Reduced fluorescence quenching of cyclodextrin-acetylene dye rotaxanes

Article abstract of DOI:10.1021/ja0584121

An acetylene dye rotaxane with α-CD has been synthesized using the Heck-Cassar-Sonogashira-Hagihara-type reaction in aqueous solution. Free dye with tetracarboxylic acids is found to be highly sensitive to various metal ions, showing high Stern-Volmer con

Full text of DOI:10.1021/ja0584121

Published on Web 05/24/2006
Reduced Fluorescence Quenching of Cyclodextrin-Acetylene Dye Rotaxanes
Jong S. Park,† James N. Wilson,‡ Kenneth I. Hardcastle,¶ Uwe H. F. Bunz,*,‡ and
Mohan Srinivasarao*,†,‡,§
School of Polymer, Textile and Fiber Engineering, School of Chemistry and Biochemistry, and Center for AdVanced
Research in Optical Microscopy (CAROM), Georgia Institute of Technology, Atlanta, Georgia 30332, and
Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322
Received December 29, 2005; E-mail: uwe.bunz@chemistry.gatech.edu; mohan@ptfe.gatech.edu
Fluorescent dyes behave differently when trapped inside the
cavity of cyclodextrins (CDs).1-5 When cyanine dyes, with extended
conjugation, were encapsulated within CDs, their fluorescence
efficiency and photostability increased.1 Upon rotaxanation, the
threaded cyanine dye also exhibited reversible redox processes, and
its kinetic stability was increased.2 The synthesis of fluorescent
stilbene and tolane rotaxanes was reported; Suzuki coupling using
a Pd catalyst in aqueous condition gave a dye rotaxane.3 PPV-
based polyrotaxanes and monomeric [2]rotaxanes have been syn-
thesized by the same method.4 Rotaxane encapsulation of fluores-
cent dyes, as in the above-mentioned examples, is a promising way
of improving the stability and brightness of luminescent and
electroluminescent materials in various applications, such as for
organic light emitting diodes and biological probes.5
In an effort to access fluorescent rotaxanes, we present the syn-
Figure 1. 1H ROESY NMR (500 MHz) spectrum of acetylene dye rotaxane
RD 3 (in DMSO-d6, 298 K).
thesis of an acetylene dye rotaxane 3, using a Pd-catalyzed reaction
of the Heck-Cassar-Sonogashira-Hagihara-type.6 In the molec-
ular structure, it corresponds to a monomer unit of the poly(para-
phenyleneethynylene)s (PPEs), which are characterized by high
fluorescence quantum yield and high oxidation potential due to the
electron-withdrawing effect of the -CtC- unit. In our synthesis,
1,4-diethynylbiphenyl 1 was reacted with 5-iodo-isophthalic acid
2 in the presence of R-CD. A water-soluble palladium (Pd) salt7
and cuprous iodide were used as catalyst; triethylamine and
potassium carbonate were added to maintain alkaline pH. After
reaction, the target molecule 3 was isolated (13% yield) by
removing residual starting materials and its uncomplexed congener
using Soxhlet extraction.
Figure 2. Side view (ORTEP) of an acetylene dye rotaxane RD 3.
H4 of the R-CD, and from b′ to H5 and H6, implying that the
narrow 6-OH rim of R-CD is closer to the isophthalic acid blocker;
the R-CD hovers over one end of the dye molecule, leaving the
other end uncovered. Aromatic protons from the uncovered part
show their peaks at the same positions as those of the free dye
(FD).
Hydrated crystals of 3 suitable for X-ray analysis were isolated
from a K2CO3 solution with 2 N HCl. An ORTEP (Oak Ridge
Thermal Ellipsoid Plot) of 3 is shown (Figure 2). It is clear that
the R-CD is displaced from the center of the threaded molecule,
and the dye is almost planar, while its main axis exhibits a slight
torsion.
There is no significant difference in the electronic absorption
and emission spectra of free and rotaxanated dyes. However, 3
exhibits higher relative fluorescence quantum yields (for FD and
3, 0.55 and 0.67 in methanol; 0.55 and 0.63 in aqueous pH 9.6
buffer, respectively). This is consistent with previous results that
CD encapsulation reduces the quenching effect of the environment
and the flexibility of the threaded chromophore, leading to an
increased quantum yield.1,4
The ROESY (Rotating Overhauser Effect Spectroscopy) spec-
trum of 3 is shown in Figure 1. Strong couplings are observed from
the aromatic protons d′ and c′ of the dumbbell to protons H3 and
† School of Polymer, Textile and Fiber Engineering, Georgia Institute of
Technology.
To investigate the sensing ability, the fluorescence intensities
of both dyes were measured in the presence of various metal ions
as quenchers. The Stern-Volmer equation (eq 1) is useful in the
‡ School of Chemistry and Biochemistry, Georgia Institute of Technology.
§ CAROM, Georgia Institute of Technology.
¶ Emory University.
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10.1021/ja0584121 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
the present FD exhibits an exceptionally high sensitivity, showing
almost the same KSV as that of conjugated polymers. This is
attributed to the presence of the four carboxylate units in FD,
implying that even monomeric acetylene dyes can exhibit a high
degree of sensitivity and function in metal sensing applications.
Obviously, once rotaxanated, 3 experiences much less fluorescence
quenching with lower KSV’s. Table 1 shows that all quenchers
exhibit a higher quenching effect on the FD; 3 does show some
quenching by these metals since its molecular structure is still the
same as that of FD, but its degree of quenching is much smaller.
The KSV difference is >100 in the case of Cu2+ and >4 for Pb2+
,
to which FD is highly sensitive.
There are two possible processes for fluorescence quenching,
namely, static and dynamic quenching.8,9 From its high sensitivity,
FD is thought to form aggregates with metal ions even in very
dilute solutions. With the addition of quenchers, the spectral shapes
of both dyes remain unchanged, even though the overall fluores-
cence intensity is decreased. The generally short emission lifetimes
of phenyleneethynylene chromophores (see Table S3) suggest that
static quenching, that is, formation of a ground state complex, is
operative in the cases of FD and the rotaxane 3. Dynamic
quenching, that is, complex formation of the excited state molecule
with the quencher, is improbable in our case.
In summary, an acetylene dye rotaxane with R-CD and its FD
analogue have been synthesized using a Heck-Cassar-Sonogash-
ira-Hagihara-type reaction. The free dye with four carboxylic acids
is highly sensitive to various metal ions, showing high Stern-
Volmer constants, KSV. As CD encapsulation protects and stabilizes
the threaded chromophore against outside quencher ions, metal-
insensitive biological tags are an obvious application for this class
of molecules.
Figure 3. Fluorescence emission spectra of FD (upper) and 3 (lower) during
quenching with CuSO4 in aqueous buffer (HEPES pH 7.2 with 10 mM
buffer strength). Concentrations of both dyes are 1.0 × 10-6 M. The insets
show the Stern-Volmer plot. The Stern-Volmer constants, KSV, were
calculated to be 2.5 × 105 M-1 (for FD) and 2.0 × 103 M-1 (for RD).
Table 1. KSV (M-1) Values Measured with the Addition of Various
Metal Ions
quenchers
FD
RD
CuSO4
Hg(OOCCH3)2
PbCl2
Pb(NO3)2
Mg(OOCCH3)2
ZnCl2
2.5 × 105
7.8 × 103
2.6 × 105
4.7 × 105
NQa
2.0 × 103
3.7 × 103
5.6 × 104
1.7 × 105
NQa
Acknowledgment. This work was supported by NSF, NTC, and
DOE (DE-FG02-04ER46141).
<10
<10
Supporting Information Available: Dye synthesis and experi-
mental details for fluorescence measurements. This material is available
free of charge via the Internet at http://pubs.acs.org.
methyl viologen
5.9 × 103
2.4 × 103
a NQ: no quenching observed.
quantitative measurement of fluorescence quenching; F0 is the initial
fluorescence intensity measured without any quencher, F is the
fluorescence intensity at a given concentration of the quencher [Q],
and KSV is the Stern-Volmer constant.8
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Plotting (F0/F) versus [Q] yields KSV value as the slope. The
more sensitive system gives a steeper plot, leading to higher KSV
values. Linear Stern-Volmer plots were observed for both dyes
when Cu2+ was used as a quencher (Figure 3). We observe that
FD is effectively quenched by copper ions, and its KSV was 2.5 ×
105 M-1. By comparison, 3 shows a much smaller KSV, 2.0 × 103
M-1, indicating that 3 is less sensitive to Cu2+ ions in solution
than FD. Similar behavior is observed for Hg2+, Pb2+, and methyl
viologen hydrate. The KSV values for these quenchers are calculated
and listed in Table 1.
The photoluminescence quenching will aid in the design of highly
sensitive chemical and biological sensors.9 Proton dissociation
(-COOH f -COO- + H+) makes the negatively charged,
fluorescent dye water-soluble and allows the binding of cationic
electron acceptors, such as metal ions (M2+), via electrostatic
interactions. Often, monomeric fluorophores are less sensitive,
giving a low KSV, when compared to their corresponding conjugated
polymeric form,8,9 which is normally able to amplify, synchronize,
and play multivalency effects in its quenching ability. However,
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