Ratiometric fluorescent pH probes based on aggregation-induced emission-active salicylaldehyde azines

Article abstract of DOI:10.1039/c4nj01908c

A series of luminescent salicylaldehyde azines (SAs) containing different electron-accepting substituents (-NO₂, -F, and -Cl), electron-donating substituents (-OMe and -NEt₂), and a π-extended system (naphthalene ring) are prepared for the application of fluorescent pH probes. These SAs inheriting the aggregation-induced emission (AIE) features display strong blue, green, and red fluorescence with large Stokes shifts in water and solid medium. Combining the advantages of AIE and the chemical reactivity of phenol towards OH^-/H⁺, most of the SAs can be used as ratiometric fluorescent pH probes with a broad pH range (2-14) in water and solid medium (test paper). Moreover, the inherent relationship between their chemical structures and AIE properties/pK_a values (7.5-13.3) is studied, which provides unequivocal insights into the design of AIE-active dyes and their applications. This journal is

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Journal Name
RSCPublishing
ARTICLE
Ratiometric fluorescent pH probes based on
aggregation-induced emission-active salicylaldehyde
azines
Citethis:DOI:10.1039/x0xx00000x
Xiaofeng Ma, Jinghui Cheng, Jiaoyan Liu, Xiangge Zhou and Haifeng Xiang*
Received 00th January 2012,
Accepted 00th January 2012
A series of luminescent salicylaldehyde azines (SAs) containing different electronꢀaccepting
substituents (−NO2, −F, and −Cl), electronꢀdonating substituents (−OMe and −NEt2), and πꢀ
DOI: 10.1039/x0xx00000x
extended system (naphthalene ring) are prepared for the application of fluorescent pH probes.
These SAs inheriting the aggregationꢀinduced emission (AIE) features display strong blue,
green, and red fluorescence with large Stokes shifts in water and solid. Combining advantages
of AIE and chemical reactivity of phenol towards OH−/H+, most of the SAs can be used as
www.rsc.org/
ratiometric fluorescent pH probes with a broad pH range (2
Moreover, the inherent relationships between their chemical structures and AIE
properties/ Ka values (7.5 13.3) are studied, which provide unequivocal insights for the
design of AIEꢀactive dyes and their applications
−14) in water and solid (test paper).
p
−
.
now, this kind of probes are much scarcer,6 because a
fluorescent dye containing only one kind of pHꢀsensitive
segment usually would tend to quench or enhance fluorescence,
whithout ratiometric fluorescence after deprotonation or
protonation.
Introduction
Protons are one of the most important targets of interest,
because many biological and geochemical processes occurring
in freshwater, seawater, and marine sediments involve strong
pH changes. Accordingly, the recognition and detection of pH
values have been an especially active research area. The most
traditional pH measurement is glass electrode, however, it has
some obvious disadvantages including single point
measurements and bulky and invasive properties for the bioꢀ
applications.1 Recently, fluorescent pH probes have attracted
considerable interests, because fluorescence detections allow
noninvasive measurements with biological objects, parallel
monitoring of multiple samples, and imaging.2 The most
common fluorescence detection is the measurement of
fluorescence intensity, but, in most cases, its determination
accuracy is frequently compromised by ambient or scattered
light, instrumental fluctuation, and background fluorescence.
Therefore, ratiometric fluorescent measurement is more
desired.3 Moreover, it usually provides the perceived colour
change, which would be useful for the rapid visual sensing. In
general, most of the reported ratiometric fluorescent pH probes
employ mixture of different fluorescent dyes4 or a single
fluorescent dye containing different multiple pHꢀsensitive
segments.1,5 Both of them might have some shortcomings, such
as unequal stability, reliability, and photobleaching of the
different dyes or segments. Another simpler and more efficient
method is to design and synthesize a single fluorescent dye that
has only one kind of pHꢀsensitive segment that can emit light at
different wavelengths after deprotonation or protonation. Up to
For pH detection applications, the preferred work conditions
are aqueous media, but it is incongruous that most organic
fluorescent dyes are hydrophobic aromatics that are barely
soluble in water. Introduction of hydrophilic groups would
improve their aqueous solubility improved, meanwhile, the
resulting amphiphilic dyes would be ready to aggregate in
water and lead to fluorescence quenching by the aggregationꢀ
caused quenching (ACQ) effect. Tang et. al. recently
discovered an exactly opposite phenomenon of aggregationꢀ
induced emission (AIE).7 Since then a large number of AIEꢀ
active dyes have been developed and used in organic lightꢀ
emitting diodes, bio/chemosensors, bioimaging, and so on.8
Our previous work focused on the synthesis, photophysical
properties, and sensing applications of salicylaldehydeꢀbased
Salen Schiff bases (Scheme 1).9 Most of these Salen Schiff
bases usually exhibit much stronger fluorescence in organic
solution than in solid or water, indicating the possible existence
of ACQ. On the other hand, salicylaldehyde azines (SAs)
(Scheme 1) explored by Tong et. al. have similar synthetic
process and chemical structures with Salen Schiff bases but
exhibit AIE effects.10 Unlike the most common AIEꢀactive dyes
of tetraphenylethene (TPE), SAs have some unique properties:
(1) two salicylaldimine moieties are connected by rotatable
N
−
N single bond rather than C
−
C bond; (2) the rotations are
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restricted in the N
−N bond only by intramolecular hydrogen depicted in Fig. 2. At room temperature, the dilute solution SA
bonds;10a (3) the intramolecular hydrogen bonds lead to keto in MeCN emits very weak fluorescence (Fig. 2a) with a large
and enol tautomers for SAs through an ultrafast excitedꢀstate Stokes shift and low quantum yield (
intramolecularꢀproton transfer (ESIPT) process.11 Furthermore, be contributed to the rotation of N
SAs have been used as fluorescent probes for the detection of ESIPT process).11 This ESIPT process is further confirmed by
Zn2+,12 Co2+,12b Cu2+, Fe3+,13 F−,14 CN−,15 protein,16 and the pronounced solvatochromic effect (Fig. S1 in Supporting
heparin.17 At present, there are only limited examples of the Information). However, if water was added to MeCN, the
AIEꢀactive dyes for fluorescent pH sensing applications in the aggregate form of SA will be generated to eliminate the above
Φ
of 0.002, which might
−
N single bond and the
literature.5a,18 In this work, we have systematically investigated effects, which results in AIE with a much higher
the fluorescent pH sensing properties of a series of AIEꢀactive volume fraction ( ) of water of 92 % (Fig. 2b). Moreover,
SAs containing different electronꢀaccepting (EA) substituents absorption spectrum of SA in MeCN/water ( = 92 %) shows an
Cl ), electronꢀdonating (ED) substituents obvious tail in the visible region, indicating the formation of
NEt2), and
ꢀextended system (naphthalene ring) aggregate nanoparticles.10a
Φ of 0.11 at a
f
f
(
(
−
−
NO2,
OMe and
−F, and −
−
π
(Scheme 1), which provide a new paradigm in the design of
ratiometric fluorescent pH probes. The photophysical properties
of most SAs have not been examined and 3-F has not been
reported.
H
O
N
N
N
N
H
O
OH HO
Salen
SA
F
OH
O2N
OH
OH
N
N
N
N
F
HO
HO
HO
NO2
3-F
3-NO2
Fig. 1 Photographs (top: under sunlight; bottom: under 360 nm UV light) of the
MeO
OH
SAs in solution (left: MeCN; right: MeCN/water) and solid.
Cl
N
N
N
N
OMe
HO
N
Cl
3-OMe
3-Cl
OH
(a) 0.30
SA in MeCN
SA in MeCN/H2O
NEt2
OMe
N
Et2N
MeO
HO
OH
0.20
0.10
0.00
N
N
4-NEt2
HO
OH
N
N
HO
Naph
4-OMe
Scheme 1 Chemical structures of Salen and SAs.
300
400
500
600
700
Results and discussion
Wavelength (nm)
Synthesis and characterization
f
0
(b)
150
Emission intensity
at 536 nm
10 %
20 %
30 %
40 %
50 %
60 %
70 %
80 %
90 %
92 %
94 %
96 %
98 %
The general method of preparation of SAs is quite
straightforward and consists of the condensation reaction of
primary hydrazine with 2 equiv of salicylaldehyde precursor in
ethanol under refluxing conditions, according to the previous
report.19 Most SAs have good solubility in organic solvents
including DMSO and MeCN but poor solubility in water.
100
50
0
0
20
40
60
80 100
f (%)
Photophysical and AIE properties
Among these SAs, the photophysical properties of SA 10a
,
4-
20
NEt2
,
and Naph12b have been examined and reported. As
450
500
550
600
650
700
shown in Fig. 1 and Table 1, except 3-OMe and 4-NEt2, all
other SAs exhibit obvious AIE effects that the fluorescence in
organic solvent is much weaker than that in solid or water. As
example, the absorption and emission spectra of SA are
Wavelength (nm)
Fig. 2 (a) Absorption and emission spectra of SA in MeCN (1.0 × 10−5 mol dm−3
)
and MeCN/H2O (1.0 × 10−5 mol dm−3, f = 92 %). (b) Emission spectra of SA
2 | J. Name., 2012, 00, 1ꢀ3
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MeCN/H2O with different
at 536 nm with different
f (
1.0 × 10−5 mol dm−3); insert: plot of emission intensity
f
.
Table 1 Photophysical data of the organic molecules at room temperature.
abs /nm (ε /dm3 mol−1 cm−1)
λ
em /nm
509
536
537
543
565
566
518
547
550
513
564
565
406
ꢀa
Φ
0.002
0.11
Medium
pKa
f /%
λ
SA
MeCN
water
solid
MeCN
water
solid
MeCN
water
solid
MeCN
water
solid
MeCN
water
solid
MeCN
water
solid
MeCN
water
solid
MeCN
water
solid
292 (2.53×104); 354 (2.15×104)
296; 400
9.3
92
3-NO2
3-F
270 (2.19×104); 364 (1.29×104); 504 (9.50×103)
395
0.004
0.04
7.5
8.2
8.5
ꢀa
90
92
94
ꢀ
300 (3.76×104); 350 (2.11×104)
402
0.002
0.05
3-Cl
302 (3.01×104); 354 (1.97×104)
302; 398
0.003
0.13
3-OMe
4-OMe
4-NEt2
Naph
302 (3.01×104); 354 (1.97×104)
316; 408
0.004
ꢀa
ꢀa
308 (1.55×104); 362 (4.71×104)
300; 340
488
507
508
480
513
529
510
530
536
0.003
0.16
9.5
13.0
13.3
92
90
80
416 (8.07×104); 432 (7.59×104)
408; 426; 456
0.13
0.10
0.30b
0.004
0.18
330 (1.51×104); 406 (3.26×104); 424 (2.74×104)
334; 428; 462
a nonemissive; b from Ref. 20a
The absorption and emission spectra of other SAs in MeCN achieve high
are shown in Fig. S2 and S3. Since, except 4-NEt2, all other short interplanar distance (
Φ
, AIEꢀactive dyes should stack closely with a
) and weak intermolecular faceꢀtoꢀ
d
SAs in MeCN have very weak fluorescence (Table 1), the face π−π interactions. The former can ensure to eliminate the
discussion will be focused on their AIE characteristics. As molecular rotation, the later would prevent the formation of
shown in Fig. 3, for aggregate SAs in MeCN/water and solid, excimer and consequently enhance fluorescence.8c,20 In order to
different substituents have different effects on the emission investigate the molecule arrangements, the xꢀray single crystals
peek (
and Naph containing ED substituents or
show blueꢀshifted emission, on the contrary, 3-F
λ
em). Compared with the simplest SA
,
4-OMe
ꢀextended system All atoms in SA are located in one plane of πꢀconjugated
3-Cl, and 3-NO2 system (Fig. 4a). As expected, the intramolecular hydrogen
,
4-NEt2
,
of SA, 4-OMe and 3-OMe are depicted in Fig 4, S4, and S5.
π
,
containing EA substituents display redꢀshifted emission. This bonds (1.90 Å) between H atom in –OH and N atom are found
substituent effect is a simple and useful tool to achieve red, in SA. Moreover, rodꢀlike SA molecules are cross stacking with
green, and blue emission (Fig. 1).
a short
d
of
~
3.45 Å (Fig. 4c). The above two factors ensure to
N and C C single bond and result
eliminate the rotation of N
−
−
3-Cl
3-F
Naph
4-NEt
SA
in AIE thereby. However, it should be noted that there still are
weak intermolecular faceꢀtoꢀface π−π interactions (overlaps in πꢀ
conjugated systems) between two neighbour SA molecules (Fig.
4-OMe
2
3-NO2
in MeCN/H2O
4b), which might be one possible reason for its moderate
Φ
(0.11) 4-OMe molecules have a similar (~3.46 Å) (Fig. 4e) but
.
d
weaker faceꢀtoꢀface π−π interactions (Fig. 4d), and thus 4-OMe
has a higher Φ (0.16) than SA. On the other hand, 3-OMe
molecules have a shorter
faceꢀtoꢀface π−π interactions (Fig. 4f), resulting in its weak
fluorescence
d (~3.30 Å) (Fig. 4g) but much stronger
in solid
.
Another interesting phenomenon is the fact that only 4-NEt2
displays strong fluorescence in both dilute MeCN and
aggregate states. In the previous work,20b the strong
fluorescence in solution was attributed to the intramolecular
500
550
600
650
Wavelength (nm)
Fig. 3 Normalized emission spectra of SAs in water and solid.
charge transfer (ICT) between D
NEt2 substituent and EA salicylaldimine moiety, However,
this can’t explain the fact that 4-OMe has strong ED
−
A system21 of the strong ED
−
It is interesting that only aggregate 3-OMe is nonemissive.
The molecule arrangements play a key role in AIE. In order to
substituents of
−
OMe as well, but it exhibits weak fluorescence
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in dilute MeCN. It was reported that 4-NEt2 powder has two
distinctive crystalline lattices with different em at 553 ( = 3.53
Å) and 529 nm
6.32 Å),20a which are consistent with our
results. The NEt2 substituents in 4-NEt2 are much bulkier than
the OMe substituents in 4-OMe, which might lead to reducing
self quenching and consequently strong fluorescence in both
λ
d
(d =
−
−
solution and solid for 4-NEt2
.
Fig. 4 Xꢀray single crystal structures of SA (a: molecule arrangement; b: top view
of faceꢀtoꢀface π−π interactions; c: side view of faceꢀtoꢀface π−π interactions), 4-
OMe (d and e), and 3-OMe (f and g).
As shown in Table 1, Naph has a highest Φ of 0.18 in
MeCN/water. Naph has a similar molecule arrangement with SA
(Fig. 5). The
d
(~3.10 Å) (Fig. 5c) of Naph molecules is the
4 | J. Name., 2012, 00, 1ꢀ3
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smallest. Moreover, unlike 3-OMe molecules, Naph molecules
almost have no faceꢀtoꢀface π−π interactions (Fig. 5b). Therefore, in
order to enhance AIE,
d should be short and faceꢀtoꢀface π−π
interactions have better be weak, even though sometimes they
are contradictory. The best way to solve such problem is to
adopt cross molecular stacking like Naph. All the above
discussion provides unequivocal insights into the inherent
relationships between AIE properties and chemical structures
and useful tools for the design of AIEꢀactive dyes.
(b)
pH
pH
450
500
550
600
650
Wavelength (nm)
2.0
(c)
Fig. 5 Xꢀray single crystal structures of Naph (a: molecule arrangement; b: top
view; c: side view).
1.5
1.0
0.5
0.0
Fluorescent pH probes
It is wellꢀknown that phenol is a weak acid with a pKa of about
10.22 Our previous work9d demonstrated that salicylaldehyde
derivatives can be used as fluorescent pH probes, however, the
measurement is not ratiometric fluorescence detection but turnꢀ
on or turnꢀoff fluorescence detection.
4
6
8
10
12
14
pH
The absorbance and fluorescence intensity of these SAs
highly depend on the pH values of the solution, and thus they
can be used as pH probes. The pH responses were measured in
the mixed solvents of MeCN/aqueous BrittonꢀRobinson (BꢀR)
buffer solution (a mixture of 0.04 mol/L H3BO3, H3PO4 and
CH3COOH in water). The concentration of SAs is 1.0 × 10−5 mol
dm−3 and the f is listed in Table 1. As example, the absorption
(Fig. S6) and emission (Fig. 6) properties of 3-Cl are strongly
dependent on pH values. The red emission intensity at 565 nm
(I565) that belongs to the protonated 3-Cl keeps constant at pH <
7.0. And then it reduces along with the increase of pH values
from 7.0 to 11.0 and finally keeps constant again at pH > 11.0.
At the same time, blueꢀgreen I515 belonging to the deprotonated
3-Cl has a totally adverse pH response, which is zero,
Fig. 6 Photographs (a) (top: under room light; bottom: under 360 nm UV light),
emission (b) spectra, and plots (c) of I565 and I515/I565 versus pH value in BꢀR
buffer solution for 3-Cl (1.0
× f = 94 %).
10−5 mol dm−3 and
Among these SAs, 3-Cl
ratiometric fluorescence responses to pH values (Fig. 7). Since
the λem difference (ꢀλem = 50 nm) between protonated (λem
565 nm) and deprotonated 3-Cl λem = 515 nm) is bigger than
that of SA ꢀλem = 23 nm, Fig. S7), 3-F ꢀλem = 38 nm, Fig.
S8), and 4-OMe ꢀλem = 16 nm, Fig. S9), 3-Cl is more suitable
, SA, 3-F, and 4-OMe exhibit
=
(
(
(
(
for ratiometric detection.
1.2
1.5
1.0
0.5
0.0
increased, and constant at pH < 7.0, 7.0−11.0, > 11.0,
respectively (Fig. 6b). Moreover, the ration between I515 and
I565 exhibits adverse pH responses with I565 (Fig. 6c). Therefore,
3-Cl provides not only a ratiometric method of detection but
also a perceived colour change (red to blueꢀgreen) for rapid
visual sensing (Fig. 6a).
SA
0.8
3-F
0.4
4
8
pH
12
4
8
pH
12
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1.0
0.5
pH
pH
3-NO
2
3-NO
2
4-NEt
2
4-OMe
0.0
4
8
12
4
8
pH
12
pH
500
550
600
650 450
500
550
600
Wavelength (nm)
Fig. 8 Emission spectra of 3-NO2 and 4-NEt2 at different pH value in BꢀR buffer
solution (1.0 = 90 %).
10−5 mol dm−3 and
×
f
4-NEt2
Naph
Except 3-OMe, the pKa values (7.5−13.3) of these SAs
(Table 1) were calculated by the fluorescence method. A
tendency of pKa is observed in the order of 3-NO2 (pKa = 7.5) <
3-F (pKa = 8.2) < 3-Cl (pKa = 8.5) < SA (pKa = 9.3) < 4-OMe
(pKa = 9.5) < 4-NEt2 (pKa = 13.0) < Naph (pKa = 13.3), which
is consistent with the fact that EA and ED substituents cause
the acidity to increase and reduce, respectively, as previously
mentioned. Understanding these inherent relations between
chemical structures and pKa values would be helpful to design
pH probes.
4
8
pH
12
4
8
pH
12
Fig. 7 Emission intensity versus pH value in BꢀR buffer solution for SAs (1.0
10− mol dm− and
ratiometric measurement, respectively).
×
5
3
f
is listed in Table 1; black and red points for intensity and
3-OMe has too weaker fluorescence to be used as fluorescent
probe. 3-NO2, 4-NEt2 (Fig. 8), and Naph (Fig. S10) serve as
turnꢀoff fluorescent pH probes but not ratiometric fluorescent
probes (Fig. 7), because they are nonemissive after
deprotonation. For example, the I566 of 3-NO2 keeps constant,
decreasing, and quenching totally at pH < 5.1, 5.1−10, > 10,
respectively, revealing that the pKa of 3-NO2 is 7.5 (Fig. 8). It is
worthwhile that the fluorescence of 4-NEt2 and Naph keeps
constant at pH 3−10 and 3−11, respective, and then decreases
upon alkalinity increase, which indicates that they can be used
as fluorescent pH probes even at very high pH values. In the
literature, such fluorescent probes for the detection of high pH
values are still scarce. The pH values of the different
subcellular compartments are 4.7
are 7.5 13.3. And thus some of them can be potentially used in
intracellular pH sensing. The pH probes with high pKa of 9 12
Our previous work demonstrated a salicylaldehydeꢀbased
Salen Schiff base that has a similar chemical structure with SAs
shows multiple different pHꢀsensitive segments including –OH
and N=C.9d For SA, as expected, adding base would lead to the
deprotonation of –OH, which can be confirmed by the 1H
nuclear magnetic resonance (1H NMR) analysis (Fig. 9). The
1H NMR signals of SA
to the OH disappear after adding base. On the contrary, there
(δ = 11.15 ppm) in (CD3)2SO belonging
−
is no obvious change in the 1H NMR signals if acid was added,
which is consistent with the fact that the emission spectra of SA
shows little change upon adding acid. These results reveal that,
in our case, SA has only one type of pHꢀsensitive segment (–
OH) to achieve ratiometric detection.
−8. The pKa values of the SAs
−
−
might be used in some special applications, such as waste water.
It is wellꢀknown that the pKa of phenol is about 10.22 SA (pKa
= 9.3) with the presence of EA salicylaldimine moiety is more
acid than phenol, because the inductive effect of EA moiety
reduces the electron density of benzene ring, resulting in that
the negative charge of phenolic hydroxyl will delocalize into
the benzene ring. Consequently, the resultant phenolate anion is
more stable and readily releases proton. On the other hand, ED
substituents stress an opposite effect to decrease the acidity.
Fig. 9 1H NMR spectra of SA (bottom) and deprotonated SA (top, adding 2 equiv.
of NaOH in D2O) in (CD3)2SO.
6 | J. Name., 2012, 00, 1ꢀ3
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The previous work10a observed that SA molecules aggregate deprotonated 3-Cl is a traditional organic dye rather than an
in water to form nanoparticles (<5 nm), which reveals SA AIEꢀactive dye, its fluorescence in solid is redꢀshifted
molecules are not dissolved but disperse in water. One of compared with that in the dilute solution.
problems of these nanoparticles without any surface protective
agents is that the disperse nanoparticles are not stable and
would precipitate through further aggregation. Therefore, the
dynamic stability of 3-Cl nanoparticles in MeCN/water was
examined (Fig. 10). After adding water to the MeCN solution
of 3-Cl, the emission of 3-Cl reaches saturation (> 95 %) in 15
minutes, indicating that 3-Cl molecules aggregate to form
nanoparticles very quick. Moreover, the nanoparticles are stable
at least 150 minutes, which is enough for sensing applications.
1.0
Fig. 11 Colour changes (top: under room light; bottom: under 360 nm UV light)
of homemade 3-Cl test papers at different pH values.
1.00
0.96
0.92
0.8
0.6
0.4
0.2
0.0
Conclusion
We have systematically synthesized and studied the
photophysical properties and pH sensing applications of a series
of salicylaldehyde azines. The chemical structures of these
salicylaldehyde azines have a significant influence on their
molecule arrangements and consequently aggregationꢀinduced
emission properties. Strong cross stacking of molecule
0
30
60
90
0
30
60
90
120
150
t (min)
3
Fig. 10 Kinetics of I565 for 3-Cl in MeCN/water (1.0
×
10−5 mol dm−
,
f
= 94 %).
arrangement with a small
interactions is a key factor to enhance aggregationꢀinduced
emission. 3-Cl 3-F, and 4-OMe exhibit aggregationꢀinduced
d and weak faceꢀtoꢀface π−π
,
pH test paper
emission at low pH values, while they become water soluble
and show blueꢀshifted fluorescence at high pH values, and thus
they can be used as ratiometric fluorescent pH probes with a
broad pH range in water and solid (test paper). Moreover, their
One of the best superiorities of AIEꢀactive dyes is the nature of
strong fluorescence in solid, and thus we tried to use 3-Cl as pH
test papers. The homemade test papers were fabricated by
dipping filter papers into the MeCN solution of 3-Cl. When the
solvent evaporates, the adsorbed 3-Cl molecules in the test
paper aggregate to emit strong red fluorescence like that in
MeCN/water solutions (Fig. 11). This aggregation is also
confirmed by the fact that the red fluorescence don’t distribute
uniformly in the test paper under UV irradiation. When the test
papers are dipped into aqueous BꢀR buffer solution with
different pH values, they exhibit different color changes under
room light and 360 nm UV light, respectively. Under room
light, the color of test papers changes from white into yellow at
high pH values, which accords with the absorption
enhancement at high pH values (Fig. S6). Under UV irradiation,
the test papers appear a much more obvious color change. The
test papers display red, yellow, and green fluorescence at pH
p
Ka values ranging from 7.5 to 13.3 can be varied and well
tuned by their chemical structures. Therefore, we believe that
these simple salicylaldehyde azines provide a new paradigm in
the design of aggregationꢀinduced emissionꢀactive dyes for
some useful sensing applications.
Experimental section
Materials and instrumentation
All reagents were purchased from commercial suppliers and
used without further purification. UV/visible absorption spectra
were recorded using a UV 765 spectrophotometer with quartz
cuvettes of 1 cm pathlength. Fluorescence spectra were
obtained using an Fꢀ7000 Fluorescence spectrophotometer
(Hitachi) at room temperature. The slit width was 2.5 or 5.0 nm
for both excitation and emission. The photon multiplier voltage
was 400 V. All SAs were prepared according to the previous
reports.19 The Xꢀray structures of SA,23a 3ꢀOMe,23b 4ꢀOMe,23c
and Naph23d were previously reported.
values of 6−9, 10−11, and 12−14, respectively. It should be
noted that the test papers emit green fluorescence at high pH
values, at the same condition, however, the MeCN/water
solutions display blueꢀgreen fluorescence (Fig. 6a). At high pH
values, 3-Cl would be deprotonated to form phenolate
consequently, which has a better solubility and less AIE effect
than protonated 3-Cl in water. This less AIE effect is coincident
with the fact that the green fluorescence from test papers
containing deprotonated 3-Cl is more uniform (Fig. 11). Since
Measurement of fluorescence quantum yield (Φ)
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Φ was measured by the optical dilute method with a standard of N,N'-bis(3-methoxyl salicylidene) hydrazine (3-OMe).19 3-OMe
quinine sulfate (Φr = 0.55, quinine in 0.05 mol dm−3 sulfuric acid) was prepared by a similar procedure as that used for SA, but with 3ꢀ
calculated by: Φs = Φr (Br/Bs) (ns/nr)2 (Ds/Dr), where the subscripts s methoxyl ꢀsalicylaldehyde (10 mmol, 1.13 g) to give pale yellow
and
r
refer to the sample and reference standard solution solid 1.08 g (75 % yield); 1H NMR (DMSOꢀd6, 400 MHz): δ (ppm)
respectively; n is the refractive index of the solvents; D is the 10.90 (s, 2H), 9.00 (s, 2H), 7.28–7.31 (m, 2H), 7.12–7.15 (m, 2H),
integrated intensity. The excitation intensity B is calculated by: B = 6.90–6.94 (m, 2H), 3.84 (s, 6H). Anal. Calcd (found): C, 63.99
1 – 10–A L, where A is the absorbance at the excitation wavelength (63.78); H, 5.37 (5.14); N, 9.33 (9.14). ESIꢀMS, m/z 300.
and L is the optical path length (L = 1 cm in all cases). The
N,N'-bis(4-methoxyl salicylidene) hydrazine (4-OMe).19 4-OMe
was prepared by a similar procedure as that used for SA, but with 4ꢀ
refractive indices of the solvents at room temperature are taken from
standard source. Errors for Φ values (± 10%) are estimated.
methoxyl ꢀsalicylaldehyde (10 mmol, 1.11 g) to give pale yellow
Measurement of pH sensing
solid 1.08 g (74 % yield); 1H NMR (DMSOꢀd6, 400 MHz): δ (ppm)
11.52 (s, 2H), 8.87 (s, 2H), 7.54–7.56 (d, 2H), 6.57–6.59 (m, 2H) ,
6.52–6.54 (d, 2H), 3.81 (s, 6H). Anal. Calcd (found): C, 63.99
(63.91); H, 5.37 (5.23); N, 9.33 (9.25). ESIꢀMS, m/z 300.
The pH responses were measured in the mixed solvents of
MeCN/aqueous BꢀR buffer solution (a mixture of 0.04 mol/L H3BO3,
H3PO4 and CH3COOH in water). The concentration of SAs is 1.0 ×
10−5 mol dm−3 and the f is listed in Table 1. All types of absorption
and fluorescence measurement were monitored 1 hour after the
addition of the probe to the buffer solution at room temperature.
N,N'-bis(4-diethylamino salicylidene) hydrazine (4-NEt2).19 4-
NEt2 was prepared by a similar procedure as that used for SA, but
with 4ꢀdiethylaminoꢀsalicylaldehyde (10 mmol, 1.93 g) to give
1
yellow solid 1.38 g (72 % yield); H NMR (DMSOꢀd6, 400 MHz): δ
Synthesis
(ppm) 11.50 (s, 2H), 8.62 (s, 2H), 7.26–7.31 (d, 2H), 6.29–6.53 (d,
N,N'-bis(salicylidene) hydrazine (SA).19 30 ml ethanol of 2H), 6.12 (s, 2H), 3.19–3.25 (q, 8H), 1.09–1.14 (t, 12H). Anal. Calcd
aqueous hydrazine (5 mmol, 0.25 g) was added into 20 ml (found): C, 69.08 (69.17); H, 7.91 (7.87); N, 14.65 (14.60). ESIꢀMS,
ethanol of salicylaldehyde (10 mmol, 1.22 g) over a period of 1 m/z 382.2.
h, and then the mixture was refluxed for 4 h. The solution was
2,2'-dihydroxy-1,1'-naphthalazine (Naph).19 Naph was prepared
concentrated to give crude solid, which was recrystallized from
by a similar procedure as that used for SA, but with 2ꢀhydroxyꢀ1ꢀ
ethanol and dried to get 0.92 g (77 % yield) of compound as a
naphthaldehyde (10 mmol, 1.72 g) to give yellow solid 1.02 g (60 %
pale yellow solid; 1H NMR (DMSOꢀd6, 400 MHz): δ (ppm) 11.13
yield); 1H NMR (DMSOꢀd6, 400 MHz): δ (ppm) 12.88 (s,2H) , 10.00
(s, 2H), 9.01 (s, 2H), 7.69–7.72 (m, 2H), 7.38–7.43 (m, 2H), 6.95–
(s, 2H) , 8.65 (d, 2H) , 8.01 (m, 2H) , 7.92 (m, 2H) , 7.63 (m, 2H) ,
7.01 (m, 4H). Anal. Calcd (found): C, 69.99 (69.74); H, 5.03 (4.86);
7.45 (m, 2H) , 7.28 (m, 2H). Anal. Calcd (found): C, 77.63 (77.91);
N, 11.66 (11.64). ESIꢀMS, m/z 240.3.
H, 4.74 (4.77); N, 8.23 (8.28). ESIꢀMS, m/z 340.1.
N,N'-bis(3-fluoro salicylidene) hydrazine (3-F).19 3-F was
prepared by a similar procedure as that used for SA, but with 3ꢀ
fluoroꢀsalicylaldehyde (10 mmol, 1.40 g) to give pale yellow solid
1.01 g (73 % yield); m.p. 265 oC. 1H NMR (DMSOꢀd6, 400 MHz): δ
(ppm) 11.31 (s, 2H), 9.09 (s, 2H), 7.54–7.56 (d, 2H), 7.38–7.43 (m,
2H), 6.96–7.01 (m, 2H). 13C NMR (DMSOꢀd6, 400 MHz): δ (ppm)
119.76 120.02 121.13 126.68 146.95 151.38 163.37. Anal. Calcd
(found): C, 60.87 (60.74); H, 3.65 (3.69); N, 10.14 (10.04). ESIꢀMS,
m/z 276.1.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21172160 and 21372169).
Notes and references
College of Chemistry, Sichuan University, Chengdu, 610041, China. Fax:
(+86) 28ꢀ8541ꢀ2291. Eꢀmail: xianghaifeng@scu.edu.cn.
Electronic Supplementary Information (ESI) available: Photophysical
properties and applications of fluorescent pH probes of SAs. See
DOI: 10.1039/b000000x/
N,N'-bis(3-chloro salicylidene) hydrazine (3-Cl).19 3-Cl was
prepared by a similar procedure as that used for SA, but with 3ꢀ
chloroꢀsalicylaldehyde (10 mmol, 1.56 g) to give pale yellow solid
1.22 g (79 % yield); 1H NMR (DMSOꢀd6, 400 MHz): δ (ppm) 11.90
(s, 2H), 9.14 (s, 2H), 7.61–7.65 (m, 4H), 7.03–7.07 (m, 2H). Anal.
Calcd (found): C, 54.39 (54.28); H, 3.26 (3.14); N, 9.06 (9.04). ESIꢀ
MS, m/z 308.
1
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