3,6-disubstituted carbazole-based bisboronic acids with unusual fluorescence transduction as enantioselective fluorescent chemosensors for tartaric acid

Article abstract of DOI:10.1021/jo8025669

Carbazole-based bisboronic acids were found to be enanti-oselective fluorescent sensors for tartaric acid. The fluorescence response toward the enantiomers of tartaric acid at neutral pH displayed enhancement/diminishment. The sensor displays an unusual f

Full text of DOI:10.1021/jo8025669

well in noncompetitive solvents but fail in protic solvents due
to the severe interference of protic media with the hydrogen
bonding receptor. In order to develop chiral fluorescent chemosen-
sors that are effective in aqueous media, especially toward
R-hydroxyl acids, which are bioactive or versatile chiral building
blocks, boronic acid based fluorescent sensors have been
designed and some of these sensors display tight binding toward
sugars or polyhydroxyl acids, such as glucose, tartaric acid,
mandelic acid or glucarate, etc.9-16
3,6-Disubstituted Carbazole-Based Bisboronic
Acids with Unusual Fluorescence Transduction as
Enantioselective Fluorescent Chemosensors for
Tartaric Acid
Feng Han,† Lina Chi,† Xiaofen Liang,† Shaomin Ji,†
Shasha Liu,‡ Fuke Zhou,† Yubo Wu,† Keli Han,§
Jianzhang Zhao,*,† and Tony D. James*,¶
We previously reported a BINOL-based boronic acid sensor
with a unique signal transduction profile where the fluorescence
intensity of the sensor is either enhanced or diminished with
the two enantiomers of tartaric acid.13 Anthracene-based sensors
were also prepared and show very high binding constants and
enantioselectivity toward tartaric acid (e.g., 6 in Scheme 1).14,15
Recently, we designed a new BINOL boronic acid, which is
enantioselective on sorbitol, but the sensor failed to show
fluorescence enhancement/diminishment toward tartaric acids.16
Herein we report new chiral boronic acid sensor 3 (Scheme
1), based on 3,6-disubstituted carbazole, that shows high
enantioselectivity toward tartaric acids and, surprisingly, a
reverse fluorescence intensity-pH relationship14,15 (i.e., dimin-
ished fluorescence intensity at acidic pH but stronger emission
at basic pH).9,10 Furthermore, fluorescence enhancement/
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, School of Physics and Optoelectronic
Technology, Dalian UniVersity of Technology,
158 Zhongshan Road, Dalian 116012, People’s Republic of
China, Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, 457 Zhongshan Road, Dalian 116023, People’s
Republic of China, and Department of Chemistry, UniVersity of
Bath, Bath BA2 7AY, U.K.
zhaojzh@dlut.edu.cn; t.d.james@bath.ac.uk
ReceiVed April 14, 2008
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47, 3577.
Carbazole-based bisboronic acids were found to be enanti-
oselective fluorescent sensors for tartaric acid. The fluores-
cence response toward the enantiomers of tartaric acid at
neutral pH displayed enhancement/diminishment. The sensor
displays an unusual fluorescence intensity-pH relationship
with diminished emission at acidic pH but enhanced emission
at basic pH. Photoinduced electron transfer (PET) from the
fluorophore to the protonated amine/phenylboronic acid unit
is proposed to be responsible for this effect, which is
rationalized by density functional theory (DFT) calculations.
(9) James, T. D.; Shinkai, S. Top. Curr. Chem. 2002, 218, 159.
(10) James, T. D. Top. Curr. Chem. 2007, 277, 107.
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45, 3829. (e) He, M.; Johnson, R. J.; Escobedo, J. O.; Beck, P. A.; Kim, K. K.;
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Mrse, A. A.; Treleaven, W. D.; Strongin, R. M. J. Am. Chem. Soc. 2002, 124,
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Commun. 2005, 1889.
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(16) Liang, X.; James, T. D.; Zhao, J. Tetrahedron 2008, 64, 1309.
Synthetic receptors and fluorescent chemosensors for enan-
tioselective recognition of chiral compounds, such as chiral
R-hydroxyl carboxylic acids, are a subject of increasing
interest.1-8 The majority of these chiral fluorescent sensors are
based on hydrogen bonding interactions.2-6 These systems work
† State Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology.
‡ School of Physics and Optoelectronic Technology, Dalian University of
Technology.
§ Dalian Institute of Chemical Physics.
¶ University of Bath.
(1) Stibor, I.; Zlatusˇkova´, P. Top. Curr. Chem. 2005, 255, 31.
10.1021/jo8025669 CCC: $40.75
Published on Web 01/14/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 1333–1336 1333

SCHEME 1. Enantioselective Fluorescent Sensors 3 and 4
for r-Hydroxyl Carboxylic Acids (Reported Sensors 5 and 6
Are Also Presented)
FIGURE 1. Normalized excitation and emission spectra of (R,
R)-3: 3.0 × 10-6 mol dm-3 of sensor in 0.05 mol dm-3 NaCl ionic
buffer (52.1% methanol in water); pH 7.5, λex ) 307 nm, λem
375 nm, 22 °C.
)
FIGURE 2. Fluorescence intensity-pH profile of (S,S)-3 in the
presence of D- and L-tartaric acid: λex ) 307 nm, λem ) 375 nm, 3.0 ×
10-6 mol dm-3 of sensor in 0.05 mol dm-3 NaCl ionic buffer (52.1%
methanol in water), c(D- and L-tartaric acid) ) 0.05 mol dm-3, 22 °C.
diminishment toward D- and L-tartaric acid was observed at
neutral pH. A reverse PET mechanism (i.e., electron transfer
(ET) from the fluorophore to the protonated amine/phenylbo-
ronic acid unit) is proposed to be responsible for the unusual
fluorescence-pH relationship and has been rationalized by
density functional theory (DFT) calculations.
Synthesis of chiral sensors 3 and 4 is outlined in Scheme 1.
Reductive amination of 3,6-diformyl-9-ethylcarbazole with (R)-
and (S)-benzylmethylamine leads to chiral amine 2 then by
reaction with 2-(2-bromobenzyl)-1,3,2-dioxaborinane to obtain
(R,R)-3 and (S,S)-3. The model monoboronic acid sensor (S)-4
was synthesized with a similar approach. The previously
reported sensor 5 is used as a model compound in DFT cal-
culation/mechanistic studies.14
the PET process leads to intensified fluorescence).13-17 How-
ever, sensor 3 gives stronger emission at basic pH (pKa of 6.55
( 0.55). This reverse PET effect is beneficial to the recognition
of tartaric acid, due to the reduced background fluorescence and
the enhanced signal transduction.13,14
Enantioselectivity is found for 3 (Figure 2). For example, in
the pH range of 2.0-6.0, (S,S)-3 gives much higher fluorescence
enhancement in the presence of L-tartaric acid, when compared
with D-tartaric acid. With (R,R)-3, a mirror effect is observed
(Figure S25). Furthermore, the fluorescence profile of enhance-
ment/diminishment previously found for the axially chiral
BINOL-based sensor (observed at acidic pH)13 is also observed
for 3, but now at neutral pH (pH 6.5-8.0). For example, at pH
7.4, the fluorescence intensity of (S,S)-3 is intensified in the
presence of L-tartaric acid, while with D-tartaric acid, the
fluorescence intensity is diminished. With (R,R)-3, a mirror
effect is observed. To the best of our knowledge, this type of
The emission and excitation fluorescence spectra of (R,R)-3
are shown in Figure 1. The excitation and the emission
wavelengths for the binding studies were selected as 307 and
375 nm, respectively.
The pH versus fluorescence intensity (IF) profile of 3 with or
without analytes is shown in Figure 2.5,6,13-16 Surprisingly, this
apparent PET chemosensor gave a reverse fluorescence response
as a function of pH, compared to normal PET sensors.9,10,13-15
Usually, a PET sensor containing a N atom gives an emission
in acidic media stronger than that in basic media, due to
protonation of the N atom in acidic media (thus suppression of
(17) de Silva, A. P.; H.; Gunaratne, Q. N.; Gunnlaugsson, T.; Huxley,
A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
1515.
1334 J. Org. Chem. Vol. 74, No. 3, 2009

FIGURE 4. Relative fluorescence intensity of (S,S)-3 versus concentra-
tion of L- and D-tartaric acid, at pH 7.4: λex ) 305 nm, λem ) 375 nm,
1.0 × 10-6 mol dm-3 of sensor in 0.05 mol dm-3 NaCl ionic buffer
(52.1% methanol in water), 25 °C.
FIGURE 3. Relative fluorescence intensity of (S,S)-3 versus concentra-
tion of L- and D-tartaric acid: λex ) 305 nm, λem ) 375 nm, pH ) 5.6,
1.0 × 10-6 mol dm-3 of sensor in 0.05 mol dm-3 NaCl ionic buffer
(52.1% methanol in water), 25 °C.
signal transduction is rarely observed, and this is only the second
example of such a unique enantioselective fluorescence signal
response.13
On the basis of the IF-pH studies (Figure 2), pH 5.6 and 7.4
were selected to further investigate the binding and enantiose-
lectivity of these systems.
The titrations of (S,S)-3 with D- and L-tartaric acid at pH 5.6
are shown in Figure 3. Higher fluorescence enhancement was
observed in the presence of L-tartaric acid versus D-tartaric acid.
The apparent binding constants for D- and L-tartaric acid are
The unusual PET behavior of sensor 3 still puzzled us (Figure
2). The fluorescence intensity-pH profile of model sensor 4, a
monoboronic acid, is similar to that of sensor 3 (Figure S27);
therefore, the reverse PET response is unlikely caused by
formation of a cyclic 1:1 binding complex, instead, the reverse
PET effect is probably due to the carbazole fluorophore. The
reverse PET response is also unlikely due to the hybridization
change of the boronic acid group12g because the boronic acid
moiety is not directly appended to the fluorophore and the
apparent pKa does not support the boronic acid hybridization
mechanism (Figure 2).14 Since the carbazole moiety is a well-
known strong electron donor,18 we envision that PET from the
carbazole moiety to the amine and boronic acid moiety is highly
possible, especially when the amine is protonated. Such an ET
from fluorophore to the positively charged N atom will quench
the fluorescence.19 Thus, at acidic pH, the fluorescence was
diminished due to the reverse PET effect. The acidity of the
boron atom of the boronic acid moiety also facilitated the
potential reverse PET.9,10
app
app
KD ) (2.31 ( 0.00) × 106 and KL ) (2.84 ( 0.00) × 106
M-1, respectively, thus the enantioselective response is KDappF(D)/
KLappF(L) ) 1.0:1.8,13 where F is fluorescence enhancement of
the sensor in the presence of D- or L-tartaric acid. With (R,R)-3,
a mirror effect is observed (Figure S30), for which KDapp ) (3.80
app
( 0.02) × 106 and KL
) (2.67 ( 0.23) × 106 M-1
,
respectively, with the enantioselective response KDappF(D)/
KLappF(L) ) 1.7:1.0.13
The binding of sensor 3 with tartaric acid at pH 7.4 was also
investigated (Figure 4). For (S,S)-3, fluorescence enhancement
is observed with L-tartaric acid, and the apparent binding
To help confirm this reverse PET mechanism, theoretical
calculations based on DFT and time-dependent DFT (TDDFT)
were performed for 4 (Figure 5, see Supporting Information
for more details).20
app
constant is KL ) (1.18 ( 0.35) × 105 M-1. With D-tartaric
acid, however, a fluorescence diminishment was observed with
app
KD ) (7.78 ( 1.51) × 104 M-1. Thus the enantioselective
response KDappF(D)/KLappF(L) is (-1.0):(+1.6). With (R,R)-3,
The frontier molecular orbitals for protonated 4 reveal that
there is ET from the fluorophore to the protonated amine unit
on excitation (see Supporting Information), which leads to
fluorescence quenching of the carbazole fluorophore.19 For
neutral sensor 4, the orbitals are localized on carbazole, for both
HOMO and LUMO, so there is no charge transfer and the
fluorescence is intensified when compared with the protonated
4. The TDDFT calculations also predict, to some extent, ET
from the amine unit to carbazole moiety for neutral 4, which is
in agreement with the normal PET effect, or a-PET (fluorophore
as the electron acceptor in PET).21 Similar results were found
a mirror effect in enantioselectivity is observed (Figure S31),
app
app
for which KD ) (3.65 ( 1.07) × 104 and KL ) (2.11 (
1.58) × 104 M-1, with the enantioselective response KDappF(D)/
app
KL F(L) is (+1.6):(-1.0). Herein we noticed the deviation
of the enantioselectivity (KDapp and KLapp) from the perfect mirror
effect;12c,13,14 this may be due to some experimental uncertain-
ties, which are difficult to eliminate completely.
Such fluorescence intensity enhancement/diminishment re-
sponses toward the enantiomers of analytes are rarely reported,13
although quenching upon anion sensing is known for normal
PET sensors.2g The exact mechanism for this unique fluores-
cence intensity enhancement/diminishment is unclear, but we
believe this effect might be more common for chiral fluorescent
sensors than we previously thought (i.e., not restricted to axially
chiral BINOL moiety).13 Our results will inspire design of new
enantioselective fluorescent sensors since the effect enhances
analytical performance.10,13
(18) (a) Galmiche, L.; Mentec, A.; Pondaven, A.; L’Her, M. New J. Chem.
2001, 25, 1148. (b) Velasco, D.; Castellanos, S.; Lo´pez, M.; Lo´pez-Calahorra,
F.; Brillas, E.; Julia, L. J. Org. Chem. 2007, 72, 7523.
(19) De, A. K.; Ganguly, T. J. Lumin. 2001, 92, 255.
(20) Frisch M. J.; Gaussian 03, revision B.05; Gaussian Inc.: Pittsburgh PA,
2003.
(21) McCarroll, M. E.; Shi, Y.; Harris, S.; Puli, S.; Kimaru, I.; Xu, R.; Wang,
L.; Dyer, D. J. J. Phys. Chem. B 2006, 110, 22991.
J. Org. Chem. Vol. 74, No. 3, 2009 1335

Experimental Section
(S,S)-3: (S,S)-2 (0.92 g, 2 mmol), 2-(2-bromobenzyl)-1,3,2-
dioxaborinane (0.50 g, 4 mmol), and K2CO3 (1.18 g, 8.56 mmol)
were mixed in dry MeCN (30 mL) and refluxed for 10 h under N2.
The mixture was cooled and the solvent evaporated, and CH2Cl2
was added. The organic layer was washed with water and dried
over MgSO4. After evaporation, the residue was purified with
column chromatography (Al2O3, CH2Cl2/MeOH, 100:1, v/v); 0.3 g
of white powder was obtained: yield 20.4%; mp 216.4-217.8 °C;
1H NMR (400 MHz, CDC13) δ (ppm) ) 7.80 (s, 2H), 7.72 (s,
2H), 7.14-7.30 (m, 20H), 4.25 (q, 2H, J ) 8.0 Hz), 4.05 (q, 2H,
J ) 8.0 Hz), 3.83 (dd, 4H, J ) 12.0 Hz), 3.58 (d, 2H, J ) 8.0 Hz),
3.30 (d, 2H, J ) 8.0 Hz), 1.52 (d, 6H, J ) 8.0 Hz), 1.32 (t, 3H, J
) 8.0 Hz); 13C NMR (100 MHz, CDC13) δ (ppm) ) 141.9, 139.6,
139.2, 136.2, 134.9, 131.4, 130.0, 129.3, 128.2, 127.6, 127.5, 127.2,
122.6, 121.8, 108.5, 57.5, 57.0, 53.8, 37.7, 15.7, 13.9; [R]25
)
D
-33.4 ( 0.9 (c ) 0.39 in MeOH); TOF MS ESI+ calcd for
C46H51B2N3O4 ([M + 2H]2+) 365.7033, found 365.7031. Anal.
Calcd for C46H49B2N3O4 ·2CH3OH: C, 72.64; H, 7.24; N, 5.29.
Found: C, 72.98; H, 6.93; N, 5.36.
FIGURE 5. Mechanism of the reverse PET effect: HOMO and LUMO
of protonated and neutral sensor 4, calculated with DFT/TDDFT at
the B3LYP/6-31G(d) level using Gaussian 03.
(R,R)-3 was prepared with the same methods: mp 218.4-219.7
1
°C; H NMR (400 MHz, CDC13) δ (ppm) ) 7.88 (s, 2H), 7.80
(br, 2H), 7.23-7.43 (m, 20H), 4.33 (q, 2H, J ) 8.0 Hz), 4.13 (q,
2H, J ) 8.0 Hz), 3.90 (dd, 4H, J ) 12 Hz), 3.63 (d, 2H, J ) 8.0
Hz), 3.38 (d, 2H, J ) 8.0 Hz), 1.60 (d, 6H, J ) 8.0 Hz), 1.41 (t,
3H, J ) 8.0 Hz); 13C NMR (100 MHz, CDC13) δ (ppm) ) 141.9,
139.7, 139.2, 136.2, 134.9, 131.4, 130.0, 129.3, 128.3, 127.7, 127.5,
for 3 (see Supporting Information). At basic pH, the PET from
carbazole to the amine unit is inhibited, but the normal PET
(i.e., ET from the amine unit to fluorophore) occurs, thus on
balance the fluorescence intensity will be intensified, but not
significantly.14 These calculations are consistent with experi-
mental results and rationalize the concept of d-PET (fluorophore
as electron donor in the PET process).21,22 On the basis of the
calculated molecular orbitals, the free energy changes (∆G°)
of the ET of protonated sensor 4 is estimated as -0.22 eV
(-21.2 kJ mol-1).21,22
To rationalize the putative d-PET effect, sensor 5, which
shows normal PET response,14 was also studied for TDDFT
calculations, and the results indicate that no ET from fluorophore
to the charged amine unit occurs. Instead, there is ET from the
N unit to the fluorophore for the neutral 5, thus it belongs to
the normal a-PET sensor. According to our putative mechanism,
neutral 5 should give weak emission whereas protonated 5 gives
intensified emission, which are in full agreement with experi-
mental observations.14
These TDDFT calculations/experimental results might be
suggesting a general role for the reverse PET effect, and it can
potentially be used to design new sensors that give improved
signal transduction at acidic pH, where the normal PET sensors
give poor signal transduction.9,10,13,14
In summary, new 3,6-disubstituted carbazole-based bisboronic
acid for enantioselective recognition of tartaric acid is reported.
The unique fluorescence response of enhancement/diminishment
toward the enantiomers of tartaric acid was found at neutral
pH. The new sensors show an unusual PET effect of diminished
fluorescence at acidic pH, by which improved signal transduc-
tion resulted. A mechanism of electron transfer from fluorophore
to the protonated amine/phenylboronic acid unit (d-PET) was
proposed and supported by DFT/TDDFT calculations. Our
future aim is to study this reverse PET effect and to explore its
potential applications in new fluorescent PET sensors.
127.2, 122.6, 121.8, 108.6, 57.5, 56.9, 53.9, 37.8, 15.8, 13.9; [R]25
D
) +34.8 ( 2.0 (c ) 0.39 in MeOH); TOF MS ES+ calcd for
C46H51B2N3O4 ([M + 2H]2+) 365.7033, found 365.7026; calcd for
C46H50B2N3O4 ([M + H]+) 730.3982, found 730.4019. Anal. Calcd
for C46H49B2N3O4 ·0.5CH3OH: C, 74.91; H, 6.89; N, 5.64. Found:
C, 74.58; H, 6.81; N, 5.53.
(S)-4 was prepared with the same methods: mp 120.7-122.5
°C; 1H NMR (400 MHz, CDC13) δ (ppm) ) 8.07 (d, 2H, J ) 8.0
Hz), 7.91 (s, 1H), 7.82 (d, 1H, J ) 8.0 Hz), 7.19-7.84 (m, 12H),
4.35 (q, 2H, J ) 8.0 Hz), 4.12 (q, 1H, J ) 8.0 Hz), 3.97 (d, 1H,
J ) 12.0 Hz), 3.86 (d, 1H, J ) 12.0 Hz), 3.60 (d, 1H, J ) 12.0
Hz), 3.52 (d, 1H, J ) 12.0 Hz), 1.59 (d, 3H, J ) 8.0 Hz), 1.42 (t,
3H, J ) 8.0 Hz); 13C NMR (100 MHz, CDC13) δ (ppm) ) 141.9,
140.3, 139.4, 139.2, 136.2, 131.4, 130.0, 129.3, 128.2, 127.6, 127.5,
127.3, 127.2, 125.7, 122.8, 121.8, 120.4, 118.8, 108.6, 108.4, 57.5,
56.9, 53.8, 37.7, 15.8, 13.9; [R]25 ) -31.6 ( 0.5 (c ) 0.78 in
D
MeOH); TOF MS ESI+ calcd for C30H32BN2O2 ([M + H]+)
463.2557, found 463.2563. Anal. Calcd for C30H31BN2O2: C, 77.93;
H, 6.76; N, 6.06. Found: C, 77.50; H, 7.18; N, 5.44.
Acknowledgment. We thank the NSFC (20642003 and
20634040), Scientific Research Foundation for the Returned
Overseas Chinese Scholars (MOE), PCSIRT(IRT0711), Cultiva-
tion Fund of the Key Scientific and Technical Innovation Project
(707016), State Key Laboratory of Fine Chemicals and Dalian
University of Technology (SFDUT07005) for support. We are
also grateful to the reviewers for their critical comments on the
manuscript.
Supporting Information Available: General experimental
methods, compound characterization data (IR, etc.), fluorescence
spectra, and DFT/TDDFT calculation details of sensors 3, 4,
and 5. This material is available free of charge via the Internet
at http://pubs.acs.org.
(22) Ueno, T.; Urano, Y.; Setsukinai, K.; Takakusa, H.; Kojima, H.; Kikuchi,
K.; Ohkubo, K.; Fukuzumi, S.; Nagano, T. J. Am. Chem. Soc. 2004, 126, 14079.
JO8025669
1336 J. Org. Chem. Vol. 74, No. 3, 2009