Cationic 3H-indolium dyes by ring-opening of benzo[1,3]oxazine

Article abstract of DOI:10.1016/j.dyepig.2013.02.001

The reaction of 2-methylbenzo[1,3]oxazine with (hetero)aromatic aldehydes under acid catalysis afforded directly polymethine dyes, formed by a thermal ring-opening of the oxazine cycle followed by condensation with the aldehydes. These dyes have the structure of a cationic thermally stable coloured open form of a photochromic benzo[1,3]oxazine and do not undergo ring-closure to afford the closed form again, even under basic treatment. All of the dyes showed reversible acidochromic properties.

Full text of DOI:10.1016/j.dyepig.2013.02.001

Dyes and Pigments 98 (2013) 93e99
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Cationic 3H-indolium dyes by ring-opening of benzo[1,3]oxazine
*
Yaroslav Prostota, Paulo J. Coelho
Centro de Química e Vila Real, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 2-methylbenzo[1,3]oxazine with (hetero)aromatic aldehydes under acid catalysis affor-
ded directly polymethine dyes, formed by a thermal ring-opening of the oxazine cycle followed by
condensation with the aldehydes. These dyes have the structure of a cationic thermally stable coloured
open form of a photochromic benzo[1,3]oxazine and do not undergo ring-closure to afford the closed
form again, even under basic treatment. All of the dyes showed reversible acidochromic properties.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 25 January 2013
Received in revised form
5 February 2013
Accepted 6 February 2013
Available online 16 February 2013
Keywords:
Oxazine
Polymethine
Ring-opening
Thermal stability
Acidochromism
3H-indolium
1. Introduction
these 2-substituted benzo[1,3]oxazines generates two different
absorption bands due to the formation of two chromophoric sys-
Benzo[1,3]oxazines are a family of molecular switches that
combine two fused heterocyclic rings, indoline and oxazine, in their
molecular skeleton [1]. These molecules exhibit thermally reversi-
ble photochromic properties with a very fast switching speed be-
tween the uncoloured (closed) and coloured (open) states [2].
UV light irradiation leads to the cleavage of the CeO bond at the
junction of the two heterocycles and consequent opening of the
[1,3]oxazine ring with formation, in less than 20 ns, of a zwitter-
tems in the same zwitterionic molecule: one located at 440 nm
assigned to the 4-nitrophenolate anion and the other around
550 nm due to the 2-substituted-3H-indolium cation with extended
conjugation (A in Scheme 2). The position of the longer wavelength
band can be tuned by the introduction of some substituents in the
aromatic ring without considerably affecting the photochromic
performance of the benzo[1,3]oxazine. Nevertheless, changes in the
phenolate chromophore or in the 3H-indolium fragment may
completely prevent the photoinduced ring-opening [6,7].
ionic species incorporating
a 3H-indolium cation and a 4-
nitrophenolate anion. This photochemical process is accompanied
by the appearance of an absorption band around 430e440 nm
typical of the 4-nitrophenolate chromophore (Scheme 1). The col-
oured species usually have a very short lifetime (<60 ns) and revert
spontaneously to the uncoloured closed benzo[1,3]oxazine with
first order kinetics. Therefore, a complete cycle can be performed
normally in less than 100 ns [3,4].
When a 2-phenylethenyl fragment is attached to the chiral C-2
atom, the photochemical ring-opening of the oxazine ring allows
the conjugation between the p-systems of the 3H-indolium and the
1H NMR studies on some benzo[1,3]oxazines revealed that the
two diastereotopic methyl groups of the indoline ring, adjacent to
the chiral C-2 centre, resonate as a single and broad peak at 1.3e
1.6 ppm. This indicates that the two enantiomers of the ring-
closed form exchange rapidly on the 1H NMR time-scale and a
thermal equilibrium between the closed and opened forms must
exits at room temperature [3]. This equilibrium can be reversibly
shifted towards the opened form by the addition of either acid of
base. Under acidic conditions (e.g. addition of CF3COOH) the
phenolate anion is protonated displacing the equilibrium position
towards the formation of the stable opened form B (Scheme 2), that
will show only the longer wavelength absorption band assigned to
the 3H-indolium polymethine-like chromophoric system. The
addition of base (few equivalents of Bu4NOH) has an opposite ef-
fect: the hydroxyl ion adds to the 3H-indolium cation with for-
mation of a stable hemiaminal which breaks the conjugation of this
2-phenylethenyl fragment leading to a polymethine-type chromo-
phore which results in a shift of the absorption maxima of the entire
molecule to longer wavelength [5]. Therefore the ring-opening of
* Corresponding author. Tel.: þ351 259 350284; fax: þ351 259 350480.
E-mail address: pcoelho@utad.pt (P.J. Coelho).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.02.001

94
Y. Prostota, P.J. Coelho / Dyes and Pigments 98 (2013) 93e99
3H-indolium
R
O
2
N
R
UV-light
Dark
2
NO2
p-nitrophenolate
N
chromophore
NO2
O
Open form
coloured
Closed form
uncoloured
Scheme 1. Photochromic equilibrium for benzo[1,3]oxazines.
part of the molecule. As a result, in basic medium, only the 440 nm
absorption band characteristic of the 4-nitrophenolate anion is
observed (C in Scheme 2). Both processes are reversible and the
benzo[1,3]oxazines can be switched between these states with no
signs of degradation [3].
The extension of the conjugation through the linkage of a sub-
stituent (R) at the 2-position of the benzo[1,3]oxazine has also an
effect on the thermal stability of the opened bichromophoric dye
leading to an increase of their lifetime when strong electron-
donating groups become part of the conjugated system: for
instance, a 4-dimethylaminophenyl group present at the chiral C-2
atom leads to a 1000 times increase on the lifetime of the photo-
generated coloured species and, accordingly, these 2-substituted
benzo[1,3]oxazines operate in the microsecond time scale [7].
However, when some conjugated systems are linked to the chiral
oxazine centre, the spontaneous opening of the oxazine ring occurs
with formation of stable dyes with a ring-opened oxazine zwit-
terionic structure [8,9]. These dyes exhibit two absorption bands in
the visible spectrum and do not undergo ring-closure even in basic
medium. Such type of stabilization of the coloured open form was
N
NO2
N
Protonated form
B
λmax = 550 nm
Red
HO
CF3COOH
N
N
O
NO2
Zwitterionic form
N
N
Two bands at 440 and 550 nm
A
Closed form
uncoloured
NO2
O
Bu4NOH
N
NO2
OH
N
Hemiaminal form
Yellow λmax = 440 nm
C
O
Scheme 2. Ring-opening of 2-(40-dimethylaminostyryl)benzo[1,3]oxazine in basic and acid medium.

Y. Prostota, P.J. Coelho / Dyes and Pigments 98 (2013) 93e99
95
N
S
N
NO2
CF3CO2
3b
HO
O
TFA
S
H
N
2b
OH
O
H
2a
O
OH
N
N
NO2
TFA
CF3CO2
1
3a
NO2
HO
O
H
TFA
2c
NH
NH
N
NO2
CF3CO2
3c
HO
Scheme 3. Synthesis of dyes 3aec.
achieved when pyrrole, indoline or julolidine systems were linked
to the benzo[1,3]oxazine and suggests that it is possible to control
the lifetime of the usually short lived open species through elec-
tronic effects [8e10]. The increase in the solvent polarity can also
increase the lifetime of the ring-opened isomers [11] or even lead to
the formation of equilibrium mixtures of the closed and open forms
at room temperature [9].
In order to study the influence of other electron-rich systems on
the equilibrium between the ring-closed and ring-opened forms
we introduced 4-hydroxyphenyl, 5-dimethylaminothiophenyl and
3-indolyl substituents on the C-2 position of the benzo[1,3]oxazine
and studied the spectroscopic and acidochromic properties of these
new compounds.
2. Results and discussion
2.1. Synthesis
The benzo[1,3]oxazine core with the fused indoline and oxazine
rings can be easily prepared by N-alkylation of 2-substituted-3H-
indoles with 2-chloromethyl-4-nitrophenol in acetonitrile followed
by spontaneous intramolecular oxazine cyclization. This reaction is
quite reliable and has been used for the synthesis of many com-
pounds having different substituents [3e6]. Alternatively, the
methyl group at C-2 of the benzo[1,3]oxazine 1 can be condensed
with aromatic aldehydes to produce ring-closed oxazines able to
produce short-lived dyes with extended conjugation after UV

96
Y. Prostota, P.J. Coelho / Dyes and Pigments 98 (2013) 93e99
O
2c
NH
H
NH
2
N
N
4c
ClO4
ClO4
λmax = 462 nm
O
S
H
N
2b
OH
S
4b
N
2
N
N
4a
ClO4
λmax = 427 nm
λmax = 558 nm
Scheme 4. Synthesis of model 3H-indolium dyes 4bec.
activation [7,9,10]. In special cases, depending on the nature of the
aromatic aldehydes, this reaction afforded directly stable dyes with
an oxazine ring-opened structure [8e10].
2.2. Acidochromic properties of dyes 3aec
The steady state absorption spectrum of dye 3a (1.0 ꢀ 10ꢁ5 M)
(Fig. 1) shows an intense absorption band in the visible range, at
438 nm, which resembles the ground state absorption of model
compound 4a (lmax ¼ 427 nm). Addition of one drop of base
(Bu4NOH, 1 M in MeOH) to a CH3CN solution of dye 3a leads to the
formation of an intense red colour that turns in few seconds to
yellow with the absorption maxima at 434 nm.
Further addition of base does not change the position of ab-
sorption maxima. These results suggests that upon addition of base
the phenolic proton of the 3H-indolium cation is removed leading
to an intermediate merocyanine structure (A) which is known to
absorb at longer wavelengths (lmax ¼ 540 nm) (Scheme 5) [12]. This
species is unstable and through proton exchange is probably con-
verted to the zwitterionic form (B) which has two chromophoric
systems, both absorbing in the same range of wavelength, around
434 nm (Scheme 5). Subsequent addition of an aqueous solution of
The condensation of 2-methylbenzo[1,3]oxazine 1 with the
aromatic aldehydes 2aec was performed in CH3CN in the presence
of trifluoroacetic acid (TFA) and afforded, after 3e12 days at reflux,
the dyes 3aec with a ring-opened oxazine structure (Scheme 3).
The NMR spectra of the opened and closed forms of benzo[1,3]
oxazines are very similar, nonetheless there are some specific res-
onances typical of each form [3,4]. The 1H NMR spectra of the new
dyes 3aec reveals resonances that can be assigned exclusively to
the ring-opened form: in particular the two methyl groups of the
indoline ring resonate at the same chemical shift, 1.73e1.46 ppm (s,
6H) which suggests that the adjacent C-2 is no longer a chiral
centre; the CH2N signal was found between 5.81 and 4.94 ppm
shifted from his usual value in the benzo[1,3]oxazines (around
4.50 ppm) [3,4]; two low field phenolic signals were found for
compound 3a at 12.15 and 11.1 ppm; a low field signal was found in
the 13C NMR spectra, between 174 and 183 ppm, which can be
assigned to C-2 of the 3H-indolium moiety [8]. 19F NMR spectros-
copy established the presence of trifluoroacetate as the counterion
which indicates that the dyes 3aec exists in the cationic form. These
data, along with the results from mass spectrometry, unambigu-
ouslyconfirmed that these dyes possess a ring-opened 3H-indolium
cationic structure and are thus probably formed by thermal ring-
opening of the oxazine cycle of compound 1 and subsequent reac-
tion of the active 2-methyl group of the 3H-indolium cationwith the
aldehydes, leading to the polymethine dyes 3aec.
1.00
0.75
0.50
0.25
0.00
OH
N
NO2
3a
c
HO
a
To compare the similarity of the chromophore skeleton, the
model 3H-indolium dyes 4bec were prepared by condensation of
1,2,3,3-tetramethyl-3H-indolium perchlorate with aldehydes 2bec
(Scheme 4). These compounds exhibit some NMR similarities with
the dyes 3bec, in particular, the low field signals at 180e173 ppm in
their 13C NMR spectra, which is characteristic of the 3H-indolium C-
2 carbon atom. The synthesis and some spectroscopic properties of
the model compound 4a have already been described in the lite-
rature [10,12].
b
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. Absorption spectra of the compound 3a in acetonitrile (1.0 ꢀ 10ꢁ5 M) (a); after
addition of 10 eq. of Bu4NþOHꢁ (b); and subsequent addition of aqueous HCl (5%) (c).

Y. Prostota, P.J. Coelho / Dyes and Pigments 98 (2013) 93e99
97
OH
OH
O
Bu4NOH
N
N
N
NO2
NO2 HCl (aq)
NO2
O
3a
B
HO
HO
A
λmax = 438 nm
λmax = 434 nm
Scheme 5. Acid-base equilibrium for dye 3a.
HCl(aq) lead to a small bathochromic shift of the lmax (þ7 nm)
probably due to the regeneration of compound 3a, to the change of
the counterion and also due to some solvatochromism.
Dye 3b shows a strong and sharp absorption band at 560 nm
which resembles the absorption of the model compound 4b
3. Conclusion
The reaction of 2-methylbenzo[1,3]oxazine with some (hetero)
aromatic aldehydes affords polymethine dyes with a conjugated
chain corresponding to cationic open form of the benzo[1,3]oxazine
system, which do not undergo intramolecular ring-closure in basic
medium. These dyes show reversible acidochromic behaviour
allowing switching between the 3H-indolium cation with extended
conjugation and the 4-nitrophenolate anion chromophores. Dye 3c
formed, after basic treatment, a stable zwitterionic bichromophoric
opened form with two absorption bands assigned to the poly-
methine 3H-indolium cation and the 4-nitrophenolate anion
chromophores.
(lmax ¼ 558 nm) and is due to the extended conjugation in the 3H-
indolium fragment (Fig. 2). Addition of base leads to the extinction
of this violet band and the appearance of a broad one at 432 nm.
The disappearance of the band at 550 nm is due to the addition
of hydroxide ion to the 3H-indolium cation to give a hemiaminal
structure, thus breaking the conjugation between the indoline and
the thiophene systems, while the new band at 420 nm can be
assigned to the formation of the 4-nitrophenolate anion. This
process is reversible and upon subsequent addition of acid the
hemiaminal is rapidly converted to the dye 3b (Scheme 6).
4. Experimental
In CH3CN solution dye 3c exhibits a broad absorption band at
486 nm that is similar to the absorption of the model 3H-indolium
dye 4c (lmax ¼ 462 nm) (Fig. 3). Addition of base leads to the
appearance of two bands at 434 and 494 nm that suggests the
formation of a zwitterionic open form of the benzo[1,3]oxazine
system with two chromophores, the 4-nitrophenolate
4.1. General
2-Nitro-5a-phenyl-6,6-dimethyl-5a,6-dihydro-12H-indolo[2,1-
b][1,3]benzooxazine
1
and 5-dimethylamino-thiophene-2-
carbaldehyde 2b were prepared according to literature pro-
cedures [1,13]. Solvents were of analytical grade and the acetic
anhydride was previously distilled. A 1 M solution Bu4NþOHꢁ in
methanol was used. All reactions were monitored by thin-layer
chromatography on aluminium plates coated with Merck silica
gel 60 F254 (0.25 mm). 1H and 13C NMR spectra were recorded at
(
lmax ¼ 434 nm) and the 3H-indolium cation (lmax ¼ 494 nm),
instead of the possible hemiaminal structure (Scheme 7).
This bichromophoric dye is remarkably stable and does not
undergo ring-closure to form
a benzo[1,3]oxazine structure.
Further addition of acid led to the appearance of a broad band at
488 nm which may correspond to the renewal of the cationic
structure for the dye 3c. The process is reversible and the dye can be
switched between these two states upon addition of base or acid.
298
K on a Bruker ARX400 spectrometer (at 400.13 and
100.62 MHz). The new compounds were determined to be >95%
pure by 1H NMR spectroscopy. IR spectra were obtained on a Uni-
cam Research Series FTIR spectrometer using KBr pellets. Wave-
numbers (lmax) are reported in cmꢁ1. UVeVis spectra were
recorded on a PerkineElmer Lambda 25 spectrophotometer in
spectral grade acetonitrile. High resolution electrospray ionization
time-of-flight (ESI-TOF) mass spectra and electron impact time-of-
flight (EI-TOF) mass spectra were measured with a VG AutoSpec M
spectrometer. Melting points were determined in open capillary
tubes in a Buchi 535 melting point apparatus and are uncorrected.
2.00
1.75
d
N
S
1.50
N
a
NO2
1.25
1.00
0.75
0.50
0.25
0.00
3b
HO
4.2. Synthesis of dyes 3aec
c
4.2.1. 1-(20-Hydroxy-50-nitro-benzyl)-3,3-dimethyl-2-[40-
hydroxystyryl]-3H-indolium trifluoroacetate 3a
b
A solution of 1 (0.20 g, 0.65 mmol), 2a (0.090 g, 0.74 mmol) and
CF3CO2H (0.023 mL, 0.30 mmol) in CH3CN (20 mL) was heated
under reflux for 3 days. After cooling to ambient temperature, the
reaction mixture was evaporated to dryness under reduced pres-
sure and the resultant residue dissolved in hot CH2Cl2 (15 mL). The
crystalline residue formed at cooling was filtered off and washed
consequently with CH2Cl2 (3 mL) and Et2O (10 mL) to afford 3a
(0.22 g, 64%) as an orange solid. Mp. 219e222 ꢂC. IR: 522, 715, 832,
200
300
400
500
600
700
Wavelength (nm)
Fig. 2. Absorption spectra of the compound 3b (a); after addition of 10 eq. of
Bu4NþOHꢁ (b); subsequent addition of aqueous HCl (5%) (c); and the model dye 4b (d)
in acetonitrile (1.0 ꢀ 10ꢁ5 M).

98
Y. Prostota, P.J. Coelho / Dyes and Pigments 98 (2013) 93e99
N
N
S
S
Bu4NOH
N
OH
N
NO2
NO2
HCl (aq)
3b
O
HO
λmax = 432 nm
λmax = 560 nm
Scheme 6. Acid-base equilibrium for dye 3b.
1162, 1230, 1517, 1568, 1664, 2987, 3083, 3393. 1H NMR (DMSO-d6):
1.79 (s, 6H), 5.90 (broad s, 2H), 6.94 (d, J ¼ 8.4, 2H), 7.02 (d, J ¼ 9.0,
1H), 7.51 (broad s, 2H), 7.71 (broad s, 2H), 7.83 (broad s, 1H), 8.07
(broad s, 2H), 8.11 (dd, J ¼ 9.0, 2.6, 1H), 8.37 (m, 1H), 8.38e8.50
(broad s, 1H), 11.06 (broad s, 1H), 12.15 (broad s, 1H). 13C NMR
(DMSO-d6): 26.0, 45.5, 51.8, 54.8, 109.8, 114.81, 116.1, 116.6, 118.4,
120.7, 122.9, 125.4, 125.9, 126.1, 128.8, 133.8, 139.4, 141.1, 142.9,
162.4, 164.0, 156.0, 183.0 19F NMR (DMSO-d6): ꢁ73.66 (CF3); HRMS
(TOF ESI): calcd for [C25H23N2O4]þ: 415.1652; found: 415.1634.
4.2.3. 1-(20-Hydroxy-50-nitro-benzyl)-3,3-dimethyl-2-[(1H-indol-
3-yl)vinyl]-3H-indolium trifluoroacetate 3c
A solution of 1 (0.20 g, 0.65 mmol), 2c (0.10 g, 0.69 mmol) and
CF3CO2H (0.023 mL, 0.30 mmol) in CH3CN (20 mL) was heated
under reflux for 9 days. After cooling to ambient temperature the
reaction mixture was evaporated to dryness under reduced pres-
sure. The resultant residue was dissolved in CH2Cl2 (25 mL), washed
with KOH (40 mL, 0.05 M), water (40 mL), dried over anhydrous
Na2SO4 and evaporated to dryness under reduced pressure. The
residue was crystallized from a mixture of EtOH:acetone:CH3CN
(5:2:1 v/v), filtered off and washed with Et2O (3 mL) to afford 3c
(0.10 g, 28%) as a red solid. Mp. 205e207 ꢂC. IR: 584, 697, 749, 815,
1027, 1089, 1141, 1189, 1219, 1293, 1415, 1463, 1571, 3083, 3373. 1H
NMR (DMSO-d6): 1.46 (s, 6H), 4.94 (s, 2H), 6.52 (d, J ¼ 16.0,1H), 6.79
(d, J ¼ 9.0,1H), 6.96 (t, J ¼ 7.0, 1H), 7.06e7.22 (m, 4H), 7.33 (d, J ¼ 7.0,
1H), 7.43 (d, J ¼ 8.0, 1H), 7.32e7.45 (br.s, 1H, merged with two other
signals), 7.85e7.87 (m, 2H), 7.91 (dd, J ¼ 9.0, 3.0, 1H), 8.05 (d, J ¼ 3.0,
1H). 13C NMR (DMSO-d6): 18.5, 23.8, 26.4, 30.6, 49.5, 110.4, 12.8,
144.0, 117.7, 120.1, 121.0, 121.2, 122.3, 122.7, 124.0, 124.7, 124.9, 127.8,
136.8, 138.4, 139.2, 145.1. 19F NMR (CDCl3): ꢁ75.21 (CF3); HRMS
(TOF ESI): calcd for [C27H24N3O3]þ: 438.1812; found: 438.1807.
4.2.2. 1-(20-Hydroxy-50-nitro-benzyl)-3,3-dimethyl-
2-[(5-dimethylaminothiophen-2-yl)vinyl]-3H-indolium
trifluoroacetate 3b
A solution of 1 (0.17 g, 0.55 mmol), 2b (0.090 g, 0.58 mmol) and
CF3CO2H (0.023 mL, 0.30 mmol) in CH3CN (20 mL) was heated
under reflux for 12 days. After cooling to ambient temperature the
crystalline residue formed was filtered off and washed conse-
quently with EtOAc (3 mL), hexanes (3 mL) and Et2O (10 mL) to
afford 3b (0.16 g, 52%) as a green solid with a metallic lustre. Mp.
222e224 ꢂC. IR: 592, 706, 749, 810, 840, 906, 1019, 1075, 1132, 1189,
1263, 1315, 1380, 1463, 1541, 1589, 1667, 3095, 3385. 1H NMR
(DMSO-d6): 1.68 (s, 6H), 3.34 (s, 6H), 5.39 (s, 2H), 6.10 (d, J ¼ 14.0,
1H), 6.77 (d, J ¼ 5.0, 1H), 7.04 (d, J ¼ 9.0, 1H), 7.25 (t, J ¼ 7, 1H), 7.37e
7.45 (m, 2H), 7.62 (d, J ¼ 7.0, 1H), 7.98 (d, J ¼ 5.0, 1H), 8.03 (m, 1H),
8.09 (d, J ¼ 9.0, 1H), 8.31 (d, J ¼ 14.0, 1H). 13C NMR (DMSO-d6): 27.1,
42.1, 43.2, 48.6, 97.9, 111.1, 113.3, 115.7, 118.0, 122.2, 124.3, 124.5,
125.6, 127.1, 128.3, 139.3, 140.3, 142.2, 142.4, 148.8, 162.0, 172.7,
174.2. 19F NMR (CDCl3): ꢁ73.39 (CF3); HRMS (TOF ESI): calcd for
[C25H26N3O3S]þ: 448.1689; found: 448.1684.
4.3. Synthesis of model dyes 4bec
4.3.1. 1,3,3-Trimethyl-2-[2-(5-dimethylaminothiophen-2-yl)vinyl]-
3H-indolium perchlorate 4b
A solution of 2b (0.11 g, 0.73 mmol) and 1,2,3,3-tetramethyl-3H-
indolium perchlorate (0.20 g, 0.73 mmol) in freshly distilled acetic
anhydride (5 mL) was heated under reflux for 3 min. The crystalline
residue formed upon cooling was filtered off, washed with EtOAc
(5 mL) and Et2O (5 mL) to afford 4b (0.20 g, 67%) as blue crystals
with a metallic lustre. Mp. 229e231 ꢂC (dec.). IR: 584, 619, 702, 797,
911, 1081, 1207, 1259, 1311, 1384, 1411, 1476, 1551, 2925, 3373. 1H
NMR (DMSO-d6): 1.66 (s, 6H), 3.31 (s, 6H), 3.61 (s, 3H), 6.04 (d,
J ¼ 13.8, 1H), 6.69 (m, 1H), 7.23 (m, 1H), 7.38 (m, 2H), 7.58 (d, J ¼ 7.0,
1H), 8.00 (m, 1H), 8.33 (d, J ¼ 13.8, 1H). 13C NMR (CDCl3): 26.8, 31.1,
42.9, 48.9, 97.5, 111.0, 111.8, 122.2, 124.8, 126.1, 128.2, 140.8, 142.5,
142.7, 148.2, 173.1, 173.9. HRMS (TOF ESI): calcd for [C29H23N2S]þ:
311.1577; found: 311.1583.
1.0
NH
d
N
b
c
NO2
3c
0.5
HO
4.3.2. 1,3,3-Trimethyl-2-[2-(1H-indol-3-yl)vinyl]-3H-indolium
perchlorate 4c
a
A solution of 3c (0.11 g, 0.73 mmol) and 1,2,3,3-tetramethyl-3H-
indolium perchlorate (0.20 g, 0.73 mmol) in freshly distilled acetic
anhydride (10 mL) was heated under reflux for 3 min. The crys-
talline residue formed upon cooling was filtered off, washed with
Et2O (5 mL) and recrystallized from CH3CN:MeOH 2:1 v/v mixture
to afford 4c (0.18 g, 61%) as a orange solid. Mp. > 300 ꢂC (dec.). IR:
562, 619, 758, 819, 928, 950, 1102, 1236, 1301, 1328, 1406, 1432,
1475, 1593, 3060, 3260. 1H NMR (DMSO-d6): 1.80 (s, 6H), 4.00 (s,
0.0
300
400
500
600
Wavelength (nm)
Fig. 3. Absorption spectra of the compound 3c (a); after addition of 10 eq. of
Bu4NþOHꢁ (b); subsequent addition of aqueous HCl (5%) (c); and the model dye 4c (d)
in acetonitrile (1.0 ꢀ 10ꢁ5 M).

Y. Prostota, P.J. Coelho / Dyes and Pigments 98 (2013) 93e99
99
NH
NH
Bu4NOH
N
N
NO2
NO2
HCl (aq)
3c
O
HO
Two bands
λmax = 434 and 494 nm
λmax = 486 nm
Scheme 7. Acid-base equilibrium for dye 3c.
[3] Tomasulo M, Sortino S, White AJP, Raymo F. Fast and stable photochromic
oxazines. J Org Chem 2005;70:8180e9.
[4] Prostota Y, Coelho PJ, Pina J, de Melo JS. Fast photochromic sterically hindered
benzo[1,3]oxazines. J Photochem Photobiol A 2010;216:59e65.
[5] Tomasulo M, Sortino S, Raymo F. Bichromophoric photochromes based on the
opening and closing of a single oxazine ring. J Org Chem 2008;73:118e26.
[6] Tomasulo M, Sortino S, Raymo F. A new family of photochromic compounds
based on the photoinduced opening and thermal closing of [1,3]oxazine rings.
J Photochem Photobiol A: Chem 2008;200:44e9.
[7] Deniz E, Tomasulo M, Sortino S, Raymo F. Substituent effects on the photo-
chromism of bichromophoric oxazines. J Phys Chem C 2009;113:8491e7.
[8] Prostota Y, Berthet J, Delbaere S, Coelho PJ. Bichromophoric dye derived from
benzo[1,3]oxazine system. Dyes Pigment 2013;96:569e73.
3H), 7.19 (d, J ¼ 16.0, 1H), 7.37e7.41 (m, 2H), 7.49 (t, J ¼ 7.4, 1H), 7.57
(t, J ¼ 7.8, 1H), 7.61e7.63 (m, 1H), 7.75 (d, J ¼ 7.9, 1H), 7.79 (d, J ¼ 7.2,
1H), 8.27 (dd, J ¼ 6.0, 3.0, 1H), 8.66 (s, 1H), 8.69 (d, J ¼ 16.0, 1H). 13
C
NMR (DMSO-d6): 26.2, 32.8, 50.7, 105.0, 113.4, 113.5, 115.8, 121.3,
122.5, 123.1, 124.3, 124.5, 127.4, 128.6, 138.3, 140.5, 142.0, 142.3,
148.8, 180.1. HRMS (TOF-ESI): calcd for [C21H21N2]þ: 301.1699;
found: 301.1696.
Acknowledgements
[9] Deniz E, Tomasulo M, Cusido J, Yildiz I, Petriella M, Bossi ML, et al. Photo-
activatable fluorophores for super-resolution imaging based on oxazine aux-
ochromes. J Phys Chem C 2012;116:6058e68.
[10] Deniz E, Cusido J, Swaminathan S, Battal M, Impellizzeri S, Sortino S, et al.
Synthesis and properties of molecular switches based on the opening and
closing of oxazine rings. J Photochem Photobiol A: Chem 2012;229:20e8.
[11] Prostota Y, Coelho PJ, Pina J, de Melo JS. Photochromic and photophysical
properties of new benzo- and naphtho[1,3]oxazine switches. Photochem
Photobiol Sci 2011;10:1346e54.
[12] Pilipchuk NV, Piryatinski YP, Kachkovsky GO, Slominskii YL, Kachkovsky OD.
Low-temperature fluorescence of oxystyryls. Dyes Pigment 2007;74:47e53.
[13] Yan P, Xie A, Wei M, Loew LM. Amino(oligo)thiophene-based environmentally
sensitive biomembrane chromophores. J Org Chem 2008;73:6587e659.
To FCT (Portugal’s Foundation for Science and Technology) and
FEDER for financial support through the research unit Centro de
Química-Vila Real (POCTI-SFA-3-616).
References
[1] Shachkus AA, Degutis YA, Urbonavichyus AG. Synthesis and study of 5a,6-
dihydro-12H-indolo[2,1-b][1,3]-benzoxazines. Chem Heterocycl Compd 1989;
25:562e5.
[2] Tomasulo M, Sortino S, Raymo FM. A fast and stable photochromic switch
based on the opening and closing of an oxazine ring. Org Lett 2005;7:1109e12.