Spiropyran-merocyanine equilibrium in presence of organic acids and bases

Article abstract of DOI:10.1080/15421400590946712

Manipulation of the rate of thermal reversion of N-functionalized merocyanines (MC) to the corresponding spiropyran isomers in the dark, was demonstrated by using both organic acid and base additives. In the presence of organic bases, increase or decrease of the rate of reaction was dependent on the solvent used. When organic acids were used, in all cases the reaction rate decreased when compared with the system containing no acid. Similar behavior was also observed in polymer films. This approach is of interest for applications in the design of optical switches and memory with enhanced stability of the stored information.

Full text of DOI:10.1080/15421400590946712

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Spiropyran-Merocyanine
Equilibrium in Presence of
Organic Acids and Bases
a
a
a
Ningning Xiao , Yu Chen , Robert Lemieux
a
b
,
Erwin Buncel , Gabriel Iftime & Peter M.
b
Kazmaier
a
Department of Chemistry , Queen's University ,
Kingston, Ontario, Canada
b
Xerox Research Centre of Canada , Mississauga,
Canada
Published online: 31 Aug 2006.
To cite this article: Ningning Xiao , Yu Chen , Robert Lemieux , Erwin Buncel , Gabriel
Iftime & Peter M. Kazmaier (2005) Spiropyran-Merocyanine Equilibrium in Presence of
Organic Acids and Bases, Molecular Crystals and Liquid Crystals, 431:1, 337-344
To link to this article: http://dx.doi.org/10.1080/15421400590946712
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Mol. Cryst. Liq. Cryst., Vol. 431, pp. 37=[337]–44=[344], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590946712
Spiropyran-Merocyanine Equilibrium in Presence
of Organic Acids and Bases
Ningning Xiao
Yu Chen
Robert Lemieux
Erwin Buncel
Department of Chemistry, Queen’s University,
Kingston, Ontario, Canada
Gabriel Iftime
Peter M. Kazmaier
Xerox Research Centre of Canada, Mississauga, Canada
Manipulation of the rate of thermal reversion of N-functionalized merocyanines
(MC) to the corresponding spiropyran isomers in the dark, was demonstrated by
using both organic acid and base additives. In the presence of organic bases,
increase or decrease of the rate of reaction was dependent on the solvent used.
When organic acids were used, in all cases the reaction rate decreased when com-
pared with the system containing no acid. Similar behavior was also observed in
polymer films. This approach is of interest for applications in the design of optical
switches and memory with enhanced stability of the stored information.
Keywords: merocyanine; optical switch; spiropyran
Spiropyrans represent one of the most promising classes of materials
for applications in molecular devices including sensors, optical
switches, signal transducers and optical memories.
When aiming to design optical switches, one of the disadvantages of
spiropyran materials is that the open state isomer (merocyanine; MC)
does not persist for a long enough time. An optical memory device
This work was supported by NSERC, Queen’s University and by Xerox Research
Centre of Canada.
Address correspondence to Gabriel Iftime, Xerox Research Centre of Canada,
2
660 Speakman Drive, Missisauga, ON, L5K 2L1, Canada. E-mail: gabriel.iftime@
xrcc.xerolabs.com
37=[337]

3
8=[338]
N. Xiao et al.
using these materials would keep the stored information only for a
relatively short period of time.
Several approaches were previously investigated in order to
achieve increased thermal stability of the merocyanine isomer.
Notable results were obtained by incorporation of silica nanoparticles
into spiropyran-containing Langmuir-Blodgett films [1], incorporation
of spirobenzopyran into siloxan matrices [2], incorporation of spiropyr-
ans with long alkyl chains into polymeric films [3], and by dissolution
of spiropyrans in fluoroalcohols [4]. In our group we have demon-
strated the stabilization of merocyanine isomer by using metal salts
[
5]. However, metal salts have limited solubility in common organic
solvents. In addition, they also have limited compatibility with many
organic polymers. Using organic acids and bases as additives instead
of metal salts may provide a wider scope for optimization of optical
switches, because of their superior miscibility with polymers in
organic solvents.
We report here our preliminary results with respect to the
investigation of the effect of organic acids and bases as additives
in spiropyran solutions. Several spiropyrans were synthesized from
the Fischer’s base or its quaternary salt by reaction with 2-hydroxy-
5
-nitrobenzaldehyde in ethanol, in the presence of a base [6].
Chemical structures of spiropyran (SP) isomers are shown in
Figure 1.
They are divided in series as a function of number of chelating
groups. Series A yields monodentate MC, while series B and C yield
bidentate and tridentate MC, respectively.
Merocyanine isomers are obtained from closed spiropyran isomers
after illumination with UV light in solution.
EXPERIMENTAL RESULTS
Addition of Bases
Solutions of a concentration of 1 ꢀ 10ꢁ4 M of spiropyrans in the given
solvent were prepared and to these various amounts of bases were
added. Two solvents were used: methanol and acetone while two bases
were investigated: tri-N-butylamine (TBA) and N,N-diisopropylethyl-
amine (dIEA). The kinetics of the thermal reversion reaction of mero-
cyanine isomer (MC) to the closed isomer (SP) were studied by
measuring the decay of absorbance at kmax of MC by using UV-VIS
absorption spectroscopy after irradiation for 90 s with UV light. Typi-
cal results are shown, for comparison, in Figure 2. Data shown are for
the MC(4.x) series and for TBA as a base.

S-M Equilibrium of Organic Acids and Bases
39=[339]
FIGURE 1 N-functionalized spiropyrans (SPs) investigated in this work. The
SP series A, B and C on irradiation with UV light afford monodentate, biden-
tate and tridentate merocyanines (MCs), respectively.
FIGURE 2 Decay of MC concentration as a function of base used in methanol
(
left side) and in acetone (right side). TBA was used as the base. Data are
shown for selected MC(4.x) compounds, i.e., spacer is four methylene groups
in length. x is 1, 2, 3 for mondentate (series A), bidentate (series B) and triden-
tate (series C) compounds, respectively.

4
0=[340]
N. Xiao et al.
The main observations when bases were added are as follows:
–
–
no spectral changes were observed when base was added;
series A shows no effect on rate of reversion with respect to solvent or
addition of base;
–
–
–
for series B and C an effect on rate was observed and this depended
on the solvent used;
in methanol, there is slight increase of the rate constant for decolora-
tion reaction;
in acetone there is a significant decrease of the rate constant when
large amount of base is added.
Addition of Acids
Trifluoro acetic acid was used in various amounts and in methanol as
solvent. The main observations are as follows:
–
–
new peaks appeared in the UV-VIS spectra at 414 nm and 314 nm,
while the peak of free merocyanine at 570 nm disappeared.
for all series (A, B and C) the rate constant for thermal reversion of
the merocyanine isomer decreased, i.e., the fading reaction was
slowed down when TFA was added.
Discussion
The key to understanding these various behaviors is the possibility of
proton transfer from Nꢁ(CH ) ꢁCOOH to the p-nitrophenoxide group
2
n
of the MC isomer. If proton transfer occurs, then it impedes thermal
decoloration through ring closure to SP. By analyzing available pKa
data on model compounds (acetic acid and p-nitrophenol) we concluded
that in the protic solvent methanol pKa(CH COOH) < pKa(p-nitro-
3
phenol) and as a result, in methanol proton transfer from RꢁCOOH
ꢁ
to ArO occurs thus impeding ring closure. On the contrary, in polar
aprotic solvents (DMSO, DMF, acetone) pKa(CH COOH) > pKa
3
(
p-nitrophenol) and as a result this proton transfer does not occur,
leaving the rate of ring closure unaffected.
In presence of organic bases in methanol, we propose the mech-
anism shown in Figure 3.
In methanol, where the proton transfer occurs, the acceleration of
thermal decoloration reaction is explained by the fact that the phenolic
proton in III (Figure 3) is abstracted by the added base yielding the
aryloxide IV, which undergoes reversion to the closed isomer.

S-M Equilibrium of Organic Acids and Bases
41=[341]
FIGURE 3 Proposed mechanism for acceleration of thermal reversion from
MC to SP in methanol when bases are added.
On the other hand, in acetone where the proton transfer does not
occur, the major isomer present is the phenoxide II (Fig. 4) which is
expected to revert rapidly to the closed isomer. Addition of tertiary
base yields the ammonium salt which forms an ion pair with
the carboxylic acid and phenoxide anions IP-IV. The latter provides
stabilization of the merocyanine (MC) isomer when base is added.
When organic acid (TFA) is added in methanol, the protonation of
phenoxide anion V-VI (Fig. 5), slows down the thermal reversion of
MC to SP in all cases. One uncertainty in experiments with TFA arises
from the possibility of reaction of TFA with methanol which was
reported previously in the literature [7]. To the extent that this occurs,
the actual concentration of TFA will be lower than expected. However,
the trend towards stabilization of the MC isomer due to protonation is
still valid. On the other hand, TFA is not known to react with other
organic solvents like THF or acetone. We are currently investigating
the effect of TFA in such solvents.

4
2=[342]
N. Xiao et al.
FIGURE 4 Proposed mechanism for slowing down of thermal reversion reac-
tion in acetone when base is added.
Polymeric Films
The next step was to extend the stabilization mechanism by organic
acids to polymer films. Polymer films were prepared on glass
substrates by spin-coating solutions containing SP(1.1)CH3, poly-
methyl-methacrylate (PMMA) and TFA in tetrahydrofuran as solvent.
As seen from Figure 6 (left side), the addition of TFA resulted in a
significant blue shift of the absorption maximum of MC isomer from
5
75 nm to about 415 nm. Decay tests after illumination with UV light
showed significantly increased stability of the MC isomer in presence
of TFA in the dark, when compared with the sample having no organic
acid added. For example, after 90 min. the sample containing TFA is
still showing an absorbance close to 100% of the initial value, while
the reference sample (no TFA) decayed to about 20% of its initial value
(
Fig. 6 right side).

S-M Equilibrium of Organic Acids and Bases
43=[343]
FIGURE 5 Mechanism in presence of acids.
þ
FIGURE 6 UV-VIS absorption spectra of MC and MCH in PMMA film (left)
and decay of colored state (MC) in PMMA films in presence of TFA and with no
TFA (right).
Conclusions
The objective of this work was to investigate new approaches for
stabilizing the MC isomer in solutions and in polymer films. We have
found that depending on the type of MC used and of the nature of

4
4=[344]
N. Xiao et al.
solvent, the addition of bases could slow down or accelerate the ther-
mal reversion reaction. In contrast, addition of acids always slowed
down the decay of MC isomer. Mechanisms explaining these different
behaviors were proposed.
Stabilization of MC isomer in polymeric films by added organic acid
was also demonstrated. This result is of interest for fabrication of
optical switches with improved bistability. Next steps in this work
involve extension this study by taking into account pKa of excited
states of the species involved in these processes.
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