A novel squaraine dye with squaramide as a scaffold and the colorimetric detection of amine

Article abstract of DOI:10.1016/j.cclet.2011.11.001

A novel unsymmetrical squaramide-linked squaraine dye (SQ) has been synthesized through squaramide 3 and semisquaraine 6. The molecular structure of SQ has been characterized by ¹H NMR, IR and MS. Due to the influence of the hydrogen bond and the solvent effect, SQ exhibits unique spectral properties compared with typical squaraine dyes. For its excellent ability of binding primary amine, SQ is a promising receptor of recognizing primary amine.

Full text of DOI:10.1016/j.cclet.2011.11.001

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 217–220
www.elsevier.com/locate/cclet
A novel squaraine dye with squaramide as a scaffold and the
colorimetric detection of amine
*
Chuan Guo Sun, Qian Lin, Nan Yan Fu
Key Laboratory of Analysis and Detection for Food Safety, Ministry of Education & Fujian Provincial Key Laboratory of Analysis and Detection
Technology for Food Safety, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China
Received 27 June 2011
Available online 21 December 2011
Abstract
A novel unsymmetrical squaramide-linked squaraine dye (SQ) has been synthesized through squaramide 3 and semisquaraine 6.
The molecular structure of SQ has been characterized by 1H NMR, IR and MS. Due to the influence of the hydrogen bond and the
solvent effect, SQ exhibits unique spectral properties compared with typical squaraine dyes. For its excellent ability of binding
primary amine, SQ is a promising receptor of recognizing primary amine.
# 2011 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
Keywords: Squaraine dye; Squaramide; Hydrogen bond; Receptor
As the growing demand for simple, rapid and flexible online analyses of a variety of chemical compounds, the
development of new colorimetric receptor that allow naked-eye detection without the use of any spectroscopic
instrument is a field of current interest.
Be a superior performance of near-infrared dyes, squaraines have been applied widely in fields such as cation
recognition [1], biochemicallabeling [2], optical recording [3], and photovoltaics [4]. Recently, Ros-Lis et al. [5]
reported a chromomeric response sensor to volatile amines and thiols. With a two-step ship-in-the-bottle way, they put
squaraine dye into the supercages of zeolite Y and got the hybrid material Z30-SQ. The blue colour of the Z30-SQ
material was faded by reacting with the thiol and amine derivatives.
Secondary squaramides have considerable potential as hydrogen bond donors and acceptors [6]. They have been
used for wide ranging applications such as anion receptor [7–9], cation recognition [10,11], chiral ligand [12,13].
Herein we reported a novel squaraine (SQ) with squaramide as a scaffold. With the influence of squaramide skeleton,
the SQ exhibits some unique spectral properties as a promising receptor of primary amine.
1. Experimental
The synthesis of SQ is shown in Scheme 1. The aniline derivatives 2 and 5 were prepared according to the literature
[14,15]. Squaramide 3 was synthesized by direct condensation of diethyl squarate and aniline derivative 2.
* Corresponding author.
E-mail address: nanyan_fu@fzu.edu.cn (N.Y. Fu).
1001-8417/$ – see front matter # 2011 Published by Elsevier B.V. on behalf of Chinese Chemical Society.
doi:10.1016/j.cclet.2011.11.001

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C.G. Sun et al. / Chinese Chemical Letters 23 (2012) 217–220
Semisquaraine 6 was prepared by condensation of aniline derivative 5 and squaric acid dichloride. SQ was synthesized
through condensation of squaramide 3 and semisquaraine 6 in n-heptanol under reduced pressure.
Squaramide 3: diethyl squarate (0.34 g, 2 mmol) was dissolved in ethanol (10 mL) and then aniline derivative 2
(0.60 g, 4 mmol) in ethanol (5 mL) was added. The resulting solution was stirred over night at ambient temperature.
The crude product was precipitated out. After filtration the filter residue was chromatographed on a column of silica
gel with chloroform/methanol (100:6, v/v) to give 3 (0.71 g, 94%) as a white solid, mp 220–221 8C. IR (KBr, cmÀ1):
3434, 3179, 2934, 1803, 1633, 1600, 1572, 1506, 1480, 1429, 1369, 1338, 744, 689. 1H NMR (400 MHz, DMSO-d6): d
2.88 (s, 6H), 3.46 (t, 4H, J = 6.2 Hz), 3.63 (br, s, 4H), 6.62 (t, 2H, J = 7.2 Hz), 6.73 (d, 4H, J = 8.4 Hz), 7.13–7.17 (m,
4H), 7.44 (br, s, 2H). 13C NMR (100 MHz, DMSO-d6): d 38.5, 41.3, 53.4, 112.5, 116.4, 129.4, 149.5, 168.5, 183.0.
ESI-MS: m/z 379.0 ([M+H]+), 377.3 ([MÀH]À).
Semisquaraine 6: to a solution of aniline derivative 5 (0.68 g, 3.3 mmol) in benzene (25 mL) was added squaric acid
dichloride (0.51 g, 3.4 mmol), and then the reaction mixture was refluxed for 6 h. After cooling to room temperature,
the mixture was poured into ice water, washed three times with water, dried with Na2SO4, filtered, and concentrated
under reduced pressure to afford dark oil which was changed into yellow solid as kept in refrigerator. The yellow solid
was dissolved in the mixture of dichloromethane (25 mL) and acetone (5 mL), 2 mol/L hydrochloric acid (3 mL) and
trifluoroacetic acid (3 mL) were added and then refluxed over night. The solvent was removed under reduced pressure
to give dark oil which was changed into yellow solid upon the addition of methanol. After filtration the crude product
was washed adequately with methanol and anhydrous ether to give 6 (0.49 g, 49%) as a yellow powder, mp 203–
204 8C (literature [16] 258 8C). 1H NMR (400 MHz, DMSO-d6): d 0.90 (t, 6H, J = 7.2 Hz, CH3), 1.27–1.36 (m, 4H,
CH2), 1.49 (s, 4H, CH2), 3.38 (s, 4 H, CH2), 6.86 (s, 2H, ArH), 7.86 (s, 2H, ArH) (literature [17] 1H NMR (300 MHz,
DMSO-d6): d 0.88 (t, 6H, J = 7.5 Hz, CH3), 1.29 (m, 4H, CH2), 1.46 (s, 4H, CH2), 3.43 (s, 4H, CH2), 6.81 (s, 2H, ArH),
7.83 (s, 2H, ArH)).
Squaraine dye (SQ): squaramide 3 (15.2 mg, 0.04 mmol) and semisquaraine 6 (24.0 mg, 0.08 mmol) were added to
a three-necked flask equipped with a Dean-stark trap. n-Heptanol (20 mL) was added and the reaction mixture was
refluxed under reduced pressure for 8 h. The solvents were removed under reduced pressure and the crude product was
chromatographed twice on a column of silica gel with chloroform/methanol (20:1, v/v) to give SQ (9.2 mg, 24%) as a
green solid, mp >300 8C. IR (KBr, cmÀ1): 3436, 2923, 1623, 1580, 1385, 1187. 1H NMR (400 MHz, CDCl3:CD3OD
(v/v) = 1:1): d 0.98 (t, 12H, J = 7.0 Hz), 1.35–1.41 (m, 8H), 1.63 (br, s, 8H), 3.14 (s, 6H), 3.57 (br, s, 8H), 3.77 (br, s,
8H), 5.34 (s, 2H), 6.70 (d, 4H, J = 8.0 Hz), 6.75 (d, 4H, J = 7.6 Hz), 8.10 (d, 4H, J = 7.6 Hz), 8.15 (d, 4H, J = 8.4 Hz).
ESI-MS: m/z 947.1 ([M+H]+) 945.3 ([MÀH]À).
2. Results and discussion
The spectra data of SQ in different solvents are listed in Table 1. Compared with bis[4-(dibutylamino)phenyl]-
squaraine (BDPSQ) of which the absorption maximum is in the range from 640 to 645 nm in various solvents, the long
wavelength band absorption maximum of SQ (in the range from 635 to 639 nm in various solvents) shows a blue-shift
of around 5 nm, indicating distinguished intermolecular hydrogen bond interactions between anilino N at squaraine
moiety and NH at squaramide moiety. With the flexible alkyl linkage between squaraine moiety and squaramide
Scheme 1. Synthesis of squaraine dye SQ. (a) 2-Bromoethylamine hydrobromide, KI, n-BuOH; (b) diethyl squarate, EtOH, reflux; (c) n-C4H9Br,
K2CO3, KI, DMF; (d) squaric acid dichloride, benzene, reflux; (e) HCl, TFA, CH2Cl2, acetone, reflux; (f) n-heptanol, À0.09 MPa, reflux.

C.G. Sun et al. / Chinese Chemical Letters 23 (2012) 217–220
219
Table 1
Absorption and emission spectral data of SQ in various solvents.
Solvent
l1max (nm)
e1 (molÀ1 cmÀ1 L)
l2max (nm)
e2 (molÀ1 cmÀ1 L)
e1:e2
Fa
lem (nm)
CH3OH
CH3CN
CH3CH2OH
CHCl3
638
635
639
639
639
639
1. 97 Â 105
1.94 Â 105
1.97 Â 105
2.32 Â 105
1.44 Â 105
2.14 Â 105
268
266
267
266
267
259
0.36 Â 105
0.41 Â 105
0.36 Â 105
0.34 Â 105
0.24 Â 105
0.45 Â 105
5.47:1
4.73:1
5.47:1
6.82:1
6.00:1
4.76:1
0.0096
0.0181
0.0219
0.072
663
660
658
655
652
653
i-PrOH
THF
0.0391
0.107
a
Fluorescence quantum yields were determined using Rhodamine B as the standard.
moiety, SQ exhibits a typical squaramide absorption (in the range from 259 to 268 nm in various solvents) in the short
wavelength band with the e value 30% higher than that of BDPSQ. Due to the structural flexibility [17] and asymmetry
[18], SQ displays a weak emission in red region (from 653 to 663 nm in organic solvents) with only around 10%
quantum yield of BDPSQ.
Fig. 1. Absorption spectra of SQ (0.6 mmol/L) upon the addition of n-butylamine (0–3.4 mmol/L) in i-propanol. Inset: the binding isotherm of SQ at
638 nm upon the addition of n-butylamine.
0.10
pyridine
L-Ala
TEA
aniline
p-nitroaniline
p-anisidine
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
p-picoline
diphenylamine
dibutyl-
amine
Fig. 2. The competition experiments of n-butylamine (3.5 mmol/L) in i-propanol solution of SQ (0.6 mmol/L) to various nucleophilic reagents
(3.5 mmol/L). Black bars represent the addition of the 3.5 mmol/L competing nucleophilic regents to a 0.6 mmol/L solution of SQ. White bars
represent the subsequent addition of 3.5 mmol/L n-butylamine to the solution.

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C.G. Sun et al. / Chinese Chemical Letters 23 (2012) 217–220
The n-butylamine titration of SQ was shown in Fig. 1. Upon the addition of amine, the absorption peak of SQ
diminished proportionally. Preliminary studies show that n-butylamine, as a strong nucleophilic reagent, can attack the
dye at the electron-deficient central four-membered squaric acid ring, which destroys the conjugation of squaraine
moiety of SQ, leading its bleaching [19–21]. The competition experiments show that other nucleophilic reagents, such
as triethylamine, aniline, p-nitroaniline, p-anisidine, diphenylamine, pyridine, p-picoline and L-ALA have low
interference in the detection of primary amine because the nucleophilicity of these reagents is not very high. Only
dibutylamine, an aliphatic secondary amine, can combine with SQ competitively and has some disturbance to the
primary amine detection. So SQ has a potential application in the selective detection of aliphatic primary amine in the
presence of tertiary amine, aromatic amine, pyridine derivative and amino acid (see Fig. 2).
In conclusion, we have synthesized a novel squaraine dye based on squaramide. With the influence of the hydrogen
bond and the solvent polarity, SQ exhibits remarkable difference in photophysical properties. Amine titration
experiment shows that SQ can act as a new receptor of recognizing primary amine in alcohol. Future studies on SQ are
still in progress.
Acknowledgments
The authors are grateful to the National Natural Science Foundation of China (No. 20702005) and the Fujian
Provincial Natural Science Foundation of China (No. 2008J0148).
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