Separately enhanced dual emissions of the amphiphilic derivative of 2-(2′-hydroxylphenyl) benzothiazole by supramolecular complexation

Article abstract of DOI:10.1039/c8sm00552d

Here, we report separately enhanced dual emissions of the amphiphilic derivative of 2-(2′-hydroxyphenyl)benzothiazole (denoted as HBT-11) by supramolecular complexation with cyclodextrins (CDs). When dispersed in water, HBT-11 shows two relatively weak emission bands, which can be assigned to the emissions of enol- and keto-forms, the two tautomers, owing to excited-state intramolecular proton transfers. Upon the addition of α-CD and β-CD, the keto- and enol-emissions, respectively, are separately enhanced; the enhancement effect is due to the formation of HBT-11/α-CD and HBT-11/β-CD complexes through multiple hydrogen bonding and host-guest interactions, respectively. It is worth to note that the keto-emission caused by the complex of HBT-11/α-CD has a much shorter wavelength compared with that of the aggregates formed by pure HBT-11. To the best of our knowledge, this is the first time that a study on keto-emission of the isolated HBT chromophore has been reported.

Full text of DOI:10.1039/c8sm00552d

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Separately enhanced dual emissions of the
amphiphilic derivative of 2-(20-hydroxylphenyl)
Cite this:DOI: 10.1039/c8sm00552d
benzothiazole by supramolecular complexation†
Chengfeng Wu, Yingzhi Jin, Dahua Li, Lan Ding, Yuzhi Xing, Kaicheng Zhang and
Bo Song
*
Here, we report separately enhanced dual emissions of the amphiphilic derivative of 2-(20-hydroxyphenyl)-
benzothiazole (denoted as HBT-11) by supramolecular complexation with cyclodextrins (CDs). When
dispersed in water, HBT-11 shows two relatively weak emission bands, which can be assigned to the
emissions of enol- and keto-forms, the two tautomers, owing to excited-state intramolecular proton
transfers. Upon the addition of a-CD and b-CD, the keto- and enol-emissions, respectively, are separately
enhanced; the enhancement effect is due to the formation of HBT-11/a-CD and HBT-11/b-CD
complexes through multiple hydrogen bonding and host–guest interactions, respectively. It is worth to
note that the keto-emission caused by the complex of HBT-11/a-CD has a much shorter wavelength
compared with that of the aggregates formed by pure HBT-11. To the best of our knowledge, this is the
first time that a study on keto-emission of the isolated HBT chromophore has been reported.
Received 17th March 2018,
Accepted 1st May 2018
DOI: 10.1039/c8sm00552d
rsc.li/soft-matter-journal
poor solvent (e.g., cyclohexane), it disperses in the form of
Introduction
aggregates and gives keto-emission.5 Notably, after being dis-
Chromophores with excited-state intramolecular proton trans- persed in a poor solvent, the emission becomes much stronger
fer (ESIPT)1–3 properties is a class of unique fluorogens, and with a big bathochromic shift due to the AIE effect; however,
they are known for large Stokes shifts (100–200 nm), low self- to the best of our knowledge, the keto-emission of isolated
absorption and dual emission features.4–6 Since first reported monomers has not been addressed in literature yet.
by Weller et al.,7 ESIPT has drawn considerable attention owing
Because of the transient transformations involved in the four-
to an intrinsic four-level photo-cycle process. Upon excitation, level photo-cycle process, HBT-based chromophores are sensitive
chromophores undergo a fast (in sub-picosecond time scale) to external stimuli26 such as pH,27–29 solvent polarity,30,31 tem-
intramolecular proton transfer and hence, molecular isomer- perature,32 and ion effects.17,33–38 Supramolecular assembly is
ization from enol- to keto-form occurs concomitantly; these two considered a powerful tool to modulate the fluorescence emis-
tautomers show distinctly different fluorescence, and the keto- sion of chromophores, which rely on inter- and intra-molecular
form gives a relatively longer wavelength emission due to the interactions.39–44 Through proper design of complexations, we
reduced lowest occupied molecular orbital.5
can even understand complicated emission processes.
2-(20-Hydroxyphenyl)benzothiazole (HBT) is known for its
To investigate the emission behavior of HBT-based chromo-
ESIPT property8–17 and recently, it has been extensively studied phores in water, we synthesized an amphiphilic derivative of
due to ‘‘aggregation-induced emission’’ (AIE),18–23 which can be HBT (denoted as HBT-11) having a terminal pyridinium group.
explained by the restriction of the intramolecular rotation When the concentration was lower than the critical micelle
between thiazole and hydroxyphenyl rings.24 It is reported that concentration (CMC),45 HBT-11 showed two emissive bands
the emission of HBT greatly depends on solvents;14,25 for at 385 and 446 nm, which were assigned to enol- and keto-
example, in a good solvent (e.g., THF), it appears in the form emissions, respectively, of isolated monomers. These two
of isolated monomers and mainly gives enol-emission and in a emissions were separately enhanced by complexation with
cyclodextrins (CDs)46 such as a-CD and b-CD.47 When the
concentration was higher than the CMC, a new fluorescence
peak appeared at 507 nm, which was assigned to the emission
of aggregated HBT-11, but this emission was regarded as the
College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Ren-ai Road 199, 907-1341, 215123 Suzhou, P. R. China.
E-mail: songbo@suda.edu.cn
keto-emission of the HBT moiety. The complexation between
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8sm00552d
HBT-11 and CDs resulted in the relatively reduced emission of
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the aggregates and enhanced emission of the isolated monomers.
This line of research supplies a method to separately modulate
the emissions of the two tautomers of HBT derivatives, and it
takes one step forward on the progress of understanding emitting
mechanisms of ESIPT chromophores.
Experimental
Materials
Anhydrous MgSO4, anhydrous Na2CO3, NaCl, NaHSO4, triethyl-
amine, thionyl chloride and pyridine were purchased from
Sinopharm Chemical Reagent Co., Ltd. Ethyl acetate, petroleum
ether, dichloromethane and methanol were purchased from
Chinasun Specialty Products Co., Ltd. CDCl3, D2O, DMSO-d6,
a-CD, 4-aminosalicylic acid and polyphosphoric acid (PPA) were
purchased from J & K Technology Co., Ltd. b-CD was bought
from TCI Development Co., Ltd (Shanghai). tert-Butyl dimethyl-
silyl chloride (TBDMSCl) and imidazole were bought from
Tokyo Kasei Kogyo Co., Ltd. 2-Aminothiophenol and 11-bromo-
undecanoic acid were purchased from Alfa Aesar Chemicals
Co., Ltd (China). Diethyl ether was obtained from Shanghai
Lingfeng Chemical Reagent Co., Ltd. Milli-Q water with resis-
tivity of 18 MO cm was produced by Direct-Q5uv manufactured
by Merck Millipore.
Scheme 1 Synthetic route of HBT-11.
6.15 (d, J = 2.1 Hz, 1H), 5.95 (s, 2H). MS (ESI): calculated for
C
13H10N2OS, m/z: 242.05; found, 243.05 [A + H]+.
Synthesis of compound B in Scheme 1. A mixture of com-
pound A (500 mg, 2.07 mmol), TBDMSCl (624 mg, 4.14 mmol)
and imidazole (282 mg, 4.14 mmol) was dissolved in 35 mL of
dry dichloromethane. The reactant was refluxed at 40 1C for 36 h
under the protection of nitrogen. After being cooled to room
temperature, the mixture was extracted with brine/dichloro-
methane for three times. The combined organic phase was dried
over anhydrous MgSO4, and the solvent was removed by a
rotavapor. The product was purified by column chromatography
on silica gel using dichloromethane/methanol (40 : 1 v/v) as the
eluent. Yield: 700 mg, 95%. dH (400 MHz, CDCl3, TMS): 8.21 (d,
J = 8.6 Hz, 1H), 8.00 (d, J = 8.2 Hz, 1H), 7.88 (d, J = 8.0 Hz, 1H),
7.44 (t, J = 7.6 Hz, 1H), 7.31 (t, J = 7.6 Hz, 1H), 6.42 (dd, J = 8.6,
1.7 Hz, 1H), 6.26 (d, J = 1.7 Hz, 1H), 3.90 (s, 2H), 1.02 (s, 9H),
0.39 (s, 6H). MS (ESI): calculated for C19H24N2OSSi, m/z: 356.14;
found, 357.14 [B + H]+.
Synthesis of compound C in Scheme 1. Thionyl chloride (5 mL)
was slowly added into a flask containing 11-bromoundecanoic
acid (148 mg, 0.56 mmol). The reaction was continuously refluxed
for 5 h. Remaining thionyl chloride was removed using a cold trap
in vacuum for 1 h and then, 11-bromoundecanoyl chloride was
obtained without further purification. To the above-mentioned
product, compound B (100 mg, 0.28 mmol) and 50 mL of
trimethylamine dissolved in 20 mL of dry dichloromethane were
Instruments
1H NMR spectra for molecular structure characterization were
recorded on Avance III 400 MHz produced by Bruker Co., Ltd
and 600 MHz NMR spectrometer manufactured by Agilent
Technologies. Mass spectra (MS) of the products were obtained
on a micro Q-TOF III mass spectrometer (Bruker Co.) in the
electrospray ionization (ESI) mode. Fluorescence spectra were
acquired on Hitachi F-2700 (Japan). Quantum yield (FF) measure-
ment was performed on a Fluorolog-3 spectrofluorometer with a
450 W Xe lamp as the light source. The time-resolved fluorescence
spectra measurement was conducted on FLS980 (Edinburgh
Instrument, U.K.). TEM images were obtained on Hitachi F-2700
(Japan) operating at 120 kV.
Synthesis
Synthesis of compound A in Scheme 1. 4-Aminosalicylic acid added. The mixture was stirred at 40 1C for 16 h under the
(1.53 g, 10 mmol) and PPA (50 mL) were added to a round protection of nitrogen. The resulting solution was neutralized
bottom flask, and the mixture was vigorously stirred for 10 min. with a saturated NaHSO4 solution, and it was extracted with
Then, 2-aminothiophenol (1.25 g, 10 mmol) was added under a dichloromethane for three times. The organic layer was dried
nitrogen atmosphere. After being stirred for 4 h at 180 1C, the over anhydrous MgSO4. After removing the solvent, the crude
solution was cooled to room temperature and then, the pH was product was purified on a silica-gel column using dichloro-
adjusted to 5.0 by a 10% Na2CO3 solution. The precipitate was methane/ethyl acetate (100 : 1 v/v) as the eluent. Yield: 50 mg,
filtered and washed with water. The solvent was removed under 37%. dH (400 MHz, DMSO-d6, TMS): 11.61 (s, 1H), 10.11 (s, 1H),
reduced pressure, and the crude product was purified by silica 8.14–8.05 (m, 2H), 8.00 (d, J = 8.2 Hz, 1H), 7.60 (s, 1H), 7.51 (t, J =
gel column chromatography using a dichloromethane/petro- 7.6 Hz, 1H), 7.41 (t, J = 7.6 Hz, 1H), 7.13 (d, J = 8.9 Hz, 1H), 3.51
leum ether mixture (4 : 1 v/v) as the eluent. A yellowish-brown (t, J = 6.7 Hz, 2H), 2.33 (t, J = 7.4 Hz, 2H), 1.85–1.70 (m, 2H), 1.63–1.56
powder (compound A) was obtained. Yield: 1.20 g, 50%. dH (m, 2H), 1.26 (s, 12H). MS (ESI): calculated for C24H29BrN2O2S,
(400 MHz, DMSO-d6, TMS): 11.71 (s, 1H), 8.02 (d, J = 7.4 Hz, m/z: 490.11; found, 491.10 [C + H]+.
1H), 7.87 (d, J = 7.7 Hz, 1H), 7.62 (d, J = 8.6 Hz, 1H), 7.53–7.40
Synthesis of HBT-11 in Scheme 1. Compound C (150 mg,
(m, 1H), 7.38–7.24 (m, 1H), 6.24 (dd, J = 8.6, 2.1 Hz, 1H), 0.31 mmol) was dissolved in 30 mL of pyridine. The mixture
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was refluxed for 24 h under nitrogen protection. After the new peak appeared at around 507 nm, corresponding to
solution was cooled to room temperature, the excess pyridine the emission of the aggregates formed by the self-assembly of
was removed under reduced pressure. The product was dis- HBT-11. The position of the new peak was close to that of the
solved in methanol and washed with diethyl ether for five times emission of the powder (the picture is shown in Fig. S9 in ESI†),
to get the target compound. Yield: 160 mg, 92%. dH (400 MHz, further confirming that this emission originated from the aggrega-
DMSO-d6, TMS): 11.61 (s, 1H), 10.11 (s, 1H), 9.08 (d, J = 5.8 Hz, tion of HBT-11. By plotting the fluorescence intensity at 507 nm
2H), 8.59 (t, J = 7.8 Hz, 1H), 8.15 (t, J = 7.0 Hz, 2H), 8.09 (dd, J = versus the corresponding concentrations (the corresponding spectra
13.1, 8.4 Hz, 2H), 8.00 (d, J = 8.1 Hz, 1H), 7.59 (s, 1H), 7.52 (t, J = are shown in Fig. S10 in ESI†), CMC of HBT-11 in an aqueous
8.0 Hz, 1H), 7.41 (t, J = 7.5 Hz, 1H), 7.13 (d, J = 8.9 Hz, 1H), 4.58 solution was determined. As shown in Fig. 1c, the linear fitting of
(t, J = 7.3 Hz, 2H), 2.33 (t, J = 7.3 Hz, 2H), 1.93–1.79 (m, 2H), the plot indicated a point of intersection at 2.3 ꢁ 10ꢀ4 mol Lꢀ1
1.67–1.51 (m, 2H), 1.27 (s, 12H). MS (ESI): calculated for which could be assigned to CMC of HBT-11.
,
C
29H34BrN3O2S m/z: 567.16; found, 488.23 [HBT-11 ꢀ Brꢀ]+.
In the previous study, it was reported that the short-
wavelength (around 400 nm) fluorescence emission in a good
solvent (e.g., THF, where HBT-based chromophores dissolve
in the form of isolated monomers) can be assigned to the
enol-emission, and the newly appeared long-wavelength (around
500 nm) fluorescence in a poor solvent48 (e.g., cyclohexane,
where it disperses in the form of aggregates) can be assigned to
the keto-emission due to ESIPT.49,50 To define these peaks,
the emissions of the HBT derivative without pyridinium salt
(compound C in the Scheme 1) in THF and cyclohexane were
measured. The fluorescence peaks were located at around 385
and 402 nm in THF and 507 nm in cyclohexane, as shown in
Fig. 1d. The peak located at around 507 in cyclohexane could be
assigned to the formation of aggregates, which was proved by
the Tyndall effect (as shown in Fig. S11 in ESI†). Cyclohexane
was a relatively poor solvent for compound C, and the emis-
sions of both monomers and aggregates were observed using
fluorescence spectra. After dispersing the compound in water by
re-precipitation (adding 1 portion of THF solution of compound
C into 9 portions of water), the peaks obtained at 385 and
402 nm were negligible, and the peak at 507 nm became very
intense. These results were comparable with those reported in
the literature. By combining the literature results with the above-
mentioned results regarding emissions of HBT-11 in aqueous
solutions, we assumed that the fluorescence peaks at around 446
and 507 nm could be assigned to the emissions of the isolated
keto-form and aggregates, respectively. The bathochromic shift
emission of the aggregates was possible because of the forma-
tion of static excimers. It is worth to note that the keto-emission
of the isolated HBT chromophore, to the best of our knowledge,
has not been addressed in the literature yet.
Results and discussion
In an aqueous solution, the emission of HBT-11 showed a
concentration-dependent feature, as shown in Fig. 1a. At low
concentrations, HBT-11 exhibited two relatively weak fluores-
cence peaks at around 385 and 446 nm, which could be assigned
to the emissions of the two tautomers, i.e., enol- and keto-forms,
respectively. A schematic illustration is presented in Fig. 1b, to
elucidate the four-level photo-cycle process and the emissions of
these two tautomers. A tautomeric transformation occurred
at the excited state, and the enol-form was converted to the
keto-form in a sub-picosecond time scale. Subsequently, the
keto-form underwent radiative emission to ground state and
then, it converted back to the initial enol-form via a reverse
proton transfer. Likewise, HBT-11 underwent fast intramolecular
proton transfer, and molecular isomerization from enol- to
keto-form occurred concomitantly. At a high concentration, a
In an aqueous solution, the fluorescence of HBT-11 is
relatively weak; however, upon the addition of a-CD and b-CD,
the keto and enol emissions can be enhanced, respectively.
As shown in Fig. 2a, when the concentration of HBT-11 is
5.0 ꢁ 10ꢀ5 mol Lꢀ1 (lower than CMC), the addition of a-CD
leads to an increase in fluorescence intensity, especially for the
emission band located at around 446 nm. In contrast, the
addition of b-CD mainly results in the intensity increase of
emission band located at 385 nm. The complexation between
HBT-11 and a-CD (or b-CD) segregates the HBT-11 molecules
and reduces the chance of its mutual collision, which results in a
decrease in non-radiative energy loss and an increase in the fluor-
escence intensity. When the concentration is 4.0 ꢁ 10ꢀ4 mol Lꢀ1
(higher than CMC), the situation is similar except for the
Fig. 1 (a) The normalized fluorescence spectra of HBT-11 in aqueous
solutions with concentrations of 5.0 ꢁ 10ꢀ5 and 4.0 ꢁ 10ꢀ4 mol Lꢀ1 and
HBT-11 powder. (b) Schematic representation of ESIPT photo-cycle of
the HBT derivative. (c) Plot of fluorescence intensity at 507 nm vs. the
concentration of HBT-11. (d) The normalized fluorescence spectra of
compound C in cyclohexane, THF/water (1 : 9, by volume) and THF
with a concentration of 5.0 ꢁ 10ꢀ5 mol Lꢀ1. The excitation wavelength
was 335 nm.
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Table 1 Photophysical properties of HBT-11 in the absence and presence
of CDs
FF (%)
t1 (ns)
f1 (%)
t2 (ns)
f2 (%)
HBT-11
+a-CD
+b-CD
1.0
3.7
3.4
1.07
1.45
1.23
99.4
11.3
25.8
6.56
2.76
2.74
0.6
88.7
74.2
fitted by bi-exponential decays. The two components t1 and t2
could be assigned to the non-radiative and radiative emissions,
respectively, and f1 and f2 were the corresponding fractions. As
for the pure HBT-11, f1 was 99.4%, which meant that the non-
irradiative emission was the dominant emission. Upon the
additions of a-CD and b-CD, f1 values decreased to 11.3% and
25.8%, respectively, and at the same time, f2 values increased to
88.7% and 74.2%, respectively, which suggested that the prob-
ability for irradiative emission was improved. These results
were consistent with the change in FF.
Separately enhanced dual emissions of HBT-11 can be due
to the complexation between HBT-11 and the two CDs. Both
a-CD and b-CD are composed of glucose residues17,46,52 and
hence, the reasons for the different emissions are the size of the
cavities. Next, two groups of experiments were carried out to
investigate the supramolecular complexation.
Fig. 2 Emission spectra of HBT-11 at the concentrations of (a) 5.0 ꢁ
10ꢀ5 mol Lꢀ1 and (b) 4.0 ꢁ 10ꢀ4 mol Lꢀ1 in the absence and presence of
a-CD and b-CD in aqueous solutions. Photographs showing the color
change from HBT-11 to HBT-11/a-CD and HBT-11/b-CD under the illumi-
nation of 365 nm light at the concentrations of (c) 5.0 ꢁ 10ꢀ5 mol Lꢀ1 and
(d) 4.0 ꢁ 10ꢀ4 mol Lꢀ1
.
appearance of a new emission at around 507 nm, as shown in
Fig. 2b. Notably, upon the addition of a-CD or b-CD, the inten-
sity of the fluorescence emission band located at around 507 nm
becomes relatively weaker than that of the other two emission
bands. The complexation between HBT-11 and a-CD (or b-CD) is
strong enough to break the self-assembled aggregates of HBT-11
in the aqueous solution; this can be proved by the relatively weak
intensity of the emission peak of the aggregates, which is
obtained at 507 nm, and the enhanced emission of the isolated
monomers upon the addition of these two CDs. The morpholo-
gical change in HBT-11 upon the addition of the two CDs can also
prove this point. Pure HBT-11 compounds form sheet-like struc-
tures, whereas in the presence of a-CD and b-CD, they exhibit
featureless and worm-like structures (Fig. S12 in ESI†). The
fluorescence enhancement effect can also be studied by the color
of the solution. As shown in Fig. 2c and d, under UV irradiation, the
colors of the aqueous solutions of HBT-11 are dark blue and olive
First, the association ratios were determined by Job’s plot.53 As
shown in Fig. 3a, the fluorescence intensity difference was plotted
versus the molar faction of HBT-11 (denoted by R) in the total
amount of HBT-11 and a-CD (or b-CD). For the convenience of
description, the concentrations of HBT-11 and a-CD (or b-CD) are
denoted by C0 and C1, respectively; R is equal to C0/(C0 + C1). The
linear fitting of the data indicated two points of intersections, and
both were located at around R = 0.5 for both a-CD and b-CD
systems. From these results, the association ratios for both
HBT-11/a-CD and HBT-11/b-CD complexes were found to be 1 :1.
After obtaining the association ratio, the binding constant
(K) can be extrapolated by adopting Benesi–Hilderbrand’s53
double reciprocal plot for a 1 : 1 complex, according to the
eqn (1), as shown in Fig. 3b.
when the concentrations are 5.0 ꢁ 10ꢀ5 and 4.0 ꢁ 10ꢀ4 mol Lꢀ1
,
1
1
1
1
¼
þ
(1)
respectively. After the additions of a-CD and b-CD, despite the
concentrations, the solutions turn dodger blue and sky blue,
respectively. The color change of the aqueous solution is consistent
with the results from the fluorescence spectra.
Fi ꢀ F0 K ꢂ C0 C1 C0
Here, Fi ꢀ F0 refers to the difference between fluorescence
intensities of HBT-11 in the presence and absence of CDs.
To understand the influence of a-CD and b-CD on the ESIPT
process of HBT-11, quantum yields (FF)51 and fluorescence life-
times were determined, and the corresponding data are listed in
Table 1. FF is independent of the concentration of HBT-11 (as
shown in Table S1 in ESI†). Considering the solution with a
concentration of 5.0 ꢁ 10ꢀ5 mol Lꢀ1 as an example, FF of pure
HBT-11 was found to be 1.0%, and the additions of a-CD and b-CD
led to an increase in FF to 3.7% and 3.4%, respectively. These data
were in good accordance with the results of enhanced emission
indicated by fluorescence spectra.
Time-resolved fluorescence spectra were obtained to analyze
the lifetime changes upon the addition of a-CD and b-CD (the
Fig. 3 (a) Job’s plots for HBT-11/a-CD and HBT-11/b-CD. The total
concentration of HBT-11/a-CD (or HBT-11/b-CD) was kept at 5.0 ꢁ
spectra are presented in Fig. S13 in the ESI†). The spectra were 10ꢀ5 mol Lꢀ1. (b) Benesi–Hilderbrand’s plots.
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and the HBT part was still ‘‘free’’. This effect was similar to
being dissolved in a good solvent, which explained the enhanced
enol-emission.
Conclusions
In summary, an amphiphilic molecule HBT-11 bearing a 2-(20-
hydroxyphenyl)benzothiazole moiety was designed and synthe-
sized. Below CMC, HBT-11 showed two weak emission bands
due to the enol- and keto-form tautomers and owing to the
ESIPT process. Supramolecular complexation with a-CD and
b-CD resulted in the separate enhancement of dual emissions.
a-CD and HBT-11 formed complexes via multiple supramole-
cular interactions, which mainly improved the keto-emission;
b-CD and HBT-11 underwent host–guest complexation, which
mainly led to the increase in enol-emission. Above CMC, the
fluorescence emission of the aggregates appeared at 507 nm.
Both a-CD and b-CD were able to break down the aggregates by
supramolecular complexation and induce a similar fluorescence
enhancement effect as mentioned above.
Fig. 4 Partial 1H NMR spectra (600 MHz, D2O, 298 K) of (a) HBT-11
(5.0 ꢁ 10ꢀ5 mol Lꢀ1), (b) HBT-11/a-CD (1 eq.), (c) a-CD, (d) HBT-11 (5.0 ꢁ
10ꢀ5 mol Lꢀ1), (e) HBT-11/b-CD (1 eq.), and (f) b-CD.
For the complexations with a-CD and b-CD, the fluorescence
intensities at 446 and 385 nm, respectively, are adopted. By
plotting 1/(Fi ꢀ F0) versus 1/C1, K is obtained, and the values for
HBT-11/a-CD and HBT-11/b-CD complexes are 3.3 ꢁ 103 and
1.7 ꢁ 103 L molꢀ1, respectively, indicating that both a-CD and
b-CD have strong supramolecular interactions with HBT-11 in
aqueous solutions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Second, 1H NMR spectra were obtained to investigate the
supramolecular interaction between HBT-11 and a-CD (or b-CD).
As shown in Fig. 4(a)–(c), upon the addition of a-CD into the
aqueous solution of HBT-11, almost all the NMR signals in the
aromatic region shifted to up-field, whereas the protons on
the methyl groups of a-CD hardly showed any apparent changes.
The NMR signals of protons 3 and 5 of a-CD are often used to
judge the encapsulation of CD with guest molecules.54,55 It was
clear that HBT-11 was not present in the cavity of a-CD. Then,
the shifts of NMR signals of the HBT segment and pyridinium
could only be explained by the interactions with the hydroxyl
groups on the rims of a-CD. Presumably, the synergistic inter-
action of hydrogen bonding between heteroatoms (such as N,
O and S) and hydroxyl groups and the dipole–dipole interaction
between pyridinium and hydroxyl groups resulted in the binding
of HBT-11 to the outside of a-CD. Combining the above-
mentioned analysis with the emission results, we speculated
that multiple supramolecular interactions, especially hydrogen
bonding interactions, facilitated the formation of the keto-form
tautomer; this is a reasonable explanation for the enhanced
keto-emission.
We thank the financial support of the National Natural Science
Foundation of China (21674075), the Natural Science Founda-
tion of Jiangsu Province (BK20161211), Key University Science
Research Project of Jiangsu Province (17KJA150007), Suzhou
Key Laboratory of Macromolecular Design and Precision Synthesis,
State and Local Joint Engineering Laboratory for Novel Func-
tional Polymeric Materials, and a Project Funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions.
Notes and references
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3 J. Zhao, S. Ji, Y. Chen, H. Guo and P. Yang, Phys. Chem.
Chem. Phys., 2012, 14, 8803–8817.
4 J. E. Kwon and S. Y. Park, Adv. Mater., 2011, 23, 3615–3642.
5 J. L. Cheng, D. Liu, W. Li, L. J. Bao and K. L. Han, J. Phys.
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As shown in Fig. 4(d)–(f), upon the addition of b-CD, the
NMR signals of protons 3 and 5 of b-CD shifted to up-field, and
the NMR signals of protons h, i, j and k shifted to down-field,
whereas the rest of the proton signals changed slightly. Clearly,
the pyridinium group was encapsulated by b-CD due to for-
mation of host–guest assemblies. The complexation of HBT-11
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