Fast photochromic sterically hindered benzo[1,3]oxazines

Article abstract of DOI:10.1016/j.jphotochem.2010.09.006

A series of new substituted benzo[1,3]oxazines presenting bulky substituents on the chiral oxazine centre were prepared from isopropyl ketones or substituted cyclohexanones. Laser irradiation of these uncoloured compounds in solution promotes the cleavage

Full text of DOI:10.1016/j.jphotochem.2010.09.006

Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Fast photochromic sterically hindered benzo[1,3]oxazines
Yaroslav Prostotaa, Paulo Jorge Coelhoa,∗, João Pinab, João Seixas de Melob
a Centro de Química - Vila Real, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal
b Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 June 2010
Received in revised form 1 September 2010
Accepted 6 September 2010
Available online 17 September 2010
A series of new substituted benzo[1,3]oxazines presenting bulky substituents on the chiral oxazine
centre were prepared from isopropyl ketones or substituted cyclohexanones. Laser irradiation of
these uncoloured compounds in solution promotes the cleavage of the C–O bond and the opening of
the [1,3]oxazine ring generating a zwitterionic species, incorporating a 3H-indolium cation and a 4-
nitrophenolate anion, that absorbs strongly at 440 nm. The photogenerated coloured open isomers are
thermally unstable and revert to the initial closed form with first order kinetics and lifetimes ranging from
13 to 68 ns. These photochromic switches are extraordinarily stable displaying no significant degradation
upon repetition of various irradiation/dark cycles.
Keywords:
Photochromism
Oxazines
Photoswitches
Laser flash photolysis
Zwitterion
© 2010 Elsevier B.V. All rights reserved.
Indoles
1. Introduction
numerous colouration/decouloration cycles with no signs of degra-
dation [5]. UV irradiation ofthese uncoloured molecules leadsto the
Photochromism is defined as a reversible colour change induced
in a compound by the action of electromagnetic radiation. The
absorption of light causes a molecular rearrangement creating an
intensely coloured structure that can revert thermally or photo-
chemically to the original uncoloured form. The most studied and
used thermally reversible molecules (T-type) belong to four main
classes: chromenes and naphthopyrans, spiropyrans, spirooxazines
and azobenzenes [1]. These compounds are used today in many
practical applications such as variable-transmission optical mate-
rials like photochromic plastic ophthalmic lenses that darken in the
sunlight, surface coatings or authentication systems [2]. New appli-
cations like optical filters, optical switches and memories are now
being explored [3]. Their photochromic properties are very sensi-
ble to structural modifications and in the last 20 years hundreds of
new photochromes have been produced, tested and patented for
industrial applications [4].
cleavage of the C–O bond and consequent opening of the oxazine
ring with formation, in few ns, of a zwiterionic isomer incorporat-
ing a 3H-indolium cation and a 4-nitrophenolate anion that absorbs
strongly around 430–440 nm (Scheme 1). The photoinduced ring
opening brings the R group on the sp3 chiral centre in conjugation
with the 3H-indolium and therefore when the R substituent has
an extended ␲-system, two chromophores are created in the same
molecule: the p-nitrophenolate anion and the 3H-indolium cation
(Scheme 1). These chromophores absorb usually in the same region
of the visible spectrum and consequently a more intense absorption
is obtained upon excitation. This photogenerated coloured isomer
reverts thermally to the uncoloured original state in 25 ns with
first-order kinetics. This particular photochromic system is remark-
ably stable and tolerates several thousands of switching cycles
without significant degradation, even in the presence of molecu-
lar oxygen. However, their application is limited due to the very
low lifetime of the coloured open species. Their application in opti-
One of the main drawbacks of T-type molecules is their
relatively low photostability: upon successive cycles of coloura-
tion/decolouration
irreversibly converted to non-photochromic molecules, leading to
gradual weakening of colour upon repeated activation.
Benzo[1,3]oxazines are a new class of thermally reversible
photochromic molecules with an exceptional ability to perform
a portion of the dye is undesirably and
3H-indolium chromophore
R
2
O
R
UV-light
Dark
N
NO2
N
p-nitrophenolate
chromophore
NO2
O
Open form
bright yellow
Closed form
uncoloured
∗
Corresponding author. Tel.: +351 259350284; fax: +351 259350480.
E-mail address: pcoelho@utad.pt (P.J. Coelho).
Scheme 1. Photochromic equilibrium for benzo[1,3]oxazines.
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.09.006

60
Y. Prostota et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
R2
a R1= isopropyl
b R1= 2,4-dimethoxyphenyl
c R1= 2,4-dimethylphenyl
d R2= CH3 R3= H
e R2= R3= CH3
f R2= Ph R3= H
R1
2
R3
2
O
N
N
NO2
NO2
3a-c
3d-f
Scheme 2. Formulae of new substituted benzo[1,3]oxazines.
Cl OH
R
1) Benzene, reflux
NO2
O
+
N
O
R
R
NH2
2) HOAC or BF3.Et2O
N
MeCN, Δ
2a-c
3a-c
1a-c
NO2
Scheme 3. Synthesis of benzo[1,3]oxazines 3a–c (a: R = isopropyl, b: R = 2,4-dimethoxyphenyl, c: R = 2,4-dimethylphenyl).
Cl OH
R2
R2
R2
NO2
1) Benzene, reflux
2) HOAC
R3
+
O
N
O
NH2
R3
N
MeCN, Δ
R3
NO2
3d-f
2d-f
1d-f
Scheme 4. Synthesis of benzo[1,3]oxazines 3d–f (d: R2 = CH3, R3 = H, e: R2 = R3 = CH3, f: R2 = Ph, R3 = H).
cal devices depends on the possibility to control and modulate the
photochromic properties in particular the colour of the photogen-
erated state, the efficiency of the photochemical transformation
and the kinetics of the thermal back reaction [6].
in general. Some years ago the synthesis of some cyclohexane-fused
benzo[1,3]oxazines was described but the photochromic proper-
ties of these molecules have not been investigated [9]. Taking into
consideration that the presence of sterically hindered substituents
near the reactive C-2 atom could slowdown the ring closure reac-
tion, we prepared and studied the photochromic properties of a
series of new benzo[1,3]oxazines (Scheme 2).
Since the open form has a strong zwitterionic character, the
introduction of some electron donating substituents in the oxazine
structure could have a significant effect on the stability of this
species and thus on the kinetics of the fading process. Raymo et al.
showed that the lifetime of the open form could be increased 400
times by the use of a p-dimethylaminophenyl substituent at the
oxazine chiral centre (position 2), while the introduction of elec-
tron donating groups on the indole aromatic ring has a negligible
effect on the ring closure kinetics [7]. The nature of the substituent
at the chiral centre can also be exploited to regulate the absorption
characteristics of the 3H-indolium cation on the photogenerated
isomer [8].
2. Synthesis
The target molecules were prepared in a 3-step synthesis
from substituted cyclohexanones or isopropyl ketones according
to Schemes 3 and 4. The Fischer condensation of the phenylhy-
drazones derived from isopropyl ketones 1a–c or cyclohexanones
1d–f under acidic conditions afforded the corresponding 3H-
indoles 2a–c and 2,2,4,4a-tetrahydrocarbazoles 2d–f in moderate
yields using modified literature procedures [10,11]. Surprisingly,
the Fischer condensation of the phenylhydrazone of the 2,4-
dimethoxyphenyl isopropyl ketone 1c, performed in acetic acid, led
to the cleavage of the ortho-methoxy group giving 3H-indole 2g in
Although many benzo[1,3]oxazines derivatives have been
obtained and their photochromic properties investigated, the intro-
duction of sterically hindered groups, such as isopropyl or ortho
substituted phenyl on the chiral centre have not been investigated
Cl OH
OMe
NNHPh
OMe
Me
Me
HO
Me
Me
2' 3'
NO2
OH
O
HOAc
3
2
OMe
MeCN
Δ, 2d
N
4'
1'
N
Δ, 2d
5'
1
6'
OMe
2g
3g
NO2
Scheme 5. Synthesis of benzo[1,3]oxazine 3g.

Y. Prostota et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
61
65% yield (Scheme 5). The position of the OH group was deduced by
3.0
2.0
1.0
0.0
430 nm
2D NMR analysis. The correlation between the methoxy group at
3.85 ppm and protons H-5ꢀ (6.51 ppm) and H-3ꢀ (6.62 ppm) found
in the NOESY spectrum is consistent with a p-CH3O group and thus
the OH group is in the ortho position. Changing the acid catalyst
to the more mild Lewis acid BF3·Et2O led to the formation of the
desired 3H-indole 2c.
The subsequent N-alkylation of the correspondent indole
derivatives 2a–g with 2-chloromethyl-4-nitrophenol in acetoni-
trile followed by spontaneous intramolecular oxazine cyclization
yielded the target compounds 3a–g (11–53% yield).
b
O
N
3f
NO2
c
a
The 1H NMR spectra of [1,3]benzooxazines 3a–g
showed some characteristic signals. Indeed, all compounds
exhibited the expected aromatic signals although the chemical
shift depends on the substituent connected to C-2. Due to the C-2
oxazine chiral centre the two adjacent methyl groups in com-
pounds 3a–c are not equivalent and therefore a set of two distinct
singlets appear between 0.83 and 1.67 ppm, while for compound
3g these methyl groups are observed at the same chemical shift
(1.60 ppm). The N–CH2 signals appear at 4.51–4.80 ppm as an AB
system, multiplet or as singlet. All compounds showed a character-
istic resonance in the 13C NMR spectra at 102–106 ppm assigned to
the C-2 (chiral) carbon atom of the oxazine ring. The existence of
this signal can be used to confirm the formation of the [1,3]oxazine
ring. The 1H NMR spectra of benzo[1,3]oxazine 3c showed two
resonances for the ortho methyl group in the 2,4-dimethylphenyl
substituent at 2.53/2.64 ppm and two signals for each of the indole
methyl groups at 0.86/0.97 and 1.57/1.66 ppm. This may indicate
the existence of two rotational isomers of 3c due to a rotation
barrier for this substituent.
250
300
350
400
450
500
λ (nm)
Fig. 1. UV–vis absorption spectra of (a) benzo[1,3]oxazine 3f in acetonitrile (b) after
the addition of Bu4NOH in acetonitrile and (c) after the subsequent addition of HCl(aq)
(5%).
substituent) shows a ∼20 nm red-shift (ꢀmax = 339 nm) for the max-
imum absorption wavelength (Fig. 2).
The oxazine cycle can be opened by the addition of base, with the
formation of a stable coloured hemiaminal compound (Scheme 6).
Addition of Bu4NOH (2 equiv.) to benzo[1,3]oxazines 3a–g acetoni-
trile solutions led to the appearance of a strong absorption between
430 and 438 nm characteristic of the 4-nitrophenolate anion [12]
due to the opening of the [1,3]oxazine ring (see Figs. 1 and 2). This
conversion is reversible and upon addition of acid we can observe
the disappearance of this band caused by the ring closure process
leading to the initial benzo[1,3]oxazine or due to the formation of
open protonated cationic form that displays a different absorption
spectra.
3. Photochromic properties
Laser irradiation of benzo[1,3]oxazines 3a–g in acetonitrile
(10−4 M), at 355 nm, lead to the fast development of an absorption
at 440 nm that can be attributed to the 4-nitrophenolate chro-
mophore, formed upon opening of the oxazine ring. Although the
benzo[1,3]oxazines absorption bands are centred at 304–339 nm
The UV spectra of benzo[1,3]oxazines 3a–f recorded in acetoni-
trile shows a strong absorption band in the range 304–319 nm
(Fig. 1 and Table 1) that has been assigned to the 4-nitrophenoxy
fragment [5] while 3g (that displays a o-hydroxy-p-methoxyphenyl
Table 1
Absorption spectroscopic data (ꢀmax and absorption coefficient) for benzo[1,3]oxazines 3a–g, ꢀmax after the addition of base and fading rate constants (kꢁ) and half-life time
(t1/2) of the photogenerated isomers after laser excitation (355 nm).
a
Benzo[1,3]oxazine
ꢀmax (ε × 103) nm (M−1 cm−1
)
ꢀmax (base)
kꢁ × 107 (s−1
)
t1/2 (ns)
3a
3b
316 (10.18)
423
3.12
32.1
R1
O
MeO
Me
N
314 (15.41)
316 (12.77)
431
427
438
5.93
7.49
1.47
16.9
13.4
68.0
OMe
NO2
3c
3g
Me
HO
339 (44.30) 415 (sh.) (11.00)
OMe
R2
R3
H
CH3
H
R2
3d
3e
3f
CH3
CH3
Ph
320 (9.34)
319 (11.79)
319 (14.84)
430
430
430
5.14
4.73
3.70
19.5
21.1
27.0
R3
O
N
NO2
a
After the addition of Bu4NOH in acetonitrile.

62
Y. Prostota et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
R
O
R
R
OH-
H+
OH-
H+
NO2
NO2
N
N
N
OH
NO2
HO
Protonated form
O
Yellow
Closed form
uncoloured
Scheme 6. Reversible ring opening and ring closing of benzo[1,3]oxazines in basic and acid medium.
0.9
0.6
0.3
0.0
OMe
OH
Light off
Light off
Light off
Light off
438 nm
0.015
0.010
0.005
0.000
-0.005
Me
N
Me
O
b
3g
c
NO2
a
Light on
0
Light on
2000
Light on
Light on
6000
250
300
350
400
450
500
4000
λ(nm)
time (ns)
Fig. 2. UV–vis absorption spectra of (a) benzo[1,3]oxazine 3g in acetonitrile (b) after
the addition of Bu4NOH in acetonitrile and (c) after the subsequent addition of HCl(aq)
(5%).
Fig. 4. Four consecutive switching cycles of benzo[1,3]oxazine 3a in acetonitrile
performed by laser excitation at 355 nm and recorded at 440 nm.
they are sufficient large for the molecules to be activated with the
355 nm laser light. The evolution of the absorbance at 440 nm indi-
cates that the formation of the open form occurs within the laser
pulse (ca. 10 ns) (Fig. 3). After the pulse, the absorbance decays
monoexponentially corresponding to the thermal ring closure of a
single opened coloured species to the initial [1,3]oxazine ring. The
full return to the initial state occurs within 50–100 ns. Therefore a
full switching cycle can be completed on a nanosecond timescale.
The ring closure rate constants of benzo[1,3]oxazine 3a–g
are presented in Table 1. The fading kinetics for compounds
3a–f are very homogeneous and vary between 3.1 × 107 s−1 and
7.5 × 107 s−1 corresponding to a half-life time of the coloured open
isomers between 13 and 68 ns. These values are in accordance to
the ones obtained with other benzo[1,3]oxazines and demonstrate
that the fading kinetics are poorly sensible to the steric effects of the
substituents on the chiral C-2 atom. However for benzo[1,3]oxazine
3g a significant higher half-life time was observed (68 ns). This may
be due to some stabilization of the 3H-indolium cation promoted
by the ortho-hydroxy group present in the C-2 phenyl substituent
that delays the ring-closing process.
Finally it is worth noting that, as can be seen from
Fig. 4, after four consecutive opening/closing cycles of the
benzo[1,3]oxazine 3a no significant variation of the maximum
absorption is observed. The very high switching speeds of these
benzo[1,3]oxazines allows to change repeatedly the absorbance
of the solution in few nanoseconds just by turning a laser on and
off.
kΔ= 5.14x107 s-1
0.030
0.015
0.000
4. Conclusion
Laser UV irradiation of a set of new benzo[1,3]oxazines present-
ing hindered substituents at the ozaxine chiral centre promotes
the cleavage of the C–O bond of the oxazine ring in few ns, leading
to the formation of coloured thermally unstable zwitterionic
species that return completely to the initial molecule in <100 ns.
The switching speeds exhibited by these molecules were poorly
sensible to the steric effect of the substituent present on the
chiral oxazine centre, however a significant stabilization of the
open form was observed for benzo[1,3]oxazine 3g which present
0
80
160
time (ns)
0.003
0.000
-0.003
Regular Residual
Fig. 3. Time evolution of the absorption at 440 nm upon laser irradiation of
benzo[1,3]oxazine 3d in acetonitrile, the first order fit to the decay (line in red)
and the regular residual plot distribution (obtained from the difference between
a
o-hydroxyphenyl substituent at C-2. These photochromic
∧
the experimental and the values generated by the fitting function (yi − yi)). (For
switches can perform repeatedly several opening/closing
cycles.
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of the article.)

Y. Prostota et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
63
5. Experimental
amount of p-toluenesulfonic acid monohydrate in dry benzene or
xylene (50 mL) was heated under reflux and the water distilled off
using a water removal trap (3–4 h). The solution was evaporated to
dryness under reduced pressure to afford the corresponding crude
phenylhydrazone that was purified by recrystallization, CC or used
in the next step without further purification.
(b) Fischer reaction: the phenylhydrazone (14 mmol) was dis-
solved in glacial acetic acid (15 mL) and heated under reflux for 4 h.
After cooling to room temperature the solution was poured into
water (50 mL) and made alkaline with NaOH(aq) (20%). The result-
ing mixture was extracted with CH2Cl2 (3 × 20 mL), washed with
water (50 mL) and dried over anhydrous Na2SO4. After removal of
the solvent under reduced pressure the residue was purified by CC
[EtOAc–petroleum ether (1:1 or 1:3 v/v)] or recrystallization.
5.1. Materials
The reactions were monitored by thin-layer chromatogra-
phy on aluminium plates precoated with Merck silica gel 60
F254 (0.25 mm). Column chromatography (CC) was performed on
silica gel 60 (70–230 mesh). The new compounds were deter-
mined to be >95% pure by 1H NMR spectroscopy. CH2Cl2 was
pre-dried under phosphorus pentoxide and distilled before use.
Ketones 1a, 1d–f are commercially available. 2,4-Dimethylphenyl
isopropyl ketone 1b was prepared according to the literature pro-
cedure [13]. 4a-Methyl-2,3,4,4a-tetrahydro-1H-carbazole 2d was
prepared according to the literature [10].
5.2. Instrumentation
5.5.1. 2-Isopropyl-3,3-dimethyl-3H-indole 2a
All compounds were characterized by IR, NMR and MS. 1H and
13C NMR spectra were recorded at 298 K in CDCl3 using a Bruker
ARX400 spectrometer (at 400.13 and 100.62 MHz). Chemical shifts
(ı) are reported in ppm and coupling constants (J) in Hz. UV–vis
spectra were recorded on a CARY 50 Varian spectrophotometer in
spectral grade acetonitrile. IR spectra were obtained on a Perkin-
Elmer FTIR 1600 spectrometer using KBr disks (wave numbers in
cm−1). Electronic impact mass spectra were measured with an
AutoSpecE spectrometer (values in m/z (%)).
Phenylhydrazone: the reaction was performed in benzene and
the crude compound (orange liquid, 96% yield) was used without
further purification. The 3H-indole 2a was purified by CC (1:1 v/v) to
afford a yellow-reddish solid that was recrystallized from hexane.
44% yield. Mp 56–58 ◦C. IR: 1049, 1312, 1665. 1H NMR: 1.32 (m,
12H). 2.91 (sept, J = 7 Hz, 1H), 7.17 (t, J = 7 Hz, 1H), 7.23 (d, J = 6 Hz,
1H), 7.26 (t, J = 8 Hz, 1H), 7.56 (d, J = 8 Hz, 1H). 13C NMR: 22.0, 22.7,
24.1, 25.4, 27.9, 54.1, 119.9, 121.0, 125.0, 127.4, 145.0, 153.6, 195.9.
MS (TOF): 115 (18), 117 (26), 130 (25), 144 (72), 145 (33), 172 (100,
[M−CH3]+), 187 (44, M+).
5.3. Laser flash photolysis
5.5.2. 3,3-Dimethyl-2-(2,4-dimethylphenyl)-3H-indole 2b
The decays for the coloured open forms were obtained by irra-
diating 0.1 mM acetonitrile solutions of benzo[1,3]oxazines 3a–g
with the third harmonic (355 nm, ∼10 ns FWHM, ∼8 mJ) of a
Nd:YAG laser (Spectra Physics) and collected at 440 nm with a laser
flash photolysis apparatus (Applied Photophysics). The detection
system is at right angles to the excitation beam and a pulsed 150 W
Xe lamp is used to analyze the absorption of the open form. The
signal is fed into a Tektronix TDS 3052B digital analyzer and trans-
ferred to an IBM RISC computer where the decays were analyzed
with appropriate software (Applied Photophysics). Further details
of this can be found in Ref. [14].
Phenylhydrazone: the reaction was performed in xylene and the
crude oily residue was dissolved in EtOAc and purified by CC (1:3
v/v) on a short silica column to afford, after solvent evaporation,
the phenylhydrazone of the 2,4-dimethylphenyl isopropyl ketone
as an orange liquid (75% yield). The 3H-indole 2b was purified by CC.
Bright-yellow solid. 82% yield. Mp 59–62 ◦C. IR: 1116, 1308, 1661,
2806, 2889. 1H NMR: 1.39 (s, 6H), 2.30 (s, 3H), 2.36 (s, 3H), 7.04 (d,
J = 8 Hz, 1H), 7.13 (s, 1H), 7.19 (d, J = 8 Hz, 1H), 7.26 (t, J = 7 Hz, 1H),
7.32–7.37 (m, 2H), 7.65 (d, J = 8 Hz, 1H). 13C NMR: 20.1, 20.2, 23.1,
55.8, 120.1, 121.2, 125.6, 125.8, 127.5, 127.6, 131.5, 131.5, 136.7,
138.4, 153.6, 187.1. MS (TOF): m/z: 133 (21), 144 (41), 234 (72),
248 (100, [M−H]+), 249 (40, M+)
5.4. Synthesis of 2,4-dimethoxylphenyl isopropyl ketone 1c
A solution of isobutyryl chloride (4.2 mL, 38 mmol) in dry CH2Cl2
(10 mL) was added dropwise over 20 min to a chilled (0 ◦C) mix-
ture of 1,4-dimethoxybenzene (5.00 g, 36.2 mmol) and anhydrous
AlCl3 (4.83 g, 36.2 mmol) in dry CH2Cl2 (50 mL)and the temperature
maintained below 0 ◦C. After the end of the addition the result-
ing solution was kept for 3 h at this temperature and then poured
into ice (100 g), stirred until the ice melted and extracted with
CH2Cl2 (3 × 20 mL). The organic phase was washed with NaOH(aq)
(5%, 50 mL), water (50 mL), dried over anhydrous Na2SO4 and then
evaporated to dryness under reduced pressure to afford ketone 1c
(6.98 g, 93% yield) as a colourless liquid. IR: 1029, 1214, 1465, 1598,
1665, 2972. 1H NMR: 1.11 (d, J = 7 Hz, 6H), 3.48 (sept, J = 7 Hz, 1H),
3.82 (s, 3H), 3.86 (s, 3H), 6.43 (d, J = 2 Hz, 1H), 6.48 (dd, J = 2.0, 9.0 Hz,
1H), 7.64 (d, J = 9 Hz, 1H). 13C NMR: 18.7, 39.5, 55.4, 55.4, 98.3, 104.9,
121.2, 132.5, 160.0, 163.8, 205.7. MS (TOF): 77 (13), 92 (9), 107 (23),
122 (22), 165 (100) (M+−CH2 CH–CH3), 166 (18), 179 (9), 208 (4,
M+).
5.5.3. 2-(2,4-Dimethoxyphenyl)-3,3-dimethyl-3H-indole 2c
Phenylhydrazone: the reaction was performed in benzene and
the crude solid residue was purified by recrystallization from
benzene–petroleum ether [1:3 v/v] to afford the phenylhydrazone
of 2,4-dimethoxylphenyl isopropyl ketone 1c as a white solid. 70%
yield. Mp 120–122 ◦C. IR: 1026, 1128, 1209, 1260, 1307, 1458, 1501,
1601, 2969, 3452. 1H NMR 1.13 (d, J = 7 Hz, 6H), 2.73 (sept, J = 7 Hz,
1H), 3.75 (s, 3H), 3.85 (s, 3H), 6.56–6.58 (m, 2H), 6.72 (t, J = 7 Hz,
1H), 6.94–7.01 (m, 4H), 7.17 (d, J = 7 Hz, 1H), 7.35 (s, 1H). 13C NMR:
20.2, 35.9, 55.3, 55.4, 99.0, 105.0, 112.5, 115.3, 118.9, 128.9, 130.0,
145.8, 150.0, 157.6, 161.3. MS (TOF): 65 (33), 77 (31), 84 (41), 91
(93), 133 (37), 149 (61), 161 (61), 164 (80), 178 (84), 206 (51), 298
[M+H]+ (100%). The synthesis of the 3H-indole 2c was performed
as described before except that the reaction was performed in the
presence of BF3·Et2O (2 equiv.). The residue was purified by recrys-
tallization from benzene to afford 3H-indole 2c as a white solid.
67% yield. Mp 117–119 ◦C. IR: 1029, 1159, 1304, 1469, 1595, 2967,
3463. 1H NMR: 1.29 (s, 6H), 3.70 (s, 3H), 3.80 (s, 3H), 6.48–6.50 (m,
2H), 7.18–7.21 (m, 2H), 7.25–7.28 (m, 2H), 7.60 (d, J = 8 Hz, 1H). 13C
NMR: 23.1, 55.2, 55.5, 98.6, 104.1, 116.9, 120.7, 121.1, 125.4, 153.6,
158.6, 161.5, 185.9. MS (TOF): 117 (31), 120 (74), 236 (23), 250 (45),
266 (66), 281 (100, M+)
5.5. General procedure for the synthesis of 3H-indoles 2a–c and
carbazoles 2d–f
(a) Synthesis of the phenylhydrazones: a solution of ketone 1
(20 mmol), phenylhydrazine (2.00 mL, 20.0 mmol) and a catalytic

64
Y. Prostota et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
5.5.4. 1,4a-Dimethyl-2,3,4,4a-tetrahydro-1H-carbazole 2e
5.6.2. 5a-(2,4-Dimethoxyphenyl)-6,6-dimethyl-2-nitro-5a,
Phenylhydrazone: the reaction was performed in benzene and
the crude compound (orange liquid, 94% yield) was used without
further purification. The carbazole 2e was purified by CC. Bright-
yellow oily solid. Mp 39–41 ◦C (26% yield). IR: 1061, 1186, 1241,
1276, 1310, 1449, 1550, 1720, 2932. 1H NMR: 1.16 (s, 3H), 1.21–1.24
(m, 4H), 1.37–1.57 (m, 3H), 2.08–2.15 (m, 1H), 2.47–2.55 (m, 1H),
2.80–2.85 (m, 1H), 7.17 (t, J = 7 Hz, 1H), 7.31 (t, J = 8 Hz, 1H), 7.38 (d,
J = 8 Hz, 1H), 7.57 (1H, d, J = 7 Hz). 13C NMR: 13.8, 14.4, 16.3, 27.8,
28.9, 29.7, 41.7, 56.00, 120.0, 122.7, 124.2, 127.3, 145.6, 154.2, 190.6.
MS (TOF): 144 (17), 184 (100, [M−CH3]+), 199 (25, M+).
6-dihydro-12H-indolo[2,1-b]benzo[1,3]oxazine 3b
After cooling to ambient temperature the solid formed
was filtered off, washed with acetonitrile (10 mL), dissolved in
acetonitrile–water [10:1 v/v] and then KOH(aq) (0.05 M, 20 mL)
was added. The solid formed on standing was filtered off, washed
with water (20 mL) and dried at air to afford benzo[1,3]oxazine 3b
(0.33 g, 53%) as a slightly yellowish solid. Mp = 171–173 ◦C. IR: 1028,
1248, 1334, 1481, 1589, 2953, 3052. 1H NMR (CDCl3): 0.85–1.01
(broad s, 3H), 1.52–1.57 (broad s, 3H), 3.78 (s, 3H), 3.79 (s, 3H),
4.57 (m, 2H), 6.45 (m, 2H), 6.62 (m, 1H), 6.82 (m, 2H), 7.14 (m, 2H),
7.89 (broad s, 1H), 7.9 (m, 2H). 13C NMR (DMSO-d6, 50 ◦C): 23.0,
49.6, 54.9, 55.4, 99.4, 104.7, 104.9, 108.1, 114.4, 117.7, 119.2, 121.0,
122.8, 122.9, 127.1, 131.3, 137.0, 140.0, 146.5, 158.7, 158.9, 160.8.
HRMS calcd for C25H24N2O5: 432.1685; found: 432.1686.
5.5.5. 4a-Phenyl-2,3,4,4a-tetrahydro-1H-carbazole 2f
Phenylhydrazone: the reaction was performed in benzene and
the crude compound (orange liquid, 96% yield) was used without
further purification. The carbazole 2f was purified by CC. yellowish
solid. 48% yield. Mp 122–124 ◦C. (lit. 124–125 ◦C [12]). IR: 1024,
1071, 1095, 1126, 1448, 1495, 1577, 1713, 2924, 3061. 1H NMR:
1.22–1.34 (m, 2H), 1.48–1.71 (m, 4H), 2.09–2.13 (m, 1H), 2.51–2.
59 (m, 1H), 3.11–3.16 (m, 1H), 2.92–2.95 (m, 1H), 7.05–7.11 (m,
4H), 7.18–7.31 (m, 4H), 7.61 (d, J = 8 Hz, 1H). 13C NMR: 21.7, 29.1,
30.4, 36.4, 62.7, 120.3, 122.1, 125.1, 126.9, 127.4, 129.1, 138.2, 147.3,
153.8, 188.8. MS (TOF): 131 (11), 204 (13), 217 (50), 246 (42), 247
(100, M+).
5.6.3. 6,6-Dimethyl-5a-(2,4-dimethylphenyl)-2-nitro-5a,
6-dihydro-12H-indolo[2,1-b]benzo[1,3]oxazine 3c
After cooling to ambient temperature the solid formed
was filtered off and recrystallized from acetone to afford
benzo[1,3]oxazine 3c (0.33 g, 42%) as a slightly yellowish solid. Mp
184–186 ◦C. IR: 1015, 1041, 1083, 1126, 1267, 1328, 1446, 1498,
1587, 1611, 2598, 3061. 1H NMR: 0.86 and 0.97 (2s, 3H, 70/30 ratio),
1.57 and 1.66 (2s, 3H, 30/70 ratio), 2.28 (s, 3H), 2.53 and 2.64 (2s,
3H, 70/30 ratio), 4.51 (m, 2H), 6.65 (d, J = 8 Hz, 1H), 6.83–6.88 (m,
2H), 6.96–7.02 (m, 2H), 7.11–7.15 (m, 2H), 7.49 and 7.61 (m, 1H,
30/70 ratio), 7.91–7.93 (m, 2H). 13C NMR: 20.5, 20.8, 22.7, 26.6, 28.2,
30.9, 40.7, 51.1, 105.7, 107.6, 108.6, 117.6, 120.1, 120.5, 122.4, 123.1,
123.6, 126.6, 127.6, 130.3, 133.7, 134.1, 137.3, 138.8, 140.8, 146.5,
158.6. HRMS: calc. for C25H24N2O3: 400.1787; found: 400.1794.
5.5.6. 2-(2-Hydroxy-4-methoxyphenyl)-3,3-dimethyl-3H-indole
2g
The reaction was performed as described before except that the
extraction was made with Et2O instead of CH2Cl2. The residue was
purified by CC (1:1 v/v) to afford 3H-indole 2g (2.36 g, 65% yield) as
a bright yellow solid. Mp 114–116 ◦C. IR: 1034, 1107, 1152, 1267,
1407, 1466, 1503, 1609, 2939, 3045. 1H NMR: 1.62 (s, 6H), 3.85
(s, 3H), 6.51 (dd, J = 3.0, 9.0 Hz, 1H), 6.62 (d, J = 3.0 Hz, 1H), 7.22
(t, J = 8 Hz, 1H), 7.31–7.36 (m, 2H), 7.53 (d, J = 7 Hz, 1H), 7.70 (d,
J = 9.0 Hz, 1H), 14.9 (s, 1H). 13C NMR: 25.2, 53.2, 55.3, 101.8, 106.5,
109.3, 119.0, 121.0, 125.5, 127.9, 129.6, 145.4, 150.4, 163.3, 164.8,
184.1. HRMS: calc. for C17H17NO2: 267.1257; found: 267.1255.
5.6.4. 15b-Methyl-8-nitro-1,3,4,15b-tetrahydro-2H,
10H[1,3]benzoxazino[2,3-k]carbazole 3d
A
solution of carbazole 2d (1.00 g, 5.4 mmol) and 2-
chloromethyl-4-nitrophenol (1.10 g, 5.95 mmol) in acetonitrile
(20 mL) was kept at ambient temperature for 3 h and then 12 h at
−5 ◦C. The solid formed was filtered off and recrystallized from
acetone to afford benzo[1,3]oxazine 3d (0.40 g) as a slightly yel-
lowish solid. The filtrate was evaporated under reduced pressure
and the residue recrystallized from acetone to obtain additional
amounts of 3d (0.34 g). Overall yield: 0.74 g, 41%. Mp 180–182 ◦C
(lit [9] 178–179 ◦C). IR: 1006, 1080, 1265, 1465, 1590, 2924. 1H
NMR: 1.26–1.56 (m, 7H), 1.71–1.79 (m, 3H), 2.23–2.27 (m, 1H),
4.57 (d, J = 18 Hz) and 4.63 (d, J = 18 Hz) (AB system, 2H), 6.61 (d,
J = 8 Hz, 1H), 6.66 (d, J = 9 Hz, 1H), 6.81 (t, J = 8 Hz, 1H), 7.05–7.25
5.6. General procedure for the synthesis of benzo[1,3]oxazines
3a–f
A solution of 3H-indole or carbazole 2a–f (1.55 mmol) and
2-chloromethyl-4-nitrophenol (0.30 g, 1.6 mmol) in acetonitrile
(10 mL) was heated under reflux for 48 h. After cooling to ambient
temperature, the solvent was evaporated under reduced pressure
and the residue purified by crystallization or CC [EtOAc–hexanes
(1:3 v/v)].
(m, 2H), 7.90 (dd, J = 3.0 and 9.0 Hz, 1H), 8.03 (d, J = 3.0 Hz, 1H). 13
NMR: 15.4, 21.2, 22.5, 27.8, 39.7, 39.8, 47.4, 102.6, 109.1, 117.8,
118.7, 120.4, 120.5, 121.5, 123.2, 127.2, 138.9, 140.2, 146.3, 159.4.
HRMS: calc. for C20H20N2O3: 336.1474; found: 336.1472.
C
5.6.1. 5a-Isopropyl-6,6-dimethyl-2-nitro-5a,6-dihydro-12H-
indolo[2,1-b]benzo[1,3]oxazine 3a
5.6.5. 4,15b-Dimethyl-8-nitro-1,3,4,15b-tetrahydro-2H,
10H[1,3]benzoxazino[2,3-k]carbazole 3e
The residue was dissolved in CH2Cl2 (40 mL) washed with
KOH(aq) (0.05 M, 10 mL), water (40 mL), dried over anhydrous
Na2SO4 and evaporated to dryness under reduced pressure. The
residue was purified by CC and finally purified by crystallization
from benzene–hexanes (1:3 v/v) to afford benzo[1,3]oxazine 3a
(0.14 g, 26%) as a white solid. Mp 125–127 ◦C. IR: 1003, 1280, 1484,
1602, 2948. 1H NMR: 0.75 (broad s, 3H), 0.85 (s, 3H), 1.48 (broad
s, 3H), 1.66 (s, 3H), 1.98 (broad s, 1H), 4.58 (s, 2H), 6.53 (d, J = 8 Hz,
1H), 6.68 (d, J = 9 Hz, 1H), 6.79 (t, J = 7 Hz, 1H), 7.07 (m, 2H), 7.91
(dd, J = 2.0, 9.0 Hz, 1H), 8.04 (d, J = 2.0 Hz, 1H). 13C NMR: 16.8, 16.9,
33.7, 39.7, 54.3, 108.0, 102.7, 118, 118.6, 119.8, 123.1, 124.0, 124.3,
127.6, 128.3, 140.3, 159.1. HRMS: calc. for C20H22N2O3: 338.1630;
found: 338.1622.
After cooling to ambient temperature the solvent was evap-
orated under reduced pressure. The residue was dissolved in
CH2Cl2 (50 mL), washed with KOH(aq) (0.05 M, 20 mL), water
(50 mL), dried over anhydrous Na2SO4 and evaporated to dryness
under reduced pressure. The residue was purified by CC to afford
benzo[1,3]oxazine 3e (0.30 g, 39%) as a slightly yellowish solid.
Mp 128–130 ◦C. IR: 1093, 1261, 1343, 1460, 1606, 2914, 3065. 1H
NMR: 0.63 (d, J = 7 Hz, 1H), 0.87 (d, J = 7 Hz, 2H), 1.24–1.96 (m, 9H),
2.20–2.27 (m, 1H), 4.53–4.64 (m, 2H), 6.56–6.69 (m, 2H), 6.79 (t,
J = 7 Hz, 2H), 7.04–7.13 (m, 2H), 7.89–7.93 (m, 2H), 8.02 (d, J = 3 Hz,
1H). 13C NMR: 8.3, 16.1, 22.0, 27.1, 27.5, 29.2, 39.0, 39.7, 51.2, 103.8,
109.2, 117.3, 118.8, 123.2, 123.6, 123.8, 125.5, 135.4, 140.1, 146.8,
159.4. HRMS: calc. for C21H22N2O3: 350.1630; found: 350.1630.

Y. Prostota et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 59–65
65
5.6.6. 15b-Phenyl-8-nitro-1,3,4,15b-tetrahydro-2H,
Acknowledgements
10H[1,3]benzoxazino[2,3-k]carbazole 3f
After cooling to ambient temperature the solid formed was fil-
tered off, washed with acetonitrile (10 mL), dissolved in a mixture
of acetonitrile–water (10:1 v/v) and then KOH(aq) (0.05 M, 20 mL)
was added. The solid formed on standing was filtered off, washed
with water (20 mL) and dried at air to afford benzo[1,3]oxazine 3f
(0.22 g, 34%) as a slightly yellowish solid. Mp 153–155 ◦C. IR: 1025,
1088, 1192, 1257, 1339, 1472, 1589, 2858, 2940, 3065. 1H NMR:
1.67–1.83 (m, 5H), 1.91–1.95 (m, 1H), 2.16–2.20 (d, J = 11 Hz, 1H),
2.58 (d, J = 14 Hz, 1H), 4.58 (d, J = 18 Hz) and 4.68 (d, J = 18 Hz) (AB
system, 2H), 6.54 (d, J = 9 Hz, 1H), 6.68 (d, J = 8 Hz, 1H), 6.76–6.80
(m, 2H), 7.12 (t, J = 6 Hz, 1H), 7.34–7.38 (m, 3H), 7.56–7.58 (m,
2H), 7.90 (dd, J = 3.0, 6 Hz, 1H), 8.07 (d, J = 3 Hz, 1H).13C NMR
(CDCl3): 21.9, 23.8, 28.5, 38.6, 39.7, 56.1, 102.1, 109.1, 117.7, 118.7,
120.5, 123.2, 123.9, 124.1, 127.0, 127.2, 127.6, 131.2, 139.3, 139.8,
140.4, 146.5, 158.9. HRMS: calc. for C25H22N2O3: 398.1630; found:
398.1630.
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), program Ciência 2008 and
for a post-doctoral grant to J. Pina (SFRH/BPD/65507/2009).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.09.006.
References
[1] (a) J.C. Crano, R. Guglielmetti (Eds.), Organic Photochromic Thermochromic
Compounds. Main Photochromic Families, vol. 1, Plenum, New York, 1998;
(b) S.K. Yesodha, C.K.S. Pillai, N. Tsutsumi, Prog. Polym. Sci. 29 (2004) 45–74.
[2] J. Crano, T. Flood, D. Knowles, A. Kumar, B. Van Gemert, Pure Appl. Chem. 68
(1996) 1395–1398.
[3] (a) A. Poscik, B. Wandelt, Synth. Met. 159 (2009) 723–728;
(b) K. Kunpeng, Y. Chen, J. Phys. Org. Chem. 23 (2009) 207–210;
(c) S. Kawata, Y. Kawata, Chem. Rev. 100 (2000) 1777–1788.
[4] S.N. Corns, S.M. Partington, A.D. Towns, Color. Technol. 125 (2009) 249–261.
[5] M. Tomasulo, S. Sortino, A.J.P. White, F. Raymo, J. Org. Chem. 70 (2005) 283–391.
[6] M.A. Petersen, E. Deniz, M.B. Nielsen, S. Sortino, M. Raymo, Eur. J. Org. Chem.
25 (2009) 4333–4339.
5.6.7. 5a-(2-Hydroxy-4-methoxyphenyl)-6,6-dimethyl-2-
nitro-5a,6-dihydro-12H-indolo[2,1b][1,3]benzoxazine 3g
After cooling to ambient temperature the solid formed was fil-
tered and recrystallized from ethanol to afford benzo[1,3]oxazine
3g (0.165 g, 11%) as a bright yellow solid. Mp 206–208 ◦C. IR:
1104, 1280, 1331, 1496, 1598, 2928. 1H NMR: 1.60 (s, 6H), 3.96
(s, 3H), 4.01 (s, 2H), 6.49 (d, J = 9 Hz, 1H), 6.81 (d, J = 9 Hz, 1H),
7.20–7.33 (m, 4H), 7.45 (d, J = 8 Hz, 1H), 7.63 (d, J = 9 Hz, 1H), 7.92
(dd, J = 3.0, 9 Hz, 1H), 8.32 (d, J = 3 Hz, 1H), 10.55 (broad s, 1H).
13C NMR: 24.4, 25.6, 52.1, 56.0, 102.4, 109.1, 116.3, 116.9, 117.6,
121.5, 124.1, 126.2, 127.3, 127.5, 127.9, 128.4, 128.8, 140.4, 144.1,
146.1, 162.1, 164.7, 183.2. HRMS: calc. for C24H22N2O5: 418.1529;
found: 418.1527.
[7] E. Deniz, M. Tomasulo, S. Sortino, F. Raymo, J. Phys. Chem. C 113 (2009)
8491–8497.
[8] M. Tomasulo, S. Sortino, F. Raymo, J. Org. Chem. 73 (2008) 118–126.
ˇ
[9] A. Sacˇkus, S. Kriksˇtolaityte, V. Martynaitis, Chem. Heterocycl. Comp. 35 (1999)
729–732.
[10] J.G. Rodrigues, A.S. Andres, J. Heterocycl. Chem. 28 (1991) 1293–1299.
[11] K.H. Pausacker, J. Chem. Soc. (1950) 621–624.
[12] M.B.S. Kirketerp, M.A. Petersen, M. Wanko, L.A.E. Leal, H. Zettergren, F.M.
Raymo, A. Rubio, M.B. Nielsen, S.B. Nielsen, ChemPhysChem 10 (2009)
1207–1209.
[13] R.H. Woudenberg, EP 1337501B1, 2001.
[14] J. Seixas de Melo, L.M. Silva, M. Kuroda, J. Chem. Phys. 115 (2001) 5625–5636.