Fast and stable photochromic oxazines

Article abstract of DOI:10.1021/jo051417w

We have designed and synthesized two photochromic compounds incorporating fused indoline and benzooxazine fragments. Variable-temperature ¹H NMR spectroscopy demonstrates that their central [1,3]oxazine ring opens thermally with free energy bar

Full text of DOI:10.1021/jo051417w

Fast and Stable Photochromic Oxazines
Massimiliano Tomasulo,† Salvatore Sortino,*,‡ Andrew J. P. White,§ and
Franc¸isco M. Raymo*,†
Center for Supramolecular Science, Department of Chemistry, University of Miami, 1301 Memorial Drive,
Florida, 33146-0431, Dipartimento di Scienze Chimiche, Universita´ di Catania, viale Andrea Doria 8,
Catania, I-95125, Italy, and Department of Chemistry, Imperial College London, South Kensington,
London, SW7 2AY, U.K.
ssortino@unict.it; fraymo@miami.edu
Received July 8, 2005
We have designed and synthesized two photochromic compounds incorporating fused indoline and
benzooxazine fragments. Variable-temperature 1H NMR spectroscopy demonstrates that their
central [1,3]oxazine ring opens thermally with free energy barriers ranging from 14 to 19 kcal
mol-1. The ring-opened species reverts rapidly to the original isomer and can only be detected after
chemical trapping. Specifically, the nucleophilic attack of a hydroxide anion to the indolium cation
of the ring-opened species prevents re-isomerization. Laser excitation of both compounds induces
the opening of the [1,3]oxazine ring in less than 6 ns with quantum yields up to 0.1. The
photoinduced ring opening generates a 4-nitrophenolate chromophore, which absorbs strongly at
440 nm. The photogenerated species reverts to the original form with a lifetime of 22 ns for both
compounds. Thus, these transformations can be exploited to interconvert the two isomers of each
species with nanosecond switching speeds. Furthermore, thousands of switching cycles can be
repeated consecutively without any sign of degradation, even in the presence of molecular oxygen.
These processes can be reproduced efficiently in poly(methyl methacrylate) matrixes. Under these
conditions, the thermal re-isomerization occurs with biexponential kinetics in submillisecond time
scales. In principle, the fast isomerization kinetics and excellent fatigue resistance of both compounds
offer the opportunity to modulate rapidly and efficiently a variety of molecular and macroscopic
properties. Thus, our molecular design can evolve into the realization of a new family of photochromic
compounds and materials with promising photoresponsive character.
Introduction
is an indoline with two methyl groups in position 3. A
common carbon atom with tetrahedral geometry connects
the two fragments, forcing their mean planes in an
orthogonal arrangement. The resulting molecules (e.g.,
SP in Figure 1) are colorless, but switch to colored
merocyanines (e.g., ME in Figure 1) upon ultraviolet
irradiation. In addition to imposing drastic absorbance
changes in the visible region, the pronounced structural
modifications associated with these photoinduced trans-
Spiropyrans combine two heterocyclic fragments in
their molecular skeleton.1-5 Often, one of them is a
benzopyran with a nitro group in position 6, and the other
† University of Miami.
‡ Universita´ di Catania.
§ Imperial College, London.
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10.1021/jo051417w CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/25/2005
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J. Org. Chem. 2005, 70, 8180-8189

Fast and Stable Photochromic Oxazines
tronic mixing and C-O bond cleavage.2 The resulting
ring-opened intermediate decays to the ground state with
microsecond time scales, producing a mixture of cis- and
trans-merocyanines.17 The subsequent ground-state cis
f trans isomerization completes the photoinduced pro-
cess in submillisecond time scales.17
The participation of a triplet state with microsecond
lifetime in the isomerization of nitrospiropyrans18 opens
competitive reaction pathways.19 These side reactions are
often irreversible and are responsible for the character-
istic photodegradation of nitrospiropyrans.3,5 In particu-
lar, the relatively long-lived triplet state encourages the
production of significant amounts of singlet oxygen
(1∆g),17f,g which, presumably, causes the oxidative degra-
dation of the photochromic species. Consistently, the
fatigue resistances of nitrospiropyrans increase consider-
ably in the presence of singlet-oxygen scavengers.20,21
The photogenerated and colored merocyanines are not
thermally stable.1-5 They revert to the original and
colorless forms after a rate-determining22 trans f cis re-
isomerization and the subsequent re-formation of the
C-O bond at the spirocenter. Thus, the macroscopic
properties (e.g., color and/or refractive index) of a nitro-
spiropyran-based material can be switched reversibly
between two states by simply turning on and off an
optical stimulation. The thermal decoloration process,
however, is relatively slow. In organic solvents at ambient
temperature, the lifetimes of the colored isomers range
from 1 to 104 s.17c,e,g It follows that the switching speeds
of photoresponsive materials based on these compounds
cannot be greater than 1 Hz.
The poor fatigue resistances and slow switching speeds
of photoresponsive materials based on nitrospiropyrans
have limited considerably their practical applications.1b,6-12
The detailed knowledge established on the fundamental
properties of these compounds,13-22 however, can be
exploited to engineer photochromic compounds with
improved isomerization kinetics and stability. In search
of viable strategies to optimize both parameters, we have
designed a new family of heterocyclic photochromes
closely related to nitrospiropyrans.23 In this article, we
report the synthesis, structural characterization, and
FIGURE 1. Interconversion of a colorless spiropyran (SP) and
a colored merocyanine (ME).
formations translate also into significant dipole moment
and molecular polarizability alterations. In fact, diverse
photoresponsive materials have been designed around
the unique properties of this class of photochromic
compounds over the past three decades.1b,6-12 Their
preparation generally relies on the integration of nitro-
spiropyrans within polymer matrixes, multilayer struc-
tures, or liquid-crystalline assemblies. The macroscopic
properties of the resulting composites can then be
regulated by operating their photochromic components
with optical stimulations. Indeed, the birefringence, color,
dichroism, and/or refractive index of materials based on
nitrospiropyrans have all been modulated successfully,
relying on these operating principles.
The photoinduced isomerization of nitrospiropyrans to
the corresponding merocyanines demands the cleavage
of the C-O bond at the spirocenter and the subsequent
cis f trans isomerization of the adjacent CdC bond. In
organic solvents at ambient temperature, the first of
these two steps occurs upon ultraviolet excitation with
picosecond time scales13-16 along the potential energy
surface of the first triplet state.17,18 Indeed, the nitro
group appended to the benzopyran fragment facilitates
intersystem crossing in the spiropyran geometry. Con-
comitantly, the small dihedral angle (15-30°) between
the axis of the 2pz orbital on the indoline nitrogen atom
and that of the adjacent σC-O orbital encourages elec-
(17) (a) Atabekyan, L. S.; Chibisov, A. K. Zh. Prikl. Spektrosk. 1987,
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(18) Quenching and sensitization experiments, coupled with the
investigation of the photoinduced re-isomerization of the colored
isomers, demonstrate the fundamental role of the triplet state in the
photoisomerization of nitrospiropyrans (ref 17). Earlier reports, how-
ever, suggest alternative mechanisms for these photoinduced trans-
formations. See refs 13-15 and (a) Irie, M.; Menju, A.; Hayashi, K.
Macromolecules 1979, 12, 1176-1180. (b) Krongauz, V.; Kiwi, J.;
Gra¨tzel, M. J. Photochem. 1980, 13, 89-97. (c) Kalisky, Y.; Williams,
D. J. Macromolecules 1984, 17, 292-296. (d) Lenoble, C.; Becker, R.
S. J. Phys. Chem. 1986, 90, 62-65. (e) Tamaki, T.; Sakuragi, M.;
Ichimura, K.; Aoki, K. Chem. Phys. Lett. 1989, 161, 23-26.
(19) Malkin, Y. N.; Krasieva, T. B.; Kuzmin, V. A. J. Photochem.
Photobiol., A 1989, 49, 75-88.
(20) Sakuragi, M.; Aoki, K.; Tamaki, T.; Ichimura, K. Bull. Chem.
Soc. Jpn. 1990, 63, 74-79.
(21) Salemi, C.; Giusti, G.; Guglielmetti, R. J. Photochem. Photobiol.,
A 1995, 86, 247-252.
(22) Sheng, Y.; Leszczynski, J.; Garcia, A. A.; Rosario, R.; Gust, D.;
Springer, J. J. Phys. Chem. B 2004, 108, 16233-16243.
(23) Tomasulo, M.; Sortino, S.; Raymo, F. M. Org. Lett. 2005, 7,
1109-1112.
(7) Crano, J. C.; Kwak, W. S.; Welch, C. N. In Applied Photochromic
Polymer Systems; McArdle, C. B., Ed.; Blackie: Glasgow, 1992; pp 31-
79.
(8) (a) Krongauz, V. In Applied Photochromic Polymer Systems;
McArdle, C. B., Ed.; Blackie: Glasgow, 1992; pp 121-173. (b) Berkovic,
G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000, 100, 1741-1754.
(9) Irie, M. In Applied Photochromic Polymer Systems; McArdle, C.
B., Ed.; Blackie: Glasgow, 1992; pp 174-206.
(10) Hibino, J.; Hashida, T.; Suzuki, M. In Photo-Refractive Materi-
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Amsterdam, 1994; pp 25-53.
(11) Kawata, S.; Kawata, Y. Chem. Rev. 2000, 100, 1777-1788.
(12) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100, 1817-1845.
(13) (a) Krysanov, S. A.; Alfimov, M. V. Chem. Phys. Lett. 1982, 91,
77-80. (b) Krysanov, S. A.; Alfimov, M. V. Laser Chem. 1984, 4, 129-
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(14) Kalisky, Y.; Orlowski, T. E.; Williams, D. J. J. Phys. Chem.
1983, 87, 5333-5338.
(15) (a) Ito, T.; Hiramatsu, M.; Hirano, I.; Ohtani, H. Macromolecules
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5502-5509.
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Tomasulo et al.
two distinct environments on the pair of indoline methyl
groups and on the two oxazine methylene protons.
1
Consistently, the H NMR spectra of both compounds
reveal pairs of singlets for the methyl protons and AB
systems for the methylene protons, when recorded in
acetonitrile-d3 at ambient temperature. For example, the
two singlets for the methyl protons (MeO and Me0) of 3a
1
appear at 1.15 and 1.51 ppm in the H NMR spectrum
(a in Figure 3) recorded at 275 K. The AB system for the
methylene protons (HO and H0) is instead centered at
4.69 ppm. Upon warming the solution, we found that the
two singlets broaden (b-f in Figure 3) and eventually
coalesce into a single peak. Similarly, the two doublets
within the AB system broaden and coalesce into a single
peak. These changes are a result of the interconversion
between the two enantiomers of 3a on the 1H NMR time
scale. This degenerate site exchange process demands the
thermal cleavage of the C-O bond at the junction of the
two heterocycles with the formation of the ring-opened
intermediate 3b (Figure 3). The kinetic parameters
(Table 1) associated with the ring-opening process can
be extracted from the analysis of the temperature de-
pendence of the line widths associated with the singlets
for MeO and Me0 in the slow-exchange regime.24 A similar
analysis can be extended to the 1H NMR spectra (Figure
S2) of 4a, which reveal essentially the same behavior.
A comparison of the rate constants (k in Table 1)
determined for the thermal ring opening of 3a and 4a in
acetonitrile-d3 reveals that the group on the chiral center
at the junction of the two heterocycles has a pronounced
influence on the kinetics of this process. Indeed, a
transition from the methyl group of 3a to the phenyl ring
of 4a translates into a decrease in k from ca. 199 to 0.4
s-1. This change corresponds to an increase in free energy
barrier (∆Gq in Table 1) of ca. 3.7 kcal mol-1. Interest-
ingly, the enthalpic term (∆Hq in Table 1) dominates ∆Gq,
while the entropic contribution (T∆Sq) at 298 K is less
than 1 kcal mol-1 for both compounds. In toluene-d8,
T∆Sq remains negligible, but ∆Hq increases by ca. 4.7 kcal
mol-1 for 3a. Consistent with the solvent-induced en-
hancement in ∆Hq, the 1H NMR spectrum of 4a does not
change significantly with temperature in toluene-d8. Two
well-defined singlets for the methyl protons of 4a can
clearly be observed even at 363 K, in agreement with the
slow ring-opening kinetics.
FIGURE 2. Synthesis of the [1,3]oxazines 3a and 4a.
photochemical properties of two members of this novel
class of photoactive molecular switches.
Results and Discussion
Design and Synthesis. The photoinduced transfor-
mation of SP into ME (Figure 1) requires submillisecond
time scales to reach completion at ambient temperature
in organic solvents.17 The ground-state cis f trans
isomerization of the CdC bond linking the indoline and
4-nitrophenolate fragments of the ring-opened form limits
the rate of the overall process.17,22 The thermal transfor-
mation of ME into SP requires several minutes under
the same experimental conditions.17 Once again, the
trans f cis isomerization of the CdC bond limits the rate
of this transformation.22 Thus, the coloration and decol-
oration kinetics can both be improved by removing the
CdC bond from the nitrospiropyran skeleton. Indeed, the
sluggish cis/trans interconversion steps are not strictly
necessary to generate a significant absorbance change
in the visible region. The photoinduced and fast cleavage
of the C-O bond at the spirocenter alone is sufficient to
produce a 4-nitrophenolate chromophore able to absorb
visible light. Under these conditions, the thermal and fast
re-formation of the C-O bond is the only requirement to
restore the original and colorless form.
On the basis of these considerations, we have designed
and synthesized the two compounds 3a and 4a (Figure
2). Specifically, the N-alkylation of the indoles 1 and 2
with 2-chloromethyl-4-nitrophenol and the subsequent
cyclization under basic conditions led to the target
molecules 3a and 4a in an overall yield of 58%. These
two compounds differ in the group on the tetrahedral
carbon atom shared by the indoline and benzooxazine
fragments. The relative orientation of these two fused
heterocycles constrains the dihedral angle between the
axis of the 2pz orbital on the indoline nitrogen atom and
that of the adjacent σC-O orbital to values comparable to
those (15-30°) of nitrospiropyrans.2 Thus, this particular
geometry is expected to favor electronic mixing between
the two orbitals and facilitate the cleavage of the C-O
bond in the excited state, as observed for nitrospiropy-
rans.2 It follows that the ultraviolet excitation of either
3a or 4a should open the central [1,3]oxazine ring to
generate a 4-nitrophenolate chromophore able to absorb
visible light.
The ring-opened intermediates 3b (Figure 3) and 4b
(Figure S2) revert to the corresponding oxazines 3a and
4a after the intramolecular attack of the 4-nitrophenolate
(24) Below the coalescence temperature, two well-separated singlets
can be observed for the methyl protons MeO and Me0 in the 1H NMR
spectra (a-c in Figure 3). Under these conditions, the line width (∆ν)
of either one of the two singlets is related to the rate constant (k) of
the degenerate site exchange process according to eq 1, where ∆ν0 is
the line width at the stopped-exchange limit (Nelson, J. H. Nuclear
Magnetic Resonance Spectroscopy; Prentice Hall: Upper Saddle River,
NJ, 2003.). Following this protocol, k can be determined at any
temperature (T) within the slow-exchange regime. A plot of ln(kT-1
)
against T-1 can then be fitted to eq 2, where R is the gas constant, to
extract the enthalpy (∆Hq) and entropy (∆Sq) of activation. Finally,
the free energy (∆Gq) of activation can be calculated at any T using eq
3.
k ) π(∆ν - ∆ν0)
∆Hq ∆Sq
(1)
k
T
ln ) -
+
+ 23.76
(2)
(3)
1H NMR Spectroscopy. The chiral center at the
junction of the two heterocycles in 3a and 4a imposes
RT
R
∆Gq ) ∆Hq - T∆Sq
8182 J. Org. Chem., Vol. 70, No. 20, 2005

Fast and Stable Photochromic Oxazines
FIGURE 3. Partial 1H NMR spectra (500 MHz, acetonitrile-d3, 5 mM) of 3a at 275 (a), 283 (b), 293 (c), 313 (d), 323 (e), and 336
K (f).
TABLE 1. Kinetic Parameters Associated with the Thermal Ring Opening of 3a and 4a at 298 Ka
solvent
compound
k (s-1
)
∆Gq (kcal mol-1
)
∆Hq (kcal mol-1
)
-∆Sq (kcal mol-1 K-1
)
acetonitrile-d3
3a
4a
3a
199 ( 7
14.31 ( 0.02
17.99 ( 0.18
19.00 ( 0.29
13.5 ( 0.2
17.4 ( 0.3
19.8 ( 2.0
0.003 ( 0.001
0.002 ( 0.001
-0.003 ( 0.006
0.4 ( 0.1
b
toluene-d8
0.081 ( 0.036
a The rate constant (k), free energy (∆Gq), enthalpy (∆Hq), and entropy (∆Sq) of activation were determined by variable-temperature 1H
NMR spectroscopy (ref 24). b In toluene-d8, the line widths of the singlets associated with the pair of methyl protons of 4a remain
approximately constant in the examined temperature range (275-363 K). As a result, the kinetic parameters for the ring opening of this
compound could not be determined.
anion to the adjacent indolium cation. Nucleophiles able
to compete intermolecularly with the ring-closing step
can therefore “trap” these short-lived intermediates. For
example, the addition of 2 equiv of Bu4NOH to a solution
of 4a results in the quantitative formation of the hemi-
aminal 4c (Figure 4). This transformation is accompanied
by pronounced changes in the 1H NMR spectrum (a and
b in Figure 4). In particular, the doublet for Ha moves
from 7.99 to 8.14 ppm, while the resonances for all the
other aromatic protons move in the opposite direction.
The largest chemical shift change is observed for the
doublet associated with Hh, which moves from 6.96 to
6.11 ppm. The AB system for the pair of diastereotopic
methylene protons Hl and Hm is retained with the
transformation of 4a into 4c, consistent with the presence
of a chiral center in both compounds. In addition, the
formation of 4c is further confirmed by the presence of a
peak at an m/z of 390 in the fast atom bombardment
mass spectrum.
The addition of Bu4NOH to a solution of 3a has similar
1
effects on the H NMR and mass spectra. Once again,
peaks for the corresponding hemiaminal 3c (Figure S3)
can be clearly observed. The methyl group in position 2
of the indolium fragment of 3b, however, is relatively
acidic. Thus, the nucleophilic attack of the hydroxide
anion to the indolium cation of 3b competes with its
J. Org. Chem, Vol. 70, No. 20, 2005 8183

Tomasulo et al.
FIGURE 4. Partial 1H NMR spectra (400 MHz, acetonitrile-d3, 10 mM) of 4a before (a) and after (b) the addition of Bu4NOH (2
equiv).
(which are coplanar to within ca. 0.08 Å). The geometry
at N(1) is pyramidal, the nitrogen lying ca. 0.36 Å out of
the plane of its substituents, and its 2pz orbital is
approximately collinear with the σ-orbital of the C(9)-
O(10) bond, the O(10)-C(9)-N(1) lone-pair dihedral
angle being ca. 9°.
Adjacent molecules in the crystal are linked by a
combination of C-H‚‚‚π, π‚‚‚π, and C-H‚‚‚O interactions.
One face of the indoline C6 ring (ring A in Figure 5) is
approached by a hydrogen atom of the benzooxazine C6
ring (ring B) [H‚‚‚A 2.97 Å, C(13)-H‚‚‚A 128°]. The other
face is close to a proton on C(20) [H‚‚‚A 2.99 Å, C(20)-
H‚‚‚A 132°]. These two C-H‚‚‚π interactions subtend an
angle of ca. 176° at the centroid of ring A. One face of
ring B is approached by a proton on C(18) [H‚‚‚B 3.13 Å,
C(18)-H‚‚‚B 173°], while the other face is involved in a
head-to-tail overlap with the nitro group of its cen-
trosymmetrically related counterpart. The corresponding
N‚‚‚centroid and mean interplanar separations are 3.67
and 3.57 Å, respectively. The oxygen atoms of the nitro
group are the recipients of a pair of weak C-H‚‚‚O
interactions. The aromatic C(3) proton approaches O(23)
[H‚‚‚O 2.48 Å, C-H‚‚‚O 176°], while a proton on C(20) is
close to O(22) [H‚‚‚O 2.65 Å, C-H‚‚‚O 174°].
The single-crystal X-ray structure of 4c (Figure 6)
confirms the opening of the oxazine ring. As was seen in
3a, the indoline fragment of 4c adopts an envelope-type
geometry, the chiral center C(9) being ca. 0.52 Å out of
the plane of the remaining indoline atoms (which are
coplanar to within ca. 0.02 Å). The geometry at the
adjacent nitrogen atom [N(1)] is again pyramidal, the
nitrogen lying ca. 0.33 Å out of the plane of its substit-
uents. Despite the oxazine ring having been opened, the
nitrogen 2pz orbital and the σ-orbital of the C(9)-O(20)
FIGURE 5. Single-crystal X-ray structure of 3a.
deprotonation, and the compound 3d (Figure S3) is
formed in parallel to the hemiaminal 3c. Consistently,
the broad resonance for the pair of diastereotopic meth-
ylene protons of 3a (a in Figure S3) is replaced by an AB
system for Ha and Hb of 3c and a singlet for HA and HB
of 3d (b in Figure S3). In addition, an AB system for the
olefinic protons HC and HD of 3d can also be observed.
From the integrals of these resonances, the ratio between
3c and 3d can be estimated to be 34:66.
X-ray Crystallography. The single-crystal X-ray
structure of 3a (Figure 5) shows a near-orthogonal
arrangement for the fused indoline and benzooxazine ring
systems, their planes, excluding the two junction atoms
[C(9) and N(1)], being inclined by ca. 87°. Both ring
systems adopt envelope-type geometries, with that for the
indoline moiety the chiral carbon C(9) being ca. 0.50 Å
out of the plane of the remaining indoline atoms (which
are coplanar to better than 0.01 Å), while for the
benzooxazine unit the nitrogen center N(1) lies ca. 0.53
Å out of the plane of the remaining benzooxazine atoms
8184 J. Org. Chem., Vol. 70, No. 20, 2005

Fast and Stable Photochromic Oxazines
FIGURE 6. Single-crystal X-ray structure of 4c showing
O-H‚‚‚O hydrogen-bonding interactions with included water
molecules. The O-H‚‚‚O hydrogen-bonding geometries [O‚‚‚O,
H‚‚‚O (Å), and O-H‚‚‚O (°)] are: (a) 2.859(2), 2.00, 160.0; (b)
2.687(3), 1.81, 164; (c) 3.135(3), 2.35, 146; (d) 2.773(3), 1.88,
170; (e) 2.760(3), 1.87, 168.
bond are still approximately collinear, the O(20)-C(9)-
N(1) lone-pair dihedral angle being ca. 11°.
The packing of 4c in the crystal is dominated by
O-H‚‚‚O hydrogen bonds involving the included water
molecules (Figure 6). There are, however, some relatively
weak C-H‚‚‚π interactions between the anion in 4c and
the included Bu4N+ cation, with both faces of ring A
[H‚‚‚A 3.31 Å, C-H‚‚‚A 143°; H‚‚‚A 3.47 Å, C-H‚‚‚A 130°;
H‚‚‚A‚‚‚H 158°] and ring B [H‚‚‚B 2.97 Å, C-H‚‚‚B 160°;
H‚‚‚B 3.39 Å, C-H‚‚‚B 130°; H‚‚‚B‚‚‚H 118°] being
approached by methylene protons. Ring C is only ap-
proached on one face [H‚‚‚C 2.97 Å, C-H‚‚‚C 152°],
presumably because the other face is partially shielded
by the adjacent methyl group [C(18)].
FIGURE 7. Zero-point corrected energies [B3LYP/6-31G(d)]
of the main stationary points on the ground-state potential
energy surface of 3a.
Molecular Modeling. To gain further understanding
on the thermal isomerization of our oxazines, we have
modeled the ground-state potential energy surface of 3a
with a combination of AM1 (Figure S6) and B3LYP/
6-31G(d) calculations. In the B3LYP/6-31G(d) geometry
of 3a (Figure 7), the length of the C-O bond at the junc-
tion of the two heterocycles is 1.470 Å. The cleavage of
this bond proceeds through a transition state that is 18.1
kcal mol-1 higher in energy than 3a. This activation en-
ergy is close to the experimental ∆Hq (Table 1) determin-
ed for the very same process in toluene-d8. According to
our calculations, the ring-opened product 3b is 17.6 kcal
mol-1 higher in energy than 3a. Thus, the ring-closing
Thus, the stationary concentrations of the ring-opened
isomers 3b and 4b are below the detection limit in both
cases, under these experimental conditions (MeCN, 298
K). After the addition of Bu4NOH to solutions of 3a and
4a, however, a band at ca. 430 nm appears in the spectra
of both species (d in Figures 8 and S7). This band
resembles the absorption (e in Figures 8 and S7) of the
model phenolate 8 (Table 2) and can be assigned to the
ring-opened products 3c/3d and 4c detected by 1H NMR
spectroscopy (Figures 4 and S3).
Transient Absorption Spectroscopy. The transient
absorption spectra, recorded 30 ns after laser excitation,
of aerated MeCN solutions of 3a and 4a show bands
centered at ca. 440 nm (a in Figures 9 and S8). In both
instances, the transient bands resemble the steady-state
ones (b in Figures 9 and S8) of 3c/3d and 4c and,
therefore, can be assigned to ground-state absorptions
of the 4-nitrophenolate chromophores of 3b and 4b. In
agreement with this assignment, singlet-oxygen mea-
surements for 4a and control experiments with 7 confirm
that the transient absorptions are not associated with a
triplet-triplet transition. Indeed, the quantum yield of
singlet oxygen is less than 0.02, and the transient
spectrum of 7 does not reveal any detectable absorption
in the nanosecond domain.25
reaction has an energy barrier of only 0.5 kcal mol-1
.
Steady-State Absorption Spectroscopy. The two
chromophoric fragments of 3a and 4a are isolated
electronically. As a result, their absorption spectra (a in
Figures 8 and S7) resemble the sum of those (b and c in
Figures 8 and S7) of model indolines (5 and 6 in Table 2)
and 4-nitroanisole (7 in Table 2). The most significant
difference is a shift to longer wavelengths for the absorp-
tion associated with the 4-nitrophenyl chromophore. This
band is centered at 307 nm in the spectrum of 7 (Table
2), but at 318 and 316 nm in those of 3a and 4a,
respectively.
The spectra of 3a and 4a (a in Figures 8 and S7) do
not show bands in the wavelength range expected for a
4-nitrophenolate chromophore (e in Figures 8 and S7).
The kinetic traces monitored at 440 nm (c in Figures
9 and S8) indicate that the ring-opened isomers are
J. Org. Chem, Vol. 70, No. 20, 2005 8185

Tomasulo et al.
FIGURE 9. Transient absorption spectrum (a) of 4a recorded
30 ns after the laser pulse (355 nm, 6 ns, 8 mJ, 0.1 mM, MeCN,
295 K) and steady-state absorption spectrum (b) of 4a and 100
equiv of Bu4NOH (0.1 mM, MeCN, 298 K). Evolution of the
absorbance at 440 nm (c) upon laser excitation of 4a and the
corresponding monoexponential curve fitting (d).
FIGURE 8. Steady-state absorption spectra (0.1 mM, MeCN,
298 K) of 3a (a), 5 (b), and 7 (c) as well as of 3a after the
addition of Bu4NOH (1 equiv) (d) and 8 (e).
TABLE 2. Absorption Wavelengths (λ) and Molar
Extinction Coefficients (E) of the Oxazines 3a and 4a and
of the Model Compounds 5-8 in MeCN at 298 Ka
original oxazines 3a and 4a. Thus, the photoinduced and
reversible interconversion between the two isomers of
each system can be achieved with nanosecond switching
speeds. Furthermore, both photochromic switches are
remarkably stable. In fact, the steady-state absorption
spectra of 4a recorded before and after 3000 excitation
cycles, operated in the presence of molecular oxygen, are
virtually identical.
compound
λ (nm)
ꢀ (mM-1 cm-1
)
3a
4a
5
318
316
283
281
307
426
10.0 ( 0.5
11.0 ( 0.6
2.2 ( 0.1
3.9 ( 0.2
11.1 ( 0.6
32.5 ( 0.9
6
7
8
The steady-state absorption spectra (a in Figures 10
and S9) of poly(methyl methacrylate) (PMMA) films
doped with 3a or 4a are essentially identical to those
recorded in MeCN (a in Figures 8 and S7). Once again,
the stationary concentrations of the ring-opened isomers
3b and 4b are negligible, and the absorption bands of
their 4-nitrophenolate chromophores cannot be detected.
In the presence of Bu4NOH, however, the characteristic
absorptions of 3c/3d and 4c at ca. 430 nm can be
observed also in PMMA (b in Figures 10 and S9). Similar
bands are evident in the transient absorption spectra of
3a and 4a (c in Figures 10 and S9) recorded 1 µs after
laser excitation. These absorptions can be assigned to the
photogenerated ring-opened isomers 3b and 4b and are
relatively short-lived.26 After 80 µs, for example, the band
at 430 nm can no longer be observed in the transient
spectrum of 4a (d in Figure 10). In addition to this
absorption, a second and more intense band at 380 nm
a The model compounds are shown in the following diagram.
The λ and ꢀ of the phenolate 8 were determined by recording the
absorption spectrum of the corresponding phenol in the presence
of Bu4NOH (4 equiv).
formed within the excitation pulse (ca. 6 ns). The corre-
sponding quantum yields can be estimated to be 0.03 for
3b and 0.1 for 4b. In both instances, the absorbance
decays monoexponentially to zero (d in Figures 9 and S8)
with a first-order rate constant of ca. 4 × 107 s-1, as the
ring-opened isomers 3b and 4b revert thermally to the
(25) The lowest triplet state of 4-nitroanisole has a subnanosecond
lifetime in MeCN. Mir, M.; Jansen, L. M. G.; Wilkinson, F.; Bourde-
lande, J. L.; Marquet, J. J. Photochem. Photobiol., A 1998, 113, 113-
117.
(26) The transient absorptions of 3a and 4a at 430 nm appear to
decay biexponentially (Figure S10). However, the signal-to-noise ratio
of the kinetic traces is relatively poor, and a reliable determination of
the associated rate constants was not possible.
8186 J. Org. Chem., Vol. 70, No. 20, 2005

Fast and Stable Photochromic Oxazines
biexponential kinetics under these experimental condi-
tions. The aggregation of their photogenerated isomers
into long-lived supramolecular assemblies is believed to
be responsible for this behavior. Presumably, similar
processes govern the spectral evolution of 3a and 4a in
PMMA. In any case, the absorbance associated with the
photogenerated species of both systems can be modulated
reversibly with millisecond switching speeds. Once again,
both photochromic switches are remarkably stable and
remain essentially unaffected after thousands of excita-
tion cycles. As an example, Figure 11 illustrates kinetic
traces recorded at 380 nm for 4a before and after 1000
switching cycles. The two profiles are virtually indistin-
guishable, indicating that the photochromic switch is,
indeed, extremely stable.
Conclusions
Indoline and benzooxazine fragments can be conve-
niently fused in a single synthetic step starting from 2-R-
3,3′-dimethyl-3H-indoles and 2-chloromethyl-4-nitrophe-
nol. The [1,3]oxazine ring in the resulting molecules
opens thermally with a free energy barrier ranging from
14 to 19 kcal mol-1. This parameter is dominated by its
enthalpic term and increases when R is changed from a
methyl to a phenyl group and when the solvent varies
from acetonitrile-d3 to toluene-d8. The fast re-isomeriza-
tion of the ring-opened species translates into a negligible
steady-state concentration. However, this isomer can be
trapped chemically and characterized in this form by
nuclear magnetic resonance spectroscopy, crystallogra-
phy, and visible absorption spectroscopy. Indeed, the
nucleophilic attack of a hydroxide anion to the indolium
cation of the ring-opened species prevents the thermal
re-isomerization.
FIGURE 10. Steady-state absorption spectra (5%, PMMA,
298 K) of 4a without (a) and with (b) Bu4NOH (7 equiv) and
transient absorption spectra of 4a recorded 1 (c) or 80 µs (d)
after the laser pulse (355 nm, 6 ns, 12 mJ, 5%, PMMA, 295
K). Evolution of the absorbance at 380 nm (e) upon laser
excitation of 4a (355 nm, 6 ns, 12 mJ, 5%, PMMA, 295 K) and
the corresponding biexponential curve fitting (f).
The ultraviolet absorption spectra of these compounds
indicate that the indoline and benzooxazine chromophores
are electronically isolated. The local excitation of the
benzooxazine fragment at 355 nm cleaves its C-O bond
at the junction of the two heterocycles in less than 6 ns
with a quantum yield up to 0.1. The photoinduced bond
cleavage generates a 4-nitrophenolate chromophore with
an intense absorption at 440 nm. The photogenerated
isomer reverts thermally to the original [1,3]oxazine with
a lifetime of 22 ns. These processes can be reproduced
successfully even in poly(methyl methacrylate) matrixes.
Under these conditions, however, the thermal re-isomer-
ization occurs in submillisecond time scales with biex-
ponential kinetics. Presumably, the aggregation of the
photogenerated isomer within the polymer matrix is
responsible for this behavior. Even in the presence of
molecular oxygen, the photoinduced isomerization and
thermal re-isomerization are not accompanied by any
detectable degradation. These molecules withstand thou-
sands of excitation cycles without any sign of decomposi-
tion in aerated acetonitrile as well as in poly(methyl
methacrylate) matrixes.
FIGURE 11. Evolution of the absorbance at 380 nm upon
laser excitation of 4a (355 nm, 6 ns, 12 mJ, 5%, PMMA, 295
K) before (a) and after (b) 1000 excitation cycles.
(c in Figures 10 and S9) is evident in the transient
spectra of both compounds. This absorption is relatively
long-lived and can still be observed after the complete
decay of the band at 430 nm (d in Figure 10). The
corresponding kinetic traces (e in Figure 10 and d in
Figure S9) show biexponential decays with rate constants
of 1 × 104 and 1 × 105 s-1 for 3a and of 3 × 103 and 5 ×
104 s-1 for 4a. Both trends parallel the behavior of
nitrospiropyrans in polymer matrixes.8,27 Indeed, the
thermal decoloration of nitrospiropyrans also follows
The fast isomerization kinetics and excellent fatigue
resistance of our oxazines offer the opportunity to modu-
late rapidly and efficiently molecular (e.g., dipole mo-
ment) and macroscopic (e.g., color) properties under the
(27) Krongauz, V. A. In Photochromism: Molecules and Systems;
Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; pp
793-821.
J. Org. Chem, Vol. 70, No. 20, 2005 8187

Tomasulo et al.
Molecular Modeling. The structure of 3a was built with
GaussView28 and optimized (AM1) with Gaussian.29 In the
resulting geometry, the length of the C-O bond at the junction
of the indoline and benzooxazine fragments is 1.469 Å. This
value was increased in 10 consecutive steps of 0.1604 Å each.
After each increment, the C-O distance was constrained and
the overall geometry was re-optimized (AM1). The profile of
the minimized energy for the C-O distance drive is illustrated
in Figure S6. The structures (3a and 3b in Figure S6)
corresponding to the two energy minima were re-optimized
[B3LYP/6-31G(d)] with no distance constrains. Frequency
calculations [B3LYP/6-31G(d)] on the resulting geometries did
not reveal imaginary frequencies, confirming that they cor-
respond to minima on the potential energy surface. The
transition state for the transformation of 3a into 3b was
identified with the QST3 protocol (AM1), using the geometry
corresponding to the energy maximum in Figure S6 in
conjunction with the corresponding reactant and product. The
resulting structures were then optimized [B3LYP/6-31G(d)]
using the TS option within Gaussian. Frequency calculations
[B3LYP/6-31G(d)] on the final geometry revealed a single
imaginary frequency, confirming that it corresponds to a
transition state on the potential energy surface. The energies
of the three stationary points were corrected to account for
the corresponding zero-point energies, using a scaling factor30
of 1.0119, and are reported in Figure 7.
influence of optical stimulations. In fact, our molecular
design can lead to the development of a new family of
photochromic compounds with unprecedented switching
speeds and remarkable stability and, eventually, evolve
into photonic materials with unique photoresponsive
character.
Experimental Section
2-Nitro-5a,6,6-trimethyl-5a,6-dihydro-12H-indolo[2,1-
b][1,3]benzooxazine (3a). A solution of 1 (291 µL, 1.8 mmol)
and 2-chloromethyl-4-nitrophenol (162 mg, 0.9 mmol) in MeCN
(5 mL) was stirred for 50 min at ambient temperature under
N2. Then, the mixture was stored in a refrigerator for 12 h.
The resulting precipitate was filtered and dissolved in H2O
(40 mL). After the addition of aqueous KOH (0.05 M, 5 mL),
the solution was extracted with Et2O (3 × 20 mL). The organic
layer was dried (MgSO4) and filtered, and the solvent was
distilled off under reduced pressure to give 3a (94 mg, 0.3
mmol) as a white solid. The mother liquor of the initial
filtration was concentrated under reduced pressure, and the
residue was purified by column chromatography [SiO2/CH2-
Cl2 f CH2Cl2/MeCO2Et (10:1)] to afford an additional amount
of 3a (60 mg, 0.2 mmol). The overall yield of 3a was 58%.
HPLC [analytical, MeCN/H2O (80:20)]: RT ) 4.5 min, PA )
1.6, APP ) 236.7 ( 1.0 nm; mp ) 180 °C; FABMS: m/z ) 311
[M + H]+; 1H NMR (500 MHz, CDCl3): δ 1.17 (3H, s), 1.52
(3H, s), 1.57 (3H, s), 4.60 (2H, s), 6.56 (1H, d, 8 Hz), 6.69 (1H,
d, 9 Hz), 6.82 (1H, t, 8 Hz), 7.07 (1H, t, 8 Hz), 7.11 (1H, d, 8
Hz), 7.92 (1H, dd, 3 and 9 Hz), 8.06 (1H, d, 3 Hz); 13C NMR
(75 MHz, CDCl3): δ 16.6, 18.9, 26.0, 40.0, 48.0, 102.8, 108.4,
118.2, 118.8, 120.5, 122.3, 123.3, 124.0, 127.6, 138.0, 140.4,
146.6, 159.1.
Steady-State Absorption Spectroscopy. The absorption
spectra were recorded either in aerated MeCN, using quartz
cells with a path length of 0.5 cm, or in PMMA matrixes. The
polymer films were prepared by spin-coating aliquots of CH2-
Cl2 solutions of PMMA (160 mg mL-1) and either 3a or 4a (8
mg mL-1) with and without Bu4NOH (7 equiv) on glass plates
at 420 rpm for 9 s. The thicknesses of the resulting films were
ca. 6 µm and were measured with a digital micrometer.
2-Nitro-5a-phenyl-6,6-dimethyl-5a,6-dihydro-12H-in-
dolo[2,1-b][1,3]benzooxazine (4a). A solution of 2 (700 mg,
3.2 mmol) and 2-chloromethyl-4-nitro-phenol (709 mg, 3.8
mmol) in MeCN (30 mL) was heated under reflux for 48 h.
After cooling to ambient temperature, the solvent was distilled
off under reduced pressure and the residue was dissolved in
CH2Cl2 (30 mL). The resulting solution was washed with
aqueous KOH (0.2 M, 15 mL) and H2O (15 mL). The organic
phase was concentrated under reduced pressure, and the
residue was purified by column chromatography [SiO2/hexane
f CH2Cl2/hexane (1:1 v/v)] to give 4a (680 mg, 58%) as a white
solid. HPLC [analytical, MeCN/H2O (95:5)]: RT ) 3.6 min,
PA ) 2.1, APP ) 261.0 ( 0.1 nm; mp ) 176 °C; FABMS: m/z
Transient Absorption Spectroscopy. The absorption
spectra were recorded with a commercial laser flash photolysis
apparatus either in aerated MeCN, using quartz cells with a
path length of 1.0 cm, or in PMMA matrixes. The excitation
source was a Nd:YAG laser (355 nm, 6 ns, 8 or 12 mJ). The
quantum yield (Φ) for the photoinduced ring opening of 3a
and 4a was determined with eq 1, using an optically matched
MeCN solution of benzophenone as standard. The quantum
yield (ΦBE) for the intersystem crossing of benzophenone is
unity, and the molar extinction coefficient (ꢀBE) for its triplet
1
) 372 [M]+; H NMR (400 MHz, CDCl3): δ 0.89 (3H, s), 1.60
absorption at 520 nm is 6.5 mM-1 cm-1 31
. The molar extinction
(3H, s), 4.53 (1H, d, 11 Hz), 4.63 (1H, d, 11 Hz), 6.73 (1H, d, 8
Hz), 6.86-6.94 (2H, m), 7.16-7.19 (2H, m), 7.38-7.42 (3H,
m), 7.54-7.65 (2H, m), 7.92-7.94 (2H, m); 13C NMR (100 MHz,
CDCl3): δ 18.6, 27.9, 41.0, 49.9, 105.5, 109.2, 118.3, 120.3,
121.0, 122.6, 123.3, 123.9, 127.9, 128.2, 128.8, 129.1, 136.1,
137.9, 141.4, 147.0, 159.3.
coefficient (ꢀ) of the ring-opened isomers at 440 nm was
estimated to be ca. 22 mM-1 cm-1 from the absorption
spectrum of 8 (e in Figure 8). The terms ø and øBE in eq 4 are
the slopes of the linear portions of plots of the photoinduced
absorbance changes, measured at the end of the pulse, for the
ring-opened isomer and the benzophenone triplet, respectively,
against the energy of the laser pulse.
X-ray Crystallography. Single crystals of 3a were grown
by vapor diffusion of i-Pr2O/hexane into a MeCN solution of
the oxazine. Single crystals of 4c were grown by vapor diffusion
of i-Pr2O into an equimolar MeCN solution of 4a and Bu4NOH.
Crystal Data for 3a: C18H18N2O3, M ) 310.34, monoclinic,
P21/n (no. 14), a ) 8.1728(5), b ) 17.7129(9), c ) 11.0657(6)
Å, â ) 101.171(5)°, V ) 1571.56(15) Å3, Z ) 4, Dc ) 1.312 g
cm-3, µ(Cu KR) ) 0.735 mm-1, T ) 173 K, colorless blocks;
2981 independent measured reflections, F2 refinement, R1 )
0.042, wR2 ) 0.113, 2718 independent observed absorption-
corrected reflections [|Fo| > 4σ(|Fo|), 2θmax ) 142°], 209
parameters. CCDC 272868.
Crystal Data for 4c: (C16H36N)(C23H21N2O4)‚2H2O, M )
667.91, monoclinic, C2/c (no. 15), a ) 37.174(2), b ) 9.2670-
(7), c ) 22.8271(17) Å, â ) 103.270(6)°, V ) 7653.9(9) Å3, Z )
8, Dc ) 1.159 g cm-3, µ(Cu KR) ) 0.616 mm-1, T ) 173 K,
yellow blocks; 7252 independent measured reflections, F2
refinement, R1 ) 0.064, wR2 ) 0.183, 5291 independent
observed absorption-corrected reflections [|Fo| > 4σ(|Fo|), 2θmax
) 143°], 538 parameters. CCDC 272869.
øꢀBEΦBE
Φ )
(4)
ø
BEꢀ
(28) GaussView 2.1; Gaussian, Inc.: Pittsburgh, PA, 1998.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7;
Gaussian, Inc.: Pittsburgh, PA, 1998.
8188 J. Org. Chem., Vol. 70, No. 20, 2005

Fast and Stable Photochromic Oxazines
The evolution of singlet oxygen (1∆g) in the course of the
photoinduced ring opening of 4a was monitored by lumines-
cence measurements in air-saturated MeCN. Upon laser
excitation of 4a, the emission of singlet oxygen at 1.27 µm was
probed orthogonally to the exciting beam with a preamplified
(low impedance) Ge photodiode (300-ns resolution) maintained
at 77 K and coupled to a long-pass silicon filter (>1.1 µm) and
an interference filter (1.27 µm). The temporal profile of the
luminescence was fitted to a monoexponential decay function
with the exclusion of the initial portion of the plot, which is
affected by the scattered excitation. The luminescence at initial
time was extrapolated from the curve fitting. The quantum
yield (Φ∆1) of singlet oxygen was determined with eq 5, using
an optically matched and air-saturated MeCN solution of
benzophenone as standard. The quantum yield (Φ∆2) of singlet
oxygen formed upon excitation of benzophenone is 0.37.32 The
terms ø1 and ø2 in eq 2 are the slopes of the linear portions of
plots of the singlet-oxygen luminescence, determined at initial
time upon excitation of 4a and benzophenone, respectively,
against the energy of the laser pulse.
ø1Φ∆2
Φ∆1
)
(5)
ø2
Acknowledgment. We thank the National Science
Foundation (CAREER Award CHE-0237578), Univer-
sity of Miami, and MIUR (Rome, Italy) for financial
support.
Supporting Information Available: General methods
and experimental procedure for the synthesis of 2 and 6,
partial 1H NMR spectra of 4a at various temperatures, partial
1H NMR spectra of 3a before and after the addition of Bu4-
NOH, single-crystal X-ray structures (ORTEP) of 3a and 4c,
ground-state potential energy profile (AM1) of 3a, computa-
tional data (B3LYP) for 3a, 3b, and the transition state,
steady-state absorption spectra of 4a before and after the
addition of Bu4NOH, transient absorption spectra of 3a, and
kinetic trace for the photoisomerization of 4a. This material
is available free of charge via the Internet at http://pubs.acs.org.
(30) Foresman, J. B.; Frisch, Æ. Exploring Chemistry with Electronic
Structure Methods; Gaussian, Inc.: Pittsburgh, PA, 1996.
(31) Bensasson, R. V.; Gramain, J. C. J. Chem. Soc., Faraday Trans.
1980, 76, 1801-1810.
(32) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref.
Data 1993, 22, 113-262.
JO051417W
J. Org. Chem, Vol. 70, No. 20, 2005 8189