Characterization of group-inclusion complexations of rhodamine derivatives with native and 2,6-di-O-methylated β-cyclodextrins

Article abstract of DOI:10.1007/s10847-020-00979-y

The inclusion complexations of rhodamine derivatives with native and 2,6-di-O-methylated β-cyclodextrins (β-CD and DM-β-CD) were studied spectrophotometrically. Rhodamine derivatives were shown to form 1:1 inclusion complexes with β-CDs by the continuous variation method. The structures of the inclusion complexes were characterized by ¹H-¹H rotating frame nuclear Overhauser effect spectroscopy (ROESY) NMR measurements. It was found that native β-CD encapsulates the xanthenyl ring of rhodamines into the cyclodextrin cavity, while DM-β-CD forms two group-in complexes (phenyl-in and xanthenyl-in (bidirectional (bimodal) inclusion complexes)) with rhodamines bearing moderately bulky functional groups. Furthermore, we demonstrated the unique thermodynamics for the group-inclusion complex formation by DM-β-CD. The quantum yields for the inclusion complexes of rhodamines were determined using a quantum measurement apparatus equipped with a half-moon unit. The results indicated that the cyclodextrin inclusion of rhodamines with the bulky amino substituents on the xanthenyl ring largely decreases the quantum yield values. Based on these results, the substituent effects on the fluorescence process for the cyclodextrin inclusion complexes of rhodamines were discussed. This study provides useful insights for the functional group recognition of native and modified β-CDs.

Full text of DOI:10.1007/s10847-020-00979-y

Journal of Inclusion Phenomena and Macrocyclic Chemistry
https://doi.org/10.1007/s10847-020-00979-y
ORIGINAL ARTICLE
Characterization of group‑inclusion complexations of rhodamine
derivatives with native and 2,6‑di‑O‑methylated β‑cyclodextrins
Yoshimi Sueishi1 · Yuki Matsumoto1 · Yuka Kimata1 · Yoshihiro Osawa2 · Naoya Inazumi3 · Tadashi Hanaya1
Received: 5 September 2019 / Accepted: 17 January 2020
© Springer Nature B.V. 2020
Abstract
The inclusion complexations of rhodamine derivatives with native and 2,6-di-O-methylated β-cyclodextrins (β-CD and
DM-β-CD) were studied spectrophotometrically. Rhodamine derivatives were shown to form 1:1 inclusion complexes with
β-CDs by the continuous variation method. The structures of the inclusion complexes were characterized by 1H-1H rotating
frame nuclear Overhauser efect spectroscopy (ROESY) NMR measurements. It was found that native β-CD encapsulates
the xanthenyl ring of rhodamines into the cyclodextrin cavity, while DM-β-CD forms two group-in complexes (phenyl-in
and xanthenyl-in (bidirectional (bimodal) inclusion complexes)) with rhodamines bearing moderately bulky functional
groups. Furthermore, we demonstrated the unique thermodynamics for the group-inclusion complex formation by DM-β-CD.
The quantum yields for the inclusion complexes of rhodamines were determined using a quantum measurement apparatus
equipped with a half-moon unit. The results indicated that the cyclodextrin inclusion of rhodamines with the bulky amino
substituents on the xanthenyl ring largely decreases the quantum yield values. Based on these results, the substituent efects
on the fuorescence process for the cyclodextrin inclusion complexes of rhodamines were discussed. This study provides
useful insights for the functional group recognition of native and modifed β-CDs.
Keywords Rhodamine dye · β-Cyclodextrins · Group-inclusion complex · Quantum yield · Substituent efect
Introduction
emission, high fuorescence quantum yield, and high stabil-
ity [6, 7]. The optical properties of rhodamines are sensitive
to the polarity and hydrogen donating ability of the medium
and can be tuned by incorporating suitable electron-donat-
ing and/or electron-accepting groups [6, 7]. Therefore, it is
benefcial to clarify the photochemical processes and solva-
tochromic behaviour of rhodamines.
Fluorescent molecules have attracted much attention for use
in the development of sensors, switches, and high-density
optical data storage units [1–3]. Rhodamine dyes are useful
fuorescence probes and fnd widespread application in the
felds of chemistry and biology [4, 5]. Among the many
reported fuorophores, rhodamines have excellent optical-
physical properties such as long-wavelength absorption and
Several limitations of organic dyes, including undesirable
aggregation, photo-bleaching, and surface adsorption, have
hampered their application as molecular biological probes
and sensors [8, 9]. These disadvantages can be addressed by
the inclusion of supermolecules, which minimizes aggre-
gation and surface adsorption [9–11]. Nau and co-workers
reported that the addition of the cucurbit[7]uril host mol-
ecule to rhodamine 6G solution resulted in several advanta-
geous efects such as enhanced brightness and aggregation
reduction [9, 12].
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10847-020-00979-y) contains
supplementary material, which is available to authorized users.
* Yoshimi Sueishi
ysueishi@okayama-u.ac.jp
1
Department of Chemistry, Faculty of Science,
Okayama University, 3-1-1 Tsushima-naka, Kita-ku,
Okayama 700-8530, Japan
Cyclodextrins have been used extensively as host mol-
ecules [13]. β-Cyclodextrin (β-CD) has an appropriate
cavity that can accommodate many aromatic compounds
and it has received much more attention than other
cyclodextrin analogues. Therefore, numerous chemically
2
Otsuka Electronics Co. Ltd, 1-10 Sasagaoka, Minakuchi-cho,
Kouka-shi, Shiga 528-0061, Japan
3
Technical Support Division, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan
Vol.:(0123456789)
1 3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
modified β-CDs have been designed and synthesized in
order to enhance the solubility and cavity hydrophobicity.
The fundamental properties of functional molecules are
improved by CD inclusion [14]. However, the effects of
such inclusion on the fundamental properties of rhoda-
mine derivatives are not well established.
Experimental
Materials
The structures of the rhodamine derivatives are shown in
Fig. 1. The rhodamines were obtained commercially and
used as received (purity > 98%): RB (Tokyo Chemical
Industry Co.); TMRM (Thermo Fisher Scientifc K. K.);
R110 (Sigma-Aldrich Chemical Company, Inc.); R123
and R101 (Santa Cruz Biotechnology, Inc.). Two kinds of
β-CDs (β-cyclodextrin (β-CD) and 2,6-di-O-methylated
β-CD (DM-β-CD)) were purchased from Nacalai Tesque and
were used without further purifcation. Water was purifed
by distillation and used as the solvent.
In this study, using five rhodamine derivatives as guest
molecules, we have examined the inclusion complexa-
tion of native and modified β-CDs. We have unveiled the
characteristic inclusion behaviour of rhodamines with the
β-CD modified by methylation of the hydroxy groups on
the rim. Based on these results, we have demonstrated the
functional group recognition ability of β-CDs. Further-
more, using a fluorescence quantum efficiency analysis
system equipped with a half-moon unit, we examined the
effects of various substituents on the quantum yields for
inclusion complexes of rhodamines with β-CDs; further,
we discussed fluorescence processes of rhodamines.
Absorption, fuorescence, and NMR measurements
The absorption spectra of rhodamines were measured on a
Shimadzu UV-1850 spectrometer. The solution temperature
Fig. 1 Structures of rhodamine
Rhodamine dyes (guest molecules)
dyes and β-cyclodextrins (native
and 2,6-methylated β-CDs)
OH
O
OH
O
O
Cl
O
ClO4
Cl
N
O
N
N
O
N
NH2
O
NH2
R110
RB
TMRM
O
O
O
O
Cl
NH2
N
O
N
NH2
O
R123
R101
-Cyclodextrins (host molecules)
OR6
-CD: R2, R3, R6 = H
DM- -CD: R3 = H
6
O
5
4
1
O
3
2
7
R2, R6 = CH3
O
OR2
R3
1 3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
was maintained at 298.0 0.2 K by means of an external cir-
culating water bath. The ϕ values of the inclusion complexes
of rhodamines were determined on an Otsuka Electronics
Quantum Efciency Analysis System (QE 1000) equipped
with a half-moon unit (Otsuka, Japan) at 298.0 0.5 K. The
quantum yield was calibrated using quinine bisulfate solu-
tion in sulfuric acid (0.05 M (1 M=1 mol dm−3)) as a stand-
ard: excitation wavelength=347.5 nm and ϕ=0.51 0.02
[15].
Results and discussion
Evaluation of complexation constants of inclusion
complexes
The typical absorption spectra for the complexation of RB
with native β-CD and modifed DM-β-CDs in the visible
region are shown in Fig. 2. The absorption intensities of RB
decreased upon the addition of β-CDs. To confrm the stoichi-
ometry for the complexation of RB with β- and DM-β-CDs,
NMR spectral shift measurements were conducted by vary-
ing the mole fraction of RB and β-CDs (continuous variation
method, Job’s method) [16]. The Job plots (Δδ(1–xH) vs xH:
xH =[β-CD]0/([β-CD]0 +[RB]0)) for the complexation of RB
with β-CDs showed a maximum at xH=0.5 (Fig. 2), suggesting
a 1:1 complex formation. Similar results were obtained for the
complexation of other rhodamine derivatives with native and
modifed β-CDs. The equilibrium for the 1:1 complexation
can be expressed as
1H-NMR spectra of the β-CD inclusion complexes with
rhodamines were recorded on a Varian 400-MR ASW spec-
trometer (400 MHz) at room temperature. Chemical shifts
are given as δ values relative to HOD (δ 4.79) as the inter-
nal standard. 2D ROESY-NMR measurements were carried
out on a Varian VNS600 spectrometer (600 MHz) at 303 K.
The mixing time for each ROESY-NMR measurement was
200 ms.
Guest + Host ⇌ Comp
(a)
(b)
Fig. 2 Absorption spectra of RB in aqueous solution contain-
ing a β-CD and b DM-β-CD at 298 K: a [RB]0 =7.8×10–3 mM,
[β-CD]0 =(1) 0, (2) 0.16, (3) 0.32, (4) 0.63, (5) 0.95, and (6) 1.3 mM;
b [RB]0 =7.8×10–3 mM, [DM-β-CD]0 =(1) 0, (2) 0.79, (3) 0.94, (4)
1.1, (5) 1.4, and (6) 1.6 mM. Down Determination of inclusion com-
plexation constants of RB with native and 2,6-methylated β-CDs.
Inset Job’s plots for the inclusion complexation of RB with β-CDs
1 3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
ROESY-NMR spectra for the inclusion complexes of other
rhodamines are shown in Figs. S2–S7 in the Supplementary
Data. In R110/β-CD, cross peaks were detected between the
inner C(3,5)-protons of the β-CD cavity and the C(1,4,5,8)-
protons in the xanthenyl group: specifically, interactions
between C(3)-H of β-CD and C(1,8)-H in the xanthenyl group,
and between C(5)-H of β-CD and C(2,7)-H in the xanthenyl
group were seen. Furthermore, there was no correlation with
the protons of the phenyl group. These observations indicated
that the xanthenyl group of R110 is encapsulated from the
wide end of β-CD. In R110/DM-β-CD, the inner protons of
DM-β-CD interacted with the C(3′,4′,5′,6′)-protons of the
phenyl ring in R110, and cross peaks arising from the C(3,5)-
protons of DM-β-CD and C(1,4,5,8)-protons of the xanthenyl
group in R110 were also observed. As suggested above, the
continuous variation method for the complexation of rhoda-
mines with β- and DM-β-CDs suggests a 1:1 complex for-
mation. Therefore, it is noteworthy that R110, which has two
bulky functional groups, forms two group-in inclusion com-
plexes, where either a phenyl or xanthenyl group is included in
the DM-β-CD cavity (phenyl-in and xanthenyl-in complexes).
On the basis of ESR experiments, Kotake and Janzen reported
that a nitroxide radical having more than one bulky functional
group forms two group-in inclusion complexes (bidirectional
inclusion complexes) [18]. Furthermore, a few NMR and theo-
retical calculation studies on inclusion complexation with CDs
assumed the presence of bidirectional inclusion complexes [19,
20], which is in agreement with our observations.
[Comp]
K =
(1)
[Guest][Host]
where Guest and Host denote rhodamines and β-CDs,
respectively. Comp denotes the complex of rhodamines with
β-CDs. Under the condition [Host]> >[Guest], the absorption
spectral data in Fig. 2 were analysed according to the Ben-
esi–Hildebrand equation for the 1:1 complex formation [17]:
[Host]0
1
1
=
[Host]0 +
(2)
(A − A0)
A∞ − A0
K[A∞ − A0]
where A and A0 are the absorption intensities. As shown
in Fig. 2, a good linear relationship between [Host]0/(A – A0)
and [Host]0 was obtained and the complexation constant K
of inclusion complex could be determined from the slope
and intercept in Eq. 2.
The K values obtained for the complexation of rhodamines
with β-CDs at various temperatures are listed in Table 1.
The K values for DM-β-CD are larger than those for native
β-CD, except for the inclusion of RB. In DM-β-CD, meth-
ylation of the OH groups on the CD rim widens the hydro-
phobic environment around the cavity. An enhancement of
the inclusion ability is responsible for larger K values in the
case of DM-β-CD inclusion. In the case of the complexation
of RB, the K values for native β-CD are larger than those
for DM-β-CD, which is attributable to the diference in the
inclusion complex structures, as explained in the next section.
The relative intensities of the cross peaks for the inclusion
complexes of rhodamines with β- and DM-β-CDs are listed in
Table S1 (Supplementary Data). In the inclusion of TMRM
and R123 by β-CD, cross peaks of the C(1,2,4,5,7,8)-protons
of rhodamines with the inner C(3,5)-protons of β-CD were
observed, while no interaction between the protons of the
phenyl group in the rhodamines and the C(3,5)-protons of
β-CD was detected, indicating that the xanthenyl moiety of
Structures of inclusion complexes with β‑CDs
ROESY-NMR experiments provide information on the posi-
tion of the guest molecules in the CD cavity. Figure 3 shows
the representative ROESY-NMR spectra of the inclusion
complexes of rhodamine R110 with β- and DM-β-CDs. The
Table 1 The complexation
constants and thermodynamic
parameters for inclusion
complex formation of
Guest
Host
10−2 K/M−1
ΔH/kJ mol−1
ΔS/kJ mol−1 K−1
288 K
298 K
308 K
RB
RB
β-CD
DM-β-CD
β-CD
DM-β-CD
β-CD
DM-β-CD
β-CD
48.1 1.0
8.14 0.04
13.9 0.2
55.5 0.9
1.40 0.01
2.36 0.05
2.46 0.03
12.1 0.1
7.66 0.15
33.6 0.4
5.83 0.14
10.3 0.2
41.5 0.6
1.21 0.03
2.06 0.04
2.06 0.02
9.91 0.07
6.85 0.09
23.7 0.5
4.51 0.04
7.93 0.07
31.2 0.8
–
1.74 0.05
1.78 0.03
8.05 0.16
6.36 0.05
−26.2 0.2
−21.8 0.8
−20.6 0.1
−21.3 0.2
−10.6 0.5
−11.3 0.6
−11.9 0.4
−15.1 0.3
−6.88 0.45
−20.5 0.7
−20.1 2.7
−11.5 0.4
−2.23 0.66
4.27 1.61
6.40 2.11
4.50 1.27
6.50 0.96
31.3 1.5
rhodamines with β-CDs
TMRM
TMRM
R110a
R110
R123
R123
R101
DM-β-CD
DM-β-CD
aThe K values for the inclusion complexation of R110 with β-CD were determined at 288, 293,
and 298 K because the complexation constants with CDs decreases with increasing temperature: K
(293 K)=1.29 0.03 M−1 for the complexation of R110 with β-CD
1 3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
Fig. 3 The fragments of
ROESY-NMR spectra for the
inclusion complexes of R110
with a β-CD and b DM-β-CD
in D2O-CD3CD (9:1, v/v)
mixture: [R110]0 =1.0 mM,
[β-CD]0 =3.0 mM, and
[DM-β-CD]0 =2.9 mM
the rhodamines is encapsulated in the β-CD cavity (Figs. S3
and S5). In the inclusion of TMRM and R123 by DM-β-CDs,
the inner protons of DM-β-CD interacted with the protons of
both the phenyl and xanthenyl groups in the rhodamines (Figs.
S4 and S6). These observations indicated that the inclusion
modes of TMRM and R123 are similar to that of R110.
1 3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
In RB/β-CD, cross peaks between the C(5)-proton of β-CD
and C(1,4,5,8)-protons of RB were detected, and the inter-
action of the C(3,5)-proton of β-CD with the -N(CH2CH3)2
protons of RB was observed (Fig. S1). These observations
indicated that the -N(CH2CH3)2 group of RB is included from
the narrow end of β-CD. In RB/DM-β-CD, cross peaks cor-
responding to the protons of bidirectional complexes were
observed, in analogy with the R110/DM-β-CD complex. Fur-
thermore, the -N(CH2CH3)2 protons strongly interacted with
the C(2)OCH3 protons on the DM-β-CD rim (Fig. S2), indicat-
ing that the -N(CH2CH3)2 group of RB is located at the narrow
end of the DM-β-CD.
The formation of bidirectional inclusion complexes sug-
gests that the apparent complexation constant K is com-
posed of two group-in inclusion equilibria, which would
be responsible for scattering in the ∆H–∆S competitive
relationship.
Fluorescence quantum yields of rhodamine
complexes
Rhodamines have a fuorescence efciency close to 100%,
but the fuorescence is strongly quenched in polar solvents
such as water [22, 23]. Measurement of the quantum yield
(ϕ) of inclusion complexes is useful for understanding the
inclusion behaviour. The ϕ values of the inclusion com-
plexes are determined on an Otsuka fuorescence spec-
trometer and are expressed by the following equation:
In the inclusion of R101 by β- and DM-β-CDs, cross peaks
arising from the protons of R101 and the inner protons of
β-CD were not observed, and the C(1,8)- and N(CH2CH3)2-
protons of R101 strongly interacted with the inner C(3)-proton
of DM-β-CD (Fig. S7 and S8, Table S1 in Supplementary
Data). These observations revealed that a part of the xanthenyl
ring in R101 is encapsulated into the DM-β-CD cavity. Based
on the NMR results, the plausible structures for the inclusion
complexes of RB, R110, and R101 with β- and DM-β-CDs are
proposed, as depicted in Fig. 4.
FPcomplex
APcomplex
FP − FPguest
AP − APguest
ꢀ =
=
(4)
where FP and AP denote the fuorescence and excitation
photon numbers of the inclusion complexes, free rhoda-
mines (guest) and apparent solutions, respectively. The FP
and AP values of the apparent and guest solutions were
measured, and the concentration of the free guest rhoda-
mine was estimated using the complexation constants K of
inclusion complexes in Table 1. The obtained ϕ values are
listed in Table 2 and can be summarized as follows: the ϕ
values decrease in the order of ϕfree(R110) ≈ ϕfree(R123
) > ϕfree(TMRM) > ϕfree(RB). The electron donor capabil-
ity of the amino groups in the xanthenyl ring increases in
the order of NH2 < N(CH3)2 < N(C2H5)2, as can be seen
from the decrease of the ionization potential (IP) val-
ues (NH2 > N(CH3)2 > N(C2H5)2) [24]. Electron transfer
from the amino groups plays an important role for the
determination of ϕ values. In RB and TMRM, ϕ values
are decreased by inclusion of β- and DM-β-CDs and the
efects of CD inclusion on ϕ values become large as the
donor capability of amino groups increases. In the R101/
DM-β-CD inclusion, the efect of inclusion on ϕ value
is not observed, which is attributable to the diference in
binding modes of inclusion complexes (inclusion of the
rigid amino group by DM-β-CD). The ϕ values of R110
with –COOH in the phenyl group is comparable to that of
R123 with –COOMe, suggesting that the inductive efect
of –COOMe through the phenyl group is insensitive to the
radiative process.
Thermodynamic parameters for inclusion
To obtain additional information on the inclusion of rhoda-
mines with β-CDs, the thermodynamic parameters (enthalpy
and entropy changes (∆H and ∆S)) for the inclusion process
were determined from van’t Hof plots by using the complexa-
tion constants K of inclusion complexes. The obtained values
are listed in Table 1. A linear relationship between the ∆H
and ∆S values is obtained for the inclusion complexation of
rhodamines with native β-CD (Fig. 5), suggesting that a single
mechanism operates in the inclusion process [21]. In the R101
inclusion by DM-β-CD, a large shift from the above linear
relationship is obtained, which is ascribed to the diference in
the free-energy changes for inclusion processes, as expected
from the diference in the structures of the inclusion complexes
(Fig. 4). In the inclusions of RB, TMRM, R123, and R110 by
DM-β-CD, the ∆H-∆S plot is considerably scattered (Fig. 5).
As suggested above, the four rhodamines form bidirec-
tional inclusion complexes with DM-β-CD. The equilibria for
the group-in inclusion complex formation are expressed as
follows:
Guest + DM-ꢀ-CD ⇌ xanthenyl-in, complexation constant K1
Guest + DM-ꢀ-CD ⇌ phenyl-in, complexation constant K2
Albeloa et al. suggested that the fuorescence quantum
yield of rhodamines is strongly reduced by the formation
of a twisted conformation in the excited state (non-radi-
ative deactivation process) [23]. Our observations indi-
cate that the inclusion of RB and TMRM having bulky
[group-in]
[xanthenyl-in] + [phenyl-in]
[Guest][DM-ꢀ-CD]
K =
=
= K1 + K2
[Guest][DM-ꢀ-CD]
(3)
1 3

Journal of Inclusion Phenomena and Macrocyclic Chemistry
Fig. 4 Plausible structures of
the inclusion complexes of RB,
R110, and R101 with β- and
DM-β-CDs
amino-substituents favours the formation of twisted non-
radiative structure, resulting in decrease of ϕ values.
In summary, the complexation constants and fuores-
cence quantum yields of inclusion complexes were deter-
mined for the inclusion complexes of substituted rhodamines
with native and 2,6-methylated β-CDs. The thermodynamic
parameters and ROESY-NMR spectra of the inclusion com-
plexes of rhodamines with DM-β-CD showed the formation
of bidirectional inclusion complexes. It was found that the
inclusion of rhodamines having bulky amino-substituents
on the xanthenyl ring favours the formation of the twisted
excited state, resulting in a decrease in ϕ values. These
results provide useful insights for functional group recogni-
tion of native and 2,6-methylated β-CDs.
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