Fast (hetero)aryl-benzothiazolium ethenes photoswitches activated by visible-light at room temperature

Article abstract of DOI:10.1016/j.dyepig.2015.02.015

A series of novel photochromic cationic stilbenes dyes bearing different aromatic systems were easily synthesized from (hetero)aromatic aldehydes. The photoisomerization of the double bond can be achieved through irradiation with ultraviolet or visible light, for few seconds, leading to the Z isomers that return much faster to the initial state than common stilbenes. Benzothiazolium dyes with electron donating substituents exhibited very fast thermal back reactions (<1 s). Pyrrole-benzothiazolium dye showed a higher lifetime of the Z isomer, in different solvents, including water (0.45-7.1 min). This cationic diarylethene can be switched between the E-Z states using visible light, in few minutes at room temperature, with a noticeable change in the visible spectrum.

Full text of DOI:10.1016/j.dyepig.2015.02.015

Dyes and Pigments 117 (2015) 163e169
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Fast (hetero)aryl-benzothiazolium ethenes photoswitches activated by
visible-light at room temperature
,
*
b
b, **
Paulo J. Coelho a , M. Cidalia R. Castro , M. Manuela M. Raposo
ꢀ
a
ꢀ
Centro de Química-Vila Real, Universidade de Tras-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal
b Centro de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel photochromic cationic stilbenes dyes bearing different aromatic systems were easily
synthesized from (hetero)aromatic aldehydes. The photoisomerization of the double bond can be ach-
ieved through irradiation with ultraviolet or visible light, for few seconds, leading to the Z isomers that
return much faster to the initial state than common stilbenes. Benzothiazolium dyes with electron
donating substituents exhibited very fast thermal back reactions (<1 s). Pyrrole-benzothiazolium dye
showed a higher lifetime of the Z isomer, in different solvents, including water (0.45e7.1 min). This
cationic diarylethene can be switched between the E-Z states using visible light, in few minutes at room
temperature, with a noticeable change in the visible spectrum.
Received 27 December 2014
Received in revised form
14 February 2015
Accepted 17 February 2015
Available online 26 February 2015
Keywords:
Diarylethenes
Photochromism
cisetrans isomerization
Molecular switches
Pyrrole
© 2015 Elsevier Ltd. All rights reserved.
Benzothiazole
1. Introduction
state. In fact it acts as versatile reagent able to reversibly induce
molecular changes [3].
Molecular switches are dynamic molecules that can be revers-
ibly shifted between two or more states through an external stimuli
such as pH, temperature, electrical current or light [1]. The
commonly used pH indicators are an example of such molecules
which change their structures and colours, upon changes in the
environmental pH. Photochromic molecular switches use light to
swap between two states which differ on the molecular structure
and on the maximum absorption wavelength or absorbance value
[2]. Schematically light promotes a reversible chemical reaction
between two isomeric forms with different absorption spectra.
Usually one or both forms absorb in the visible region.
The use of light to promote these reversible changes has several
advantages: it is an external stimulus which can be switched on and
off easily and quickly; allows the selective activation of a localized
area of a material without any direct contact with it; it can be used
simultaneously for the molecules activation and for monitoring its
There are many different molecular systems that exhibit pho-
toswitchable properties, involving diverse chemical reactions. In
naphthopyrans and spiropyrans the UV light promotes the breaking
of a CeO bond and opening of the pyran ring with formation of
planar conjugated species that absorb intensively in the visible
region [4]. In azobenzenes the switching is associated with the E-Z
photoisomerization of the N]N bond [5] and in salicylic Schiff
bases there is an intramolecular proton transfer [6]. In all these
molecules a stable isomer is photochemically converted into a less
stable species which can revert thermally to the initial state e T-
type photochromism. Diaryl cyclopentenes are a different class of
photochromic molecules where both forms are thermally stable
and thus both “forward and back” steps are promoted photo-
chemically although with different wavelengths [7]. Due to the high
thermal stability of these molecules and the high quantum yield of
both steps they have been intensively studied for application as
optical memories and writing systems [8].
Recently some photoswitches have been studied for biological
applications related to the photocontrol of molecular processes
[9]. In particular, different photochromic molecules have been
attached to macromolecules and the isomerization of the light-
* Corresponding author. Tel.: þ351 259 350284; fax: þ351 259 350480.
** Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382.
E-mail addresses: pcoelho@utad.pt (P.J. Coelho), mfox@quimica.uminho.pt
(M.M.M. Raposo).
http://dx.doi.org/10.1016/j.dyepig.2015.02.015
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

164
P.J. Coelho et al. / Dyes and Pigments 117 (2015) 163e169
active photochromic moiety used to induce conformational
changes in the macromolecule [10]. However, since UV light is
harmful to biological environments, visible-light activated
photochromic molecules have gained an enhanced importance
[11]. Besides safe, visible light, especially of longer wavelength has
a deeper tissue penetration than UV light, which is important for
in vivo applications. Azobenzenes have received considerable
attention since the E-Z photoisomerization of the N]N double
bond is very easy to perform using visible light and it is a very
clean reaction [9b,12].
Stilbenes (diphenylethenes) can also undergo E-Z photo-
isomerization. UV or visible irradiation of the thermodynamically
more stable trans-isomer produces the cis-isomer that returns to
the initial form either thermally of through irradiation with light of
a different wavelength. Unsubstituted diphenylethene absorbs only
in the UV region but the introduction of substituents can promote a
bathochromic shift of the lmax towards longer wavelengths
allowing the activation using visible light. However, contrary to
2. Results and discussion
2.1. Synthesis of diarylethene dyes 3aef
Diarylethenes bearing positive charged benzothiazolium group
linked to aromatic (phenyl, naphthyl) 3aeb and heteroaromatic
(pyrrole, thiophene, (benzo)indole) systems 3cef were synthesised
through condensation of 2,3-dimethylbenzo[d]thiazol-3-ium io-
dide 1 with aldehydes 2aef, under reflux in ethanol during
6 h (Scheme 1). After this time all compounds precipitated from the
reaction mixture and were washed with cold ethanol to give the
pure diarylethene derivatives 3aef as coloured (orange to dark
purple) dyes in fair to good yields (13e62 %) (Table 1).
The structures of diarylethenes 3aef were unambiguously
confirmed by their analytical and spectral data. The most charac-
teristic signals in the 1H NMR spectra of this family of compounds
were those corresponding to the CH]CH ethene protons at
7.51e8.06 (Hb) and 7.95e8.81 (Ha) ppm with coupling constants
between 14.4 and 16.0 Hz indicating their E conformation, and the
NeCH3 protons of the benzothiazolium ring which resonate be-
tween 3.96 and 4.37 ppm. The NeH of the benzoindole heterocyclic
system of dye 3e was found at 13.28 ppm. Additionally, compounds
3bed exhibit one singlet assigned to the dimethylamino group
(3.06e3.24 ppm, 6H) and dye 3f display one singlet due to the
NeCH3 protons of the pyrrole system (3.89 ppm, 3H).
azobenzenes, there is a competing reaction involving a 6
p elec-
trocyclization of the Z-isomer to “close-ring” form (dihy-
a
drophenanthrene) which is quite prone to oxidation by molecular
oxygen leading irreversibly to phenanthrene [13]. Nevertheless,
this reaction can be prevented placing substituents at the ortho
position of the phenyl rings or using heteroaromatic ring systems
[14].
The photoinduced transecis isomerization of stilbenes, exten-
sively studied in the last decades [15], is typically a slow process
even though the speed of the thermal back process (Z-E) is variable
(from minutes to several days) depending on the structure of the
compound, temperature, solvent, pH, etc. Consistently the activa-
tion energy for the isomerization in stilbenes is around 35e42 kcal/
mol, higher than azobenzenes (21e24 kcal/mol) or aromatic Schiff
bases (15 kcal/mol) [16]. Although much research has been per-
formed on simple stilbenes, far less attention has been devoted to
diarylethenes bearing heteroaromatic rings even though some
interesting results have been achieved: the photoisomerization in
small ring heteroaromatic diarylethenes is faster than with the
parent diphenylethenes; the use of azotated heterocyclic rings al-
lows the introduction of a permanent positive charge on the ni-
trogen atom which generates a powerful electron withdrawing
effect that leads to even faster switching speeds and significant
bathochromic shift of the absorption bands allowing the activation
of these systems with visible light. On the other hand positive
charged diarylethenes are water soluble which is important for
biological applications [9e11].
2.2. Electronic structure analysis
The 1H NMR chemical shifts of diarylethene derivatives 3aef,
bearing different aromatic or heterocyclic donor moieties, show
significant changes that can be accounted to a charge separation in
the ground state (Table 2). The chemical shifts of the aromatic
protons of the benzothiazolium ring (H4eH8) in compounds 3bed,
bearing strong electron donor groups (dimethylaminoaryl) linked
to the vinyl bridge, exhibit signals that are upfield relative to the
unsubstituted phenyl derivative 3a indicating charge transfer from
the donor to the acceptor group. These data confirms the pushepull
character of these diarylethene derivatives with a significant
intramolecular charge transfer from the functionalized donor aryl
(phenyl, naphthyl) or heteroaryl (pirrolyl, thienyl and benzoindolyl)
moieties to the methylbenzothiazolium acceptor group.
2.3. UVevisible and photochromic properties
This series of (hetero)aromatic ethene dyes includes six dyes
Recently the photochemistry of several styrylbenzothiazoles
dyes has been described. These compounds can be activated by
ultraviolet or visible light but the speed of the photo-
isomerization is too low: styrylbenzothiazoles linked to phenyl,
naphthyl or phenanthryl rings take more than 90 min to reach to
the photostationary state which is thermally stable at room
temperature for many days [17]. Prior studies, have shown that
the substitution of the benzothiazole by a benzothiazolium
cation can increase the speed of the back thermal cisetrans and
provide a strong bathochromic shift leading to faster E-Z pho-
toswitches although the lifetime of the cis-isomer still ranged
from minutes to hours depending on the solvent [18]. In order to
study the possibility to prepare faster diaryl ethene dyes, acti-
vated by visible light and soluble in aqueous media, we prepared
a series of new diarylethenes bearing positive charged benzo-
thiazolium group linked to functionalized aromatic and hetero-
aromatic systems, such as phenyl, naphthyl, pyrrole, thiophene
and benzoindole (Scheme 1) and evaluated their photochromic
properties under continuous visible irradiation at room
temperature.
(3aef) bearing aromatic (phenyl, p-dimethylaminophenyl, 4-
dimethylaminonaphthyl)
or
heteroaromatic
(4-
dimethylaminothienyl, 1-methylpyrrolyl and benzoindolyl) donor
moieties linked to the methylbenzothiazolium acceptor group
through a vinyl bridge (Scheme 1).
2-Styrylbenzothiazole has the main absorption band at 337 nm
in CH3CN [16]. The introduction of a positive charge on the nitrogen
atom of the benzothiazole generates 2-styrylbenzothiazolium 3a
which exhibits a significant bathochromic shift of the absorption
band to 372 nm with some absorption around 400 nm and there-
fore concentrated solutions of this compound are coloured. Further
introduction of an electron donor p-dimethyl group in the phenyl
ring shifts the lmax to 519 nm and thus diluted solutions of ben-
zothiazolium 3b are deeply red (in the web version) (Fig. 1). Ben-
zothiazolium 3c with a 4-dimethylaminonaphthyl group absorbs
also at longer wavelengths with a very large band that extends from
400 to 600 nm with two maxima at 519 and 560 nm due to more
extensive electronic delocalization.
Electron-rich heterocycles such as pyrrole and thiophene can be
used in heterocyclic systems not only as
p-bridges but also as

P.J. Coelho et al. / Dyes and Pigments 117 (2015) 163e169
165
Scheme 1. Synthesis of aryl- and heteoaryl-benzothiazolium dyes 3aef.
auxiliary donating groups. In fact, changing the phenyl ring by
pirrolyl (3f, lmax ¼ 463 nm) or by 5-dimethylaminothienyl (3d,
lmax ¼ 562 nm) induced very high bathochromic shifts: 91 nm (3f)
or 190 nm (3d) due to their higher electron donor ability. Therefore,
compound 3d exhibit a violet colour in solution (Fig. 1). Changing
the phenyl ring (3a) by benzoindolyl (3e, lmax ¼ 447 nm) led also to
considerable shift (77 nm) of the maximum absorption band
possibly due to different effects such as more extensive electron
delocalization as well as higher electron donor ability of this groups
compared to phenyl ring.
The major absorption band of these dyes is due to the
pep*
transition of the E isomer, although a small amount of the Z-isomer
is probably also present in equilibrium. Heating the solutions of all
compounds to 60 ꢀC led to a decrease in the intensity of this band,
without the appearance of new bands or an isosbestic point, which
suggests a small thermal conversion of part of the trans isomer into
the cis that probably has the same lmax with a smaller extinction
coefficient or absorbs at a different wavelength with a much
smaller extinction coefficient. The lower absorbance of the Z-iso-
mer is due to the loss of planarity owing to a considerable increase
of the steric hindrance upon double bond rotation (Fig. 2). This new
state is stable at 60 ꢀC and upon cooling to 20 ꢀC the absorbance
returned to the initial value indicating a thermal equilibrium be-
tween two species with different absorption spectra [19].
Table 1
Yields, maximum wavelength of absorption and extinction coefficient for diary-
lethene dyes 3aef in acetonitrile.
Dye
h
(%)
lmax (nm)
ε (Mꢁ1 cmꢁ1
)
3a
3b
3c
3d
3e
3f
33
35
52
13
18
62
372
519
519
562
447
463
4.3 ꢂ 104
4.6 ꢂ 104
1.2 ꢂ 104
2.0 ꢂ 105
3.2 ꢂ 104
2.9 ꢂ 104
The transecis isomerization can also be achieved by a photo-
chemical process at room temperature. Styryl benzothiazolium 3a
Table 2
1H chemical shifts (ppm) of compounds 3aef in deuterated DMSO.
Dye NþCH3 C]CHa C]CHb
4-H
5-H
6-H 7-H
3a
3b
3c
3d
3e
3f
4.37
4.22
4.34
3.96
4.31
4.21
8.22
8.06
8.81
8.06
8.52
7.95
8.06
7.62
7.96
8.44
8.28
8.49e8.51 7.70
7.89
7.78
7.81 8.25e8.28
7.60 8.07e8.09
7.60 8.38
7.82e7.83 8.08e8.10 7.63
7.69
7.51
7.49 7.82e7.83
8.41e8.43 7.63e7.73 7.54 8.04e8.06
8.30 7.78 7.67 8.08e8.11
Fig. 1. Samples of dyes 3aef in acetonitrile solution.

166
P.J. Coelho et al. / Dyes and Pigments 117 (2015) 163e169
Fig. 4. UVeVis/Dark cycles for benzothiazolium 3a in acetonitrile measured at lmax.
experimental conditions no competitive irreversible reactions are
occurring (Fig. 4).
Since compounds 3bef have the main absorption band in the
visible region they can be activated with visible light. Continuous
visible light irradiation of 2.5 ꢂ 10ꢁ5 M acetonitrile solutions of
Fig. 2. UVeVis spectra of dye 3e at 20 ꢀC and 60 ꢀC.
compounds 3bee for 30 s using a long pass filter (l > 420 nm) led to
a very small decrease of the absorption at lmax (<1%) indicating a
very small E-Z conversion. The low efficiency of the photo-
isomerization is probably due to an overlapping of the absorption
bands of both E and Z isomers in the visible region, being thus both
activated by visible light at the same time using the same wave-
length, and is also indicative of a very fast thermal cisetrans process
rather than some structural hindrance to the photoisomerization.
In fact, under these continuous irradiation conditions, the decrease
in the absorbance at lmax is inversely proportional to the rate of the
thermal cisetrans conversion. The continuous irradiation technique
is suitable for slow systems with lifetime of the thermally unstable
form higher than 1 s. For these fast switches, bearing electron
donating substituents in one of the aromatic ring, the flash
photolysis technique is more appropriate. We have noticed that azo
compounds and imines with heteroaromatic rings exhibit also
faster thermal double bond isomerization than phenyl derivatives
[20].
Benzothiazolium dye 3f linked to a pyrrole ring showed more
interesting properties. Visible irradiation of the yellow solution of
this dye in acetonitrile, for 30 s, led to a significant spectral change
characterized by a decrease of the band at 463 nm and a small
increase at 288 nm due to a partial conversion (around 35%) of the
trans isomer into the cis (Fig. 5). However, the solution maintains its
yellow colouration.
absorbs strongly in the UV region with lmax ¼ 372 nm and can be
activated by UV light. Fig. 3 illustrates the effect of continuous
UVevis irradiation (ozone free xenon lamp) on the absorption
spectra of this compound at 20 ꢀC. The absorbance at 372 nm
gradually decreased, reaching a photo-stationary state after 30 s.
This was accompanied by a very small enhancement at 288 nm,
with an isobestic point at 319 nm, due to the formation of the Z-
isomer. The absorbance variation at 372 nm indicates that
approximately 50% of the E-isomer was converted to the Z-isomer.
After illumination is ceased the system reverts back to its initial
state in 5 min. The kinetics of the thermal Z-E isomerization can be
determined recording the absorbance variation at lmax in time. The
colouration curb (Fig. 4) fits a first order kinetics and can be
described by a growing mono-exponential equation which suggests
that we have two species in equilibrium. The thermal rate constant,
k ¼ 0.54 minꢁ1 indicates an half-life time of 1.3 min for the cis
isomer. This procedure was repeated several times and the
behaviour was reproducible indicating that under these
The thermal relaxation time of the cis isomer of benzothiazo-
lium 3f in acetonitrile at 20 ꢀC was calculated from the colouration
curb (Fig. 6). The rate constant, 0.20 minꢁ1 indicates that the half-
time life of this photoisomer is 3.5 min. The activation energy of the
thermal relaxation, Ea ¼ 17.6 kcal, was calculated from the variation
of the thermal rate constant (kZ-E) with temperature and is lower
than that observed with common stilbenes being similar to the one
observed for 1-naphthyl-2-(2-benzothiazolium)ethene [17] con-
firming that the presence of a benzothiazolium facilitates the
isomerization of the double bond leading to faster photoswitches.
The nature of the solvent has some influence on the photo-
chromic behaviour of these dyes (Table 3). It was found that less
polar solvents like dichloromethane or chloroform led to a bath-
ochromic shift of the lmax. Although the percentage of the E-Z
conversion was near the same in all the solvents tested, a clear
Fig. 3. UVeVis absorption spectra of styryl benzothiazolium 3a in acetonitrile before
and after UVeVis irradiation (2.5 ꢂ 10ꢁ5 M).

P.J. Coelho et al. / Dyes and Pigments 117 (2015) 163e169
167
Fig. 5. Evolution of the UVeVis spectrum of pyrrole benzothiazolium 3f upon visible
Fig. 7. Photoisomerization of benzothiazolium 3f at 465 nm at 20 ꢀC in acetone.
irradiation.
3. Conclusion
A set of diaryl ethenes bearing a benzothiazolium acceptor
moiety was synthesized, in fair to good yields, using available re-
agents and simple experimental procedures. The photo-
isomerization of the double bond can be achieved through
irradiation with UV or visible light although for the diaryl ethenes
bearing electron donating substituents the thermal back reaction
was too fast to be measured under continuous irradiation condi-
tions. However, the lifetime of the cis isomer of the benzothiazo-
lium dye linked to a pyrrole ring is considerable higher (3.5 min)
and thus this diarylethene can be switched between both trans and
cis states, at room temperature, with a noticeable change in the
visible spectrum. The speed of the thermal back cisetrans reaction
is very sensitive to the solvent's nature being faster in less polar
solvents like ethyl acetate or acetone. The photoisomerization can
also be performed in water although the back reaction was slower.
Fig. 6. Photoisomerization of benzothiazolium 3f (463 nm) at 20 ꢀC in acetonitrile.
4. Experimental section
acceleration of the thermal cisetrans isomerization rate was
observed in less polar solvents. In acetone the switching between
the two isomers is quite fast allowing to perform a complete Vis
irradiation/dark cycle in 4 min at room temperature (Fig. 7). The
photoisomerization of dyes 3aef can also be performed in water
with similar rate constants which is important for biological
applications.
4.1. Materials
Benzaldehyde, 4-(dimethylamino)benzaldehyde, 4-(dimethyla-
mino)-1-naphthaldehyde, 1H-benzo[g]indole-3-carbaldehyde, 1-
methyl-1H-pyrrole-2-carbaldehyde, 2-methylbenzothiazole, 5-
(dimethylamino)thiophene-2-carbaldehyde and methyl iodide
Table 3
Spectrokinetic properties under continuous visible irradiation: maximum wavelength of absorption (lmax), maximum absorbance (Amax), absorbance variation (
bleaching rate (kD) and half-time life (t1/2) of diarylethenes 3a and 3f in different solvents.
DAbs), thermal
Dye
Solvent
lmax (nm)
Amax
D
Abs
kD (minꢁ1
)
t1/2 (min)
Acetonitrile
Water
372
370
0.54
0.90
0.24 (44%)
0.56 (62%)
0.54
0.38
1.3
1.8
S
N
I
3a
3f
AcOEt
CHCl3
CH2Cl2
Acetone
Acetonitrile
Ethanol
Methanol
Water
468
480
479
465
463
469
466
459
0.34
1.03
0.98
0.99
0.79
0.82
0.74
0.74
0.10 (33%)
0.32 (31%)
0.33 (32%)
0.32 (32%)
0.28 (35%)
0.27 (33%)
0.23 (31%)
0.26 (36%)
1.54
0.75
0.13
1.1
0.20
0.13
0.12
0.097
0.45
0.92
5.3
0.63
3.5
5.3
5.8
7.1
S
N
I
N

168
P.J. Coelho et al. / Dyes and Pigments 117 (2015) 163e169
were purchased from Aldrich, Acros or Fluka and used as received.
TLC analyses were carried out on 0.25 mm thick precoated silica
plates (Merck Fertigplatten Kieselgel 60 F254) and spots were
visualized under UV light.
7.96 (d, 1H, J ¼ 15.2 Hz, C]CHb) 8.18e8.22 (m, 2H, 50- and 80-H),
8.38 (dd, 1H, J ¼ 8.0 and J ¼ 0.4 Hz, 7-H), 8.44 (dd, 1H, J ¼ 8.4 Hz, 20-
H), 8.49e8.51 (m, 1H, 4-H), 8.81 (d, 1H, J ¼ 15.2 Hz, C]CHa). 13C
NMR (DMSO-d6)
d 44.3 (2C), 45.8, 111.5, 112.8, 116.5, 122.9, 123.6,
123.9, 125.3, 125.5, 126.5, 127.5, 127.7, 128.0, 129.1, 129.2, 133.0,
4.2. Instruments
142.0, 144.6, 155.7, 171.4. lmax
¼
519 nm (Acetonitrile,
2726, 1557, 1498, 1339, 1299,
ε ¼ 1.2 ꢂ 104 Mꢁ1 cmꢁ1). IR (Nujol):
n
NMR spectra were obtained on a Varian Unity Plus Spectrometer
1278, 1245, 1198, 1128, 1065, 1021 cmꢁ1. MS (ESI) m/z (%) ¼ 345
([M]þ, 100), 329 (5). HMRS: m/z (ESI) for C22H21N2S; calcd:
345.14200; found: 345.14183.
at an operating frequency of 300 MHz (1H NMR) and 75.4 MHz (13
C
NMR) or a Bruker Avance III 400 at an operating frequency of
400 MHz (1H NMR) and 100.6 MHz (13C NMR) using the solvent
peak as internal reference at 25 ꢀC. All chemical shifts are given in
ppm using dH Me4Si ¼ 0 ppm as reference and J values are given in
Hz. Assignments were made by comparison of chemical shifts, peak
multiplicities and J values and were supported by spin decoupling-
double resonance and bidimensional heteronuclear HMBC and
HMQC correlation techniques. IR spectra were determined on a
BOMEM MB 104 spectrophotometer using nujol. Mass spectrom-
etry analyses were performed at the “C.A.C.T.I. e Unidad de
Espectrometria de Masas” at the University of Vigo, Spain. All
melting points were measured on a Gallenkamp melting point
apparatus and are uncorrected.
4.3.2.4. 2-((E)-2-[5-N,N-Dimethylthiophen]vinyl)-3-methyl-1,3-
benzothiazol-3-ium iodide 3d. Purple solid (13%). Mp 264e265 ꢀC.
1H NMR (DMSO-d6) 3.24 (s, 6H, N(CH3)2), 3.96 (s, 3H, NþCH3), 6.50
d
(d, 1H, J ¼ 4.8 Hz, 40-H), 6.60e6.64 (m, 1H, 30-H), 7.49 (dt, 1H, J ¼ 7.4
and J ¼ 1.0 Hz, 6-H), 7.63 (dt, 1H, J ¼ 7.4 and J ¼ 1.0 Hz, 5-H)
7.82e7.83 (m, 2H, C]CHb 7-H), 8.06 (d, 1H, J ¼ 14.4 Hz, C]CHa),
,
8.08e8.10 (m, 1H, 4-H). 13C NMR (DMSO-d6)
d 34.2, 42.6 (2C), 98.3,
109.0, 114.3, 123.2, 123.9, 125.4, 125.9, 128.2, 141.5, 141.8, 145.2,
167.5, 170.7. lmax ¼ 562 nm (Acetonitrile, ε ¼ 2.0 ꢂ 105 Mꢁ1 cmꢁ1).
IR (Nujol):
n 2726, 1597, 1553, 1316, 1278, 1251, 1201, 1167, 1148,
1080,1028 cmꢁ1. MS (ESI) m/z (%) ¼ 301 ([M]þ,100), 268 (5). HMRS:
m/z (ESI) for C16H17N2S2; calcd: 301.08277; found: 301.08264.
4.3. Synthesis
4.3.2.5. 2-((E)-2-[1H-Benzo[g]indole]vinyl)-3-methyl-1,3-
4.3.1. Synthesis of 2,3-dimethylbenzo[d]thiazol-3-ium iodide 1
To a solution of 2-methylbenzothiazole (1.0 mmol) in acetoni-
trile (10 ml) was added methyl iodide (6.0 mmol). The solution was
stirred at reflux for 6 h. The precipitate was filtered and washed
with acetonitrile to give the pure white salt.
benzothiazol-3-ium iodide 3e. Dark orange solid (18%). Mp
295e296 ꢀC. 1H NMR (DMSO-d6) 4.31 (s, 3H, NþCH3), 7.54 (dt, 1H,
d
J ¼ 6.8 and J ¼ 1.2 Hz, 6-H), 7.63e7.73 (m, 2H, 5- and 70-H), 7.69 (d,
1H, J ¼ 15.4 Hz, C]CHb) 7.78e7.83 (m, 2H, 60- and 80-H), 8.04e8.06
(m, 1H, 7-H), 8.16 (d, 1H, J ¼ 8.6 Hz, 50-H), 8.34 (dd, 1H, J ¼ 8.0 and
J ¼ 0.4 Hz, 90-H), 8.35 (d, 1H, J ¼ 8.6 Hz, 40-H), 8.41e8.43 (m, 1H, 4-
H), 8.52 (d, 1H, J ¼ 15.4 Hz, C]CHa), 8.55 (d, 1H, J ¼ 3.2 Hz, 20-H),
4.3.2. General procedure for the synthesis of ethenes 3aef
A solution of aldehyde 2a-f (1 mmol), benzothiazolium salt 1
(1 mmol) and piperidine (1 drop) in ethanol (10 ml) was heated at
reflux for 6 h. After this time the solid that precipitated from the
reaction mixture was filtered and washed with cold ethanol to give
the pure products.
13.28 (br s, 1H, NH). 13C NMR (DMSO-d6)
d 35.7, 107.0, 115.5, 116.1,
119.8, 120.7, 121.4, 121.8, 122.7, 123.9, 125.0, 126.4, 126.8, 127.6,
128.5, 128.9, 130.5, 132.6, 133.5, 141.9, 143.9, 172.1. lmax ¼ 447 nm
(Acetonitrile, ε ¼ 3.2 ꢂ 104 Mꢁ1 cmꢁ1). IR (Nujol):
n 2725, 1583,
1321, 1286, 1267, 1236, 1194, 1125, 1097 cmꢁ1. MS (ESI) m/z
(%) ¼ 341 ([M]þ, 100), 325 (4). HMRS: m/z (ESI) for C22H17N2S;
calcd: 341.11070; found: 341.11044.
4.3.2.1. 2-[(E)-2-Phenylvinyl]-3-methyl-1,3-benzothiazol-3-ium io-
dide 3a. Brown solid (33%). Mp 208e210 ꢀC. 1H NMR (DMSO-d6)
d
4.37 (s, 3H, NþCH3); 7.55e7.58 (m, 3H, 30-, 40- and 50-H), 7.81 (dt,
4. 3. 2. 6. 2-((E)-2-(1-Methyl-1H-pyrrol-20-yl)vinyl)-3-
1H, J ¼ 7.4 and J ¼ 1.2 Hz, 6-H), 7.89 (dt, 1H, J ¼ 7.4 and J ¼ 1.2 Hz, 5-
H), 8.05e8.07 (m, 2H, 20- and 60-H), 8.06 (d, 1H, J ¼ 16.0 Hz, C]CHb)
8.22 (d, 1H, J ¼ 16.0 Hz, C]CHa), 8.25e8.28 (m, 1H, 7-H), 8.44 (dd,
1H, J ¼ 7.6 and J ¼ 1.2 Hz, 4-H). lmax ¼ 372 nm (Acetonitrile,
methylbenzothiazolium iodide 3f. Orange solid (62%). Mp
238e239 ꢀC. 1H NMR (DMSO-d6)
s 3.89 (s, 3H, CH3); 4.21 (s, 3H,
N þ CH3); 6.37e6.39 (m, 1H, 40-H), 7.37e7.38 (m, 1H, 30-H), 7.45 (dd,
1H, J ¼ 4.2 and J ¼ 1.5 Hz, 50-H) 7.51 (d, 1H, J ¼ 15.2 Hz, C]CHb), 7.67
(dt, 1H, J ¼ 7.7 and J ¼ 1.1 Hz, 6-H), 7.78 (dt, 1H, J ¼ 7.7 and J ¼ 1.1 Hz,
5-H), 7.95 (d, 1H, J ¼ 15.2 Hz, C]CHa), 8.08e8.11 (m, 1H, 7-H), 8.30
ε ¼ 4.3 ꢂ 104 Mꢁ1 cmꢁ1). IR (Nujol):
n 2727, 1988, 1904, 1805, 1607,
1590, 1573, 1504, 1273, 1247, 1209, 1159, 1140 cmꢁ1
.
(dd,1H, J ¼ 8.1 and J ¼ 0.9 Hz, 4-H). 13C NMR (DMSO-d6)
s 34.1, 35.6,
4.3.2.2. 2-((E)-2-[4-(Dimethylamino)phenyl]vinyl)-3-methyl-1,3-
106.1, 111.7, 115.9, 118.0, 123.8, 126.8, 127.5, 128.9, 130.4, 132.9, 136.5,
benzothiazol-3-ium iodide 3b. Dark brown solid (35%). Mp
141.9, 171.0. lmax
¼
466 nm (Acetonitrile,
ε
¼
463
2725, 1599, 1583, 1528, 1345,
1301, 1274, 1237, 1149, 1062 cmꢁ1
. Elemental analysis for
252e254 ꢀC. 1H NMR (DMSO-d6)
d
3.10 (s, 6H, N(CH3)2); 4.22 (s, 3H,
ε ¼ 2.9 ꢂ 104 Mꢁ1 cmꢁ1) IR (Nujol):
n
NþCH3); 6.63 (d, 2H, J ¼ 8.8 Hz, 30- and 50-H), 7.62 (d,1H, J ¼ 15.4 Hz,
C]CHb), 7.67 (dt, 1H, J ¼ 7.8 and J ¼ 0.9 Hz, 6-H), 7.78 (dt, 1H, J ¼ 7.8
and J ¼ 0.9 Hz, 5-H), 7.90 (d, 2H, J ¼ 8.8 Hz, 20- and 60-H), 8.06 (d,1H,
J ¼ 15.4 Hz, C]CHa), 8.07e8.09 (m, 1H, 7-H), 8.28 (dd, 1H, J ¼ 8.0
C15H15IN2S: Found: C, 47.39; H, 3.92; N, 7.44; S, 8.17. % CHNS re-
quires: C, 47.13; H, 3.96; N, 7.33; S, 8.39.
and
J
¼
0.9 Hz, 4-H). lmax
¼
519 nm (Acetonitrile,
n 2757, 2675, 2475, 2347, 1651,
4.4. Photochromic measurements
ε ¼ 4.6 ꢂ 104 Mꢁ1 cmꢁ1). IR (Nujol):
1611, 1564, 1525, 1330, 1294, 1269, 1162, 1067, 1033, cmꢁ1
.
The photochromic properties of the materials were studied by
UVeVis using a CARY 50 Varian spectrophotometer equipped with
a 50 W Ozone free Xenon lamp (6255 Oriel Instruments). The light
from the UV lamp was filtered using a water filter (61945 Oriel
Instruments) and then carried to the spectrophotometer holder at
the right angle to the monitoring beam using an optical fibre sys-
tem. For irradiation in the visible region a long-pass filter (Schott
4.3.2.3. 2-((E)-2-[4-N,N-Dimethylnaphthalen]vinyl)-3-methyl-1,3-
benzothiazol-3-ium iodide 3c. Dark violet solid (52%). Mp
229e230 ꢀC. 1H NMR (DMSO-d6)
d 3.04 (s, 6H N(CH3)2), 4.34 (s, 3H,
NþCH3); 7.19 (d, 1H, J ¼ 8.4 Hz, 30-H), 7.60 (dt, 1H, J ¼ 7.0 and
J ¼ 1.3 Hz, 6-H), 7.70 (dt, 1H, J ¼ 7.0 and J ¼ 1.3 Hz, 5-H), 7.76 (dt. 1H,
J ¼ 8.0 and J ¼ 1.2 Hz, 60-H), 7.85 (dt, 1H, J ¼ 8.0 and J ¼ 1.2 Hz, 70-H),
GG 420,
l
> 420 nm) filter was also used. A thermostated (20 ꢀC)

P.J. Coelho et al. / Dyes and Pigments 117 (2015) 163e169
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~
^
Thanks are due to the Fundaçao para a Ciencia e Tecnologia
(Portugal) and FEDER-COMPETE for financial support through the
research units Centro de Química e Universidade do Minho and
Centro de QuímicaeVila Real, Project PEst-C/QUI/UI0686/2013
(FCOMP-01-0124-FEDER-037302 and a PhD grant to M. C. R. Cas-
tro (SFRH/BD/78037/2011). The NMR spectrometer Bruker Avance
III 400 is part of the National NMR Network and was purchased
within the framework of the National Program for Scientific Re-
equipment, with funds from FCT. We are also grateful to the Insti-
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