The synthesis, crystal structure and enhanced blue fluorescence emission of novel antipyrine derivatives

Article abstract of DOI:10.1016/j.saa.2012.06.047

Four new antipyrine derivatives were synthesized and characterized by elemental analysis, FT-IR, ¹H NMR, ¹³C NMR spectroscopy, and a representative compound 1b was confirmed based on the X-ray crystallographic analysis. The antipyrine-phenylboronic acid 1b crystallizes in the monoclinic space group P2₁/c, and displays an E configuration about the CN double bond. The absorption and fluorescence spectra of these compounds were investigated. Replacement of a six-membered phenyl group with a five-membered thienyl unit in the antipyrine derivatives resulted in a bathochromic shift approximately 21 nm, while the bithienyl system caused a strong bathochromic effect of 53 nm relative to the substituted phenyl groups. The bithienyl-antipyrine hybrid 1c displayed a turn-on fluorescence response to water, acetic acid (HOAc) and sulfuric acid (H₂SO₄) in ethanol solution, but no fluorescence response toward alkali. Whereas the free compound was very weakly fluorescent in ethanol, the addition of water, HOAc and H₂SO₄ leads to an appearance of strong blue fluorescence and a dramatic increase of emission intensity.

Full text of DOI:10.1016/j.saa.2012.06.047

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The synthesis, crystal structure and enhanced blue fluorescence emission
of novel antipyrine derivatives
a
a
a
b
Yi-Feng Sun a,b, , He-Ping Wang , Yong Chen , Zhi-Yong Chen , Ji-Kun Li
⇑
a Guangdong Provincial Public Laboratory of Analysis and Testing Technology, China National Analytical Center, Guangzhou 510070, PR China
b School of Chemistry and Chemical Engineering, Taishan University, Taian 271021, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
Novel antipyrine derivatives have
been synthesized and characterized.
X-Ray crystal structures of
compound 1b have been
"
determined.
"
Compound 1c exhibits interesting
fluorescence enhancement behavior.
a r t i c l e i n f o
a b s t r a c t
Four new antipyrine derivatives were synthesized and characterized by elemental analysis, FT–IR, 1H
NMR, 13C NMR spectroscopy, and a representative compound 1b was confirmed based on the X-ray crys-
tallographic analysis. The antipyrine–phenylboronic acid 1b crystallizes in the monoclinic space group
P21/c, and displays an E configuration about the C@N double bond. The absorption and fluorescence spec-
tra of these compounds were investigated. Replacement of a six-membered phenyl group with a five-
membered thienyl unit in the antipyrine derivatives resulted in a bathochromic shift approximately
21 nm, while the bithienyl system caused a strong bathochromic effect of 53 nm relative to the substi-
tuted phenyl groups. The bithienyl-antipyrine hybrid 1c displayed a turn-on fluorescence response to
water, acetic acid (HOAc) and sulfuric acid (H2SO4) in ethanol solution, but no fluorescence response
toward alkali. Whereas the free compound was very weakly fluorescent in ethanol, the addition of water,
HOAc and H2SO4 leads to an appearance of strong blue fluorescence and a dramatic increase of emission
intensity.
Article history:
Received 1 December 2011
Received in revised form 16 June 2012
Accepted 25 June 2012
Available online 10 July 2012
Keywords:
Antipyrine
2,2’-Bithiophene
Phenylboronic acid
Fluorescence enhancement
Synthesis
Crystal structure
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
antibacterial, antipyretic, analgesic, anti-inflammatory as well as
antitumor activity [1,2]. At the same time, they have also been suc-
Antipyrine derivatives continue to generate great interest to
medicine and science stemming from their wide range of pharma-
cological activities and clinical applications, including antifungal,
cessfully used as a test for the effects of other drugs on the activity
of drug-metabolising enzymes in the liver [3,4], as a chromogenic
reagent for determination of bromate, urea [5], nitrate and phenol
[6,7], and as chelating agents for coordinating, concentrating, and
extracting of specific metal cations [8–13]. Much work has been fo-
cused mainly on the synthesis, structure characterization and
biological evaluation of antipyrine derivatives [14–20], whereas
only a few interesting fluorescence probes/chemosensors based
⇑
Corresponding author at: School of Chemistry and Chemical Engineering,
Taishan University, Taian 271021, PR China. Tel.: +86 20 37656300; fax: +86 20
87686511.
E-mail address: sunyf50@yahoo.com.cn (Y.-F. Sun).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.06.047

Y.-F. Sun et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
569
on antipyrine and involving different mechanisms to induce spec-
tral changes have been developed [21,22].
was heated under reflux for 3 h. After cooling, the solvent was re-
moved under reduced pressure and the solid residues were recrys-
tallized from ethanol to yield the pure products.
In our recent papers we reported on the synthesis and crystal
structures of some antipyrine derivatives [23–25]. More recently,
we found that a bithienyl-coumarin derivative displays three pri-
mary color emission bands, having maxima at 404, 524, and
697 nm, respectively, roughly corresponding to blue, greenish blue,
and red light [26]. In view of the considerable importance of these
compounds, we are thus interested in extending these studies to
design and make materials with potential technological applica-
tions. Here, we wish to report the synthesis and spectroscopic
characterization of new antipyrine derivatives. Specially, a bithie-
nyl-antipyrine derivative (1c) has been synthesized, which is ex-
pected to possess some unique optical properties. Additionally,
the molecular structure of 1b was studied by the X-ray diffraction.
The structures of target molecules are shown in Fig. 1.
1,5-Dimethyl-4-(3,5-di-tert-butyl-4-hydroxybenzylidene)amino-2-
phenyl-1H- pyrazol-3(2H)-one (1a)
Colorless crystals, yield 69%; mp 223–225 °C; 1H NMR
(300 MHz, CDCl3/TMS) d: 1.39 (s, 18H), 2.39 (s, 3H), 3.03 (s, 3H),
5.40 (s, 1H), 7.17–7.41 (m, 5H), 7.64 (d, J = 2.7 Hz, 2H), 9.61 (s,
13
1H).
C NMR (75 MHz, CDCl3/TMS) d: 10.25, 30.10, 30.24, 34.38,
36.16, 122.79, 124.02, 125.20, 126.56, 127.66, 129.09, 129.28,
135.04, 136.02, 151.52, 156.27, 158.59, 161.14. IR (KBr, cmÀ1
) m:
3537 (OH), 1647 (C@O), 1594 (C@N), 1109 (Ar–OH). Anal. calcd
for C26H33N3O2: C 74.43, H 7.93, N 10.02; found: C 74.49, H 7.85,
N 10.08.
Experimental
4-(1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-
yl)phenylboronic acid (1b)
General
Pale yellow crystals, yield 63%; mp 248–250 °C; 1H NMR
(300 MHz, DMSO-d6/TMS) d: 2.47 (s, 3H), 3.19 (s, 3H), 7.36 (m,
1H NMR and 13C NMR spectra were taken on a Bruker AVANCE-
300 NMR spectrometer and chemical shifts expressed as d (ppm)
values with TMS as internal standard. Infrared spectra were mea-
sured over the region 400–4000 cmÀ1 with a Nicolet Magna 760
FT–IR instrument, with 4 cmÀ1 resolution. The samples were
examined as KBr pellets. Element analysis was taken with a Per-
kin–Elmer 240 analyzer. Absorption spectra were determined on
a Hitachi U-3900 UV–Vis scanning spectrophotometer. Fluores-
cence spectra measurements were performed on a Hitachi F-
2500 spectrofluorimeter and the slit widths were 5 nm for both
excitation and emission. Single crystal was characterized by Bruker
Smart 1000 CCD X-ray single-crystal diffractometer. The melting
points were determined with a WRS-1A melting point apparatus
and are uncorrected. All reagents and chemicals are commercially
available and used without further purification.
3H), 7.55 (m, 2H), 7.77 (d, J = 8.1 Hz, 2H), 7.88 (d, J = 8.0 Hz, 2H),
13
8.15 (s, 2H), 9.65 (s, 1H).
C NMR (75 MHz, DMSO-d6/TMS) d:
9.74, 35.27, 116.31, 124.62, 126.14, 126.89, 129.11, 134.39,
134.52, 138.85, 152.20, 154.36, 159.54. IR (KBr, cmÀ1
: 3386
(OH), 1614 (C@O), 1590 (C@N), 1333 (B–O). Anal. calcd for
18H18BN3O3: C 64.50, H 5.41, N 12.54; found: C 67.42, H 5.56, N
)
m
C
12.47.
1,5-Dimethyl-2-phenyl-4-(2-(5-(2-thienyl)thienyl)
methylidene)amino-1H-pyrazo-3(2H)-one (1c)
Yellow solid, yield 55%; mp 193–195 °C; 1H NMR (300 MHz,
CDCl3/TMS) d: 2.37 (s, 3H), 3.05 (s, 3H), 6.95 (m, 1H), 7.06 (d,
J = 3.6 Hz, 1H), 7.18 (m, 3H), 7.28 (m, 3H), 7.40 (t, J = 8.0 Hz, 2H),
13
9.78 (s, 1H).
C NMR (75 MHz, CDCl3/TMS) d: 9.13, 34.79,
117.32, 123.15, 123.36, 123.40, 124.11, 125.94, 126.97, 128.18,
130.31, 133.70, 136.52, 139.16, 142.57, 149.34, 150.44, 159.79. IR
Synthesis of the antipyrine derivatives (1)
(KBr, cmÀ1
)
m: 1641 (C@O), 1579 (C@N). Anal. calcd for
A mixture of 4-aminoantipyrine (2 mmol) and the appropriate
aryl or heteroaryl aldehydes (2 mmol) anhydrous ethanol (30 mL)
C20H17N3OS2: C 63.30, H 4.52, N 11.07; found: C 63.36, H 4.58, N
11.01.
H C
3
CH
H C
3
CH
CH
CH
3
H C
3
CH
3
3
3
3
CH
N
N
N
N
N
N
3
N
N
OH
B
H C
H C
3
3
S
S
N
N
N
S
N
HO
O
O
O
HO
O
HO
H C
B
OH
CH
3
3
CH
3
1c
1a
1b
1d
H C
3
CH
H C
3
CH
H C
CH
3
3
3
3
N
N
N
N
N
N
H CO
3
N
N
N
O
O
O
H CH C
3
2
HO
HO
OH
N
CH CH
3
O
O
OCH
OH
3
2
HMPP
DPTP
CA
O
O
O
O
O
O
S
O
S
S
S
S
N
O
S
O
O
O
TTPBP
TTPNP
BTCE
Fig. 1. The structures of the antipyrine and bithienyl derivatives.

570
Y.-F. Sun et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
5-(1,5-Dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)-2-
thiophene boronic acid (1d)
substituted aryl or heteroaryl aldehydes with 4-aminoantipyrine
in anhydrous ethanol.
Green solid, yield 68%; mp > 250 °C; 1H NMR (300 MHz, DMSO-
d6/TMS) d: 2.41 (s, 3H), 3.17 (s, 3H), 7.38 (m, 3H), 7.47 (d, J = 3.9 Hz,
All compounds have been fully characterized spectroscopically.
The IR spectra of these compounds show sharp and intense peaks
in the range of 1579–1594 and 1610–1647 cmÀ1 which can be
attributed to mC@N and mC@O stretching modes respectively. The
broad IR signal centered at 3349–3537 cmÀ1 obtained for the three
compounds 1a, 1b and 1d can be ascribed to the OH functional
groups.
1H), 7.53 (t, J = 7.6 Hz, 2H), 7.65 (t, J = 3.6 Hz, 1H), 8.33 (s, 2H), 9.70
13
(s, 1H).
C NMR (75 MHz, DMSO-d6/TMS) d: 9.66, 35.26, 116.08,
124.65, 126.94, 129.13, 130.98, 134.46, 136.44, 148.26, 149.16,
151.63, 159.56. IR (KBr, cmÀ1
)
m
: 3349 (OH), 1622 (C@O), 1581
(C@N), 1321 (B–O). Anal. calcd for C16H16BN3O3S: C 56.32, H
4.73, N 12.32; found: C 56.38 H 4.81, N 12.26.
The signals of protons displayed chemical shifts and multiplic-
ities corresponding to their surroundings. The 1H NMR spectra of
all compounds showed a well distinguishable intensive singlet sig-
nal at d = 9.61–9.78 ppm for the imino proton (CH@N). Addition-
ally, the 1a gave a characteristic singlet at d ꢀ 5.40 ppm in CDCl3,
which was assigned to the OH proton. Obviously, in the 1H NMR
spectra the B(OH)2 group signal was observed as a slightly broad-
ened singlet at d 8.15 ppm for 1b, at d 8.33 ppm for 1d.
X-ray crystallography
X-ray quality crystals of 1b were obtained by evaporation of eth-
anol solution. The diffraction data were collected on a Bruker Smart
Apex 1000 CCD X-ray single crystal diffractometer with a graphite
monochromated Mo Ka radiation (k = 0.071073 nm) at 298(2) K.
The structure was solved by direct methods with SHELXS-97 pro-
gram and refinements on F2 were performed with SHELXL-97 pro-
gram by full-matrix least-squares techniques with anisotropic
thermal parameters for the non-hydrogen atoms. All H atoms were
initially located in a difference Fourier map. The methyl H atoms
were then constrained to an ideal geometry, with C–H = 0.096 nm
and Uiso(H) = 1.5Ueq(C). The hydroxyl H atom was treated as a riding
atom, with O–H = 0.082 nm and Uiso(H) = 1.5Ueq(O). All other H
atoms were placed in geometrically idealized positions and con-
strained to ride on their parent atoms with C–H distances
0.093 nm and Uiso(H) = 1.2Ueq(C). A summary of the crystallographic
data and structure refinement details is compiled in Table 1.
Crystal structure of 1b
Compound 1b crystallizes in monoclinic space group P21/c with
four molecules in the unit cell. As can be seen from Fig. 2, the mol-
ecule 1b adopts an E configuration about the central C@N double
bond and exists in the imine form, as observed in the similar anti-
pyrine Schiff base analogues, namely, 4-[(1E)-(4-Hydroxy-3,5-
dimethoxy benzylidene)-amino]-1,5-dimethyl-2-phenyl-2,3-dihy-
dro-1H-pyrazol-3-one (HMPP) [23] and 1,5-Dimethyl-2-phenyl-4-
[(1E)-(2,3,4-trihydroxybenzylidene)amino]-1H-pyrazol-3(2H)-one
(DPTP) [25] (Fig. 1).
The dihedral angle between the pyrazole ring and the substi-
tuted phenyl ring (C1–C6) is 16.9°, larger than the value of 7.5°
found in HMPP, but smaller than the value of 25.3° in DPTP. The
mean planes of the pyrazole and phenyl (C13–C18) rings make a
dihedral angle of 117.2°. Therefore, the molecule 1b is not planar.
Obviously, the N3–C7 (0.1273(2) nm) and O3–C9 (0.12412(19) nm)
bonds feature C@N and C@O double bonds.
Results and discussion
Synthesis and characterization
The antipyrine derivatives (Fig. 1) were easily prepared with
55–69% yield by refluxing equimolar amounts of the properly
On the other hand, the phenyl ring (C1–C6) and the boron coor-
dination group, which adopts a trigonal planar configuration, are
not coplanar with a dihedral angle of 11.3°. Moreover, B1, O1
and O2 deviate from the phenyl ring (C1–C6) mean plane by
0.00249, 0.02853 and 0.01378 nm, respectively. The B1–O1
(0.1344(2) nm) and B1–O2 (0.1361(2) nm) distances are somewhat
shorter than the bonds in 2-[2-(2-Methoxy-ethoxy)-ethoxy]-phen-
ylboronic acid (0.1358(2) and 0.1381(2) nm) [27]. Additionally, the
bond angles at the trigonal planar boron atoms are somewhat dis-
torted from ideal values in a manner which makes all of the hydro-
gen bonds more linear. The C1–B1–O1 angle (117.39(16)°) is
significantly smaller than C1–B1–O2 (123.62(15)°). And, the O1–
B1–O2 angle is 118.93(15)°. Similar geometry has been observed
in related organoboron acids [27,28].
Table 1
Crystal data and structure refinement.
Compound
1b
Empirical formula
Formula weight
Temperature (K)
Crystal system,
Space group
Unit cell dimensions
a (nm)
C
18H18BN3O3
335.16
298(2)
Monoclinic
P21/c
1.3990(6)
0.6624(3)
1.8751(9)
90
102.444(7)
90
1.6967(14), 4
1.312
0.090
704
b (nm)
c (nm)
a
(°)
b (°)
c
(°)
Volume (nm3), Z
Dcalc (Mg/m3)
Absorption coefficient (mmÀ1
F (000)
)
Crystal size (mm)
h range for data collection(°)
Limiting indices
0.23 Â 0.16 Â 0.12
2.40–25.05
À16 6 h 6 13
À7 6 k 6 7
À15 6 l 6 22
8425/3001 [Rint = 0.0283]
0.9893 and 0.9796
3001/0/229
1.039
Reflections collected/unique
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
R1 = 0.0393; wR2 = 0.0998
R1 = 0.0489; wR2 = 0.1067
0.0110(14)
211 and À149
845710
Extinction coefficient
Largest diff. peak and hole (e.nmÀ3
CCDC
)
Fig. 2. The molecular structure of 1b, showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown
as small spheres of arbitrary radius.

Y.-F. Sun et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
571
Fig. 3. The one-dimensional structure formed via hydrogen bonds in 1b. The dashed lines indicate hydrogen bonds.
Generally, analogous to carboxylic acids, in boronic acids also,
the hydroxyl groups may participate in hydrogen bonds. For exam-
ple, the most common structural motif of phenylboronic acids is a
cyclic dimer [29], which is formed by two units linked by intermo-
lecular hydrogen bonds. As might be expected, such a basic subunit
is also formed in the crystal structure of 1b.
hydrogen bonds and
network structure (Fig. 4).
p–p stacking interactions to form an extended
Absorption and fluorescence spectra
The structures of the target molecules are shown in Fig. 1. All
molecules (1a–1d) contain an antipyrine core. These molecules
are related, but their chemical structures are differentiated by
introducing various functional groups like substituted phenyl,
thienyl and bithienyl into the antipyrine core. Such structural
modification could be expected to result in some changes in
The molecules are linked by pair-wise self complementary O1–
H1. . .O2 (O1–H1. . .O2i: O–H = 0.0820 nm, H1. . .O2 = 0.2054 nm,
O1. . .O2 = 0.2836 nm, O1–H1. . .O2 = 159°; symmetry code: (i) –
x À 1, Ày + 2, Àz + 1) hydrogen bonding interactions into a centro-
symmetric dimer, with the formation of an eight-membered R22(8)
ring motif as shown in Fig. 3. Further, the dimers are linked by inter-
molecular O–H. . .O and C–H. . .O hydrogen bonds (O2–H2. . .O3ii:
O–H = 0.0820 nm, H1. . .O2 = 0.2029 nm, O1. . .O2 = 0.2823 nm,
O1–H1. . .O2 = 163°; C6–H6. . .O3ii: C–H = 0.0930 nm, H6. . .O3 =
0.2555 nm, C6. . .O3 = 0.3319 nm, C6–H6. . .O3 = 140°; symmetry
code: (ii) Àx, Ày + 2, Àz + 1) into a one-dimensional structure
extending along the a-axis (Fig. 3), which are further held together
by the weak intermolecular C–H. . .N (C11–H11A. . .N3iii:
C–H = 0.0960 nm, H11A. . .N3 = 0.2610 nm, C11. . .N3 = 0.3565 nm,
C11–H11A. . .N3 = 173°; symmetry code: (i) Àx, y + 3/2, Àz + 1/2)
the
spectra.
These molecules are soluble in common organic solvents such
p-conjugated length and in the absorption and emission
as ethanol, dichloromethane and N,N-dimethylformamide. Among
these molecules, the dilute ethanol solution of 1c has the deepest
color and appears yellow under natural light. UV–vis absorption
spectra of these molecules in diluted ethanol solutions are given
in Fig. 5, and the UV data are summarized in Table 2. Two to three
absorption peaks could be observed in the absorption spectra of
these four molecules in the wavelength range from 220 to
Fig. 4. A packing diagram for 1b, viewed down the b axis. Dashed lines indicate hydrogen bonds.

572
Y.-F. Sun et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
Fig. 6. Fluorescence spectra of compounds
1 in ethanol solution (excitation
wavelength: 310 nm for 1a, 1b and 1d, 350 nm for 1c).
Fig. 5. Absorption spectra of compounds 1 in ethanol solution.
p-conjugated system in 1c was also confirmed by the observation
of a strong broad absorption in its UV–vis spectrum.
Table 2
Additionally, as one would expect from the joining of the pyra-
zole ring to the conjugated bithienyl framework with a CH@N
bridge, in comparison to that for bithiophene (kmax = 303 nm)
[30], kmax for 1c is red-shifted 84 to 387 nm. At the same time,
the absorption bands of 1c are red-shifted of 7–10 nm as compared
to the bithienyl-containing Schiff base BTCE with two maximum
centered at 380 and 250 nm [31]. In contrast, kmax of 1c is consid-
erably blue-shifted with respect to those of coumarin-bithienyl hy-
brids (3-(3-(2-(thiophen-2-yl)thiophene-5-yl) prop-2-enoyl) -2H-
1-benzopyrane-2-one (TTPBP) (kmax = 436 nm) and 3-(3-(2-(thio-
Absorption and emission data for 1 in ethanol.
Concentration/10À5
2.24
M
ka/nm
e
/MÀ1 cmÀ1
ke/nm
1a
1b
334
236
334
260
239
387
260
355
276
241
30000
21900
29100
21300
20400
35800
30100
24100
19100
15000
402
2.46
404
1c
2.16
2.17
441
405
1d
phene-2-yl)
thiophene-5-yl)prop-2-enoyl)-2H-1-naphtho[2,1-
b]pyran-2-one (TTPNP) (kmax = 445 nm)) [32] and coumarin–anti-
pyrine hybrid (CA) (kmax = 453 nm) [22], indicating the presence
of a more extended
hybrids.
p-conjugated system in three coumarin
500 nm. The first intense absorption band at longest wavelength is
located at around 334–387 nm.
The fluorescence properties of compounds were investigated in
ethanol solution at room temperature (excitation wavelength:
310 nm for 1a, 1b and 1d, 350 nm for 1c). The fluorescence spectra
are shown in Fig. 6, and the fluorescence spectral data are summa-
rized in Table 2. As expected, all antipyrine derivatives exhibit
rather weak fluorescence due to the isomerization around the
C@N bond, and have similar fluorescence spectra. The emission
maxima for 1a, 1b, 1c and 1d appear at 402, 404, 441 and
405 nm, respectively. In comparison with 1a, 1b and 1d, 1c shows
a ꢀ36 nm red shift in kmax. But, weak emission of 1c is blue-shifted
as compared to that observed for coumarin–antipyrine hybrid CA
(500 nm) [22].
For the maximum absorption peaks of the first absorption band,
there is
a consequence: 1c (387 nm) > 1d (355 nm) > 1a, 1b
(334 nm) (Fig. 5 and Table 2). As revealed in Fig. 5, clearly, both
1a and 1b have the same value of kmax, while both 1b and 1d have
the similar spectral shape, owing to their very similar structure.
Furthermore, we noticed that when replacing 3,5-di-tert-butyl-4-
hydroxyphenyl with a boronophenyl group, a change in the shorter
wavelength part of the spectrum is observed. Concomitantly, the
replacement of six-membered phenyl group with five-membered
thienyl unit resulted in a red shift of approximately 21 nm in kmax
.
Considering such a bathochromic shift, one can say that the elec-
tronic effect of thienyl group is stronger than that of the phenyl
As will be shown, however, it is found that 1c exhibits interest-
ing fluorescence multi-responses to water, HOAc and H2SO4.
group. In fact, it is known that thiophene rings are
p-excessive,
and have a greater diene character than phenyl rings. Thus, thio-
phene rings are thought to facilitate changes in charge density
upon excitation as a consequence of reduced aromaticity, which
might explain the bathochromic shift.
Fluorescence enhancement behaviour of 1c
The effect of solvents on the fluorescence of 1c is investigated
systematically by using aqueous binary solutions as the medium.
In ethanol, 1c showed interesting response sensitivities when
water was added. In comparison to the data recorded in pure eth-
anol solution, the addition of water brought marked changes to the
emission spectra of 1c, although the absorption was slightly af-
fected by the presence of water. Fluorescence emission wavelength
and intensity of the solutions with different water fractions (vol%),
as well as the fluorescence emission response photograph of 1c in
ethanol solution in the absence and presence of water are shown in
It is worth noting that derivative 1c featuring electron-rich bit-
hienyl substituent, shows only two absorption bands at around
387 and 260 nm, and its value of kmax with a high extinction coef-
ficient (e
max = 35800 MÀ1 cmÀ1) is considerably more red-shifted
than those of other antipyrine derivatives (Fig. 5 and Table 2). This
is apparently related to the greater electron-donating ability of the
end bithienyl unit. Therefore, it could be suggested that more
electrons and more extended -conjugated system may be in-
volved in 1c than those in others. The presence of an extended
p-
p

Y.-F. Sun et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
573
Fig. 9. Fluorescence spectra changes of 1c in ethanol solution (2.16 Â 10À6 M) upon
addition of increasing concentrations of H2SO4 (0–56.40 Â 10À5 M). Inset: fluores-
cence intensity at 438 nm as a function of H2SO4 concentration.
and then reduced (Fig. 7). Concurrently, a red-shift in the fluores-
cence maximum was observed (Fig. 7). From the fluorescence spec-
tra of 1c in ethanol with varying amounts of water, it could be seen
that, when the water fraction was increased from 0% to 20%, the
fluorescence intensity for 1c shows only a slight increase. Upon
the increasing of water content from 30% to 80%, 1c exhibits a
strong blue fluorescence emission peak with a significant fluores-
cence enhancement and a small red shift in its emission wave-
length (from 441 to 451 nm). However, an obvious drop in the
overall intensity and a slight red shift in its emission wavelength
(from 451 to 454 nm) are observed when the water fraction was
further increased from 80% to 90%. This means that more than
80% water content is not effective in enhancing the fluorescence
properties. It was clear that compound 1c had sensitive responses
towards changes in the water composition in the aqueous solvent
systems.
Fig. 7. (a) Fluorescence spectra of 1c in ethanol–water mixtures (1.08 Â 10À6 M).
(b) Fluorescence intensity at 451 nm as
a function of water fraction. Inset:
photograph of 1c in ethanol with and without addition of water under irradiation
at 365 nm.
Additionally, this investigation has also been extended to the
ethanol-heavy water (D2O) systems. Similarly, in the case of etha-
nol–D2O mixture (VD2O = 80%) we also observe the enhanced blue
fluorescence emission.
Fig. 7. As the water fraction increased, the change of fluorescence
emission intensity exhibited a two-step process: first enhanced,
Besides, responses of 1c to acid (e.g. HOAc and H2SO4) were also
investigated. As can be seen in Figs. 8 and 9, upon the addition of
acid, compound 1c shows significant fluorescence enhancement,
and exhibits a strong blue fluorescent emission peak at ca. 438–
440 nm, which is blue-shifted with respect to the peak observed
in the presence of water.
In sharp contrast, these fluorescence properties were not ob-
served in the cases of 1a, 1b, and 1d. Further testing, meanwhile,
revealed that the fluorescence of 1c was scarcely influenced by
the addition of alkali (e.g. sodium hydroxide (NaOH) and tetrabu-
tylammonium hydroxide (TBAH)).
As is mentioned above, addition of water to 1c does not pro-
mote significant changes in the UV–vis spectrum in comparison
with that observed in pure ethanol solution. However, the presence
of H2SO4 had obvious effect on the absorption spectra of 1c. More-
over, the absorption behavior of 1c in the presence of H2SO4 also
depends on the amount of H2SO4 added. Fig. 10 shows the effect
of varying amounts of H2SO4 on the absorption spectra. From
Fig. 10, one observes that the addition of a small amount of
H2SO4 (9.4 Â 10À5 M) to the system does not alter the shape and
wavelength maximum of the absorption spectra for 1c but can re-
duce the absorbance obviously. Further, with the increase in the
amount of H2SO4, the observed two absorption bands at around
Fig. 8. Fluorescence spectra changes of 1c in ethanol solution (1.08 Â 10À6 M) upon
addition of increasing concentrations of HOAc (0–33.60 Â 10À4 M). Inset: fluores-
cence intensity at 440 nm as a function of HOAc concentration.

574
Y.-F. Sun et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 568–574
Acknowledgement
This work is supported by the Nature Science Foundation of
Shandong Province of China (No. ZR2010BM032).
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