Bichromophoric dye derived from benzo[1,3]oxazine system

Article abstract of DOI:10.1016/j.dyepig.2012.09.017

The reaction of 2-methylbenzo[1,3]oxazine with julolidine-9-carbaldehyde under acid catalysis afforded an highly coloured blue dye with an intense absorption at 591 nm. NMR and UV-Vis analysis showed that this compound has an opened oxazine structure with a polymethine-type chromophore, corresponding to a protonated thermally stable coloured form of photochromic benzo[1,3]oxazines that are known to be unstable at room temperature with lifetimes in the ns timescale. In basic medium this dye is converted into a stable opened zwitterionic form of photochromic benzo[1,3]oxazines with two absorption maxima at 410 and 587 nm assigned to conjugated 3H-indolium and 4-nitrophenolate chromophores respectively.

Full text of DOI:10.1016/j.dyepig.2012.09.017

Dyes and Pigments 96 (2013) 569e573
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Bichromophoric dye derived from benzo[1,3]oxazine system
,
Yaroslav Prostota a, Jerome Berthet b, Stephanie Delbaere b, Paulo J. Coelho a
*
a Centro de Química e Vila Real, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal
b Université Lille Nord de France, CNRS UMR 8516, UDSL, Faculté des Sciences Pharmaceutiques et Biologiques, F-59006 Lille, France
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 julolidine-9-carbaldehyde under acid catalysis afforded
an highly coloured blue dye with an intense absorption at 591 nm. NMR and UVeVis analysis showed
that this compound has an opened oxazine structure with a polymethine-type chromophore, corre-
sponding to a protonated thermally stable coloured form of photochromic benzo[1,3]oxazines that are
known to be unstable at room temperature with lifetimes in the ns timescale. In basic medium this dye is
converted into a stable opened zwitterionic form of photochromic benzo[1,3]oxazines with two
absorption maxima at 410 and 587 nm assigned to conjugated 3H-indolium and 4-nitrophenolate
chromophores respectively.
Received 13 July 2012
Received in revised form
24 September 2012
Accepted 26 September 2012
Available online 4 October 2012
Keywords:
Oxazines
Polymethines
Photochromism
Zwitterion
Ó 2012 Elsevier Ltd. All rights reserved.
Ring opening
Julolidine
1. Introduction
group both chromophores absorb in the visible region near
440 nm and a single band is observed, however when a 4-dime-
Benzo[1,3]oxazines are an important class of molecules
switches with exceptional fatigue resistance and fast switching
speeds. Although their first synthesis was reported more than two
decades ago [1], their photochromic properties were discovered
only in 2005 [2]. UV light irradiation of these uncoloured molecules
leads in less than 6 ns to the heterolytic cleavage of the CeO bond
and consequent opening of the [1,3]oxazine ring with formation of
a coloured zwitterionic species incorporating a 3H-indolium cation
and a 4-nitrophenolate that absorbs strongly around 430e440 nm
(Scheme 1). After the light pulse, the photogenerated coloured
isomer reverts thermally to the uncoloured original state, usually
within 15e60 ns with first-order kinetics. This particular photo-
chromic system is remarkably stable and tolerates several thousand
of switching cycles without significant degradation, even in the
presence of molecular oxygen [2].
thylaminostyryl group is attached at C-2, the ring-opened photo-
isomer shows two different absorption bands located at 440 nm and
550 nm assigned to the 4-nitrophenolate anion and the conjugated
4-dimethylaminostyryl-3H-indolium cation, respectively [3].
On the other hand, due to the zwitterionic character of the open
form, these strong donating groups, at the 2-position of the 3H-
indolium cation, have a significant effect on the stability of the
coloured open form, increasing their lifetime up to 1000 times. For
these benzo[1,3]oxazines with extended conjugation, the switching
between the two forms can be performed in the
ms timescale [3]. In
contrast, substituents such as p-nitrophenyldiazenyl, on the para
position of the phenolate chromophore, biphenyl on the oxazine
chiral centre or methoxy, nitro and trans-stilbenylvinyl on the 3H-
indole fragment, prevents the photoinduced ring opening of the
oxazine ring [4,5].
The nature of the R group has an important effect on the
photochromic properties of these compounds. If R is an aromatic
ring, the opening of the oxazine ring generates two chromophores:
the yellow 4-nitrophenolate and the 2-aryl-3H-indolium cation that
absorbs between 430 and 590 nm depending upon the substituents
present in the aromatic ring. When R is a 4-dimethylaminophenyl
As expected, the stability of the open form is also sensitive to the
solvent polarity and increases with a transition from acetonitrile to
ethanol or DMSO. In DMSO, a solvent with an high ability to
complex organic cations, the lifetime of the zwitterionic open form
of a naphtho [1,3]oxazine increased to 159 ms [6]. The increase in
the solvent polarity can also lead to a shift in the equilibrium
between the ring-closed and the ring-opened isomers: the conju-
gated linkage of a coumarin to the oxazine chiral C-2 atom led to
a photochromic oxazine that exists predominantly in the closed
form in acetonitrile, but the corresponding steady state absorption
* Corresponding author. Fax: þ351 259350480.
E-mail address: pcoelho@utad.pt (P.J. Coelho).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.09.017

570
Y. Prostota et al. / Dyes and Pigments 96 (2013) 569e573
3H-indolium
chromophore
N
R
N
2
N
O
R
UV-light
Dark
NO2
N
O
O
O
2
p-nitrophenolate
chromophore
N
N
NO2
O
CF3CO2H
CH3CN
Open form
coloured
Closed form
uncoloured
1
NO2
NO2
Δ, 6d
Scheme 1. Photochromic equilibrium for benzo[1,3]oxazines.
Scheme 3. Synthesis of 2-styrylbenzo[1,3]oxazines.
spectra spectrum in methanol shows the characteristic absorption
of the 3H-indolium chromophore at 586 nm suggesting the presence
of both ring-closed and ring-opened isomers at equilibrium in
a 93:7 ratio [7].
The [1,3]oxazine cycle of these compounds can also be reversibly
opened in acid or basic medium [8]. The addition of Bu4NOH to
benzo[1,3]oxazines leads to the opening of the oxazine ring fol-
lowed by further reaction with the hydroxyl ion, with formation of
a stable hemiaminal compound with the concomitant appearance
of a strong absorption around 440 nm characteristic of the
4-nitrophenolate anion (Scheme 2).
2-methylbenzo[1,3]oxazine 1 with 4-dimethylaminobenzaldehyde,
in the presence of trifluoroacetic acid (0.30 eq) for 6 days in
acetonitrile under reflux, is known to produce, after basic treat-
ment with aqueous KOH solution, a photochromic oxazine with
extended conjugation [3] (Scheme 3). Recently, Deniz et al. re-
ported the synthesis of a series of oxazines using this methodology
but without any basic treatment at the end. The reactions were
carried out in CH3CN or EtOH using even an excess of TFA. After
solvent evaporation the residue was dissolved in CH2Cl2 and the
addition of hexane or Et2O caused the precipitation of the oxazines
[7,12].
In strong acidic conditions (e.g. addition of CF3COOH) the [1,3]
oxazine ring opens to generate a coloured conjugated fragment and
Following the same procedure, an acetonitrile solution of
a
protonated 4-nitrophenol. This transformation allows the
methyl oxazine 1, julolidine-9-carbaldehyde
2 (1.0 eq) and
conjugation between the -system of the 3H-indolium fragment
p
CF3COOH (1.0 eq) was heated at reflux. Within 3 h we observed the
formation of a deep blue solution, from which a green crystalline
precipitate started to form. After 3 days at reflux the precipitate was
isolated by filtration (Scheme 4). The same result was observed
when a catalytic amount of TFA (0.34 eq) was used.
Mass spectrometry (ESI-TOF) analysis of this solid indicated the
molecular formula of the product as C31H32N3O3. The 1H NMR
spectrum (DMSO) of this compound showed all the expected
aromatic signals for a benzo[1,3]oxazine but with an usual low field
signal for the CH2N protons at 5.68 ppm whereas in known benzo
[1,3]oxazines these protons resonate around 4.4e4.7 ppm. The
presence of a phenolic proton around 12.1 ppm and the absence of
the expected ethylenic signals around 6 ppm were also noteworthy.
In acetonitrile solution this compound shows a strong absorption at
and the aromatic substituent at C-2 leading to a polymethine-type
chromophore resulting in a shift in the absorption to longer
wavelengths in the visible region (lmax ¼ 550 nm), while the 4-
nitrophenol has the main absorption in the UV region (Scheme 2).
The chemical and photochemical behaviour of these compounds
suggests that it is possible to increase the stability of the zwitter-
ionic open form, especially through the introduction of conjugated
substituents with strong electron-donating character. A significant
decrease in the ultrafast switching speed between the ring-closed
and the ring-opened isomers could lead to the visual observation
of the photochromic phenomena at room temperature and thus
expand the practical applications of these compounds. With this
idea in mind we explored the introduction, at carbon C-2, of
a conjugated chain bearing the julolidine group which is known to
have a strong electron-donating character probably related to the
restricted rotation around the C(sp2)eN bond [9,10].
591 nm (
3
¼ 1.8 ꢀ 105 Mꢁ1 cmꢁ1), which resembles the ground state
absorption of the model compound 4, but no band at 440 nm,
attributable to the 4-nitrophenolate was observed (Fig. 1). These
results suggest the opening of the oxazine ring and the formation of
a coloured polymethine indolium cation with an extended conju-
gation, linked to the uncoloured protonated phenolate 3.
2. Results and discussion
The structure of this new blue dye was unambiguously estab-
lished using 2D NMR experiments. In particular 1He1H scalar
correlations were measured in COSY experiment (DMSO) between
protons H-8 at 7.19 ppm and H-9 at 8.19 ppm with a coupling
constant of 15.2 Hz and from H-4 (7.74 ppm) up to H-7 (7.49 ppm)
via H-5 and H-6. The two aromatic julolidine protons H-11 appear as
A variety of substituted benzo[1,3]oxazines can be easily
prepared by N-alkylation of 3,3-dimethyl-3H-indoles with 2-
chloromethyl-4-nitrophenol in acetonitrile followed by sponta-
neous intramolecular oxazine cyclization [11]. Alternatively, the 2-
methylbenzo[1,3]oxazine 1 can be condensed with aldehydes to
produce oxazines with extended conjugation [3,7]. The reaction of
N
N
N
NO2
N
2
N
O
Bu4NOH
CF3COOH
NO2
OH
O
N
HO
NO2
Protonated form
Red λ = 550 nm
Closed form
uncoloured
λ = 440 nm
Yellow
Scheme 2. Ring opening of 2-(40-dimethylaminostyryl)benzo[1,3]oxazine in basic and acid medium.

Y. Prostota et al. / Dyes and Pigments 96 (2013) 569e573
571
O
14
13
N
N
11
9
H
4
8
12
5
6
11
3
N
O
O
2
2
N
N
N
7
17
CF3CO2H
CH3CN
Δ, 3d
NO2
1
16
3
NO2
NO2
HO
15
Scheme 4. Synthesis of the blue dye 3.
Fig. 1. Absorption spectra of the blue salt 3 and the model compound 4 in acetonitrile (1.0 ꢀ 10ꢁ5 M).
a very broad signal at 7.63 ppm at r.t. but became distinct from each
other at 223 K in CDCl3. The HMBC spectrum evidences significant
long-range CeH correlations between carbon C-2 at 178.3 ppm and
the CH2N protons at 5.68 ppm, the C(CH3)2 methylenic protons at
1.76 ppm and H-9 at 8.19 ppm confirming the opening of the oxa-
zine ring and the presence of the indolium cation. 19F NMR spec-
troscopy confirmed the presence of trifluoroacetate as the
counteranion in compound 3. It is worth mentioning that the
closed benzo[1,3]oxazines have a characteristic resonance around
102e106 ppm for the oxazine quiral carbon C-2 which, in this open
form appears at 178.3 ppm, a value compatible with a positive
charged 3H-indolium fragment.
To promote the closure of the oxazine ring in compound 3 we
tried its reaction with bases. While refluxing an acetonitrile solu-
tion of 3 and NEt3 (1 eq) led only to the decomposition of the
starting material, when a CHCl3 solution of the blue dye 3 was
treated with an aqueous solution of KOH, at room temperature, the
organic phase turned reddish and after work up the polar dye 5 was
isolated.
The UVeVis spectra of compound 5 showed two strong
absorption bands in the visible spectrum, at 410 nm and 587 nm,
which can be assigned to the phenolate anion and to the conjugated
3H-indolium cation respectively (Fig. 2). This suggests the forma-
tion of a stable ring opened zwitterionic form of the benzo[1,3]
oxazine (Scheme 5). This hypothesis was confirmed by the 1H NMR
spectrum (in acetone-d6) of the dye 5 which showed similar
resonances as the blue dye 3. The main differences were an high
field shift of almost all signals, in particular the CH2N protons (from
5.60 to 4.86 ppm), protons H-8 (from 7.31 to 6.26 ppm) and H-9
(from 8.30 to 6.79 ppm), proton H-15 of the phenolic fragment
(from 7.52 to 6.97 ppm) and the absence of any exchangeable
protons, which are indicative of the presence of a phenolic anion in
the structure (Fig. 3).
The change in the absorption spectrum between dyes 3 and 5
can be seen in Fig. 2. Addition of Bu4NOH (10 eq) to the blue dye 3
leads to the decrease in the band at 591 nm and the rising of the
band at 410 nm suggesting the presence of two chromophores: the
phenolate anion and the conjugated 3H-indolium cation. This
spectrum is identical to a sample of the zwitterionic compound 5.
2.0
591
Dye 3
Dye + 10 eq (Bu NO H)
3
4
1.5
1.0
0.5
0.0
Dye + 100 eq (Bu4NO H)
3
587
410
λ (nm )
Fig. 2. UVeVis spectra of dye 3 in acetonitrile (1.0 ꢀ 10ꢁ5 M) before and after addition
of Bu4NOH (10 and 100 eq).

572
Y. Prostota et al. / Dyes and Pigments 96 (2013) 569e573
N
N
N
8
9
Bu4NOH
OH
Bu4NOH
100 eq
N
O
N
O
N
10 eq
NO2
NO2
NO2
HCl
HCl
3
HO
5
15
Scheme 5. Acidochromism of dye 3.
Fig. 3. 1H spectra of (a) compound 3 and (b) compound 5 in acetone-d6.
Addition of an excess of base (Bu4NOH, 100 eq) lead to a yellow
solution where the absorption at 591 nm nearly disappeared,
indicating probably the formation of a coloured hemiaminal with
only the phenolate chromophore. These transformations are
reversible: addition of an excess of HCl leads to the disappearance
of the phenolate band at 410 nm and the emerging of the band at
591 nm of compound 3 (Scheme 5).
These results indicate that dye 3 was probably formed by
condensation of the starting 2-methylbenzo[1,3]oxazine 1 with
julolidine-9-carbaldehyde, catalysed by the TFA acid, leading to an
unstable closed benzo[1,3]oxazine, that spontaneously opens the
oxazine ring leading to the open form 5 which under the action of
the TFA and water, formed in the condensation, is converted to the
dye 3.
The fact that in diluted basic medium compound 5 does not
cyclize back to the closed form, contrary to that observed with
common benzo[1,3]oxazines [8,11], points towards an increased
stability introduced by the julolidine residue. Studies on triaryl-
methane dyes, shows that the electron-donating ability of the
julolidine moiety is stronger than the 4-(N,N-dimethylamino)
phenyl group [13,14]. Julolidine dyes usually exhibit pronounced
bathochromic shifts as well an increase of the absorption band
intensity [15e18]. While in N,N-dimethylaniline the nitrogen
atom has sp3 hybridization, in the julolidine, due to the bridging
by the methylene groups, the nitrogen exists preferentially in
the sp2 hybridization which permits a more efficient conju-
gation [19].
the uncoloured state in few ns, the introduction of a conjugated
julolidine moiety in the structure lead to the overture of the oxa-
zine ring with formation of a stable cationic blue dye with
a conjugated 3H-indolium chromophore. Upon addition of base this
dye is easily transformed into a zwitterionic compound with two
chromophoric systems, absorbing at 410 and 587 nm, corre-
sponding to a very stable opened form of the benzo[1,3]oxazine
photochromic system, which is unable to undergo the intra-
molecular [1,3]oxazine ring closure.
4. Experimental
The reactions were monitored by thin-layer chromatography on
aluminium plates coated with Merck silica gel 60 F254 (0.25 mm).
Melting points were determined in open capillary tubes in a Buchi
530 melting point apparatus and are uncorrected. All compounds
were characterized by NMR and MS. 1H, 19F and 13C NMR and 2D
NMR spectra were recorded at 298 K in CDCl3, acetone-d6 and
DMSO-d6 using a Bruker ARX400 spectrometer (at 400.13 and
100.62 MHz) or a Bruker Avance-500 MHz spectrometer. Hetero-
nuclear 1He13C HSQC and HMBC experiments were carried out
using standard procedures. Chemical shifts (d) are reported in ppm
and coupling constants (J) in Hz. IR spectra were obtained on
a PerkineElmer FTIR 1600 spectrometer using KBr pellets. Wave-
numbers (nmax) are reported in cmꢁ1. UVeVis spectra were recor-
ded on a CARY 50 Varian spectrophotometer in spectral grade
acetonitrile. Low and high resolution electrospray ionization e time
of flight mass spectra (TOF-ESI) were measured with an AutoSpecE
spectrometer. All compounds were determined to be >95% pure by
1H NMR spectroscopy. Julolidine-9-carbaldehyde [9], starting
benzo[1,3]oxazine 1 [1] and model compound 4 [16] were prepared
according to literature procedures.
3. Conclusion
While the coloured ring opened forms of benzo[1,3]oxazines,
obtained upon UV excitation, are thermally unstable and return to

Y. Prostota et al. / Dyes and Pigments 96 (2013) 569e573
573
4.1. 1-(2-Hydroxy-5-nitrobenzyl)-3,3-dimethyl-2-[2-(2,3,6,7-
Acknowledgements
tetrahydro-1H,5H-benzo[i,j] quinolizin-9-yl)vinyl]-3H-indolium
trifluoroacetate 3
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). The 500 MHz NMR facilities
were funded by the Région Nord-Pas de Calais (France), the Min-
istère de la Jeunesse de l’Education Nationale et de la Recherche
(MJENR) and the Fonds Européens de Développement Régional
(FEDER).
A solution of 2-methylbenzo[1,3]oxazine 1 (0.30 g, 0.97 mmol),
julolidine-9-carbaldehyde (0.20 g, 1.00 mmol) and CF3CO2H
(0.074 mL, 0.97 mmol) in CH3CN (20 mL) was refluxed for 3 days.
The crystalline residue formed upon cooling was filtered off and
washed consequently with cold CH3CN (5 mL), i-PrOH (10 mL) and
Et2O (15 mL) to afford 3 as a deep green solid with a metal shining
(0.33 g, 56%). Mp. 220e222 ꢂC. IR: 593, 748, 808, 914, 1024, 1149,
1254, 1314, 1500, 1534, 1561, 1676, 2941. 1H NMR (DMSO-d6): 1.76
(s, 6H), 1.88e1.90 (m, 4H), 2.70e2.72 (m, 4H), 3.43e3.46 (m, 4H),
5.68 (s, 2H), 7.07 (d, J ¼ 9.0 Hz, 1H), 7.19 (d, J ¼ 15 Hz, 1H), 7.37 (t,
J ¼ 7.0 Hz, 1H), 7.41e7.49 (m, 3H), 7.63 (broad s, 2H), 7.74 (d,
J ¼ 7.0 Hz, 1H), 8.09e8.14 (m, 2H), 8.19 (d, J ¼ 15 Hz, 1H). 13C NMR
(DMSO-d6): 20.3, 26.7, 26.9, 43.5, 50.1, 102.7, 112.8, 115.9, 116.2,
118.2, 121.7, 121.9, 122.6, 124.5, 125.8, 126.3, 128.5, 139.4, 141.5, 141.7,
150.0, 153.7, 157.7, 157.9, 162.1, 177.8. 19F NMR (CDCl3): ꢁ73.4 (CF3).
HRMS (TOF ESI): calcd for [C31H32N3O3]þ 494.2438; found:
494.2434. When 0.34 molar equivalents of TFA were used, the same
crystalline solid was obtained in 43% yield.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.dyepig.2012.09.017.
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4.3. 4-Nitro-2-(3,3-dimethyl-2-[2-(2,3,6,7-tetrahydro-1H,5H-
benzo[i,j]quinolizin-9-yl)vinyl]-3H-indolium)phenolate 5
The trifluoroacetate 3H-indolium salt 3 (0.10 g, 0.18 mmol) was
dissolved in CHCl3 (75 mL). This solution was washed with aq. KOH
(100 mL, 0.01 M) water (100 mL), dried over anhydrous Na2SO4 and
evaporated to dryness under reduced pressure to afford the
phenolate 5 (0.05 g, 61%) as a deep-blue solid. Mp. 147e149 ꢂC
(dec.). IR: 639, 748, 923, 1014, 1089, 1154, 1259, 1314, 1509, 1574,
2835, 2925. 1H NMR (acetone-d6): 1.42 (s, 6H), 1.91 (quintuplet,
J ¼ 6.1 Hz, 4H), 2.68 (t, J ¼ 6.2 Hz, 4H), 3.19 (t, J ¼ 5.7 Hz 4H), 4.86 (s,
2H), 6.27 (d, J ¼ 15.6 Hz, 1H), 6.7e7.2 (m, 5H), 7.11 (t, J ¼ 7.7 Hz, 1H),
7.22 (d, J ¼ 7.2 Hz, 1H), 7.98 (dd, J ¼ 9.1 Hz, J ¼ 3.0 Hz, 1H), 8.11 (d,
J ¼ 2.8 Hz, 1H). 13C NMR (acetone-d6): 21.6, 23.4, 27.4, 40.3, 52.1,
109.0, 116.3, 117.5, 120.5, 121.0, 122.1, 123.6, 126.5, 127.4, 135.3, 137.6,
139.6, 140.2, 146.6, 160.3, 181.3. HRMS (TOF-ESI): calcd for
C31H32N3O3: 494.2438 [M þ H]þ; found: 494.2437.
[19] Barker CC, Hallas G. Steric effects in di- and tri-arylmethanes. Part IX. Elec-
tronic absorption spectra of julolidine (2,3,6,7-tetrahydro-1H,5H-benzo[ij]
quinolizine) and kairoline (1-methyl-1,2,3,4-tetrahydroquinoline) analogues
of Michler’s hydrol blue, malachite green, crystal violet, and Michler’s ketone.
J Chem Soc B 1969:1068e71.