The synthesis and the solvent and substituent effect on the spectroscopic characteristic of 3-ethyl-2-(p-substitued styryl)benzothiazolium iodides

Article abstract of DOI:10.1016/j.saa.2004.12.014

The photophysical and photochemical properties of p-substitued 2-styryl-ethylbenzothiazolium iodides, possessing different electron-withdrawing or electron-donating groups are described. The dyes were prepared by the condensation of 3-ethyl-2-methylbenzothiazole salts with p-substituted benzaldehydes. The synthesis of suitable substrates is presented as well. We describe here the absorption, emission spectra and the luminescence quantum yield of hemicyanine dyes (SH) measured in 11 different organic solvents of varying polarity. Molecular structure of the synthesized dyes was established by ¹H NMR, electronic absorption and fluorescence spectrometry. The spectral data confirmed that all the compounds exist in E-configuration of their styryl residues. The planar molecular conformation is typical for the compounds with five-membered side aromatic moieties (for example benzothiazole). The compounds possessing N-alkyl substituent in phenyl ring, in contrast to the compounds with other substituents, exhibit low fluorescence quantum yield in THF solution. This indicates that for N-alkyl derivatives the non-radiative processes are much more effective than the radiative ones. The electronic absorption and fluorescence emission spectra of tested dyes demonstrate high sensitivity to the nature of substituent introduced into the aromatic ring.

Full text of DOI:10.1016/j.saa.2004.12.014

Spectrochimica Acta Part A 62 (2005) 115–125
The synthesis and the solvent and substituent effect on the
spectroscopic characteristic of 3-ethyl-2-(p-substitued
styryl)benzothiazolium iodides
∗
∗
Janina Kabatc , Beata J e˛ drzejewska, Piotr Orli n´ ski, Jerzy P a˛ czkowski
University of Technology and Agriculture, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
Received 20 October 2004; received in revised form 8 December 2004; accepted 10 December 2004
Abstract
The photophysical and photochemical properties of p-substitued 2-styryl-ethylbenzothiazolium iodides, possessing different electron-
withdrawing or electron-donating groups are described. The dyes were prepared by the condensation of 3-ethyl-2-methylbenzothiazole
salts with p-substituted benzaldehydes. The synthesis of suitable substrates is presented as well. We describe here the absorption, emission
spectra and the luminescence quantum yield of hemicyanine dyes (SH) measured in 11 different organic solvents of varying polarity. Molecular
1
structure of the synthesized dyes was established by H NMR, electronic absorption and fluorescence spectrometry. The spectral data confirmed
that all the compounds exist in E-configuration of their styryl residues. The planar molecular conformation is typical for the compounds with
five-membered side aromatic moieties (for example benzothiazole). The compounds possessing N-alkyl substituent in phenyl ring, in contrast
to the compounds with other substituents, exhibit low fluorescence quantum yield in THF solution. This indicates that for N-alkyl derivatives
the non-radiative processes are much more effective than the radiative ones. The electronic absorption and fluorescence emission spectra of
tested dyes demonstrate high sensitivity to the nature of substituent introduced into the aromatic ring.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Hemicyanine dyes; Benzothiazole derivatives; Synthesis; Spectroscopic properties; Stokes shift
1
. Introduction
photonic properties suitable for many technological applica-
tions.
A lot of classes of fluorescent organic dyes have found
The presence of heteroatom (nitrogen, oxygen or sulphur)
in above-mentioned molecules usually does not reduce the
brightness of their fluorescence, but brings considerable
effect onto other optical characteristics: for example, molar
extinction coefficient and band position in the absorption and
fluorescence spectra. The changes in optical properties of
the organic molecules upon their chemical modification can
be foreseen by quantum-chemical simulations. However, the
fluorescence quantum yield is a poorly predictable parameter
for most organic molecules. For this reason, the experimental
discovery of new class of fluorescent compounds retains
its importance up to the present [1]. The p-substitued
styrylbenzothiazole dyes with electron donor–acceptor
moieties on opposite sides of styryl bond are particularly
attractive for their spectral sensitivity towards local host
environment and optical and electronic properties. When the
their successful application in science and technology.
The best known, among them, are xanthenes, coumarins,
naphtalimides, cyanines, various aryl-azoles, acridines and
phenoazines [1].
Synthesis of new organic molecules and characterization
of their photophysical molecular properties in different
environments (pure solvents and molecular assemblies)
are necessary prerequisites for further research in modern
technologies.
Conjugated organic dye molecules are recognized to be
important materials having novel, promising, electronic and
∗
Corresponding authors. Tel.: +48 52 3749045; fax: +48 52 3749009.
E-mail addresses: nina@atr.bydgoszcz.pl (J. Kabatc),
paczek@atr.bydgoszcz.pl (J. P a˛ czkowski).
1
386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.12.014

1
16
J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
two ends are different, the molecules can have a permanent
electric dipole and show non-linear optical behavior.
Polymethine dyes are moderately fluorescent and they ab-
sorb and emit light mostly in the visible region of the optical
spectrum which is a function of the chain length and a struc-
ture of the terminatingmoieties. It is knownthat an elongation
of the polymethine chain increases the fluorescence quantum
yields, but, as it grows further, it causes a decrease of the
fluorescence efficiency [2]. The knowledge of the fluores-
cence quantum efficiencies of organic molecules in solution
is fundamental in many important applications in physical
chemistry and photonics [3].
A number of styryl dyes (styrene-like dyes with differ-
ent aromatic groups on opposite sides of styryl double bond)
have been synthesized and investigated [3–6]. Our main task,
in the present work, was the synthesis and the study of the
chemical structure and fluorescence properties of the 18 rep-
resentatives of the 3-ethyl-2-(p-substitued styryl) benzothia-
zolium iodides, that differ by their electron donor properties
of substituent present in the aromatic moiety. In this paper,
we report the photophysical properties of the group of fluo-
rescent benzothiazole based styryl dyes, called hemicyanine
dyes, in organic solvents.
2.2.1. 3-Ethyl-2-methylbenzothiazole iodide
3-Ethyl-2-methylbenzothiazole iodide was prepared by
Hamer’s method [8]. The mixture of 2-methylbenzothiazole
◦
(1 mmol) and ethyl iodide (1.2 mmol) was heated at 100 C
for 40 h. After the extraction with dry ether, the crude product
was recrystalized from absolute ethanol, washed with ether
and dried (yield 70%).
2.2.2. 3-Ethyl-2-(p-substitued styryl)benzothiazolium
iodides
A mixture of 3-ethyl-2-methylbenzothiazole iodide
(1 mmol), p-substitued benzaldehyde (1 mmol) and acetic an-
hydride (20 ml) was refluxed for 20 min, than poured into a
warm solution of potassium iodide (4 mmol) in water (20 ml).
The precipitated product was filtered, washed with water and
recrystallized from methanol.
2.3. Measurements
For the measurements of absorption and emission spectra
−5
and fluorescence quantum yield (Φ) the 1.0 × 10 M SH
solutions in various solvents were presented. At such concen-
tration no aggregation or self-absorption effects of the dyes
were observed [9]. Absorption spectra were recorded at room
temperature using a Varian Cary 3E spectrophotometer, and
fluorescence spectra were obtained with a Hitachi F-4500
spectrofluorimeter. Absorption and emission spectra were
recorded with a spectroscopic quality: acetonitrile (MeCN),
acetone, chloroform, dimethylformamide (DMF), dimethyl-
sulfoxide (DMSO), dichloromethane, 1,2-dichloroethane,
tetrahydrofuran (THF) and ethyl acetate (EtOAc). The
fluorescence measurements were performed at an ambient
temperature.The fluorescence quantum yields for the dyes
in ethanol were determined as follows. The fluorescence
spectrum of a dilute (<25 ␮M) dye solution was recorded by
excitation at the absorption band maximum of the standard.
A dilute Rhodamine B in ethanol (Φ = 0.55 [10]) was used
as reference for dyes SH1–SH11. The fluorescence spectrum
of Rhodamine B was obtained by excitation at its absorption
peak at 530 nm. Whereas the fluorescence quantum yields
of SH12–SH21 dyes were determined by comparing with
Coumarin I fluorescence in ethanol which has been shown to
have a quantum yield of 0.64 [11]. In this case the solutions
2
. Experimental
2
.1. Materials
2-Methylbenzothiazole, Rhodamine B, Coumarin I, ethyl
iodide, p-substitued benzaldehydes and solvents were ob-
tained from Aldrich. Aldehydes, as substrates for synthesis
of SH3–SH11 dyes were synthesized according to the proce-
dure given by Gawinecki et al. [7].
3-Ethyl-2-(p-substitued styryl)benzothiazolium iodides
were synthesized in our laboratory using procedure described
in a previous report [8].
2
.2. Synthesis
A general route for the synthesis of the prepared
hemicyanine dyes is shown in Scheme 1.
Scheme 1.

J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
117
of tested dyes have absorbances around 0.1 at the excitation
wavelength (418 nm). The quantum yield of the tested dye
(Φdye) was calculated using the equation
The structure and purity of the preparated compounds
were confirmed by NMR spectroscopy and thin layer chro-
matography.
1
The H NMR spectra are in good accordance with the
I
A
dye ref
chemical structure expected for synthesized dyes (Table 1).
Φ
= Φref
(1)
dye
1
I A
ref dye
It is noteworthy that the H NMR spectra display two
characteristic doublets localized at chemical shifts about 7
and 8 ppm. They are attributed to both vinyl hydrogen atoms.
The coupling between these protons (J ≈ 15 Hz), indicates
the trans form of the ground state of all synthesized com-
pounds. There is no evidence for the formation of the cis-
structure, probably because of steric hindrance caused by the
aromatic groups [12]. The 3-ethyl-2-methylbenzothiazolium
iodide is a grey-color powder, whereas the hemicyanines are
intensely yellow–orange–red. All the samples obtained, ac-
cording to thin-layer chromatography (silica gel 60, F-254),
with the use of methanol–acetone mixtures (2:1) as eluent,
were pure.
Benzothiazole based styryl dyes studied display several
specific properties that are similar to other styryl dyes
reported in literature [3–6]. All styryl dyes tested in this
study are charged and have a common structural feature,
namely, possess an electron donor and electron acceptor
moiety located on opposite sides of a styryl double bond. As
the NMR spectra have shown, the tested styryl dyes in the
ground state exist as trans isomer.
An important problem, to be addressed interpreting the
fluorescence spectroscopic behavior in organic solvents, is
the identification of the structure or structures of the dye in
various solvents. Fluorescing molecules with rigid ring struc-
tures (like anthracene) retain the same structure in ground and
excited state in any organic solvent. Fluorescing molecules
of type studied in this work possess the possibility of two
isomeric structures because of the flexibility about the styryl
bond.
where Φref is the fluorescence quantum yield of reference
Rhodamine B or Coumarin I) in ethanol, Adye and Aref
(
are the absorbances of the dye and reference samples at
their excitation wavelengths, Idye and Iref are the areas
arbitrary units of the corrected fluorescence spectra (plotted
in frequency scale) for the dyes and reference samples,
respectively (integrated areas of fluorescence spectrum).
1
The H NMR spectra were recorded with the use of
a Varian spectrometer Gemini 200 operating at 200 MHz.
Dimethylsulfoxide (DMSO) was used as the solvent and
tetramethylsilane (TMS) as internal standard.
Melting points (uncorrected) were determined on the
Bo e¨ thius apparatus.
The reduction potentials of hemicyanine dyes were mea-
sured by cyclic voltammetry. An electroanalitycal MTM sys-
tem model EA9C-4z (Cracow, Poland), equippedwitha small
volume cell, was used for the measurements. A 1 mm plat-
inum disc electrode was used as the working electrode, a
Pt wire constituted the counter electrode and an Ag–AgCl
electrode served as the reference electrode. The supporting
electrolyte was 0.1 M tetrabutylammonium perchlorate in dry
acetonitrile.
3
. Results and discussion
The structures of the 18 styryl benzothiazole (SH) dyes
synthesized (Scheme 2) have one type of electron acceptor
group (benzothiazole) but differ in a structure of phenyl moi-
ety substituent.
The hemicyanine iodides SH were synthesized by a two-
step reaction (Scheme 1).
Scheme 3 shows possible structures of the dyes under the
study.
The alkylation of the nitrogen atom of 2-methylbenzot-
hiazole was the first step of synthesis. Next, the 3-ethyl-
2-methylbenzothiazolium iodide was condensed with p-
substitued benzaldehyde via Knoevenagel reaction using
pyridine or piperidine as the catalyst. The condensation for
the entire series of N-alkyl substitued 2-methylbenzothiazole
with 18 different benzaldehydes was performed. The yield
of the reaction varied in the range of 20–84%. To our best
knowledge, the majority of the synthesized compounds have
not been described in literature, yet.
Scheme 2.
Scheme 3.

1
18
J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
Table 1
Characteristic of 3-ethyl-2-(p-substituted styryl)benzothiazolium iodides
◦
1
Dye
Melting point ( C)
226
Molecular formula
C26H28N2O3S2
Positions of signals H-NMR spectra, σ (ppm) and coupling constants J (Hz)
SH 1
1.392–1.462 (3H,CH3); 2.281 (3H, CH3); 3.114(6H, CH3); 4.815–4.851 (2H,
N
CH2); 7.585–7.662 (J = 15.4 Hz, 1H); 8.055–8.13 (J = 15 Hz, 1H); 6.824–6.867 (2H);
.085–7.126 (2H); 7.451–7.491 (2H); 7.676–7.788 (2H); 7.911–7.956 (2H); 8.158–8.322
2H)
1.049–1.190 (6H, CH3); 1.382–1.45 (3H, CH3); 3.405–3.522 (4H, CH2); 4.805–4.840
2H, CH2); 6.796–6.842 (2H); 7.535–7.610 (J = 15 Hz, 1H); 8.023–8.100 (J = 15.4 Hz,
H); 7.660–7.770 (2H); 7.776–7.931 (2H); 8.141–8.308 (2H)
0.896–0.968 (6H, CH3); 1.298–1.418 (8H, CH2); 1.452–1.554 (3H, CH3); 3.408–3.482
7
(
SH 2
SH 4
232
121
C21H25IN2S
C25H33ISN2
(
1
(
(
(
4H, CH2); 4.798–4.834 (2H, N CH2);7.535–7.610 (J=15 Hz, 1H); 8.032–8.107
J = 15 Hz, 1H); 6.785–6.830 (2H); 7.629–7.815 (1H); 7.882–7.925 (2H); 8.145–8.312
2H)
SH 7
230
C21H23IN2S
1.401–1.473 (3H, CH3); 2.481–2.571 (4H, CH2); 3.077 (4H, N-CH2); 4.740–4.836 (2H,
CH2); 7.702–7.777 (J = 15 Hz, 1H);8.031–8.105 (J = 14.8 Hz, 1H); 6.808–6.853 (2H);
.111–7.189 (1H); 7.540–7.686 (1H); 7.833–7.933 (1H); 8.137–8.214 (2H); 8.275–8.324
1H)
1.393–1.463 (3H, CH3); 1.626 (6H, CH2); 3.510 (4H, CH2); 4.798–4.868 (2H,
CH2); 7.613–7.690 (J = 15.4 Hz, 1H); 8.046–8.123 (J = 15.4 Hz, 1H); 7.035–7.078
2H); 7.653–7.831 (2H);7.944–8.899 (2H); 8.14–8.337 (2H)
1.373–1.442 (3H, CH3); 2.956 (4H, N CH2); 3.027–3.111 (4H, CH2); 3.620–3.703
N
7
(
SH 8
238
241
236
230
251
C22H25IN2S
C20H21IN2S
C21H23IN2S
C23H25IN2S
C25H25NO3S2
N
(
SH 9
(
(
3H, CH3); 4.766–4802 (2H, N CH2); 7.467–7.543 (J = 15.2 Hz, 1H); 7.966–8.042
J = 15.2 Hz, 1H); 6.564–6.607 (1H); 7.641–7.852 (4H); 8.063–8.287 (2H)
SH 10
SH 11
SH 12
1.384–1.452 (3H, CH3); 1.909 (2H, CH2); 2.735–2.795 (2H, CH2); 3.056 (3H,
CH3); 3.417–3.471 (2H, CH2); 4.785–4.822 (2H, CH2); 7.615–7.692 (J = 15.4 Hz, 1H);
7.969–7.8.044 (J = 15 Hz, 1H); 6.686–6.674 (1H); 7.653–7.805 (4H); 8.081–8.297 (2H)
1.369–1.439 (3H, CH3); 1.904–1.921 (4H, CH2); 2.508–2.764 (4H, CH2); 3.379–3.407
(4H, N CH2); 4.745–4.781 (2H, N CH2) 7.396-7.471 (J=15 Hz, 1H); 7.867–7.941
(J = 14.8 Hz, 1H); 7.523 (2H); 7.621–7.739 (2H);8.034–8.263 (2H)
1.433–1.504 (3H, CH3); 2.281–2.410 (3H, CH3);2.490–2.508 (3H, CH3); 4.919–5.017
(2H, N CH2); 7.457–7.378 (J = 15.8 Hz, 1H); 7.961–8.040 (J = 15.8 Hz, 1H); 7.086–7.126
(2H); 7.420–7.487 (2H); 7.584–7.658 (1H); 7.760–7.920 (2H); 7.974–8.015 (2H); 8.197
(1H); 8.276–8.468 (2H)
SH 13
239
C26H27NO3S2
1.186–1.262 (3H, CH3); 1.433–1.504 (3H, CH3); 2.280 (2H, CH2); 2.649–2.762
(3H, CH3); 4.957–4.993 (2H, N CH2); 7.405–7.479 (J = 14.8 Hz, 1H); 7.920–7.994
(J = 14.8 Hz, 1H); 7.083–7.126 (1H); 7.447–7.488 (2H); 7.761–7.884 (2H); 7.928–7.965
(1H); 8.036–8.044 (2H); 8.205–8.330 (2H); 8.423–8.469 (1H)
SH 14
SH 15
248
239
C30H27NO3S2
C25H24NO4S2
1.459–1.531 (3H, CH3); 2.280 (3H, CH3); 4.991–.027 (2H, N H2); 7.806–7.866
J = 12 Hz, 1H); 8.214–8.280 (J = 13.2 Hz, 1H); 7.082–7.124 (2H); 7.451–7.577 (4H);
.847–8.067 (6H); 8.146–8.172 (2H); 8.309–8.357 (2H); 8.443–8.488 (1H)
1.425–1.495 (3H, CH3); 2.276 (3H, CH3); 3.881 (3H, CH3); 4.925–4.961 (2H,
CH2); 7.078–7.155 (J = 15.4 Hz, 1H); 7.733–7.803 (J = 15.4 Hz, 1H); 7.082–7.120
4H); 7.455–7.495 (1H); 7.765–7.932 (2H); 8.056–8.100 (1H); 8.181–8.296 (2H);
.391–8.438 (2 H)
1.335–1.492 (6H, CH3); 2.280 (3H, CH3); 4.147–4.18 (2H, O CH2); 4.920–4.956
2H, N CH2); 7.827–7.924 (J = 15.6 Hz, 1H); 8.181–8.258 (1H); 7.086–7.488 (1H);
.769–7.819 (1H); 7.846–8.088 (1H); 8.291–8.390 (1H)
1.413–1.483 (3H, CH3); 4.889–4.925 (2H, N H2); 6.948–6.910 (1H); 7.711–7.786
J = 15 Hz, 1H); 7.949–8.126 (J = 15.4 Hz, 1H); 7.748–7.882 (2H); 7.786–7.846 (2H);
.822–7.882 (1H); 8.222–8.263 (1H); 8.410–8.370 (1H); 10.623 (1H)
1.435–1.507 (3H, CH3); 4.957–4.993 (2H, N CH2); 7.896–7.973 (J = 15.4 Hz, 1H);
(
7
N
(
8
SH 16
SH 17
SH 18
258
204
241
C26H26NO4S2
C17H16INOS
C17H15INFS
(
7
(
7
8
8
.231–8.309 (J = 15.6 Hz, 1H); 7.397–7.486 (2H); 7.802–7.858 (2H); 8.052–8.215 (2H);
.341–8.480 (2H)
SH 19
SH 20
SH 21
257
273
254
C17H15INBrS
C17H15IN2O2S
C19H18INOS
1.445–1.517 (3H, CH3); 4.696–5.005 (2H, N CH2); 7.781–7.859 (J = 15.6 Hz, 1H);
.049–8.128 (J = 15.8. 1H); 7.824–8.065 (4H); 8.128–8.211 (2H); 8.291–8.494 (2H)
1.468–1.538 (3H, CH3); 4.989–5.060 (2H, N CH2); 7.900–7.977 (J = 15.4 Hz, 1H);
.422–8.497 (J = 15 Hz, 1H); 7.815–7.970 (1H); 8.505–8.544 (1H); 8.288–8.399 (6H)
1.456–1.527 (3H, CH3); 2.323 (3H, CH3); 4.973–5.011 (2H, N CH2); 7.984–8.073
J = 15.8 Hz, 1H); 8.239–8.316 (J = 15.4 Hz, 1H); 7.334–7.387 (2H); 7.814–7.866 (1H);
8
8
(
8.073 (1H); 8.130–8.173 (2H); 8.353–8.486 (2H)
◦
The spectra were measured in DMSO-d 6 with TMS as internal standard at 25 C.

J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
119
It should be emphasized that the quinoid structure might
also be considered as intramolecular charge transfer (ICT)
state. In principle the trans and quinoid structures can be
treated as mesomeric resonance structures, with the positive
charge localized on different atoms, on the benzothiazole ni-
trogen in the case of trans isomer and on the nitrogen atom
of the substituent of styryl moiety for quinoid form [13].
This is possible only for SH1–SH11 dyes. For this group of
molecules, solvents may interact differently with the two iso-
mers making possible the presence of either trans or quinoid
orbothforms. SituationislesscomplexforSH12–SH21dyes.
Here, there is no possibility of the formation of quinoidal
structure of the dye. However, it is difficult to predict the role
of a solvent in stabilization of excited states for this group of
the dyes. Therefore, one has to collect experimental spectro-
scopic data in a variety of organic solvents to identify multiple
structures of the dye in the ground and excited states.
Figs. 1 and 2). If the broadening of the spectra is understood
as deriving from the population of the thermally available
conformers, then according to Rettig and coworkers [15], the
observed results can be taken as the evidence that some of
these conformers are non-emissive (demonstrated by a re-
duction of fluorescence band width). The twisting around the
molecule bonds gives this type of a conformer. In the case
of the twist of the olefinic double bond the obtained isomer
should be deactivated mainly by radiationless processes be-
cause the energy gap between its excited state and the ground
state is very small [15].
The absorption and emission spectra of SH dyes are sensi-
tive to the solvent which is attributed to the difference in the
dipole moment in the ground and excited states. The spectra
showed considerable spectral shifts in applied organic sol-
vents. The difference in energy between the absorbed and
emitted radiation is known as the Stokes shift [1]. Stokes
The main physicochemical data for the compounds under
the study are collected in Tables 2 and 3 .
The SH dyes absorb strongly in the visible region and
are characterized by a molar absorption coefficient of about
shift (νab − ν ), is one of the quantitative parameters which
fl
is useful to understand the origin of the variation of spectral
shift in organic solvents.
The Lippert equation is a simple and the most widely
used expression to explain the general solvent effects (due
to the dielectric constant (␧) and the refractive index (n) of
the solvent). The Stokes shift is dependent on the orienta-
tional polarizability of a solvent (ꢀf), which is linearly re-
lated to this. The Stokes shift values obtained for hemicya-
nine dyes tested are also given in Table 3. The highest value
4
−1
−1
3
× 10 M cm , which is in good agreement with those
given in the literature for other similar type of the dyes [14].
The high value of the molar absorption coefficient indicates
an extensive conjugation of ␲-electrons suggesting a planar
structure of the dye molecule in its ground state.
The absorption and fluorescence emission spectra of
SH1–SH16 were measured in several organic solvents (1,4-
dioxane, benzene, chloroform, ethyl acetate, tetrahydrofuran
THF, 1,2-dichloromethane, 1,2-dichloroethane, acetone, ace-
tonitrile, N,N-dimethylformamide DMF, dimethylsulfoxide
DMSO). The normalized absorption and emission spectra of
two selected dyes, in three different solvents, are presented
in Figs. 1 and 2 for illustration.
−
1
of Stokes shift (>2200 up to 7000 cm ) was observed in ace-
−1
tonitrile, DMFandDMSO, andthelowest(1000–1600 cm
in dichloromethane.
)
It is clearly visible that dyes possessing strong electron
donor substituents such as N,N-dialkylamino groups have
smaller Stokes shift than those possessing weak electron-
donor groups or strong electron acceptor groups. In this case
−1
The absorption and emission spectra of SHs are affected
by the organic solvents. In general, there is a blue shift ob-
served in absorption maxima of SHs tested with increasing
polarity of the solvent (Table 3). The compounds we study are
ionic dyes which exhibit a polar character in the ground state.
The solvent molecules are oriented in such a way as required
by the polar character of the host molecule. During the tran-
sition, which occurs within a very small time interval, only
the electrons have the time to change position. The excited
molecules, in which the electric dipole has been weakened
and has been reoriented, are now within a solvent cage that
is no longer adopted to the electronic requirements of the
excited molecule, since the solvent cage is suitable for the
electronic distribution in the ground state molecule. Thus, a
polar solvent creates a stabilizing solvent cage around these
ionic dyes in the ground state, but a destabilizing solvent cage
for the excited state. The transition energy increases with in-
creasing solvent polarity. An increase in solvent polarity re-
sults in hypsochromic shift of the charge transfer band, i.e.
to shorter wavelengths.
the Stokes shift varies in the range from 3000 to 7000 cm
.
A typical magnitude of Stokes shift in some symmetrical cya-
nines, due to a change in dipole moments, varies in the range
of 2500–5000 cm [14].
The application of the Lippert and Mataga theory helps to
understand better the observed properties [13]. According to
this theory, the specific solvent effects can be expressed by
two equations. Eq. (2) describes the behavior of the absorp-
tion band by the following expression:
−
1
ꢀ
ꢁ
2
2µꢀ (µꢀ − µꢀ ) ε − 1
g
e
g
1 n − 1
CT
vac
ab
∼
hc ν¯ = hc ν¯
−
−
ab
3
2
a
2ε + 1 2 2n + 1
0
(2)
wherehc ν¯ aC bT andhc ν¯ av ba c aretheenergiesrelatedtothespectral
position of the CT absorption maxima in solution and to the
value extrapolated to the gas-phase respectively; µꢀg and µꢀe
are the dipole moments of the solute in the ground and excited
state; a0 is the effective radius of the Onsager’s cavity [16],
ε is the static dielectric constant, and n is the refractive index
of the solvent.
The fluorescence emission spectra band are somewhat
narrower than the absorption spectra (see Table 3 and

1
20
J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
Table 2
Data of absorption, fluorescence spectra and electrochemical properties of tested dyes
Dye
R1
Absorption
Fluorescence
E0 (red) (V)
E0 (ox) (V)
−
1
−1
λ (nm)
ε (M cm )
λ (nm)
Φ (± 0.1)
SH 1
SH 2
521
536
30600
597
597
0.015
−0.97
−1.00
0.61
0.65
21400
0.009
SH 4
SH 7
SH 8
538
535
527
15600
41800
35700
601
600
603
0.013
0.006
0.008
−0.90
−1.07
−0.75
–
–
1.01
SH 9
554
549
34800
46300
611
606
0.003
0.004
−0.92
−0.98
–
–
SH 10
SH 11
SH 12
569
388
66000
22400
620
515
0.005
0.226
−1.08
−0.99
–
–
SH 13
SH 14
SH 15
SH 16
SH 17
SH 18
SH 19
SH 20
SH 21
388
31500
34600
37700
31900
24400
36000
26300
11000
24400
480
514
507
510
574
470
473
–
0.172
0.726
0.322
0.160
0.086
0.125
0.131
0.116
0.184
−1.02
−1.00
−1.11
−0.90
−1.03
−0.93
−1.07
−0.94
−0.97
–
403
–
413
–
416
0.98
–
426, 554
376
0.63
0.92
–
381
370
382
472
–
The presented data were recorded in ethyl acetate solution. The fluorescence quantum yield was determined in THF solution.
The second equation describes the solvatochromic effect
on the spectral position of the CT fluorescence spectra band
and is given by:
(i) the negative value of 2µꢀ (µꢀ − µꢀ ) term suggests that
g
e
g
µꢀ < µꢀ ; (ii) the positive value of 2µꢀ (µꢀ − µꢀ ) term allows
e
g
e
e
g
to conclude that for relaxing molecules µꢀ > µꢀ .
e
g
ꢀ
ꢁ
Combination of mentioned above relationships indicates
the following possibilities: (i) the dipole moment of the
ground state, form which molecule is excited, is different
to that from which excited molecule relaxes; (ii) the emis-
sion occurs from the molecule with dipole moment differ-
ent from that obtained after the Franck–Condon excitation;
(iii) the emission occurs from the molecule characterized
by the dipole moment different from that from which is
excited.
2
2
µꢀe(µꢀe − µꢀg) ε − 1
1 n − 1
CT
vac
∼
hc ν¯ fl = hc ν¯ fl
−
−
3
2
a
2ε + 1 2 2n + 1
0
(3)
where hc ν¯ flC T and hc ν¯ flv ac are the spectral positions of the
solvent equilibrated CT fluorescence maxima and the value
extrapolated to the gas-phase correspondingly. This type of
treatment of solvent effect for selected dyes is shown in Fig. 3.
The analysis of the trends presented for SH2 and SH12
It is known, that a linear dependence of Stokes shift and
ꢀf (polarizability parameter [13]) for all solvents validates
(Fig. 3) (similarly behave other dyes) allows to conclude that:

J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
121
Table 3
Steady state spectral properties of dyes SH solutions
−
1
−1
−1
−1
−1
Dye
Solvent
λab (nm)
ε (M cm
)
λfl (nm)
ꢀν (cm
)
FWHMab (cm
)
FWHMfl (cm
)
SH 1
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
529
533
542
521
527
549
546
522
522
525
525
48700
43900
71200
30600
53600
86400
75000
59100
62400
55100
54600
597
601
592
597
600
595
598
598
599
606
609
2150
2120
1560
2430
2310
1410
1600
2450
2450
2540
2630
2470
2500
2100
2590
2480
1840
1950
2490
2520
2570
2630
1240
1270
1190
1250
1220
1110
1140
1220
1240
1200
1230
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
SH 2
SH 4
SH 7
SH 8
SH 9
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
542
555
559
536
544
557
557
533
530
532
535
16600
43600
75200
21400
69100
84800
76600
59300
58200
54300
53000
602
603
598
597
601
606
601
607
600
606
609
1830
1440
1170
1900
1740
1460
1310
2290
2280
2280
2280
2100
1900
1660
2050
2160
1760
1640
2310
2300
2380
2400
1020
1000
1180
1010
1020
1110
1010
1250
960
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
1070
1090
DMSO
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
541
558
561
538
545
563
560
536
535
536
537
15600
94500
103300
15600
13840
116400
104200
81800
83800
75800
72300
602
605
603
601
605
605
606
604
603
609
611
1860
1390
1240
1950
1820
1240
1350
2100
2120
2230
2260
2010
1910
2180
2160
2110
1660
1690
2240
2230
2330
2330
1210
1100
1060
1140
1100
1040
1070
1100
1110
1140
1170
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
533
552
554
535
541
558
555
531
531
535
536
43600
93700
81000
41800
42100
95100
86100
62300
66400
59000
54300
597
603
598
600
601
600
610
602
599
605
603
2020
1530
1330
2030
1850
1270
1620
2210
2130
2170
2060
2270
2420
1800
2310
2080
1620
1720
2280
2340
2390
2330
1170
880
1070
950
930
950
1020
910
2060
970
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
930
1,4-Dioksan
Benzene
Chloroform
Ethyl acetate
THF
538
545
557
527
536
555
556
524
522
524
524
35400
34500
34600
35700
35200
34500
34500
35700
35700
35400
35400
597
605
601
603
608
605
605
605
605
610
613
1830
1820
1320
2400
2200
1500
1470
2540
2620
2690
2780
3340
4360
2010
2730
2260
2070
2080
2770
2750
2910
2920
1170
1230
1100
1210
1200
1150
1150
1220
1220
1250
1190
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
1,4-Dioksan
Benzene
Chloroform
Ethyl acetate
THF
563
571
571
554
558
14200
115600
59000
34800
43000
609
616
610
611
615
1330
1280
1130
1680
1650
1930
2460
1700
2340
2240
1150
1140
1080
1150
1150

1
22
J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
Table 3 (Continued )
−
1
−1
−1
−1
−1
FWHMfl (cm )
Dye
Solvent
Dichloromethane
,2-Dichloroetane
Acetone
Acetonitrile
DMF
λab (nm)
ε (M cm
)
λfl (nm)
ꢀν (cm
)
FWHMab (cm
)
573
570
548
546
550
552
70000
58000
51400
46900
41000
41200
614
612
613
613
618
623
1160
1220
1930
2000
1990
2070
1650
1670
2320
2310
2470
2440
1070
1180
1120
1120
1180
1250
1
DMSO
SH 10
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
556
569
569
549
554
569
567
544
543
545
547
12700
86000
89800
46300
58200
108600
85800
69900
69500
64700
62400
604
611
606
606
611
611
611
611
610
616
619
1420
1210
1070
1720
1680
1220
1270
2010
2030
2120
2130
1800
1720
1590
2230
2140
1200
1680
2270
2220
2340
2390
1160
1100
1030
1130
1100
1020
1030
1100
1090
1110
1120
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
SH 11
SH 12
SH 13
SH 14
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
577
584
587
569
573
587
585
566
565
567
568
30900
154700
100500
66000
34900
119900
94100
77400
76200
69800
61200
616
624
621
620
622
625
622
622
622
626
629
1100
1090
930
1670
1850
1350
1910
1730
1310
1450
1910
1900
1980
2030
1160
1080
1000
1090
1060
970
1130
1070
1050
1150
1100
1440
1370
1030
1020
1600
1630
1670
1700
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
392
395
400
388
391
402
401
387
387
390
386
18300
27500
26500
22400
21400
28800
28700
25500
10000
24100
17200
514
517
517
515
519
483
523
523
519
524
527
6060
5960
5640
6340
6310
4180
5820
6730
6560
6540
6930
3390
3030
3200
5870
4540
3250
3190
7250
8470
3530
4360
2930
2980
2620
2690
2920
2170
2530
2640
2760
2660
2650
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
392
391
400
388
393
403
402
387
381
389
391
26900
33300
33800
31500
30300
31900
36700
34500
17400
32800
33300
484
479
514
480
480
–
489
479
475
483
482
4840
4700
5550
4950
4620
–
4440
4980
5210
5000
4830
3260
3040
3100
3350
3200
2970
3100
5950
6720
3250
3290
3340
3610
2640
3090
3210
–
3090
3040
3580
3120
3180
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
408
416
418
403
406
421
420
402
393
404
406
29500
147500
37400
34600
34400
41300
40300
40500
16300
35900
38800
516
517
513
514
518
500
525
520
516
524
525
5140
4690
4450
5370
5330
3760
4760
5650
6080
5660
5590
3460
3830
3400
3520
3330
3320
3370
4560
8240
3420
8490
3160
3020
2660
2890
2750
1160
2590
2680
2960
2620
2650
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO

J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
123
Table 3 (Continued )
−
1
−1
−1
−1
−1
Dye
Solvent
λab (nm)
ε (M cm
)
λfl (nm)
ꢀν (cm
)
FWHMab (cm
)
FWHMfl (cm
)
SH 15
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
THF
420
423
429
413
419
432
430
414
418
414
416
32400
19000
37800
37700
34900
43700
42000
40300
35000
39300
37800
512
511
510
507
512
521
511
511
507
514
514
4270
4070
3690
4490
4320
3950
3690
4590
4180
4690
4580
3390
3250
3190
3420
3260
3100
3110
3300
3230
3350
3310
2700
2710
2970
2580
2480
2380
2420
2440
3070
2430
2420
Dichloromethane
1,2-Dichloroetane
Acetone
Acetonitrile
DMF
DMSO
ꢀν – Stokes shift, FWHMab – absorption full width at half maximum, FWHMfl – fluorescence full width at half maximum.
the Lippert–Mataga equation and confirms that the dye has
a single structure in the ground and excited states and the
slope depends upon the change in dipole moment upon exci-
tation [13]. This correlation predicts also that the Stokes shift
should linearly increase with the orientational polarizability.
In all tested dyes a linear correlation, with good coefficient
fit, between Stokes shift and ꢀf was observed for all solvents
used with expection of the halogenated solvents. The most
important deviation of the spectroscopic result is observed
in chloroform, dichloromethane and 1,2-dichloroethane. In
above-mentioned solvents the Stokes shift is very low when
compared to the solvents of similar polarity and viscosity.
The experimental results strongly suggest that there is a
specific interaction between SH dye and halogenated sol-
vents. The nature of this interaction and the structure of the
dye–solvent complex are not clear at present. Similar results
were obtained for styrylpyridinium dyes tested by Mishra
et al. [13].
Generally, we observe a negative solvatochromism in the
absorption (blue-shift in absorption with increasing polarity
of the solvent), a positive solvatochromism in the emission
(red-shift in the emission with increasing polarity of solvent).
The fluorescence quantum yield of eighteen SH dyes
is reported in Table 2. The fluorescence quantum yield of
tested dyes is in the range from 3 × 10 to 0.7. The highest
fluorescence quantum yield among the eighteen dyes studied
demonstrates SH14 (Table 2).
The electrochemical reduction and oxidation potentials
for the SH dyes were determined by cyclic voltammetry using
platinum electrode in acetonitrile solution. The one electron
oxidation of SH1–SH21 occurs in the range of 0.61–1.06 V.
The one electron reduction of these dyes occurs in the range
from −1.11 to −0.895 V. The electrochemical oxidation and
reduction peaks were quasi-reversible indicating that the
oxidized and reduced dye molecules are kinetically unstable
[4].
−
3
−6
Fig. 1. Absorption and emission spectra of 1 × 10 M solution of SH1 in: ethyl acetate (full line), acetone (dashed line) and DMF (dotted line). All the spectra
were measured in a 10 mm thick quartz-cuvette.

1
24
J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
Fig. 2. Absorption and emission spectra of 1 × 10− M solution of SH16 in: ethyl acetate (full line), acetone (dashed line) and DMF (dotted line). All the
6
spectra were measured in a 10 mm thick quartz-cuvette.
Fig. 3. Solvatochromic shift related to the CT absorption (open signs) and fluorescence maxima (solid signs) for SH2 and SH12, respectively, as a function of
solvent polarity.
4
. Conclusions
iazolium iodides as fluorescence probes for various pur-
poses. The absorption, fluorescence emission spectra of
studied hemicyanine dyes were investigated in organic
non-protic solvents. The intramolecular charge transfer from
side aromatic ring to the benzothiazole moiety takes place
at excitation of hemicyanine molecules. The electronic and
steric effects of substituents have their clear reflection in
the electronic spectra of several representatives of the series
studied, which is demonstrated by the increase of the Stokes
shifts.
The synthesis and the spectroscopic characterization
of newly synthesized hemicyanine dyes, 3-ethyl-(2-p-
substitued styryl)benzothiazole iodides (SH) are reported.
The simple method of synthesis of this class of the dyes opens
wide possibility for easy modification of their chemical struc-
tures and properties in the desired direction.
The results presented in this paper suggest good prospects
for the using of 3-ethyl-2-(p-substitued styryl)benzoth-

J. Kabatc et al. / Spectrochimica Acta Part A 62 (2005) 115–125
[5] L.M. Loew, L.L. Simpson, Biophys. J. 34 (1981) 353.
125
Acknowledgment
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7
717.
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[
[
This work was supported by the State Committee for Sci-
entific Research (KBN) (grant nos. 4 T09A 16024 and BW
[
(
2
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9] U. Narang, Ch.F. Zhao, J.D. Bhawalkar, F.V. Bright, P.N. Prasad, J.
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