Bichromophoric photochromes based on the opening and closing of a single oxazine ring

Article abstract of DOI:10.1021/jo7017119

(Figure Presented) We have designed bichromophoric photochromes based on the photoinduced opening and thermal closing of a [1,3]oxazine ring. In particular, we have synthesized six compounds incorporating fused 3H-indole and 4-nitrophenoxy fragments and pendant biphenyl, styryl, biphenylvinyl, or stilbenylvinyl groups. The laser excitation of two of these six molecules cleaves a [C-O] bond and opens their [1,3]oxazine ring in less than 6 ns with quantum yields of 0.08 and 0.28, respectively. This photoinduced process generates simultaneously a 4-nitrophenolate anion and a 3H-indolium cation. Both chromophores absorb in the same region of the electromagnetic spectrum. As a result, an intense band appears at ca. 440 nm upon the photoinduced opening of the [1,3]oxazine ring. In both instances, the photogenerated species switches back to the original isomer with first-order kinetics and lifetimes of 38 and 140 ns, respectively. Both compounds have excellent fatigue resistances and retain their photochemical behavior within rigid poly-(methyl methacrylate) matrices. However, the thermal reisomerization within the polymer matrix is significantly slower and requires several microseconds to occur. The other four compounds do not undergo ring opening upon excitation under otherwise identical experimental conditions. Indeed, either photoinduced electron transfer or intersystem crossing compete successfully with the ring-opening process.

Full text of DOI:10.1021/jo7017119

Bichromophoric Photochromes Based on the Opening and Closing
of a Single Oxazine Ring
Massimiliano Tomasulo,† Salvatore Sortino,*,‡ and Franc¸isco M. Raymo*,†
Center for Supramolecular Science, Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe,
Miami, Florida 33146-0431, and Dipartimento di Scienze Chimiche, UniVersita´ di Catania,
Viale Andrea Doria 8, Catania, I-95125, Italy
ssortino@unict.it; fraymo@miami.edu
ReceiVed September 6, 2007
We have designed bichromophoric photochromes based on the photoinduced opening and thermal closing
of a [1,3]oxazine ring. In particular, we have synthesized six compounds incorporating fused 3H-indole
and 4-nitrophenoxy fragments and pendant biphenyl, styryl, biphenylvinyl, or stilbenylvinyl groups. The
laser excitation of two of these six molecules cleaves a [C-O] bond and opens their [1,3]oxazine ring
in less than 6 ns with quantum yields of 0.08 and 0.28, respectively. This photoinduced process generates
simultaneously a 4-nitrophenolate anion and a 3H-indolium cation. Both chromophores absorb in the
same region of the electromagnetic spectrum. As a result, an intense band appears at ca. 440 nm upon
the photoinduced opening of the [1,3]oxazine ring. In both instances, the photogenerated species switches
back to the original isomer with first-order kinetics and lifetimes of 38 and 140 ns, respectively. Both
compounds have excellent fatigue resistances and retain their photochemical behavior within rigid poly-
(methyl methacrylate) matrices. However, the thermal reisomerization within the polymer matrix is
significantly slower and requires several microseconds to occur. The other four compounds do not undergo
ring opening upon excitation under otherwise identical experimental conditions. Indeed, either photoinduced
electron transfer or intersystem crossing compete successfully with the ring-opening process.
Introduction
polymer matrices, or even crystals and translate into drastic
alterations of the absorption coefficient and refractive index of
the overall material. In fact, photonic materials and devices have
already been designed around the absorptive and dispersive
effects associated with this particular class of functional
compounds.6-9
In most instances, photochromic transformations are based
on unimolecular photochemical reactions involving ring-closing/
opening steps or cis/trans isomerizations.1-5 Generally, these
processes require a single chromophore to absorb a photon and
undergo significant stereoelectronic modifications, which even-
The unique properties of photochromic compounds continue
to stimulate the development of a diversity of functional
molecules with photoresponsive character.1-5 Indeed, these
molecular switches respond to optical stimulations with sig-
nificant, but reversible, structural and electronic changes. These
modifications at the molecular level can occur in liquid solutions,
† University of Miami.
‡ Universita´ di Catania.
(1) Dorion, G. H.; Wiebe, A. F. Photochromism; Focal Press: New York,
1970.
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York, 1990.
(4) Bouas-Laurent, H., Du¨rr, H., Eds. Photochromism: Molecules and
Systems; Elsevier: Amsterdam, 1990.
(6) McArdle, C. B., Ed. Applied Photochromic Polymer Systems;
Blackie: Glasgow, 1992.
(7) Irie, M., Ed. Photo-ReactiVe Materials for Ultrahigh Density Optical
Memory; Elsevier: Amsterdam, 1994.
(5) Crano, J. C., Guglielmetti, R., Eds. Organic Photochromic and
Thermochromic Compounds; Plenum Press: New York, 1999.
(8) Irie, M., Ed. Chem. ReV. 2000, 100, 1683-1890.
(9) Raymo, F. M.; Tomasulo, M. Chem. Eur. J. 2006, 12, 3186-3193.
10.1021/jo7017119 CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/06/2007
118
J. Org. Chem. 2008, 73, 118-126

Bichromophoric Photochromes
FIGURE 1. Photoinduced and reversible interconversion of the [1,3]-
oxazine 1a and zwitterion 1b.
tually culminate into a color change. For example, we have
demonstrated that the laser excitation of the 4-nitrophenoxy
chromophore of 1a (Figure 1) at 355 nm results in the cleavage
of the [C-O] bond in less than 6 ns with a quantum yield of
0.10 in acetonitrile at 20 °C.10 The photoinduced opening of
the [1,3]oxazine ring generates the 4-nitrophenolate chro-
mophore of 1b (Figure 1) with the concomitant development
of an absorption band centered at 440 nm. The photogenerated
isomer 1b reverts thermally to the original species 1a with a
lifetime of 25 ns. This particular photochromic system is
remarkably stable and tolerates thousands of switching cycles
with no sign of degradation, even in the presence of molecular
oxygen. Furthermore, the photoinduced and reversible inter-
conversion of 1a and 1b also occurs in rigid poly(methyl
methacrylate) matrices with concomitant changes in the absorp-
tion coefficient of the doped polymer.
In search of alternative structural designs for photochromic
compounds, molecular switches incorporating pairs of identical
photoresponsive chromophores have also been developed.11-17
In these systems, the photoinduced switching of the two identical
subunits requires the absorption of two independent photons in
two distinct photochemical events. In principle, bichromophoric
photochromes can also be designed to operate in response to a
single photon in a single photochemical reaction. Specifically,
a photochromic transformation can be engineered to generate a
pair of distinct chromophores after the absorption of only one
photon. On the basis of these considerations, we have envisaged
FIGURE 2. Synthesis of the [1,3]oxazine 4a.
the possibility of modifying the structural design of our original
photochromic oxazine 1a (Figure 1) to generate a new family
of bichromophoric photochromes. In this paper, we report the
synthesis of six bichromophoric switches, based on this general
design logic, together with their photochemical and photophysi-
cal properties.
Results and Discussion
Design and Synthesis. The photoinduced opening of the [1,3]-
oxazine ring of 1a (Figure 1) generates a 4-nitrophenolate
chromophore able to absorb in the visible region.10 Additionally,
the cleavage of the [C-O] bond at the junction of the two
heterocyclic fragments of 1a brings the adjacent phenyl ring in
conjugation with the 3H-indolium cation of 1b. In principle,
this transformation can be exploited to generate another chro-
mophore able to absorb in the visible region in addition to the
4-nitrophenolate anion. Indeed, the phenyl substituent of 1a can
be replaced with an extended π-system in order to enhance
significantly the conjugation of the 3H-indolium cation formed
upon photoinduced opening of the [1,3]oxazine ring. Alterna-
tively, conjugated substituents can be introduced in the para
position, relative to the nitrogen atom, on the phenylene ring
of the 3H-indole fragment of 1a, once again, with the ultimate
goal of generating an extended π-system able to absorb in the
visible region, after the opening of the [1,3]oxazine ring. On
the basis of these considerations, we have designed and
synthesized six photochromic oxazines incorporating biphenyl
(4a in Figure 2), styryl (6a in Figure 3), biphenylvinyl (7a in
Figure 3), or stilbenylvinyl (8a, 13a, and 14a in Figures 3 and
4) substituents.
(10) (a) Tomasulo, M.; Sortino, S.; Raymo, F. M. Org. Lett. 2005, 7,
1109-1112. (b) Tomasulo, M.; Sortino, S.; White, A. J. P.; Raymo, F. M.
J. Org. Chem. 2005, 70, 8180-8189.
(11) Bleisinger, H.; Scheidhauer, P.; Du¨rr, H.; Wintgens, V.; Valat, P.;
Kossanyi, J. J. Org. Chem. 1998, 63, 990-1000.
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Chem. Soc. 1999, 121, 2601-2602. (b) Mitchell, R. H.; Ward, T. R.; Chen,
Y.; Wang, Y.; Dibble, P. W.; Weerawarna, S.; Marsella, M. J.; Almutairi,
A.; Wang, Z.-Q. J. Am. Chem. Soc. 2003, 125, 2974-2988. (c) Mitchell,
R. H.; Bandyopadhyay, S. Org. Lett. 2004, 6, 1729-1732. (d) Straight, S.
D.; Andre´asson, J.; Kodis, G.; Bandyopadhyay, S.; Mitchell, R. H.; Moore,
T. A.; Moore, A. L.; Gust, D. J. Am. Chem. Soc. 2005, 127, 9403-9409.
(e) Mitchell, R. H.; Bohne, C.; Wang, Y.; Bandyopadhyay, S.; Wozniak,
C. B. J. Org. Chem. 2006, 17, 327-336.
(13) (a) Yassar, A.; Jaafari, H.; Rebie`re-Galy, N.; Frigoli, M.; Moustrou,
C.; Samat, A.; Guglielmetti, R. Eur. Phys. J. 2002, 18, 3-8. (b) Yassar,
A.; Rebie`re-Galy, N.; Frigoli, M.; Moustrou, C.; Samat, A.; Guglielmetti,
R.; Jaafari, A. Synth. Met. 2001, 124, 23-27.
(14) (a) Zhao, W.; Carreira, E. M. J. Am. Chem. Soc. 2002, 124, 1582-
1583. (b) Zhao, W.; Carreira, E. M. Org. Lett. 2006, 8, 99-102.
(15) (a) Kawai, T.; Sasaki, T.; Irie, M. Chem. Commun. 2001, 711-
712. (b) Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2001, 123, 9896-9897.
(c) Higashiguchi, K.; Matsuda, K.; Irie, M. Angew. Chem., Int. Ed. 2003,
42, 3537-3540. (d) Kobatake, S.; Irie, M. Tetrahedron 2003, 59, 8359-
8364. (e) Higashiguchi, K.; Matsuda, K.; Tanifuji, N.; Irie, M. J. Am. Chem.
Soc. 2005, 127, 8922-8923.
(16) Guo, X.; Zhang, D.; Zhou, Y.; Zhu, D. J. Org. Chem. 2003, 68,
5681-5687.
(17) (a) Chen, B.-Z.; Wang, M.-Z.; Luo, Q.-F.; Tian, H. Synth. Met. 2003,
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We have prepared 4a (Figure 2) in three steps, with an overall
yield of 38%, starting from biphenyl. Specifically, we have
acylated biphenyl with isobutyryl chloride in the presence of
J. Org. Chem, Vol. 73, No. 1, 2008 119

Tomasulo et al.
FIGURE 3. Synthesis of the [1,3]oxazines 6a, 7a, and 8a.
FIGURE 5. Model compounds 15-18.
FIGURE 6. Steady-state absorption spectra (0.1 mM, MeCN, 20 °C)
of 4a (a), 5a (b), and 15 (c).
oxazine skeleton of 11a and 12a. Finally, we have coupled these
molecules with 1-vinyl-trans-stilbene in the presence of tri-
ethylamine and palladium(II) bisacetate to afford the target
oxazines 13a and 14a.
Steady-State Absorption Spectroscopy. The absorption
spectra of the oxazines 4a, 6a, 7a, and 8a are approximately
the sum of those of their constituent chromophores (5a in Figure
3 and 15-18 in Figure 5). For example, the spectrum of 4a (a
in Figure 6) shows one band at 308 nm for the 4-nitrophenoxy
chromophore and another at 248 nm for the biphenyl appendage.
Both bands closely resemble the absorptions observed in the
spectra of the model compounds 5a and 15 (b and c in Figure
6) respectively. The other three oxazines 6a, 7a, and 8a show
essentially the same behavior (Table 1 and Figure S5, Supporting
Information). Thus, the 4-nitrophenoxy chromophore and the
adjacent π-system have negligible interactions in the ground
state of all four compounds.
The [1,3]oxazine ring of 4a, 6a, 7a, and 8a opens to generate
the anionic hemiaminals 4c, 6c, 7c, and 8c (Figure 7),
respectively, in the presence of tetrabutylammonium hydroxide.
This transformation causes the appearance of a band in the
visible region for the 4-nitrophenolate chromophore of the
resulting hemiaminals. For example, the addition of tetrabuty-
lammonium hydroxide to 4a encourages the evolution of a band
at 428 nm (a and b in Figures 8) that resembles the absorption
of the tetrabutylammonium salt of 4-nitrophenolate (c in Figure
8). Once again, the other three oxazines 6a, 7a, and 8a show
essentially the same behavior upon addition of tetrabutylam-
monium hydroxide (Figure S16, Supporting Information).
The [1,3]oxazine ring of 4a, 6a, 7a, and 8a opens to generate
the 3H-indolium cations 4d, 6d, 7d and 8d (Figure 7),
respectively, in the presence of trifluoracetic acid. This trans-
formation extends the conjugation of the π-system appended
FIGURE 4. Synthesis of the [1,3]oxazines 13a and 14a.
aluminum chloride and condensed the resulting ketone 2 with
phenyl hydrazine under acidic conditions. We have then reacted
the 3H-indole 3 with 4-nitro-2-chloromethylphenol to afford the
target molecule 4a. Instead, we have prepared 6a, 7a, and 8a
(Figure 3) in a single step, with yields of 76, 40, or 53%,
respectively, starting from the known oxazine 5a.10b,18 In
particular, we have condensed 5a with benzaldehyde, 4-phe-
nylbenzaldehyde, or trans-4-(2-phenylethylene)benzaldehyde
under acidic conditions to give 6a, 7a, or 8a, respectively.19
We have synthesized 13a and 14a (Figure 4) in three steps,
with overall yields of 21 and 13%, starting from isopropyl
methyl ketone and isopropyl phenyl ketone, respectively. Indeed,
we have condensed both ketones with 4-iodophenyl hydrazine
to generate the 3H-indoles 9 and 10. We have then reacted these
compounds with 4-nitro-2-chloromethylphenol to assemble the
(18) (a) Shachkus, A. A.; Degutis, J.; Jezerskaite, A. In Chemistry of
Heterocyclic Compounds; Kovac, J., Zalupsky, P., Eds.; Elsevier: Amster-
dam, 1987; Vol. 35, pp 518-520. (b) Shachkus, A. A.; Degutis, J.;
Urbonavichyus, A. G. Khim. Geterotskil. Soed. 1989, 5, 672-676; Chem.
Heterocycl. Compd. 1989, 562-565.
(19) Tomasulo, M.; Sortino, S.; Raymo, F. M. AdV. Mater. 2007, 19, in
press.
120 J. Org. Chem., Vol. 73, No. 1, 2008

Bichromophoric Photochromes
TABLE 1. Spectroscopic Dataa for the Oxazines 4a, 6a, 7a, 8a,
13a, and 14a and Their Model Compounds
in the oxazines 6a, 7a and 8a (Figure S21, Supporting
Information). A residual emission can only be observed for the
biphenyl appendage of 4a (a and b in Figure 11). Presumably,
the transfer of one electron from the nitrogen atom of the 3H-
indole fragment of these oxazines to the adjacent fluorophore
upon excitation is responsible for quenching. Indeed, the redox
potentials22 of 15-18 and 23 (Table 2) suggest that this
photoinduced electron-transfer process is exoergonic for the four
oxazines with free energy changes more negative than -0.2
eV.23
In agreement with a quenching mechanism based on photo-
induced electron transfer, the transformation of 4a, 6a, 7a, and
8a into the hemiaminal anions of 4c, 6c, 7c, and 8c, after the
addition of tetrabutylammonium hydroxide, has negligible
influence on their emission spectra. In fact, the 3H-indole
fragment retains its electron-rich character after ring opening24
and, thus, can still transfer an electron to the adjacent fluoro-
phores upon excitation. Instead, the treatment of 4a, 6a, 7a,
and 8a with trifluoracetic acid imposes a positive charge on
the nitrogen atom of the resulting 3H-indolium cations 4d, 6d,
7d, and 8d. Furthermore, this process brings the 3H-indolium
cation in conjugation with the adjacent π-system to generate
chromophores similar to the model compounds 19-22. Con-
sistently, the conversion of 4a, 7a, and 8a into 4d, 7d, and 8d
causes the appearance of broad and weak bands in the emission
spectra (Figure S22, Supporting Information), which closely
resemble the emissions of 19, 21, and 22.25
λA (nm)
ꢀ (mM-1 cm-1
)
λE (nm)
ΦF
ΦP
τ (ns)
1a
4a
5a
6a
7a
8a
13a
14a
15
16
17
18
19
20
21
22
316
248, 308
318
244, 315
248
330
368
367
248
247
276
325
327
383
412
426
11.0
26.2, 9.3
10.0
16.6, 9.7
36.1
50.5
57.6
58.4
16.2
14.3
31.7
30.2
14.5
28.1
35.1
33.3
0.10
25
314
0.02
0.03
25
0.08
0.28
38
140
424
429
313
305
334
408
0.02
0.02
0.15
0.16
0.65
0.28
a The absorption wavelength (λA), molar extinction coefficient (ꢀ) at λA,
emission wavelength (λE), fluorescence quantum yield (ΦF), and quantum
yield (ΦP) for the photochromic transformation and lifetime (τ) of the
photogenerated isomer were measured in MeCN at 20 °C. The λA, ꢀ, ΦP,
and τ for 1a and 5a are from ref 10b. The ΦF of 15-17 are from refs 20
and 21. The error in the determination of ΦF, ΦP, and τ is ca. 15%.
to the 3H-indolium fragment with a concomitant bathochromic
shift in absorption. For example, the band of the biphenyl
appendage of 7a shifts to 421 nm (a and b in Figure 9) upon
addition of acid to resemble the absorption of the model
compound 21 (c in Figure 9 and Figure 10). A similar effect
can be observed in the spectra of 4a, 6a, and 8a (Figure S17,
Supporting Information) upon addition of trifluoracetic acid.
Indeed, the absorption of the π-system appended to their 3H-
indolium fragment moves to 308, 388, and 460 nm with the
formation of 4d, 6d, and 8d, respectively, to resemble the bands
of the model compounds 19, 20, and 22 (Figure 10).
The two oxazines 13a and 14a emit at ca. 425 nm with a
quantum yield of 0.02 (Table 1, Figure S23, Supporting
Information). Their bands are bathochromically shifted by ca.
20 nm relative to the emission of 18, as a result of the
conjugation of the stilbenylvinyl appendage with the 3H-indole
fragment. In both instances, the opening of the [1,3]oxazine ring
with either tetrabutylammonium hydroxide or trifluoroacetic acid
has negligible influence on the emission behavior.
The spectra of the oxazines 13a and 14a (Figure 4) differ
from the sum of those of their constituent components (1a or
5a in Figures 1 or 3 and 18 in Figure 5). Both compounds show
a bathochromic shift of 42 nm relative to 18 (Table 1, Figure
S18, Supporting Information), indicating that the stilbenylvinyl
appendage and the phenylene ring of the 3H-indole interact in
the ground state. As observed for 4a, 6a, 7a, and 8a, the addition
of either tetrabutylammonium hydroxide or trifluoracetic acid
opens the [1,3]oxazine ring of 13a and 14a. The formation of
13b and 14b (Figure 7) is accompanied, once again, by the
appearance of the characteristic absorption for the 4-nitrophe-
nolate chromophore (Figure S19, Supporting Information).
Instead, the generation of 13d and 14d (Figure 7) has a modest
influence on the absorption spectrum. Specifically, the main
band of 13a and 14a at 367 nm decreases slightly in intensity
and broadens with the formation of 13d and 14d (Figure S20,
Supporting Information), suggesting that the additional double
bond on the 3H-indolium cation has a modest influence on the
absorption characteristics of the adjacent stilbenylvinylphenylene
chromophore.
Transient Absorption Spectroscopy. The laser excitation
of the 4-nitrophenoxy chromophore of 1a at 355 nm opens the
[1,3]oxazine ring with a quantum yield of 0.10 (Table 1).10 This
process is accompanied by the appearance of a band at 440 nm
for the ground-state absorption of the 4-nitrophenolate chro-
mophore (c in Figure 8) of the photogenerated isomer 1b. This
species has a lifetime of 25 ns and reverts thermally to the
original isomer with the concomitant monoexponential decay
of the 4-nitrophenolate band. In contrast, the transient absorption
spectra of 4a and 6a do not show any significant signal upon
excitation, under otherwise identical experimental conditions.
The redox potentials of the model compounds 15, 16, and 24
(Table 2) suggest that electron transfer from the biphenyl or
styryl appendages of 4a or 6a, respectively, to the excited
4-nitrophenoxy fragment is exoergonic with a free energy
change more negative than -0.5 eV.23 Presumably, this pho-
toinduced electron-transfer process competes successfully with
the opening of the [1,3]oxazine ring and prevents the formation
of the zwitterions 4b and 6b.
Steady-State Emission Spectroscopy. The model com-
pounds 15-18 emit upon ultraviolet excitation with quantum
yields ranging from 0.15 to 0.65 (Table 1).20,21 Instead, the
fluorescence of these extended π-systems is effectively quenched
(22) Bezuglyi, V. D.; Ponomarev, Yu. P. J. Anal. Chem. USSR 1969,
24, 324-328.
(23) The free energy changes for the photoinduced electron transfer
processes were estimated with the Rehm-Weller equation (Rehm, D.;
Weller, A. Isr. J. Chem. 1970, 8, 259-271) and are listed in Tables S1 and
S2, Supporting Information.
(20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry; Dekker: New York, 1993.
(21) Tanaka, K.; Pescitelli, G.; Nakanishi, K.; Berova, N. Monatsh. Chem.
2005, 136, 367-395.
(24) Tomasulo, M.; Yildiz, I.; Kaanumalle, S. L.; Raymo, F. M. Langmuir
2006, 22, 10284-10290.
(25) The spectra of 6a recorded after treatment with trifluoroacetic acid
and the one of the model compound 20 do not show any significant emission.
J. Org. Chem, Vol. 73, No. 1, 2008 121

Tomasulo et al.
FIGURE 7. Opening of the [1,3]oxazine ring of a in the presence of either Bu4NOH or CF3CO2H to form either the anionic hemiaminal c or the
cationic 3H-indolium d.
FIGURE 8. Steady-state absorption spectra (0.1 mM, MeCN, 20 °C)
of 4a before (a) and after (b) the addition of Bu4NOH (10 equiv) and
of 4-nitrophenol (c) after the addition of Bu4NOH (4 equiv).
FIGURE 10. Model compounds 19-22.
FIGURE 11. Steady-state emission spectra (0.01 mM, MeCN, 20 °C,
λEx ) 249 nm) of 15 (a) and 4a (b).
FIGURE 9. Steady-state absorption spectra (0.01 mM, MeCN, 20 °C)
of 7a before (a) and after (b) the addition of CF3CO2H (90 equiv) and
of 21 (c).
ground-state absorptions in the same region of wavelengths27
(c in Figures 8 and 9), and thus, the photoinduced absorbance
change observed for 7a is approximately twice that for 1a.
Specifically, the transition from the monochromophoric pho-
tochrome 1a to the bichromophoric photochrome 7a results in
an enhancement in coloration efficiency of 1.8 ( 0.3.28
The photogenerated isomer 7b reverts thermally to the
original one 7a on a nanosecond time scale with a concomitant
decay in the visible absorbance. The monoexponential fitting
of the temporal absorbance evolution (a and b in Figure 13)
In analogy to the photochemical behavior of 1a, the laser
excitation of 7a opens the [1,3]oxazine ring with a quantum
yield of 0.08.26 In both instances, the transient absorption
spectrum recorded 30 ns after excitation shows an intense band
in the visible region. However, the absorption observed for 7a
(a in Figure 12) is more intense and narrower than that
determined for an optically matched solution of 1a (b in Figure
12) under the same conditions. Indeed, the photoinduced
transformation of 1a into 1b generates one chromophore only,
while the conversion of 7a into 7b produces two chromophoric
fragments. In particular, the opening of the [1,3]oxazine ring
of 7a generates a 3H-indolium cation with an extended π-system
together with the 4-nitrophenolate anion. Both species have
(27) In principle, the two chromophores could be designed to absorb at
different wavelengths. Under these conditions, the photoinduced opening
of the [1,3]oxazine ring could be exploited to generate two separate bands
in the visible region of the absorption spectrum.
(28) The coloration efficiency is the ratio between the number of photons
absorbed by the photogenerated isomer at a fixed visible wavelength and
the number of photons absorbed by the original isomer at the excitation
wavelength.
(26) The quantum yield was determined using an optically-matched
MeCN solution of 1a as standard, as described in the supporting information.
122 J. Org. Chem., Vol. 73, No. 1, 2008

Bichromophoric Photochromes
TABLE 2. Electrochemical Dataa for the Model Compounds
15-18, 23, and 24
E
Ox (V vs Ag/AgCl)
ERed (V vs Ag/AgCl)
15
16
17
18
23
24
+1.80
+1.70
+1.20
+1.18
+0.74
+2.36
-2.43
-2.58
-2.23
-2.58
-2.36
-1.22
a The oxidation (EOx) and reduction (ERed) potentials of 15-18 are from
ref 22. The EOx and ERed of 23 and 24 were measured by cyclic voltammetry
in a degassed MeCN solution of Bu4NPF6 (0.1 M), using a glassy carbon
working electrode, a platinum counter electrode, and a Ag/AgCl reference
electrode. The structures of the model compounds 23 and 24 are shown in
the following diagram:
FIGURE 14. Transient absorption spectra (0.01 mM, MeCN, 20 °C)
of 13a recorded 0.08 (a), 2 (b), 5 (c), and 10 µs (d) after the laser
excitation (355 nm, 8 mJ).
the photogenerated isomer and delay the reisomerization kinetics
relative to 1b. In addition, the photoinduced and reversible
interconversion of 7a and 7b tolerates multiple switching cycles
with no sign of decomposition. Indeed, the steady-state absorp-
tion spectra of 7a recorded before and after 400 consecutive
excitation cycles are essentially identical.
The laser excitation of 8a results in the opening of the [1,3]-
oxazine ring, as observed for 1a and 7a, with a quantum yield
of 0.28.26 Once again, the photoinduced process is accompanied
by the appearance of a band in the visible region (Figure S25,
Supporting Information), which corresponds to ground-state
absorptions of the 4-nitrophenolate and 3H-indolium chro-
mophores of the photogenerated isomer 8b. This species reverts
thermally to the original one with a concomitant decay in the
visible absorbance. In this instance, however, the monoexpo-
nential fitting of the temporal evolution of the absorbance
indicates the lifetime of the photogenerated isomer to be 140
ns (Figure S26, Supporting Information). This value is consider-
ably longer than those determined for 1b and 7b and is,
presumably, a result of the extended conjugation of the 3H-
indolium cation of 8b, which stabilizes this species relative to
8a and delays the ring closing step.
FIGURE 12. Transient absorption spectra of 7a (0.05 mM, MeCN,
20 °C, a) and of an optically matched solution of 1a (b) recorded 30
ns after the laser excitation (355 nm, 8 mJ).
The photochemical behavior of 13a is significantly different
from that of 1a, 5a, 7a, and 8a. Indeed, the laser excitation of
13a does not result in the opening of the [1,3]oxazine ring, under
otherwise identical experimental conditions. A transient absorp-
tion at 510 nm (a in Figure 14) is observed instead of the
characteristic band at ca. 440 nm for the 4-nitrophenolate
chromophore. This band resembles the triplet-triplet absorption
of phenylvinylstilbene29 and decays monoexponentially on a
microsecond time scale (a-d in Figure 14). The nonlinear curve
fitting of the temporal evolution of this absorption (Figure 15)
indicates the lifetime of the transient species to be 2 µs. Thus,
the excitation of 13a encourages intersystem crossing instead
of the opening of the [1,3]oxazine ring.
FIGURE 13. Temporal evolution of the absorbance at 430 nm after
the excitation (0.05 mM, MeCN, 20 °C, 355 nm, 8 mJ) of 7a (a) and
the corresponding monoexponential fitting (b).
indicates the lifetime of the photogenerated isomer to be 38 ns.
This value is slightly longer than that determined for 1b, which
is only 25 ns.10 Presumably, the biphenylvinyl appendage
conjugated to the 3H-indolium cation of 7b tends to stabilize
(29) Hara, M.; Samori, S.; Xichen, C.; Fujitsuka, M.; Majima, T. J. Org.
Chem. 2005, 70, 4370-4374.
J. Org. Chem, Vol. 73, No. 1, 2008 123

Tomasulo et al.
FIGURE 15. Temporal evolution of the absorbance at 510 nm after
the excitation (0.01 mM, MeCN, 20 °C, 355 nm, 8 mJ) of 13a (a) and
the corresponding monoexponential fitting (b).
FIGURE 17. Temporal evolution of the absorbance at 430 nm after
the excitation a poly(methyl methacrylate) film doped with 7a (a) and
the corresponding biexponential fitting (b).
Conclusions
We have synthesized six compounds incorporating 3H-indole
and benzooxazine heterocycles fused to each other. In four
molecules, a biphenyl, styryl, biphenylvinyl, or stilbenylvinyl
appendage is attached to the chiral center at the junction of the
two heterocyclic fragments. Their steady-state absorption spectra
demonstrate that the aromatic appendages have negligible
ground-state interactions with the two heterocyclic fragments.
Instead, the emission of the aromatic fluorophores is almost
completely suppressed. The redox potentials of model com-
pounds suggest that photoinduced electron transfer from the 3H-
indole component to the excited fluorophores is responsible for
quenching. The selective excitation of the 4-nitrophenoxy
chromophore, embedded in the benzooxazine heterocycle of the
two compounds incorporating the biphenylvinyl and stilbenylvi-
nyl appendages, cleaves a [C-O] bond to open a [1,3]oxazine
ring in less than 6 ns and with quantum yields of 0.08 and 0.28,
respectively. In both instances, this photoinduced process
generates a 4-nitrophenolate anion and brings the biphenylvinyl
and stilbenylvinyl groups in conjugation with a 3H-indolium
cation. The resulting anionic and cationic chromophores absorb
in the same region of wavelengths, and thus, an intense band at
ca. 440 nm appears in the absorption spectrum upon excitation.
The photogenerated isomers of both compounds revert to the
original species after the thermal closing of the [1,3]oxazine
ring with lifetimes of 38 and 140 ns, respectively. Thus, a full
switching cycle can be completed on a nanosecond time scale
with these two photochromic systems. In addition, both mol-
ecules can be operated within rigid poly(methyl methacrylate)
matrices. Under these conditions, however, the thermal rei-
somerization process is slower and requires several microsec-
onds to occur. The selective excitation of the 4-nitrophenoxy
chromophore, embedded in the benzooxazine heterocycle of the
two compounds incorporating the biphenylvinyl and stilbenylvi-
nyl groups, does not lead to ring opening. The redox potentials
of model compounds suggest that photoinduced electron transfer
FIGURE 16. Transient absorption spectra of a poly(methyl meth-
acrylate) film doped with 7a recorded 3 (a), 17 (b), 22 (c), and 33 µs
(d) after the laser excitation (355 nm, 8 mJ).
The oxazines 7a and 8a can be trapped within rigid poly-
(methyl methacrylate) matrices and operated under these condi-
tions with a similar response to that observed in acetonitrile
solution. Specifically, the laser excitation of poly(methyl
methacrylate) films with a thickness of ca. 6 µm and containing
either 7a or 8a induces the opening of the [1,3]oxazine ring
with the appearance of the characteristic transient absorption
of the photogenerated isomer 7b or 8b (Figure 16, Figure S20,
Supporting Information). This band fades with the reisomer-
ization of the zwitterionic isomer back to the original one (a-d
in Figure 16 and Figure S27, Supporting Information), as
observed in acetonitrile. However, the reisomerization process
is significantly slower in the polymer matrix. In particular, the
visible absorbance decays biexponentially with lifetimes of 10
and 90 µs for 7a (Figure 17) and 2 and 25 µs for 8a (Figure
S28, Supporting Information), as observed for 1a under the same
experimental conditions.10b This behavior parallels the biexpo-
nential kinetics for the thermal decoloration of nitrospiropyrans
in polymer matrices and is, presumably, a result of the
aggregation of the photogenerated isomers into relatively long-
lived supramolecular assemblies.30
(30) (a) Krongauz, V. A. In ref 4, pp 793-821, and in ref 6, pp 121-
173. (b) Berkovic, G.; Krongauz, V. A.; Weiss, V. In ref 8, pp 1741-
1754.
124 J. Org. Chem., Vol. 73, No. 1, 2008

Bichromophoric Photochromes
449.1860, found 449.1856. 1H NMR (300 MHz, CDCl3): δ ) 0.91
(3H, s), 1.63 (3H, s), 4.59 (1H, d, 14 Hz), 4.67 (1H, d, 14 Hz),
6.75 (1H, d, 8 Hz), 6.88-6.95 (2H, m), 7.14-7.21 (2H, m), 7.38
(1H, d, 7 Hz), 7.43-7.48 (2H, m), 7.60-7.72 (6H, m), 7.95-7.98
(2H, m). 13C NMR (100 MHz, CD3CN): δ ) 18.6, 27.9, 41.0,
50.0, 105.5, 109.3, 118.3, 120.3, 121.1, 122.6, 123.4, 124.0, 127.3,
127.4, 127.4, 128.6, 129.1, 135.0, 137.9, 140.5, 141.9, 142.0, 146.9,
159.3.
form the biphenyl and styryl groups to the 4-nitrophenoxy
fragment competes successfully with the ring opening process.
In the other two compounds investigated, a stilbenylvinyl group
is connected to the phenylene ring of the 3H-indole heterocycle
to from an extended π-system. The resulting chromophore
absorbs at wavelengths longer than vinylstilbene, preventing the
selective excitation of the 4-nitrophenoxy fragment. As a result,
the corresponding transient absorption spectrum shows a triplet-
triplet absorption for the extended π-system instead of the
characteristic ground-state absorption of the ring-opened isomer.
In summary, we have designed and synthesized a family of
heterocyclic compounds and demonstrated that photochromic
transformations with fast switching speeds and excellent fatigue
resistances can be implemented in solution and within polymer
films with, at least, two of these molecules. Furthermore, the
modular design of these compounds offers the opportunity to
photoinduce the simultaneous formation of two chromophores
able to absorb in the visible region in a single photochemical
event. Thus, photochromic materials with multichromophoric
response can, in principle, emerge from our innovative design.
2-Nitro-5a-(2-phenylethylene)-6,6-dimethyl-5a,6-dihydro-12H-
indolo[2,1-b][1,3]benzooxazine (6a). A solution of 5a (155 mg,
0.5 mmol), benzaldehyde (150 µL, 1.5 mmol), and CF3CO2H
(125µL, 0.15 mmol) in MeCN (20 mL) was heated under reflux
and Ar for 7 d. After the solution was cooled to ambient
temperature, the solvent was distilled off under reduced pressure,
and the residue was dissolved in CH2Cl2 (25 mL) and washed with
H2O (20 mL). The solvent of the organic phase was distilled off
under reduced pressure, and the residue was purified by column
chromatography [SiO2: hexanes/MeCO2Et (4:1, v/v)] to give 6a
(153 mg, 76%) as a yellowish solid. Mp ) 107 °C. HPLC [MeCN]:
tR ) 4.0 min (254 nm), PA ) 1.6 (254 nm), APP ) 223.1 ( 0.5
nm. FABMS: m/z ) 399 [M + H]+. HRMS: m/z calcd for [M +
1
H]+ C25H23N2O3 399.1703, found 399.1699. H NMR (300 MHz,
CD3CN): δ ) 1.33 (6H, bs), 4.67 (2H, s), 6.48 (1H, d, 16 Hz),
6.73 (1H, d, 8 Hz), 6.81-6.93 (3H, m), 7.04-7.16 (1H, m), 7.18
(1H, dd, 1 and 8 Hz), 7.31-7.34 (3H, m), 7.46-7.50 (2H, m),
7.97 (1H, dd, 3 and 9 Hz), 8.07 (1H, d, 3 Hz). 13C NMR (100
MHz, CDCl3): δ ) 23.1, 32.0, 41.1, 50.4, 109.2, 118.1, 120.4,
121.1, 122.7, 123.6, 124.3, 124.4, 127.3, 128.1, 128.2, 128.9, 129.1,
136.6, 138.6, 146.8, 156.9.
Experimental Section
1-(4-Phenylphenylene)-2-methyl-propan-1-one (2). Sublimed
AlCl3 (2.92 g, 22 mmol) was added to a solution of biphenyl (2.35
g, 15 mmol) in CH2Cl2 (75 mL) maintained at 0 °C under Ar. Then,
a solution of isobutyryl chloride in CH2Cl2 (1:10 v/v, 19.5 mL, 18
mmol) was added dropwise over 30 min. The temperature was
maintained at 0 °C for a further 2 h and allowed to warm to ambient
conditions over the course of 12 h. After the addition of H2O (10
mL), the mixture was stirred for 10 min and extracted with CH2-
Cl2 (150 mL). The organic phase was washed with H2O (50 mL),
and the solvent was distilled off under reduced pressure to afford
2 (3.42 g, 100%) as a yellowish solid. FABMS: m/z ) 225 [M +
2-Nitro-5a-(2-(4-phenylphenylene)ethylene)-6,6-dimethyl-5a,6-
dihydro-12H-indolo[2,1-b][1,3]benzooxazine (7a). A solution of
5a (100 mg, 0.3 mmol), 4-phenylbenzaldehyde (175 mg, 1.0 mmol),
and CF3CO2H (80 µL, 0.1 mmol) in MeCN (15 mL) was heated
under reflux and Ar for 7 d. After the solution was cooled to
ambient temperature, the solvent was distilled off under reduced
pressure. The solid residue was dissolved in CH2Cl2 (5 mL), and
the solution was diluted with hexane (50 mL). The resulting
precipitate was filtered off and crystallized from PhMe (10 mL) to
give 7a (60 mg, 40%) as a orange solid. Mp ) 194 °C. HPLC
(MeCN): tR ) 4.1 min (278 nm), PA ) 1.4 (278 nm), APP )
236.7 ( 0.3 nm. FABMS: m/z ) 475 [M + H]+. HRMS: m/z
1
H]+. H NMR (300 MHz, CDCl3): δ ) 1.29 (6H, d, 7 Hz), 3.64
(1H, sep, 7 Hz), 7.41-7.53 (3H, m), 7.64-7.68 (2H, m), 7.70-
7.74 (2H, m), 8.06-8.10 (2H, m). 13C NMR (100 MHz, CDCl3):
δ ) 19.0, 31.7, 127.3, 128.3, 129.0, 129.1, 132.0, 140.0, 145.5,
204.3.
1
2-(4-Phenylphenylene)-3,3-dimethyl-3H-indole (3). A solution
of 2 (2.20 g, 10 mmol) and phenyl hydrazine (1.10 g, 10 mmol) in
MeCO2H (35 mL) was heated under reflux and Ar for 30 h. After
the solution was cooled to ambient temperature, aqueous KOH (1M,
50 mL) was added, and the mixture was extracted with CH2Cl2 (4
× 25 mL). The solvent of the organic phase was distilled off under
reduced pressure, and the residue was purified by column chro-
matography [SiO2: hexane f CH2Cl2/hexane (3:1, v/v)] to afford
3 (2.30 g, 78%) as a red solid. FABMS: m/z ) 298 [M + H]+. 1H
NMR (400 MHz, CDCl3): δ ) 1.68 (6H, s), 7.35 (1H, dd, 1 and
7 Hz), 7.42-7.46 (3H, m), 7.50-7.54 (2H, m), 7.71-7.74 (2H,
m), 7.78-7.82 (3H, m), 8.31-8.35 (2H, m). 13C NMR (100 MHz,
CDCl3): δ ) 25.0, 53.6, 121.0, 126.0, 127.0, 127.2, 127.9, 128.0,
128.9, 129.0, 129.2, 132.5, 140.6, 143.0, 147.8, 153.6, 182.9.
2-Nitro-5a-(4-phenylphenylene)-6,6-dimethyl-5a,6-dihydro-
12H-indolo[2,1-b][1,3]benzooxazine (4a). A solution of 4-nitro-
2-chloromethylphenol (0.43 g, 2 mmol) and 3 (0.62 g, 2 mmol) in
MeCN (35 mL) was heated under reflux and Ar for 5 d. After the
solution was cooled to ambient temperature, the solvent was distilled
off under reduced pressure, and the residue was dissolved in CH2-
Cl2 (50 mL). The resulting solution was extracted with NaOH (1M,
5 mL) and H2O (2 × 15 mL). The solvent of the organic phase
was distilled off under reduced pressure, and the residue was
purified by column chromatography [SiO2: CH2Cl2/hexane (1:1,
v/v) f CH2Cl2/hexane (3:1, v/v)] to afford 4a (0.38 g, 41%) as a
white solid. Mp ) 170 °C. HPLC [MeCN]: tR ) 4.7 min (270
nm), PA ) 1.0 (270 nm), APP ) 271.6 ( 0.2 nm. FABMS: m/z
) 447 [M + H]+. HRMS: m/z calcd for [M + H]+ C29H25N2O3
calcd for [M + H]+ C31H27N2O3 475.2016, found 475.2024. H
NMR (500 MHz, CDCl3): δ ) 1.27 (6H, s), 4.61 (2H, s), 6.42
(1H, d, 16 Hz), 6.62 (1H, d, 7 Hz), 6.85-6.91 (3H, m), 7.11-7.16
(2H, m), 7.36 (1H, t, 7 Hz), 7.35-7.38 (4H, m), 7.58-7.60 (4H,
m), 7.99 (1H, dd, 3 and 9 Hz), 8.02 (1H, d, 3 Hz). 13C NMR (100
MHz, CDCl3): δ ) 30.1, 41.1, 50.5, 104.1, 109.2, 118.1, 120.4,
121.1, 122.7, 123.6, 124.3, 124.4, 127.4, 127.7, 127.8, 128.0, 128.1,
129.2, 134.9, 136.1, 138.6, 140.8, 141.1, 141.9, 146.8, 159.6.
2-Nitro-5a-(2-(4-(2-phenylethylene)phenylene)ethylene)-6,6-
dimethyl-5a,6-dihydro-12H-indolo[2,1-b][1,3]benzooxazine (8a).
A solution of 5a (100 mg, 0.3 mmol), trans-4-(2-phenylethylene)-
benzaldehyde (200 mg, 1.0 mmol), and CF3CO2H (80 µL, 0.01
mmol) in MeCN (15 mL) was heated under reflux and Ar for 7 d.
After the solution was cooled to ambient temperature, the solvent
was distilled off under reduced pressure. The residue was dissolved
in CH2Cl2 (25 mL) and washed with H2O (15 mL). The solvent of
the organic phase was distilled off under reduced pressure, and the
residue was dissolved in CHCl3 (6 mL). The solution was diluted
with hexane (85 mL), and the resulting precipitate was filtered off
to yield 8a (85 mg, 53%). Mp ) 188 °C. HPLC (MeCN): tR
)
4.4 min (300 nm), PA ) 1.4 (300 nm), APP ) 302.2 ( 0.4 nm.
FABMS: m/z ) 501 [M + H]+. HRMS: m/z calcd for [M + H]+
C33H29N2O3 501.2173, found 501.2167. 1H NMR (500 MHz,
CDCl3): δ ) 1.27 (6H, bs), 4.61 (2H, s), 6.39 (1H, d, 16 Hz),
6.65 (1H, d, 8 Hz), 6.82 (1H, d, 16 Hz), 6.88-6.91 (2H, m), 7.11-
7.16 (4H, m), 7.36-7.42 (5H, m), 7.49-7.54 (4H, m), 8.00 (1H,
dd, 3 and 9 Hz), 8.03 (1H, d, 3 Hz). 13C NMR (100 MHz, CDCl3):
δ ) 23.1, 32.0, 41.1, 50.6, 104.2, 109.2, 118.1, 120.4, 121.1, 122.7,
J. Org. Chem, Vol. 73, No. 1, 2008 125

Tomasulo et al.
123.6, 124.1, 124.4, 127.0, 127.2, 127.6, 128.0, 128.1, 128.3, 129.1,
129.7, 135.1, 136.1, 137.5, 138.1, 138.6, 141.1, 159.6.
2-Nitro-5a,6,6-trimethyl-5a,6-dihydro-8-(2-(4-(2-phenylethyl-
ene)phenylene)ethylene)-12H-indolo[2,1-b][1,3]benzooxazine (13a).
A mixture of 11a (38 mg, 0.1 mmol), 1-vinyl-trans-stilbene (17
mg, 0.1 mmol), and Pd(MeCO2)2 (20 mg, 0.1 mmol) in degassed
Et3N (20 mL) was heated at 105 °C under Ar for 1 d. After being
cooled to ambient temperature, the mixture was diluted with CH2-
Cl2 (30 mL) and washed with H2O (40 mL). The solvent of the
organic phase was distilled off under reduced pressure, and the
residue was purified by column chromatography [SiO2: hexane f
CH2Cl2/hexane (1:1, v/v)] to afford 13a (28 mg, 66%) as a yellow
solid. HPLC [MeCN]: tR ) 4.2 min (350 nm), PA ) 1.4 (350
nm), APP ) 283.1 ( 0.5 nm. FABMS: m/z ) 515 [M+ 1]+.
HRMS: m/z calcd for [M + H]+ C34H31N2O3 515.2329, found
5-Iodo-2,3,3-trimethyl-3H-indole (9). A solution of isopropyl
methyl ketone (113 µL, 1.1 mmol) and 4-iodophenyl hydrazine (234
mg, 1.0 mmol) in MeCO2H (20 mL) was heated under reflux for
12 h. After being cooled to ambient temperature, the reaction
mixture was diluted with H2O (50 mL) and extracted with CH2Cl2
(2 × 30 mL). The solvent of the organic phase was distilled off
under reduced pressure, and the residue was dissolved in CH2Cl2
(8 mL). The addition of hexane (90 mL) caused the formation of
a precipitate, which was filtered off to afford 9 (225 mg, 79%) as
a purple oil. FABMS: m/z ) 386 [M + 1]+. 1H NMR (300 MHz,
CDCl3): δ ) 1.16 (6H, s), 2.14 (3H, s), 7.17 (1H, d, 8 Hz), 7.46-
7.50 (2H, m). 13C NMR (100 MHz, CDCl3): δ ) 15.6, 23.3, 54.4,
90.5, 122.1, 131.1, 138.3, 148.4, 153.3, 189.0.
5-Iodo-2-phenyl-3,3-dimethyl-3H-indole (10). A solution of
isopropyl phenyl ketone (193 µL, 1.3 mmol) and 4-iodophenyl
hydrazine (275 mg, 1.2 mmol) in MeCO2H (25 mL) was heated
under reflux for 3 d. After being cooled to ambient temperature,
the reaction mixture was diluted with H2O (50 mL) and extracted
with CH2Cl2 (2 × 30 mL). The solvent of the organic phase was
distilled off under reduced pressure, and the residue was dissolved
in CH2Cl2 (9 mL). The addition of hexane (90 mL) caused the
formation of a precipitate, which was filtered off to afford 10 (175
mg, 43%) as a purple oil. FABMS: m/z ) 348 [M + 1]+. 1H NMR
(300 MHz, CDCl3): δ ) 1.59 (6H, s), 7.46-7.50 (5H, m), 7.68
(1H, s), 8.12-8.15 (2H, m). 13C NMR (100 MHz, CDCl3): δ )
25.0, 54.3, 91.3, 120.6, 123.1, 128.8, 129.1, 130.5, 131.2, 137.3,
139.7, 150.3, 183.9.
1
515.2321. H NMR (500 MHz, CDCl3): δ ) 1.25 (3H, s), 1.57
(3H, s), 1.61 (3H, s), 4.61 (2H, s), 6.58 (1H, d, 9 Hz), 6.75 (1H, d,
9 Hz), 6.97 (1H, d, 16 Hz), 7.07 (1H, d, 16 Hz),7.14-7.15 (2H,
m), 7.23-7.26 (1H, m), 7.36-7.40 (4H, m), 7.47-7.54 (6H, m),
7.98 (1H, dd, 3 and 9 Hz), 8.10 (1H, d, 3 Hz). 13C NMR (100
MHz, CDCl3): δ ) 16.9, 19.3, 26.5, 40.6, 48.4, 103.2, 108.9, 118.6,
119.1, 120.4, 120.9, 122.7, 123.7, 124.5, 125.8, 126.9, 127.2, 127.5,
128.0, 128.6, 128.7, 129.1, 130.7, 136.5, 137.6, 137.8, 139.1, 140.9,
147.0, 159.3.
2-Nitro-5a-phenyl-6,6-dimethyl-5a,6-dihydro-8-(2-(4-(2-phe-
nylethylene)phenylene)ethylene)-12H-indolo[2,1-b][1,3]-
benzooxazine (14a). A mixture of 12a (45 mg, 0.1 mmol), 1-vinyl-
trans-stilbene (39 mg, 0.2 mmol), and Pd(MeCO2)2 (13 mg, 0.1
mmol) in degassed Et3N (20 mL) was heated at 105 °C under Ar
for 15 h. After being cooled to ambient temperature, the mixture
was diluted with CH2Cl2 (30 mL) and washed with H2O (40 mL).
The solvent of the organic phase was distilled off under reduced
pressure, and the residue was purified by column chromatography
[SiO2: hexane f CH2Cl2/hexane (1:2, v/v)] to afford 14a (44 mg,
52%) as a yellow solid. HPLC [MeCN]: tR ) 4.3 min (360 nm),
PA ) 1.3 (360 nm), APP ) 332.5 ( 0.3 nm. FABMS: m/z ) 577
[M]+. HRMS: m/z calcd for [M + H]+ C39H33N2O3 577.2486,
2-Nitro-5a,6,6-trimethyl-5a,6-dihydro-8-iodo-12H-indolo[2,
1-b][1,3]benzooxazine (11a). A solution of 9 (93 mg, 0.3 mmol)
and 4-nitro-2-chloromethylphenol (58 mg, 0.3 mmol) in MeCN (25
mL) was heated under reflux and Ar for 3 d. After the solution
was cooled to ambient temperature, the solvent was distilled off
under reduced pressure. The residue was dissolved in CH2Cl2 (30
mL) and washed with H2O (2 × 10 mL). The solvent of the organic
phase was distilled off under reduced pressure, and the residue was
purified by column chromatography [SiO2: CH2Cl2/hexane (1:10
f 1:1, v/v)] to afford 11a (56 mg, 41%) as a pale yellow solid.
1
found 577.2484. H NMR (500 MHz, CDCl3): δ ) 0.89 (3H, s),
1.65 (3H, s), 4.56 (1H, d, 18 Hz), 4.65 (1H, d, 18 Hz), 6.73 (1H,
d, 8 Hz), 6.91 (1H, d, 9 Hz), 7.00 (1H, d, 16 Hz), 7.10-7.13 (3H,
m), 7.30-7.32 (2H, m), 7.37-7.44 (6H, m), 7.48-7.55 (6H,
m),7.62-7.67 (2H, m), 7.93-7.96 (2H, m). 13C NMR (100 MHz,
CDCl3): δ ) 18.8, 28.1, 41.4, 50.1, 105.7, 109.6, 118.5, 120.4,
120.6, 123.5, 124.2, 126.0, 126.8, 126.9, 127.2, 127.5, 128.0, 128.3,
128.7, 128.8, 129.1, 129.4, 131.0, 136.0, 136.6, 137.6, 137.8, 138.8,
141.3, 147.2, 159.3.
1
FABMS: m/z ) 436 [M]+. H NMR (500 MHz, CDCl3): δ )
1.19 (3H, s), 1.52 (3H, s), 1.58 (3H, s), 4.56 (1H, d, 18 Hz), 4.62
(1H, d, 18 Hz), 6.38 (1H, d, 9 Hz), 6.75 (1H, d, 9 Hz), 7.36-7.38
(2H, m), 7.98 (1H, dd, 3 and 9 Hz), 8.07 (1H, d, 3 Hz). 13C NMR
(100 MHz, CDCl3): δ ) 16.8, 19.1, 26.3, 40.5, 48.5, 82.6, 102.9,
111.1, 118.6, 118.9, 123.7, 124.6, 131.7, 136.7, 141.3, 147.0, 159.2.
2-Nitro-5a-phenyl-6,6-dimethyl-5a,6-dihydro-8-iodo-12H-in-
dolo[2,1-b][1,3]benzooxazine (12a). A solution of 10 (85 mg, 0.3
mmol) and 4-nitro-2-chloromethylphenol (41 mg, 0.2 mmol) in
MeCN (30 mL) was heated under reflux and Ar for 4 d. After the
solution was cooled to ambient temperature, the solvent was distilled
off under reduced pressure. The residue was dissolved in CH2Cl2
(30 mL) and washed with H2O (10 mL). The solvent of the organic
phase was distilled off under reduced pressure, and the residue was
purified by column chromatography [SiO2: CH2Cl2/hexane (1:1,
v/v)] to afford 12a (66 mg, 60%) as a yellowish solid. FABMS:
m/z ) 498 [M]+. 1H NMR (400 MHz, CDCl3): δ ) 0.83 (3H, s),
1.56 (3H, s), 4.52 (1H, d, 18 Hz), 4.59 (1H, d, 18 Hz), 6.52 (1H,
d, 8 Hz), 6.90 (1H, d, 9 Hz), 7.39-7.46 (5H, m), 7.55-7.67 (2H,
m), 7.90 (1H, d, 3 Hz), 7.95 (1H, dd, 3 and 9 Hz). 13C NMR (100
MHz, CDCl3): δ ) 18.8, 27.9, 41.2, 50.1, 105.4, 111.8, 118.5,
120.1, 123.5, 124.3, 128.3, 128.7, 129.1, 129.5, 131.9, 135.7, 136.8,
140.9, 141.3, 147.1, 159.1.
Acknowledgment. We thank the National Science Founda-
tion (CAREER Award CHE-0237578) and the University of
Miami for financial support.
Supporting Information Available: General methods and
experimental procedures for the synthesis of 19-22; HPLC traces
of 4a, 6a, 7a, 8a, 13a, and 14a; 1H NMR spectra of 4a, 6a, 7a, 8a,
13a, and 14a; steady-state absorption spectra of 4a, 6a, 7a, 8a,
13a, and 14a; steady-state emission spectra of 4a, 6a, 7a, 8a, 13a,
and 14a; determination of the quantum yields for the photochromic
transformations; free energy changes for the photoinduced electron-
transfer processes; transient absorption spectra of 8a. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO7017119
126 J. Org. Chem., Vol. 73, No. 1, 2008