A photoswitchable bichromophoric oxazine with fast switching speeds and excellent fatigue resistance

Article abstract of DOI:10.1139/V10-070

In search of strategies to regulate the photochemical and photophysical properties of photochromic oxazines, we designed a multichromophoric compound incorporating 3H-indole, benzooxazine, and 2-(4-dimethylaminophenyl)ethynyl fragments. We synthesized thi

Full text of DOI:10.1139/V10-070

110
A photoswitchable bichromophoric oxazine with
fast switching speeds and excellent fatigue
resistance
Erhan Deniz, Stefania Impellizzeri, Salvatore Sortino, and Franc¸isco M. Raymo
Abstract: In search of strategies to regulate the photochemical and photophysical properties of photochromic oxazines, we
designed a multichromophoric compound incorporating 3H-indole, benzooxazine, and 2-(4-dimethylaminophenyl)ethynyl
fragments. We synthesized this molecule in two steps in an overall yield of 51%, starting from commercial precursors.
The ultraviolet irradiation of this photochrome opens a [1,3]oxazine ring in less than 6 ns to generate a zwitterionic isomer
with a quantum yield of 0.10. In particular, the photoinduced ring opening generates a 4-nitrophenolate anion and a 3H-
indolium cation. Additionally, this process brings the 2-(4-dimethylaminophenyl)ethynyl appendage into conjugation with
the 3H-indolium cation. As a result, two distinct bands for the anionic and cationic fragments of the photogenerated zwit-
terion appear in the visible region of the absorption spectrum. The photogenerated isomer has a lifetime of 2 ms and
switches back to the original form with first-order kinetics. Furthermore, this bichromophoric photochrome tolerates hun-
dreds of switching cycles with no sign of degradation and can be operated within rigid polymer matrices. Thus, this partic-
ular structural design can lead to the development of a new family of bichromophoric photochromes and photoresponsive
materials with microsecond switching times and excellent fatigue resistances.
Key words: heterocycles, molecular switches, oxazines, photochromism, photoisomerization.
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Resume : Dans le cadre d’une recherche sur des strategies qui pourraient permettre de regulariser les proprietes photochi-
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miques et photophysiques d’oxazines photochromiques, on a developpe un compose multichromophorique comportant des
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fragments 3H-indole, benzooxazine et 2-(4-dimethylaminophenyl)ethynyle. Cette molecule a ete synthetisee en deux eta-
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pes, avec un rendement de 51 %, a partir de precurseurs commerciaux. L’irradiation ultraviolette de ce photochrome pro-
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voque l’ouverture d’un noyau [1,3]diazine en moins de 6 ns pour conduire a la formation d’un isomere zwitterionique
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avec un rendement quantique de 0,10. En particulier, l’ouverture de cycle photoinduite genere un anion 4-nitrophenolate et
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un cation 3H-indolium. De plus, ce processus conduit le groupe 2-(4-dimethylaminophenyl)ethynyle a se placer en conju-
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gaison avec le cation 3H-indolium. Il en resulte que la region visible du spectre d’absorption comporte deux bandes dis-
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tinctes, l’une pour le fragment anionique et l’autre pour le fragment cationique du zwitterion photogenere. Le temps de vie
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de l’isomere photogenere est de 2 ms et il retourne a la forme originale avec une cinetique du premier ordre. De plus, ce
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photochrome bichromophorique tolere des centaines de cycles sans signe apparent de degradation et il peut etre utilise
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dans des matrices de polymeres rigides. Cette structure particuliere pourrait donc conduire au developpement d’une nou-
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velle famille de photochromes bichromophoriques et de materiaux photosensibles ayant des temps de commutation de
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l’ordre de la microseconde et une excellente capacite a resister a la fatigue.
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Mots-cles : heterocycles, commutateurs moleculaires, oxazines, photochromie, photoisomerisation.
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[Traduit par la Redaction]
center. All these transformations alter the electronic struc-
ture of the photoresponsive compound and result in signifi-
cant changes in the absorption spectrum. In addition, these
structural alterations at the molecular level can translate
into noticeable changes in refractive index. As a result, pho-
tochromic transformations can be exploited to modulate the
absorption coefficient and refractive index of a substrate
under the influence of optical stimulations. Indeed, a diver-
Introduction
The term photochromism refers to the photoinduced and
reversible change in color of a collection of molecules.1–5
The process can be a consequence of photoinduced ring
opening–closing steps, cis–trans isomerizations, intramolecu-
lar proton transfers, intermolecular electron transfers, cyclo-
additions, or the reorganization of ligands around a metal
Received 19 February 2010. Accepted 1 May 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 17 December
2010.
This article is part of a Special Issue dedicated to Professor J. C. Scaiano.
E. Deniz, S. Impellizzeri, and F.M. Raymo.1 University of Miami Department of Chemistry, 1301 Memorial Drive, Coral Gables, FL
33146-0431, USA.
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S. Sortino. Dipartimento di Scienze Chimiche, Universita di Catania, viale Andrea Doria 8, Catania, I-95125, Italy.
1Corresponding author (e-mail: fraymo@miami.edu).
Can. J. Chem. 89: 110–116 (2011)
doi:10.1139/V10-070
Published by NRC Research Press

Deniz et al.
111
sity of photoresponsive materials and devices have already
been designed around the absorptive and dispersive effects
associated with photochromic compounds.6–8 In fact, prom-
ising examples of all-optical logic gates,9–13 optical limit-
ers,14–18 photoresponsive filters,19 and photoswitchable
probes20–26 have all been implemented on the basis of the
photochromism associated with certain organic compounds.
Nitrospiropyrans switch from a colorless to a colored state
under ultraviolet irradiation and revert thermally to the orig-
inal form.27–31 Because of their photochemical behavior and
their synthetic accessibility, they have become one of the
most popular classes of photochromic compounds. However,
their switching speeds are relatively slow and a full switch-
ing cycle can only be completed on a timescale of minutes.
Furthermore, the involvement of the triplet state in the pho-
toisomerization mechanism tends to encourage degradation
and these compounds can only tolerate a few tens of switch-
ing cycles. In search of strategies to improve their switching
speeds and fatigue resistances, we developed a new family
of photochromic compounds based on the photoinduced
opening and thermal closing of [1,3]oxazine rings.32 Our
molecules switch from a colorless to a colored state in less
than 6 ns upon ultraviolet excitation and revert to the origi-
nal form on a nanosecond timescale. In addition, they sur-
vive thousands of switching cycles unaffected, even in the
presence of molecular oxygen, and can be operated in solu-
tion as well as within rigid polymer matrices. The integra-
tion of these compounds into photonic materials and
devices, however, requires a basic understanding of the ster-
eoelectronic factors regulating their spectroscopic signature
and photochromism. In particular, we need to learn how
their molecular skeleton should be modified to tune the
color of the photogenerated state, the timescales of the iso-
merization processes, and the quantum efficiency of their
coloration without compromising their excellent fatigue re-
sistance.
Laser excitation of 1a (Fig. 1) at 355 nm opens its
[1,3]oxazine ring in less than 6 ns to generate the zwitter-
ionic isomer 1b with a quantum yield of 0.10 in acetonitrile
at 20 8C.32a,32b The original state 1a does not absorb in the
visible region, while the 4-nitrophenolate chromophore of
the photogenerated species 1b absorbs at 440 nm. Therefore,
the photoinduced transformation of 1a into 1b is accompa-
nied by the appearance of a transient band in the visible re-
gion of the absorption spectrum. This band decays
monoexponentially on a nanosecond timescale with the ther-
mal reisomerization of 1b back to 1a.
The introduction of a 2-(4-dimethylaminophenyl)ethynyl
group in place of the phenyl ring of 1a generates a bichro-
mophoric switch in the shape of 2a.32g Specifically, the pho-
toinduced opening of the [1,3]oxazine ring of this compound
brings the 2-(4-dimethylaminophenyl)ethynyl appendage
into conjugation with the 3H-indolium cation. The resulting
chromophore absorbs at 540 nm and, hence, the formation
of 2b is accompanied by the appearance of a transient band
centered at this wavelength together with one at 440 nm for
the 4-nitrophenolate anion. Both bands decay monoexponen-
tially on a microsecond timescale and with identical ki-
netics, as 2b reisomerizes thermally to 2a. Thus, structural
modifications of the group (R2 in Fig. 1) attached to the chi-
ral center at the junction of the two heterocyclic fragments
Fig. 1. Photoinduced transformation of the [1,3]oxazines 1a–4a into
the zwitterions 1b–4b.
of our oxazines can be exploited to regulate the absorption
properties of their photogenerated states in the visible region
and impose bichromophoric character on them. However,
the transition from 1a to 2a has a depressive effect on the
photoisomerization efficiency and the quantum yield de-
creases from 0.10 to 0.07.
The introduction of a fluorine substituent in the para posi-
tion (R1 in Fig. 1) relative to the nitrogen atom of the 3H-
indole fragment of 1a facilitates the photoinduced opening
of the [1,3]oxazine ring. Specifically, the quantum yield in-
creases from 0.10 to 0.29 with the transition from 1a to
3a.32g On the basis of these considerations, we envisaged
the possibility of introducing a fluorine substituent also on
the 3H-indole fragment of the bichromophoric switch 2a to
improve the quantum yield for its photoisomerization. In
particular, we designed compound 4a and here we report its
synthesis, spectroscopic characterization, and photochemical
properties in solution and within a rigid polymer matrix.
Results and discussion
Synthesis
We synthesized 4a in two steps (Fig. 2), starting from
commercial precursors, with an overall yield of 51%. Spe-
cifically, the condensation of 2,3,3’-trimethyl-5-fluoro-3H-
indole with N,N-dimethyl-4-aminobenzaldehyde under acidic
conditions gave the extended 3H-indole 5, which was re-
acted with 2-chloromethyl-4-nitrophenol to afford the target
oxazine 4a. Similarly, the reaction of 5 with methyl iodide
produced the iodide salt of the model 3H-indolium cation 6.
Steady-state absorption spectroscopy
The spectrum of 4a reveals a band centered at 306 nm (a
and d in Fig. 3) for the overlapping absorptions of the 2-(4-
dimethylaminophenyl)ethynyl and 4-nitrophenoxy chromo-
phores. Upon addition of acid, the [1,3]oxazine ring of 4a
opens to generate the cationic species 4c (Fig. 4). This trans-
formation extends the conjugation of the 2-(4-dimethyl-
aminophenyl)ethynyl appendage and shifts its absorption to
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Can. J. Chem. Vol. 89, 2011
Fig. 2. Synthesis of the [1,3]oxazine 4a and model compound 6.
Fig. 3. Steady-state absorption spectra of solutions (0.1 mmol/L,
MeCN, 20 8C) of 4a before (a and d) and after the addition of
either CF3CO2H (1 equiv., b) or Bu4NOH (5 equiv., e), 6 (c), and
4-nitrophenol after the addition of Bu4NOH (4 equiv., f).
two transient bands at 420 and 540 nm (a in Fig. 5) in the
absorption spectrum. They resemble those observed in the
steady-state spectra of 4d and 4c, respectively (e and b in
Fig. 3) and correspond to ground-state absorptions of the 4-
nitrophenolate anion and 3H-indolium cation, respectively,
of 4b. Thus, the [1,3]oxazine ring of 4a opens upon excita-
tion to generate the bichromophoric isomer 4b, in analogy to
the behavior of 2a and 2b. However, the transition from 2a
to 4a results in an increase in quantum yield from 0.07 to
0.10.33 Indeed, plots (b and c in Fig. 5) of the absorbance
change measured at 540 nm upon laser excitation against
the pulse energy show a noticeable increase in slope on
going from 2a to 4a, consistent with an enhancement in
quantum efficiency. Thus, the presence of a fluorine sub-
stituent on the 3H-indolium fragment of 4a facilitates the
photochromic transformation. Presumably, this effect is a
consequence of the reduced electron density on the nitrogen
atom of the 3H-indole fragment of 4a, relative to that of 2a,
with the suppression of competitive photoinduced electron
transfer to the adjacent nitro group. Indeed, the cyclic vol-
tammograms of the model compounds 7 and 8 (Fig. 6) dem-
onstrate that the oxidation potential of the 3H-indole
fragment shifts by 70 mV in the positive direction with the
introduction of an electron-withdrawing fluorine substitu-
ent.34
The photogenerated isomer 4b reverts spontaneously to 4a
on a microsecond timescale. As a result, the two bands of
4b decay with identical first-order kinetics. Curve fitting of
both temporal profiles (d and e in Fig. 5) reveals the lifetime
of 4b to be 2 ms in acetonitrile at 20 8C. This value is iden-
tical to that of 2b, demonstrating that the fluorine substituent
has no influence on the reisomerization kinetics. Thus, a full
switching cycle can be completed in a few microseconds
with both photochromic systems. In addition, they tolerate
hundreds of switching cycles with no sign of degradation.
Specifically, the steady-state absorption spectra recorded be-
fore and after 300 excitation cycles are virtually identical,
indicating the lack of any significant degradation under
these experimental conditions.
The bichromophoric photochrome 4a can be incorporated
within a rigid poly(methyl methacrylate) (PMMA) matrix on
the basis of a spin-coating procedure. The laser excitation of
the resulting film at 355 nm is also accompanied by the ap-
pearance within 6 ns of two transient bands (a in Fig. 7).
Once again, these bands resemble those observed in the
steady-state spectra of 4d and 4c in solution and correspond
to ground-state absorptions of the 4-nitrophenolate anion and
559 nm (b in Fig. 3). Indeed, this band resembles that of the
model 3H-indolium cation 6 (c in Fig. 3).
The addition of base also opens the [1,3]oxazine ring of
4a to generate the anionic species 4d. This process causes
the appearance of a band at 427 nm (e in Fig. 3), corre-
sponding to the 4-nitrophenolate chromophore of 4d. In
fact, this absorption is reminiscent of that associated with
the tetrabutylammonium salt of 4-nitrophenolate (f in
Fig. 3).
Transient absorption spectroscopy
In acetonitrile at 20 8C, the laser excitation of 4a at
355 nm is accompanied by the appearance within 6 ns of
Published by NRC Research Press

Deniz et al.
113
Fig. 4. Transformation of the [1,3]oxazine 4a into the cation 4c or anion 4d under the influence of acid or base, respectively.
3H-indolium cation of 4b. As observed in solution, the pho-
togenerated isomer 4b reverts spontaneously back to 4a also
within the PMMA matrix. Concomitantly, both absorption
bands decay with the reisomerization process, though with
biexponential kinetics in this instance. Curve fitting of the
corresponding temporal profiles (b in Fig. 7) indicates life-
times of 4 and 130 ms. The biexponential decay is consistent
with that observed for 2b in PMMA and suggests the coex-
istence of two main environments for the photochromic dop-
ants within the polymer matrix.
cording to literature procedures.32a,32g All reactions were
monitored by thin-layer chromatography, using aluminum
sheets coated with silica (60, F254). Fast atom bombardment
mass spectra (FABMS) were recorded with a VG Mass Lab
Trio-2 spectrometer in a 3-nitrobenzyl alcohol matrix. Nu-
clear magnetic resonance (NMR) spectra were recorded
with Bruker Avance 400 or 500 spectrometers. Steady-state
absorption spectra were recorded with a Varian Cary 100
Bio spectrometer, using quartz cells with a path length of
0.5 cm. Time-resolved absorption spectra were recorded
with a Luzchem Research mLFP–111 spectrometer, after ex-
citation with a Continuum Surelite II–10 Nd–YAG laser
(pulse width = 6 ns (FWHM), wavelength = 355 nm). The
photochromic films were prepared by spin-coating a CH2Cl2
solution of 4a (1.0 mg/mL) and PMMA (52 mg/mL) on a
glass plate at 420 rpm for 9 s. The thickness (6 mm) of the
resulting films was measured with a Tencor Instruments 10-
00090 surface profilometer. Cyclic voltammograms were re-
corded with a CH Instruments 660 workstation in degassed
MeCN solutions of Bu4NPF6 (0.1 mol/L) under an atmos-
phere of Ar, using a glassy-carbon working electrode, a plat-
inum counter electrode, and a Ag/AgCl reference electrode.
Conclusions
Bichromophoric photochromes can be synthesized in two
steps, starting from commercial precursors, by fusing
3H-indole and benzooxazine fragments and appending a 2-
(4-dimethylaminophenyl)ethynyl group to the chiral center
at the junction of the two heterocycles. The ultraviolet exci-
tation of these compounds opens their [1,3]oxazine ring in
less than 6 ns to produce a zwitterionic isomer, incorporat-
ing a 4-nitrophenolate anion and a 3H-indolium cation.
These two chromophoric fragments absorb at 440 and
540 nm, respectively, and, therefore, the photoinduced trans-
formation is accompanied by the appearance of two bands in
the visible region of the absorption spectrum. The introduc-
tion of a fluorine substituent in the para position, relative to
the nitrogen atom, of the 3H-indole heterocycle increases
the photoisomerization quantum yield. Indeed, the electron-
withdrawing character of the substituent reduces the electron
density on the nitrogen atom of the 3H-indole heterocycle
and facilitates the photoinduced ring-opening process. In-
stead, this particular substituent has essentially no influence
on the lifetime of the photogenerated isomer, which reverts
to the original form with first-order kinetics on a microsec-
ond timescale. In fact, these bichromophoric photochromes
can be switched back and forth between their two states in
a few microseconds and tolerate hundreds of switching
cycles with no sign of degradation. In addition, they can be
operated within rigid PMMA matrices, though with slower
reisomerization kinetics than in acetonitrile solution. Thus,
our structural design for the realization of bichromophoric
photochromes can lead to the development of novel photo-
responsive materials with fast switching speeds and excel-
lent fatigue resistances.
Synthesis of 5
A solution of 2,3,3’-trimethyl-5-fluoro-3H-indole (1.09 g,
6.15 mmol), N,N-dimethyl-4-aminobenzaldehyde (0.92 g,
6.15 mmol), and HBr (0.5 mL, 33% in AcOH) in EtOH
(10 mL) was heated under reflux for 12 h. After cooling to
ambient temperature, the solvent was distilled off under re-
duced pressure. The residue was dissolved in CH2Cl2
(30 mL) and washed with a saturated aqueous solution of
NaHCO3 (2 Â 20 mL). The organic phase was dried over
Na2SO4 and the solvent was distilled off under reduced pres-
1
sure to give 5 (1.6 g, 85%) as an orange solid. H NMR
(CDCl3) d : 1.44 (6H, s), 2.98 (6H, s), 6.69 (2H, d, J =
8 Hz), 6.85 (1H, d, J = 16 Hz), 7.00 (2H, t, J = 7 Hz),
7.49–7.51 (3H, m), 7.66 (1H, d, J = 16 Hz). 13C NMR
(CDCl3) d: 24.4, 40.6, 53.4, 109.3, 112.5, 115.0, 121.0,
124.3, 129.4, 139.0, 148.9, 150.7, 151.6, 160.3, 162.7,
184.3. FABMS m/z: 310 [M + H]+.
Synthesis of 4a
A solution of 5 (278 mg, 0.9 mmol) and 2-chloromethyl-
4-nitrophenol (169 mg, 0.9 mmol) in MeCN (20 mL) was
heated under reflux for 24 h. After cooling to ambient tem-
perature, the solvent was distilled off under reduced pressure
and the residue was dissolved in CH2Cl2 (3 mL). The addi-
tion of Et2O (20 mL) caused the precipitation of a purple
solid. The solid was dissolved in CH2Cl2 (30 mL) and
washed with H2O (20 mL). The organic phase was dried
over Na2SO4 and the solvent was distilled off under reduced
Experimental procedures
Materials and methods
Chemicals were purchased from commercial sources and
used as received with the exception of MeCN, which was
distilled over CaH2. Compounds 7 and 8 were prepared ac-
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114
Can. J. Chem. Vol. 89, 2011
Fig. 5. Transient absorption spectrum (0.04 mmol/L, MeCN, 20 8C)
of 4a (a) recorded 0.1 ms after laser excitation (355 nm, 6 ns,
12 mJ). Absorbance change (0.04 mmol/L, MeCN, 20 8C) at
540 nm recorded 0.1 ms after laser excitation (355 nm, 6 ns) of 2a
(b) and 4a (c) against the pulse energy. Temporal evolution of the
absorbance (0.04 mmol/L, MeCN, 20 8C) at 440 (d) and 540 nm (e)
after excitation (355 nm, 6 ns, 12 mJ) of 4a and the corresponding
monoexponential fittings.
Fig. 6. Model compounds 7 and 8.
Fig. 7. Transient absorption spectrum (2% w/w, PMMA, 20 8C) of
4a (a) recorded 0.1 ms after laser excitation (355 nm, 6 ns, 12 mJ).
Temporal evolution of the absorbance at 640 nm (b) after excitation
and the corresponding biexponential fitting.
(4H, m), 7.20 (1H, d, J = 8 Hz), 7.44 (2H, d, J = 8 Hz), 7.95
(1H, d, J = 9 Hz), 8.10 (1H, s). 13C NMR (DMSO) d: 23.9,
24.8, 26.2, 45.0, 45.4, 50.8, 111.1, 111.3, 111.9, 112.8,
114.7, 118.3, 121.6, 123.8, 125.4, 130.2, 139.6, 141.9,
142.6, 152.2, 155.0, 157.9, 160.2, 190.7. FABMS m/z: 461
[M + H]+
Synthesis of 6
1
pressure to give 4a (248 mg, 60%) as a purple solid. H
NMR (DMSO) d: 1.34 (6H, s), 2.93 (6H, s), 4.85 (2H, s),
6.42 (1H, d, J = 16 Hz), 6.67 (2H, d, J = 9 Hz), 6.88–6.95
A solution of 5 (120 mg, 0.39 mmol) and MeI (113 mg,
0.8 mmol) in MeCN (10 mL) was heated under reflux for
48 h. After cooling to ambient temperature, the solvent was
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Deniz et al.
115
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distilled off under reduced pressure and the residue was dis-
solved in CH2Cl2 (3 mL). The addition of Et2O (20 mL)
caused the precipitation of a purple solid, which was filtered
1
off to give 6 (141 mg, 80%). H NMR (CDCl3) d: 1.79 (6H,
(17) Sun, Y.-P.; Riggs, J. E. Int. Rev. Phys. Chem. 1999, 18, 43.
doi:10.1080/014423599230008.
(18) Ganeev, R. A. J. Opt. A: Pure Appl. Opt. 2005, 7, 717.
doi:10.1088/1464-4258/7/12/004.
(19) Crano, J. C.; Kwak, W. S.; Welch C. N. Spirooxazines and
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s), 3.19 (6H, s), 4.18 (3H, s), 6.79 (2H, d, J = 8 Hz), 7.18
(2H, d, J = 8 Hz), 7.26 (1H, d, J = 20 Hz), 7.44-–7.47 (1H,
m), 8.04–8.08 (3H, m). 13C NMR (CDCl3) d: 28.0, 36.2,
40.9, 51.4, 105.5, 110.8, 113.1, 114.9, 116.5, 122.9, 135.8,
138.3, 144.4, 155.1, 155.8, 163.9, 179.4. FABMS: m/z =
323 [M – I]+.
Acknowledgments
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CHE-0749840) and MIUR (PRIN 2008) for financial sup-
port.
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(33) The quantum yield of 4a was determined using 2a as stan-
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(34) The oxidation potentials of 7 and 8 are +1.16 and +1.23 V
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