection limit of 0.29 mmol Lꢀ1 as well as a short
response time of only 100 seconds to cyanide was obtained for
BDBM in aqueous media, promising its practical applications
for detecting cyanide in drinking water. Notably, our results not
only provide a successful example of the ratiometric uorescent
sensor for water in organic solvent and cyanide in aqueous
media, but also demonstrate practical applications of our p-
conjugated molecules. We envisage that the molecular design
concept and the practical applications presented in this work
can promote future research in the eld and enable further
development of novel bio/chemosensors with intriguing opto-
electronic properties and outstanding performance.
Computational method
We employed density functional theory (DFT) calculations to
optimize the geometries of compounds BDBM and BDM, using
the hybrid B3LYP functional68 and the double-zeta basis set 6-
31G(d,p).69 Grimme's dispersion correction (version D3)70 was
employed in DFT calculations. At the optimized geometries,
frequency analyses were performed to conrm that these
geometries are true minima on the potential energy surface.
Time-dependent (TD) DFT calculations were also carried out to
provide insight into molecular orbital compositions of the
excited singlet states, using the range-separated CAM-B3LYP
functional71 and the triple-zeta basis set 6-311+G(2d,p)72 where
diffuse functions are included. Solvent effects of CH2Cl2 were
taken into account by the polarizable continuum model (PCM)73
in TD-DFT calculations. All calculations were carried out using
the Gaussian09 program package.74
Experimental section
Synthesis
Materials
4-(1,1,10,10-Tetramethyl-30-(p-tolyl)-1H,10H-[2,20-biinden]-3-yl)-
benzaldehyde (2). A solution of 1 (5.00 g, 10.7 mmol), N-bro-
mosuccinimide (NBS) (2.30 g, 12.8 mmol) and azodiisobutyr-
onitrile (AIBN) (0.08 g, 0.5 mmol) in carbon tetrachloride (100
mL) was heated for 10 h under reux. The reaction mixture was
cool to room temperature and rotary evaporated to a residue. The
residue was heated in ethanol (20 mL) for 1 h under reux, and
then cool to room temperature, and followed by vacuum ltra-
tion gave a yellow solid. The solid was heated in DMSO (30 mL)
in 150 ꢂC until no hydrogen bromide gas released. Aer cooling
to room temperature, the solution was poured into the distilled
water (100 mL) to precipitate the product. Then the precipitate
was ltered and puried by silica gel column chromatography
using petroleum ether–CH2Cl2 as eluent. A yellow-green solid
was obtained (yield: 0.95 g, 18%). 1H NMR (400 MHz, DMSO-d6,
d): 9.96 (s, 1H), 7.80 (d, 2H, J ¼ 8.2 Hz), 7.43 (dd, 2H, J1 ¼ 10.8 Hz,
J2 ¼ 7.0 Hz), 7.33 (d, 2H, J ¼ 8.1 Hz), 7.28–7.15 (m, 4H), 7.08 (dd,
4H, J1 ¼ 9.0 Hz, J2 ¼ 6.1 Hz), 7.00 (d, 2H, J ¼ 8.1 Hz), 2.28 (s, 3H),
1.51 (d, 6H, J ¼ 5.3 Hz), 1.10 (d, 6H, J1 ¼ 11.3 Hz). 13C NMR (100
MHz, CDCl3, d): 192.02, 154.81, 154.73, 149.92, 146.31, 143.16,
142.92, 141.72, 141.26, 141.11, 136.69, 134.65, 133.03, 129.63,
128.98, 128.74, 126.38, 126.34, 125.90, 125.80, 121.13, 120.93,
120.89, 120.36, 53.44, 53.14, 28.54, 28.44, 24.67, 24.49, 21.20.
HRMS (ESI, m/z): [M + H]+ calcd for C36H33O, 481.2531; found,
481.2525.
Compounds 1 and 3 were synthesized according to previous
method.12 Other reactants were purchased commercially, and
used as received without further purication. Dioxane and THF
were pre-dried over 4 A molecular sieves and distilled under
nitrogen atmosphere from sodium and benzophenone imme-
˚
˚
diately prior to use. Other solvents were dried with 4 A molec-
ular sieves prior to use. Double distilled water from a Millipore
Milli-Q plus system was used throughout the work. Cyanide
anion was in the form of tetrabutylammonium (TBA) salts,
solutions of other anions were prepared from the correspond-
ing sodium salts.
Instrumentation
1H NMR and 13C NMR spectra were recorded on Brucker AM-
400 spectrometers. Chemical shis were reported in ppm rela-
tive to a tetramethylsilane (TMS) standard in CDCl3 or DMSO-
d6. Mass spectra (MS) were obtained on a Waters LCT Premier
XE spectrometer. UV-vis absorption spectra were performed on
a Varian Cray 500 spectrophotometer and uorescence spectra
were recorded on a Varian Cray Eclipse. Quantum yields of
samples were recorded on a Horiba Fluoromax-4 with a cali-
brated integrating sphere system. The melting points were
measured on an X4 Micro-melting point apparatus.
2-(4-(1,1,10,10-Tetramethyl-30-(p-tolyl)-1H,10H-[2,20-biinden]-
3-yl)benzylidene)malononitrile (BDM). The mixture of excess
malononitrile, ammonium acetate (0.08 g, 1.0 mmol) and 2
(0.50 g, 1.0 mmol) was dissolved in acetic acid (50 mL) and
reuxed under an nitrogen atmosphere for 6 h. Aer cooling to
room temperature, the mixture was poured into water, and the
precipitation was puried by silica gel chromatography using
petroleum ether–CH2Cl2 as eluent. A deep yellow solid was
Evaluation of solvatochromism
The effects of solvents on the emission features can be evalu-
ated by the relationship between the solvent polarity parameter
(Df) and Stokes shi (Dn) of the absorption and emission
maxima (Lippert–Mataga equation).
À
Á
2Df
hca3
2
Dnhnab ꢀ nem
¼
me ꢀ mg þ const
(1)
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1
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