A fast and stable photochromic switch based on the opening and closing of an oxazine ring

Article abstract of DOI:10.1021/ol050045a

(Chemical Equation Presented) We have designed a molecular switch based on the photoinduced opening and thermal closing of an oxazine ring. Ultraviolet excitation of this molecule induces the cleavage of a [C-O] bond to form a p-nitrophenolate chromophore

Full text of DOI:10.1021/ol050045a

ORGANIC
LETTERS
2005
Vol. 7, No. 6
1109-1112
A Fast and Stable Photochromic Switch
Based on the Opening and Closing of
an Oxazine Ring
Massimiliano Tomasulo,† Salvatore Sortino,*,‡ and Francisco M. Raymo*,†
¸
Center for Supramolecular Science, Department of Chemistry, UniVersity of Miami,
1301 Memorial DriVe, Coral Gables, 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 January 10, 2005
ABSTRACT
We have designed a molecular switch based on the photoinduced opening and thermal closing of an oxazine ring. Ultraviolet excitation of this
molecule induces the cleavage of a [C O] bond to form a p-nitrophenolate chromophore in less than 10 ns with a quantum yield of ca. 0.1.
−
The photogenerated isomer reverts thermally to the original oxazine within 50 ns. Our photochromic switch survives more than 3000 excitation
cycles without decomposing, even in air-saturated solutions.
Photochromic compounds change their structural and elec-
tronic properties in response to optical stimulations.1 The
photogenerated state reverts to the original form either
thermally or after the application of a second optical input
differing in wavelength from the first. Thermally reversible
photochromes offer the opportunity to alter and reset the state
of an output property (e.g., color or refractive index) by
simply turning on and off a light source, since they do not
retain the influence of the incident radiation. The ability to
restore the output level and the lack of memory effects are
essential conditions for the implementation of combinational
logic functions.2 As a result, these compounds have emerged
as possible building blocks for the construction of molecular
logic gates.3 Indeed, recent investigations have demonstrated
that collections of photochromic compounds in solution4-10
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† University of Miami.
‡ Universita´ di Catania.
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Balzani, V. Chem. Eur. J. 1998, 4, 1184-1191. (b) Roque, A.; Pina, F.;
Alves, S.; Ballardini, R.; Maestri, M.; Balzani, V. J. Mater. Chem. 1999,
9, 2265-2269. (c) Pina, F.; Melo, M. J.; Maestri, M.; Passaniti, P.; Balzani,
V. J. Am. Chem. Soc. 2000, 122, 4496-4498.
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10.1021/ol050045a CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/18/2005

Figure 2. Partial 1H NMR spectra (500 MHz, CD3CN, 5 mM) of
OX at 30 (a), 60 (b), and 70 °C (c).
Figure 1. Reversible interconversion of a spiropyran (SP) and a
merocyanine (ME) and of an oxazine (OX) and an indolium (IN).
onds.18 The thermal transformation of the resulting mero-
cyanine (e.g., ME in Figure 1) back to the original spiropyran
is slowed significantly by a necessary trans f cis reisomer-
ization step. For example, ME switches back to SP in
minutes with a rate constant of ca. 25 × 10-4 s-1 in MeCN
at 25 °C.18f
or within rigid matrixes11,12 can reproduce logic functions
relying on the interplay of optical signals. In particular, we
have implemented molecular logic gates7,12 on the basis of
the reversible isomerization of spiropyrans.13 At the present
stage of their development, however, our rudimentary
molecular switches suffer at least two major limitations. The
thermal reisomerization of our spiropyran is relatively slow.7f
Thus, the output level can only be restored after a delay of
several minutes, once the optical input is turned off.
Furthermore, our spiropyran tolerates a limited number of
switching cycles.12b
The photoinduced isomerization of spiropyrans (e.g., SP
in Figure 1) involves two consecutive steps.13b,c,f Upon
ultraviolet excitation, the [C-O] bond at the spirocenter
cleaves in picoseconds.14-17 Then, the adjacent [CdC] bond
switches from a cis to a trans configuration in microsec-
On the basis of these observations, we have designed the
[1,3]oxazine OX (Figure 1). In analogy to SP, ultraviolet
irradiation of OX should induce the cleavage of the [C-O]
bond, involving the tertiary carbon of the indoline fragment,
with the formation of a p-nitrophenolate chromophore. The
resulting indolium IN (Figure 1) lacks the central double
bond of ME. Thus, the rate of the thermal transformation of
IN back into OX should not be limited by the relatively slow
trans f cis reisomerization associated with ME.
We have synthesized OX in two steps (Figure S1,
Supporting Information) with an overall yield of 49%. The
first step involves the condensation of phenylhydrazine and
i-propylphenyl ketone in the presence of p-toluenesulfonic
acid. The resulting 2-phenyl-3,3′-dimethyl-3H-indole is then
reacted with 2-chloromethyl-4-nitrophenol to produce the
target compound OX, after treatment with aqueous potassium
hydroxide.
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The chiral center of OX imposes two distinct environments
on the two methyl groups. Consistently, the 1H NMR
spectrum (spectrum a in Figure 2), recorded in CD3CN at
30 °C, reveals two distinct singlets for the two sets of methyl
protons. Upon warming the solution, these signals broaden
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Org. Lett., Vol. 7, No. 6, 2005

which is “trapped” in the form of the hemiaminal HE (Figure
4) after the attachment of the nucleophilic hydroxide anion
to the electrophilic carbon of the indolium cation. Consis-
tently, the fast atom bombardment mass spectrum recorded
at this point shows a peak at m/z 390 for HE.
1
In agreement with the formation of HE, the H NMR
spectrum of OX (spectrum a in Figure 4) changes dramati-
cally after the addition of Bu4NOH (spectrum b in Figure
4). In particular, the chemical shift of the proton Ha increases
by 0.15 ppm with the transformation of OX into HE. The
signals of the other aromatic protons (Hb-Hh), instead, move
in the opposite direction. The largest change is observed for
the resonances associated with the proton Hh, whose chemical
shift decreases by 0.85 ppm. Furthermore, the AB system
associated with the diastereotopic pair of methylene protons
Hl and Hm is maintained, confirming the presence of a chiral
center also in HE.
Laser flash photolysis measurements confirm the photo-
induced opening of the oxazine ring with the formation of
IN. Indeed, the transient absorption spectrum of OX
(spectrum e in Figure 3), recorded 30 ns after laser excitation,
shows a band at 440 nm, which can be assigned to a
p-nitrophenolate chromophore (spectra c and d in Figure 3).
In agreement with this assignment, control experiments with
p-nitroanisole exclude a possible association of this transient
absorption with the triplet state of the p-nitrophenoxy
fragment of OX. In fact, no transient absorptions are
observed for p-nitroanisole under identical experimental
conditions.22 Furthermore, the quantum yield of singlet
oxygen is less than 0.02 when the photoinduced conversion
of OX into IN is performed in air-saturated MeCN.
The evolution of the absorbance at 440 nm (Figure 5)
indicates that the formation of IN occurs within the laser
pulse (ca. 6 ns). The quantum yield for the photoinduced
conversion of OX into IN is ca. 0.1. A kinetic analysis of
the absorbance decay at 440 nm shows that the photogener-
ated isomer IN reverts thermally to OX with a first-order
rate constant of (46 ( 1) × 106 s-1.23 Therefore, the original
state is fully restored within ca. 50 ns. Furthermore, the
reversible interconversion of OX and IN is not accompanied
by any photodegradation, even in air-saturated solutions.
Indeed, the steady-state and transient absorption spectra of
OX recorded before (spectra a and e in Figure 3) and after
more than 3000 laser pulses are identical. Thus, virtually all
OX is recovered in full after thousands of excitation cycles.
The remarkable photochemical stability of OX agrees with
the inability of this compound to sensitize efficiently the
formation of singlet oxygen, which is responsible in part for
the degradation of spiropyrans.18e,f,24-26
Figure 3. Steady-state absorption spectra (MeCN, 20 °C, 0.1 mM)
of OX (a), p-nitroanisole (b), potassium p-nitrophenolate (c), OX
after the addition of Bu4NOH (10 equiv) and continuous irradiation
(365 nm, 400 µW cm-2) for 10 min (d). Transient absorption
spectrum (MeCN, 22 °C, 0.1 mM) of OX (e) recorded 30 ns after
a laser pulse (355 nm, 6 ns, ∼8 mJ).
considerably (spectra b and c in Figure 2), indicating that
1
the two enantiomers of OX interconvert on the H NMR
time scale under these experimental conditions. The analysis
of the temperature dependence of the line width19 of both
singlets reveals the rate constant for the degenerate site
exchange process to be 0.4 ( 0.1 s-1 at 25 °C.20
The interconversion of the two enantiomers of OX
demands the thermal opening of the oxazine ring with the
formation of IN. Nonetheless, the stationary concentration
of IN is negligible, and the steady-state absorption spectrum
(spectrum a in Figure 3) reveals only a band at 308 nm for
the p-nitrophenoxy chromophore of OX. Indeed, this absorp-
tion resembles the one observed for p-nitroanisole (spectrum
b in Figure 3), under identical experimental conditions. The
characteristic band at 429 nm (spectrum c in Figure 3)
expected for the p-nitrophenolate component of IN cannot,
instead, be detected (spectrum a in Figure 3). After the
addition of Bu4NOH and continuous irradiation, however,
an intense absorption (spectrum d in Figure 3) for a
p-nitrophenolate chromophore appears in the spectrum.21
Thus, the excitation of OX encourages the formation of IN,
The trace in Figure 5 demonstrates that the absorbance at
440 nm can be altered and reset with nanosecond switching
(21) The band for the p-nitrophenolate chromophore develops also if
the solution of OX and Bu4NOH is not irradiated. However, the thermal
process is significantly slower than the photoinduced transformation.
(22) The triplet state of p-nitroanisole has a subnanosecond lifetime in
MeCN: Mir, M.; Jansen, L. M. G.; Wilkinson, F.; Bourdelande, J. L.;
Marquet, J. J. Photochem. Photobiol. A 1998, 113, 113-117.
(23) Kinetic analyses at different wavelengths within the transient
absorption (e in Figure 3) gave essentially the same value for the rate
constant.
(19) Nelson, J. H. Nuclear Magnetic Resonance Spectroscopy; Prentice
Hall: New York, 2003.
(20) The free-energy barrier is 18.0 ( 0.2 kcal mol-1, and the associated
enthalpy and entropy of activation are 17.4 ( 0.3 kcal mol-1 and -0.002
( 0.001 kcal mol-1 K-1, respectively.
Org. Lett., Vol. 7, No. 6, 2005
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1
Figure 4. Partial H NMR spectra (400 MHz, CD3CN, 10 mM) of OX before (a) and after (b) the addition of Bu4NOH (2 equiv) and
thermal equilibration.
speeds by turning a laser on and off. The short time scales
of these processes correspond to an improVement of 10 orders
of magnitude over any of our earlier all-optical processing
schemes.7,12 The elimination of the sluggish trans f cis step,
limiting the thermal reisomerization of spiropyrans, has
translated into this dramatic decrease in switching times. In
addition, this structural modification has conferred remark-
able stability on the photoresponsive molecular skeleton.
Furthermore, the significant changes in dipole moment and
molecular polarizability accompanying the photoisomeriza-
tion can, in principle, be exploited to photoregulate a diversity
of material properties with unprecedented switching speeds.1e
Thus, our molecular design for the realization of fast and
stable photochromic compounds can evolve into the develop-
ment of a new family of photoresponsive materials.
Acknowledgment. We thank the National Science Foun-
dation (CAREER Award CHE-0237578), the University of
Miami, and the MIUR (Italy) for financial support.
Supporting Information Available: Experimental pro-
cedures. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL050045A
(24) Malkin, Ya. N.; Krasieva, T. B.; Kuzmin, V. A. J. Photochem.
Photobiol. A 1989, 49, 75-88.
Figure 5. Three consecutive switching cycles of OX (MeCN, 22
°C, 0.1 mM) performed by laser excitation (355 nm, 6 ns, ∼8 mJ)
and followed by monitoring the absorbance of IN at 440 nm.
(25) Sakuragi, M.; Aoki, K.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc.
Jpn. 1990, 63, 74-79.
(26) Salemi, C.; Giusti, R.; Guglielmetti, R. J. Photochem. Photobiol.
A. 1995, 86, 247-252.
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