Supramolecular chemistry of pyronines B and Y, β-cyclodextrin and linked β-cyclodextrin dimers

Article abstract of DOI:10.1071/CH09467

The complexation of cationic pyronine B(PB⁺) and pyronine Y(PY⁺) by β-cyclodextrin (CD) and two linked βCD dimers, N,N′-bis((2^AS,3^AS)-3^A-deoxy-β- cyclodextrin-3^A-yl)succinamide, 33βCD₂

Full text of DOI:10.1071/CH09467

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2010, 63, 687–692
Supramolecular Chemistry of Pyronines B and Y,
β-Cyclodextrin and Linked β-Cyclodextrin Dimers
Huy T. Ngo,A Philip Clements,A Christopher J. Easton,B Duc-Truc Pham,A
and Stephen F. LincolnA,C
ASchool of Chemistry and Physics, University of Adelaide, Adelaide, SA 5005, Australia.
BResearch School of Chemistry, Australian National University, Canberra, ACT 0200, Australia.
CCorresponding author. Email: stephen.lincoln@adelaide.edu.au
ThecomplexationofcationicpyronineB(PB+)andpyronineY(PY+)byβ-cyclodextrin(βCD)andtwolinkedβCDdimers,
N,Nꢀ-bis((2AS,3AS)-3A-deoxy-β-cyclodextrin-3A-yl)succinamide, 33βCD2suc, and N,Nꢀ-bis(6A-deoxy-β-cyclodextrin-
6A-yl)succinamide, 66βCD2suc, in aqueous solution has been studied by UV-vis, fluorescence, and 1H NMR spectroscopy.
The complexation constants for the 1:1 complexes: βCD.PB+, 33βCD2suc.PB+, 66βCD2suc.PB+, and the analogous PY+
complexes are reported as are the dimerization constants for PB+ and PY+. The modes of complexation, dimerization,
and fluorescence quenching are discussed.
Manuscript received: 2 September 2009.
Manuscript accepted: 7 January 2010.
Introduction
system and analogous equilibria apply to the other five systems.
The derived equilibrium constants K1 and Kd appear in Table 1.
There is reasonable agreement between the K1 derived through
all three techniques.
Native and modified cyclodextrins act as robust hosts for a
wide variety of guest species in water.[1–4] Consequently, they
are at the centre of supramolecular chemistry, where secondary
bonding between interacting host and guest species modifies
the chemical and physical behaviour of both in proportion to
the strength of interaction. Native cyclodextins and a range of
their modified forms are biocompatible which, in combination
with their supramolecular host characteristics, results in many
thousands of tonnes being used in industry annually.[5–8]
Often in supramolecular systems, several competing equi-
libria exist, as exemplified by host-guest complexation and
guest aggregation. In this study, we simultaneously quan-
tify the complexation of the dimerizing cationic pyronines
B and Y, PB+ and PY+, by β-cyclodextrin, βCD, and
the modified cyclodextrins N,Nꢀ-bis((2AS,3AS)-3A-deoxy-β-
cyclodextrin-3A-yl)succinamide, 33βCD2suc, and N,Nꢀ-bis(6A-
deoxy-β-cyclodextrin-6A-yl)succinamide, 66βCD2suc, inwhich
a succinamide linker joins two βCDs through either the CA3 or
CA6 carbons of altropyranose and glucopyranose units, respec-
tively (Fig. 1).[9] These systems are chosen because the structural
differences among the host and guest species are expected to
give insight into host-guest complexation of PB+ and PY+, their
dimerization, and the factors affecting their fluorescence, which
is the subject of debate.[10–16]
UV-vis and Fluorescence Studies
The variations of the UV-vis and fluorescence spectra of PB+
andPY+ withincreasingconcentrationsofβCD, 33βCD2sucand
66βCD2suc, are typified by the data for the 33βCD2suc/PB+
system shown in Figs 2 and 3, respectively. A red shift of
the absorption and emission maxima of PB+ occurs together
with a decrease in both absorbance and fluorescence inten-
sity as [33βCD2suc]total increases. The isosbestic point at
556 nm (Fig. 2) is consistent with PB+ existing predomi-
nantly in the free and 33βCD2suc.PB+ complexed states, in
an equilibrium characterized by K1 (Eqn 1). An algorithm
for the formation of the 1:1 complex 33βCD2suc.PB+ best-
fits the data at 0.5 nm intervals, in the range 500–590 nm,
to yield K1 = 4200 200 dm3 mol−1. The decrease in fluo-
rescence of PB+ with increasing [33βCD2suc]total in Fig. 3
was similarly best-fitted over the range 540–650 nm, to give
K1 = 4600 200 dm3 mol−1. The K1 for the other five systems
(Table 1) were similarly derived.
The K1 for the complexation of PB+ and PY+ by 33βCD2suc
and 66βCD2suc are slightly more than twice the K1 for complex-
ation by βCD. This indicates that 33βCD2suc and 66βCD2suc
have little more than a statistical advantage in forming host-guest
complexes over βCD. However, PB+ is complexed approxi-
mately five times more strongly than PY+, consistent with
the ethyl groups extending the hydrophobicity of PB+ by a
greater amount to interact more strongly with the hydrophobic
annuli of βCD, 33βCD2suc, and 66βCD2suc, than do the methyl
groups of PY+. The differing stereochemistries of 33βCD2suc
and 66βCD2suc have little effect on complexation, despite the
Results and Discussion
Equilibria
K1
33βCD2suc + PB+
33βCD2.PB+
(PB+)2
(1)
(2)
−−−ꢀ
ꢁ−−−
Kd
2PB+
−−−ꢀ
ꢁ−−−
Equilibria (1) and (2), characterized by UV-vis, fluorescence,
and 1H NMR spectroscopy, dominate the 33βCD2suc/PB+
© CSIRO 2010
10.1071/CH09467
0004-9425/10/040687

688
H. T. Ngo et al.
of PY+ become significant. The H1–H6 resonances of PB+
systematically shift upfield as [PB+]total increases (Fig. 4), con-
sistent with the formation of the (PB+)2 dimer characterized
by a dimerization constant, Kd = 100 10 mol dm−3. Similar
upfield shifts occur for the H1–H5 resonances of PY+ con-
sistent with the formation of (PY+)2, with a corresponding
Kd = 260 10 mol dm−3. These upfield shifts are greatest for
thePB+ andPY+ aromaticH1–H4 resonances, withtheethyland
methyl proton resonance shifts being significantly less. Accord-
ingly, both Kd are derived from the larger δ variations of H1–H4,
which are more accurately determined. This is consistent with
H1–H4 experiencing an increased electron density in (PB+)2
and (PY+)2, reflecting their position in relation to the aromatic
π electron density of the adjacent PB+ or PY+, as approxi-
mately shown in Fig. 5. The ethyl and methyl groups experience
a lesser change in electron density as they are further from
the aromatic π electron density. (For PB+ H1–H6 δ = 8.334,
7.680, 7.108, 6.822, 3.657, and 1.310 ppm, respectively, with
the upfield shift of (PB+)2 being 1.247, 1.030, 0.849, 1.161,
0.397, and 0.251 ppm, respectively, as derived from the fitting of
a dimerization algorithm to the observed δ data. For PY+ H1–H5
δ = 8.305, 7.630, 7.039, 6.654, and 3.238 ppm, respectively, with
the upfield shift of (PY+)2 being 0.973, 0.082, 0.713, 1.057, and
0.033 ppm.)
HO
3
4
OH
βCD
2
1
ꢀ
O
5
O
6
OH
7
O
C3A
C3A
NH
NH
βCD
βCD
O
33βCD2suc
O
C6A
C6A
NH
NH
βCD
βCD
O
66βCD2suc
CH3
H3C
6
5
4
ꢁ
CH3
O
1
H3C
N
N
3
2
Pyronine B, PBꢁ
For (PY+)2, Kd is 2.6 times that of (PB+)2, which suggests
that of the factors likely to cause differences in dimerization:
hydrophobic attraction, charge repulsion, hydration changes,
and steric hindrance, the last is the most obvious, with the
bulkier ethyl groups of PB+ causing greater steric hindrance
than the methyl groups of PY+ (at the much lower concentrations
used in the UV-vis and fluorescence studies dimer formation is
negligible).
5
CH3
H3C
N
4
ꢁ
O
N
H3C
CH3
3
2
1
Pyronine Y, PYꢁ
N
SO3ꢂ
TNSꢂ, R ꢀ CH3; BNSꢂ, R ꢀ C(CH3)3
R
The complexations of PB+ and PY+ (2.00 × 10−3 mol dm−3
)
by βCD, 33βCD2suc, and 66βCD2suc over the concentration
range0–5.00 × 10−3 mol dm−3 werealsostudiedunderthesame
conditions as the dimerizations. A downfield chemical shift
occurredforallPB+ andPY+ protonresonances, consistentwith
the change from the aqueous environment of their uncomplexed
states to the more hydrophobic environment of their complexed
states. For the 66βCD2suc/PB+ system, the best fit to these data
was obtained for an algorithm representing equilibria (1) and (2)
as shown in Fig. 6. Analogous algorithms provide the best fits to
the data from the other five systems.
Fig. 1. Structures of βCD, 33βCD2suc, 66βCD2suc, PB+, PY+, BNS−,
and TNS−.
inversions at the C2 and C3 carbons of both of the substituted
altropyranose units of the former. The simplest explanation of
these observations is that the dominant 33βCD2suc.PB+ and
66βCD2suc.PB+ complexes, and their PY+ analogues, have
either guest complexed in a single βCD annulus in a similar
way to that in βCD.PB+ and βCD.PY+.
The downfield shifts of H1 and H6 of PB+ in βCD.PB+,
33βCD2suc.PB+, and 66βCD2suc.PB+ from H1 and H6 of
free PB+ are 0.174 and 0.094, 0.165 and 0.087, and 0.190
and 0.082 ppm, respectively; determined simultaneously with
the derivation of K1 from the observed δ data. The downfield
shifts of H1 and H5 of PY+ in βCD.PY+, 33βCD2suc.PY+, and
66βCD2suc.PY+, from the corresponding values for H1 and H5
of free PY+, are 0.078 and 0.016, 0.212 and 0.097, and 0.165
and 0.032 ppm, respectively. Because the H1 and H6 of PB+, and
the H1 and H5 of PY+, are the most distant from each other in the
two pyronines, they are likely to exhibit the largest difference in
change in δ due to differences in interaction upon complexation
by βCD, 33βCD2suc, and 66βCD2suc. In the first three cases,
the downfield shift of H1 is about twice that of H6. In the second
three cases, the downfield shift of H1 varies from two to five
times that of H5. This is consistent with an equilibrium exist-
ing between βCD.PB+ isomers (a) and (c), in which one pair of
PB+ ethyl groups reside in the βCD annulus and isomer, and (b),
in which the PB+ xanthene entity is centred in the βCD annu-
lus (Fig. 7), such that both H1 and H6 experience the electronic
AsimilarrelationshipholdsfortheK1 characterizingtheanal-
ogous BNS− and TNS− systems (Fig. 1),[17,18] where the more
hydrophobic t-butyl group of BNS− interacts more strongly than
does the methyl group of TNS−. The magnitudes of K1 for the
PB+ and PY+ systems are one to four orders of magnitude less
than K1 for the BNS− andTNS− systems.This reflects the struc-
turaldifferencesbetweenthetwotypesofguest, andmayindicate
that while the positive charge of PB+ and PY+ is delocalized
over two dialkylamino groups, the negative charge of BNS− and
TNS− is largely localized on the sulfonate group. It is notice-
able that BNS− and TNS− complex substantially more strongly
than PB+ and PY+, and that the K1 of 66βCD2suc.BNS− and
66βCD2suc.TNS− are consistent with substantial cooperativity
in complexation between the two linked βCD annuli.
1H NMR Studies of the Dimerization and Complexation
of PB+ and PY+
At the higher concentrations required for 1H NMR studies
dimerization of PB+ (Eqn 2) and an analogous dimerization

Supramolecular Chemistry of Pyronines B and Y
689
Table 1. Equilibrium constants for the 1:1 host/guest complexes (K1), determined by UV-vis, fluorescence, and
1H NMR spectroscopyA
Complex
UV-visB K1 [dm3 mol−1
]
FluorescenceB K1 [dm3 mol−1
]
1H NMRC K1 [dm3 mol−1
]
βCD.PB+
1660 80
4200 200
4500 200
370 20
760 40
780 40
55400D
2000 100
4600 200
4900 200
320 30
830 40
740 40
46500D
1500 150
4400 200
5400 200
320 70
500 50
500 50
33βCD2suc.PB+
66βCD2suc.PB+
βCD.PY+
33βCD2suc.PY+
66βCD2suc.PY+
βCD.BNS−
33βCD2suc.BNS−
66βCD2suc.BNS−
βCD.TNS−
186000D
1250000D
3020E
110000D
3300000D
3300E
33βCD2suc.TNS−
66βCD2suc.TNS−
10700E
16100E
9600E
12500E
AFor PB+, PY+, and BNS−, Kd = 100 10, 260 10, and 265 dm3 mol−1, respectively.
BIn aqueous 1.00 × 10−4 mol dm−3 hydrochloric acid at I = 0.10 mol dm−3 (NaCl) and 298.2 K.
CIn 1.00 × 10−4 mol dm−3 hydrochloric acid D2O solution at I = 0.10 mol dm−3 (NaCl) and 298.2 K.
DIn aqueous phosphate buffer at pH 7.0 and I = 0.1 mol dm−3 and 298.2 K from ref. [17].
EIn aqueous phosphate buffer at pH 7.0 and I = 0.1 mol dm−3 and 298.2 K from ref. [18].
600
400
200
0
0.6
0.4
0.2
0.0
450
475
500
525
550
575
600
550
575
600
625
650
Wavelength [nm]
Wavelength [nm]
Fig. 2. UV-vis absorbance spectra of PB+ alone (6.51 × 10−6 mol dm−3
)
Fig. 3. Emission spectra of PB+ alone (6.19 × 10−7 mol dm−3) and
in the presence of increasing concentrations of 33βCD2suc (rang-
and in the presence of increasing concentrations of 33βCD2suc
(ranging from 0.00 to 1.97 × 10−3 mol dm−3) in aqueous hydrochlo-
ing from 0.00 to 1.54 × 10−3 mol dm−3
) in aqueous hydrochloric
ric acid (1.00 × 10−4 mol dm−3
,
I = 0.10 mol dm−3 NaCl) at
acid (1.00 × 10−4 mol dm−3, I = 0.10 mol dm−3 NaCl) at 298.2 K. The
excitation wavelength λex = 515 nm. The excitation and emission slit
widths = 5 nm. The arrow indicates the direction of relative fluores-
cence emission change as [33βCD2suc]total increases. The λmax = 568 nm
(656 a.u.) and 572 nm (431 a.u.) for the free and complexed PB+ species,
respectively.
298.2 K. The arrow indicates the direction of absorbance change
as [33βCD2suc]total increases. An isosbestic point occurs at
556 nm. λmax = 553 nm (ε = 1.07 × 105 dm3 mol−1 cm−1
) and 557 nm
(ε = 1.01 × 105 dm3 mol−1 cm−1) for the free and complexed PB+ species,
respectively.
environment of the βCD annulus (the resulting inequivalence of
the two N-diethyl groups is not observed in the 1H spectra as the
complex lifetimes are short on the 1H NMR timescale). Alterna-
tively, a dominant βCD.PB+ complex with a structure midway
between either (a) and (b), or (b) and (c), or both may exist.
Similar possibilities exist for the complexation of PB+ in single
βCD annuli of 33βCD2suc.PB+ and 66βCD2suc.PB+ and for
the analogous PY+ systems.
of PB+ (those arising from the PB+ H5 protons are obscured
due their chemical shift being similar to those of βCD protons)
as seen in Fig. 8. This is consistent with the two sets of interact-
ing protons being within 400 p.m. of each other and the ethyl
groups of PB+ and the attached aromatic ring being partially
within the βCD annulus of the dominant βCD.PB+ host-guest
complex. Becausetheratioofmethylprotonstoaromaticprotons
is 6:1, the cross-peaks arising from the methyl protons will be the
more intense if all other factors are the same. Similar spectra are
observed for the 33βCD2suc.PB+ (Fig. 9) and 66βCD2suc.PB+
host-guest complexes. Similar cross-peaks are also observed for
the analogous PY+ solutions, but are weaker, consistent with the
PY+ complexes exhibiting lower K1 values. These data corre-
late well with either of the models proposed, on the basis of the
2D ROESY 1H NMR Studies
Solutions of PB+ (2.00 × 10−3 mol dm−3) in the presence of
either double the concentration of native βCD, or equimolar
1
in either of the linked βCD dimers in D2O solution, show H
ROESY NMR cross-peaks arising from interactions between the
βCD H3, H5, and H6 annular protons and the H6 and H4 protons

690
H. T. Ngo et al.
(a)
100
90
80
70
60
50
40
30
20
10
0
CH3
H3C
8.25
8.10
7.95
7.80
7.65
7.50
ꢁ
N
N
N
H3C
N
O
CH3
b
c
(b)
H3C
6
CH3
βCD
5
a
4
ꢁ
H3C
O
N
CH3
3
0.000
0.005
[PBꢁ]total [mol dmꢂ3
0.010
0.015
1
2
]
(c)
CH3
H3C
Fig. 4. The left ordinate shows the variation of δ 1H (300 MHz) of the aro-
matic H1 proton of PB+ as [PB+]total increases from 2.00 × 10−4 mol dm−3
to 2.00 × 10−2 mol dm−3 in D2O (1.00 × 10−4 mol dm−3 hydrochloric acid,
I = 0.10 mol dm−3 NaCl) at 298.2 K. The circles represent experimental
data. The solid curve a, shows the simultaneous best fit of the algorithm for
dimerization of PB+ to the δ variations of protons H1–H4. The right-hand
ordinate shows the percentage speciation relative to [PB+]total. Curve b
shows the percentage of [PB+] and curve c represents twice the percentage
of [(PB+)2].
ꢁ
CH3
N
O
H3C
Fig. 7. Possible equilibria between βCD.PB+ isomers (a)–(c).
βCD
PBꢁ
H6
βCD
H2ꢂH6
H1
PBꢁ
H1 H2H3 H4
H3C
6
CH3
5
4
2
ꢁ
2
N
O
1
CH3
ꢃ
N
H3C
F2
[ppm]
3
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
Kd
H3C
CH3
ꢁ
H3C
H3C
O
N
N
CH3
CH3
N
O
N
ꢁ
H3C
CH3
Fig. 5. Dimerization of PB+ to form (PB+)2.
8
7
6
5
4
3
2
F1 [ppm]
Fig. 8. 2D 1H ROESY NMR (600 MHz) spectrum of PB+
(2.00 × 10−3 mol dm−3
) with two molar equivalent βCD in D2O
100
90
80
70
60
50
40
30
20
10
0
(1.00 × 10−4 mol dm−3 hydrochloric acid, I = 0.10 mol dm−3 NaCl)
at 298.2 K, with a mixing time of 300 ms. Cross-peaks between H6 of PB+
and H3, H5, and H6 of βCD; and between H4 of PB+ and H5 of βCD,
are enclosed in rectangles. The other cross-peaks arise from interactions
between the H1 and H2–H6 of βCD and between H3, H4 and H6 and H5
of PB+.
8.44
8.34
8.24
8.14
8.04
7.94
a
d
b
PB+ and PY+ chemical shift variations discussed in the previous
section.
c
The deductions from the 1H NMR data concerning the com-
plexation of PB+ and PY+ are relevant to interpretation of
the resulting fluorescence quenching. Both the model for com-
plexation (Fig. 7) and the alternative model, where a dominant
βCD.PB+ complex with a structure midway between either
(a) and (b), or (b) and (c), or both may exist, are likely to alter
the symmetry of the charge distribution represented by the PB+
and PY+ resonance structures (d)–(g) (Fig. 10), through which
all bonds share partial double bond character. Complexation by
βCD, 33(βCD)2suc, and 66(βCD)2suc results in part of PB+
and PY+ being in the hydrophobic βCD annulus and part in the
aqueous environment. As both dialkylamino groups cannot be
0.000 0.001 0.002 0.003 0.004 0.005
[66βCD2suc]total [mol dmꢂ3
]
Fig. 6. Left ordinate: variation of δ 1H (300 MHz) of the aromatic H1
of PB+ (2.00 × 10−3 mol dm−3) with [66βCD2suc]total (ranging from 0 to
5.00 × 10−3 mol dm−3) in D2O (1.00 × 10−4 mol dm−3 hydrochloric acid,
I = 0.10 mol dm−3 NaCl) at 298.2 K. The circles are the experimental data
and the solid curve a is the best fit of the algorithm incorporating (PB+)2
and 66βCD2suc.PB+ to the δ variations of protons H1–H4 and H6. Right
ordinate: speciation relative to [PB+]total, curve b is the percentage of [PB+],
curve c is twice the percentage of [(PB+)2], and curve d is the percentage
of [66βCD2suc.PB+].

Supramolecular Chemistry of Pyronines B and Y
691
PBꢁ
H6
suc
is located at the centre of the xanthene entity as a consequence of
stabilization by the electron rich environment generated by the
ether oxygens of the βCD annulus. This accentuates a structural
change in the amino groups towards a tetrahedral stereochem-
istry, which engenders non-radiative deactivation. This model
was proposed in the absence of evidence for the substantial
interaction of the dialkylamino groups of PB+ and PY+ with
the interior of the βCD annulus shown to occur in the present
study. In view of this it is apparent that Reija’s model represents
a component of the two alternative models proposed here for
introducing asymmetry into the charge distribution in PB+ and
PY+ upon complexation by βCD, 33βCD2suc, and 66βCD2suc,
and consequent fluorescence quenching.
βCD
H2ꢂH6
PBꢁ
H1 2 H3
HOD
H4 βCD
PBꢁ
H5
H
CH2
H1
F1
[ppm]
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
Conclusions
The complexation of PB+ and PY+ by βCD, 33βCD2suc, and
66βCD2suc have been characterized in aqueous solution by
UV-vis, fluorescence, and 1H NMR spectroscopy. By com-
parison with the stabilities of the βCD.PB+ and βCD.PY+,
66βCD2suc.PB+, 33βCD2suc.PB+, and the analogous PY+
complexes, are slightly more than twice as stable, consistent
with a statistical enhancement in stability and little coopera-
tivity between the two linked βCD annuli in complexing PB+
and PY+. However, PB+ is complexed approximately five times
more strongly than PY+, consistent with the greater hydropho-
bicity of the PB+ ethyl groups interacting more strongly with
the hydrophobic annuli of βCD, 33βCD2suc, and 66βCD2suc,
than do the methyl groups of PY+. The fluorescence quenching
of PB+ and PY+ in the complexes is attributed to a change in
their charge distribution, such that non-emissive relaxation of
the excited state occurs through an dialkylamino group, assum-
ing a tetrahedral stereochemistry in the complexes than is the
case in free PB+ and PY+. The dimerizations of PB+ and PY+,
which occurs at the higher concentrations required for 1H NMR
studies, have also been characterized.
8
7
6
5
4
3
2
F2 [ppm]
Fig. 9. 2D 1H ROESY NMR (600 MHz) spectrum of PB+
(2.00 × 10−3 mol dm−3
)
and equimolar 33(βCD)2suc in D2O
(1.00 × 10−4 mol dm−3 hydrochloric acid, I = 0.10 mol dm−3 NaCl)
at 298.2 K, with a mixing time of 300 ms. The cross-peaks enclosed in
rectangles arise from interactions between H6 of PB+ and H3, H5, H6 of
βCD; and between H4 of PB+ and H5 of βCD. The other cross-peaks arise
from interactions between the H1 and H2–H6 of 33(βCD)2suc and between
H3, H4 and H6 and H5 of PB+.
(d)
R
(e)
R
R
N
R
N
R
N
R
N
ꢁ
O
O
O
R
R
ꢁ
(f)
(g)
R
R
N
R
N
R
N
R
N
ꢁ
O
ꢁ
R
R
R
Experimental
Fig. 10. Resonance structures (d)–(g) of PB+, R = CH2CH3, and PY+,
R = CH3.
Materials
Pyronine B (Sigma) was purchased as the 95% pure salt
(PB)2Fe2Cl8 and was twice recrystallized from water before
use.[19] The commercially obtained pyronine Y chloride salt
contained ∼40% impurities by weight. These water insolu-
ble impurities were filtered from an aqueous slurry with a
0.45 µm filter before use.[12] β-Cyclodextrin (Nihon Shokuhin
Kako Co) was used without further purification and 33βCD2suc
and 66βCD2suc were synthesized as previously described.[20]
Hydrochloric acid (Ajax) and sodium chloride (Merck) were
used to maintain constant acidity and ionic strength. Water was
purified with a Milli-Q system. All organic solvents were of
HPLC grade.
simultaneously in the βCD annulus, both PB+ and PY+ experi-
ence an environmental asymmetry which is likely to introduce
an asymmetry in charge distribution. As complexation causes
fluorescence quenching the resulting change in charge distribu-
tion must increase the probability of non-radiative decay of the
excited states of PB+ and PY+.
Two studies in a range of solvents are consistent with the
most probable non-radiative decay for PB+ and PY+ occur-
ring through a two-state mechanism, where the fluorescent
planar states, resembling (d) and (f), are in equilibrium with
non-emissive states, resembling (e) and (g), in which nitrogen
assumes a tetrahedral stereochemistry such that an S1–S0 inter-
nal conversion occurs.[14,15] A similar pathway for non-radiative
transitions of excited electronic states of PB+ and PY+ to their
ground states provides a plausible explanation for the decrease
in fluorescence shown by PB+ and PY+ upon complexation by
βCD, 33(βCD)2suc, and 66(βCD)2suc.
Solutions were prepared from fresh stock solutions and
were 1.00 × 10−4 mol dm−3 in hydrochloric acid, to prevent the
base hydrolysis of PB+ and PY+,[16] and 0.10 mol dm−3 NaCl
to maintain a constant ionic strength. The concentrations of
PY+ stock solutions were estimated using the reported molar
absorptivity at 546 nm of ε = 8.1 × 104 mol−1 dm3 cm−1 [21]
.
Aqueous solutions for UV-visible and fluorescence studies were
6.0 × 10−6 mol dm−3 in PB+ and 9.0 × 10−6 mol dm−3 in PY+;
and 6.0 × 10−7 mol dm−3 of PB+ and 9.0 × 10−7 mol dm−3 of
PY+, respectively. The βCD and linked βCD dimer concentra-
tions varied over wide ranges, as indicated in the figure captions.
Solutions for 1H NMR experiments were prepared in D2O
A model has also been proposed by Reija for the decreased
fluorescence of PB+ and PY+ upon complexation by βCD.[16]
It is postulated that the xanthene entities of PB+ and PY+ are
completely complexed inside the βCD annulus to form a non-
emissive, charge-transfer excited state where the positive charge

692
H. T. Ngo et al.
1.00 × 10−4 mol dm−3 in hydrochloric acid and 0.10 mol dm−3
in NaCl. The concentrations of PB+ and PY+ solutions
for dimerization studies ranged from 1.0 × 10−3 mol dm−3 to
2.0 × 10−2 mol dm−3. For complexation studies the concen-
trations of PB+ and PY+ were 2.0 × 10−3 mol dm−3, while
those of the βCD and dimer hosts were varied over the range
algorithms to these data to derive complexation constants and
speciation plots; (ii) 1H NMR chemical shift variations of PB+
and PY+ and fitting of algorithms to these data to derive dimer-
ization and complexation constants; (iii) 2D ROESY 1H NMR
spectra and a Table of 1H NMR chemical shifts. This material is
available on the Journal’s website.
1
0–5.0 × 10−3 mol dm−3. For the 2D H NMR ROESY experi-
ments, each sample was 2.0 × 10−3 mol dm−3 in either PB+ or
PY+ and in either βCD or a linked βCD dimer.
Acknowledgement
Support of this study by the Australian Research Council and the University
of Adelaide, and the award of an Endeavour Postgraduate Award to H.T.N.
are gratefully acknowledged.
Instrumental
UV-vis spectra were run on a Varian Cary 5000 spectrophoto-
meter, using matched quartz cells with a 1 cm path length, in
a cell block with a constant temperature of 298.2 K. Solutions
were equilibrated at this temperature before scanning. The scan
rate was 600 nm min−1 and the data interval was 0.5 nm. Fluo-
rescence spectra were run on a Varian Cary Eclipse fluorimeter.
The solutions were equilibrated in a 1 cm path length quartz
cell in a thermostatted 298.2 K cell block. The excitation and
References
[1] J. Szejtli, Chem. Rev. 1998, 98, 1743. doi:10.1021/CR970022C
[2] C. J. Easton, S. F. Lincoln, Modified Cyclodextrins: Scaffolds andTem-
plates for Supramolecular Chemistry 1999 (Imperial College Press:
London).
[3] A. Harada, Acc. Chem. Res. 2001, 34, 456. doi:10.1021/AR000174L
[4] Y. Liu, Y. Chen, Acc. Chem. Res. 2006, 39, 681. doi:10.1021/
AR0502275
[5] J. Szejtli, Chem. Rev. 1998, 98, 1743. doi:10.1021/CR970022C
[6] A. R. Hedges, Chem. Rev. 1998, 98, 2035. doi:10.1021/CR970014W
[7] E. M. M. Del Valle, Process Biochem. 2004, 39, 1033. doi:10.1016/
S0032-9592(03)00258-9
[8] T. Loftsson, D. Duchêne, Int. J. Pharmaceutics 2007, 329, 1.
doi:10.1016/J.IJPHARM.2006.10.044
emission slit widths were 5 nm, the scan rate was 120 nm min−1
,
and the data interval was 0.5 nm. 2D 1H ROESY NMR spectra
were recorded on a Varian Inova 600 spectrometer operating at
599.957 MHz, using a standard pulse sequence with a mixing
time of 300 ms.
Data Analysis
[9] J. H. Coates, C. J. Easton, S. J. van Eyk, S. F. Lincoln, B. L. May, C. B.
Whalland, M. L. Williams, J. Chem. Soc., Perkin Trans. 1 1990, 2619.
doi:10.1039/P19900002619
[10] K. Fujiki, C. Iwanaga, M. Koizumi, Bull. Chem. Soc. Jpn. 1962, 35,
185. doi:10.1246/BCSJ.35.185
[11] R. L. Schiller, S. F. Lincoln, J. H. Coates, J. Chem. Soc., FaradayTrans.
I 1986, 82, 2123 (in the text “annular radii” should read “annular
diameters”). doi:10.1039/F19868202123
[12] R. L. Schiller, S. F. Lincoln, J. H. Coates, J. Chem. Soc., Faraday
Trans. I 1987, 83, 3237. doi:10.1039/F19878303237
[13] R. L. Schiller, S. F. Lincoln, J. H. Coates, J. Incl. Phenom. 1987, 5,
59. doi:10.1007/BF00656403
[14] Y. Onganer, E. L. Quitevis, J. Phys. Chem. 1992, 96, 7996.
doi:10.1021/J100199A033
[15] B. Acemioglu, M. Arik, Y. Onganer, J. Lumin. 2002, 97, 153.
doi:10.1016/S0022-2313(02)00218-1
[16] B. Reija, W.Al-Soufi, M. Novo, J. V. Tato, J. Phys. Chem. B 2005, 109,
1364. doi:10.1021/JP046587B
[17] D.-T. Pham, P. Clements, C. J. Easton, J. Papageorgiou, B. L. May,
S. F. Lincoln, New J. Chem. 2008, 32, 712. doi:10.1039/B715985D
[18] D.-T. Pham, P. Clements, C. J. Easton, J. Papageorgiou, B. L.
May, S. F. Lincoln, Supramol. Chem. 2009, 21, 510. doi:10.1080/
10610270802406579
[19] E. M. Chamberlin, B. F. Powell, D. E. Williams, J. Conn, J. Org. Chem.
1962, 27, 2263. doi:10.1021/JO01053A538
[20] C. J. Easton, S. J. van Eyk, S. F. Lincoln, B. L. May, J. Papageorgiou,
M. J. Williams, Aust. J. Chem. 1997, 50, 9. doi:10.1071/C96168
[21] L. P. Glanneschi, T. Kurucsev, J. Chem. Soc., Faraday Trans. II 1974,
70, 1334. doi:10.1039/F29747001334
[22] R. A. Binstead, B. Jung, A. D. Zuberbuhler, SPECFIT/32, v3.0.39(b)
2007 (Spectrum Software Associates: Marlborough, MA, USA).
[23] C. Frassineti, S. Ghelli, P. Gans, A. Sabatini, S. Moruzzi, A. Vacca,
Anal. Biochem. 1995, 231, 374. doi:10.1006/ABIO.1995.9984
[24] P. Gans, A. Sabatini, A. Vacca, HypNMR, v3.1.5 2004 (Protonic
Software).
The K1 for the 1:1 host-guest complexes of either PB+ or PY+
with the βCD and linked βCD dimer hosts were derived by simul-
taneously fitting the absorbance variation typified by Fig. 1 over
a wide wavelength range at 0.5 nm intervals to Eqn 3:
A = εPB[PB+] + ε33βCD2suc[33βCD2suc]
+ ε33βCD2suc.PB[33βCD2suc.PB+],
(3)
where A is the absorbance and εPB, ε33βCD2suc, ε33βCD2suc.PB
are the molar absorbances of the PB+, 33βCD2suc, and
33βCD2suc.PB+, respectively. Analogous equations apply for
the absorbance variation of the other five systems and for the flu-
orescence variations all six systems. The SPECFIT/32 protocol
was used in the fitting procedure.[22] The dimerization constants,
Kd, for PB+ and PY+ were derived by simultaneously fitting the
variation of the 1H chemical shifts, δexp, of H1–H4 as [PB+]total
and [PY+]total increased to Eqn 4, where the third right hand
term is absent, to the experimental data using the HypNMR 2003
program as typified by Fig. 4.[23,24] The K1 for all six systems
were similarly derived by fitting 1H chemical shift variations for
H1–H4 to Eqn 4 for the 66βCD2suc.PB+ system (Fig. 6) and
analogous equations for the other five systems.
δexp = δPB[PB+] + δPB2[(PB+)2]
+ δβCD2suc.PB[66βCD2suc.PB+]
(4)
Accessory Publication
Electronic supplementary material is available showing: (i) UV-
vis and fluorescence changes of PB+ and PY+ and fitting of