Tetraphenylethene-functionalized diketopyrrolopyrrole solid state emissive molecules: Enhanced emission in the solid state and as a fluorescent probe for cyanide detection

Article abstract of DOI:10.1039/c6ra10073b

Two conjugates of tetraphenylethene (TPE) and diketopyrrolopyrrole (DPP) were synthesized and their fluorescence properties were investigated. Both DPP1 and DPP2 show intense fluorescence and large Stokes shifts in both solution and solid state. In dilute

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Tetraphenylethene -functionalized diketopyrrolopyrrole solid state
emissive molecules: enhanced emission in the solid state and as a
fluorescent probe for cyanide detection
Lingyun Wang a*, Linhui Zhu a, Lin Li b*, Derong Cao a
a
School of Chemistry and Chemical Engineering, State Key Laboratory of
Luminescent Materials and Devices, South China University of Technology,
Guangzhou, China, 510641
b
School of Food Science and Engineering, South China University of Technology,
Guangzhou, China, 510641
*Corresponding author: Tel. +86 20 87110245; fax: +86 20 87110245. Eꢀmail:
lingyun@scut.edu.cn; felinli@scut.edu.cn
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Abstract
Two conjugates of tetraphenylethene (TPE) and diketopyrrolopyrrole (DPP) were
synthesized and their fluorescence properties were investigated. Both DPP1 and
DPP2 show intense fluorescence and large Stokes shifts in both solution and solid
state. In dilute THF, DPP1 and DPP2 show emission peaks at 606 nm and 632 nm
with Φf 32% and 11%, respectively. However, their powder samples for DPP1 and
DPP2 emit at 650 nm and 615 nm with Φf 13% and 7%, respectively. Moreover,
DPP2 shows remarkable selectivity and specificity toward CN− over other anions.
The color changed from pink to colorless when CN− was added to DPP2 in THF. The
maximum emission intensity at 632 nm is quenched 94.6%, which constitutes the
fluorescence signature for cyanide detection. The detection limit is 0.3 ꢁM using the
fluorescence spectra changes, which is far lower than the WHO guideline of 1.9 ꢁM.
Keywords, Chemosensors; Cyanide anion; Tetraphenylethene; Diketopyrrolopyrrole;
Aggregationꢀinduced emission (AIE)
2

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1. Introduction
As strongly fluorescent heterocyclic pigments, diketopyrrolopyrrole (DPP) and its
derivatives have been extensively applied in fluorescent probes,1 and their structures
can be easily optimized through variations of substituents at the 2,5ꢀ and 3,6ꢀpositions.
They exhibit large extinction coefficients, high fluorescence quantum in dilute
solution, brilliant redꢀfluorescence emission and exceptional light, weather, and heat
stability. As a consequence of the wellꢀconjugated DPP core, however, they display a
typical aggregationꢀcaused quenching (ACQ) effect. It is a great challenge to sustain
the efficient emission of DPPꢀbased compounds in aggregated states due to the serious
intermolecular π–π stacking, making realization of their full potential a daunting task.
To overcome this problem, it is a rational strategy to attach bulky or
aggregationꢀinduced emission (AIE) substituents onto DPP to hinder the ACQ
effect.2ꢀ4 Despite much effort, highly efficient solid emitters based on DPP are very
limited.
Tetraphenylethene (TPE) and its derivatives are representatives of AIE active
molecules, which possess prominent AIE properties and enjoy the easiness to be
synthesized.5 TPE is also used to modify traditional luminons such as triphenylamine,
pyrene, and perylenebisimide and successfully converted them from ACQ to AIE
molecules.6 However, in 2014, Tang’s group reported direct linking of TPE to DPP
could not convert the DPP derivative from an ACQ to an AIE active molecule and
obtained a DPPꢀbased fluorescent material with high quantum efficiency in the solid
state.7 The assimilation of DPP to TPE played a key role. Thus, it is a challenging task
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to examine the availability of tuning the emission performance of DPP by rational
modifications.
Promoted by the previous report on conjugates of DPP and TPE by Tang’s group,7
we were interested in the further modification of this skeleton to tune its
photophysical properties and explore potential applications. It is reasonable that the
insertion of a spacer between DPP and TPE moieties may break the assimilation effect.
A series of excellent studies have shown that the twisting phenyl ring plays a key role
in the organic photo/electronics.8 Moreover, the double bond is an essential feature for
many AIEꢀactive materials and its rigidity in the solidꢀstate prevents the surrounding
groups from excessive rotational or vibrational motions.9 For instance, in our previous
work, creation of highly efficient solid emitter by decorating a diphenylꢀDPP core
with anthracenone by carbonꢀcarbon double bond linker was achieved. 4 Herein, two
conjugates containing a diphenylꢀDPP core and two TPE arms were synthesized,
where the electronic communication between DPP and TPE is crucial for tuning their
photo/electronic properties. For DPP1, vinyl group, a πꢀconjugate building block is
used as the spacer, which can extend conjugate structure and also induce redꢀshifted
emission. For DPP2, an extended πꢀdeficient acrylonitrile spacer was used to blocking
the good planarity to optimize its optical properties. The fluorescent behaviors of
DPP1 and DPP2 in solution and aggregated sate were investigated, revealing both
dyes with intense fluorescence both in solution and solid state. Due to presence of
πꢀdeficient acrylonitrile spacers in DPP2 and exceptional nucleophilicity of cyanide,
DPP2 can be used as a colorimetric and fluorescent probe for detection of cyanide
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ions based on possible nucleophilic addition of CN− to the double bond. DPP2
showed remarkable selectivity and specificity toward CN− over other anions.
2. Experimental
2.1. Chemicals and instruments
Nuclear magnetic resonance spectra were recorded on Bruker Avance III 400 MHz
and chemical shifts are expressed in ppm using TMS as an internal standard. The
UVꢀvis absorption spectra were recorded using a Helios Alpha UVꢀVis scanning
spectrophotometer. FLuorescence spectra were obtained with a Hitachi Fꢀ4500 FL
spectrophotometer with quartz cuvette (path length = 1cm). XRD patterns were
collected on a D8 ADVANCE (Bruker).
The absolute fluorescence quantum yield was measured using a Hamamatsu
absolute PL quantum yield spectrometer C11347, which consists of an excitation light
source based on a xenon arc lamp, an integrating sphere used to measure all luminous
flux, and a highꢀsensitivity multichannel detector. The integrating spheres are
generally used to collect the emitted light. The use of integrating spheres has usually
required a laser as the excitation source in combination with a fibre coupled CCD
camera or a calibrated photodiode as the luminescence detectors. We can fit an
integrating sphere into the sample chamber of both the FluoroLog and FluoroMax
Spectrofluorometers to measure solid state photoluminescence quantum yields. This
approach significantly simplifies the experimental method as the need for special
equipment on the excitation and detection side is relaxed.
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−
−
−
−
The THF solutions of anions (F−, Cl−, Br−, I−, AcO−, HSO4 , H2PO4 , NO3 , ClO4 )
were prepared from their tetrabutylammonium salts.
Benzophenone,
phenyl(pꢀtolyl)methanone
were purchased
and
(2ꢀbromoetheneꢀ1,1,2ꢀtriyl)tribenzene
from
Sigma.
2,5ꢀDioctylꢀ3,6ꢀbis(4’ꢀformylphenyl)pyrrolo[3,4ꢀc] pyrroleꢀ1,4ꢀdione (compound 4)
and
(2Z,2’Z)ꢀ3,3’ꢀ((2,5ꢀdioctylꢀ3,6ꢀdioxoꢀ2,3,5,6ꢀtetrahydropyrrolo[3,4ꢀc]pyrroleꢀ1,4ꢀdiyl
)bis(4,1ꢀphenylene))bis(2ꢀ(4ꢀ(1,2,2ꢀtriphenylvinyl)phenyl)acrylonitrile) (compound 8)
were
(2ꢀ(pꢀtolyl)etheneꢀ1,1,2ꢀtriyl)tribenzene
(2ꢀ(4ꢀ(bromomethyl)phenyl)etheneꢀ1,1,2ꢀtriyl)tribenzene
synthesized
according
to
our
published
literature.4
(compound
5)
(compound
6)
and
4,4,5,5ꢀtetramethylꢀ2ꢀ(1,2,2ꢀtriphenylvinyl)ꢀ1,3,2ꢀdioxaborolane (compound 9) was
synthesized according to the literatures reported, respectively.10ꢀ12 The recognition
between DPP2 and different anions was investigated by UV–Vis and fluorescence
spectroscopy in THF solution at room temperature. Other solvents were obtained from
commercially available resources without further purification.
The stock solution of DPP2 in THF and anions was at a concentration of 10.0 mM.
After the DPP2 and anions with desired concentrations were mixed, they were
measured by UV–Vis and fluorescence spectroscopy.
2.2. Synthesis
2.2.1 Synthesis of compound 7
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A mixture of compound 6 (650 mg, 1.53 mmol), triphenylphosphine (1.203 g, 4.6
mmol) and DMF (10 mL) were added into a 25 mL roundꢀbottomed flask. The flask
was evacuated under vacuum and plushed with dry nitrogen three times. After stirring
0
at 95 C for 24 h, the solution was cooled to room temperature and poured into large
amount of diethyl ether. The 1.030 g white precipitate compound 7 was collected in
98% yield. m.p. 262ꢀ263 0C.
1HꢀNMR (CDCl3, 400 MHz, δ), 7.79ꢀ7.60 (m, 15H, PhꢀH), 7.10ꢀ6.83 (m, 19H,
PhꢀH), 5.31 (d, 2H, ꢀCH2). 13CꢀNMR (CDCl3, 100 MHz, δ), 162.5, 144.2, 143.5,
143.0, 142.9, 141.5, 139.8, 135.0, 134.3, 134.2, 131.6, 131.2, 131.1, 130.8, 130.8,
130.1, 130.0, 127.7, 127.6, 127.6, 126.6, 126.6, 126.5, 124.8, 117.9, 117.1, 36.5.
HRMS (ESI, m/z), [MꢀBr]+ calcd for (C45H36P), 607.7268, found, 607.7261.
2.2.2 Synthesis of compound DPP1
A mixture of compound 4 (57 mg, 0.1 mmol), compound 7 (170 mg, 0.3 mmol)，
K2CO3 (55 mg，0.4 mmol) and 1,4ꢀdioxane (5 mL) were added into a 25 mL
roundꢀbottomed flask. The flask was evacuated under vacuum and flushed with dry
nitrogen three times. After 95 0C stirring for 3 h, the solution was cooled to room
temperature. The solution was poured into water and extracted with dichloromethane.
The organic layer was dried over anhydrous magnesium sulfate. After filtration and
solvent evaporation, the residue was purified by column chromatography, using
petroleum ether/ dichloromethane/ ethyl acetate (15:5:1 by volume) as eluent.
Compound DPP1 (59 mg) was obtained as a bright red solid in 48% yield. m.p.
238ꢀ239 0C.
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1HꢀNMR (CDCl3, 400 MHz, δ), 7.84 (d, 4H, PhꢀH)), 7.61 (d, 4H, olefin H)), 7.29 (s,
4H, PhꢀH)), 7.13ꢀ7.05(m, 38H, PhꢀH), 3.79(t, 4H, NꢀCH2), 1.64ꢀ1.21 (m, 30H ꢀCH2,
ꢀCH3). 13CꢀNMR (CDCl3, 100 MHz, δ), 162.8, 147.8, 143.9, 143.6, 141.3, 140.4,
140.2, 134.8, 131.8, 131.4, 131.3, 130.6, 129.1, 127.8, 127.7, 127.6, 127.3, 127.0,
126.7, 126.6, 126.5, 126.1, 109.9, 42.1, 31.7, 29.4, 29., 29.0, 26.7, 22.6, 14.0. [M]+
calcd. for (C90H84O2), 1225.64, found, 1225.68.
2.2.3 Synthesis of compound DPP2
A mixture of compound 8 (77 mg, 0.1 mmol), compound 9 (115 mg, 0.3 mmol),
and Pd(PPh3)4 (2 mg) were added into a 100 mL two necked roundꢀbottomed flask.
The flask was evacuated under vacuum and flushed with dry nitrogen three times.
THF (10 mL) and potassium carbonate solution (2 M, 5 mL) were injected into the
flask and the mixture was refluxed overnight. The solution was poured into water and
extracted with dichloromethane. The organic layer was dried over anhydrous
magnesium sulfate. After filtration and solvent evaporation, the residue was purified
by column chromatography, using dichloromethane/ petroleum ether (1.2:1 by volume)
as eluent. Compound DPP2 (37 mg) was obtained as a bright red solid in 29% yield.
m.p. 227ꢀ228 0C.
1HꢀNMR (CDCl3, 400 MHz, δ), 8.03 (d, 4H, PhꢀH)), 7.94 (d, 4H, PhꢀH)), 7.50 (s,
2H, olefin H)), 7.46(d, 4H, PhꢀH)), 7.14ꢀ7.05 (m, 34H, PhꢀH)), 3.79(t, 4H, NꢀCH2),
1.62ꢀ1.21 (m, 30H, ꢀCH2, ꢀCH3). 13CꢀNMR (CDCl3, 100 MHz, δ), 161.5, 146.5, 144.5,
142.3, 142.2, 141.2, 138.8, 138.6, 135.2, 131.1, 130.9, 130.3, 130.3, 130.2, 128.6,
128.2, 126.9, 126.8, 126.6, 125.8, 125.7, 124.4, 116.6, 112.1, 109.7, 41.2, 30.7, 28.5,
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28.0, 28.0, 25.7, 21.5, 13.04. [M +Na]+ calcd. for (C92H82N4NaO2), 1297.644, found,
1297.439.
2.3 Computations
Calculations reported in this paper were performed using Gaussian 09 program.
Geometry optimizations for DPP1 and DPP2 were determined with Becke’s three
parameter density functional (B3LYP) method using the 6ꢀ311GIJd, p) basis set. The
local minimum for the optimized structure was confirmed through absence of negative
frequency.
3. Results and discussions
3.1 Design and synthesis
The molecular structures of DPP1 and DPP2 and their synthetic routes are shown
in Scheme 1. Intermediate (compound 4) was prepared according to the literature.4
To increase the solubility, the lactam groups were alkylated upon reaction with
1ꢀbromoctane. DPP1 was prepared by Wittig reaction between compound 4 and
TPEꢀbased ylide (compound 7). To introduce πꢀdeficient acrylonitrile spacer,
compound 4 reacted with papraꢀbromobenzylcyanide to yield compound 8. DPP2
was prepared by Suzuki coupling reaction between compound
8
and
1,2,2ꢀtriphenylethene boronic ester (compound 9). The purity of the resultants and
intermediates has been confirmed by NMR spectroscopy and MS spectrometry
(Figures S1–S9, Supporting Information). Due to the Nꢀmodified octyl chain, DPP1
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and DPP2 have good solubility in common organic solvents. This allows their
electronic structures to be evaluated with optical absorption, fluorescent emission
measurements in solution such as THF, chloroform (CHCl3), however, they display
poor solubility in hexane and polar solvents such as methanol and water.
3.2 Photophysical properties of DPP1 and DPP2 in solution and solid state
In THF, the maximum absorption band of DPP2 at 526 nm shows a red shift of 13
nm compared to that of DPP1 (513 nm) due to the electronꢀwithdrawing cyano groups
of DPP2, which is typical for the S0–S1 transition of DPP chromophore (Fig. S10a,
Supporting Information). The second absorption band (300–400 nm) for DPP1 and
DPP2 with peak maximum at 355 and 340 nm, respectively, is due to a π–π*
transition involving both the DPP and TPE units. In the fluorescent spectra, the
maximum emission wavelength of DPP2 is observed at 632 nm, which is 26 nm
longer than that of DPP1 (Fig. S10b, Supporting Information). It is interestingly
found that both DPP1 and DPP2 exhibit a large Stokes shift even up to 100 nm. The
results demonstrate that the electronꢀdonating TPE substituent introduced to the DPP
core by different spacer affects the electronic structure of DPP1 and DPP2.
The emission study of DPP1 in different solvents indicates that the luminescent
properties are slightly affected by solvent polarity (Fig. S11, Supporting Information).
The emission band has no obvious redꢀshift, but its intensity is reduced with an
increase in the solvent polarity. The FL spectra of DPP2 show similar features as
compared to that of DPP1 in different solvents (Fig. S12, Supporting Information).
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The property that we are most interested in is their fluorescent behaviors in
aggregated states. So, we investigated the fluorescent behaviors of DPP1 and DPP2
in CHCl3/ hexane, CHCl3/ methanol and THF/water with different volume percentage
of hexane (fh), methanol (fm), and water (fw), respectively. Typically, Fig. 1a shows the
FL spectra of DPP1 in CHCl3/ hexane. In CHCl3, DPP1 emits orange fluorescence
(λem = 605 nm) and the quantum efficiency (Φf) is 33%. With increase of fh, the strong
emission of orange characteristic band is gradually enhanced. Φf is 44% at fh of 90%
(Fig. 1b). Besides the increase in PL intensity, the emission peak showed slightly
blueꢀshift from 605 nm to 602 nm with the increase of fh from 0 to 90%. The emission
photographs of DPP1 in CHCl3/hexane mixtures with different fh values are shown in
Fig. 1c. Different fluorescence effect of DPP1 in CHCl3/methanol mixtures is present
(Fig. 2). When fm <40%, the emission intensity monotonously increases to negligible.
When fm >40%, aggregates form in the system and the emission intensity greatly
decreases. The Φf is only 9% at fm of 90%.
As demonstrated by the FL spectra in Fig. 3, in THF solution, DPP1 emits orange
fluorescence (λem = 605 nm) with Φf of 32%. When relative low fraction of water (fw,
by volume, 40%), a nonꢀsolvent to DPP1, is introduced into the THF solution,
slightly enhancement in emission intensity is present. When fw reaches 50%, the
emission intensity decreases greatly. The change in fluorescence intensity vs. fw is
shown in Fig. 3b. The Φf is only 3% at fw of 90%. The decrease of quantum efficiency
against the increase of fw can be ascribed to the formation of aggregates in the
THF–water mixtures, which quenches the fluorescence via the intermolecular π–π
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interaction. Besides the decrease in emission intensity, the emission peak redꢀshifted
from 605 to 619 nm with the increase of fw from 0 to 90%. Such an observation
clearly indicates that the hydrophobic DPP1 molecules had formed large aggregates,
as both water/THF are hydrophilic solvents and only those smaller ones in the
suspension contributed to the emission. To understand mode of aggregation, we have
performed UVꢀvis absorption spectra of DPP1 in THF/water (Fig. S13, Supporting
Information). Aggregation effect in which a reduction in the peak intensity and
significant redꢀshift of the absorption maximum along with loss of fine structure are
seen in THF/water as compared to bands in THF alone. The absorption tails extending
well into the long wavelength region indicate that DPP1 aggregate into particles in
the presence of water, as it is well known that the Mie effect of nanoparticles can
cause such levelꢀoff tails in the absorption spectra.13
The PL spectra of DPP2 clearly show similar features as compared to that of DPP1
in different solvent mixtures. As shown in Fig. S14 (Supporting Information), the
emission of DPP2 in the CHCl3 solution is strong with an emission maximum at 628
nm (Φf= 15%). When hexane, a nonꢀ polar and poor solvent to DPP2, is introduced
into the CHCl3 solution, the PL signal is gradually enhanced upon increasing hexane/
CHCl3 ratios. When fh reaches 80%, the PL intensity is boosted to the maximum. The
quantum efficiency of DPP2 is 1.3ꢀfold higher than its original emission intensity in
CHCl3. However, the emission intensity monotonously decreases when polar and poor
solvent of methanol is added to CHCl3 solution (Fig. S15, Supporting Information).
The Φf is only 4% at fm of 90%. It is noted that the emission peak gradually blueꢀshifts
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from 628 to 624 nm when fm is increased from 0 to 90%. Such a spectral shift can be
explained as follows: with increased methanol content, DPP2 molecules start to
aggregate, which generates less polar microenvironment for the molecules inside the
aggregates. The reduced environment polarity leads to the observed blue shift in the
emission spectrum.14 The similar behavior was also observed for THF/water mixtures,
only 4% Φf was shown in presence of 90% water (Fig. S16, Supporting Information).
Usually, emission of DPP derivatives is never observed in the solid state, because
the emission is completely quenched by πꢀπ interaction in aggregation. As expected,
no emission was observed for pure compound 4 as shown in Fig. S17 (Supporting
Information). It is intriguing to note that DPP1 and DPP2 still retain intense
fluorescence even in the powder form. This observation prompted us to investigate
their photophysical properties in the solid state in details. The emission spectra,
together with photographs taken under UV illumination, are shown in Fig. 4. Upon
exciting at 365 nm, DPP1 in powder emits strongly in the red region. With respect to
corresponding emission at 606 nm in THF solution, the fluorescence maxima peak at
650 nm for DPP1 in powder is obviously redꢀshifted. However, it is quite surprising
that the emission maximum of DPP2 in powder is located at 615 nm, which is
hypochromic shift of 17 nm than that in THF. This indicates that the DPP2
fluorophores are still in their isolated form in the powder without heavy
intermolecular interactions, as strong π–π overlaps in the aggregated or solid state
always causes delocalization of the excited states and induces emission at a longer
wavelength.15 However, this is not ture for DPP1. Evidently, the highly distorted and
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rigidified conformation of the TPE unit in DPP1 and DPP2 may lead to an extended
π–π interplanar distance and/or inefficient π–π overlap when molecules are
aggregated.
The fluorescence quantum efficiency (ΦF) of the samples was estimated using
Rhodamine 6G (ΦF=95% in ethanol) as standard solution and ΦF of the solid film was
measured using an integratingꢀsphere photometer. DPP1 (10ꢀ5 M) in cast film shows a
fluorescence quantum yield (ΦF) 13%, which is lower than its ΦF in CHCl3 and THF
i.e. 33%, 32% respectively. Interestingly, DPP2 shows quite different behavior than
DPP1. It is found that the ΦF of DPP2 in cast film is 7%, which is lower than ΦF in
CHCl3 and THF is 15%, 11%, respectively. However, ΦF of DPP1 and DPP2 in solid
state are higher than that of DPPꢀtriphenylamine dyes (ΦF=0.03%, 0.1%),3b DPPꢀTPE
dye with donorꢀacceptor effect (ΦF= 8.6%)7 and others.16 It should be noted that the
introduction of phenylacrylonitrile leads to a larger Stokes shift but a lower ΦF both in
solution and soldꢀstate as compared with DPP1. The corresponding photophysical
data are shown in Table 1.
The morphology of DPP1 and DPP2 powders were further investigated by Xꢀray
diffraction (XRD), which show many sharp and strong peaks, indicative of their
crystalline nature (Fig. 5). In our case, the molecules of DPP1 and DPP2 may steadily
cluster together to form ordered microstructures and hard to be changed due to the
restriction of the crystal lattice, and this may make the crystals stronger emitters
because of the weakened π–π interactions among the fluorophores. These results will
provide the way to tune the performance of organic luminogens by making subtle
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balance between AIE and ACQ units; and on the other hand, derived redꢀemitting
molecules with good quantum efficiency.
3.3 Theoretical calculations
The redꢀshift of emission of DPP1 and DPP2 relative to compound 4
(DPPꢀaldehyde in THF, λab = 497 nm, λem = 592 nm) is associated with the segmental
electronic conjugation between DPP and TPE moieties, as predicted by theoretical
calculation (Fig. S18, Supporting Information). In the free state of DPP1, the dihedral
angle between the vinyl bridge and the adjacent phenyl in the TPE moiety is about
2.9660, suggesting the good conjugation between the TPE and DPP moieties. In
comparison with DPP1, the large dihedral angle between the acrylonitrile bridge and
the adjacent phenyl in the TPE moiety is 26.7390 for DPP2. Moreover, the dihedral
angles between the vinyl bridge and adjacent another phenyl adjacent to DPP core
are 4.1740 and 9.6370 for DPP1 and DPP2, respectively. These results indicate that
the addition of the acrylonitrile groups between DPP and two TPE moieties reduces
planarity of DPP2.
To further elucidate the effect of the spacer on the electronic structures and thus the
photophysical properties of the DPP dyes, we conducted total energy calculation using
density functional theory (DFT) calculations at the B3LYP/631G(d) level of theory.
The density maps of their molecular orbitals are shown in Fig. 6, and the calculated
data are summarized in Table 1. DPP1 has a HOMO delocalized over the
vinylbenzene moiety and the entire TPE arms, whereas its LUMO is mostly localized
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on the central DPP and vinylbenzene moieties. The HOMO and LUMO energy levels
are ꢀ5.17 eV and ꢀ1.69 eV, respectively. It is noteworthy that in sharp contrast to the
remarkable contribution of vinylbenzene moiety to the LUMO for DPP1, DPP2 has a
HOMO delocalized over the entire TPE arms, whereas its LUMO is mostly localized
on the central DPP, benzene and acrylonitrile moieties. The HOMO and LUMO
energy levels are ꢀ5.38 eV and ꢀ2.21 eV, respectively. The decreased HOMO–LUMO
gap of DPP2 should be responsible for the redꢀshifted main emissive band relative to
that of DPP1. All the calculated results are in good agreement with the experimental
results.
3.4. Detection of cyanide with DPP2
Cyanide has been continuously of concern all over the world due to its high toxicity
and widespread use in industrial processes such as acrylic fiber manufacturing,
metallurgy, herbicide production, and silver or gold extraction.17 Therefore, it is of
great significance to find convenient and efficient detection methods at the
environmental and biological levels. The burgeoning field of reactionꢀbased indicators
has made progress in this area because of the unique reactivity of CN− toward a
variety of organic functional groups including C=O, C=N, C=C, and so on.18 The
irreversible formation of chemical bonds can provide chemodosimetric information
and develop ratiometric fluorescent probes. It is well known that general Michael
acceptors, the doubly activated acceptors, in which two electronꢀwithdrawing groups
are attached to the C=C group, are more reactive, and allow the Michael addition
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reaction to occur under mild condition. Clearly, to improve the sensitivity, it is
necessary to enhance the reactivity of the probe to cyanide. With this in mind, we
recently reported chemodosimeter approaches based on diketopyrrolopyrrole, where
electron withdrawing cyano and ester groups or 1,3ꢀindanedione to afford doubly
activated Michael addition type probes for the colorimetric fluorescent detection of
cyanide with high sensitivity and good selectivity.19
The interaction of DPP1 in THF with cyanide through absorption and fluorescence
spectra titration experiments was carried out. As shown in Figs. S19ꢀS20 (Supporting
Information), no changes could be found in presence of cyanide. DPP1 is inactive to
cyanide due to low reactivity of vinyl group in DPP1. However, DPP2 may have
great potential for detection of cyanide due to presence of πꢀdeficient
phenylacrylonitrile section, electron withdrawing cyano group connected with the
αꢀposition of the ethylenic bond. The interaction of DPP2 in THF with cyanide
through absorption and fluorescence spectra titration experiments was carried out.
DPP2 exhibits main absorption bands centered at 336 and 525 nm, respectively (Fig.
7). Upon addition of 0–24.0 equiv. of cyanide, both bands gradually decrease and the
one at 525 nm eventually vanishes at the saturation point (24.0 equiv.). Two new
bands at 312 nm and 360 nm are present. This change suggests that the large
πꢀconjugation is interrupted, accompanied by the formation of [DPP2–CN]2ꢀ due to
the nucleophilic attack of cyanide toward the acrylonitrile groups of DPP2. Moreover,
the solution faded and became almost colorless, which could be clearly observed by
naked eye (Fig. 7 inset).
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As expected, the fluorescence emission of DPP2 is quenched by the nucleophilic
addition reaction between cyanide and acrylonitrile group. Upon excitation at 520 nm,
a broadened emission band at around 632 nm is detected, which gradually decreases
upon addition of cyanide and became saturated with 52.0 equiv. of CN− (Fig. 8).
Finally, 94.6% PL intensity at 632 nm is quenched. Additionally, a linear relationship
between the fluorescent intensity at 632 nm and the concentration of CN− in the low
concentration region is obtained (Fig. S21, Supporting Information). The detection
limit is as low as 0.3 ꢁM, which is much lower than the maximum contaminant level
for CN− in drinking water (1.9 ꢁM) set by the World Health Organization (WHO).20
In comparison with reported probes,21 the detection limit of DPP2 is moderate for
fluorescence detection limits to cyanide.
For the purpose of evaluating selectivity of DPP2 to cyanide, the absorption
spectral change of DPP2 upon addition of other anions was also investigated.
Dramatic change of the absorption spectrum induced by CN− was observed, while
almost no changes could be found in presence of other anions, including F−, Cl−, Br−,
−
−
−
I−, AcO−, HSO4 , H2PO4 , NO3 , ClO4− (Fig. 9a). More importantly, the color change
from pink to colorless can be clearly observed by the naked eye in presence of CN−,
while other anions did not induce any significant color change, which suggested that
nakedꢀeye selective detection of CN− became possible (Fig. 9b).
Meanwhile, only CN− renders a remarkable “TurnꢀOff” fluorescence response,
whereas all other anions reveal a negligible change in the fluorescent spectra of DPP2
(Fig. 10a). In the presence of 52 equivalents of CN −, an emission peak at 632 nm with
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94.6% fluorescence quenching is observed. Fig. 10b showed the photographs of
DPP2 in the presence of different anions under excitation at 365 nm using a portable
UV lamp. The disappearance of intense red color of the solution upon interaction of
DPP2 with CN− is present. However, other anions do not induce any significant
emission color change.
Another important feature of DPP2 is its high selectivity toward the CN− in
presence of other competitive anions. Changes of fluorescence response of DPP2 (10
ꢁM) caused by CN− (52 equiv) and miscellaneous competing species (52 equiv) were
recorded in Fig. S22 (Supporting Information). As can be seen, these competitive
species, did not lead to any significant interference. In the presence of these ions, the
CN− still produced similar optical spectral changes. These results showed that the
selectivity of DPP2 toward CN− was not affected by the presence of other anions.
Fig. S23 (Supporting Information) showed the timeꢀdependent absorbance at 525
nm of DPP2 in the presence of CN− (24 equiv.) at room temperature in THF.
Generally, reactionꢀbased probes suffer from a long response time. In our case, the
response of DPP2 to CN− is found to be fast. The absorption peak of DPP2 at 525 nm
gradually decreased when 24 equiv. of CN− is added with time passing. After 15
minutes the UVꢀVis spectra of DPP2 are unchangeable, indicating nucleophilic
addition reaction between the vinyl group and CN− is completed, which is impressive
as many reported cyanide probes require high equivalents of cyanide and long
reaction time to reach a maximal spectral signal.22ꢀ31
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3.5 Practical application
Active materialsꢀbased test strips represent a group of convenient probing substrate
for practical utilization. DPP2ꢀbased test strip was thus fabricated by immersing filter
paper into the THF solution of DPP2 (1.0 × 10−3 M) and drying in air, which was
energyꢀ and costꢀeffective. The corresponding probing experiments were carried out
subsequently. The results indicated that this protocol really took effect. The obvious
color change from pink to colorless was observed by immersing this test strips in
solutions of CN−, exhibiting colorimetric changes differentiable to naked eyes (Fig.
11a). As shown in Fig. 11b, when the DPP2ꢀexposed test strip was immersed into
solutions of CN− (0.001 M), strong red fluorescence disappeared and easily
distinguished.
Conclusions
In summary, we described synthesis of two new TPE substituted DPP derivatives
(DPP1 and DPP2) and studied comparative fluorescence behaviors. Typically, DPP1
in dilute THF solution shows the characteristic PL features (λem =606 nm) with Φf
32%. Its powder sample emits red fluorescence (λem =650 nm, Φf = 13%). DPP2 in
dilute THF solution has Φf = 11% (λem =632 nm) and its powder sample emits red PL
(λem =615 nm, Φf = 7%). Moreover, DPP2 displays high selectivity and sensitivity
toward CN−. The strong red emission of DPP2 was completely quenched in the
presence of CN−. However, other anions did not induce any significant change. Test
strips for use in practical applications are successfully realized.
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Acknowledgements
The supports by National Basic Research Program of China (2012CB720801),
National Natural Science Foundation of China (No. 21274045), the Fundamental
Research Funds for the Central Universities (2015ZZ037) and the Natural Science
Foundation of Guangdong Province (2015A030313209, 2016A030311034) are
gratefully acknowledged.
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H
N
O
O
O
O
CHO
CN
O
O
O
CH2OH
CH2OH
CH
CH
O
O
O
CH
t-C5H11ONa
t-C5H11OH
NH2SO3H
O
N
H
O
2
CN
O
1
C8H17
C8H17
N
O
O
N
O
CH
CHO
C8H17Br
O
HCl/H2O
THF
CH
OHC
O
t-C4H9OK
N
O
O
N
C8H17
3
C8H17
4
NBS, BPO
CCl4
Zn, TiCl4
THF
O +
O
5
6
Br
Ph3P
DMF
CH2P(Ph)3Br
7
C8H17
N
O
K2CO3
Spacer
+
4
7
O
N
1,4-dioxane
C8H17
Spacer
DPP1
C8H17
N
C8H17
N
O
CHO
Br
CN
O
CN
Br
OHC
Br
O
O
N
NC
O
N
C8H17
C8H17
4
8
B
O
Br
PdCl2 (PPh3) 2, Ph3P
toluene
C8H17
N
9
NC
O
Spacer
Pd (PPh3) 4, K2CO3
THF
O
N
8+ 9
CN
Spacer
C8H17
DPP2
Scheme 1. Synthesis of DPP1 and DPP2.
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Table 1. Photophysical data of DPP1 and DPP2 in solution and solid state.
THF a
Solidꢀstate
b
c
HOMO LUMO gap λab(nm) λem (nm) Φf
λem (nm)
Φf
(ev)
ꢀ5.17
ꢀ5.38
(ev)
ꢀ1.69
ꢀ2.21
(ev)
3.48
3.17
513
526
605
632
32% 650
11% 615
13%
7%
DPP1
DPP2
a
0
b
Measured at a concentration of 10 ꢁM at 25 C; estimated using Rhodamine 6G
(ΦF=95% in ethanol) as reference; c Absolute quantum yield determined by calibrated
integrating sphere systems.
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Fig.1 (a) The photoluminescence (PL) spectra, (b) PL intensity vs fh and (c) emission
photographs of DPP1 in CHCl3/hexane mixtures with different fh values.
29