1,3-Bis(phenylamino)squaraine - Photophysical and photochemical properties

Article abstract of DOI:10.1016/j.dyepig.2015.12.027

A novel squarylium dye possessing donor-π-acceptor structure that features a fixed imine phenyl ring as an electron donor and 3,4-dihydroxycyclobut-3-ene-1,2-dione moiety as an electron acceptor was synthesized. The structure of the dye was elucidated by means of NMR and IR spectroscopy and elemental analysis. The compound was studied using steady-state absorption and emission spectroscopy, as well as the time-resolved spectroscopy. The measurements were carried out in solvents of different polarity. The dye absorbs in the visible light region about 400 nm and emits light with low fluorescence quantum yield, below 1 × 10^-3 M. An increases polarity of solvent results in a blue-shift of absorption band and red-shift of emission band, respectively. The rate constants of radiative and non-radiative deactivation of excited state depend on the viscosity of solvent used. The bimolecular quenching of excited singlet state of squaraine depends on the rate of diffusion process.

Full text of DOI:10.1016/j.dyepig.2015.12.027

Dyes and Pigments 127 (2016) 179e186
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
1,3-Bis(phenylamino)squaraine e Photophysical and photochemical
properties
,
Janina Kabatc a *, Katarzyna Kostrzewska a, Katarzyna Jurek a, Robert Dobosz a,
Łukasz Orzeł b
a UTP University of Science and Technology, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
b Jagiellonian University, Faculty of Chemistry, Ingardena 3, 30-060 Cracow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel squarylium dye possessing donorepeacceptor structure that features a fixed imine phenyl ring
Received 15 November 2015
Received in revised form
18 December 2015
Accepted 22 December 2015
Available online 8 January 2016
as an electron donor and 3,4-dihydroxycyclobut-3-ene-1,2-dione moiety as an electron acceptor was
synthesized. The structure of the dye was elucidated by means of NMR and IR spectroscopy and
elemental analysis. The compound was studied using steady-state absorption and emission spectroscopy,
as well as the time-resolved spectroscopy. The measurements were carried out in solvents of different
polarity. The dye absorbs in the visible light region about 400 nm and emits light with low fluorescence
quantum yield, below 1 ꢀ 10ꢁ3 M. An increases polarity of solvent results in a blue-shift of absorption
band and red-shift of emission band, respectively.
The rate constants of radiative and non-radiative deactivation of excited state depend on the viscosity
of solvent used. The bimolecular quenching of excited singlet state of squaraine depends on the rate of
diffusion process.
Keywords:
Squarylium dye
Synthesis
Physicochemical properties
Spectroscopic studies
Quenching of fluorescence
Radiative and non-radiative deactivation
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
that 1,3-bis(aminophenyl)squaraine is polarized, with the phenyl-
imino moiety being an electron donor (D) and the central C4O2 unit
Squarylium dyes often also called squaraines are derived from
quadratic acid (squaric acid). According to IUPAC nomenclature
they are derivatives of 1,2-dihydroxycyclobuten-3,4-dione.
being an electron acceptor. The squaraines also form a sol-
uteesolvent complex in organic solvents, the equilibrium constant
of which is dependent on DeAeD charge-transfer character of the
squaraine [2]. The next feature of squaraine is the emission from
the three different excited states, e.g. free squaraine, soluteesolvent
complex, and a twisted excited state as a result of CeC bond rota-
tion [2].
Squaraines belong to so-called photosensitizing dyes, that
absorb strongly in the visible light region and are very important
for various imaging applications and solar energy conversion [2,3].
The dye under study was synthesized by a condensation reac-
tion of squaric acid and aniline. A characteristic feature of this
compound is a presence of nitrogen atom in the system of conju-
gated double bonds. There are not any amine and iminium termi-
nals in dye structure. Form our knowledge, such compounds are
very rare described in literature. Therefore, in order to improve the
knowledge on the physicochemical properties of squaraines, the
synthesis and spectroscopic properties of 1,3-bis(phenylamino)
squaraine are described in article.
There are two types of bis-substituted derivatives of squaric
acid: 1,2-bis donor substituted and 1,3-bis donor substituted. The
merocyanines belong to the first group are essentially and have no
distinctive properties, whereas the second type represents an
unique group of chromophore, which is neither a merocyanines nor
cyanine and has exceptional light absorption characteristics [1]. The
most of very important squarylium dyes possess an arylamine or a
heterocyclic moiety as the end electron donating groups. The
symmetric DeAeD (donoreacceptoredonor) arrangement of
squaraine dyes has an interesting effect on the formation of intra-
molecular charge-transfer states. Basing on the Kamat's and et al.
studies [2] on the photochemistry of bis[4-(dimethylamino)phenyl]
squaraine and quantum chemical calculations may be suggested
* Corresponding author. Tel.: þ48 52 374 9112; fax: þ48 52 374 9005.
E-mail address: nina@utp.edu.pl (J. Kabatc).
http://dx.doi.org/10.1016/j.dyepig.2015.12.027
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

180
J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
2. Experimental
of fluorescence decay signals with a resolution of few picoseconds
possible. The dye was studied at concentration able to provide
equivalent absorbance at 370 nm (0.2 in the 10 mm cell) to be
obtained. The fluorescence decay was fitted to two exponentials.
The fluorescence quenching measurements were performed
using a single-photon counting system UVeVIS-NIR Fluorolog 3
Spectrofluorimeter (Horiba Jobin Yvon). The apparatus uses a
picosecond diode laser (370 nm) generating pulses of about 50 ps
for the excitation. Short laser pulses in combination with a fast
microchannel plate photodetector and ultrafast electronics make a
successful analysis of fluorescence decay signals in the range of
single picoseconds possible. The dye was studied at a concentration
able to provide equivalent absorbance at 370 nm (0.2 in the 10 mm
cell). The rate constant for the quenching of 1,3-bis(phenylamino)
squaraine by all quenchers under studies were determined in 1-
methyl-2-pyrrolidinone. The concentration of dye was
2 ꢀ 10ꢁ5 M and that of the quenchers was in the range from
1 ꢀ 10ꢁ4 M to 5.0 ꢀ 10ꢁ3 M. The fluorescence quenching at 440 nm
was measured in deaerated solution by bubbling with argon.
2.1. Materials and general methods
All reagents and solvents (spectroscopic grade) were purchased
from Aldrich (Poland) and used without further purification.
1H spectra was recorded in DMSO-d6 on a Bruker Ascend™
spectrometer operating at 400 MHz. Chemical shifts are reported in
ppm using tetramethylsilane TMS as an internal standard. Coupling
constants (J) are given in Hz.
IR spectra were recorded on a Bruker Vector 22 FTIR Spectro-
photometer (Germany). Samples were prepared by mixing FTIR-
grade KBr (SigmaeAldrich) with 1% (w/w) of dye, and grinding to
a fine powder. Spectra were recorded over the 400e4000 cmꢁ1
range. Characteristic absorptions are given in cmꢁ1
.
The elemental analysis was made with a Vario MACRO 11.45-
0000, Elemental Analyzer System GmbH (Germany), operating
with the software VARIOEL 5.14.4.22.
€
Melting point was measured on the Boethius apparatus (type
PHMK 05, Germany).
Absorption and emission spectra were recorded at room tem-
perature using an Agilent Technology UVevis Cary 60 Spectro-
photometer, a Hitachi F-7000 spectrofluorimeter and UVeVIS-NIR
Fluorolog 3 Spectrofluorimeter (Horiba Jobin Yvon), respectively.
The spectra were recorded in following solvents: water (H2O),
dimethylsulfoxide (DMSO), acetonitrile (CH3CN), N,N-dime-
thylformamide (DMF), 1-methyl-2-pyrrolidinone (MP), methanol
(MeOH), ethanol (EtOH), acetone, tetrahydrofuran (THF) and
diethyl ether. The final concentration of the dye in the solution was
1.0 ꢀ 10ꢁ5 M. The spectroscopic measurements were performed in
mentioned above solvents containing 10% of 1-methyl-2-
pyrrolidinone. For this purpose a suitable amount of the dye was
dissolved in 1-methyl-2-pyrrolidinone, than 0.1 mL of the
concentrated (ca. 1 mM) stock solution was added to a 10 mL
volumetric flask containing spectroscopic grade solvents under the
study. They were characterized by a static dielectric constant (ε)
and a refractive index (n) at 20 ꢂC. The solvent polarity function
f(ε,n) is given by Eq. (1) [1].
2.2. Synthesis
1,3-Bis(phenylamino)squaraine was synthesized according with
general method: squaric acid (0.290 g, 2.5 mmol) was heated under
reflux in a mixture of 1-butanol (40 mL) and toluene (20 mL), and
water was distilled off azeotropically using a DeaneStark trap. After
1 h, aniline (5 mmol) was added and the reaction mixture refluxed
for additional 4 h. The suspension was then cooled to room tem-
perature and the solvent removed on a rotary evaporator. The
residue was crystallized from 1-butanol and the solid dried in an
oven at 50 ꢂC [1,6].
Dye was obtained as a yellow solid (0.38 g, 59%), mp. 328 ꢂC.
1H NMR (DMSO-d6),
d (ppm): 7.1196e7.1566 (t, 2H, Ar);
7.3625e7.4021 (t, 4H, Ar); 7.7874e7.8073 (d, J ¼ 7.96 Hz, 4H, Ar);
11.2766 (s, 2H, eNHe).
IR (KBr)
n
(cmꢁ1): 3087.87, 3058.58, 2992.99, 2967.42 (]CeH);
2777.99, 2746.84, 2680.45 (NH2þ); 1615.30, 1591.09 (C]O);
1552.29 (NeH); 1499.64, 1450.55, 1425.97, 1410.77 (CeC); 1337.62,
1317.78, 1294.63 (NeC); 1261.90, 1245.62, 1184.03, 1164.73, 1145.56,
1098.39, 1079.20 (CeO); 825.69, 751.13, 688.38 (“oop”); 825.69
(substituent para).
"
#
ꢀ
ꢁ
ꢀ
ꢁ
2n2 þ 1
n2 ꢁ 1
n2 þ 2
ðε ꢁ 1Þ
ðε þ 2Þ
ꢀ
ꢁ
ꢀ
ꢁ
f ðε; nÞ ¼
$
ꢁ
(1)
n2 þ 2
Anal. Calcd. for C16H12N2O2: C, 72.73; H, 4.545; N, 10.606. Found:
C, 72.66; H, 4.558; N, 10.561.
The fluorescence quantum yield for the dye in solvents were
determined as follows. The fluorescence spectra of a diluted dye
solution (A z0.1 at 366 nm) was recorded by excitation at the
maximum of absorption band of the standard. Dilute Coumarin I
2.3. Computational details
Gaussian 09 [7] software package was used for calculations.
For the optimization of geometry of the ground and first excited
state of the SQ dye molecule the B3LYP/6-311þþG(d,p) method was
used. Frequencies analysis were performer at the same level. All
calculations were performed in the gas phase (no intermolecular
interactions were considered). For visualization of the surfaces of
molecular orbitals the Gabedit software [8] was used.
(
Ф
¼ 0.64) was used as referene [5]. The fluorescence spectra of
Coumarin I was obtained by excitation at 366 nm. The fluorescence
quantum yield of dye (fdye) was calculated using Eq. (2):
n2dye
IdyeAref
fdye ¼ fref
$
$
(2)
n2ref
Iref Adye
where: fref is the fluorescence quantum yield of the reference, Adye
and Aref are the absorbance of the dye and reference at the exci-
tation wavelength, Idye and Iref are the integrated emission intensity
for the dye and reference, ndye and nref are the refractive indexes of
the solvents used for dissolve of dye and reference, respectively.
The fluorescence lifetimes were measured using a single-photon
3. Results and discussion
3.1. Synthesis
Structure and purity of 1,3-bis(phenylamino)squaraine was
confirmed by FTIR, 1H NMR spectroscopy, elemental analysis and
thin layer chromatography. The data were found be in good
agreement with the structure of dye. The 1H NMR spectra of dye
shows characteristic bands in the region of
region the signals characteristic for the protons in benzene ring,
cyclobut-3-ene-1,2-dione ring, and the imino group are present.
counting system UVeVIS-NIR Fluorolog
3 Spectrofluorimeter
(Horiba Jobin Yvon). The apparatus utilizes for the excitation a
picosecond diode laser generating pulses of about 55 ps at 370 nm.
Short laser pulses in combination with a fast microchannel plate
photodetector and ultrafast electronics make a successful analysis
d
¼ 1.0e9 ppm. In the

J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
181
The IR spectra indicates some characteristic bands for the dye
obtained. For example, the aromatic stretching vibrations for CeN
and C]C bonds were observed in the frequency range from
1295 cmꢁ1 to 1500 cmꢁ1 and for CeH in the range from 2967 cmꢁ1
to 3088 cmꢁ1. The stretching vibrations for C]O were observed in
generated by tripletetriplet sensitization [2]. It is well known, that
properties of ground and excited state compounds depend on the
properties of surrounding solvent. The molecules of solvent may
interact (electrostatic interaction, hydrogen bonding, Van der
Waals interaction, etc.) with molecules of dissolved compound.
Therefore, the effect of solvent on the properties of compound in its
ground and excited state was studied. The solvents used differ
polarity and viscosity. Fig. 4 presents some fluorescence spectra of
1,3-bis(phenylamino)squaraine recorded at room temperature in
solvents of different polarity.
The values of half-width of emission band are much higher than
that of absorption band. The broadening of emission band may be
result of contribution from more than one form of excited state of
squarylium dye. The spectroscopic characteristic of 1,3-
bis(phenylamino)squaraine is presented in Table 1.
the frequency range from 1615 cmꢁ1 to 1592 cmꢁ1
.
3.2. Ground state of squarylium dye
The presence of an electron donating end group attached to the
squaric acid results in delocalized nature of
p-electron system.
Fig. 1 presents the surfaces of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) defined by quantum chemical calculations.
Dirk C. et al. used X-ray crystallographic analysis also confirmed
As it is seen from data presented in Table 1, the spectroscopic
properties of 1,3-bis(phenylamino)squaraine depend on the po-
larity of solvent used. An increase of solvent polarity leads to blue
shift of absorption spectra and red shift of fluorescence band. The
reason of effect of solvent polarity on the absorption and emission
properties of dye is different energy of solvation process of mole-
cule in its ground state and FranckeCondon excited state. The
negative solvatochromism is caused by better stabilization of the
ground state by polar solvents in comparison with excited state.
The molecular orbitals taking part in intramolecular charge transfer
were shown above (see Fig. 1). The highest occupied molecular
orbital (HOMO) is dominated by cyclobut-3-ene-1,2-dione moiety
with partially negative charge located at the oxygen atom on cen-
tral four-member ring. The lowest unoccupied molecular orbital
(LUMO) is dominated by azomethine groups. The electronic exci-
tation of squarylium dye leads to the displacement of electron
density from electron donating part of molecule to an electron
accepting part of molecule. Therefore, the excited state is less polar
than the ground state, and hence less stabilized by polar solvents.
As a result the negative solvatochromism is expected [4]. The
negative solvatochromism observed confirms that the ground state
of 1,3-bis(phenylamino)squaraine is more polar than the charge-
transfer excited state, and hence lower dipole moment value of
the excited state as compared to the ground state is evident [13]. It
should be also pointed out that the maximum of emission band in
each of the solvents used at room temperature shows a large red
shift, more than ca. 4000 cmꢁ1 with respect to the absorption band.
The high value of Stokes shift indicates a significant charge redis-
tribution upon excitation [4,13].
the delocalization of p-electrons in squarylium dye structure [9].
In solution squarylium dyes exist in form of two neutral reso-
nance structures. These compounds may also undergo deprotona-
tion giving monoanion or/and dianions. Fig. 2 presents the possible
structures of squarylium dye.
The ability to the deprotonation strongly depends on the type of
substituent in p-position of phenyl ring [1]. The presence of
electron-withdrawing group leads to mono- or dianions formation.
The squarylium dye under study without any electron-
withdrawing group exists only as neutral form. Fig. 3 displays
some electronic absorption spectra of 1,3-bis(phenylamino)squar-
aine recorded at room temperature in solvents of different polarity.
The absorption spectra are broad and have the half-widths of
about 3000 cmꢁ1. The molar extinction coefficient values are not
high and ranging from 0.2 ꢀ 104 Mꢁ1 cmꢁ1 to 1.2 ꢀ 104 Mꢁ1 cmꢁ1
.
The absorption band around 400 nm is due to
p / p* transition.
The position of absorption band depends on the polarity of solvent.
3.3. Excited state of squarylium dye
The irradiation of a dye with the visible light leads to an excited
singlet state formation. The dye in its excited state undergoes
different deactivation processes. Generally, they are radiative or
non-radiative. The fluorescence and phosphorescence belong to
radiative but internal conversion and intersystem crossing are non-
radiative processes. An excited singlet state may also be self-
quenched or quenched by other molecules in bimolecular reac-
tion. The fluorescence quenching also belongs to non-radiative
processes. For other squarylium dyes, it has been already found
that the triplet state cannot be directly populated and that the
intersystem crossing quantum yields are poor (FISC < 0.001)
[2,10e12]. The excited triplet state of squaraine dye can by
There is observed a linear correlation between spectroscopic
properties, e.g. maximum of both absorption and fluorescence
bands, Stokes shift and solvent polarity function (f), for dye studied
(Fig. 5).
There are several empirical polarity scales which may be applied
to investigate the solvatochromism, and different empirical solvent
polarity scale, for example: Dimroth-Reichardt's [14], Kamlet-Taft's
[15], Lippert-Mataga's [16], Bakhshiev's [17], Kawski-Chamma-
Viallet's [18], McRae's and Suppan's scales [19]. The Lippert-Mata-
ga's, Bakhshiev's, Kawski-Chamma-Viallet's, McRae's and Suppan's
polarity scales all based on the both the dielectric constant and the
refractive index of solvents gave remarkable linear correlations for
1,3-bis(phenylamino)squaraine (Fig. 6). The results obtained
confirmed a charge redistribution upon photoexcitation. As antic-
ipated, the largest magnitude of the Stokes shift was observed in
the most polar solvents and this variation is indicative of a major
change in the dipole moment value referring to a geometry of the
excited state different to that of the ground state.
Other characteristic feature of excited state is its lifetime. Fig. 7
shows the fluorescence decay observed for 1,3-bis(phenylamino)
squaraine in 1-methyl-2-pyrrolidinone solution.
Fig. 1. The presentation of structure, HOMO (left) and LUMO (right) orbitals for 1,3-
bis(phenylamino)squaraine.

182
J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
O
O
O
O
NH
NH
NH
NH
Resonanse structures
O
NH
N
O
Monoanion structure
O
O
O
N
N
N
N
O
Resonanse structures of dianion
Fig. 2. Presentation of structures of resonance form, mono- and dianion of neutral squarylium dye.
1,00
Water
DMSO
MeCN
DMF
1,00
0,75
0,50
0,25
0,00
Water
DMSO
MeCN
MeOH
0,75
MP
EtOH
Acetone
THF
MeOH
EtOH
Acetone
THF
Ether
0,50
0,25
0,00
Ether
400
500
600
700
350
400
450
500
Wavelength [nm]
Wavelength [nm]
Fig. 4. Fluorescence spectra of 1,3-bis(phenylamino)squaraine recorded at room
temperature in solvents of different polarity, lEX ¼ 380 nm.
Fig. 3. The electronic absorption spectra of 1,3-bis(phenylamino)squaraine recorded at
room temperature in solvents of different polarity.
The fluorescence decay observed was fitted to two exponential
curve. The dye exists in two conformers that differ in fluorescence
lifetime described by the corresponding components of two
exponential models, such as t1 and t2 together with the corre-
sponding amplitudes (B1 and B2). This feature results from inter-
action of 1,3-bis(phenylamino)squaraine with surrounding solvent.
On the basis of fitting of two exponential curve, the fluorescence
lifetimes of two conformers were designated. The fluorescence
lifetime of conformer appearing predominantly (over 70%) is 2.6 ns.
While, the fluorescence lifetime of longer living conformer is equal
10.04 ns.
As noted above, the broadening of emission band in comparison
with the absorption band observed for dye under study confirmed
the contribution from more than one form of excited state of this
dye.
It is well known, that the quantum yield of deactivation of
excited state is the sum of quantum yields of all deactivation
processes. As it was mentioned above the fluorescence is one of
deactivation processes. Basing on the data presented in Table 1 it is
seen that dye under study possess a low fluorescence quantum
yield ranging from 0.00043 to 0.0051 which also depends on sol-
vent polarity. The inspection of data presented in Table 1 shows also
that the fluorescence quantum yield increases with decrease of
solvent polarity (from water to methanol). This behavior is a result
of interaction of polarized dye molecule in excited state and sol-
vents [13]. Moreover the lowest fluorescence quantum yield in
water, ethanol that belong to polar protic solvents may be result of
hydrogen bond formation between dye molecule and surrounding
solvent. The hydrogen bonding leads to an additional non-radiative
deactivation process.
Taking into account both the fluorescence lifetime (t) and
fluorescence quantum yield (ffl), the rate constants of radiative (kr)
and non-radiative deactivation (knr) processes have been calculated
using the Eqs. (3) and (4).

J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
183
Table 1
Spectroscopic characteristic of 1,3-bis(phenylamino)squaraine in solvents of different polarity.
Parameter
Solvent
H2O
DMSO
MeCN
DMF
MP
MeOH
EtOH
Acetone
THF
Ether
lab [nm]
374
0.304
504
13,729
6900
4.3
0.167
3.859
0.9136
378
0.555
504
5857
6614
11.0
0.448
3.856
0.8407
390
0.36
496
7082
5480
5.4
0.208
3.858
0.8630
393
0.76
502
4179
5525
28.0
1.088
3.805
0.8357
393
0.19
495
4831
5243
46.0
1.798
3.843
0.8131
390
11.0
498
7297
5561
51.0
1.978
3.841
0.8547
393
11.2
492
7684
5120
6.7
0.26
3.858
0.8126
398
11.3
490
7554
4717
8.0
0.31
3.858
0.7903
401
0.69
477
14,353
3973
10.0
0.402
3.857
0.5491
400
0.53
484
8853
4339
7.4
0.287
3.858
0.3766
ε [104 Mꢁ1 cmꢁ1
lfl [nm]
]
FWHMfl [cmꢁ1
]
Stokes shift [cmꢁ1
]
ffl [ꢀ104]
kr [106 sꢁ1
]
knr [108 sꢁ1
f(ε, n)a
]
FWHMfl e the full with at half of fluorescence maximum.
a
Measured at 20 ꢂC.
ffl
t
kr ¼
(3)
25000
20000
15000
ꢂ
ꢃ
1 ꢁ ffl
knr
¼
(4)
t
The values of radiative and non-radiative rate constants are
summarized in Table 1. It was found that in polar protic solvents:
water, methanol and ethanol the radiative and non-radiative rate
constants do not depend on viscosity of solvent. As it is shown in
Fig. 8, the radiative rate constant depends on the viscosity of sol-
vent (expect to water, methanol and ethanol).
Therefore one can conclude, that the electronic structure of the
fluorescence state is sensitive to the environmental perturbation
induced by solvent.
As it was mentioned above, the excited state of dye molecule
may be quenched by different molecules in bimolecular reaction. In
next step, we study the changes in the fluorescence intensity and
fluorescence lifetime under increasing concentration of quencher.
The following compounds: tetramethylammonium n-butyl-
triphenylborate (B2), diphenyliodonium chloride (I1) and diphe-
nyliodonium hexafluorophosphate (I2) were used. The structure of
quencher is shown in Fig. 9.
Absorption
Fluorescence
Stokes shift
10000
5000
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
Solvent polarity parameter, f
Fig. 5. Correlation between spectroscopic properties: (maximum of both absorption
and fluorescence bands, Stokes shift) and solvent polarity function, f.
The influence of quencher on the fluorescence intensity and
fluorescence lifetime are present in Fig. 10 and Table 2, respectively.
From the comparison of the results obtained for the chromo-
phore alone and chromophore in the presence of quenchers the
shortening of the average fluorescence lifetime was observed. The
shortening of the fluorescence lifetime is interpreted as a result of
interaction of dye with quencher molecule.
An addition of borate salt and iodonium salt results in a signif-
icant decrease of the fluorescence intensity and decrease of lifetime
of the excited singlet state of dye. Basing on this, it should be noted,
that in a presence of suitable quencher the fluorescence state of 1,3-
bis(phenylamino)squaraine is quenched. The results obtained from
fluorescence quenching experiments were analyzed with use the
SterneVolmer relationship Eq. (5).
8000
7000
6000
Solvent polarity function, f
5000
I
I0
¼ 1 þ KSV ½Qꢃ ¼ 1 þ kqt½Qꢃ
(5)
4000
Lippert-Mattaga
Bakshiev
where: I0 and I are the fluorescence intensities of squarylium dye in
absence and presence of quencher, respectively; KSV is the
SterneVolmer constant, characterized the collision interaction of
quencher molecules (Q) with the excited state of fluorophore, kq is
the quenching rate constant.
0,0
0,2
0,4
0,6
0,8
1,0
Solvent polarity function, f
From the fluorescence lifetime
relationship of SterneVolmer plot (Fig. 11), one can calculated the
kq value.
t and the slope of the linear
Fig. 6. Lippert-Mataga's, Bakhshiev's correlations plots for 1,3-bis(phenylamino)
squaraine in different polarities solvents. Inset: McRae's and Suppan's solvatochromic
correlations plots for dye studied in different polarities solvents.

184
J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
Fig. 7. The fluorescence decay recorded for 1,3-bis(phenylamino)squaraine in 1-methyl-2-pyrrolidinone as a solvent, lEX 370 nm.
From an inspection of the data presented in Fig. 11 it is seen the
From the fluorescence lifetime measurements it is seen, that the
deviation from linearity of the SterneVolmer relationship. Taking
this into account, one can conclude that fluorescence quenching
may occur via dynamic and static mechanism. If the fluorescence
quenching occurs by a mixed mechanism (dynamic-static), the
quencher molecules interact with the dye molecules in both the
ground state and excited state, the SterneVolmer equation assumes
the form:
decrease of the fluorescence lifetime and changes in the fluores-
cence intensity of dye under study are observed in whole range of
quencher concentration. This confirmed the dynamic mechanism
of fluorescence quenching. Therefore, the broken lines for all the
three slopes in Fig. 11 can be explained as an evidence of hetero-
geneity of conjugate formation (namely, two different conjugates
are formed with different accessibility for the quenchers). As a
result, one part of dye is quenched almost immediately with in-
crease in the quencher concentration. The second, has low
quencher accessibility and its fluorescence lifetime is long even at
high quencher concentrations.
I
¼ ð1 þ KSV ½QꢃÞ$ð1 þ K½QꢃÞ
(6)
I0
and the relationship I0/I ¼ f([Q]) becomes nonlinear.

J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
185
Table 2
Effect of the quencher concentration on the fluorescence lifetime of squarylium dye.
2,0x106
1,5x106
1,0x106
5,0x105
0,0
Concentration of
quencher [M]
B2
I1
I2
t
[ns]
t
[s]
t
[ns]
0
8.0552
6.6514
6.2689
6.0698
5.5764
5.5571
5.3186
5.236
5.0116
4.9441
4.9263
4.6455
4.2855
3.7565
3.4134
3.1794
8.0552
7.5734
7.2019
6.9457
6.7155
6.464
6.2186
5.9575
5.7081
5.5116
5.3816
5.0397
4.6269
3.8739
3.3821
3.1321
8.0552
4.7261
4.62
1.0 ꢀ 10ꢁ4
2.0 ꢀ 10ꢁ4
3.0 ꢀ 10ꢁ4
4.0 ꢀ 10ꢁ4
5.0 ꢀ 10ꢁ4
6.0 ꢀ 10ꢁ4
7.0 ꢀ 10ꢁ4
8.0 ꢀ 10ꢁ4
9.0 ꢀ 10ꢁ4
1.0 ꢀ 10ꢁ3
1.5 ꢀ 10ꢁ3
2.0 ꢀ 10ꢁ3
3.0 ꢀ 10ꢁ3
4.0 ꢀ 10ꢁ3
5.0 ꢀ 10ꢁ3
4.5713
4.5018
4.4241
4.2558
4.2837
4.2115
4.1566
4.0706
3.8678
3.6827
3.3825
3.0513
2.9366
0,0
0,5
1,0
1,5
2,0
Viscosity [mPa*s]
lEX ¼ 370 nm, lEM ¼ 440 nm.
Fig. 8. The radiative rate constants as a function of viscosity of aprotic solvents
calculated for 1,3-bis(phenylamino)squaraine.
where: fi is a fraction of the total fluorescence, v is the constant of
static quenching.
The fluorescence quenching rate constant calculated for all
quenchers are different for low concentration of quencher than that
for higher concentrations. The slopes of SterneVolmer linear rela-
tionship observed (the quenching constant value) are as follows:
622.53 Mꢁ1, 510.71 Mꢁ1, 295,05 Mꢁ1, 284.21 Mꢁ1, 314.22 Mꢁ1 and
168.60 Mꢁ1 for B2, I1, I2 at concentration below 1 ꢀ 10ꢁ3 M, and for
B2, I1, I2 at concentration above 1 ꢀ 10ꢁ3 M, respectively. Taking
into account the fraction of conformer in the total fluorescence
(0.71 and 0.29 for f1 and f2, respectively), the rate constants of
exited singlet state quenching are equal 54.87 ꢀ 109 Mꢁ1 sꢁ1 (B2),
45.01 ꢀ 109 Mꢁ1 sꢁ1 (I1) and 26.17 ꢀ 109 Mꢁ1 sꢁ1 (I2) for the
concentration of quenchers below 1 ꢀ 10ꢁ3 M. For concentrations
of quenchers above 1 ꢀ 10ꢁ3 M, the rate constants of exited singlet
state quenching are equal 10.23 ꢀ 109 Mꢁ1 sꢁ1, 11.31 ꢀ 109 Mꢁ1 sꢁ1
and 6.7 ꢀ 109 Mꢁ1 sꢁ1 for B2, I1 and I2, respectively. The calculated
rate constants are close to the rate constant for diffusion-controlled
reaction. The quenching of the excited singlet state of the dye by
onium salts is much more efficient in the range of low concentra-
tions (below 1 ꢀ 10ꢁ3 M). The rate constant of quenching by
diphenyliodonium hexafluorophosphate is lower than that
observed for similar salt diphenyliodonium chloride. It is probably
due to a higher counter ion in salt I2 than in diphenyliodonium
chloride I1, which reduces the mobility of the salt I2.
I
I
B
N(CH )
3 4
Cl
PF
6
B2
I1
I2
Fig. 9. Structures and abbreviation of quenchers studied.
The influence of borate salt and iodonium salts on the rate of
fluorescence decay of the dye suggests that the primary photore-
action occurs between the dye and borate salt and both iodonium
salts, respectively. This phenomena may be result of photoinduced
electron transfer process occurred between excited dye molecule
and quencher in ground state. During photoinduced electron-
transfer process squaraine may act as an electron donor or an
electron acceptor depending on the electrochemical properties of
both dye and quencher [2]. This process may be very useful during
application of this dye in dyeing photoinitiating systems for pho-
topolymerization of different monomers.
Fig. 10. The effect of diphenyliodonium on the fluorescence intensity of 1,3-
bis(phenylamino)squaraine in 1-methyl-2-pyrrolidinone as a solvent.
If there is more than one fluorophore, their individual partici-
pation should be taken into account. In such a case, the general
SterneVolmer equation assumes form:
4. Conclusion
The synthesis and spectroscopic properties of 1,3-
bis(phenylamino)squaraine were studied in solvents of different
polarity. The negative solvatochromism in the absorption spectra
and bathochromic shifts of fluorescence spectra were observed
with increasing the polarity of solvents. This fact is achieved with
n
X
I
I0
fi
¼
(7)
i¼1 1 þ KSV ½QꢃeV½Qꢃ

186
J. Kabatc et al. / Dyes and Pigments 127 (2016) 179e186
ꢀ
One of us (R.D.) is very much indebted to CI TASK Gdansk, for
supply of computer time and providing programs.
2,8
2,4
2,0
1,6
1,2
0,8
B2
I1
I2
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This work was supported by The National Science Centre (NCN)
(Cracow, Poland), Grant No. 2013/11/B/ST5/01281.