The synthesis and spectroscopic studies of new aniline-based squarylium dyes

Article abstract of DOI:10.1016/j.dyepig.2016.05.027

The synthesis and spectroscopic properties of series of new aniline-based squarine dyes have been presented. The effect of solvent polarity, hydrogen bonding and substituent on spectroscopic properties of 1,3-bis(phenylamino)squarine (SQG1), 1,3-bis(4-bro

Full text of DOI:10.1016/j.dyepig.2016.05.027

Dyes and Pigments 133 (2016) 273e279
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and spectroscopic studies of new aniline-based
squarylium dyes
Katarzyna Jurek*, Janina Kabatc, Katarzyna Kostrzewska
UTP, University of Science and Technology, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and spectroscopic properties of series of new anilineebased squarine dyes have been
presented. The effect of solvent polarity, hydrogen bonding and substituent on spectroscopic properties
of 1,3-bis(phenylamino)squarine (SQG1), 1,3-bis(4-bromophenylamino)squarine (SQG2), 1,3-bis(4-
Received 16 April 2016
Received in revised form
14 May 2016
Accepted 17 May 2016
Available online 3 June 2016
ethylphenylamino)squarine
nitrophenylamino)squarine
iodophenylamino)squarine
(SQG3), 1,3-bis(4-ethoxyphenylamino)squarine
(SQG4), 1,3-bis(4-
(SQG5),
(SQG7),
1,3-bis(4-chlorophenylamino)squarine
1,3-bis(4-aminobenzosulfo)squarine
(SQG6),
(SQG8)
1,3-bis(4-
1,3-bis(4-
methylphenylamino)squarine (SQG9) 1,3-bis(4-hydroxyphenylamino)squarine (SQK1) in ten selected
solvents were studied.
Keywords:
Synthesis
Aniline based squarine dyes
Spectroscopic properties
Effect of substituent
Solvatochromism
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
2. Materials and methods
Development of new dyes, which have a panchromatic or
abroad absorption bands in visible region, make a possibility of
application them in wide modern technologies such as radiation
curing [1e10], imaging and optics technologies, medicine [11,12],
electronics, nanotechnology and new advances materials produce,
photovoltaics, photodynamic therapy and pharmaceutical industry
[13e19]. The squarylium dyes possess a D-A-D structure classified
to polymethine dyes including the butenedione core linked with
two the same substituent in symmetrical squarine [20] and with
two different groups in asymmetrical squarine [21].
The squarylium dyes demonstrated three resonance structures
with all-aromatic zwitterion and two tautomeric cyanine type
structures [22].
In present paper, the series of p-aniline based squarine dyes are
synthesized and their spectroscopic properties in different solvents
are described. The effect of both solvents and substituents on the
absorption properties of p-substituted aniline based squarine dyes
has been determined.
2.1. Synthesis
The substrates used for synthesis of squarine dyes were pur-
chased from Sigma-Aldrich (Poland) and were used without further
purification. The squarylium dyes: 1,3-bis(phenylamino)squarine
(SQG1), 1,3-bis(4-bromophenylamino)squarine (SQG2), 1,3-bis(4-
ethylphenylamino)squarine (SQG3), 1,3-bis(4-ethoxyphenylamino)
squarine (SQG4), 1,3-bis(4-nitrophenylamino)squarine (SQG5), 1,3-
bis(4-chlorophenylamino)squarine
(SQG6),
1,3-bis(4-
iodophenylamino)squarine (SQG7), 1,3-bis(4-aminobenzosulfo)
squarine (SQG8), 1,3-bis(4-methylphenylamino)squarine (SQG9);
1,3-bis(4-hydroxyphenylamino)squarine (SQK1) were synthesized
according to literature procedure described in work [20]. The
general route of synthesis is shown in Scheme 1.
2.1.1. General procedure for synthesis of SQG1, SQG2, SQG3, SQG4,
SQG6, SQG7, SQG9, SQK1
The synthesis of squarine dyes is based on the condensation
reaction of squaric acid with p-substituted aniline derivatives. 1,2-
Dihydroxycyclobuten-3,4-dione (2.5 mmol, 0.29 g) was heated
under reflux in a mixture of 1-butanol (40 mL) and toluene (20 mL),
and water was distilled off azeotropically using a Dean-Stark trap.
After 1 h, the appropriate p-aniline (5 mmol) was added and the
reaction mixture refluxed for additional 4 h [20]. Reaction mixture
* Corresponding author.
E-mail addresses: katarzyna.jurek@utp.edu.pl (K. Jurek), nina@utp.edu.pl
(J. Kabatc).
http://dx.doi.org/10.1016/j.dyepig.2016.05.027
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

274
K. Jurek et al. / Dyes and Pigments 133 (2016) 273e279
Scheme 1. Synthesis of squarine dyes.
was cooled and the solvent removed on a Büchner funnel. The solid
dried in room temperature.
7.7422e7.7638 (d, J z 8.64 Hz, 4H, Ar); 11.4480 (s, 2H, -NH-). IR
(KBr; cmꢁ1): 3060.26, 2965.23 (¼C-H); 2774.56, 2741.75, 2644.58
(NHþ2 ); 1618.99, 1589.15 (C=O); 1547.16 (N-H); 1492.33,
1429.17,1412.46 (C-C); 1334.54, 1310.78, 1280.86 (N-C); 964.57,
940.83, 840.67 (¼C-H); 826.24 (substituent para); 512.23 (Br-C).
Anal. Calcd. for C16H10N2O2Br2: C, 45.50; H, 2.370; N, 6.635. Found:
C, 45.18; H, 2.754; N, 6.649.
1,3-Bis(phenylamino)squarine (SQG1) was prepared with gen-
eral procedure from aniline (5 mmol; 0.45 mL) gave as solid yellow
(0.38 g, 58.15%.), mp. 328 ꢀC. 1H NMR (DMSOed6),
d (ppm):
7.1196e7.1566 (t, 2H, Ar); 7.3625e7.4021 (t, 4H, Ar); 7.7874e7.8073
(d, J z 7.96 Hz, 4H, Ar); 11.2766 (s, 2H, -NH-). IR (KBr; cmꢁ1):
3087.87, 3058.58, 2992.99, 2967.42 (¼C-H); 2777.99, 2746.84,
2680.45 (NHþ2 ); 1615.30, 1591.09 (C=O); 1552.29 (N-H); 1499.64,
1450.55, 1425.97, 1410.77 (C-C); 1337.62, 1317.78, 1294.63 (N-C);
825.69, 751.13, 688.38 (¼C-H); 825.69 (substituent para). Anal.
Calcd. for C16H12N2O2: C, 72.73; H, 4.545; N,10.606. Found: C, 68.36;
H, 4.958; N, 9.461.
1,3-Bis(4-ethylphenylamino)squarine was prepared from p-
ethylaniline (SQG3) (5 mmol; 0.62 mL) by general procedure.
Compounds was obtained as a yellow solid (0.61 g, 76.32%.), mp.
289 ꢀC. 1H NMR (DMSOed6),
d (ppm): 1.1729e1.2107 (t, 6H, -CH3);
2.5712e2.6286 (m, 4H, -CH-); 7.1965e7.2175 (d, J z 8.4 Hz, 4H, Ar);
7.6723e7.6930 (d, J z 8.28 Hz, 4H, Ar); 11.1395 (s, 2H, -NH-). IR
(KBr; cmꢁ1): 3065.18, 2964.93 (¼C-H); 2777.26, 2751.12, 2648.68
(NH.þ2 ); 1615.03,1588.77 (C=O); 1549.01 (N-H); 1428.83,1410.09 (C-
C); 1313.66 (N-C); 847.17; 829.15, 763.42, 728.41 (¼C-H); 829.15
(substituent para). Anal. Calcd. for C20H20N2O2: C, 75.00; H, 6.250;
1,3-Bis(4-bromophenylamino)squarine (SQG2) was prepared
from p-bromoaniline (5 mmol; 0.86 g) using general procedure.
Dye was obtained as a yellow solid (0.80 g, 75.67%.), mp. 359 ꢀC. 1H
NMR (DMSOed6),
d (ppm): 7.5555e7.5775 (d, J z 8.08 Hz, 4H, Ar);

K. Jurek et al. / Dyes and Pigments 133 (2016) 273e279
275
N, 8.750. Found: C, 73.36; H, 6.081; N, 8.438.
2.1.1.2. Synthesis of 1,3-bis(4-nitrophenylamino)squarine (SQG8).
The squarine dye SQG8 was synthesized by general procedure
presented in point 2.1.1. After 1 h of refluxing, sulfanilic acid
(5 mmol; 0.865 g) [20] dissolved in boiling water (16 mL) was
added to the reaction mixture. The reaction was continued under
reflux for additional 2 h. Next, 1-butanol (20 mL) and toluene
(10 mL) was added and refluxed for 2 h. Reaction mixture was
cooled and the solvent removed on a Büchner funnel. The solid
dried in room temperature. Dye was obtained as a light yellow solid
(0.66 g, 61.81%), mp. 277 ꢀC. IR (KBr; cmꢁ1): 2648.54 (¼C-H);
1670.83, 1631.72, 1602.08 (NHþ2 ); 1577.63, 1548.04, 1522.20 (C=O);
1499.71, 1457.66, 1424.02 (N-H); 1499.71, 1457.66, 1424.02 (C-C);
1319.72 (N-C); 882.13, 846.85, 836.52, 829.82, 819.85, 801.96 (¼C-
H); 836.52 (substituent para); 1034.79, 1009.90, 686.37 (R-SO3H).
Anal. Calcd. for C16H12N2O8S2: C, 45.28; H, 2.830; N, 6.604. Found: C,
41.30; H, 4.572; N, 7.833.
1,3-Bis(4-ethoxyphenylamino)squarine was (SQG4) synthesized
using general procedure from p-ethoxyaniline (5 mmol; 0.64 mL).
Dye was obtained as a yellow solid (0.66 g, 74.47%), mp. 308 ꢀC. 1H
NMR (DMSOed6),
d (ppm): 1.2905e1.3484 (m, 6H, -CH3);
3.9769e4.0638 (m, 4H, -CH-); 6.9379e6.9476 (d, J z 3.88 Hz, 4H,
Ar); 7.6760e7.6991 (d, J z 9.24 Hz, 4H, Ar); 11.0144 (s, 2H, -NH-). IR
(KBr; cmꢁ1): 3069.38, 2976.36 (¼C-H); 2779.34, 2753.10, 2655.32
(NHþ2 ); 1623.70, 1593.62, 1554.77 (C=O); 1515.44 (N-H); 1435.20 (C-
C); 1301.78 (N-C); 895.04, 835.96, 810.75, 762.21 (¼C-H); 835.96
(substituent para). Anal. Calcd. for C20H20N2O4: C, 68.18; H, 5.682;
N, 7.955. Found: C, 65.73; H, 6.066; N, 7.334.
1,3-Bis(4-chlorophenylamino)squarine (SQG6) was prepared
using general procedure from p-chloroaniline (5 mmol; 0.638 g).
Dye was obtained as a green-yellow solid (0.67 g, 80.44%.), mp.
328 ꢀC. IR (KBr; cmꢁ1): 3061.95, 2968.05 (¼C-H); 2777.79, 2740.81,
2646.24 (NHþ2 ); 1615.50, 1589.25 (C=O); 1548.56 (N-H); 1494.97,
1429.04, 1413.49 (C-C); 1335.19, 1310.93, 1281.98 (N-C); 840.09,
829.40, 802.64, 762.80, 702.63 (¼C-H); 829.40 (substituent para);
671.76, 628.69 (Cl-C). Anal. Calcd. for C16H10N2O2Cl2: C, 57.66; H,
3.003; N, 8.408. Found: C, 55.13; H, 3.800; N, 7.647.
1,3-Bis(4-iodophenylamino)squarine (SQG7) was synthesized
by general procedure from p-iodoaniline (5 mmol; 0.638 g). Dye
was prepared as a green-yellow solid (1.01 g, 78.56%), mp. 328 ꢀC. IR
(KBr; cmꢁ1): 3055.85, 2960.85 (¼C-H); 2739.80, 2654.22 (NHþ2 );
1616.48, 1589.60, 1559.44 (C=O); 1543.16 (N-H); 1489.01, 1429.71,
1411.60 (C-C); 1339.71 (N-C); 849.74, 841.83, 822.22, 795.59,
762.86, 696.92, 668.49 (¼C-H); 822.22 (substituent para); 507.43 (I-
C). Anal. Calcd. for C16H10N2O2I2: C, 37.21; H, 1.938; N, 5.426. Found:
C, 37.24; H, 1.965; N, 5.468.
2.2. Materials
Aniline, p-bromoaniline, p-ethylaniline, p-ethoxyaniline, p-
chloroaniline, p-iodoaniline, p-toluidyne, p-aminophenol, p-nitro-
aniline, sulfanilic acid, diethyl ether, tetrahydrofuran (THF),
acetone, acetonitrile, ethanol, methanol, dimethylsulfoxide
(DMSO), distilled water, toluene, 1-butanol, N,N-dimethylforma-
mide (DMF), 1-methyl-2-pyrrolidinone (MP) were purchased from
Sigma-Aldrich (Poland). All chemicals were of analytical grade.
2.3. Measurements
2.3.1. Nuclear magnetic resonance spectroscopy
The 1H and 13C NMR spectra were recorded used an Ascend III
spectrometer operating at 400 MHz, Bruker (USA). Dimethylsulf-
oxide (DMSO-d6) was used as a solvent and tetramethylsilane as
internal standard.
1,3-Bis(4-methylphenylamino)squarine (SQG9) was prepared
from p-toluidine (5 mmol; 0.535 g) using general procedure. Dye
was obtained as a yellow solid (0.56 g, 77.05%.), mp. 336 ꢀC. IR (KBr;
cmꢁ1): 3070.10, 3037.83, 2982.92 (¼C-H); 2776.71, 2748.12,
2660.00 (NHþ2 ); 1581.56, 1551.18 (C=O); 1516.00 (N-H); 1428.80 (C-
C); 1336.36, 1320.96, 1292.49 (N-C); 840.91, 813.08, 795.61, 764.85,
741.14 (¼C-H); 813.08 (substituent para). Anal. Calcd. for
2.3.2. Fourier transform infrared spectroscopy FTIR
FTIR spectra of obtained dyes were recorded on spectrometer
Vector 22 Bruker USA in KBr pellets with frequency range
C
18H16N2O2: C, 73.97; H, 5.479; N, 9.589. Found: C, 73.36; H, 5.632;
N, 9.444.
1,3-Bis(4-hydroxyphenylamino)squarine (SQK1) was synthe-
4000e400 cmꢁ1
.
sized from p-aminophenol (5 mmol; 0.545 g) follow general pro-
2.3.3. Melting point measurements
Melting points (uncorrected) were determined on the Boeothius
apparatus e PGH Rundfunk, Fernsehen Niederdorf KR, Stollberg/E.
cedure. Dye was obtained as a yellow solid (0.42 g, 56.76%), mp.
€
328 ꢀC. 1H NMR (DMSOed6),
d (ppm): 6.7908e6.8126 (d,
J z 8,72 Hz, 4H, Ar); 7.6358e7.6536 (d, J z 7,12 Hz, 4H, Ar); 9.4518
(s, 2H, -OH); 11.0821 (s, 2H, -NH-). IR (KBr; cmꢁ1): 3424.46, (O-H);
3094.80, 2968.70, 2932.08 (¼C-H); 1599.27, 1563.37 (C=O); 1515.74
(N-H); 1446.70, 1428.97 (C-C); 1307.73 (N-C); 755,49 (¼C-H);
828,13 (substituent para).
2.3.4. Elemental analysis
The elemental analysis was made with
11.45e0000, Elementar Analyser System GmbH (Germany), oper-
a Vario MACRO
ating with the software VARIOEL 5.14.4.22.
2.3.5. Spectroscopic measurements
2.1.1.1. Synthesis of 1,3-bis(4-nitrophenylamino)squarine (SQG5).
The squarine dye SQG5 was synthesized by general procedure
presented in point 2.1.1 from p-nitroaniline (5 mmol; 0.69 g).
Additionally after 4 h, further amount of 1-butanol (20 mL) and
toluene (10 mL) was added and the reaction mixture was refluxed
for 4 h. Reaction mixture was cooled and the solvent removed on a
Büchner funnel. The solid dried in room temperature. Dye was
obtained as an orange solid (0.57 g, 84.16%), mp. 356 ꢀC. IR (KBr;
cmꢁ1): 3078.32, 2993.04, 2968.24 (¼C-H); 2651.35 (NHþ2 ); 1586.81,
1560.89 (C=O); 1511.76 (N-H); 1407.43 (C-C); 1339.48, 1311.26 (N-
C); 847.17; 843.61, 832.37, 808.31, 782.52, 764.52, 750.18, 685.84
(¼C-H); 860.75 (substituent para); 1339.48, 1311.26 (N-O). Anal.
Calcd. for C16H10N4O6: C, 54.24; H, 2.825; N, 15.819. Found: C,52.47;
H, 3.680; N, 14.677.
An influence of substituent on absorption spectra and sol-
vatochromic properties of dyes were measurement on a Cary 50
spectrophotometer (Varian) in quartz cuvette 1 cm.
3. Results and discussion
The spectroscopic properties of squarine dyes under study in
different solvents are presented in Table 1.
As shown in Fig. 1, the hypsochromic shift of absorption
maximum of SQG5 in different solvents as an effect of increasing
polarity of solvents from nonpolar diethyl ether to polar protic
solvent, such as methanol and water was observed. The same effect
was observed for all other p-substituted aniline derivatives.

276
K. Jurek et al. / Dyes and Pigments 133 (2016) 273e279
diethyl ether
THF
acetone
1,0
0,8
0,6
0,4
0,2
0,0
ethanol
methanol
1-methyl-2-pyrrolidinone
acetonitrile
DMF
DMSO
water
350
400
450
500
550
600
650
700
Wavelength [nm]
Fig. 1. Normalized absorption spectra of squarine dye (SQG5) in different solvents.
The effect of substituent was also study in different solvents.
Generally, the red shift of absorption band is observed for all p-
substituted aminosquarines in comparison with the dye without
any substituent (SQG1). The stronger bathochromic effect is caused
by strong electron-accepting substituents. For example, the intro-
duction of nitro group into the dye structure leads to shift of ab-
sorption band about 70 nm in comparison to the unsubstituted
squarine. In the case of the dyes possessing an electron donating
group the observed bathochromic shift changes from 6 nm to
10 nm for ethyl and hydroxyl groups, respectively. The absorption
spectra of all synthesized dyes in diethyl ether are presented in
Fig. 2.
The effect of solvent polarity and hydrogen bonding on sqary-
lium p-aniline derivatives were studied using the linear solvation
energy correlation described by Kamlet Taft equation presented
below (Eq. (1)) [23]:
v ¼ v0 þ a
a
þ b
b
þ sp*
(1)
where:
b
a
is the scale of the solvent hydrogen bond donor acidity,
is the scale of the solvent hydrogen bond basicity, p* is a
SQG1
1,0
0,8
0,6
0,4
0,2
0,0
SQG2
SQG3
SQG4
SQG5
SQG6
SQG7
SQG9
SQK1
350
400
450
500
550
600
650
Wavelength [nm]
Fig. 2. Normalized absorption spectra of squarine dyes in dimethyl ether.

K. Jurek et al. / Dyes and Pigments 133 (2016) 273e279
277
Table 2
Table 4
Kamlet-Taft solvent parameters.
Hammett substituents constant.
Solvent
a
b
p
*
Substituent
sp
Diethyl ether
Tetrahydrofurane (THF)
Acetone
Ethanol
Methanol
1-Methy1-2-pyrrolidinone (MP)
Acetonitryle
Dimethylformamide (DMF)
Dimethyl sulfoxide (DMSO)
Water
0
0
0.47
0.55
0.43
0.75
0.66
0.77
0.4
0.96
0.76
0.47
0.27
0.58
0.71
0.54
0.6
0.92
0.75
0.88
1
H
Br
C2H5
OC2H5
NO2
Cl
0
0.23
ꢁ0.15
ꢁ0.24
0.78
0.23
0.18
0.08
0.86
0.98
0
0.19
0
I
SO3H
CH3
OH
0.35
0
1.17
ꢁ0.17
1.09
ꢁ0.37
measure of the solvent dipolarity/polarizability and n0 is the
regression value of the solute properties in reference solvent
cyclohexane. The regression coefficient a, b and s in Eq. (1)
measure the relative susceptibility of the solvent dependent of
solute property (absorption frequencies) to the indicated solvent
of solvatochromic parameters are due to solvent dipolarity/polar-
izability which indicate by non-specific solute-solvent interactions.
The absorption frequencies were correlated by Hammett equa-
tion (Eq. (2)) using sp substituent constants (Table 4) [24]:
parameters. The Kamlet eTaft
a
,
b
and p* solvent parameters are
v ¼ v0 þ rsp
(2)
listed in Table 2.
The multiple linear regression are used to find correlations of
the spectroscopic data of studied squarine dyes for ten selected
solvents. The results of multiple regression are listed in Table 3.
The positive sign of coefficient s and a for all squarine dyes
without SQG8 indicates hypsochromic shift with increasing polar-
izability of solvents. This suggest the stabilization of the ground
state relative to the electronically excited state. The negative sign of
all coefficients in Kamlet-Taft equation for SQG8 shows a hyp-
sochromic shift with increasing hydrogen bond acidity of solvents.
The percentage contribution of solvatochromic parameters
(Table 3) for p-aniline derivatives of squarylium dyes with strong
electron-withdrawing substituent as a nitro group and strong
where:
r is a coefficient of sensitivity of the absorption frequencies
to substituent effects. The correlation parameters of Hammett
equation for all solvent are presented in Table 5.
Electron-donating substituents decrease the
p-
p* transition
energy and the bathochromic shift of absorption maximum are
observed. Similar to the electron-withdrawing substituents leads to
the bathochromic shifts and are observed with increasing of
electron-accepting properties. The correlation between absorption
frequency and Hammett substituents constant compared to ami-
nosquarine without any substituent are presented in Fig. 3 for
electron-withdrawing group and in Fig. 4 for electron-donating
group introduced in para position of aniline.
electro-donating substituent as
a hydroxyl group and other
electron-withdrawing groups shows that the major contribution of
solvatochromism is due to solvent acidity hydrogen bond and are
the results of specific solute-solvent interactions. The opposite to
other electron-donating groups, the major percentage contribution
The positive slope for electron-donating substituents (Fig. 3)
and negative slope for electron-withdrawing substituents (Fig. 4)
with satisfactory correlation (Table 5) were observed for all studied
solvents excluding water were only negative slope were observed
Table 3
Multilinear regression fits and percentage contribution of solvatochromic parameters of Kamlet-Taft equation.
Substituent
n0
a
b
s
R
Standard error
F
(103 cmꢁ1
)
(103 cmꢁ1
)
(103 cmꢁ1
)
(103 cmꢁ1
)
P(a
)
P(b
)
P(p*)
%
%
%
H
24.45
23.36
24.25
24.18
20.41
23.43
23.48
27.04
24.40
24.40
0.65
35.65
1.83
36.04
1.89
35.62
1.9
35.33
3.29
35.77
1.84
36.14
1.64
35.84
ꢁ0.077
40.36
1.51
35.60
1.13
ꢁ0.062
24.17
ꢁ2.30
30.24
ꢁ2.63
26.77
ꢁ3.33
24.63
ꢁ4.17
30.07
ꢁ2.10
31.17
ꢁ2.36
28.95
ꢁ2.43
29.78
ꢁ2.18
26.67
ꢁ1.39
36.95
1.19
40.18
3.55
33.72
2.90
37.60
3.28
40.04
2.29
34.15
3.39
32.69
3.32
35.21
ꢁ0.53
29.86
2.42
37.74
1.23
0.85
0.9
0.58
0.72
5.20
8.83
8.21
8.30
Br
C2H5
OC2H5
NO2
Cl
0.89
0.89
0.96
0.89
0.92
0.62
0.92
0.94
0.71
0.71
0.88
24.94
8.31
0.70
I
0.77
11.35
0.86
SO3H
CH3
OH
ꢁ0.062
0.78
11.72
16.07
0.83
36.02
27.03

278
K. Jurek et al. / Dyes and Pigments 133 (2016) 273e279
Table 5
Linear regression fits of substituent constants in all solvents of Hammett equation.
Solvent
n0
r
R
Standard error
Standard errora
F
ra
Ra
Fa
a
V0
(103 cmꢁ1
)
(103 cmꢁ1
)
Diethyl ether
25.13
25.21
24.93
25.41
25.07
25.42
25.43
25.99
25.60
25.98
25.62
26.56
25.49
26.20
25.49
26.59
25.13
26.20
28.75
28.75
2.09
ꢁ4.92
1.58
0.84
0.92
0.86
0.81
0.77
0.96
0.86
0.89
0.73
0.91
0.83
0.91
0.91
0.91
0.84
0.91
0.83
0.84
0.58
0.58
0.45
0.67
0.30
0.38
0.41
0.44
0.43
0.32
0.54
0.66
0.24
0.47
0.26
0.66
0.66
0.66
0.45
0.59
1.39
1.39
31.79
53.06
27.02
23.54
14.73
124.53
18.77
32.45
8.46
49.67
15.24
52.99
31.56
39.62
22.23
40.06
20.77
27.40
11.31
11.31
Tetrahydrofuran (THF)
Acetone
ꢁ4.97
1.57
ꢁ4.87
1.88
Ethanol
ꢁ5.49
Methanol
1.59
ꢁ4.70
4.39
1-Methyl-2-pyrrolidinone (MP)
Acetonitrile
ꢁ9.16
1.50
ꢁ6.68
3.13
Dimethylformamide (DMF)
Dimethyl sulfoxide (DMSO)
Water
ꢁ8.73
2.08
ꢁ8.16
ꢁ4.69
ꢁ4.69
a
The linear correlation for electron-withdrawing substituents.
Fig. 4. Correlation between nmax and sp for squarine dyes with electron donating
group in different solvents, inset correlation between nmax and sp for squarine dyes in
water.
Fig. 3. Correlation between nmax and sp for squarine dyes with electron withdrawing
group in different solvents.
energy increase with decrease electron-donating constant and
electron-withdrawing constant of all substituent due to hydrogen
substituents in para position.
for all substituents.
4. Conclusions
Acknowledgements
The series of aniline-based squarine dyes were synthesized and
their spectroscopic properties were characterized in ten selected
solvents. The effect of solvent show that the electron-donating
group presented the major percentage contribution of sol-
vatochromic parameters due to solvent dipolarity/polarizability.
Opposite to the strong electron-attracting substituent, strong
electro-donating substituent and electron-withdrawing groups
which shows the major contribution of solvatochromism due to
This work was supported by The National Science Centre (NCN
Cracow, Poland) Grant No. 2013/11/B/ST5/01281.
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