Enantioselective recognition of tartaric acids with ethynylated carbazole-based chiral bisboronic acid chemosensors with improved response at acidic pH

Article abstract of DOI:10.1007/s10895-011-0898-z

Chiral bisboronic acid chemosensors based on ethynylated carbazole were prepared. The chiral chemosensors show red-shifted emission than the chemosensors with unsubstituted carbazole fluorophore. a-PET effect was found for the chemosensors, which is different from our previous observation of the d-PET effect for boronic acid chemosensors based on carbazole. Enantioselective recognition of tartaric acids was implicated with these chemosensors. Consecutive fluorescence emission enhancement/diminishment were observed with increasing the concentration of the tartaric acids, which is tentatively assigned to the transition of the binding stoichiometry from 1:1 binding to 1:2 binding. In particularly interesting is the improved fluorescence response at acidic pH for recognition of tartaric acids, which is rarely observed for a-PET chemosensors. We propose that the sensing is due to hybrid mechanism of a-PET/d-PET and conformational restriction upon binding. Our results will be useful for design of chiral boronic acid chemosensors with improved fluorescence response at acidic pH, which are rarely reported.

Full text of DOI:10.1007/s10895-011-0898-z

J Fluoresc (2011) 21:1979–1986
DOI 10.1007/s10895-011-0898-z
ORIGINAL PAPER
Enantioselective Recognition of Tartaric Acids
with Ethynylated Carbazole-Based Chiral Bisboronic
Acid Chemosensors with Improved Response at Acidic pH
Yifan Liu & Xin Zhang & Huimin Guo & Yubo Wu &
Qiuting Li & Liping Liu & Jianzhang Zhao
Received: 22 March 2011 /Accepted: 9 May 2011 /Published online: 19 May 2011
#
Springer Science+Business Media, LLC 2011
Abstract Chiral bisboronic acid chemosensors based on
ethynylated carbazole were prepared. The chiral chemo-
sensors show red-shifted emission than the chemosensors
with unsubstituted carbazole fluorophore. a-PET effect was
found for the chemosensors, which is different from our
previous observation of the d-PET effect for boronic acid
chemosensors based on carbazole. Enantioselective recogni-
tion of tartaric acids was implicated with these chemosensors.
Consecutive fluorescence emission enhancement/diminish-
ment were observed with increasing the concentration of the
tartaric acids, which is tentatively assigned to the transition of
the binding stoichiometry from 1:1 binding to 1:2 binding. In
particularly interesting is the improved fluorescence response
at acidic pH for recognition of tartaric acids, which is rarely
observed for a-PET chemosensors. We propose that the
sensing is due to hybrid mechanism of a-PET/d-PET
and conformational restriction upon binding. Our results
will be useful for design of chiral boronic acid chemo-
sensors with improved fluorescence response at acidic
pH, which are rarely reported.
Introduction
Fluorescent molecular chemosensors have attracted much
attention due to their applications in chemical, biological and
environmental science [1–13]. Boronic acid chemosensors
for selective detection of sugars or hydroxyl acids, such as
tartaric acid, are of particular interesting due to the covalent
bonding nature of the interaction between the boronic acid
chemosensors and the analytes, thus the chemosensors can
be used in aqueous media [14–22]. It should be pointed out
that a large amount of chiral molecular chemosensors are
based on hydrogen bonding, thus the recognition can only be
carried out in non-protic organic solvents [23].
We have been interested in study of boronic acid
chemosensors for a while [24–32]. Besides the binding
sites and the fluorophore of the molecular chemosensors,
the sensing mechanism is also critical for a successful
chemosensor design [31]. One of the most popular sensing
mechanism of the molecular chemosensors, including
boronic acid chemosensors, is the photo-induced electron
transfer (PET), usually with fluorophore as the electron
acceptor and a proximate alkyl N atom (not conjugated to
the fluorophore) as the electron donor, and the switch of the
fluorescence [8]. This kind of PET is termed as a-PET
(a for acceptor of the PET process). Quenched emission at
basic media is usually observed for a-PET chemosensors
because the PET effect (quenching effect) at neutral and
basic pH is significant [8]. At acidic pH, however, the
N atom is protonated and the otherwise transferable electron
on the N atom is fixed, thus PET is inhibited, and the
fluorescence of the fluorophore is switched on. As a result,
the recognition of analytes with a-PET chemosensors,
especially the Wulff type of boronic acid chemosensors, is
.
.
Keywords Chiral boronic acid Enantioselective
.
.
Fluorescent chemosensors Tartaric acid PET
:
:
:
:
:
:
Y. Liu X. Zhang H. Guo Y. Wu Q. Li L. Liu J. Zhao (*)
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology,
Dalian 116024, People’s Republic of China
e-mail: zhaojzh@dlut.edu.cn
H. Guo
e-mail: guohm@dlut.edu.cn

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J Fluoresc (2011) 21:1979–1986
only effective at neutral or basic pH region [8]. Serious
interference exists in the acidic media due to the proton-
ation of the N atom, i.e. the background emission of the
chemosensors is strong, and as a result, the fluorescence
transduction at acidic pH upon binding with analytes is
small [24–27] (chemosensor 7 and 8, Scheme 1). However,
some analytes, such as tartaric acid or mandelic acid,
require recognition at acidic pH because the binding of the
boronic acid chemosensors with these analytes at acidic pH
is much stronger than that at neutral or basic pH.
Unfortunately, the fluorescence relay of the normal a-PET
boronic acid chemosensors upon binding with hydroxyl
acid analytes is poor at acidic pH [25].
Recently we devised the d-PET boronic acid chemo-
sensors with carbazole as the fluorophore (chemosensor 9,
10, and 11, Scheme 1) [30–32]. In this kind of chemo-
sensors, the fluorophore serves as the electron donor and
the protonated amine/boronic acid groups as the electron
acceptor of the PET process. This is in stark contrast to the
normal a-PET chemosensors. d-PET chemosensors give
emission intensity-pH profiles which are opposite to the
normal a-PET chemosensors, i.e. d-PET chemosensors
show relatively diminished emission at acidic pH (due to
the quenching effect of the d-PET) and intensified
emission at neutral/basic pH. The chiral recognition of
tartaric acid is achieved with the d-PET boronic acid
chemosensors [30]. The fluorescence response is signifi-
cant at acid pH [30]. However, up to date the number of
the d-PET boronic acid chemosensors is very limited and
little is known on the fluorophores that can induce the
d-PET effect [30–32].
Herein we prepared ethynylated carbazole based chiral
bisboronic acid chemosensors (S,S-1 and R,R-1, Scheme 2).
The emission of the chemosensors is red-shifted compared
to the chemosensors that based on carbazole [30–32].
Different from our previous d-PET chemosensors based on
carbazole [30–32], these new chemosensors show the
normal a-PET profile. We found that the recognition of
tartaric acid is enantioselective, i.e. the chiral chemosensors
give different fluorescence response to the enantiomers
of the tartaric acids. Interestingly, we found that the
fluorescence response at acidic pH range is significant.
This kind of response is rare and the only reported
example is boronic acid chemosensors based on anthra-
cene [35]. We propose that the sensing mechanism of the
chemosensor is hybrid a-PET/d-PET mechanism, and the
conformational restriction upon binding with analytes.
Furthermore, we found a consecutive fluorescence
enhancement/diminishment upon increasing the tartaric
acid concentration, which is proposed to be due to the
transition of the binding stoichiometry from 1:1 binding to
1:2 binding. The binding complex of the chemosensor
with tartaric acid was optimized by DFT calculations. Our
result will be useful for the design of chiral boronic acid
chemosensors with improved fluorescence response at
acidic pH, which is rarely reported [8].
Scheme 1 Typical a-PET (7, 8,
12) and d-PET (9, 10 and 11)
fluorescent boronic acid
chemosensors
N
(OH)2B
B(OH)2
N
N
N
B(OH)2
O
O
B(OH)2
S,S-8 and R,R-8
R1
OH
B
OH
N
N
N
R3
N
N
HO
OH
B
B
OH
OH
10: R1 = -NMe2, R2 = F
11: R1 = -NMe2, R2 = H
12: R1 = H, R2 = F
S,S-9 and R,R-9

J Fluoresc (2011) 21:1979–1986
1981
Scheme 2 Synthesis of the S,
S- and R,R- fluorescent boronic
acid chemosensor 1. i) KI,
KIO3, CH3COOH, reflux,
10 min, 45%; ii) NaH, DMF,
n-C4H9Br, room temperature,
1 h, 72% iii) POCl3, DMF,
CH2Cl2, reflux, 8 h, 63%. iv)
Pd(PPh3)4, CuI, NEt3,
I
CHO
I
I
iii)
i)
ii)
N
4
N
3
N
H
N
H
2
CHO
3-ethynylbenzaldehyde, argon
atmosphere, 60 °C, 8 h,
79.8%; v) ethanol, THF,
NH
N
S-1-phenylethylamine or
iv)
CHO
H
v)
R-1-phenylethylamine, reflux,
6 h, then methanol, THF,
NaBH3CN, room temperature,
1 h; vi) acetonitrile, K2CO3,
2-(2-bromomethylphenyl)-1,3,
2-dioxaborinane, reflux, 10 h,
11.7-12.6%
N
N
5
S,S- or R,R-6
(R)-
H
N
HO
B
OH
B
OH
HO
N
vi)
H
(R)-
N
S,S- or R,R-1
Experimental
9-butyl-6-[2-(3-formylphenyl)ethynyl]-9H-carbazole-3
-carbaldehyde (5)
Materials and General Methods
9-Butyl-6-iodocarbazole-3-carbaldehyde (400.0 mg,
1.06 mmol) and Pd(PPh3)4 (120.0 mg, 0.1 mmol) were
dissolved in triethylamine (8 mL) under argon atmosphere.
3-ethynylbenzaldehyde (198 mg, 1.52 mmol) was added to
the above solution and CuI (29.0 mg, 0.15 mmol) was
added. The mixture was stirred at 60 °C for 8 h. After
removal of triethylamine in vacuo, the residue was
purified with column chromatography (silica gel;
dichloromethane/methanol, 100:1, V/V) to give
320.0 mg (yield: 79.8 %) yellow solid. M. p. 122.9-
All the chemicals are analytical pure and were used as received.
NMR spectra were taken on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with Q-TOF
Micro MS spectrometer. Fluorescence spectra were recorded on
a JASCO FP-6500 or on a Sanco 970 CRT spectrofluorometer.
3-Iodocarbazole (2)
3-Iodocarbazole was synthesized according to a literature
procedure [33].
1
123.3 °C. H NMR (400 MHz, CDCl3) δ 10.12 (s, 1H),
10.05 (s, 1H), 8.62 (s, 1H), 8.37 (s, 1H), 8.04-8.09
(m, 2H), 7.82-7.87 (m, 2H), 7.71 (d, 1H, J=8.4 Hz), 7.56
(t, 1H, J=7.6 Hz), 7.50 (d, 2H, J=8.4 Hz), 7.45 (d, 1H,
J=8.4 Hz), 4.36 (t, 2H, J=6.8 Hz), 1.86-1.93 (m, 2H),
1.40-1.45 (m, 2H), 0.98 (t, 3H, J=7.2 Hz). ESI-HRMS
9-Butyl-3-iodocarbazole (3) and 9-Butyl-6-iodocarbazole-3
-carbaldehyde (4)
9-Butyl-3-iodocarbazole and 9-Butyl-6-iodocarbazole-3-
carbaldehyde were prepared by a literature procedure [34].
+
(C26H21NO2 ): calcd 379.1572, found 379.1577.

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J Fluoresc (2011) 21:1979–1986
N-((3-(2-(3-((1-phenylethylamino)methyl)-9-butyl-9H-carbazol
-6-yl)ethynyl)phenyl)methyl)-1-phenylethanamine (S, S-6
or R, R-6)
(m, 2H), 1.59 (d, 6H, J=7.2 Hz), 1.34-1.43 (m, 2H), 0.95
(t, 3H, J=7.2 Hz). 13C NMR (100 MHz, CDCl3/CD3OD) δ
141.9, 141.6, 140.5, 140.3, 137.8, 136.2, 132.7, 131.4,
130.4, 130.0, 129.3, 129.2, 129.1, 128.8, 128.5, 128.4,
128.0, 127.8, 127.7, 127.4, 127.2, 124.1, 123.9, 122.8,
121.8, 113.3, 109.0, 108.9, 91.0, 87.6, 54.0, 53.6, 43.2, 31.3,
29.5, 20.7, 15.7, 14.0. ESI-HRMS ([C58H61B2N3O4 + 2H]2+):
calcd 443.7503, found 443.7483.
5 (150.0 mg, 0.4 mmol) and S-1-phenylethylamine
(288.0 mg, 3.2 mmol) were dissolved in ethanol/THF
(3:2, V/V). The mixture was refluxed with stirring for 6 h
under N2. The solvent was removed under reduced
pressure, the residue was dissolved in 10 mL of MeOH/
THF, NaBH3CN (252.0 mg, 4.0 mmol) was added in
several portions and the mixture was stirred for 1 h at room
temperature. The solvent was removed and the residue was
taken up with dichloromethane (DCM), the organic phase
was washed with brine and dried over Na2SO4, DCM was
removed and the residue was purified with column
chromatography (silica gel, DCM/MeOH, 20:1, v/v). A
light yellow oil of S,S-6 was obtained in quantitative yield.
1H NMR (400 MHz, CDCl3) δ 8.17 (s, 1H), 8.00 (s, 1H),
7.53 (s, 1H), 7.46-7.50 (m, 6H), 7.40-7.43 (m, 7H), 7.28-
7.30 (m, 2H), 7.14-7.17 (m, 2H), 4.13-4.17 (q, 1H), 4.09
(t, 2H, J=7.6 Hz), 4.01 (d, 1H, J=6.8 Hz), 3.85-3.96
(m, 2H), 3.60-3.68 (m, 2H), 1.70-1.76 (m, 2H), 1.66 (d, 3H,
J=6.8 Hz), 1.58 (d, 3H, J=6.8 Hz), 1.30-1.38 (m, 2H),
0.91 (t, 3H, J=7.6 Hz). ESI-HRMS ([C42H43N3 + H]+):
calcd 590.3535, found 590.3518.
R,R-1 was prepared with the same method of S,S-1.
24.0 mg of yellow powder (R,R-1) was obtained, yield:
1
11.7 %. m.p. 118.9-119.4 °C. H NMR (400 MHz, CDCl3/
CD3OD) δ 8.25 (s, 1H), 7.86 (s,1H), 7.81 (s, 2H), 7.61
(d, 1H, J=8.4 Hz), 7.40-7.45 (m, 6H), 7.33-7.39 (m, 12H),
7.17-7.28 (m, 5H), 4.30 (t, 2H, J=7.2 Hz), 3.93-4.16
(m, 5H), 3.61-3.71 (m, 5H), 1.81-1.89 (m, 2H), 1.60 (d, 6H,
J=7.2 Hz), 1.37-1.42 (m, 2H), 0.95 (t, 3H, J=7.2 Hz). 13C
NMR (100 MHz, CDCl3/CD3OD) δ 142.0, 141.6, 140.7,
140.4, 137.1, 136.5, 132.9, 131.6, 130.5, 130.1, 129.7,
129.6, 129.4, 129.0, 128.6, 128.5, 128.0, 127.8, 127.7,
127.5, 127.2, 124.2, 124.0, 122.5, 122.0, 113.5, 109.0,
108.9, 91.1, 87.7, 54.0, 53.7, 43.3, 31.3, 29.9, 20.7, 16.1,
14.0. ESI-HRMS ([C58H61B2N3O4 + 2H]2+): calcd
443.7503, found 443.7501.
1
R, R-6 was prepared with same method of S,S-6. H
Results and Discussions
NMR (400 MHz, CDCl3) δ 8.17 (s, 1H), 7.95 (s, 1 H), 7.50
(s, 1H), 7.40-7.50 (m, 7H), 7.32-7.39 (m, 6H), 7.24-7.28
(m, 2H), 7.14-7.19 (m, 2H), 4.12-4.17 (q, 1H), 4.05 (t, 2H,
J=7.2 Hz), 4.00 (d, 1H, J=6.8 Hz), 3.84-3.92 (m, 2H),
3.61-3.69 (m, 2H), 1.68-1.74 (m, 2H), 1.66 (d, 3H,
J=6.8 Hz), 1.52 (d, 3H, J=6.8 Hz), 1.24-1.33 (m, 2H),
0.87 (t, 3H, J=7.2 Hz). ESI-HRMS ([C42H43N3 + 2H]2+):
calcd 295.6807, found 295.6812.
Synthesis Previously we devised chiral d-PET chemosensor
9 with unsubstituted carbazole as the fluorophore
(Scheme 1) [30]. In a later study we designed chemosensors
10–12 (Scheme 1) [31]. Interestingly, with introduce of
ethynylene subunit to the carbazole fluorophore, the d-PET
effect was switched to a-PET effect, which is probably due
to the electron-deficient feature of the ethynylene moiety.
Herein we design the bisboronic acid chemosensors based
on the scaffold of ethynylated carbazole. The binding
pocket of the new chiral chemosensor is larger than the
previous chiral bisboronic acid chemosensors [31].
The synthesis is with carbazole as the starting material
(Scheme 2), with iodination and formylation, the iodo-
aldehyde 4 was obtained. Then with Sonogashira coupling
reaction, the bis-aldehyde 5 was obtained. With reductive
amination and introduce of the binding moiety of boronic
acid moieties, the chiral chemosensor 1 (S,S-1 and R,R-1)
was obtained in moderate yields.
S, S-1 or R, R-1
S,S-6 (140.0 mg, 0.24 mmol), 2-(2-bromomethylphenyl)-
1,3,2-dioxaborinane (390.0 mg, 1.31 mmol) and K2CO3
(620.0 mg, 3.86 mmol) were mixed in acetonitrile (15 mL),
the mixture was refluxed for 10 h under N2. The mixture
was cooled and DCM was added. The organic layer was
washed with water and dried over anhydrous Na2SO4. The
solvent was removed under reduced pressure and the
residue was purified with column chromatography (Al2O3,
DCM/MeOH, 15:1, V/V). 26.0 mg of yellow powder
(S,S-1) was obtained, yield: 12.6 %. M.p. 116.5-117.2 °C.
1H NMR (400 MHz, CDCl3/CD3OD) δ 8.27 (s, 1H), 7.88
(s, 1H), 7.81 (d, 2H, J=6.0 Hz), 7.61 (d, 1H, J=8.8 Hz),
7.39-7.45 (m, 6H), 7.31-7.37 (m, 12H), 7.16-7.27 (m, 5H),
4.28 (t, 2H, J=7.2 Hz), 4.07-4.16 (m, 2H), 3.93-4.00
(m, 2H), 3.84 (d, 1H, J=13.2 Hz), 3.59-3.69 (m, 3H), 3.51
(d, 1H, J=12.4 Hz), 3.33 (d, 1H, J=13.6 Hz), 1.81-1.88
Fluorescence of the Chemosensors The excitation and the
emission spectra of S,S-1 were studied (Fig. 1). Broad
structureless excitation and emission bands were observed,
which are different from the structured excitation/emission
bands of the previously reported carbazole-based chemo-
sensor [30, 31]. Furthermore, the emission is red-shifted by
ca. 15 nm compared to a previously reported boronic acid

J Fluoresc (2011) 21:1979–1986
1983
acidic pH is in stark contrast to the normal a-PET boronic
acid chemosensors [24–27]. Furthermore, we found that the
recognition of tartaric acids with the chemosensor 1 is
enantioselective. For example, The S,S-1 gives larger
response to L-tartaric acid than the D-tartaric acid. With
R,R-1, however, the profile is reversed. This mirror effect is
typical enantioselective response for the chiral boronic acid
chemosensors.
Previously we reported a boronic acid chemosensor
based on anthracene/naphthalethylamine which shows
improved fluorescence transduction at acidic pH, due to
the intramolecular interaction between the chirogenic center
(naphthalene subunit) and the fluorophore [27]. For the new
chemosensor 1, however, such an interaction is unlikely.
Thus we propose that the sensing mechanism is a hybrid
d-PET and a-PET. The a-PET effect will be suppressed
upon binding with analytes, thus the emission was
enhanced at acidic pH [30–32]. Furthermore, the restriction
of the conformation upon binding with analytes may also
enhance the fluorescence emission at acidic pH.
The binding titration of the chemosensor 1 with L- and
D-tartaric acid was studied (Fig. 3). Enantioselectivity was
observed and the result is in consistent with the pH titration
(Fig. 2). With S,S-1, larger fluorescence enhancement was
observed with the L-tartaric acid. With R,R-1, however,
higher fluorescence response was observed with the
D-tartaric acid. Moderate enantioselectivity was observed
for the recognition of tartaric acid with chemosensor 1 at
pH 4.0. For example, the binding constants of S,S-1 toward
D- and L-tartaric acids are (4.64 0.70)×105 M-1 and
(5.60 0.47)×105 M-1, respectively. The enantioselectivity
is KD/KL=1.0 : 1.2. With R,R-1, the binding constants
toward D- and L-tartaric acid are (1.04 0.05)×106 M-1 and
(8.68 0.49)×105 M-1, respectively. Thus the enantioselec-
tivity is KD/KL=1.2 : 1.0. These reversed kD/kL ratios
indicated the enantioselectivity of the binding constants.
We noticed the binding constants are slightly smaller than
the previous carbazole-based chiral boronic acid chemo-
sensors [30], which is tentatively attributed to the larger
binding pocket of chemosensor 1 than chemosensor 9
1.0
Excitation
0.8
Emission
0.6
0.4
0.2
0.0
200
300
400
500
600
Wavelength / nm
Fig. 1 Normalized excitation and emission spectra of S,S-1. lex=343 nm,
em=391 nm. 1.0×10-6 mol dm-3 of chemosensor in 0.05 mol dm-3 NaCl
ionic buffer (52.0% methanol in water). pH 7.5, 20 °C
l
chemosensor based on the non-ethynylated carbazole [30].
These results demonstrated that the electron communication
between the periphery phenyl subunit and the carbazole
core is efficient.
The pH titration of the chemosensors in the absence and
presence of tartaric acids were studied (Fig. 2). Enhanced
emission at acid pH but diminished emission at neutral and
basic pH were observed. Thus the chemosensors can be
identified as a-PET chemosensors [25, 31]. The pKa of
S,S-1 is 5.71 0.08. similar pKa was observed for R,R-2
(5.81 0.05). The pKa of the chemosensors changed in the
presence of tartaric acid. For example, the pKa of S,S-1
changed to 7.03 0.06 (D-tartaric acid) or 7.15 0.08
(D-tartaric acid). For R,R-1, the pKa changed to 7.33 0.04
(D-tartaric acid) or 6.99 0.06 (L-tartaric acid).
We noticed that the contrast ratio of the chemosensors
are relatively smaller (only ca. 2.0-fold) compared to the
typical a-PET chemosensors based on anthracene fluoro-
phore (ca. 8-fold) [25, 35]. The pH titration of the
chemosensors in the presence of D- and L-tartaric acid
shows that the recognition at acidic pH region is significant.
This improved fluorescence response upon binding at
Fig. 2 Fluorescence intensity-
pH profiles of (a) S,S-1 and (b)
R,R-1 in the presence of D- and
L-tartaric acid. lex=330 nm,
lem=389 nm, 1.0×10-6 mol
dm-3 of chemosensors in
600
500
400
300
200
100
500
blank S,S-1
blank R,R-1
a
b
S,S-1 + D-acid
R,R-1 + D-acid
S,S-1 + L-acid
400
300
200
100
R,R-1 + L-acid
0.05 mol dm-3 NaCl ionic buffer
(52.0% methanol in water),
c(D- and L-tartaric acid)=1.25×
10-2 mol dm-3. 20 °C
2
4
6
8
10
12
2
4
6
8
10
12
pH
pH

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J Fluoresc (2011) 21:1979–1986
Fig. 3 Relative fluorescence
intensity of S,S-1 and R,R-1
versus concentration of D- and
L-tartaric acid. lex=330 nm,
lem=389 nm, pH=4.0, 1.0×
10-6 mol dm-3 of chemosensor
in 0.05 mol dm-3 NaCl ionic
buffer (52.0% methanol
2.0
1.8
1.6
1.4
1.2
1.0
2.0
1.8
1.6
1.4
1.2
1.0
a
S,S-1
b
S,S-1
L-tartaric acid
L-tartaric acid
in water). 20 °C
D-tartaric acid
D-tartaric acid
0.000
0.004
0.008
0.012
0.0000
0.0001
c[tartaric acid] / mol dm-3
0.0002
c[tartaric acid] / mol dm-3
2.0
1.8
1.6
1.4
1.2
1.0
2.0
1.8
1.6
1.4
1.2
1.0
c
d
R,R-1
R,R-1
D-tartaric acid
D-tartaric acid
L-tartaric acid
L-tartaric acid
0.000
0.004
0.008
0.012
0.0000
0.0001
c[tartaric acid] / mol dm-3
0.0002
c[tartaric acid] / mol dm-3
(Scheme 1) [30]. The enantioselective fluorescence response
with R,R-1 is [kDF(D)]/[kLF(L)]=1.0 : 2.0. With S,S-1, the
enantioselective fluorescence response with R,R-1 is
[kDF(D)]/[kLF(L)]=1.6 : 1.0, where k is binding constants
with D- or L-tartaric acid, F is the fluorescence enhancement
in the presence of D- or L-tartaric acid.
DFT Optimization of the Geometry of the Binding
Complexes The binding complexes of S,S-1-L-tartaric acid
and S,S-1-D-tartaric acid were optimized (the results of the
complex S,S-1-L-tartaric acid was presented in Fig. 4). The
binding complex was considered as a methanol-inserted
Furthermore, we observed a consecutive emission
enhancement/diminishment with increasing the tartaric acid
concentration. Similar result is found for the recognition of
L- and D-tartaric acid with S,S-1 and R,R-1 at pH 6.0. The
decrease of the fluorescence at higher tartaric acid concen-
tration is not due to the quenching effect of tartaric acid.
The fluorescence enhancement is due to the binding of
tartaric acid with boronic acid, thus the suppressed PET
effect via B-N interaction, either the direct or the
intramolecular hydrogen bonding [36, 37]. Previously we
observed such emission enhancement/diminishment for a
BINOL based boronic acid chemosensor [24]. We propose
the emission enhancement/diminishment is due to the
transition of the binding stoichiometry from 1:1 binding
to 1:2 binding. 1:1 cyclic binding shows extra emission
enhancement than the 1:2 opened form binding. In order to
elucidate the fluorescence enhancing mechanism, the
binding complex of the enantiomers were optimized.
O2
B2
H2
N2
B1
O1
N1
H1
Fig. 4 Optimized structure of the binding complex of S,S-1-(R)-L-tartaric
acid. Calculated by DFT at the B3LYP/6-31G(d) level using
Gaussian 09W

J Fluoresc (2011) 21:1979–1986
1985
zwitterionic structure [36, 37]. The optimized intramolecular
hydrogen binding structures are in agreement with the
experimental results [36, 37]. For example, the hydrogen
bond N(1)-H(1)⋅⋅⋅⋅O(1) shows bond length of N-O: 2.553Å,
H-O: 1.455Å and the angle N-H⋅⋅⋅O is 163 °.
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