Two coupled photochromic systems of 3′,4′-(methylenedioxy) flavylium: Kinetic and thermodynamic characterization

Article abstract of DOI:10.1039/b405346j

The chemistry and photochemistry of 3′,4′-(methylenedioxy) flavylium was studied by means of UV-Vis spectrophotometry, ¹H NMR, stopped flow, and continuous irradiation, in acidic and basic aqueous solutions. Six species were identified: the flavylium cation, AH⁺; the hemiketal, B; the cis- and the trans-chalcones, Cc and Ct, and their ionized forms. Cc^- and Ct^-. These species define a multiequilibria network whose kinetics and thermodynamics were completely characterized. The two pairs of chalcones define two coupled photochromic systems, respectively in acidic and basic media, both allowing cycles capable of writing, reading and erasing to be defined.

Full text of DOI:10.1039/b405346j

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P A P E R
Two coupled photochromic systems of
30,40-(methylenedioxy)flavylium: kinetic and
thermodynamic characterizationw
´ ´
Damian Fernandez, Filipe Folgosa, A. Jorge Parola and Fernando Pina*
´
REQUIMTE/CQFB, Departamento de Quımica, Faculdade de Ciencias e Tecnologia,
Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. E-mail: fjp@dq.fct.unl.pt
ˆ
Received (in Durham, UK) 8th April 2004, Accepted 4th June 2004
First published as an Advance Article on the web 16th August 2004
The chemistry and photochemistry of 30,40-(methylenedioxy)flavylium was studied by means of UV–Vis
1
spectrophotometry, H NMR, stopped flow, and continuous irradiation, in acidic and basic aqueous
solutions. Six species were identified: the flavylium cation, AH1; the hemiketal, B; the cis- and the
trans-chalcones, Cc and Ct, and their ionized forms, Ccꢀ and Ctꢀ. These species define a multiequilibria
network whose kinetics and thermodynamics were completely characterized. The two pairs of chalcones
define two coupled photochromic systems, respectively in acidic and basic media, both allowing cycles
capable of writing, reading and erasing to be defined.
ing to Scheme 1, AH1 should be the prevalent species in very
Introduction
acidic solutions; B, Cc and Ct are present at intermediate pH
values and, Ccꢀ and Ctꢀ, at more basic pH values. In terms of
the kinetics of conversion between the aforementioned species,
deprotonation is the fastest process whose rate can not be
followed by stopped flow methods. On the other hand, tauto-
merization and hydration can be easily followed by this last
technique. Finally, the thermal cis–trans isomerizations are
slow enough to the monitored by a standard UV–Vis spectro-
photometer.
One important feature of photochromic compounds is their
capacity to switch, by the action of light, between two different
states exhibiting different molecular shapes and electronic
configurations.1,2 A photon is simultaneously a bit of informa-
tion and a quantum of energy. In Nature, the information of
the light captured by a photochromic compound is transferred
and amplified by the complex machinery of the living organ-
isms, leading, for example, to vision, phototropism or photo-
movements.
In recent years, the photochromic properties of synthetic
flavylium compounds, in particular their multistate/multifunc-
tional properties, have been the object of a systematic study.3,4
The most important feature of these photochromic compounds
is their pH dependent photoresponse. This behaviour permits
one to obtain different states exhibiting different properties
according to sequences of pH and light stimuli.
Another interesting aspect of synthetic flavylium compounds
is the possibility of tuning their thermal and photochemical
properties, by changing the nature and position of the sub-
stituents. In the present work, the multistate/multifunctional
behaviour of the 2-aryl-substituted benzo[b]pyrylium salt 30,40-
(methylenedioxy)flavylium tetrafluoroborate is reported. It
was observed that this compound shows a network of chemical
equilibria similar to those observed for other reported flavy-
lium salts,3–8 Scheme 1. However, the presence of two kinetic
barriers between the two pairs of chalcones confers great
novelty on this system, with the possibility of defining two
pH coupled write–read–erase cycles.
Results and discussion
Thermodynamic equilibrium
In the pH range 1–4, the spectral modifications observed for
30,40-(methylenedioxy)flavylium are reported in Fig. 1a. At
pH ¼ 1.0, the dominant species is the flavylium cation, whose
absorption decreases with increasing pH, leading to the
In acidic aqueous solution, it is possible to distinguish four
species for this compound: the flavylium cation (AH1); the
hemiketal (B), obtained by hydration of AH1; the cis-chalcone
(Cc), resulting from tautomerization of B; and the trans-
chalcone (Ct), due to the cis–trans isomerization of Cc. In basic
aqueous solutions, ionized cis- and trans-chalcones (Ccꢀ and
Ctꢀ), obtained by deprotonation of the hydroxyl group, can
be observed in the equilibrium or as transient species. Accord-
{ Electronic supplementary information (ESI) available: experimental
data. See http://www.rsc.org/suppdata/nj/b4/b405346j/
Scheme 1
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 2 1 – 1 2 2 6
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Fig. 1 Spectral variations of 30,40-(methylenedioxy)flavylium 2.0 ꢁ
10ꢀ5 M at the equilibrium: a) pH range 1 to 4, AH1 transforming
into Ct; b) pH range 7 to 11, Ct ionizing to Ctꢀ.
Fig.
2
Spectral variations of 30,40-(methylenedioxy)flavylium
2.0 ꢁ 10ꢀ5 M in the pseudo equilibrium: a) pH range 1 to 6, AH1
transforming into B þ Cc; b) pH range 7 to 12, Cc ionizing to Ccꢀ.
characteristic spectrum of the trans-chalcone at pH ¼ 4.0, see
decreases with increasing pH, with the concomitant appear-
ance of a new absorption band which can be assigned to the
cis-chalcone, in equilibrium with the hemiketal, see Fig. 2a.
When higher pH values are considered, Fig. 2b, the absorp-
tion spectra of cis-chalcone leads to another absorption band
whose characteristics clearly indicate that the ionized cis-
chalcone is forming. This assignment was confirmed by 1H
NMR, as referred to above.
The absorption spectra shown in Fig. 2 can be maintained
without major modifications for a fairly long period of time at
room temperature. This is due to the existence of large kinetic
barriers between Cc and Ct at intermediate pH values and, Ccꢀ
and Ctꢀ at basic pH values. This situation also permits
consideration of a pseudo equilibrium, described by eqns. (3)
and (4), which is very useful to fully characterize the system.
eqn. (1).
AH1 " Ct þ H1
The trans-chalcone can be deprotonated at higher pH values,
as reported in Fig. 1b, according to eqn. (2).
pKa1 ¼ 2.75
(1)
Ct " Ctꢀ þ H1
pKCt ¼ 8.60
(2)
At the thermodynamic equilibrium, only three species AH1
(pH o 1), Ct (4 o pH o 7) and, Ctꢀ (pH 4 10) are observed.
These results were confirmed by 1H NMR. At pH ¼ 1.0, the 1H
NMR spectrum is compatible with the existence of a single
species that corresponds to the flavylium cation (see ESIw). At
basic pH values, by dissolving 30,40-(methylenedioxy)flavylium
1
at pH ¼ 11, a H NMR spectrum is initially obtained that is
compatible with the presence of Ccꢀ, as shown by the value of 3
JH3–H4 ¼ 12.4 Hz, characteristic of ionized cis-chalcones (see
ESI).6 This solution, either by heating at 60 1C (see below, Fig.
3b) or irradiating at 313 nm (see below, Fig. 7c), leads to the
formation of a new species that is assigned to Ctꢀ on the basis
of the large scalar coupling constant between protons 3 and 4, 3
JH3–H4 ¼ 15.7 Hz, characteristic of conjugated trans double
bonds (see ESI).5,6
AH1 " B þ Cc þ H1
B þ Cc " Ccꢀ þ H1
pKa2 ¼ 3.75
pKa3 ¼ 9.50
(3)
(4)
Thermal kinetics
Fig. 3 shows the spectral modifications corresponding to the
conversion of the pseudo equilibrium into the final equili-
brium. At pH ¼ 6.3, the Cc species is converted into the Ct
species. In order to confirm that the conversion was almost
complete, a pH jump on the final Ct solution to pH ¼ 1.0 was
carried out. No significant spectral modifications were ob-
served, showing that the amount of Cc at the equilibrium at
pH ¼ 6.3 is negligible (otherwise Cc would rapidly be converted
Pseudo equilibrium
In Fig. 2, the spectral variations obtained immediately upon
preparation of a set of solutions at different pH values are
shown. In the pH range up to pH ¼ 6, the spectra clearly show
that the characteristic absorption band of flavylium cation
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Fig. 4 a) Traces of the absorbance measured at 459 nm (AH1
absorption) obtained upon a pH jump from solutions freshly prepared
at pH ¼ 6.0 to more acidic values; b) representation of the pH
dependent process as a function of proton concentration.
Fig.
3
Spectral modifications of 30,40-(methylenedioxy)flavylium
2.0 ꢁ 10ꢀ5 M, showing the conversion of species at pseudo equilibrium
to the final equilibrium: a) Cc isomerizes to Ct in ca. 17 min, at
pH ¼ 6.3 and 60 1C; b) Ccꢀ isomerizes to Ctꢀ in ca. 40 min, at
pH ¼ 12.0 and 60 1C.
of 1.01 ꢁ 104
M
ꢀ1sꢀ1. As pointed out by McClelland and
Gedge,9 these results can be interpreted by considering B in
fast exchange with Cc at the pseudo equilibrium (pH ¼ 6.0). On
this basis, the more strongly pH dependent process (ii) can
be attributed to the hydration–dehydration reaction
(AH1 þ H2O " B þ H1), whose rate is kh þ kꢀh[H1] and, by
into AH1). At room temperature, the conversion of the Ct
species at pH ¼ 1.0 into the thermodynamically stable AH1
takes several days to be completed.
An identical process can be observed at pH ¼ 12.0. In this
case, the Ccꢀ species obtained during the pseudo equilibrium is
converted into its Ctꢀ isomer. Acidification to pH 1 of this
solution leads to the absorption spectrum of Ct without forma-
tion of any AH1, which indicates that the thermal conversion
from Ccꢀ to Ctꢀ is also complete in these conditions.
The determination of the activation energies for the two
kinetic barriers, between Cc and Ct in moderately acidic media
and, between Ccꢀ and Ctꢀ in basic media, was carried out
through Arrhenius plots, leading to values of 97 kJ molꢀ1 and
106 kJ molꢀ1, respectively.
consequence k ¼ 1.01 ꢁ 104
M
ꢀ1sꢀ1. Unfortunately, kh can
ꢀh
not be calculated with accuracy from this experiment, but a
value of kh ¼ 1.2 sꢀ1 can be obtained from the thermodynamic
equilibrium constant, Kh ¼ kh/kꢀh, see below. On the other
hand, the slightly pH dependent kinetic process (i) should be
due to the tautomerization reaction (B " Cc). At higher pH
values, the hydration–dehydration reaction becomes the rate
determining step of the global process and consequently only
one pH dependent trace, corresponding to a first order kinetic
process, is observed. Inspection of Fig. 4a clearly shows that at
pH ¼ 1.6 and pH ¼ 2.2, the amplitude of the faster process
(hydration–dehydration) is practically the same, as expected if
all the B species is consumed to give AH1. In addition, the
formation of different final plateaus in the slow process is a
consequence of the decrease of the AH1 species at the final
pseudo equilibrium, with increasing pH, according to the data
reported in Fig. 2a. One important aspect of Fig. 4a is that the
amplitudes of the traces of both processes reported at pH ¼ 1.6
and 2.1, allow calculation of the pseudo equilibrium constant
between B and Cc, Kt ¼ 0.49. McClelland and Gedge observed
that in the case of 2-(phenyl)benzo[b]pyrylium (unsubstituted
flavylium) and the parent compounds substituted in position 40
with methoxy or methyl groups, the tautomerization rate is
Stopped flow experiments
In order to characterize the kinetic processes occurring in the
present system, a series of pH jumps were performed and the
respective variations of the absorbance followed by stopped
flow methods. A solution of the compound freshly prepared at
pH ¼ 6.0 (containing B and Cc at equilibrium) was submitted
to pH jumps to lower pH values, Fig. 4a. When the final pH is
lower than ca. 2.5, two different processes can be identified,
both following first order kinetic laws: i) one of the processes is
slightly dependent on the final pH, with a rate constant of 0.84
sꢀ1 at pH ¼ 2.1 and 1.5 sꢀ1 at pH ¼ 1.6; ii) the other process
varies linearly with proton concentration (Fig. 4b) with a slope
constituted by one term which is pH independent, 1.78 sꢀ1
,
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Table 1 Equilibrium constants for 30,40-(methylenedioxy)flavylium in
aqueous solutions, at 23.0 1C
Kh
Kt
Ki
KCc
KCt
K0i
1.2 ꢁ 10ꢀ4
M
0.49
30
9.6 ꢁ 10ꢀ10
M
2.5 ꢁ 10ꢀ9
M
75
Energy level diagram and mole fraction distribution
To completely characterize the network of reactions repre-
sented in Scheme 1, equilibrium constants for each process
must be evaluated. Taking into account reactions (1)–(4) and
the fast equilibrium between B and Cc, it is easy to demonstrate
(see ESIw) that the fittings on the insets of Figs. 1 and 2,
together with the value of Kt ¼ 0.49 calculated from the
stopped flow experiments, allows us to obtain all the relevant
thermodynamic data for the system, Table 1.
The self consistency of the data can be tested by calculating
the value of the equilibrium constant pKa1 ¼ 2.71 from the
data of Table 1, which compares well with the experimental
value of 2.75.
The equilibrium constants of Table 1 can be combined (i)
with the data from the stopped flow analysis to obtain rate
constants for the hydration–dehydration and tautomerization
processes and, (ii) with the data from thermal cis–trans iso-
merization kinetics to obtain rate constants for the two cis–
trans isomerizations. The kinetic data are collected in Table 2.
As reported previously for other flavylium salts, the data
shown in Table 1 allow an energy level diagram of the network
to be obtained, Scheme 2.3 In this particular case, the scheme
was extended to species existing at basic pH values. This type
of representation allows to visualize which species should be
predicted for a given pH value at the equilibrium, as well as at
the pseudo equilibrium. Another alternative to visualise the
system is the representation of the mole fraction distribution of
the equilibrium and pseudo equilibrium, shown in Fig. 6. In
this figure, the full lines represent the mole fraction distribution
at the equilibrium, while the dotted lines the same for the
pseudo equilibrium. If, for instance, the compound is dissolved
at pH ¼ 6, the system evolves from a mixture of ca. 65%
B þ 35% Cc at the pseudo equilibrium to a final equilibrium
containing only Ct.
Fig. 5 a) Traces of the absorbance measured at 459 nm (AH1
absorption) obtained upon a pH jump from solutions freshly prepared
at pH ¼ 1.0 to the basic pH region; b) representation of the pH
dependent process as a function of the hydroxide concentration.
0.96 sꢀ1 and 0.46 sꢀ1, respectively, and another one which
increases linearly with increasing proton concentration, 42, 39
and 44 Mꢀ1sꢀ1.9 In the present system, we obtained for the pH
independent term 0.54 sꢀ1, and for the other term, 38 Mꢀ1sꢀ1
.
Photochemistry
Taking into account that in our experiment the equilibrium
between Cc and B is not reached, since B is immediately
consumed to give AH1, kꢀt should be equal to 0.54 sꢀ1, leading
to kt ¼ 0.26 sꢀ1, from the tautomeric equilibrium constant, Kt,
determined above, see Table 2.
The compound 30,40-(methylenedioxy)flavylium presents two
photochromic systems depending on pH, i.e., Cc " Ct (Sys-
tem I), and Ccꢀ " Ctꢀ (System II), each one exhibiting a
different behaviour. The photochemistry of System I is re-
ported in Figs. 7a and 7b. As mentioned above, the thermal
barrier permits the Ct form to be obtained in a metastable
situation, even in very acidic solutions. However, irradiation at
313 nm of the Ct species (pH ¼ 1.3) leads to the formation of
AH1, the thermodynamically stable species at this pH value,
with a quantum yield of F ¼ 0.02. On the other hand, irradia-
tion at 313 nm and pH ¼ 6.3 of the B þ Cc species (pseudo
equilibrium) leads to the formation of a photostationary state
between Cc and the Ct species, since acidification to pH ¼ 1.0
of the photostationary state gives rise to the formation of some
flavylium cation. The amount of flavylium cation permits
calculation of the ratio Cc/Ct ¼ 0.27 at the photostationary
state. The quantum yield of the photochemical conversion of
Cc into Ct, calculated from the initial points, is F ¼ 0.19. From
this value and the molar absorption coefficients of both species
at 313 nm, the quantum yield of the reverse photoreaction
Ct - Cc is calculated as F ¼ 0.02, in total agreement with the
value obtained for the same photoreaction at pH ¼ 1.3. At
basic pH values, photochromic system 2 (Fig. 7c) is character-
ized by a Ccꢀ - Ctꢀ quantum yield of 0.10 (pH ¼ 12). This
Another series of pH jumps from equilibrated solutions at
pH ¼ 1.0 to less acidic pH values was also performed. For
example, if the final pH after the pH jump is 2.9, a single first
order process with rate constant equal to 0.90 sꢀ1 was mea-
sured. At this pH value, kobs for the hydration–dehydration
reaction is predicted to be ca. 1.2 þ 1.01 ꢁ 104 ꢁ 10ꢀ2.9
¼ 14 sꢀ1. Consequently, the rate determining step of the global
process is the tautomerization reaction, whose calculated rate
is 0.26 þ (0.54 þ 38 ꢁ 10ꢀ2.9) ¼ 0.85 sꢀ1, in good agreement
with the experimental value. Finally, the traces of the pH
jumps from solutions at pH ¼ 1.0 (AH1) to basic pH values,
Fig. 5a, can be fitted according to a first order kinetic process,
with rates that are linearly dependent on the hydroxyl
concentration, with a slope of 2 ꢁ 105
M
ꢀ1sꢀ1 and an inter-
cept of 1.2 sꢀ1, see Fig. 5b. These constants compare well
with those previously reported for the compound 40-methoxy-
flavylium,9 1.8 ꢁ 105
M
ꢀ1sꢀ1 and 1.38 sꢀ1, respectively. The
value of the intercept should reflect the hydration reaction
due to water attack, and as expected it is coincident
with kh.
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Table 2 Rate constants for 30,40-(methylenedioxy)flavylium in aqueous solution
kh/sꢀ1Mꢀ1a
k
ꢀh/sꢀ1a
kt/sꢀ1a
k
ꢀt/sꢀ1a
0.54
ki/sꢀ1b
k
ꢀi/sꢀ1b
k0i/sꢀ1c
k0ꢀi/sꢀ1c
7.1 ꢁ 10ꢀ7
1.2
1.01 ꢁ 104
0.26
5.8 ꢁ 10ꢀ6
1.9 ꢁ 10ꢀ7
5.3 ꢁ 10ꢀ5
c
a
b
At 23 1C. At 40 1C, assuming that the equilibrium constant is the same as the one determined at 23 1C and pH ¼ 6.0. The same as b) at
pH ¼ 11.8.
Scheme 2
photoreaction proceeds to completion, since acidification of
the final irradiated solution to pH ¼ 1.0 shows no presence of
flavylium.
Two coupled photochromic systems
The network of thermal and photochemical reactions of the
compound 30,40-(methylenedioxy)flavylium is shown in Fig. 8.
In the pH range 1 to 6, a cycle capable of write–read–erase can
be conceived. The starting point is the metastable Ct species at
pH ¼ 1.0. Using near-UV light, Ct is converted into AH1. The
system can be erased by a pH jump to pH ¼ 6.0 in order
produce Cc (in equilibrium with B), which is thermally recon-
verted into Ct. To prepare the system for a new cycle, a second
pH jump to 1 should be performed. The other photochromic
system is eventually more interesting because there is the
possibility of two consecutive write steps. Starting for example
with Ccꢀ at pH ¼ 11, the first write step consists of the
irradiation of this species that totally converts into Ctꢀ.
The system can be erased through a pH jump to 1 leading to
the metastable Ct species. At this point a second write step (the
same as the previous cycle) is available, converting the system
into AH1. This last reaction can also be carried out thermally.
Finally, the system should be submitted to a second pH jump
to the starting pH value, so that Ccꢀ species is recovered.
Experimental
30,40-(Methylenedioxy)flavylium tetrafluoroborate was pre-
pared from condensation of 30,40-methylenedioxyacetophe-
none (0.49 g, 3 mmol) and salicylaldehyde (0.37 g, 3 mmol),
on the basis of a method described by Katritzky for similar
Fig. 6 Mole fraction distribution of the several species of the com-
pound 30,40-(methylenedioxy)flavylium as a function of pH: full lines:
thermodynamic equilibrium; dotted lines: pseudo equilibrium.
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 2 2 1 – 1 2 2 6
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Fig. 7 Spectral variations upon irradiation of at 313 nm of 30,40-(methylenedioxy)flavylium 2.0 ꢁ 10ꢀ5 M: a) Ct form at pH ¼ 1.3; b) B þ Cc species
at pH ¼ 6.3; c) Ccꢀ species at pH ¼ 12.0. Acidification to pH ¼ 1.0 at the end of the photochemical reaction leads to the formation of some AH1 in
the case of b) (trace line) but not in the case of c) (not shown).
compounds.10 The reagents were dissolved in acetic acid
(5 mL) and tetrafluoroboric acid (1 mL) was added. Acetic
anhydride (4 mL) was then added, keeping the temperature
below 60 1C (B10 min). The solution was stirred overnight.
The red solid that precipitated was filtered, washed with ethyl
acetate, and dried under vacuum; 0.27 g (27%). Elemental
analysis (Thermo Finnigan–CE Instruments EA 1112): found
(calc. for C16H11BF4O3) %C 56.4 (56.8), %H 2.9 (3.2). 1H
NMR: see ESI.w
The stopped flow experiments were collected in a SFM-300
spectrophotometer, controlled by a MPS-60 unit (Bio-Logic)
and the data were collected by a TIDAS diode array (J&M),
with wavelength range between 300 and 1100 nm, all connected
to a computer. The standard cuvette has an observation path
length of 1 cm. For this experiments the dead time of each
shot was previously determined to be 6.4 ms with a 7 mL sꢀ1
flow rate.
All experiments were carried out in aqueous solutions. The
pH was adjusted by addition of HCl and NaOH, or buffer, and
was measured using a Meterlab pHM240 pH meter from Radio-
meter Copenhagen.
UV–Vis absorption spectra were recorded on a Shimadzu
UV2501-PC spectrophotometer.
Light excitation was carried out using a medium-pressure
mercury arc lamp (Helius), and the excitation bands were iso-
lated with interference filters (Oriel). The incident light inten-
sity was measured by ferrioxalate actinometry.11
Acknowledgements
Financial support from Fundac¸ ao para a Ciencia e Tecnologia
and FEDER (POCTI/QUI/47357/2002), Portugal; DF ac-
knowledges the financial support provided through the Eur-
opean Community’s Human Potential Programme under
contract HPRN-CT-2000-00029 [Molecular Level Devices
and Machines].
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Fig. 8 The network of thermal and photochemical reactions of the
compound 30,40-(methylenedioxy)flavylium allows two write–read–
erase cycles to be defined.
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