interested in developing new fluorescence sensors selective
for saccharides employing a modular approach.11,14,15 The
basic idea was to break a sensor into three components:
receptor units, linker units, and fluorophore units. The
approach requires the selection and synthesis of a set of
molecular binding blocks from which the selective fluores-
cent sensors can be easily constructed. The quick assembly
of a diverse selection of fluorescent sensors will require that
the receptor and fluorophore units are linked to core units
using the minimum of synthetic linkage reactions. The use
of common reactions means the synthetic routes toward the
new sensors will be convergent. Our modular system 1
contains two phenylboronic acid groups (for selectivity), one
pyrene group (for fluorophore), and hexamethylene (for
linker).11 The modular nature of 1 makes it easy to vary both
the fluorophore and linker length. The choice of linker is
very important because it determines the selectivity for a
particular saccharide. Our research has demonstrated that
hexamethylene was the best linker length to obtain D-glucose
selectivity.11,15
possible to observe the long wavelength excimer emission
due to π-π stacking of phenanthrene and pyrene.
To confirm that the π-π stacking of sensor 2 is only
intramolecular and not intermolecular we plotted the absorp-
tion versus concentration of 2 and 3 + 4 in pH 8.21 buffer
(52.1 wt % methanol in water with KCl, 0.01000 mol dm-3;
KH2PO4, 0.002752 mol dm-3; Na2HPO4, 0.002757 mol
dm-3).18 The plots for sensor 2 and the mixture of sensor 3
+ 4 are linear until 2.0 × 10-5mol dm-3 (ꢀ ) 2.12 × 104
dm3 mol-1 (λmax 342 nm) for sensor 2 and 4.59 × 104 dm3
mol-1 (λmax 342 nm) for sensors 3 + 4), clearly demonstrat-
ing that the π-π stacking of sensor 2 is only intramolecular.
Fluorescence titrations of 2 (2.5 × 10-6 mol dm-3) excited
at λex 299 nm (phenanthrene) and λex 342 nm (pyrene) were
performed with different saccharides in pH 8.21 buffer. The
fluorescence intensity of sensor 2 at 417 nm increased with
added saccharide when excited at 299 and 342 nm, while
the excimer emission at 460 nm decreased with added
saccharide. The excimer emission change indicates that the
fluorophore stacking of phenanthrene and pyrene is broken
on saccharide binding.
Our aim with this research was to apply the modular design
to prepare a saccharide sensing system using fluorescence
energy transfer. Fluorescence energy transfer is the transfer
of excited-state energy from a donor to an acceptor. The
transfer occurs as a result of transition dipole-dipole
interactions between the donor-acceptor pair.16 In this paper
we report on a new fluorescence sensor 2 that has two
phenylboronic acid groups, hexamethylene linker, and two
different fluorophore groups (phenanthrene and pyrene).
At an excitation wavelength of 299 nm (phenanthrene)
no emission was observed at 369 nm (phenanthrene), but
emission was observed at 417 nm (pyrene) (Figure 1). This
result implies that the excited energy of phenanthrene (donor)
was transferred to pyrene (acceptor), so that only the emission
spectra of pyrene was observed.
Our idea with this system was to investigate the efficiency
of energy transfer (ET) from phenanthrene to pyrene as a
function of saccharide binding. A similar concept has
previously been employed in the construction of a fluorescent
calix[4]arene sodium sensor.17
The excitation and emission wavelengths of phenanthrene
3 (donor) are 299 and 369 nm, respectively, while the
excitation and emission wavelengths of pyrene 4 (acceptor)
are 342 and 397 nm, respectively. The emission wavelength
of phenanthrene 3 (369 nm) and excitation wavelength of
pyrene 4 (342 nm) overlap. These observations suggest that
intramolecular energy transfer from phenanthrene to pyrene
can take place in modular sensor 2. In addition it is also
Figure 1. Fluorescence spectral change of 2 (2.5 × 10-6 mol dm-3
)
with different concentrations of D-glucose in pH 8.21 buffer (λex
(6) Wiskur, S. L.; Lavigne, J. J.; Ait-Haddou, H.; Lynch, V.; Chiu, Y.
H.; Canary, J. W.; Anslyn, E. V. Org. Lett. 2001, 3, 1311.
(7) Tong, A.-J.; Yamauchi, A.; Hayashita, T.; Zhang, Z.-Y.; Smith, B.
D.; Teramae, N. Anal. Chem. 2001, 73, 1530.
(8) Springsteen, G.; Wang, B. Chem. Commun. 2001, 1608.
(9) DiCesare, N.; Lakowicz, J. R. Chem. Commun. 2001, 2022.
(10) Arimori, S.; Bosch, L. I.; Ward, C. J.; James, T. D. Tetrahedron
Lett. 2001, 42, 4553.
(11) Arimori, S.; Bell, M. L.; Oh, C. S.; Frimat, K. A.; James, T. D.
Chem. Commun. 2001, 1836.
(12) Cao, H.; Diaz, D. I.; Di Cesare, N.; Lakowicz, J. R.; Heagy, M. D.
Org. Lett. 2002, 4, 1503.
299 nm).
The stability constants (K) of sensors 2(λex 299 nm), 2(λex
342 nm), 3, and 4 were calculated by fitting the emission
wavelengths at 417, 417, 367, and 397 nm versus concentra-
tion of saccharide curves and are given in Table 1.14,19 The
stability constants K for diboronic acid sensor 2 (λex 299
and 342 nm) with D-glucose were enhanced relative to those
of monoboronic acid sensors 3 and 4, while the stability
(13) Eggert, H.; Frederiksen, J.; Morin, C.; Norrild, J. C. J. Org. Chem.
1999, 64, 3846.
(14) Cooper, C. R.; James, T. D. J. Chem. Soc., Perkin Trans. 1 2000,
963.
(15) Arimori, S.; Bell, M. L.; Oh, C. S.; Frimat, K. A.; James, T. D. J.
Chem. Soc., Perkin Trans. 1 2002, 802.
(16) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum:
New York, 1999.
(18) Perrin, D. D.; Dempsey, B. Buffers for pH and Metal Ion Control;
Chapman and Hall: London, UK, 1974.
(19) The K values were analyzed in KaleidaGraph using nonlinear
(Levenberg-Marquardt algorithm) curve fitting. The errors reported are
the standard errors obtained from the best fit.
(17) Jin, T. Chem. Commun. 1999, 2491.
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Org. Lett., Vol. 4, No. 24, 2002
Arimori, Susumu
