The B-N bond controls the balance between locally excited (LE) and twisted internal charge transfer (TICT) states observed for aniline based fluorescent saccharide sensors

Article abstract of DOI:10.1016/j.tetlet.2004.01.112

Three boronic acid based saccharide sensors with an aniline fluorophore have been prepared. One of the systems (1a) contains an intramolecular boron-nitrogen (B-N) bond and displays fluorescence due to both LE and TICT states. The other two systems (1b an

Full text of DOI:10.1016/j.tetlet.2004.01.112

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2859–2862
The B–N bond controls the balance between locally excited (LE)
and twisted internal charge transfer (TICT) states observed for
aniline based fluorescent saccharide sensors
Laurence I. Bosch,a Mary F. Mahonb and Tony D. Jamesa,*
aDepartment of Chemistry, University of Bath, Bath BA2 7AY, UK
bBath Chemical Crystallography, Department of Chemistry, University of Bath, Bath BA2 7AY, UK
Received 24 November 2003; revised 15 January 2004; accepted 23 January 2004
Abstract—Three boronic acid based saccharide sensors with an aniline fluorophore have been prepared. One of the systems (1a)
contains an intramolecular boron–nitrogen (B–N) bond and displays fluorescence due to both LE and TICT states. The other two
systems (1b and c) have no B–N bond and only show fluorescence due to the LE state.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Boronic acid receptors with the capacity to bind sac-
charides selectively and signal this event by altering their
optical signature have attracted considerable interest in
recent years.1–9 Boronic acids are known to bind sac-
charides via covalent interactions in aqueous media. The
most common interaction is with the cis-1,2- or -1,3-
diols of saccharides to form five- or six-membered rings,
respectively.10
hν 274 nm
hν 274 nm
OH
S
OH
B
O
OH
OH
O
B
OH
S
N
NH
A
B
404 nm
362 nm
We recently published our findings on compound 1a a
monoboronic fluorescent sensor, this system shows large
shifts in the emission wavelength on saccharide bind-
ing.11 When saccharides interact with sensor 1a in
aqueous solution at pH 8.21 the emission maxima at
404 nm shifts to 362 nm (kex ¼ 274 nm).
Scheme 1. Proposed fluorescent species.
at 362 nm when excited at 274 nm (Scheme 1). The dif-
ferent fluorescent properties of species A and B can be
ascribed to locally excited (LE) and twisted internal
charge transfer (TICT) states of the aniline fluoro-
phore.12–15 Species B only shows the normal band
associated with the LE state since the nitrogen lone pair
is free to conjugate with the p-system while species A
shows the anomalous band associated with the TICT
The species responsible for the observed fluorescence
properties were previously assigned (Scheme 1). Species
A contains a B–N bond and when excited at 274 nm
emits at 404 nm. On addition of a saccharide the B–N
bond is broken to form boronate species B, which emits
OH
B
OH
NH
HO
NH
NH
B
B
OH
OH
HO
1b
1c
1a
Keywords: Fluorescence sensors; Boronic acid; Saccharide.
* Corresponding author. Tel.: +44-1225-383810; fax: +44-1225-386231; e-mail: t.d.james@bath.ac.uk
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.112

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L. I. Bosch et al. / Tetrahedron Letters 45 (2004) 2859–2862
state since the lone pair is coordinated with the boron
and perpendicular to the p-system.
aqueous solution in the presence of 0.05 mol dmÀ3
sodium chloride osmotic buffer) were determined from
the fluorescence intensity versus pH profiles in the
From these investigations we realised that the species B
emitting at 362 nm does not contain a B–N bond, but
does shown fluorescent enhancement. Therefore, we
decided to investigate systems where formation of a B–
N bond was not possible. Compounds 1a–c were pre-
pared according to Scheme 2 from readily available
starting materials, aniline and 2-, 3- or 4-formylbenz-
eneboronic acid.16 For compound 1a we were also able
to obtain crystals suitable for X-ray analysis.17 The
structure is shown in Figure 1. It is clear from Figure 1
that the boronic acid forms a cyclic trimeric anhydride,
where only one of the units contains a B–N bond
absence and presence of D
-fructose (0.05 mol dmÀ3). The
pKa of compounds 1a–c were: 10.20 0.01, 9.30 0.03
and 9.58 0.02, respectively, and in the presence of
-fructose (0.05 mol dmÀ3) were 7.96 0.06, 6.95 0.04
D
and 7.22 0.02, respectively. The observed shift in pKa
to lower values on saccharide binding is in agreement
with previous work.2
The fluorescence titration of 1a (2.0 · 10À5 mol dmÀ3), 1b
(1.0 · 10À5 mol dmÀ3) and 1c (1.0 · 10À5 mol dmÀ3) with
different saccharides were carried out in a pH 8.21 buffer
(52.1 wt %
0.01000 mol dmÀ3
methanol
;
in
KH2PO4,
water
with
KCl,
;
(1.747(2) A). This value is similar to those found in
0.002752 mol dmÀ3
ꢀ
similar cyclic boroxins18 and within the 1.5–1.8 A range
Na2HPO4, 0.002757 mol dmÀ3).20 The fluorescence
spectra of 1c in the presence of D-fructose (0–
ꢀ
expected for a strong B–N bond.19 The structure sup-
ports Scheme 1 in that it illustrates that a subtle balance
exists for formation of a B–N bond.
0.1 mol dmÀ3) are shown in Figure 2.
The stability constants (K) of fluorescence sensors 1a–c
with -fructose, -glucose, -galactose and -mannose
For compounds 1b and 1c when excited at 240 and
244 nm, respectively, an emission at 350 nm was
observed (excitation at 274 nm resulted in no emission).
Also, when compound 1a was excited at 244 nm only an
emission at 360 nm was observed. In all these cases only
the LE state is formed.
D
D
D
D
were calculated by fitting the emission intensity at 350 or
360 nm (kex ¼ 240 and 244 nm) versus concentration of
saccharides (Fig. 3).21 The stability constants for sensors
1a–c calculated from these titrations are given in Table
1.
The pKa of compounds 1a–c (1.0 · 10À5 mol dmÀ3
33.3 wt % (1a) and 52.1 wt % (1b and c) methanolic
,
The fluorescence enhancements obtained for 1a–c on the
addition of D-fructose are 15-, 18- and 25-fold, respec-
tively. We believe that these large fluorescence
enhancements can be attributed to fluorescence recovery
of the aniline fluorophore. With these systems in the
absence of saccharides the normal fluorescence of the
LE state of the aniline donor is quenched by energy
transfer to the phenylboronic acid acceptor. When sac-
charides are added, a negatively charged boronate anion
is formed (cf. species B from Scheme 1), under these
conditions energy transfer from the aniline donor is
unfavourable and fluorescence recovery of the LE state
of the aniline donor is observed.
HO
HO
OH
NH2
B
B
NH
OH
i,ii
+
CHO
1a-c
Scheme 2. Reagents and conditions: (i) EtOH/PhCH3 reflux (1a) or
MeOH, rt (1b–c); (ii) NaBH4, 1a 40% (two steps), 1b and 1c both 45%
(two steps).
700
600
500
400
300
200
100
0
300
320
340
360
380
400
420
440
460
Wavelength / nm
Figure 2. Fluorescence spectra change of 1c (1.0 · 10À5 mol dmÀ3) with
different concentrations of
-fructose (0–0.1 mol dmÀ3) in 52.1 wt %
MeOH pH 8.21 phosphate buffer. kex ¼ 244 nm.
Figure 1. Crystal structure of 1a. Ellipsoids are illustrated at the 30%
D
probability level. Hydrogen atoms omitted for clarity.

L. I. Bosch et al. / Tetrahedron Letters 45 (2004) 2859–2862
2861
National Mass Spectrometry Service in Swansea. TDJ
would also like to thank Professor A. P. de Silva from
the School of Chemistry at the Queen’s University Bel-
fast for helpful discussion.
30
25
20
15
10
5
References and notes
1. de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.;
Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice,
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9014.
5. Tong, A.-J.; Yamauchi, A.; Hayashita, T.; Zhang,
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1530.
0
0
0.02
0.04
0.06
0.08
0.1
[Saccharide] /M
Figure 3. Relative fluorescence intensity versus saccharide concentra-
tion profile of 1c with ( -fructose, ( -glucose, (j) -galactose,
-mannose. The measurement conditions are the same as those in
)
r
D
)
D
D
ꢀ
6. Karnati, V. V.; Gao, X.; Gao, S.; Yang, W.; Ni, W.;
Sankar, S.; Wang, B. Bioorg. Med. Chem. Lett. 2002, 12,
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(N)
D
Figure 2. kex ¼ 244 nm, kem ¼ 350 nm.
7. DiCesare, N.; Lakowicz, J. R. Chem. Commun. 2001,
2022.
8. Cao, H.; Diaz, D. I.; Di Cesare, N.; Lakowicz, J. R.;
Heagy, M. D. Org. Lett. 2002, 4, 1503.
9. Eggert, H.; Frederiksen, J.; Morin, C.; Norrild, J. C.
J. Org. Chem. 1999, 64, 3846.
10. Lorand, J. P.; Edwards, J. O. J. Org. Chem. 1959, 24, 769.
11. Arimori, S.; Bosch, L. I.; Ward, C. J.; James, T. D.
Tetrahedron Lett. 2001, 42, 4553.
Table 1. Stability constant K (coefficient of determination; r2) for
saccharide complexes of fluorescent sensors 1a–c, in pH 8.21 buffer at
kex ¼ 244 nm (sensors 1a and c) and 240 nm (sensor 1b)
K molÀ1 dm3
Saccharides
1a
1b
1c
D
D
D
D
-Fructose
-Glucose
79.2 1.7
(0.99)
212.1 6.9
(0.99)
128.6 2.6
(0.99)
12. Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.;
Cowley, D. J.; Baumann, W. Nouv. J. Chim. 1979, 3, 443.
13. Rettig, W.; Chandross, E. A. J. Am. Chem. Soc. 1985, 107,
5617.
14. Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971.
15. Joedicke, C. J.; Luethi, H. P. J. Am. Chem. Soc. 2003, 125,
252.
6.4 0.4
(0.99)
8.7
(0.99)
1
6.7 0.5
(0.99)
-Galactose
-Mannose
14.2 0.6
(0.99)
26.6 1.317.7 0.3
(0.99)
16.2 0.8
(0.99)
(0.99)
7.9 01.3413
(0.99)
(0.99)
16. Selected data for 1a: mp: 125–129 ꢁC; found: C, 74.70; H,
5.97; N, 6.63. C13H14BNO2–H2O requires C, 74.72; H,
5.80; N, 6.70%; dH (300 MHz; CD3OD) 4.21 (2H, s, CH2),
6.60–6.70 (3H, m, ArH), 6.95–7.25 (6H, m, ArH); dC
(75 MHz; CD3OD) 51.6, 117.4 (2C), 121.5, 126.9, 127.9,
129.8, 130.2 (2C), 132.7, 146.0, 149.1; m=z (FAB) 497.3
([M+2(3-HOCH2C6H4NO2))2H2O]þ, 100%).
Selected data for 1b: mp: 166 ꢁC; found: C, 73.90; H, 5.56;
This was confirmed by the quantum yield measurements of
compounds 1a–c. The quantum yield U of aniline is 0.0922
(in methanol), and the measured quantum yield U of 1a is
0.0082, 1b is 0.0087 and 1c is 0.0070 (in methanol).23;24
N, 6.72. C13H14BNO2–H2O + 0.03CHCl
requires C,
3
73.60; H, 5.70; N, 6.58%); (HRMS: found [M+2(3-
HOCH2C6H4NO2))2H2O]þ, 497.1776. C27H24BN3O6
requires 497.1758); dH (300 MHz; CD3OD) 4.31 (2H, s,
CH2), 6.55–6.65 (3H, m, ArH), 7.00–7.10 (2H, m, ArH),
7.30 (1H, m, ArH), 7.42 (1H, m, ArH), 7.60 (1H, m, ArH),
7.76 (1H, m, ArH); dC (75 MHz; CD3OD) approximately
49.0 carbon masked by CD3OD, 114.5, 118.3, 129.1,
130.3, 130.8, 133.8, 134.4, 140.8, 150.5; m=z (FAB) 498.1
([M+H+2(3-HOCH2C6H4NO2)–2H2O]þ, 100%).
Selected data for 1c: mp: 164 ꢁC; found: C, 73.90; H, 5.88;
N, 6.15. C13H14BNO2–H2O+0.03CHCl3 requires C, 73.60;
H, 5.70; N, 6.58%); (HRMS: found [M+2 (3-
HOCH2C6H4NO2)–2H2O]þ, 497.1772. C27H24BN3O6
requires 497.1758); dH (300 MHz; CD3OD) 4.30 (2H, s,
CH2–NH), 6.55–6.65 (3H, m, ArH), 7.00–7.10 (2H, m,
ArH), 7.30–7.40 (2H, m, ArH), 7.56 (1H, m, ArH), 7.69
(1H, m, ArH); dC (75 MHz; CD3OD) approximately 49.0
carbon masked by CD3OD, 114.5, 118.4, 127.9, 130.3,
135.5, 144.1, 150.5; m=z (FAB) 497.1 ([M+2(3-
HOCH2C6H4NO2)–2H2O]þ, 100%).
In conclusion, we have prepared two new systems (1b and
c), which display large fluorescence enhancements on
saccharide binding. The fluorescence changes observed
for the LE state at 350 or 360 nm for all three systems
(1a–c) has been ascribed to the fluorescence recovery of
the aniline fluorophore. We are confident that these
discoveries will lead to the development of improved
boronic acid based fluorescent saccharide sensors. Our
ongoing research is directed towards exploiting these
findings in other saccharide selective systems.
Acknowledgements
Financial support from the Royal Society and Beck-
man–Coulter are gratefully acknowledged. We would
also like to thank the University of Bath and the EPSRC

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L. I. Bosch et al. / Tetrahedron Letters 45 (2004) 2859–2862
17. Crystallographic data (excluding structure factors) for
the structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplemen-
tary publication numbers CCDC 228172. Copies of the
data can be obtained, free of charge, on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
[fax: 144-(0)1223-336033 or e-mail: deposit@ccdc.cam.
ac.uk].
23. Fluorescence and Phosphorescence Spectroscopy: Physio-
chemical Principles and Practice; Schulman, S. G., Ed.;
Pergamon: Oxford, 1977; 59.
24. The areas under the fluorescence spectra were calculated
using KALEIDAGRAPH version 3.51 for PC, published by
Synergy Software and developed by Abelbeck Software,
2457 Perikiomen Avenue, Reading, PA 19606. The quan-
tum yields of sensors 1a–1c were then calculated by
comparison with aniline as standard using the following
equation:23
18. Norrild, J. C.; Sotofte, I. J. Chem. Soc., Perkin. Trans. 2
2002, 303.
19. Hopfl, H. J. Organomet. Chem. 1999, 581, 129.
20. Perrin, D. D.; Dempsey, B. Buffers for pH and Metal Ion
Control; Chapman and Hall: London, 1974.
21. The K were analysed in KALEIDAGRAPH using nonlinear
(Levenberg–Marquardt algorithm) curve fitting. The
errors reported are the standard errors obtained from
the best fit.
Us ¼ Uan  ðFAs=FAanÞ Â ðAan=AsÞ
U is quantum yield; FA is the area under the curve of the
fluorescence peak; A is the absorbance of each sensor at
244 nm (for 1a and c) and 240 nm for 1c. The subscripts s
and an are the abbreviations for sensors (1a–c) and the
aniline reference, respectively. U ¼ 0:09 (in methanol)22 is
used as the reference quantum yield of aniline in methanol.
22. Perichet, G.; Chapelon, R.; Pouyet, B. J. Photochem. 1980,
13, 67.