Spectral study on the inclusion complex of cryptophane-E and CHCl3

Article abstract of DOI:10.1016/j.saa.2009.10.004

Cryptophane-E was synthesized from vanillin by a three-step method, and its absorption and fluorescence spectroscopic properties were determined. Two absorption bands at about 245-260 and 280-290 nm were observed for cryptophane-E and the fluorescence emission maxima were at 320-330 nm depending on the solvent used. The interaction of cryptophane-E with CHCl₃ was studied in detail by absorption and fluorescence spectroscopies. The results showed that cryptophane-E and CHCl₃ can easily form a stable 1:1 host-guest inclusion complex. Their binding constant (K) was determined by Benesi-Hildebrand equation and the nonlinear least squares fit method. The binding constant is largest in ethyl acetate, followed by dioxane and with acetonitrile as the smallest. In addition, the effect of guest volume on the host-guest inclusion complex was investigated. Guest molecules including CH₂Cl₂ and CCl₄ were unable to form inclusion complex with cryptophane-E because of sizes mismatching with the host cavity.

Full text of DOI:10.1016/j.saa.2009.10.004

Spectrochimica Acta Part A 75 (2010) 157–161
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectral study on the inclusion complex of cryptophane-E and CHCl3
Caihong Zhanga, Weili Shena, Ruying Fana, Guomei Zhanga, Shaomin Shuanga,
Chuan Donga,∗, Martin M.F. Choib,∗∗
a Center of Environmental Science and Engineering Research, School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cryptophane-E was synthesized from vanillin by a three-step method, and its absorption and fluorescence
spectroscopic properties were determined. Two absorption bands at about 245–260 and 280–290 nm
were observed for cryptophane-E and the fluorescence emission maxima were at 320–330 nm depending
on the solvent used. The interaction of cryptophane-E with CHCl3 was studied in detail by absorption and
fluorescence spectroscopies. The results showed that cryptophane-E and CHCl3 can easily form a stable
1:1 host–guest inclusion complex. Their binding constant (K) was determined by Benesi–Hildebrand
equation and the nonlinear least squares fit method. The binding constant is largest in ethyl acetate,
followed by dioxane and with acetonitrile as the smallest. In addition, the effect of guest volume on the
host–guest inclusion complex was investigated. Guest molecules including CH2Cl2 and CCl4 were unable
to form inclusion complex with cryptophane-E because of sizes mismatching with the host cavity.
© 2009 Elsevier B.V. All rights reserved.
Received 30 June 2009
Received in revised form
28 September 2009
Accepted 8 October 2009
Keywords:
Cryptophane-E
CHCl3
Fluorescence
Absorption
Inclusion complex
1. Introduction
advanced solid state NMR spectroscopy. Their results show that
the mobilities of the encaged guest in the solid state and in the
In recent years, the design of hosts capable of binding neutral
substrates has attracted much attention in supramolecular chem-
istry, and, in this context, the capture of the smallest aliphatic
hydrocarbons still remains a challenging problem [1–9]. Cryp-
tophanes are a group of interesting cage-like hosts consisted of
two rigid cone-shaped units (cyclotriveratrylene) linked by three
O–Z–O bridges [10–12]. They are well suited for inclusion of
molecules of various sizes and shapes because the dimensions
of the host cavity can be easily modified by simple structural
changes. For instance, NMR studies and CPK models indicate that
cryptophane-A, with three –OCH2CH2O– bridges, preferentially
binds CH4 and CH2Cl2 [13,14] whereas cryptophane-E with a larger
cavity of three –OCH2CH2CH2O– bridges, as shown in Scheme 1,
prefers to bind CHCl3 [15–17].
The complex of cryptophane-E and CHCl3 has been stud-
ied by NMR spectroscopy and computational chemistry methods
[15–18]. It is found that CHCl3 behaves as an integral part of the
cryptophane-E host after inclusion. The inclusion complex is rela-
tively stable and the binding constant was estimated to be 470 M−1
in (CDCl2)2 at 300 K by NMR. Tosner et al. [19] also reported the
formation of cryptophane-E and CHCl3 inclusion complex using
isotropic liquid solution were very similar. Although their tech-
niques can provide valuable information on the binding constant
of the inclusion complex, NMR spectrometers are still relatively
expensive and not commonly available in most laboratories. As
such, cheaper equipment such as UV–vis absorption and fluo-
rescence spectrophotometers for studying the inclusion complex
behavior of host–guest interaction should be explored. To the best
of our knowledge, there are very few studies on cryptophane-E and
halogenated compounds using fluorescence and absorption spec-
troscopies.
In this article, the fundamental spectroscopic characteristics of
cryptophane-E in different organic solvents, the inclusion complex
of cryptophane-E and CHCl3, binding constants, solvent effect on
the complex and interaction of cryptophane-E with CH2Cl2 and
CCl4 have been investigated in detail. Our results are to some extent
consistent with the reported NMR spectroscopic data. These fun-
damental data and the spectral method should provide valuable
information for studying the structure, dynamics, and ordering
behavior of the host–guest inclusion complexes.
2. Experimental
2.1. Materials
∗
Corresponding author. Tel.: +86 351 7018842; fax: +86 351 7011688.
Corresponding author. Tel.: +852 34117839; fax: +852 34117348.
E-mail addresses: dc@sxu.edu.cn (C. Dong), mfchoi@hkbu.edu.hk (M.M.F. Choi).
∗∗
Acetonitrile, dibromoethane, diethyl ether, dioxane, ethanol,
ethyl acetate, methanol, sodium borohydride (NaBH4), and vanillin
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.10.004

158
C. Zhang et al. / Spectrochimica Acta Part A 75 (2010) 157–161
Fig. 1. UV–vis absorption spectra of cryptophane-E (3.21 × 10−5 mol L−1) in differ-
ent organic solvents.
∼245–260 and ∼280–300 nm depending on the type of solvent
used. The absorption intensity of cryptophane-E in CHCl3 is high-
est, followed by acetonitrile, dioxane and ethyl acetate. The lowest
absorption intensities are in diethyl ether, ethanol and methanol.
The absorption bands of diethyl ether, dioxane, ethanol, and
methanol and are not as distinctive (∼245–260 nm) as that of ace-
tonitrile, CHCl3 and ethyl acetate. The more obvious and distinctive
absorption bands of cryptophane-E in CHCl3 are possibly attributed
to the formation of inclusion complex of cryptophane-E and CHCl3.
Scheme 1. The structure of cryptophane-E.
were purchased from Beijing Chemical Plant (Beijing, China).
Organic solvents were purified before use and were checked to
ensure they contained no fluorescence impurities at the work-
ing excitation wavelengths (292–298 nm). Other chemicals and
reagents for the synthesis of cryptophane-E were used as received
unless otherwise stated.
2.2. Synthesis of cryptophane-E
3.2. Inclusion complex of cryptophane-E and CHCl3 determined
by absorption
Cryptophane-E was synthesized from vanillin using
a
three-step method [20]. The compound 1,3-bis(4-formyl-2-
methoxyphenoxy)propane was prepared by reacting vanillin with
1/2 equivalents of dibromoethane. The bis-aldehyde derivative was
then reduced to its corresponding bis-vanillyl alcohol by NaBH4
in methanol. The ring closure reaction of benzylic alcohol was
performed in a 1.4–1.5 mmol L−1 formic acid solution at 60 ◦C for
3 h. The reaction mixture was purified by column chromatography
on silica gel to obtain 13.0% yield of cryptophane-E. The 1HNMR
(CDCl3) data of cryptophane-E were acquired: ı = 6.70 (s, 6H, Ar),
6.62 (s, 6H, Ar), 4.65 (d, 6H, CHa), 4.04 (m, 12H, OCH2), 3.87 (s,
18H, OCH3), 3.50 (d, 6H, CHe), and 2.31 (m, 6H, CH2). Satisfactory
It has been reported that cryptophanes can form inclusion com-
plexes with various halogenomethanes [14–16]. The absorption
spectra of cryptophane-E with various CHCl3 concentrations in
ethyl acetate, dioxane and acetonitrile are depicted in Fig. 2(a)–(c),
respectively. The absorption intensity of cryptophane-E decreases
with the increase in CHCl3 concentration. The spectral data at
their absorption peak maxima were then analyzed with the
Benesi–Hildebrand equation [21] as depicted in the insets of
Fig. 2. The binding constants were found to be 542.2, 316.7
and 189.6 M−1 in ethyl acetate, dioxane and acetonitrile, respec-
tively. The linear correlation coefficients (r) were higher than
0.993. The results suggest that the decrease in absorption inten-
sity is mainly attributed to the formation of host–guest inclusion
complexes between cryptophane-E and CHCl3. The binding con-
stants decrease in the trend: ethyl acetate > dioxane > acetonitrile,
demonstrating that different solvents can affect the binding of
cryptophane-E and CHCl3. The binding constant is smallest in ace-
tontitrile, possibly attributed to its competition with CHCl3 for
binding with cryptophane-E. In addition, as dioxane has similar
structure to cryptophane-E which has a rigid framework of three
“–OCH2CH2CH2O–” bridges, it is possible that dioxane competes
with cryptophane-E for interacting with CHCl3; as a result, the
binding constant in dioxane is lower.
elemental analysis (C, H) was obtained, Anal. Calcd for C57H60O12
:
C, 73.06; H, 6.45; O, 20.49. Found: C, 72.18; H, 6.03; O, 20.62. These
data were consistent with the literature values [20].
2.3. Instrumentation
Fluorescence spectra were taken on a F4500 Hitachi spectroflu-
orometer (Tokyo, Japan). Both excitation and emission slits were
set at 5 nm. All the experiments were carried out at 20 1 ◦C. 1H
NMR spectra were done on a Bruker Avance DRX 300 MHz nuclear
magnetic resonance spectrometer (Fällanden, Switzerland).
3. Results and discussion
3.3. Fluorescence spectra of cryptophane-E in various organic
solvents
3.1. Absorption spectra of cryptophane-E in different organic
solvents
Fig.
3
displays the fluorescence emission spectra of
Fig. 1 displays the UV–vis absorption spectra of cryptophane-E
(3.21 × 10−5 mol L−1) in different organic solvents including ace-
tonitrile, CHCl3, diethyl ether, dioxane, ethanol, ethyl acetate,
and methanol. Cryptophane-E shows two absorption peaks at
cryptophane-E (2.91 × 10−5 mol L−1
) in different organic sol-
vents including acetonitrile, diethyl ether, dioxane, ethanol, ethyl
acetate, and methanol. Table 1 summarizes their fluorescence
excitation (ꢀex) and emission (ꢀem) maxima. The ꢀex and ꢀem of

C. Zhang et al. / Spectrochimica Acta Part A 75 (2010) 157–161
159
Fig. 3. Fluorescence emission spectra of cryptophane-E (2.91 × 10−5 mol L−1) in dif-
ferent organic solvents. Excitation wavelengths are at their corresponding excitation
maxima.
cryptophane-E in these solvents were 292–298 and 322–330 nm,
respectively. It seems that cryptophane-E does not vary too much
with the change in solvent polarity, possibly attributing to the rigid
nature of the molecule. This shows that solvent polarity has not
much effect on ꢀex and ꢀem under different solvent systems in our
work, which is different from the literatures that the ꢀex and ꢀem
are slightly red-shifted in more lipophilic solvent systems [22,23].
The fluorescence intensity of cryptophane-E in these solvents fol-
lows the trend: dioxane > acetonitrile > methanol > ethanol > ethyl
acetate > diethyl ether; however, this trend is still not clearly
understood.
3.4. Inclusion complex of cryptophane-E and CHCl3 studied by
fluorescence
Cryptophane-E displays good fluorescence in ethyl acetate as
depicted in Fig. 4. It has a ꢀem of 325 nm when excited at 296 nm.
When CHCl3 was added to the cryptophane-E solution, the fluores-
cence intensity decreased and the emission spectrum also slightly
shifted hypsochromically. This can be explained by the fact that an
inclusion complex of cryptophane-E and CHCl3 was formed which
could affect the fluorescence yield and spectral characteristics of
cryptophane-E.
The binding constant can be estimated by the least squares fit
to the experimental data obtained from the fluorescence titrations
[24] as follows:
ꢀ
ꢃ
ꢁ
ꢂ
1
2
1
K
ꢁF =
˛
H
+ G
+
[
]
[ ]
0
0
Fig. 2. A (a) UV–vis absorption spectra of cryptophane-E (3.21 × 10−5 mol L−1
)
with CHCl3 in ethyl acetate. The concentrations of CHCl3 are (1) 0.00 mol L−1
,
,
,
ꢄ
(2) 2.01 × 10−4 mol L−1
(5) 8.38 × 10−4 mol L−1
,
,
(3) 4.19 × 10−4 mol L−1
(6) 1.05 × 10−3 mol L−1
,
,
(4) 6.29 × 10−4 mol L−1
(7) 1.68 × 10−3 mol L−1
ꢁ
ꢂ
−
˛
H
+ G
[
+
2 − 4 H
G ˛
]0[ ]
0
1
K
2
2
[
]
]
[
(8) 2.09 × 10−3 mol L−1
,
and (9) 2.51 × 10−3 mol L−1
. The inset displays the
0
0
Benesi–Hildebrand plot and the absorption values were taken at 288 nm. (b) UV–vis
absorption spectra of cryptophane-E (6.42 × 10−5 mol L−1) with CHCl3 in diox-
where [H]0 and [G]0 are the initial concentrations of host
cryptophane-E and guest CHCl3, respectively. ꢁF denotes the
change of the fluorescence intensity of cryptophane-E with the
ane. The concentrations of CHCl3 are (1) 0.00 mol L−1
,
(2) 1.42 × 10−4 mol L−1
,
,
(3) 2.83 × 10−3 mol L−1
,
(4) 4.52 × 10−3 mol L−1
,
(5) 5.66 × 10−3 mol L−1
(6) 7.08 × 10−3 mol L−1
,
(7) 8.49 × 10−3 mol L−1
,
(8) 9.91 × 10−3 mol L−1
,
.
(9)
The
1.13 × 10−2 mol L−1
,
(10) 1.27 × 10−2 mol L−1
,
and (11) 1.41 × 10−2 mol L−1
inset displays the Benesi–Hildebrand plot and the absorption values were taken
Table 1
at 294 nm. (c) UV–vis absorption spectra of cryptophane-E (6.42 × 10−5 mol L−1
)
,
,
The fluorescence excitation (ꢀex) and emission (ꢀem) maxima of cryptophane-E in
various organic solvents.
with CHCl3 in acetonitrile. The concentrations of CHCl3 are (1) 0.00 mol L−1
(2) 3.16 × 10−4 mol L−1
,
(3) 6.31 × 10−4 mol L−1
,
(4) 9.47 × 10−4 mol L−1
(5) 1.26 × 10−3 mol L−1
,
(6) 1.88 × 10−3 mol L−1
,
(7) 1.89 × 10−3 mol L−1
, (8)
Solvent
Acetonitrile Diethyl Dioxane Methanol Ethyl
Ethanol
2.21 × 10−3 mol L−1
3.16 × 10−3 mol L−1
,
.
(9) 2.52 × 10−3 mol L−1
,
(10) 2.84 × 10−3 mol L−1
, and (11)
ether
acetate
The inset displays the Benesi–Hildebrand plot and the
ꢀex (nm)
295
295
324
298
330
292
322
296
325
292
324
absorption values were taken at 291 nm.
ꢀem (nm) 323

160
C. Zhang et al. / Spectrochimica Acta Part A 75 (2010) 157–161
Fig. 4. Fluorescence quenching effect of CHCl3 on the emission intensity
of cryptophane-E (2.07 × 10−5 mol L−1
)
in ethyl acetate. (1) 0.00 mol L−1
,
,
(2) 4.20 × 10−4 mol L−1
,
(3) 9.21 × 10−4 mol L−1
,
(4) 1.50 × 10−3 mol L−1
(5) 2.16 × 10−3 mol L−1
,
(6) 2.91 × 10−3 mol L−1
,
(7) 3.74 × 10−3 mol L−1
,
(8)
(11)
Scheme 2. The optimal structure of the cryptophane-E–CHCl3 complex simulated
by the computer modeling.
4.66 × 10−3 mol L−1
,
(9) 5.66 × 10−3 mol L−1
,
(10) 6.74 × 10−3 mol L−1
,
7.90 × 10−3 mol L−1, and (12) 9.15 × 10−3 mol L−1 CHCl3. Excitation wavelength is
296 nm. The inset displays the Stern–Volmer plot for cryptophane-E with different
CHCl3 concentrations.
3.5. Molecular modeling studies
The cryptophane-E–CHCl3 complex was simulated by the
molecular dynamic calculation using the Computer Molecular
Modeling System CS Chem 3D Pro 7.0 (CambridgeSoft, Cambridge,
MA, USA). The optimal configuration of the cryptophane-E–CHCl3
complex that has the lowest total energy is displayed in Scheme 2.
This result shows that CHCl3 can penetrate into the cavity of
cryptophane-E to form a 1:1 host–guest system.
addition of CHCl3. ˛ is a sensitive factor of the structure change of
the complex cryptophane-E–CHCl3 at the interactive course. The
binding constant was estimated to be 106 9 M−1 and r is 0.9972
as shown in Fig. 5. The result of nonlinear fitting is good, indicat-
ing that the inclusion complex was formed with a stoichiometric
ratio of 1:1. The binding constants were found to be 46 2 and
18 1 M−1 in dioxane and acetonitrile, respectively.
In addition, the inclusion complex of cryptophane-E–CHCl3 was
studied by 1HNMR in (CDCl2)2 at 300 K [17]. The peaks at ıf 7.28
and ıc 2.84 ppm corresponding to the free and complexed CHCl3
molecules, respectively slowly exchanged at this temperature. The
binding constant of the complex was more than 100 M−1 by 1HNMR
and was in good agreement with our result by fluorescence. X-
ray crystallography confirmed that cryptophane-E–CHCl3 was a 1:1
host–guest [17]. Our result shows some similarity with the reported
results [17].
3.6. Solvent effect on the inclusion complex of cryptophane-E and
CHCl3
The binding constants from the absorption and fluores-
cence data were found to follow the same trend: ethyl
acetate > dioxane > acetonitrile; however, there are some slight dif-
ferences in the binding constants obtained by these two methods.
This is probably due to differences in calculation methods. Com-
paring with ethyl acetate, the acetonitrile molecules are smaller
and can compete with the guest CHCl3 molecules causing the
decrease in the binding constant. By contrast, ethyl acetate is too
bulky to enter the cavity of cryptophane-E; thus, the binding con-
stant is comparatively larger. In addition, cryptophane-E has a rigid
framework with three “–OCH2CH2CH2O–” bridges and its structure
shows some similarities with dioxane. Therefore, dioxane can be a
good candidate to compete with cryptophane-E for interacting with
CHCl3. This can explain why the binding constant of cryptophane-
E and CHCl3 in dioxane solvent is smaller than ethyl acetate. In
essence, the formation of inclusion complex of cryptophane-E and
CHCl3 not only depends on the host–guest interaction but also
encompasses solvent–guest and solvent–host interactions.
3.7. Effect of guest volume
The interaction of cryptophane-E with smaller (CH2Cl2) or larger
(CCl4) guests was examined by fluorescence spectroscopy and their
occupancy factors [13] as depicted in Table 2. When CH2Cl2 was
added to cryptophane-E in ethyl acetate solvent, the fluorescence
intensity decreased and the binding constant was determined to
Fig. 5. Nonlinear curve fits for the cryptophane-E–CHCl3 complex in ethyl acetate.

C. Zhang et al. / Spectrochimica Acta Part A 75 (2010) 157–161
161
Table 2
the cavity of cryptophane-E and so they cannot form stable inclu-
sion complexes with cryptophane-E. Our proposed absorption and
fluorescence methods can provide simple and cheaper ways of
investigating the inclusion complex.
The guest volume (Vvdw) and the occupancy factor of guest in cryptophane-E.
Guest
CH2Cl2
CHCl3
106
CCl4
K (M−1
Vvdw (Å3)
Occupancy factor
)
2.0 0.2
55.6
0.48
9
–
86.8
0.74
72.2
0.62
Acknowledgements
The van der Waals volume of the cavity for cryptophane-E is 117 Å3.
The work described in this paper was supported by the
National Nature Science Foundation Key Project (50534100) and
the Youth Science Foundation of Shanxi Province (2007021007 and
2008021011).
be 2.0 0.2 M−1 which is much smaller than the binding constant
of cryptophane-E and CHCl3. CCl4 does not cause any fluores-
cence quenching on cryptophane-E. These results indicated that the
larger CCl4 guest cannot penetrate into the cavity of cryptophane-
E while the smaller CH2Cl2 guest can only weakly interact with
cryptophane-E as their sizes do not match well with the cavity
of cryptophane-E. The cavity of cryptophane-E is estimated to be
117 Å3 according to the reference by Mecozzi and Rebek [25]. The
occupancy factor is 0.62 [13,25] for one molecule of CHCl3 in one
cryptophane-E, which is quite reasonable for such a complex, sig-
nifying that it is neither too tight nor too loose. The cryptophane-E
and CHCl3 complex thus represents a van der Waals molecule.
CH2Cl2 has an occupancy factor of 0.48 which is 20% smaller than
CHCl3. Thus, it has more space to move in and out of the cavity
of cryptophane-E, resulting in an unstable inclusion complex. It is
more difficultfor CCl4 toforminclusioncomplexwith cryptophane-
E as it has an occupancy factor of 0.74 [13,25] which is too large for
the cavity of cryptophane-E. The results of cryptophane-E to guests
fit within the range of the widely quoted rule: the ratio of the guest
volume to the host volume, is in the range of 0.55 0.09 [25]. These
findings are also supported by the CPK model that the six –OCH3
groups of the host can obstruct CCl4 molecules from entering the
cavity of cryptophane-E.
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In summary, cryptophane-E was synthesized and possesses
fluorescence property. Its host property can be studied using
molecular spectral methods. Our experimental results demonstrate
that the formation of cryptophane-E–CHCl3 inclusion complex
is governed by the good matching size of the guest with the
cryptophane-E cavity. Ethyl acetate is a favorable solvent for study-
ing the cryptophane-E–CHCl3 inclusion complex but acetonitrile
and dioxane are not ideal as they can compete with CHCl3. In addi-
tion, the larger CCl4 or smaller CH2Cl2 guest molecules do not suit
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