Fluorescent behaviour in host-guest interactions. Part 3. Fluorescent sensing for organic guests using three types of amino-β-cyclodextrins

Article abstract of DOI:10.1039/b108204n

The pH- and temperature-dependent fluorescence behaviour in aqueous solution of three amino-β-cyclodextrins (amino-β-CDx), 1, 2 and 3, bearing an amide-linked naphthalene probe has been investigated. The emission intensity at λ_max(em) of 1 decreased dramatically with increasing temperature. The pH-dependent fluorescence spectrum was also recorded. Operation of the in-out equilibrium of the naphthalene probe of 1 was mainly analyzed using ¹H NMR and circular dichroism spectra. The application of 1 to organic-guest sensing is demonstrated by several examples. These findings suggest that the new host molecule, 1, will be an excellent CDx-based fluorescent sensor for temperature, pH and neutral organic guests.

Full text of DOI:10.1039/b108204n

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Fluorescent behaviour in host–guest interactions. Part 3.†
Fluorescent sensing for organic guests using three types of amino-
ꢀ
-cyclodextrins‡
a
b
a
2
Yasushi Takenaka, Miwako Higashi and Noboru Yoshida*
a
Laboratory of Molecular Functional Chemistry, Division of Material Science, Graduate
School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
Center for Cooperative Research and Development, Ibaraki University, Hitachi 316-8511,
Japan
b
Received (in Cambridge, UK) 10th September 2001, Accepted 21st November 2001
First published as an Advance Article on the web 17th January 2002
The pH- and temperature-dependent fluorescence behaviour in aqueous solution of three amino-β-cyclodextrins
(
amino-β-CDx), 1, 2 and 3, bearing an amide-linked naphthalene probe has been investigated. The emission intensity
at λmax(em) of 1 decreased dramatically with increasing temperature. The pH-dependent fluorescence spectrum was
1
also recorded. Operation of the in–out equilibrium of the naphthalene probe of 1 was mainly analyzed using H
NMR and circular dichroism spectra. The application of 1 to organic-guest sensing is demonstrated by several
examples. These findings suggest that the new host molecule, 1, will be an excellent CDx-based fluorescent sensor for
temperature, pH and neutral organic guests.
Introduction
Fluorescent artificial cyclodextrins have been received con-
8c,9 10
siderable attention in sensory,
biochemical and photo-
Cyclodextrins (CDx) as a supramolecular host have an internal
hydrophobic cavity like a “molecular flask” providing a non-
polar environment for various types of guest molecules in
11
electronic applications. The CDx cavity as a binding site and
the appended fluorophore as a signaling unit with the spacer
group are indispensable for the substrate specific-responsive
function. Pyrene-appended cyclodextrin derivatives have often
been used because of the larger change in fluorescence intensity
1
aqueous solution. The recent use of CDx as building blocks for
the construction of supramolecular species such as rotaxanes,
dendrimers and for application in lipophilic chiral sensing
in solution represents a new, contemporary field in CDx
9k
after complexation with substrate, but their solubility in water
was not adequate. In the present study, we prepared water-
soluble naphthalene-appended amino-β-cyclodextrin hosts
1, 2 and 3) to carry out the guest sensing in aqueous solution.
2–4
chemistry. The molecular recognition process in aqueous
solution of several anionic azo compounds by α-cyclodextrin
has been a subject of our particular attention as a dynamic
(
5–7
model for enzyme–substrate processes.
Also, weak inter-
actions such as electrostatic forces have been found to be effec-
tive in selectivity and enhancement of the binding properties
8
of some amino-β-cyclodextrins. Strong binding of doubly-
2ϩ
protonated β-CDxenH2
with fluorescent 2-(4-toluidino)-
naphthalene-6-sulfonate (TNS) anion was observed owing to
Ϫ
the electrostatic interactions between the –SO group of TNS
3
ϩ
2
ϩ
3
8c
and the –NH –CH CH –NH moiety of β-CDxenH2.
2
2
Experimental
Materials and synthesis
†
‡
For Part 2, see ref. 13.
Electronic supplementary information (ESI) available: Figs. S1–S3.
Guaranteed-grade β-cyclodextrin (Wako Pure Chemical Ind.
Ltd.) was used without further purification. Monotosylated
See http://www.rsc.org/suppdata/p2/b1/b108204n/
DOI: 10.1039/b108204n
J. Chem. Soc., Perkin Trans. 2, 2002, 615–620
This journal is © The Royal Society of Chemistry 2002
615

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β-CDxots at the O-6 position on the -glucopyranosyl rings of
β-CDx was synthesized by the reaction of β-CDx with toluene-
p-sulfonyl chloride in pyridine at room temperature for 1.5 h.
The β-CDxots thus obtained was dissolved in excess ethylene-
diamine (Tokyo Kasei) and the solution was heated at 70 ЊC for
The protons, H-6 and H-7, at the head moiety of the
naphthalene probe show a relatively large upfield-shift on
decreasing the temperature. Furthermore, the well-separated
signals arising from the anomeric protons (C -H) of the CDx
1
ring of 1 became simplified as a more coalescent anomeric
resonance. These results suggest that the head moiety of 1 may
be less tightly included within the CDx cavity (self-inclusion)
at the lower temperature and moved outside the cavity at the
higher temperature.
1
.5 h with stirring. The reaction mixture was poured into a large
amount of acetone and the resultant white precipitate was
collected and dried in vacuo. Purification of mono(6-N-
(
2-aminoethylamino-6-deoxy)-β-cyclodextrin (β-CDxen) as a
precursor of the functionalized β-CDx derivatives was carried
Fig. 1 shows the induced circular dichroism (ICD) spectra
out by ion-exchange column chromatography through cation-
Ϫ3
exchange resin (Toyo-pearl 650M; 0.05 mol dm NH HCO3
4
aqueous solution as eluent). The purified β-CDxen was then
coupled with 1- and 2-naphthoic acid and naphthalene-1-acetic
acid by the usual N,NЈ-dicyclohexylcarbodiimide (DCC)
method and precipitation with acetone and cation-exchange
chromatography gave 1, 2 and 3, respectively. The elemental
analysis of 1, 2 and 3 was satisfactory, although solvent water
molecules were usually bound. Found for 1: C, 48.06; H, 6.51;
N, 1.87. Calc. for C H N O ؒ3H O: C, 47.69; H, 6.40; N,
5
5
82
2
35
2
ϩ
1
2
.02%. MS (FAB) m/z 1331 [M ϩ H] . H NMR (D O, 400
2
MHz) δH 3.3–3.9 (42H, CDx ring protons), 4.8–5.0 (7H, 1-H)
and 7.4–8.0 (7H, aromatic protons). Found for 2: C, 46.95; H,
6
.29; N, 2.00. Calc. for C H N O ؒ3H O: C, 47.69; H, 6.40;
55
82
2
35
2
ϩ
1
N, 2.02%. MS (FAB) m/z 1331 [M ϩ H] . H NMR (D O, 400
2
MHz) δH 3.3–3.9 (42H, CDx ring protons), 4.8–5.0 (7H, 1-H)
and 7.6–8.3 (7H, aromatic protons). Found for 3: C, 45.72; H,
6
.52; N, 1.98. Calc. for C H N O ؒ5H O: C, 46.86; H, 6.60;
56
84
2
35
2
ϩ
1
N, 1.95%. MS (FAB) m/z 1345 [M ϩ H] . H NMR (D O, 400
2
Fig. 1 Circular dichroism spectra of 1 at various temperatures: 25 (a);
MHz) δH 2.5–2.7 (2H, CH ), 3.3–3.8 (COCH , CDx ring pro-
tons), 4.8–5.0 (7H, 1-H) and 7.4–7.9 (7H, aromatic protons).
4
3
Ϫ1
Ϫ1
2
2
4
0 (b); 60 (c); 80 (d) ЊC. θ/10 dm mol cm .
of 1 at various temperatures. The ICD spectral changes provide
Measurements
precise structural information such as the orientation of the
12
chromophore in the CDx cavity. There are two main bands
at 223(s) and 290(vw) nm which are attributed to the long-
and short-axis polarized π–π* transition of the naphthalene
Binding constants (K ) of the inclusion complexes were deter-
mined fluorometrically using a Shimadzu RF-5300PC record-
f
8c
ing fluorescence spectrometer. The fluorescence measurements
were performed by excitation at 295 nm. The pH values in
solution were determined using a HORIBA pH meter B-112.
The temperature was maintained between 10 and 80 ЊC
using an external circulating water bath (Thomas Kagaku
Co. Ltd., TRL-108H). Circular dichroism spectra1 were
12
chromophore. If the long-axis polarized π–π* transition is
almost parallel to the symmetry axis of CDx, a relatively strong
positive ICD sign should be observed as shown in Fig. 1. Upon
an increase in the temperature, the ICD intensity of 1 at 223 nm
tends to decrease due to the disinclusion of the naphthalene
chromophore (Scheme 1).
2
measured with a JASCO J-600C circular dichrometer. The
1
H NMR spectra were obtained at various temperatures with
a JEOL EX400 FT NMR spectrometer [with 2,2-dimethyl-
2
-silapentane-5-sulfonate sodium salt (DSS) as an external
reference].
Result and discussion
Temperature-dependent fluorescence spectra of 1, 2 and 3
Strongly temperature-responsive fluorescence spectra of 1 in
aqueous solution were observed over the temperature range
between 25 and 80 ЊC as shown in Fig. S1. This phenomenon
13
has been also found in other amino-β-cyclodextrins. Excita-
tion at 295 nm would produce a broad emission spectrum
in both 1 and 2 (λmax(em) ∼378 and 360 nm respectively). On the
other hand, the fluorescence spectrum of 3, which has no π-
conjugation with a methylene spacer between the carbonyl
group and the naphthalene chromophore, shows a sharp peak
at 330 nm with 340 (sh, m) and 355 (sh, w) nm, which is similar
to that of 1-methylnaphthalene. Hamai and Hatamiya reported
that such monomer fluorescence of 1-methylnaphthalene
Scheme 1 In–out equilibrium for the naphthalene probe of 1.
These findings are in accord with the NMR shifts at higher
temperatures. At ca. 290 nm, a weak negative ICD sign
was observed, suggesting that the short-axis polarized π–π*
transition would be perpendicular to the CDx axis at the
lower temperature. Fig. S3 shows the comparison of the tem-
perature-dependence of fluorescence intensity (I ) at λmax(em)
f
14
is slightly enhanced by increasing the β-CDx concentration.
between 1, 2 and 3.
At higher temperatures (70–80 ЊC), the fluorescence of 1 is
effectively reduced (Fig. S1). In order to clarify the strong
temperature-dependent fluorescence of 1, we measured the
It is noteworthy that the control compound 3, which has no
π-conjugation system, shows a smaller temperature-dependence
in its intensity. Therefore, compound 3 is not suitable as
a fluorescence sensor upon binding the guest molecule (vide
infra).
1
H NMR spectra of 1 at various temperatures between 10 and
6
0 ЊC (Fig. S2).
6
16 J. Chem. Soc., Perkin Trans. 2, 2002, 615–620

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pH-Dependent fluorescence spectra of 1, 2 and 3
Fluorescent amino-β-cyclodextrin derivatives (1, 2 and 3)
have one amine site that can be protonated. Protonation of
the amine site linked to the fluorophore may enhance the
fluorescence of 1, 2 and 3 owing to chelation-enhanced
15
fluorescence1 or a photo-induced electron-transfer (PET)
6
mechanism. Fig.
2 shows the pH-dependence of the
fluorescence intensity (I ) at λmax(em) of 1, 2 and 3.
f
4
3
Ϫ1
Fig. 3 Circular dichroism spectra of 1 at various pH. θ/10 dm mol
Ϫ1
cm
.
Fig. 2 Plots of fluorescence intensity at λ (em) vs. pH for 1, 2 and 3
max
Ϫ5
Ϫ3
at 25 ЊC. [1] = [2] = [3] = 1.25 × 10 mol dm
.
The drastic decrease in I upon increasing the pH from 6 to 8
f
could be ascribed to the deprotonation of the amine protons as
shown in Scheme 2. Since the ICD spectra of 1 at pH 4, 7 and 10
Fig. 4 Fluorescence spectra of 1 in aqueous solutions containing
Ϫ5
various concentrations of cyclohexanol (G1), [1] = 1.25 × 10 mol
Scheme 2 Protolytic equilibrium of the amine proton of 1.
Ϫ3
dm . The excitation (λex = 295 nm) and emission bandwidth were set at
5
.0 and 3.0 nm.
almost coincide with each other (Fig. 3), no conformational
change would occur upon protonation–deprotonation at the
amine site. The fluorescence of 3 is only slightly sensitive to the
pH in the solution.
fluorophore of 1 moves partially outside the CDx cavity upon
binding with G1. The sensitivity of this host–guest system is
limited by competition between complexation with the guest
9a
and self-inclusion of the fluorophore in the CDx cavity.
Fluorescent sensing and guest-binding mechanism
In order to elucidate the relationship between guest inclusion
and the in–out equilibrium for the naphthalene probe of 1, the
ICD changes of 1 at various G1 concentrations were measured
(Fig. 5). The positive ICD sign of 1 at 223 nm, which is attri-
buted to the long-axis polarization of the naphthalene probe,
decreases drastically upon addition of G1. Further addition of
G1 would produce the negative ICD sign in this region. On the
other hand, the negative ICD sign at 290 nm becomes a positive
The application of fluorophore-appended amino-β-
cyclodextrin derivatives (1, 2 and 3) to neutral guest sensing is
described in this section using several organic guest systems
(
G1–G4).
A typical example of the fluorescence spectral change of 1
upon addition of cyclohexanol (G1) at a constant pH is shown
in Fig. 4. A gradual decrease in I indicates that the naphthalene
f
J. Chem. Soc., Perkin Trans. 2, 2002, 615–620
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In all cases, the binding of guests (G1–G3) in the CDx cavity
of 1 resulted in a large decrease in I , indicating that the host 1
f
would be a excellent neutral-guest sensor.
Fig. 6 shows the 1 : 1 (= host : guest) binding curves of 1 with
G1 and G2 at various temperatures. The solid lines denote the
theoretical curves that could be obtained using a curve-fitting
method. Fig. 7 shows the fluorescence responses of 1 to the
various temperatures and G3 concentrations.
The low binding constant for the 1–G1 system (Table 1) indi-
cates a less tight inclusion. Furthermore, steric hindrance
between the naphthalene probe of 1 and the guest G1 would
Ϫ1
lead to such a lower value (K = 107 M ) compared with that
(
f
Ϫ1
K = 2000 M ) found for the dimethylaminobenzoyl-modified
f
9g
β-CDx–G1 system. On the other hand, more compatible
inclusion with adamantan-1-ol (G2) results in very high binding
constants. Interestingly, the decrease in I (∆I ) of 2 with G2 and
f
f
G3 was found to be too small to determine the exact K values,
f
but their stability order would be almost similar to that in the
1
-G2 and 1-G3 systems. No appreciable ∆I was observed in the
f
2
–G1 and control compound 3 systems.
Fig. 5 Circular dichroism spectra of 1 containing various concen-
trations of cyclohexanol (G1).
Thermodynamic parameters
From the van’t Hoff plots (log K vs. 1/T ), we could evaluate
f
value at the higher G1 concentrations. These ICD changes
suggest clearly that the long-axis and short-axis polarizations
of the naphthalene probe of 1 would be brought nearly parallel
and perpendicular to the CDx cavity, respectively, as shown in
Scheme 3. Thus, the ICD method is a powerful tool to estimate
the thermodynamic parameters such as ∆HЊ and ∆SЊ for the
binding of 1 with G1, G2 and G3. The free energy changes
(∆GЊ) calculated from the slope and the intercept are listed in
Table 1. It is well known that the inclusion reaction by native
CDx is enthalpically favored (∆Hincl < 0) and the standard
5b,5c,18
entropy (∆SЊincl) is either negative or positive.
Therefore,
the negative ∆HЊ values in our cases (Table 1) indicate that the
binding of 1 with G1–G3 is exothermic. Compared with the
1
–G1 system, the complexation of 1 with G2 and G3 is
enthalpically less favorable but entropically more favorable. It
is noteworthy that the comparative contribution of the positive
∆SЊ to the Gibbs energy term ∆GЊ found in both the 1–G2 and
1
–G3 systems may result from a change in solvent structure in
the bulky guests G2 and G3 and/or a conformational change
in 1 upon binding with the bulky guests.
NMR sensing using the control compound 3
Although strongly temperature-dependent fluorescence spectra
were observed in both the 1 and 2 systems as discussed above,
the changes in their H NMR spectra were relatively small. On
Scheme 3
1
the conformational changes upon binding between aromatic
ring appended CDx and organic guests.
the other hand, the temperature-dependence of the fluorescence
spectrum of 3 was quite small, but its H NMR spectrum was
17
1
Fig. 6 Binding curves for the interaction of 1 with G1 (a) and G2 (b) at various temperatures.
6
18
J. Chem. Soc., Perkin Trans. 2, 2002, 615–620

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Table 1 Binding constants and thermodynamic parameters for the inclusion reaction of host 1 with various guest molecules
Ϫ1
Ϫ3
Ϫ1
Ϫ1
Ϫ1
Ϫ1
Guest
K /mol dm
∆GЊ/kcal mol
∆HЊ/kcal mol
∆SЊ/cal mol
K
f
Cyclohexanol (G1)
107 (293 K)
9800 (293 K)
1149 (303 K)
Ϫ2.77
Ϫ5.53
Ϫ4.24
Ϫ4.38 ± 0.02
Ϫ2.65 ± 0.48
Ϫ2.33 ± 0.71
Ϫ5.41 ± 0.07
9.50 ± 1.59
6.30 ± 2.38
Adamantan-1-ol (G2)
(
Ϫ)-Borneol (G3)
found to be strongly temperature-dependent. Comparison of
1
the H NMR spectra of 2-naphthylacetic acid (2-NA) and its β-
CDx inclusion complex (β-CDx–2-NA) shown in Fig. 8(a) with
1
the H NMR spectrum of 3 at 30 ЊC [Fig. 8(b)] indicates that
the naphthalene probe in 3 would be included within the CDx
cavity at lower temperature.
The signals in the β-CDx–2-NA complex display some
broadening and large upfield shifts of up to 0.2 ppm for the
protons, H –H , inside the cavity. The H , H and H protons of
5
8
1
3
4
2
-NA at the methylene site remain well-separated signals. As the
temperature is increased in solution, the protons H –H in the
5
8
head group of 3 tend to shift downfield owing to disinclusion
from the CDx cavity, while the protons, H , H and H , in the
1
3
4
vicinity of the methyleneamide linkage shift a little.
The flexible motion of the naphthalene probe of 3 would be
applicable to the NMR sensing of the guest molecules. Fig. 9
1
shows the H NMR spectra of 3 in the absence and presence of
adamantanecarboxylate anion (G4).
Upon inclusion with G4, the head protons, H –H , of 3
5
8
experience a large downfield shift, suggesting that the naphtha-
lene probe moves outside the CDx cavity. Work towards the H
Fig. 7 Fluorescence sensing by 1 for temperature and (Ϫ)-borneol
1
(
G3) concentrations.
1
Fig. 8 H NMR spectra of the aromatic region of β-CDx–2-NA and 3. 2-NA and β-CDx–2-NA complex at 30 ЊC in D O (a). 3 at various
2
temperatures in D O (b).
2
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shows the highest sensitivity towards neutral-guest sensing
in aqueous solution. It has been demonstrated that the
fluorescence properties of 1, 2 and 3 are strongly dependent on
the temperature and pH in solution. Their fluorescent responses
were also controlled by the geometrical position and π-
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(
(
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2
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