CO2 triggering and controlling orthogonally multiresponsive photochromic systems

Article abstract of DOI:10.1021/ja1013322

We report a new generic method of reversibly controlling the photochromism of spiropyrans. It was found that the photochromic effect of spiropyrans can be reversibly switched on and off by addition and removal of carbon dioxide (CO₂) to spiropyran in alcohol solutions containing an amidine (i.e., DBU) that acts as a CO₂ sensitizer. Spiropyrans are not photochromic in the presence of DBU but photochromic when CO₂ is subsequently added to the solution. The CO₂ is readily removed by inert gas bubbling, thus allowing facile activation and deactivation of the photochromic effect. Carbon dioxide, without the presence of the sensitizing amidine, had no effect on photochromism of the spiropyrans. Other photochromic dyes classes such as spirooxazines and chromenes are not affected by this CO₂/DBU stimulus. As a result, orthogonal activation of mixtures of spirooxazines and spiropyrans was achieved to provide four color states (clear, yellow, green, and blue) by varying the combinations of the stimuli of UV, visible light, CO ₂, and CO₂ depleted. This finding now permits the many applications using spiropyrans to be CO₂ responsive.

Full text of DOI:10.1021/ja1013322

Published on Web 07/19/2010
CO Triggering and Controlling Orthogonally Multiresponsive
2
Photochromic Systems
,
†
,‡
†,§
‡
Tamim A. Darwish,* Richard A. Evans,* Michael James, Nino Malic,
†
,†
Gerry Triani, and Tracey L. Hanley*
Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee
DC, New South Wales 2232, Australia, CSIRO Materials Science and Engineering, Bag 10
Clayton, South Victoria 3169, Australia, and School of Chemistry, UniVersity of New South
Wales, Sydney, New South Wales 2052, Australia
Received February 14, 2010; E-mail: tamim.darwish@ansto.gov.au; richard.evans@csiro.au;
tracey.hanley@ansto.gov.au
Abstract: We report a new generic method of reversibly controlling the photochromism of spiropyrans. It
was found that the photochromic effect of spiropyrans can be reversibly switched on and off by addition
and removal of carbon dioxide (CO
that acts as a CO sensitizer. Spiropyrans are not photochromic in the presence of DBU but photochromic
when CO is subsequently added to the solution. The CO is readily removed by inert gas bubbling,
2
) to spiropyran in alcohol solutions containing an amidine (i.e., DBU)
2
2
2
thus allowing facile activation and deactivation of the photochromic effect. Carbon dioxide, without the
presence of the sensitizing amidine, had no effect on photochromism of the spiropyrans. Other
photochromic dyes classes such as spirooxazines and chromenes are not affected by this CO /DBU
2
stimulus. As a result, orthogonal activation of mixtures of spirooxazines and spiropyrans was achieved
to provide four color states (clear, yellow, green, and blue) by varying the combinations of the stimuli
of UV, visible light, CO
2 2
, and CO depleted. This finding now permits the many applications using
spiropyrans to be CO responsive.
2
7
8
Introduction
attachment, self-assembly, modulation of the fluorescence of
9,10
organic and inorganic nanoparticles, micro- and macroscopic
ordering, photoswitching of DNA-binding, recognition and
quantification of amino acids, molecular logic devices,
electro-optical devices, molecular and supramolecular logic
Chemical processes from the natural world have inspired
scientists to continue synthesis of new controllable stimuli-
responsive molecules to mimic the complexity of biological
1
1
12
1
3
14
1
5
1,2
systems. Such systems would ideally include multiple stimuli
and responses which are independently addressable. Potential
applications are not limited to biomimicry, but stimuli response
can also be of use in information technology where molecules
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†
Australian Nuclear Science and Technology Organisation (ANSTO).
CSIRO Materials Science and Engineering.
University of New South Wales.
‡
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0748 9 J. AM. CHEM. SOC. 2010, 132, 10748–10755
10.1021/ja1013322  2010 American Chemical Society

Reversible Control of Photochromism of Spiropyrans
A R T I C L E S
1
6
switches, multifunctional artificial receptors, and photoswit-
Chart 1. Photochromic Compounds Used in This Study Showing
the Spiro and Merocyanine Forms of Each Dye
1
7
chable biomaterials.
Conventionally, photochromic compounds have been used
as optical dyes, and as such the majority of work has focused
18
on this application. More recent studies have shown that these
molecules are very sensitive to their immediate chemical
environment. In a previous report it was demonstrated that
the isomerization speed of photochromic dyes in rigid
polymeric matrices was enhanced by the attachment of short-
1
9
chain oligomers to the dye. In this current study, a system
was developed where photochromism can be reversibly
2
deactivated, and benign stimuli, such as CO , can be used
together with UV or visible light. The system can be used to
effect selectively the structural switching of spiropyran systems
when present in multicomponent photochromic solutions.
Introducing new dimensions to selectively trigger spiropyran
photochromic molecules allows the design of multicomponent
stimuli response materials that can respond to multiple inputs
and produce multiple outputs. This essentially means the
development of logic gate molecular systems with extended
logic circuits and truth tables which have been attracting
2
0
considerable interest lately.
This study reports on a reversible multistimuli response
system that combines both photochromism and the switchable
properties of a cyclic amidine (1,8-diazabicyclo[5.4.0]undec-
solution) as switching triggers maintains the concentration
of species in solution and allows for continued, reversible
switching. The generic applicability of this method is
demonstrated by the use of both a commercially available
7
2
-ene (DBU)), in the presence and absence of CO to control
the optical responses of spiropyran systems. It was demon-
strated that DBU, a non-nucleophilic base, added to an
alcohol solution of a photochromic spiropyran dye causes
N-methyl spiropyran (SP-NO
spiropyran with a propanol group at the indoline nitrogen
SP-1) (Chart 1). The latter molecule allowed the use of NMR
2
) and a purposely synthesized
the deactivation of photochromism and activation of CO
responsiveness. The use of CO and N (to remove CO from
2
2
2
2
(
spectroscopy to probe the different colored species produced
by the different stimuli used.
(
7) Higuchi, A.; Hamamura, A.; Shindo, Y.; Kitamura, H.; Yoon, B. O.;
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(
Materials and Methods
2
453.
General Materials and Methods. All solvents used were purified
(
9) Zhu, M.; Zhu, L.; Han, J. J.; Wu, W.; Hurst, J. K.; Li, A. D. Q. J. Am.
Chem. Soc. 2006, 128, 4303.
21
by literature methods. Chemicals and reagents of the highest grade
commercially available were used without further purification.
DBU and methanol were purchased from Aldrich and used as
received without drying. 1′,3′,-dihydro-8-methoxy-1, 3′, 3′-trimethyl-
(
10) (a) Medintz, I. L.; Trammell, S. A.; Mattoussi, H.; Mauro, J. M. J. Am.
Chem. Soc. 2004, 126, 30. (b) Zhu, L. Y.; Wu, W. W.; Zhu, M. Q.;
Han, J. J.; Hurst, J. K.; Li, A. D. Q. J. Am. Chem. Soc. 2007, 129,
3
524.
6
-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indole] (N-methyl spiropy-
(
(
(
11) Wu, S. Z.; Lu, J. R.; Zeng, F.; Chen, Y. N.; Tong, Z. Macromolecules
ran, SP-NO ) and 1,3-dihydro-3,3-dimethyl-1-(2-methylpropyl)spiro
2
2
007, 40, 5060.
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Soc. 2008, 130, 11836.
[2H-indole-2,3′-[3H]-naphth[2,1-b][1,4]oxazine] (SOX) are com-
mercially available and were purchased from Sigma-Aldrich and
James Robinson and were used without further purification.
CO and N gases used in this study were purged directly into
2 2
the solutions inside either the NMR tube or the UV-vis
cuvette before recording the spectra.
13) (a) Shao, N.; Jin, J. Y.; Cheung, S. M.; Yang, R. H.; Chan, W. H.;
Mo, T. Angew. Chem., Int. Ed. 2006, 45, 4944. (b) Shao, N.; Jin, J.;
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(
(
(
14) Kompa, K. L.; Levine, R. D. Proc. Natl. Acad. Sci. U.S.A. 2001, 98,
4
10.
Synthesis of SP-1 and CRM. Details of the experimental
procedures for the synthesis of compounds SP-1 and CRM are
15) Favaro, G.; Chidichimo, G.; Formoso, P.; Manfredi, S.; Mazzucato,
U.; Romani, A. J. Photochem. Photobiol. A 2001, 140, 229.
described in the Supporting Information (SI).
16) Raymo, F. M.; Giordani, S. Proc. Natl. Acad. Sci. US.A. 2002, 99,
Instruments. 1H NMR (400 MHz) and 13C NMR (100.6 MHz)
4
941.
spectra were recorded on a Bruker 400 MHz instrument at ∼25 °C.
(
(
(
17) Willner, I. Acc. Chem. Res. 1997, 30, 347.
1
18) Higgins, S. Chem. Br. 2003, (June), 26–29.
In H NMR spectra, chemical shifts (ppm) were referenced to
). In 13C NMR
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G. K.; Yee, L. H.; Ball, G. E.; Lewis, D. A. Nat. Mater. 2005, 4, 249.
20) (a) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651.
residual solvent protons (3.31 ppm in MeOH-d
spectra, chemical shifts (ppm) were referenced to the carbon signal
of the deuterated solvent (49.0 ppm in MeOH-d ). Solutions for
4
(
4
(
b) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. J.
NMR measurements were made to concentrations of 16.7 mg/mL.
Org. Chem. 2003, 68, 4158. (c) Andreasson, J.; Straight, S. D.;
Bandyopadhyay, S.; Mitchell, R. H.; Moore, T. A.; Moore, A. L.;
Gust, D. J Phys. Chem. C 2007, 111, 14274. (d) Straight, S. D.; Liddell,
P. A.; Terazono, Y.; Moore, T. A.; Moore, A. L.; Gust, D. AdV. Funct.
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Chem., Int. Ed. 2008, 47, 1–5. (f) Green, K. A.; Cifuentes, M. P.;
Corkery, T. C.; Samoc, M.; Humphrey, M. G. Angew. Chem., Int.
Ed. 2009, 48, 7867. (g) Andreasson, J.; Pischel, U. Chem. Soc. ReV.
UV-visible absorption spectra were measured, from 200-800 nm
-1
at a scan rate of 600 nm s , on a Cary-50 spectrometer fitted with
a peltier temperature control cell. Solutions for UV-visible
measurements were made to concentrations of 0.05 mg/mL, unless
(21) Perrin, D. D.; Armarego, W. L.; Perrin, D. R. Purification of
Laboratory Chemcials, 2nd ed.; Pergamon: New York, 1980.
2
010, 39, 174–188.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010 10749

A R T I C L E S
Darwish et al.
2
Table 1. Composition of Solutions of N-Methyl Spiropyran (SP-NO ), N-Propanol Spiropyran (SP-1) and Spirooxazine (SOX)
a
22
Slightly colored due to the existence of small amount of the photomerocyanine form at equilibrium which decolorizes under direct visible light.
b
c
d
The solution decolorizes when exposed to visible light. The solution does not change color when exposed to either UV or visible light. CRM
2 2
showed the same results as SOX; i.e., blue color in the presence of UV, DBU, CO , and N (see Supporting Information for photos: Figure SI1).
otherwise specified. White light and UV illuminations were
conducted using an Abet Technologies arc lamp fitted with a water
filter and either a 420 nm Schott glass cutoff filter for white light
or an Edmund Optics U-340 band-pass filter for UV. Samples were
temperature equilibrated to 25 °C and then illuminated for 5 min
prior to measurement.
chromic molecules gave blue-color solutions which decolorized
when exposed to visible light. Addition of DBU, CO , or N
2
2
did not affect the merocyanine colored forms of both
molecules. The results for the SOX dye are summarized in
Table 1; while those results for the CRM dye are shown in
the SI, Figure SI1. The noteworthy difference among these pyran
compounds is that, when illuminated by UV light, the spiroox-
Results and Discussion
2
3
24
azine and chromene molecules take quinoidal open forms,
Table 1 describes the solutions of photochromic molecules
investigated in this study, along with the stimuli used to trigger
switching. This table also indicates whether the solutions are
in a photoactive state, as well as their color and λmax after
switching.
25
while the spiropyran molecule prefers the zwitterionic open
form, having a positively charged indoline (indolium) fragment
and a negatively charged phenolate oxygen (Chart 1). This
suggests that DBU interacts with the zwitterionic merocyanine
form of SP-NO2, while having no effect on the neutral
merocyanine forms of the other two dyes.
SP-NO
behavior when DBU, UV light, visible light, CO
introduced to each system, and the results are summarized in
Table 1. The UV-vis absorption spectroscopy of SP-NO
2
and SP-1 in methanol (S-1 and S-6) exhibited similar
2
and N were
2
The acid-base susceptibility and color-indicating properties
of spiropyrans have been reported before, where change in color
is reversible upon acidification, neutralization, and
2
system is described in the Supporting Information, Figure SI2,
while that of SP-1 is described below. This essentially dem-
onstrates the reversible loss of photochromism of both spiro-
20a
basification. The addition of acid causes spiropyrans to open,
and protonation takes place at the phenolate oxygen (pH ) 3)
leading to a yellow solution (MC-H) due to the loss of charge
delocalization between the indoline and benzopyran halves of
2
pyran molecules with DBU, unless CO is present. In contrast,
the spirooxazine (SOX) and the chromene (CRM) dyes, shown
in Chart 1, retained their photochromism when DBU was added
to the methanol solutions. Upon UV irradiation, both photo-
(
23) Berthet, J.; Delbaere, S.; Lokshin, V.; Bochu, C.; Samat, A.;
Guglielmetti, R.; Vermeersch, G. Photochem. Photobiol. Sci. 2002,
1
, 333.
(
22) Unlike SP-1, the decolorization of SP-NO
2
in methanol under visible
(24) Berthet, J.; Delbaere, S.; Levi, D.; Samat, A.; Guglielmetti, R.;
Vermeersch, G. Photochem. Photobiol. Sci. 2002, 665.
(25) Inouye, M.; Ueno, M.; Kitao, T.; Tsuchiya, K. J. Am. Chem. Soc.
1990, 112, 8977.
light is very slow. In this case, toluene was added to methanol to reduce
the polarity of the solution for the ease of taking photos of the
colourless solutions when exposed to visible light.
1
0750 J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010

Reversible Control of Photochromism of Spiropyrans
A R T I C L E S
2
Figure 1. Acid-base reversible reactions of SP-NO and with DBU complexation.
the molecule (Figure 1a). These species remain photochromic
the negative charge on the phenolate oxygen from its interaction
with the countercation DBUH and allows the re-establishment
of the zwitterionic form of the photochromic molecule. It was
2
evident that CO /DBU mixture in methanol is not acidic enough
(pH ) 6.8) to protonate the phenolate oxygen and thus the MC
species is present in solution and does not proceed to the MC-H
species. In fact, upon dropwise addition of 1 M HCl to the above
20a
+
and decolorize in visible light. Neutralizing the acidic yellow
solution with a base regenerates the red merocyanine form (MC)
at the neutralization point (Figure 1b); however, this is not the
same chemistry as reported here in this study. The addition of
CO
did not show any change in color (i.e., remained purple),
suggesting that CO is not acidic enough to protonate the
phenolate oxygen of the MC form of SP-NO . The yellow
nonphotochromic solution (S-3) was produced when DBU, a
non-nucleophilic base, was added to SP-NO in alcohol solution
2 2
to SP-NO in methanol (pH ) 5.4) in the absence of DBU
2
2
CO /DBU methanol solution, the color changed from purple to
2
bright yellow (pH ) 3) with the appearance of a new absorption
band at 346 nm (broad) (see SI: Figure SI3c). This is different
to the absorption band attributed to the yellow solution S-3 that
results from the addition of DBU to solution S-2 (λabs ) 432
nm) (see SI: Figures SI2 and SI3a).
2
(
pH ) 13) (Figure 1c). We propose that this occurs through
the interaction of the deprotonated solvent (methanol) with the
highly electrophilic indolium fragment of the open MC form
In an attempt to obtain a clearer picture of the molecular
interactions in these solutions, NMR spectroscopy was used.
+
of SP-NO
2
. The conjugate acid of DBU (i.e., DBUH ) then acts
1
13
as a weak ligand and electrostatically interacts with the negatively
charged phenolate oxygen on the benzopyran fragment of the
molecule, trapping the delocalization of the electrons between the
two parts of the molecule. Bleached yellow solutions have been
reported when a base was directly added to MC forms of some
spiropyrans, however these species remained unidentified due to
However, the H and C NMR spectra of SP-NO
2
with DBU
in MeOH-d were too complicated to make clear assignments.
4
The complexity of these NMR spectra was likely to be due to
solvent exchange and the production of isomeric species.
Therefore, a new photochromic spiropyran molecule was
synthesized with a propanol linker attached to the indoline
nitrogen, N-propoanol spiropyran (SP-1), in an attempt to affect
intramolecular interactions. This was anticipated to generate
less isomeric and more stable species that would be easier
to characterize by NMR spectroscopy. A solution of SP-1 in
26
the complexity of the associated NMR spectra. This is likely to
be due to the addition of the nucleophilic base to the reactive
indolium fragment in the MC form of the molecule.
The iminium carbon atom adjacent to the positively charged
nitrogen in the indolium fragment of the zwitterionic MC species
3
3
methanol (S-6) demonstrated typical photochromic behavior
(Scheme 1). When irradiated with UV light (S-7), it exhibited
an intense red color with an absorbance centered at 530 nm,
indicating the formation of the MC form (Figure 2). When left
in the dark following exposure to UV, this absorption band
decreased in intensity (see SI: Figures SI4 and SI5) with the
(
formerly the spiro carbon) is highly electrophilic and susceptible
2
7
to nucleophilic attack. Shiraishi et al. recently reported the
nucloephilic addition of CN anion to the iminium carbon of
-
the indolium fragment of the MC form of SP-NO
2
. Intermo-
lecular and intramolecular nucleophilic addition is common for
2
8
indolium molecules in general.
Recently, it has been demonstrated that the exposure of amidines
(26) Hobley, J.; Malatesta, V.; Millini, R.; Montanari, L.; Parker, W. O. N.
29
30
Phys. Chem. Chem. Phys. 1999, 1, 3259.
mixed with water or alcohol to gaseous CO
2
, results in
(
(
27) Shiraishi, Y.; Adachi, K.; Itoh, M.; Hirai, T. Org. Lett. 2009, 11, 3482.
28) (a) Bergman, J.; Koch, E.; Pelcman, B. Tetrahedron Lett. 1995, 36,
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F. M. J. Org. Chem. 2008, 73, 118.
exothermic reactions that produce bicarbonate or alkylcarbonate
29
salts (eq 1). NMR spectroscopy and conductivity measurements
of the reaction between DBU and CO detected rapid formation
of bicarbonate salt of DBU (i.e., [DBUH ][HCO
DBU (not dried) is exposed to CO
2
+
-
3
]) when wet
(
(
(
29) Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.; Liotta,
C. L. J. Org. Chem. 2005, 70, 5335.
2
.
30) Jessop, P. G.; Heldebrant, D. J.; Li, X. W.; Eckert, C. A.; Liotta, C. L.
Nature 2005, 436, 1102.
+
-
3
31) Although methyl carbonate salt of DBU (i.e. [DBUH ][CH OCOO ])
could also be produced, we will refer to this as the bicarbonate
+
-
[
3
DBUH ] [HCO ] species since wet (not dried) DBU and methanol
were used in the experiments. Also, it has been reported that methyl
carbonate salts are extremely sensitive to small amounts of acid, and
it is very likely that the presence of water and CO
formation of the bicarbonate adduct. Alternative reactions, such as
the electrophilic addition of CO to the phenolate anion in the presence
of DBU is not possible as phenols did not form aryl carbonate even
under high pressures of CO as reported in ref 32.
2
will favor the
2
When CO
2
is added to solution S-3 (SP-NO
2
+ DBU in
+
methanol) to give S-4, it reacts with the DBUTDBUH
equilibrium, acting as a weak ligand with the phenolate oxygen,
to give the bicarbonate adduct [DBUH ][HCO
The consumption of DBU/DBUH by the added CO
2
(
(
32) Munshi, P.; Main, D. A.; Linehan, J. C.; Tai, C. C.; Jessop, P. G.
J. Am. Chem. Soc. 2002, 124, 7963.
+
-
31
3
] (pH ) 6.8).
33) Ethanol and isopropanol were also used as alcohol solvents, and similar
effects were observed.
+
2
liberates
J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010 10751

A R T I C L E S
Darwish et al.
2
Scheme 1. Photochromic and CO Switching between SP and MC Forms of SP-1, and the DBU Complex MCox-DBU
3
5
simultaneous growth of an absorbance at 267 nm associated
with the SP form. It is noted that the absorption band at 530
nm does not disappear entirely and reaches an equilibrium
intensity indicating some of the SP-1 remains in the open MC
form. The absorption spectrum of SP-1 in methanol at 530 nm
increases linearly with the concentration of the dye (see SI:
solution of MCox-DBU (S-8). Correspondingly in the UV-vis
spectrum (Figure 2, S-8), the total disappearance of the
absorbance at 530 nm and the appearance of an absorption band
at 413 nm were observed. The latter grew with time in the dark
at the expense of the SP absorbance at 267 nm (see SI: Figure
SI8). Ultimately, neither visible nor ultraviolet light made any
3
6
changes to the color of the resulting yellow solution. Modeling
the increase in intensity of the 413 nm band (MCox-DBU) with
Figure SI6). From this correlation and the equilibrium constant
1
(
K
1
) 0.1 ( 0.03), determined by H NMR spectroscopy (see
-
5
-1
time gave a rate constant kobs ) 1 × 10
s
(see SI: Figure
SI: NMR Spectroscopy Results), the molar extinction coefficient
3
-1
-1
SI9). This is the same order of magnitude as k , the rate constant
of MC can be estimated to be 18 ( 6 × 10 M cm . The
1
for the thermal interconversion of SP to MC. This suggests
time dependence of the absorbance at 530 nm (see SI: Figure
that DBU interacts with the MC rapidly and the observed rate
constant of this reaction depends on the slow thermal intercon-
version of the SP to MC. The latter is consumed by the added
DBU and thus shifts the SP T MC equilibrium to the right, leading
ultimately to the consumption of the SP species and the formation
of the MCox-DBU (Scheme 1). In fact, the disappearance of the
530 nm band corresponding to MC upon the addition of DBU
happens instantaneously and could not be followed either by
UV-vis or by NMR (shown below). Additionally, when DBU
was added to SP-1 in methanol, the pale-yellow color of the
solution did not increase in intensity as long as the solution was
kept under direct white light. The yellow color only increased in
intensity when the solution was kept in the dark where the
interconversion of SP to MC can occur, and an equilibrium was
SI7) indicates that the rate constant (k
interconversion of SP form into MC form and the rate constant
1
) for the thermal
-5
-1
(
(
k
-1) for the opposite process are (1.5 ( 0.5) × 10
s
and
-4
-1
34
1.5 ( 0.5) × 10 s , respectively, at 298 K.
When at least an equimolar amount of DBU was added to
the red MC solution, it produced a yellow nonphotochromic
(
1
34) Molar extinction coefficient (ε530) and rate constants (k and k-1) for
the thermal equilibrium of SP T MC were calculated according to
the method described in detail by Raymo et. al in ref 20b.
(
35) A competing effect is anticipated to exist between the solvent hydroxyl
group (MeOH) and the propanol linker on the SP-1; however,
intramolecular interaction is believed to be thermodynamically more
favored which produces exclusively MCox-DBU. Molar extinction
3
-1
coefficient of MCox-DBU was estimated to be (26.1 ( 1) × 10 M
-
1
cm (see SI, Figure SI25).
(
36) When SP-1 was dissolved in toluene to which a small amount of DBU
was added, the toluene solution turned quickly yellow in color with
the loss of photochromism (see SI: Figure SI24). This suggests the
involvement of the propanol linker in the photochromic deactivation
through its intramolecular cyclization at the iminium carbon center in
the open merocyanine form. To reproduce the photochromic activity
Figure 2. UV-vis spectra for SP-1 with the curves corresponding to the
different solutions. S-6: black (SP form); S-7: red (a mixture of ∼18%
MC form and ∼82% SP form, calculated using εMC); S-8: yellow (MCox-
DBU) with DBU after three days in the dark; S-9: brown curve (MCox-
of this solution, CO was bubbled in the toluene solution; however,
2
the solubility of CO in toluene was not sufficient unless wet toluene
2
2
DBU + CO ) upon addition of CO
2
(resulting in ∼86% of MC form: the
amount of MC form immediately starts to diminish with time and continues
until it reaches equilibrium state (see SI: Figure SI10)). The inset shows
the different colors under the different stimuli.
was used (i.e., 50 µL of H
O in 3 mL of toluene) or some methanol
responsiveness similar to that in the systems
2
is added. This showed CO
described above.
2
1
0752 J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010

Reversible Control of Photochromism of Spiropyrans
A R T I C L E S
reached after 3 days (as indicated by UV-vis and NMR). When
the same solution was irradiated by UV light for 10 min following
the addition of DBU, the yellow color increased in intensity and
reached equilibrium rapidly. Therefore, production of MCox-DBU
only occurs when MC is present in solution.
When the yellow solution of MCox-DBU was briefly purged
(
2
for a few seconds) with CO (producing S-9), the solution
turned red again, reforming the MC species. The UV-vis
spectrum (Figure 2, S-9) shows the immediate disappearance
of the 413 nm absorption band and the instantaneous reappear-
ance of the 530 nm band associated with MC and the 267 nm
band associated with SP due to the re-establishment of the SP
T MC equilibrium. Probing the subsequent evolution of these
signature absorption bands with time (see SI: Figure SI10)
indicates an initial increase in the amount of MC, before a
decrease in the quantity of MC and an increase in SP over
time until an equilibrium state was reached. Solution S-9
regained its photochromism and could be switched between
colorless SP and red MC forms as long as the solution was
kept under an atmosphere of CO
2
. When solution S-9 was
purged for approximately one minute with N
2
to remove CO
2
3
7
from the solution, the band at 530 nm disappeared along
with the return of the yellow color and the reappearance of
the absorption band at the 413 nm band, corresponding to
1
nonphotochromic MCox-DBU.
Figure 3. Partial H NMR spectra (400 MHz, MeOH-d , 298 K) of the
4
1H NMR spectroscopy was also used to probe the same
SP, MC, and MCox-DBU forms of SP-1 under gas and photochromic
switching conditions. (a) SP-1 in MeOH-d equilibrated in the dark for
4
one day, showing SP signals and a small amount of MC (“*”); (b) after
addition of DBU and equilibration in the dark for 3 days, showing MCox-
progression of photochromic and molecular switching of SP-1
under the action of DBU and CO /N (Figure 3). This enabled
2 2
the determination of the detailed molecular structures at each
stage and showed the reproducibility of the species when the
different stimuli were used. Figure 3a is the spectrum associated
DBU; (c) after addition of CO , showing the disappearance of signals of
MCox-DBU and the appearance of MC and SP signals; (d) the same
solution after 355 min in the dark showing mostly SP; (e) after purging
2
2
with N showing the re-emergence of MCox-DBU signals. For full spectra,
4
with solution S-6 in MeOH-d (equilibrated for one day in the
see SI Figure SI16.
dark) showing mainly peaks attributed to SP with a small
1
amount of MC (indicated by “*” ∼10% by H NMR, which
1
) determined by NMR spectroscopy were (a) the propanol
corresponds to equilibrium constant K ) 0.1 ( 0.03), which
1
linker in MC form that had cyclized at the former spiro carbon
in the indoline fragment to form a six-membered 1,3-oxazine
ring and an open pyran fragment; (b) the trans configuration of
the bridging carbon-carbon double bond; (c) the neutral charge
on the indoline nitrogen which is otherwise positively charged
in MC form; and (d) the negatively charged phenolate oxygen.
is enough to give a pale-red color to the solution due to the
very high molar absorption coefficient of this form of the
molecule. The structure of the SP form was completely
1
13
characterized by H and C NMR (see SI: Figures SI11-SI13
and Table S1). Upon addition of an equimolar amount of DBU
1
(
S-8), the H NMR spectrum showed the complete disappearance
1
Examination of the H NMR signals associated with DBU
of the signals attributed to MC and the formation of a new set
of signals associated with the new MCox-DBU species. As
observed in the UV-vis data, when solution S-8 was kept in
the dark at room temperature over a period of three days,
showed an evolution over a three-day period during the
formation of MCox-DBU. The presence of positively charged
amidinium species was concluded from the observed H/D
isotope exchange between the deuterated solvent (MeOD) and
the allylic amidinium protons (see SI: NMR Characterization
of Complexed DBU and Table SI3), which became labile due to
the positively charged nitrogen atoms. The presence of a positively
charged amidinium cation and a negatively charged phenolate
oxygen anion suggests the presence of electrostatic interaction
between these two charged species. Similar ion-pairing interactions
have been previously proposed for the merocyanine forms of
spiropyrans when binding polar and charged molecules, such as
1
residual H NMR signals from the SP form gradually disap-
peared, leaving a spectrum composed almost entirely of MCox-
1
13
DBU (Figure 3b). H and C, 1D and 2D NMR spectral
analyses confirmed the structures of the existing species in situ.
The spiro structures of SP and MCox-DBU and the planar
structure of MC species can be simply followed by probing
1
the H NMR of the geminal methyl groups on the indoline ring
(
Figure 3). These two methyl groups are nonequivalent in the
spiro structures, thus giving two different pairs of resonances,
i.e. 1.26 and 1.16 ppm for SP, and 1.35 and 0.98 ppm for
MCox-DBU. While these methyl groups become equivalent in
the planar structure of MC, they therefore produce only one
resonance at 1.82 ppm (for more characterization see notes in
SI: NMR Spectroscopy Results, Figures SI14-SI23 and Table
S2). Key structural information relating to MCox-DBU (Scheme
13
38
39
amino acids, ionic liquids, and metal ions.
Solution S-8 containing MCox-DBU was purged with CO
in the NMR tube before being sealed, producing solution S-9.
2
(
(
38) (a) Wu, Y.; Sasaki, T.; Kazushi, K.; Seo, T.; Sakurai, K. J. Phys.
Chem. B 2008, 112, 7530–7536. (b) Zhang, S.; Zhang, Q.; Ye, B.; Li,
X.; Zhang, X.; Deng, Y. J. Phys. Chem. B 2009, 113, 6012–6019.
39) (a) Inouye, M.; Ueno, M.; Kitao, T. J. Am. Chem. Soc. 1990, 112,
8977–8979. (b) Inouye, M. J. Inclusion Phenom. Mol. Recognit. Chem.
(
37) Heat (40-60 °C) can be used to speed up the depletion of CO
2
from
1998, 32, 223–233. (c) Shao, N.; Jin, J. Y.; Wang, H.; Zhang, Y.;
Yang, R. H.; Chan, W. H. Anal. Chem. 2008, 80, 3466–3475.
the solution.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010 10753

A R T I C L E S
Darwish et al.
Table 2. Optical Responses of a Dual Dye System of SP-NO
2
and SOX with DBU in Methanol Using the Different Stimuli Combinations and
Showing the Reversibility of the System
a
See ref 22.
2
Scheme 2. Mixture of 1:1 SPNO and SOX in Methanol with DBU
Exposed to a Combination of Lights and Gases Showing the
Optical Responses
1
The H NMR signals of the MCox-DBU species decreased
rapidly and the ratio of compounds MCox-DBU: SP: MC were
observed to evolve from 196:10:1 to 72:22:123 within the first
two minutes after the addition of CO
2
and then continued to
change to reach equilibrium after ∼6 h with a ratio of 10:184:
4
0
1
6. The corresponding signals of the MC and SP forms re-
emerged due to the SP T MC equilibrium (Figure 3c,d). The
rate of disappearance of MC to give SP (going from c to d in
1
Figure 3) was followed by H NMR (see SI Figure SI19) and
-4
-1
showed a first-order rate constant (kobs) of 2 × 10 s , which
was the same as that observed using UV-vis in the absence of
DBU and CO
2
. This indicates that the presence of DBU and
CO in solution has essentially no effect on the photochromic
2
1
3
switchability of SP and MC. C NMR signals from solutions
of SP-1 in methanol were also used to probe the evolution of
molecular species and complexation under the above conditions
a combination of optical responses. For example, in a mixture
(
see SI: Figures SI17 and SI18). Addition of CO
2
to the yellow
of SP-NO
amount of DBU (N-1, Table 2), the photochromism of SOX
or CRM) is retained where it can switch colors in the
presence of UV and visible lights, while the photochromism of
SP-NO is switched off and can be reversibly switched on by the
addition and removal of CO gas. This means the system becomes
2
and SOX (or CRM) in methanol with a small
solution S-8 (SP-1 and DBU in methanol), giving S-9, led to
1
3
the appearance of a new C resonance at 161.4 ppm which
(
+
-
3
was attributed to the bicarbonate carbon in [DBUH ][HCO ]
(
2
9
see SI Figure SI18) which has been reported by Jessop et al.
Purging of solution S-9 with N removed the CO from solution
producing S-10) and restored the H NMR signals of the
2
2
2
2
1
(
responsive to a combination of the different optical and gaseous
triggers. The results of exposure of the mixed solution (N-1) to
combinations of stimuli are tabulated in Table 2 and Scheme 2.
The orthogonal dimensions of the stimuli reported in this
study can allow the design of a three (or more)-dye system.
3
7
nonphotochromic MCox-DBU species (Figure 4e). This was
1
3
accompanied by the disappearance of the carbonate C signal
at 161.4 ppm (see SI Figure SI18). Hence, the photochromic
2
switching of SP-1 (and SP-NO ) can be turned on or off by the
presence or absence of CO , respectively.
2
For example, in a mixture of SOX, SP-NO
methanol solution with DBU, the photochromic activity of the
later two dyes can be controlled by CO while the first dye
2
, and SP-1 in
The insensitivity of the photomerocyanine forms of CRM
and SOX to the addition of DBU makes them good candidates
to use in a mixture with spiropyrans for the design of
multicomponent dye systems. In these systems, distinct dye
components can be triggered separately using the different
2
remains photochromic. From such multistate molecular switch
systems, combinational logic circuit can be designed similar to
2
0a,41
those reported else where,
combinations.
yet with many more possible
2 2
stimuli (i.e., UV light, visible light, CO , and N ) to produce
(
40) From this ratio, the equilbrium constant between SP and MC species
Conclusion
(
K
eq ) [MC]/[SP]) is calculated to be 0.08, which is comparable to
K
1
) 0.1 ( 0.03 calculated for the same equilbrium reaction in the
We have demonstrated that DBU can reversibly switch off
the spiropyran’s photochromism unless CO is present. By
reversibly switching the basicity of DBU using CO or nitrogen
gases, one can control the different responses of spiropyran
absence of any DBU and CO
and CO
2
. This shows that the presence of DBU
2
2
has no effect on the equilibrium ratio of the two species and
that the initial equilibrium system can be restored by addition of CO
to DBU.
2
2
1
0754 J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010

Reversible Control of Photochromism of Spiropyrans
A R T I C L E S
molecules and switch the photochromism on and off. Multire-
sponsive photochromic systems have been demonstrated by
not contribute to contamination or dilution effects. Furthermore,
CO is a mild reagent and is present in low concentrations in
2
using SP-NO
photochromic response of SP-NO
addition and removal of CO gas from the systems. Importantly,
2
and SOX in methanol solution, where the
numerous biological systems. The increasing use of spiropyrans
cited in this work makes the discovery of molecular switching
2
can be controlled by the
2
by amidine/CO addition of great importance. Its general use
2
this work demonstrates that the addition of DBU can possibly
be applied to any spiropyran system and opens the way for
additional CO control into systems such as those described in
2
and applicability suggest additional possible applications in
bioresponsive materials based on CO
biological environments.
2
gradients found in
the introduction, and more generally to any photochromic system
with an active indolium fragment. Photochromic oxazines and
bichromophoric photochromes (which have been reported
Acknowledgment. T.A.D. thanks ANSTO Postdoctoral fellow-
ship for funding. Part of this work was conducted at The Australian
National Deuteration Facility at ANSTO, which is cofunded by
the National Collaborative Research Infrastructure Strategy, an
Australian government initiative.
4
2
recently by Raymo et al. ) would possibly be good candidates
for such application due to the presence of indolium cation in
their open states. Carbon dioxide is an excellent stimulus for
photochromic systems as it is easily added and removed; it does
Supporting Information Available: Details on the synthesis
1
13
of SP-1 and CRM, UV-vis spectroscopy, H and C (1D and
D) NMR spectroscopy characterizations (including spectra) of
(
41) Giordani, S.; Cejas, M. A.; Raymo, F. M. Tetrahedron 2004, 60,
0973.
42) (a) Tomasulo, M.; Soritino, S.; Ryamo, F. M. Org. Lett. 2005, 7, 1109–
112. (b) Tomasulo, M.; Soritino, S.; White, A. J. P.; Ryamo, F. M.
1
2
(
the different species, and photographs of solutions. This material
is available free of charge via the Internet at http://pubs.acs.org
1
J. Org. Chem. 2006, 71, 744–753. (c) Tomasulo, M.; Soritino, S.;
Ryamo, F. M. AdV. Mater. 2008, 20, 832–835. (d) Tomasulo, M.;
Soritino, S.; Ryamo, F. M. J. Org. Chem. 2008, 73, 118–126.
JA1013322
J. AM. CHEM. SOC. 9 VOL. 132, NO. 31, 2010 10755