Synthesis, crystal structures and spectroscopic characterizations of two difluoroboradiaza-s-indacene dyes

Article abstract of DOI:10.1016/j.molstruc.2006.05.014

Two new 8-(4-dimethylaminophenyl) substituted boron-dipyrromethene (BDP) dyes: 1,2,6,7-tetraethyl-4,4-difluoro-3,5-dimethyl-8-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene (5) and 4,4-difluoro-1,2,3,5,6,7-hexamethyl-8-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene (6) have been synthesized and their crystal structures are determined by X-ray diffraction analysis. The torsion angles of 8-phenyl ring and the indacene plane are 79.61° for 5 and 87.56° for 6, respectively. The absorption and steady-state fluorescence properties in different solvents have been investigated. The fluorescence quantum yields of the two dyes are lower in more polar solvents, which can be ascribed to the intramolecular charge transfer from the aniline to the BDP unit. Protonation of the aniline moiety at low pH in MeOH-H₂O (1:1 v/v) solution causes a large fluorescence enhancement. The pK_a values calculated from the pH dependent fluorescence emission spectra are 3.14 for 5 and 3.09 for 6, respectively. They are suitable as pH probes excitable with visible light.

Full text of DOI:10.1016/j.molstruc.2006.05.014

Journal of Molecular Structure 827 (2007) 130–136
www.elsevier.com/locate/molstruc
Synthesis, crystal structures and spectroscopic characterizations of two
difluoroboradiaza-s-indacene dyes
a
a,
a,c
a
b
*
Yan-Hong Yu , Zhen Shen , Hai-Yun Xu , Yan-Wei Wang , Tetsuo Okujima ,
b
a
a
Noboru Ono , Yi-Zhi Li , Xiao-Zeng You
a
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, China
b
Department of Chemistry, Faculty of Science, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan
c
Department of Chemistry, Huainan Normal College, Huainan 232001, China
Received 10 April 2006; received in revised form 1 May 2006; accepted 10 May 2006
Available online 21 June 2006
Abstract
Two new 8-(4-dimethylaminophenyl) substituted boron-dipyrromethene (BDP) dyes: 1,2,6,7-tetraethyl-4,4-difluoro-3,5-dimethyl-
-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene (5) and 4,4-difluoro-1,2,3,5,6,7-hexamethyl-8-(4-dimethylaminophenyl)-
-bora-3a,4a-diaza-s-indacene (6) have been synthesized and their crystal structures are determined by X-ray diffraction analysis.
8
4
The torsion angles of 8-phenyl ring and the indacene plane are 79.61ꢀ for 5 and 87.56ꢀ for 6, respectively. The absorption and
steady-state fluorescence properties in different solvents have been investigated. The fluorescence quantum yields of the two dyes
are lower in more polar solvents, which can be ascribed to the intramolecular charge transfer from the aniline to the BDP unit.
Protonation of the aniline moiety at low pH in MeOH–H O (1:1 v/v) solution causes a large fluorescence enhancement. The
2
pK
suitable as pH probes excitable with visible light.
2006 Elsevier B.V. All rights reserved.
a
values calculated from the pH dependent fluorescence emission spectra are 3.14 for 5 and 3.09 for 6, respectively. They are
ꢁ
Keywords: Boron–dipyrromethene; Fluorescence; Crystal structure; pH probe
1
. Introduction
,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (boron-dip-
cent probes have been synthesized for proton or metal
ion detections [3,9]. BDP-based fluorophores bearing
dialkyl-aminophenyl [25–28], phenolic [29], naphtholic
and calyx [4]-arene [30] subunits at meso-position have
been investigated as pH sensors as they showed fluores-
cence off/on switching behavior upon protonation/
deprotonation. However, the crystal structures of BDP
dyes reported so far are still rare [2,31–38]. Herein we
describe the crystal structures of two newly synthesized
BDP dyes: 1,2,6,7-tetraethyl-4,4-difluoro-3,5-dimethyl-
8-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene
(5) and 4,4-difluoro-1,2,3,5,6,7-hexamethyl-8-(4-dimethyl-
aminophenyl)-4-bora-3a,4a-diaza-s-indacene (6). Their
absorption and steady-state fluorescence properties have
been investigated.
4
yrromethene, BDP) dyes are strongly fluorescent materi-
als that possess many valuable photophysical properties,
such as high photostability, high absorption coefficients,
high fluorescent quantum yield and they are photoexcit-
able with visible light [1]. They have found numerous
applications in many different areas, such as laser dyes
[
2–8], fluorescent sensors [9–16], optical devices [17–23]
and molecular probes for biochemical research [24].
Recently, an increasing number of BDP-based fluores-
*
Corresponding author. Tel.: +86 25 8368 6679; fax: +86 25 8331 4502.
E-mail address: zshen@nju.edu.cn (Z. Shen).
0
022-2860/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.05.014

Y.-H. Yu et al. / Journal of Molecular Structure 827 (2007) 130–136
131
2
. Experimental
as the internal standard. The MALDI-TOF mass spectra
were measured on an Applied Biosystems Voyager De pro
spectrometer. UV–visible spectra were carried out on a
UV-3100 spectrophotometer. Fluorescence spectra were
measured on an Aminco Bowman 2 Luminescence spectro-
photometer with a xenon arc lamp as the light source. The
2
.1. Instruments
Elemental analyses for C, H and N were performed on a
Perkin-Elmer 240C elemental analyzer. Melting points were
measured with a Reichert Thermometer apparatus without
correction. H NMR and C NMR spectra were recorded
on a Bruker DRX500 spectrometer at ambient temperature.
NMR chemical shifts are expressed in ppm relative to TMS
fluorescence quantum yields (/ ) were determined relative
f
1
13
to rhodamine 6G in water (kex = 488 nm, / = 0.76) using
f
dilute solutions with an absorbance below 0.1 at the excita-
tion wavelength (k ) [39]. Fluorimetric titration measure-
ex
ments were done in MeOH–H O (1:1 v/v) by gradually
2
+
ꢀ3
adding microlitre of H (c = 10 M of HClO ) to the dye
4
Table 1
ꢀ7
solution (c = 10 M).
Crystal data and structure refinement for 5 and 6
5
6
2.2. Reagents and materials
Empirical formula
C
27
H36BF
2
N
3
C
23 2 3
H30BF N O
F
w
451.40
413.31
All syntheses were carried out under nitrogen. Unless
Crystal size (mm)
Crystal system
Space group
Z
0.22 · 0.26 · 0.28
monoclinic
C2/c
0.24 · 0.24 · 0.32
monoclinic
C2/c
otherwise noted, all reagents or solvents were of analytical
grade and were used as received. Dry CH Cl was obtained
2
2
4
4
by refluxing and distilling over CaH under nitrogen. Dry
2
˚
˚
˚
a/A
b/A
c/A
19.290(6)
13.721(5)
11.075(4)
90
122.447(5)
90
2473.7(15)
1.212
0.081
17.747(4)
13.633(3)
11.676(3)
90
129.747(4)
90
2172.0(9)
1.264
0.089
THF was distilled from sodium/benzophenone. Triethyl-
amine was obtained by simple distillation. The solvents
used for photophysical measurement were of spectroscopic
grade and used without further purification.
a(deg)
b(deg)
c(deg)
V/A
q (calc) (g/cm )
˚
3
3
2.3. Crystal structure determination
ꢀ1
lcalc(mm
)
T (K)
F(000)
293(2)
968
0.98; 0.97
293(2)
880
0.98; 0.98
Single crystals of
0.24 · 0.24 · 0.32) were obtained by slow diffusion of
methanol into the corresponding solutions in CHCl at
5
(0.22 · 0.26 · 0.28) and
6
(
Transmission factors
max; min)
Scan type
3
(
room temperature and were glued to a thin glass fiber with
adhesive, respectively. The unit cell parameters and data
were collected at 293 K on a Bruker Smart Apex CCD dif-
fractometer with graphite monochromated Mo Ka radia-
/ and x
/ and x
2.11–26.00
2127
1515 (0.028)
semi-empirical
full-matrix
H range for data collection (ꢀ) 1.94–26.00
Total reflections
Measured reflections (R(int))
Absorption correction
Refinement method
2417
1548 (0.044)
semi-empirical
full-matrix
least-squares on F least-squares on F
156
0.0612, 0.1109
1.056
0.17 andꢀ0.39
˚
tion (k = 0.71073 A) using the x-2h scan mode. The data
2
2
were corrected for Lorenz and polarization effects. The
two structures were solved by the direct methods and
Parameters refined
147
2
R
1
,wR
2
0.0587, 0.1009
1.504
0.30 and ꢀ0.29
refined on F by full-matrix least-squares methods using
2
Goodness-of-fit on F
Largest diff. peak
and hole/(eA
SHELXTL-2000. All nonhydrogen atoms were refined
anisotropically. All hydrogen atoms were treated as riding
on their attached positions. All calculations and molecular
˚
ꢀ3
)
NMe2
NMe
2
R1
R2
R1
R2
R1
R1
CHO
LiAlH , THF
4
2
R2
R
CO Et
2
N+
N
H
reflux
1. TFA
. DDQ
. TEA, BF OEt
N
N
H
B-
2
3
.
3
2
F F
1
3
1
2
5: R = Et, R = Et
1
2
3: R = Et, R = Et
1
2
: R = Et, R = Et
1
3
1
2
6: R = CH , R = CH
3 3
1
2
4: R = CH , R = CH
3
: R = CH , R = CF
3
3
3
Scheme 1. Synthetic procedures for compounds 5 and 6. DDQ: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

132
Y.-H. Yu et al. / Journal of Molecular Structure 827 (2007) 130–136
graphics were carried out on a computer using the SHEL-
XTL-2000 program package [33]. Crystallographic data for
Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: (internat.) +44 1223/336
033; E-mail: deposit@ccdc.cam.ac.uk].
5
and 6 are summarized in Table 1.
CCDC-297327 for 5 and CCDC-297326 for 6 contain
the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.ca-
m.ac.uk/conts/retrieving.html [or from the Cambridge
3
. Results and discussion
3.1. Synthesis of 1,2,6,7-tetraethyl-4,4-difluoro-3,5-dimethyl-
8-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene
(5) and 4,4-difluoro-1,2,3,5,6,7-hexamethyl-8-(4-dimethyl-
aminophenyl)-4-bora-3a,4a-diaza-s-indacene (6)
The synthetic routes for 5 and 6 are outlined in Scheme
1
. Ethyl 3,4-diethylpyrrole-2-carboxylate (1) and ethyl 3-
(trifluoromethyl)-4-methylpyrrole-2-carboxylate (2) were
prepared according to published procedures [40,41]. Our
purpose to introduce the electron-withdrawing CF group
3
to pyrrole is to tune the redox properties of the resulting
BDP dye. However, reduction of 2 with excess of LiAlH4
in dry THF under reflux for 3 h gave 2,3,4-trimethylpyrrole
Table 2
˚
Selected bond lengths (A) and bond angle (ꢀ) of 5 and 6
5
6
Bond lengths
F(1)–B
N(1)–B
1.387(3)
1.552(3)
1.383(3)
1.338(4)
1.370(4)
1.420(3)
1.439(4)
1.403(3)
1.384(3)
1.497(3)
1.399(3)
1.517(4)
1.493(3)
1.478(4)
1.391(2)
1.358(3)
1.407(3)
F(1)–B
N(1)–B
1.407(3)
1.516(3)
1.403(3)
1.340(3)
1.422(4)
1.490(3)
1.380(3)
1.418(2)
1.381(3)
1.516(3)
1.380(3)
1.537(3)
1.501(3)
1.400(4)
1.388(3)
1.388(3)
1.332(3)
N(1)–C(1)
N(1)–C(4)
N(3)–C(9)
N(3)–C(10)
C(1)–C(2)
C(1)–C(5)
C(2)–C(3)
C(2)–C(11)
C(3)–C(4)
C(3)–C(13)
C(4)–C(15)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
N(1)–C(1)
N(1)–C(4)
N(2)–C(9)
N(2)–C(10)
C(1)–C(2)
C(1)–C(5)
C(2)–C(3)
C(2)–C(11)
C(3)–C(4)
C(3)–C(13)
C(4)–C(15)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
C(8)–C(9)
Fig. 1. ORTEP view of the molecular structure of 5 with thermal
ellipsoids shown at 30%.
Bond angles
F(1)–B–F(1A)
F(1)–B–N(1)
F(1)–B–N(1a)
N(1)–B–N(1a)
C(1)–N(1)–C(4)
C(1)–N(1)–B
109.4(3)
F(1)–B–F(1A)
F(1)–B–N(1)
F(1)–B–N(1a)
N(1)–B–N(1a)
C(1)–N(1)–C(4)
C(1)–N(1)–B
104.8(3)
110.65(9)
109.62(10)
106.9(3)
109.3(2)
125.3(2)
125.4(2)
120.70(13)
107.3(2)
120.9(2)
109.2(2)
123.0(2)
131.8(2)
120.5(2)
119.76(14)
121.50(13)
106.1(2)
108.2(2)
127.0(2)
111.16(9)
110.22(10)
109.2(3)
107.29(16)
124.98(19)
127.67(19)
118.24(12)
107.18(17)
120.49(18)
110.49(18)
123.00(17)
132.3(2)
C(4)–N(1)–B
C(4)–N(1)–B
C(9)–N(3)–C(10)
N(1)–C(1)–C(2)
N(1)–C(1)–C(5)
N(1)–C(4)–C(3)
N(1)–C(4)–C(15)
C(2)–C(1)–C(5)
C(1)–C(5)–C(1A)
C(1)–C(5)–C(6)
C(5)–C(6)–C(7)
C(1)–C(2)–C(3)
C(1)–C(2)–C(3)
C(2)–C(3)–C(13)
C(9)–N(2)–C(10)
N(1)–C(1)–C(2)
N(1)–C(1)–C(5)
N(1)–C(4)–C(3)
N(1)–C(4)–C(15)
C(2)–C(1)–C(5)
C(1)–C(5)–C(1A)
C(1)–C(5)–C(6)
C(5)–C(6)–C(7)
C(1)–C(2)–C(3)
C(1)–C(2)–C(3)
C(2)–C(3)–C(13)
119.7(2)
120.15(12)
123.52(13)
108.91(19)
106.12(17)
128.01(19)
Fig. 2. ORTEP view of the molecular structure of 6 with thermal
ellipsoids shown at 30%.

Y.-H. Yu et al. / Journal of Molecular Structure 827 (2007) 130–136
133
˚
(
4). 2-Methyl-3,4-diethyl pyrrole (3) was obtained by the
whereas the bond length of B–F for 5 (1.387(3) A) is shorter
˚
reduction of 1 with excess of LiAlH under the same con-
than that of 6 (1.407(3) A). The N–B–N bond angle of 5
4
ditions. Compounds 5 and 6 were prepared according to
a published procedure [17] by condensation of 4-dimethyl-
aminophenyl aldehyde with 3 or 4 using trifluoroacetic acid
(106.9(3)ꢀ) is relatively smaller than that of 6 (109.2(3)ꢀ),
while the F–B–F bond angle of 5 (109.4(3)ꢀ) is longer than
that of 6 (104.8(3)ꢀ). These data are in good agreement with
the corresponding values of BF N tetrahedron configura-
(
6
TFA) as a catalyst. After oxidation with 2,3-dichloro-5,
-dicyano-1,4-benzoquinone (DDQ) and treatment with
2
2
tions in their analogues [2].
The bond length of N(1)–C(4)/N(1A)–C(4A) is
triethylamine and boron trifluoride etherate, the crude
product was purified by silica-gel column chromatography
˚
˚
1.338(4) A for 5 and 1.340(3) A for 6, which is significantly
and recrystallized from CHCl /hexane. Compound 5:
Orange red crystals; 34% yield; mp > 250 ꢀC; H NMR
shorter than the bond length of N(1)–C(1)/N(1A)–C(1A)
3
1
˚
˚
(1.383(3) A) for 5 and (1.403(3) A) for 6. The central six-
membered ring containing B–N1–C1–C5–C1A–N1A is
almost coplanar with the adjacent five-membered rings with
(
(
(
(
CDCl , 500 MHz) d: 0.71 (t, 6H), 1.05 (t, 6H), 1.66–1.71
q, 4H), 2.29–2.34 (q, 4H), 2.54 (s, 6H), 3.04 (s, 6H), 6.90
3
˚
m, 2H), 7.15 (m, 2H); MALDI-TOF MS: m/z: 451.96
the average deviation from the mean plane being 0.02 A for
+
˚
M ); Anal. Calcd for C H BF N : C, 71.84; H, 8.04;
5 and 0.0194 A for 6, indicating strong p-electron delocaliza-
2
7
36
2
3
N, 9.31. Found: C, 71.95; H, 8.07; N, 9.51; UV–vis
CHCl ) k /nm 377, 491 and 523. Compound 6: Orange
tion within the indacene plane. The two fluorine atoms in the
two compounds occupy apical positions relative to the mean
plane of the indacene ring system. The meso-phenyl ring is
almost perpendicular to the indacene plane with torsion
angles being 87.56ꢀ and 79.61ꢀ for 5 and 6, respectively.
The molecular packing in the crystal structure of 5 and 6
is given in Figs. 3 and 4. All the molecules are parallel to
each other with head-to-tail orientation. The meso-phenyl
ring of one molecule is nearly orthogonal to the indacene
plane of the other molecule, therefore no p–p stacking
interactions can be observed.
(
3
max
1
red crystals; 45% yield; mp > 250 ꢀC; H NMR (CDCl ,
3
5
00 MHz) d: 1.38 (s, 6H), 1.87 (s, 6H), 2.53 (s, 6H), 3.08
(
s, 6H), 6.90 (m, 2H), 7.10 (m, 2H); MALDI-TOF MS:
+
m/z: 395.38 (M ); Anal. Calcd for C H BF N : C,
2
3
28
2
3
6
1
9.88; H, 7.14; N, 10.63. Found: C, 69.96; H, 7.09; N,
0.55; UV–vis (CHCl ) kmax/nm 376, 492 and 520.
3
3
.2. Structural descriptions for 5 and 6
The molecular conformations of 5 and 6 are shown in
Figs. 1 and 2, respectively. Selected geometrical parameters
of 5 and 6 are listed in Table 2.
3.3. Absorption and fluorescent spectroscopy of 5 and 6
Compounds 5 and 6 are crystallized in the monoclinic
space group C2/c. The bond length of B–N for 5
The absorption and steady-state fluorescence emission
spectra of 5 and 6 in hexane are given in Fig. 5. The
absorption spectra of 5 and 6 have similar band features
˚
˚
(
1.552(3) A) is relatively longer than that of 6 (1.516(3) A),
Fig. 3. Molecular packing in the crystal structure of 5 along the a-axis.

134
Y.-H. Yu et al. / Journal of Molecular Structure 827 (2007) 130–136
Fig. 4. Molecular packing in the crystal structure of 6 along the c-axis.
1
00
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
80
60
40
20
0
3
00
400
500
600
700
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Fig. 5. The UV–vis absorption and fluorescence emission spectra of 5 (left) and 6 (right) in hexane (kex = 495 nm).
with a strong, narrow band centered at 520–523 nm and a
shoulder on the high-energy side centered at about 493 nm,
which is typical for the conventional BDP derivatives car-
rying only alkyl substituents on the indacene core. In addi-
tion, a considerably weaker and broad absorption band is
observed at about 380 nm, which can be ascribed to the
S0 fi S2 transition [17]. The absorption maximum of 5
and 6 is not affected by the solvent polarity (Table 3).
The fact that the absorption spectra of 5 and 6 are similar
to those of the classical BDP dyes indicates the absence of
donor-acceptor interaction in the ground state, which is
due to the decoupled structure between the indacene plane
and meso-subunit as has been observed in their crystal
structures.
In nonpolar solvent hexane, the emission spectra of 5
and 6 display mirror symmetry to their absorption spec-
tra (Fig. 5) with high fluorescence quantum yields
(Table 3). Increasing the solvent polarity to methanol,
the emission intensities of 5 and 6 are heavily quenched,
while the maximum of the bands are slightly shifted to
the red. The / values of 5 and 6 are very low in ace-
f
tonitrile and methanol. This is due to the activation of
an excited-state intramolecular charge transfer (ICT)
from the meso-aniline donor to the BDP fluorophore
+
[42]. Gradually adding
H
to their solutions in
MeOH–H O (1:1 v/v), the emission intensity increases
2
significantly while the emission maxima centered at
540 nm remain unchanged (Fig. 6). Protonation of the
tertiary amine function [9] drastically changes the elec-
tron-donating properties of the aniline group and is
In contrast to the absorption spectra, the emission
behaviors of 5 and 6 are solvent dependent (Table 3).

Y.-H. Yu et al. / Journal of Molecular Structure 827 (2007) 130–136
135
Table 3
the steady-state fluorescence data F with n, K , F
a max
Absorption and emission properties of 5, 6 in different solvents
and Fmin as freely adjustable parameters gives values
of n close to 1, indicating that one proton is bound
per aniline molecule. Therefore, n is kept fixed at 1 in
the final curve fitting, from which the estimated values
ꢀ
1
ꢀ1
)
BDP Solvent
k
abs (max/nm) e (M cm
k
em (max/nm)
/
f
5
6
Hexane 523
115000
81800
131000
127000
78000
184000
222000
169000
537
533
541
540
535
535
534
539
0.64
0.43
0.007
0.005
0.71
0.50
0.012
0.004
CH
MeCN
2
Cl
2
523
523
of pK are obtained as 3.14 for 5 and 3.09 for 6, respec-
a
MeOH 522
Hexane 520
tively. Such a high proton-induced fluorescence enhance-
ment makes 5 and 6 into very sensitive fluorescent
probes for pH measurements.
CH
MeCN
2
Cl
2
520
520
MeOH 521
þ n
F
max½H ꢁ þ F min
K
a
F ¼
ð1Þ
þ n
K þ ½H ꢁ
a
expected to switch off the ICT to the boron-dipyrro-
methene moiety. The ground-state acidity constant Ka
values of 5 and 6 are determined by fluorimetric titra-
tion as a function of pH using the fluorescence emission
spectra (Fig. 7). Nonlinear curve fitting Eq. (1) to the
fluorescence data (F) recorded as a function of pH yield
4. Conclusion
In summary, we have prepared and determined the
structures of two new difluoroboradiaza-s-indacene dyes
containing an electron-donating aniline entity at the
meso-position. In the polar solvent MeOH/H O, both
compounds exhibit a large fluorescent enhancement
upon increasing the acidity of the solution. They can
+
values of n (the stoichiometry of H binding to the ani-
2
line), K , the fluorescence signals F
imal and minimal [H ], respectively. Fitting Eq. (1) to
and Fmin at max-
a
max
+
5
2.0
PH
1
2
2.46
.93
.29
PH
2.12
4
3
2
1
0
2.36
.57
2
2
2
2
3
3
3
3
3
3
3
3
4
4
.61
.74
.83
.95
.06
.16
.27
.39
.48
.57
.70
.88
.00
.18
1.5
2
2.78
2.99
3
.17
3.29
.47
3.69
1
.0
3
3
3
.86
.98
0.5
0.0
4.06
4.09
4
.51
550
600
650
550
600
650
Wavelength/nm
Wavelength/nm
Fig. 6. Fluorescence emission spectra (kem = 488 nm) of 5 (left) and 6 (right) in MeOH–H
2
O (1:1 v/v) as a function of pH.
5
2
1
1
0
0
.0
.5
.0
.5
.0
4
3
2
1
0
0
.000
0.005
0.010
0.015
0.020
0.025
0
.000
0.004
0.008
0.012
0.016
+
+
[H ] / M
[
H ] / M
+
Fig. 7. Fluorescence titration curve of 5 (left) and 6 (right) in MeOH–H
2
O (1:1 v/v) as a functional of [H ] (kem = 540 nm, kex = 488 nm). The solid line
represents the best fit of Eq. (1) with n = 1.

136
Y.-H. Yu et al. / Journal of Molecular Structure 827 (2007) 130–136
be used as fluorescent pH probes excitable at visible
light.
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