Table 1 Rate constants for the thermal equilibration between 1a and 1b in aqueous solutions containing 80 mM NaC at 25 1C, measured for the
relaxation of 1b to 1a (k
0
1
, k
2
) after the sample was irradiated at 338 nm and measured for the relaxation of 1a to 1b after the sample was exposed to
0
ambient light (k and k )
1
2
ꢂ3
ꢂ1
ꢂ5
ꢂ1
0
ꢂ3
ꢂ1
0
ꢂ5
ꢂ1
[
NaCl]/M
k
1
/10 min
k
2
/10 min
k =10 min
k =10 min
1
2
0
1
.20
.0
5.6 ꢁ 0.1 (3)
5.4 ꢁ 0.8 (3)
3 ꢁ 1 (3)
5.4 ꢁ 0.2 (4)
8.4 ꢁ 0.3 (4)
5.1 ꢁ 0.8 (4)
2.6 ꢁ 0.7 (4)
8.74 ꢁ 0.06 (3)
The value in parentheses corresponds to the number of independent experiments; errors correspond to standard deviations.
The decoloration of 1b occurs thermally and photochemically.
The relative photochemical decoloration was not studied
because this process overlaps with the thermal decoloration.
The isomers 1a and 1b are in thermal equilibrium and for this
reason the relaxation rate constant for equilibration, which
corresponds to the sum of the rate constant for the 1a - 1b
stable as the aggregate size increases. This result is in line with
the lower polarity derived from the absorption maxima at the
higher NaC concentrations.
In summary, the use of bile salt aggregates is a viable
strategy to develop photochromic systems with aqueous
solubility without the need for synthetic modification. Bile
salts are more adaptable than rigid host systems and therefore
capable of solubilizing photochromic compounds with very
different structures and shapes without inhibiting solubilization
and photochromic activity.
1
5
f b
(k ) and 1b - 1a (k ) processes, can be obtained by either
forming an excess of 1a or 1b with irradiation of visible or UV
light, respectively (Fig. 2). The kinetics were measured at 517
nm. In the case of 1b in excess, a fast decay was observed
followed by a slow decay, while in the case of 1a in excess a
growth at 517 nm was followed by a decay (Fig. 2). The data
were fit to the sum of two exponentials (eqn (1)) and the same
rate constants were recovered from the kinetics for 1a or 1b in
excess (Table 1), indicating that the kinetic processes measured
were coupled by the equilibration between the two isomers.
We thank the Xerox Foundation for a University Affairs
Committee grant and the Natural Sciences and Engineering
Research Council of Canada (NSERC) for a Collaborative
Research and Development grant.
Notes and references
ꢂk
1
t
ꢂk
2
t
A = a
1
e
+ a
2
e
(1)
1
M. Irie, editor, Chem. Rev., 2000, 100, 1683–1890; H. Bouas-
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¨
The slow kinetics (k ) led to the disappearance of 1b over time
2
¨
regardless of which isomer, i.e. 1a or 1b, was prepared in
excess, suggesting that compound 1 was decomposing. The
same type of kinetics was observed for a synthetically modified
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2
3
F. M. Raymo and M. Tomasulo, Chem. Soc. Rev., 2005, 34,
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6
water soluble spiropyran, and the decomposition was assigned
to the hydrolysis of 1. Indeed, after long periods of time we
measured spectral changes (Fig. S9w) that were not due to the
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region as those observed for the hydrolysis product for the
2
4 T. Hirose, K. Matsuda and M. Irie, J. Org. Chem., 2006, 71,
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7
2
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U. Al-Atar, R. Fernandes, B. Johnsen, D. Baillie and
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6
reaction in water. The decomposition rate constant for the
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ꢂ3
ꢂ1
higher (2–4 ꢃ 10 min , pH 7–8) than observed for 1 in
the presence of NaC aggregates, suggesting that the NaC
aggregates protect 1 from hydrolysis.
8
J. Zhou, S. Q. Huang and B. Huang, J. Photochem. Photobiol., A,
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The thermal re-equilibration rate constants in the NaC
aggregates (k
ꢂ2
1
)
were lower than observed in ethanol
2
008, 200, 83–89; M. Takeshita, N. Kato, S. Kawauchi,
ꢂ1 15
(
2.3 ꢃ 10 min ), in line with a more polar environment
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The structure of the bile salt can be modulated with the
concentration of bile salt or NaCl, where the addition of NaCl
9
1
0 G. Favaro, F. Ortica and V. Malatesta, J. Chem. Soc., Faraday
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1
6
is known to increase the size of the bile salt aggregates.
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change in size had three effects on the photochromic behavior
of 1. (i) The larger aggregates led to a decrease in the
decomposition rate constant of 1. (ii) The larger aggregates
led to an increase of B60% in the re-equilibration rate
1
1
1
2 N. N. Tsikurina, Y. P. Strokach and Z. N. Markina, Kolloidn. Zh.,
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b and 1a was observed at the higher concentration of NaCl
inset Fig. 2). The equilibrium constant is equal to k /k while
the observed rate constant corresponds to the sum of these two
rate constants (k + k ). Observations (ii) and (iii) show that
1
). (iii) A lower equilibrium concentration between
1
(
f
b
1
6 N. A. Mazer, M. C. Carey, R. F. Kwasnick and G. B. Benedek,
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f
b
kb increases in the larger aggregates suggesting that 1b is less
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Chem. Commun., 2010, 46, 1941–1943 | 1943