Dicyanomethylene-4H-pyran-based NIR fluorescent ratiometric chemosensor for pH measurement

Article abstract of DOI:10.1007/s11164-018-3334-z

Abstract: A D-π-A featured dicyanomethylene-4H-pyran-based dye (DCM-OH) is designed as a pH-sensitive fluorescent chemosensor. In neutral conditions, DCM-OH shows a long absorption wavelength at 431?nm attributed to the DCM chromophore via the intramolecular charge transfer (ICT), but very weak fluorescence around 574?nm. While pH increases from 7.15 to 11.00, DCM-OH shows a bathochromic absorption band at 574?nm due to the increase of the ICT process from the oxygen anion group, shifting the emission band to the NIR region. A DFT calculation has been performed to confirm the ICT enhancement. In this way, an obvious ratiometric fluorescent signal (I₆₉₂/I₅₇₄) allows the ratiometric detection of pH, induced by a large Stokes shift of about 118?nm. The fluorescent chemosensor exhibits high sensitivity to H⁺ with a pK_a value of 7.21. The strong change in fluorescence intensity with pH allows the determination of pH over a much wider range (typically 7–11). Furthermore, the reversible absorption and fluorescence conversion was also performed with protonation and deprotonation, which makes DCM-OH a simple naked-eye sensitive NIR fluorescent chemosensor for pH measurement.

Full text of DOI:10.1007/s11164-018-3334-z

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3334-z
Dicyanomethylene-4H-pyran-based NIR fluorescent
ratiometric chemosensor for pH measurement
1
,2
3
1
Jinfeng Yang
•
Meng Li
•
Wei-Hong Zhu
Received: 15 October 2017 / Accepted: 17 February 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A D-p-A featured dicyanomethylene-4H-pyran-based dye (DCM-OH) is
designed as a pH-sensitive fluorescent chemosensor. In neutral conditions, DCM-OH
shows a long absorption wavelength at 431 nm attributed to the DCM chromophore via
the intramolecular charge transfer (ICT), but very weak fluorescence around 574 nm.
While pH increases from 7.15 to 11.00, DCM-OH shows a bathochromic absorption
band at 574 nm due to the increase of the ICT process from the oxygen anion group,
shifting the emission band to the NIR region. A DFT calculation has been performed to
confirm the ICT enhancement. In this way, an obvious ratiometric fluorescent signal
(
I692/I574) allows the ratiometric detection of pH, induced by a large Stokes shift of about
?
18 nm. The fluorescent chemosensor exhibits high sensitivity to H with a pK value of
.21. The strong change in fluorescence intensity with pH allows the determination of
1
7
a
pH over a much wider range (typically 7–11). Furthermore, the reversible absorption and
fluorescence conversion was also performed with protonation and deprotonation, which
makes DCM-OH a simple naked-eye sensitive NIR fluorescent chemosensor for pH
measurement.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3334-z) contains supplementary material, which is available to authorized users.
&
Wei-Hong Zhu
whzhu@ecust.edu.cn
1
Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and
Molecular Engineering, East China University of Science and Technology, Shanghai 200237,
China
2
3
College of Chemistry and Chemical Engineering, Key Laboratory for Green Processing of
Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi 832000, China
Department of Environmental Science and Engineering, North China Electric Power University,
Baoding 071003, China
123

J. Yang et al.
Graphical Abstract
Keywords Fluorescent pH sensor Á Dicyanomethylene-4H-pyran Á Intramolecular
charge transfer Á Ratiometric response
Introduction
Fluorescent chemosensors have been extensively reported due to their high
sensitivity, simplicity and non-invasive imaging [1–4]. It is particularly desirable to
develop fluorescent sensors for pH measurement because pH is one of the most
important parameters in clinical analysis, pharmacy, industry, the environment and
life sciences [5–8]. In fact, the protonation and deprotonation of biomolecules are
frequently involved in many cellular events, such as cell growth, cellular
proliferation, endocytosis, apoptosis and other processes [9–11]. The detection of
pH is extremely significant since intracellular or extracellular pH can reflect severe
inflammation and diseases. In addition, pH plays an important role in monitoring
food production, freshwater or wastewater treatment procedures and marine
sediments [12–15]. Currently, pH fluorescent sensors are widely reported due to
their short response time, simple operation, low cost and excellent sensitivity.
Intensive efforts have been devoted to pH measurement mainly based on the
fluorescence mechanisms of photo-induced electron transfer, intramolecular charge
transfer (ICT), and fluorescence resonance energy transfer [16–18]. Most fluores-
cent chemosensors show turn-off or turn-on behavior [19–21], and some fluorescent
chemosensors’ emission and absorption are in the ultraviolet–visible (UV–Vis)
region. These pH fluorescent chemosensors have disadvantages such as the
influences of sensor concentration, instrumental instability and environment
interference. Therefore, ratiometric fluorescent sensors are particularly interesting
because the ratio of fluorescence intensities at two different wavelengths can
eliminate most or all ambiguities [22–24]. However, those sensors with short-
wavelength excitation and emission spectra have low signal-to-noise ratios and
autofluorescence of biomolecules. Therefore, the ideal ratiometric fluorescent
chemosensors are preferable to display a large wavelength shift in the near-infrared
(
NIR) region [25, 26].
Longer p-electron systems, especially electron donor-p-conjugate–electron
acceptor (D-p-A) chromophores, are distinct gateways to design NIR fluorescent
1
23

Dicyanomethylene-4H-pyran-based NIR fluorescent…
sensors [27–29]. Attributed to its ICT, dicyanomethylene-4H-pyran (DCM) is an
excellent candidate for NIR fluorophore. Meanwhile, DCM derivatives are very
promising due to their broad absorption, long emission wavelength, large Stokes
shift, and relatively high fluorescent quantum yield [30, 31]. Thus, DCM-based
derivatives, such as NIR fluorescent labels, have been used in various fields
including ion recognition [29, 32, 33], logic memory [34, 35], fluorescent molecular
switches [36], solar cells [37, 38], fluorescent sensors [39, 40] and in vivo imaging
[
41–45]. There is a high interest in developing DCM-type ratiometric fluorimetry
and lighting up NIR fluorescent chemosensors, and a DCM-OH has been
synthesized as shown in Scheme 1, whose optical characters for determining pH
have been investigated.
Experimental
Materials and instruments
Toluene, dichloromethane (CH Cl ), chloroform (CHCl ), petroleum ether and
2
2
3
acetic acid were purchased from Chinese Chemical Reagent Shanghai Titan
Scientific. All solvents were of analytical grade. Nuclear magnetic resonance
1
13
hydrogen ( H NMR) and nuclear magnetic resonance carbon ( C NMR) spectra in
CDCl3 were measured on a Brucker AV-400 spectrometer (Germany) with
tetramethyl silane (TMS) as an internal standard. Ultraviolet–visible (UV–Vis)
spectra were obtained by using a Varian Cary 50 spectrophotometer (1 cm quartz
cell) at 25 °C. Fluorescence spectra were recorded on a Varian Cary Eclipse
fluorescence spectrophotometer (1 cm quartz cell) at 25 °C, and all fluorescence
spectra were uncorrected by the photo-multiplier tube response. The width of the slit
was 5 nm. TLC analyses were performed on silica gel plates, and column
chromatography was conducted over silica gel (mesh 200–300), both of which were
obtained from the Qingdao Ocean Chemicals (China).
1
-(2-Hydroxypheny)-1,3-butadione (1)
o-Hydroxyacetophenone (5.0 g, 36.7 mmol) and 30 mL of ethyl acetate were put
into a 100-mL rouind-bottom flask. Then, sodium (4.0 g, 170.0 mmol) was added to
the flask and the mixture was stirred for 2.5 h at room temperature. After being
filtered, the residue was dissolved in water, and the solution was acidified with
acetic acid to neutral. After 24 h, compound 1 was obtained in 54% yield.
Scheme 1 Synthetic route of DCM-OH: (i) CH
3
COOC
2
H
5
, Na, 54% yield; (ii) HOAc, H
yield; (iii) Ac O, CNCH CN, 30% yield; and (iv) 4-hydroxybenzaldehyde, piperidine/CH CN, 48% yield
2 4
SO , 92%
2
2
3
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J. Yang et al.
2
-Methyl-4-one-4H-chromen (2)
Compound 1 (3.0 g, 16.8 mmol) in acetic acid (30 mL) was refluxed in the presence
of 2.0 mL sulfuric acid for 0.5 h. The reaction mixture was poured onto crushed ice,
and neutralized with sodium carbonate. The mixture was extracted with
dichloromethane. After removing solvents and recrystallized from ethanol,
1
compound 2 was obtained as a gray solid in 92% yield. H NMR (400 MHz,
CDCl , ppm): d 2.39 (s, 3H, –CH ), 6.18 (s, 1H, –CH =), 7.42 (m, 2H, phenyl-H),
3
3
7
.64 (d, J = 8.0 Hz, 1H, phenyl-H), 8.17 (d, J = 8.0 Hz, 1H, phenyl-H).
4
-(Dicyanomethylene)-2-methyl-chromone (3)
Compound 2 (2.0 g, 12.5 mmol) was added to 10.0 mL acetic anhydride of
containing malononitrile (1.0 g, 15.1 mmol). After refluxing for 13 h, the mixture
was concentrated under reduced pressure. Then, the residue was refluxed for 0.5 h
in 30 mL of water. Finally, the solution was filtered to give the crude product, which
1
was recrystallized from ethanol as a gray solid in 30% yield. H NMR (400 MHz,
CDCl , ppm): d 2.45 (s, 3H, –CH ), 6.73 (s, 1H, –CH =), 7.46 (m, 2H, phenyl-H),
3
3
7
.73 (m, 1H, phenyl-H), 8.94 (dd, J = 8.6, J = 1.2 Hz, 1H, phenyl-H).
1 2
DCM-OH
Compound 3 (131.2 mg, 0.63 mmol) and 4-hydroxybenzaldehyde (76.9 mg,
.63 mmol) were taken in 30.0 mL anhydrous toluene under argon, and then
0
piperidine (1.0 mL) and acetic acid (0.5 mL) were added. The contents were
refluxed, and monitored by TLC (dichloromethane: petroleum ether). After
refluxing for 13 h, the solvent was removed in a rotary evaporator, and the
obtained crude product was purified by column chromatography on silica gel with
1
dichloromethane to afford the desired product in 52% yield. H NMR (400 MHz,
DMSO-d ): d 6.85 (d, J = 8.8 Hz, 1H), 6.96 (s, 1 H), 7.30 (d, J = 16.0 Hz, 1H),
6
1
7
.59 (m, 3H), 7.70 (d, J = 16.0 Hz, 1H), 7.79 (d, J = 7.6 Hz,1H), 7.93 (m, 1 H),
1
.73 (d, d, J = 8.4 Hz, J = 1.2 Hz, 1 H), 10.16 (s, 1 H). C NMR (100 MHz,
3
8
1
2
CDCl , ppm): d = 165.25, 164.14, 158.15, 157.26, 144.51, 140.53, 135.59, 131.33,
3
1
H] Calculated for C H N O 312.0928, found 311.0820.
29.84, 124.25, 122.65, 122.35, 121.30, 121.15, 110.96, 64.27. HR-MS (m/z): [M–
?
2
0 12 2 2
Results and discussion
Design and synthesis of DCM-OH
As shown in Scheme 1 and Scheme S1 (Supporting Information online), the key
intermediate 3 was first obtained by using a condensation reaction of malononitrile
and 2-methyl-4-one-4H-chromen 2, which was synthesized from 1-(2-hydrox-
ypheny)-1,3-butadione 1 via Vilsmeier formylation in 92% yield. Then, the D-p-A
type pyran derivative DCM-OH was obtained from the reaction of 2-(3,5,5-
1
23

Dicyanomethylene-4H-pyran-based NIR fluorescent…
trimethylcyclohex-2-enylidene) malononitrile 3 with 4-hydroxybenzaldehyde. Their
1
chemical structures were well characterized by H NMR, C NMR, and HRMS. In
13
1
H NMR spectra (S1), the characteristic signal at d 6.96 ppm corresponds to the
hydrogen on the pyranyl ring. This is further confirmed by the appearance of a
1
3
signal at d 64.3 ppm in the C NMR spectra. The presence of a singlet at d
1
0.16 ppm in H NMR spectrum was attributed to the phenol hydrogen of benzene
1
ring. The presence of (CH=) protons resonance at about d 7.93 and 7.30
demonstrates the successful synthesis of DCM-OH. Furthermore, the characteristic
coupling constant (J = 16.0 Hz) of alkene protons is indicative of the predominant
1
3
trans isomer. These correspond to the appearance of carbon signals in the C NMR
-
?
-
spectra (S2). The [M-H ] peak of HR-MS corresponding to the DCM-O well
supports our proposed mechanism (S3).
Photophysical properties
-
5
Figure 1 displays the absorption spectra of DCM-OH (1 9 10 M) in several
solvents of different polarity. The absorption peak at kmax 450 nm is attributed to
the ICT from the 4H-pyran in all the solvents. For aprotic polar solvents, such as
dimethyl sulfoxide and N,N-dimethyl formamide (DMF), there are serious
disturbances of the solvation effect. In the UV–vis spectra around 650 nm, an
absorption band was seen for DMSO and DMF, which might belong to the
deprotonated form. And the DMF was unstable in the presence of strong bases such
as sodium hydroxide or strong acid such as hydrochloric acid. Moreover, the
absorbance decrease with increasing solvent polarity should be attributed to a highly
polar-excited state population (ICT) state [46]. Considering the solubility of the
DCM-OH in water, the investigation of absorption and emission spectra with pH
value in the mixed solvent, C H OH–H O (6:4, v/v), was carried out.
2
5
2
The absorption and emission spectra were measured with varying pH values.
Firstly, the solution of free probe DCM-OH shows one major absorption peak at
around 450 nm, with another one at about 431 nm. The interaction of pH value for
DCM-OH is investigated by means of spectrometric titration experiments using
either acid or base. Both the absorption and emission spectra was barely changed
Fig. 1 Absorption spectra of
DCM-OH (1.0 9 10 M) in
different solvents
-5
123

J. Yang et al.
-
when HClO was added stepwise to the solution of DCM-OH (1 9 10 M) at
5
4
room temperature. However, when NaOH was added stepwise to the solution of
DCM-OH, a new absorption peak at kmax 550 nm rose and the absorption peak at
kmax 450 nm fell. Obviously, the bathochromic shifted absorption spectra were
observed from acidic to basic conditions as shown in Fig. 2.
Next, the fluorescent titration experiments were investigated. There existed a
weak emission band at 574 nm upon excitation at isobestic point 494 nm.
Simultaneously, a new emission band at 692 nm was enhanced with the increase
of pH, during which the fluorescence intensity at 692 nm progressively increased,
and the intensity at 574 nm gradually decreased (Fig. 3). Additionally, the
maximum emission not only shifted by 118 nm but the color of the solution also
changed obviously from yellow to purple with protonation and deprotonation. As a
consequence, the ICT-based fluorescent chemosensor DCM–OH can monitor pH in
ratiometric applications, specifically as a ‘naked-eye’ pH sensor.
pKaÀpH ¼ log½ðAmaxÀAÞ=ðAÀAminފ
ð1Þ
As shown in Fig. 4, no absorption spectra change was observed at pH from 3 to 6.
At a higher pH, however, the absorbance at 450 nm clearly decreased, along with an
increasing broad band at 550 nm. In contrast, upon alkalization from pH 7 to 11, the
absorption band of DCM-OH was red-shifted about 100 nm. The plots of
absorbance versus pH exhibit S-shaped calibration graphs and the acidity constant
pK is calculated to be 7.18 by Henderson–Hasselbalch Eq. (1) based on the titration
a
data [7, 17], where A is the observed absorbance at a fixed wavelength, and Amax and
Amin are the maximum and minimal absorbance, respectively.
pKaÀpH ¼ log½ðFmaxÀFÞ=ðFÀFminފ
ð2Þ
The determined pK from Eq. 2 was also verified by fitting the individual
a
fluorescence titration curve. The pK value of 7.21 was almost in agreement with the
a
data calculated from absorption changes, where F is the observed fluorescent
intensity at a fixed wavelength, and Fmax and Fmin are the maximum and minimal
fluorescent intensity, respectively. As shown in Fig. 5, below pH 7 and above pH 9,
the intensity remained almost constant. The fact is that there were no concentration
Fig. 2 Changes in absorption
spectra of DCM-OH
-
5
(
H
0
1.0 9 10 M in C
O, v/v = 6:4) titrated by
.01 M HClO and 0.01 M
NaOH in the pH range of
.08–11.00
2 5
H OH/
2
4
3
1
23

Dicyanomethylene-4H-pyran-based NIR fluorescent…
Fig. 3 Changes in fluorescence
spectra of DCM-OH
-
5
(
H
0
1.0 9 10 M in C
O, v/v = 6:4) titrated by
.01 M HClO and 0.01 M
NaOH in the pH range of
.08–11.00 (excited at the
2 5
H OH/
2
4
3
isobestic point of 483 nm)
Fig. 4 Plots of absorption (A550
and A450 are the absorbance at
wavelengths of 550 and 450 nm,
respectively)
Fig. 5 Plots of emission (I692
and I574 are the emission
intensity at wavelengths of 692
and 574 nm, respectively)
effects in the experiments for that pH range. Moreover, it is obvious that, in the
transition range (pH 7–9), there should be a drastic change in the emission intensity.
In addition, an obvious ratiometric fluorescent signal (I692/I574) is observed (Fig. 6)
which provides us with another way to monitor pH. Upon deprotonation, the
fluorescence intensity enhancement (FE = I692/I574) is calculated by the emission
maximum at 692 nm to that at 574 nm. The ratio of I692/I574 recorded in the
examined pH interval (from 8 to 11) is more than 20 times and the maximum FE is
7
1
9. From acidification to alkaline media, the maximum emission shift is about
18 nm. The Stokes shift remained almost constant above pH 8. Correspondingly,
the solution changes obviously from yellow to purple. Figure 7 shows absorption
123

J. Yang et al.
Fig. 6 The ratio of the
fluorescence intensity at 692 nm
to that at 574 nm with various
pH
-5
Fig. 7 a The color changes of DCM-OH (1.0 9 10 M in C
HClO
indicator paper dipping into 0.01 M HClO
2
H
OH/H O, v/v = 6:4) titrated by 0.01 M
5 2
4
and 0.01 M NaOH in the pH range of 3.08–11.00. b Visible color changes of DCM-OH by
(a) and 0.01 M NaOH solution (b) for 3 min
4
photographs of DCM-OH with protonation and deprotonation. These results
indicate that the dye DCM-OH is an excellent ratiometric fluorescent chemosensor
[
47, 48], and also a ‘‘naked-eye’’ NIR probe for determining pH changes. However,
the system is operable in EtOH/H O (60/40 v/v), and such a high level of organic
2
solvent could weaken the system in practical applications.
It is known that DCM moiety, with a typical D-p-A system, possesses an
excellent ICT effect. Initially, DCM-OH possesses an ICT structure with absorption
at kmax 450 nm and emission spectra at kmax 574 nm. The alkali state is a ‘‘push–
pull’’ p electron system with the oxygen anion as a stronger donor than the phenolic
group, and DCM as an acceptor. Deprotonation creates a stronger phenolate donor,
and DCM-OH exhibits an absorption band at kmax 550 nm and an emission band at
kmax 692 nm along with a strong fluorescent intensity. The remarkable changes in
both absorption and fluorescence spectra should be mainly attributed to the intensive
ICT process resulting from a stronger efficient electron-push capacity of the oxygen
anion (Fig. 8). Moreover, the structural change is taking place in the pH range of
7–8. Furthermore, the fluorescent intensity at kmax 692 nm increases to the
maximum within 3 min and then stays stable during the testing time. As a result, the
fluorescence can be repeatedly modulated by pH.
Finally, the HOMO and LUMO frontier orbitals of DCM-OH are shown in
Fig. 9. With the DFT calculation, the separation energy (De = eLUMO - eHOMO) is
5
.05 eV. It is obvious that the orbit modeling of DCM-OH is the ‘trans-’
configuration. The conjugation is clearly predicted by the calculations of HOMO,
since the electron density is mainly centered at the conjugated system by benzene
1
23

Dicyanomethylene-4H-pyran-based NIR fluorescent…
Fig. 8 Conversion mechanism of DCM-OH
Fig. 9 Frontier molecular orbitals of DCM-OH
antenna. The LUMO, as expected, spread over the whole molecule backbone
including the benzene moiety.
Conclusions
We have developed a ratiometric NIR fluorescent chemosensor for determining pH
changes over a wide pH scale. The large Stokes shift from both absorption and
emission spectra makes DCM-OH available to the NIR region. The proposed
strategy by modulation of pH via protonation or deprotonation provides a promising
methodology for design of ratiometric fluorimetry. Furthermore, DCM-OH displays
-
a color change from yellow to purple upon the addition of OH , which can serve as
a ‘‘naked-eye’’ chemosensor. Meanwhile, for the D-p-A featured DCM-OH, the
deprotonation can increase the ICT process comntributing to a stronger electron-
push capacity of the oxygen anion than the phenolic hydroxyl group. Correspond-
-
ingly, the DFT calculation demonstrates that a stronger ICT process from DCM-O
is possuible. As demonstrated, DCM-OH is a promising colorimetric pH sensitive
chemosensor in the NIR region.
Acknowledgements This work was supported by Natural Science Foundation of China (21788102, and
21607044), and Natural Science Foundation of Hebei Province (B2017502069).
123

J. Yang et al.
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