Photo- and pH-induced transformations of flavylium cation: 'Write-lock- read-unlock-erase' cycles

Article abstract of DOI:10.1002/(sici)1099-0690(199911)1999:11<3199::aid-ejoc3199>3.0.co;2-x

The structural transformations of flavylium ion in aqueous solutions caused by pH jumps and photoexcitation have been investigated. At pH < 1, the stable form is the colored cationic species (AH⁺). By increasing pH, the concentration of AH⁺ decreases and, at pH = 5, this form is no longer present. The species obtained immediately after a pH jump undergo transformation processes with pH-dependent rate constants. At pH = 5.2 and 20 °C, the final product is the uncolored trans-chalcone (C(t)). This form can be transformed by light excitation into the cis-chalcone (C(c)) isomer, which is in equilibrium with the hemiacetal form (B(2)). This mixture is relatively inert due to the existence of a kinetic barrier that slows down the back thermal isomerization of C(c) to the stable C(t) form. Such a back reaction to C(t) can be totally prevented if the irradiated solution is submitted to a pH jump to pH = 1, which transforms the photoproducts into the stable AH⁺ species. In basic solution, two more species were detected, namely the anionic forms C(c)^- and C(t)^- of the cis and trans chalcone. C(t)^- is a stable, not photosensitive and luminescent species, whereas C(c)^- is not stable, being converted into C(t)^- in the dark. The photochemical and pH- induced transformations of the flavylium cation in the pH range 0-11 can be taken as a basis to design write-lock-read-unlockerase cycles for an optical molecular-level memory with multiple readout capacity.

Full text of DOI:10.1002/(sici)1099-0690(199911)1999:11<3199::aid-ejoc3199>3.0.co;2-x

FULL PAPER
Photo- and pH-Induced Transformations of Flavylium Cation:
“Write–Lock–Read–Unlock-Erase” Cycles
[a]
[a]
[b]
[b]
˜
Fernando Pina,* Maria Joao Melo, Mauro Maestri,* Paolo Passaniti,
Nadia Camaioni,[c] and Vincenzo Balzani[b]
Dedicated to Professor Fritz Vögtle on the occasion of his 60th birthday
Keywords: Flavylium salts / Photochromism / Reaction mechanism / Substituent effects
The structural transformations of flavylium ion in aqueous
Ct form. Such a back reaction to Ct can be totally prevented
solutions caused by pH jumps and photoexcitation have been if the irradiated solution is submitted to a pH jump to pH =
investigated. At pH Ͻ 1, the stable form is the colored
cationic species (AH+). By increasing pH, the concentration
of AH+ decreases and, at pH = 5, this form is no longer
present. The species obtained immediately after a pH jump
undergo transformation processes with pH-dependent rate
constants. At pH = 5.2 and 20 °C, the final product is the
uncolored trans-chalcone (Ct). This form can be transformed
by light excitation into the cis-chalcone (Cc) isomer, which is
1, which transforms the photoproducts into the stable AH+
species. In basic solution, two more species were detected,
–
namely the anionic forms Cc and Ct– of the cis and trans
chalcone. Ct– is a stable, not photosensitive and luminescent
–
species, whereas Cc is not stable, being converted into Ct–
in the dark. The photochemical and pH-induced
transformations of the flavylium cation in the pH range 0–11
can be taken as a basis to design write–lock–read–unlock–
in equilibrium with the hemiacetal form (B2). This mixture is erase cycles for an optical molecular-level memory with
relatively inert due to the existence of a kinetic barrier that
multiple readout capacity.
slows down the back thermal isomerization of Cc to the stable
complex structural transformations as represented in
Scheme 1. Such transformations, which are accompanied
by changes in the absorption and emission spectra, can be
induced by pH changes and/or by light excitation and can
be studied by using pH jump and continuous irradiation
as well as flash photolysis techniques.[14Ϫ16] Of particular
interest is the reversible photoinduced isomerization of the
thermally stable (in a certain pH range) trans-chalcone form
which, in the case of the 4Ј-methoxyflavylium ion, is the
starting point of a writeϪlockϪreadϪunlockϪerase pho-
tochromic cycle for information processing at the molecular
level.[7] More recently we have shown that, in the case of
4Ј-hydroxyflavylium ion, the light- and pH-induced trans-
formations can also be taken as a basis for simple logic
operations and create a remarkably intricate network of
chemical processes.[8]
Introduction
Anthocyanins are the largest water-soluble group of
natural molecules responsible for the brilliant colours found
in plants, flowers, and fruits.[1Ϫ3] Anthocyanins are attract-
ive and highly desirable as food colourants (jams, wines,
etc.),[1] and their farmaceutical properties are also begin-
ning to be exploited (e.g., for the treatment of cardiovascu-
lar diseases).[4]
Synthetic flavylium salts, exhibiting a basic chemical
structure identical to anthocyanins, are excellent model
compounds for the study of the chemical behaviour of natu-
ral anthocyanins and promising candidates for the use in
food-coloring industry.[2] Interesting is also the recent dis-
covery[7Ϫ10] that flavylium ions represent examples of mul-
tistate/multifunctional compounds potentially useful for the
design of molecular-level system for information pro-
cessing.[11Ϫ13]
Previous studies[7Ϫ10] have clearly shown that substitu-
ents in the 4Ј- or 7-positions have a strong influence in de-
termining the thermal and photochemical reactivity of the
different forms originating from synthetic flavylium ions. In
order to extend the knowledge of the reversible transform-
ation processes that take place in this family of compounds,
we have performed an investigation on the behaviour of the
unsubstituted flavylium cation. The thermal reactions of
this compound in acidic and neutral solutions had already
been investigated by McClelland and Gedge.[5] By using the
pH-jump technique, these authors were able to obtain the
values of the kinetic and thermodynamic constants con-
necting the various forms which are involved in the ob-
Systematic investigations[5][6] have shown that in acidic
and neutral aqueous solution flavylium ions can undergo
[a]
´
Departamento de Quımica,
´
Centro de Quımica Fina e Biotecnologia,
Universidade Nova de Lisboa,
P-2825 Monte de Caparica, Portugal
E-mail: fjp@dq.fct.unl.pt
[b]
[c]
Dipartimento di Chimica “G. Ciamician”,
`
Universita di Bologna
Via Selmi 2, I-40126 Bologna, Italy
E-mail: mmaestri@ciam.unibo.it
Istituto FRAE-CNR, via Gobetti 101, I-40129 Bologna, Italy
E-mail: camaioni@bofra3.frae.bo.cnr.it
Eur. J. Org. Chem. 1999, 3199Ϫ3207
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/1111Ϫ3199 $ 17.50ϩ.50/0
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F. Pina, M. J. Melo, M. Maestri, P. Passaniti, N. Camaioni, V. Balzani
FULL PAPER
served transformations. In this work, we have (i) reexam- the sum of the concentrations of B2, Cc, and Ct and molar
ined the thermal reactions of the unsubstituted flavylium fraction given by Equation (8):[15a]
cation in acidic and neutral solution, (ii) extended the study
to basic solutions, and (iii) investigated the effect of con-
tinuous and pulsed photoexcitation. The results have then
been elaborated on the basis of a previously reported kine-
tic method[14,15a] to obtain all the kinetic and thermo-
dynamic parameters that control the complex network of
observed reactions.
The individual expressions of the molar fractions of each
component of CB can be easily calculated as shown in
ref.[15a] If the pH is sufficiently high to consider all the com-
pounds in the form of CB (β ϭ 1), the relationships shown
Results and Discussion
Equilibria in the Dark
in Equation (9) apply:
It is well-known that in aqueous solution synthetic flavyl-
ium ions can undergo structural transformations involving
several forms connected by equlibria that can be displaced
by pH changes and light excitation.[1Ϫ10] As we will see be-
low, in the case of the unsubstituted flavylium ion as many
as seven species can be involved (Scheme 1). McClelland
and Gedge[5] investigated the thermal reactions of the flavy-
lium cation in acidic and neutral aqueous solutions and
identified the five species connected by the following equili-
bria:
Equations (1) to (7) concern the thermodynamic equilib-
rium. For several flavylium compounds the rates of the
thermal cis-trans isomerization reactions [Equation (4)] are
much slower than the rates of the reactions involved in the
other equilibria. In such a case, a pseudo-equilibrium can
be attained and Equations (10) to (13) can be used to evalu-
ate the pseudoequilibrium constant, indicated by K , and
a
the molar fractions:
Such species are the flavylium cation AHϩ, two hemiace-
tal forms B2 and B4 obtained by hydration of the flavylium
cation in the 2 or 4 position, respectively, the cis-chalcone
species Cc formed from hemiacetal B2 through a tautomeric
process, and the trans-chalcone form Ct resulting from the
isomerization of cis-chalcone. McClellandЈs results indicate
that the B4 species is always a small fraction (ca 1%) of B2,
so that B4 will be neglected in the following discussion.
As shown previously[14,15a] the molar fraction of the
acidic form AHϩ can be obtained from Equation (5):
We have reexamined the thermal behaviour of the unsub-
stituted flavylium cation extending our study to basic solu-
tions and we have found spectral evidence for the existence
Ϫ
of two more species, namely the anionic forms Cc and
CtϪ of the cis and trans chalcone.
Acid Solutions: In strongly acid solution (e.g., pH ϭ 0),
the flavylium cation AHϩ is the thermodynamically stable
form of the system. Its absorption spectrum (Figure 1a,
spectrum I) shows an intense band with λmax ϭ 394 nm
(ε ϭ 34000 Ϫ1 cmϪ1). On increasing the pH of the solu-
tion by a pH jump to pH ϭ 5.2, the band at 394 nm disap-
pears within the mixing time and the spectrum II of Figure
1a is obtained. In agreement with McClelland and Gedge,[5]
we associate this spectral changes with the transformation
of AHϩ into Cc and B2 (Scheme 1). Figure 2 shows the
spectral variations obtained after 5 minutes upon a series
where Co is the total concentration and KЈa is given by
Equation (6):
Equation (5) accounts for the complex equilibria de- of pH jumps from 1.0 to different final pH values. These
scribed by Equations 2 to 4 in terms of a single acidϪbase spectra are due to the presence of the three species AHϩ,
equilibrium [Equation (7)] between the acidic species AHϩ B2, and Cc, whose relative amounts depend on the final pH
and a conjugated base CB having a concentration equal to of the solution. The inset of Figure 2 shows the molar frac-
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Eur. J. Org. Chem. 1999, 3199Ϫ3207

Photo- and pH-Induced Transformations of Flavylium Cation
FULL PAPER
Scheme 1. Structural transformations taking place in flavylium compounds. The compound investigated in this paper is the unsubstituted
member of the family (R7 ϭ R4 ϭ R4Ј ϭ H)
tion of AHϩ as a function of the final pH, calculated from
the absorbance changes at 394 nm. A best fitting procedure
on this plot gives a pKa value of 3.0 for the dissociation
equilibrium of AHϩ to give the B2 and Cc species. On a
much longer time scale (half life 16 hours), a stable product
is formed the spectrum of which (curve III in Figure 1a)
shows absorption maxima at 302 and 354 nm. This second,
slower equilibration process is assigned to the conversion of
AHϩ into the thermodynamically stable product Ct through
the Cc/B2 mixture (Scheme 1). Figure 3 reports the spectral
variations observed as function of time upon a pH jump to
pH ϭ 2.6 and shows that the observed conversion follows
first order kinetics. A similar behaviour is observed at any
other pH value in the acid region. The values of the rate
constants are 4.2 ϫ 10Ϫ6 sϪ1 at pH ϭ 2.6, 9.8 ϫ 10Ϫ6 sϪ1
at pH ϭ 3.7, and 1.5 ϫ 10Ϫ5 sϪ1 at pH ϭ 5.0, showing that
the rate of formation of Ct is pH dependent. The molar
fraction of Ct present at the final equilibrium at the various
pH values and Equation (8) allow us to calculate the equi-
librium constant between AHϩ and Ct (pKЈa ϭ 1.64). The
reaction leading to Ct is also temperature dependent and
exhibits an activation energy of 89 kJmolϪ1 at pH ϭ 5.2.
Figure 1. Absorption spectra of the flavylium ion under different
experimental conditions. (a) Acidic or neutral solutions: I, pH ϭ
0, where only the thermodynamically stable species AHϩ is present;
II, pseudoequilibrium after a pH jump to pH ϭ 5.2, where the Cc
and B2 species are present; III, spectrum of the thermodynamically
stable product Ct formed upon equilibration after a pH jump to
pH ϭ 5.2. (b) basic solutions: III, see above; IV, spectrum obtained
after a pH jump from 5.2 to 11.0, assigned to the form CtϪ; V,
spectrum obtained immediately after a pH jump from 1.0 to 11.0,
Ϫ
assigned to the form Cc
.
shows the molar fraction distributions of the various spec-
Using the kinetic treatment illustrated above, the data ob- ies present at 20°C in aqueous solution in the pH range
tained by McClelland and Gedge,[5] and the results of our 0Ϫ7. The thermodynamically stable species are AHϩ at pH
investigation we have obtained the plots of Figure 4 which < 1 and Ct at pH > 3, while B2 and Cc are present as transi-
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F. Pina, M. J. Melo, M. Maestri, P. Passaniti, N. Camaioni, V. Balzani
FULL PAPER
pH jump from pH ϭ 5.2 to pH ϭ 11.0 of a solution of Ct
causes dramatic spectral variations ending with the spec-
trum IV of Figure 1b. We associate these spectral changes
with the deprotonation of Ct to give CtϪ (Scheme 1). Figure
6 shows the spectral variations obtained upon a series of
pH jumps from 5.2. to different final pH values. The inset
of Figure 6 shows the molar fraction of Ct as a function of
pH. A best fitting procedure on this plot gives a pKa of 8.5
for the deprotonation equilibrium of Ct.
Figure 2. Absorption spectra of aqueous solutions of the flavylium
cation (3.9 ϫ 10Ϫ5 ) as a function of pH, few minutes after the
pH jumps. The inset shows the molar fraction of AHϩ as a function
of the final pH, calculated from the absorbance changes at 394 nm
Figure 3. Spectral variations observed in the dark for an aqueous
solution of the flavylium cation (2.5 ϫ 10Ϫ5 ) upon a pH jump
from pH ϭ 1.0 to pH ϭ 2.6 as a function of time. The inset shows
that the change in absorbance at 394 nm, i.e. the conversion of the
AHϩ, Cc, and B2 mixture to Ct, follows a first order kinetics.
ent species between pH 2 and 7. The behaviour of the sys-
tem in the pH range 0Ϫ7 can be clearly understood on the
basis of the energy-level diagram shown in Figure 5.
Figure 5. Energy-level diagram for the species involved in the trans-
formation of the flavylium cation in acidic solutions
Figure 6. Absorption spectra of dark equilibrated aqueous solu-
tions of Ct (6.0 ϫ 10Ϫ5 ) as a function of pH. The inset shows the
molar fraction of the Ct species as a function of pH, calculated
from the absorbance changes at 432 nm, and the equilibrium con-
stant for the deprotonation of Ct
Figure 4. Molar fraction distributions of the species involved in the
transformation of the flavylium cation in acidic or neutral region.
Solid lines refer to the species involved in the thermodynamic equ-
ilibrium. Dashed lines refer to the pseudoequilibrium obtained by
pH jump from strongly acidic conditions to higher pH values or
by exciting Ct by flash light.
A jump from pH ϭ 1.0 to pH ϭ 11.0 of a solution of
Basic Solutions: As reported above, at pH ϭ 5.2 the AHϩ causes spectral changes during the mixing time yield-
thermodynamically stable species is the Ct form the spec- ing the spectrum V of Figure 1b. Subsequently, in the mi-
trum of which is again reported in Figure 1b, curve III. A nutes time domain, the system undergoes the spectral vara-
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Photo- and pH-Induced Transformations of Flavylium Cation
FULL PAPER
tions reported in Figure 7. The final spectrum is identical
to the spectrum of CtϪ obtained by a pH jump to 11.0 of
a Ct solution. A possible explanation for this behaviour is
the following: the pH jump to 11.0 causes a fast transform-
ation of AHϩ to CcϪ, that subsequently undergoes a slower
conversion process to CtϪ. A similar behaviour was pre-
viously observed for the 4Ј-hydroxyflavylium ion in basic
solution. Attempts were made to measure the pKa for the
dissociation equilibrium of Cc to give CcϪ, but no clear re-
sult was obtained, probably because of the concomitant oc-
currence of the processes indicated in equations 14Ϫ16 that
prevents a clear assignment of the spectral variations. How-
ever, an approximate value for such an equilibrium constant
was estimated to be about 8.
Figure 8. Spectral variations observed upon continuous irradiation
(313 nm) of dark equilibrated aqueous solutions of Ct as a function
of time (initial time increments ϭ 30 s). (a) pH ϭ 2.0, concentration
4.7 ϫ 10Ϫ5 ; (b) pH ϭ 5.2, concentration 8.0 ϫ 10Ϫ5

to its pseudo equilibration with B2 and AHϩ. The relative
concentrations of the photoproducts Cc, B2, and AHϩ are
controlled by the pH of the solution (Figure 4, dashed
lines). At pH < 1, the only photoproduct is AHϩ. At
pH > 5 no AHϩ, is present in the mixture. Moreover, in-
spection of Figure 8b shows that at pH ϭ 5.2 the systems
reaches a photostationary state whose spectrum is different
from that expected for the mixture Cc/B2 at this pH (Figure
2). These results are consistent with an uncomplete photo-
conversion of Ct, caused by the concomitant Cc Ǟ Ct back
photoisomerization (see below) that takes place as the Cc
concentration increases. The quantum yield of the Ct Ǟ Cc
photoisomerization, measured after 1 minute of irradiation,
is 0.05 at both pH 2.0 and 5.2.
Figure 7. Spectral variations observed after a pH jump from 1.0 to
11.0 for an aqueous solution of the flavylium cation (6.0 ϫ 10Ϫ5 )
as a function of time. The inset shows that the change in absor-
bance at 440 nm follows a first order kinetics
As we have seen above, a pH jump from 1.0 to 5.2 on
AHϩ solutions leads immediately to an equilibrated mixture
of B2 and Cc that lasts for a time period sufficiently long to
allow the study of its photochemical behavior. Continuous
irradiation of this mixture at 313 nm causes an absorbance
increase at 302 and 345 nm, indicating the formation of Ct.
The apparent photochemical quantum yield for this reac-
tion, based on the total light absorbed by the mixture, is
ca. 0.1. According to Scheme 1 it is clear that only the Cc
species can be responsible for the photoreaction leading to
Photochemical Experiments
No spectral variation was observed upon 1 hour ir- Ct. Since at pH ϭ 5.2 Cc constitutes about 6% of the mix-
radiation of solutions of flavylium ion at pH ϭ 1.0, showing ture and presumably absorbs a comparable fraction of the
that AHϩ is photochemically stable. The same result was total absorbed light, it follows that the actual quantum
obtained in the case of CtϪ, whereas Ct and Cc were found yield of the cisǞtrans photoisomerization is likely close to
to undergo a reversible photoisomerization reaction.
unity.
Continuous Irradiation: Continuous irradiation of Ct with
Flash Photolysis: As pointed out in previous papers, flash
313 or 365 nm light causes spectral variations at all the pH photolysis experiments can be most useful to confirm and
values in the range 1Ϫ7. Figure 8 shows the absorbance complement the results obtained by the pH jump measure-
changes observed at pH 2.0 and 5.2 under irradiation with ments.[16]
313 nm light. Such spectral changes are the reverse of those
Dark equilibrated solutions of flavylium ion were ir-
observed in the thermal reactions which follow the pH radiated with a flash lamp (lifetime ca. 0.1 s) and the
jumps of AHϩ solutions from pH ϭ 1 to the same pH of changes in absorbance were monitored as a function of
the irradiated solution (see, e.g., Figure 3).
time. In Figure 9 the traces obtained at 394 nm (absorption
As shown by the flash photolysis experiments described maximum of AHϩ) are shown for the pH values 2.0 and
below, the primary product of the photochemical reaction 5.2. Like in the case of 4Ј-methoxyflavylium,[7] the ab-
of Ct is Cc (Scheme 1), which then undergoes the previously sorbance decay traces clearly show the presence of three
discussed thermal reactions leading in the second time scale consecutive kinetic processes. The first one, that can be bet-
Eur. J. Org. Chem. 1999, 3199Ϫ3207
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F. Pina, M. J. Melo, M. Maestri, P. Passaniti, N. Camaioni, V. Balzani
FULL PAPER
ter seen at pH ϭ 5.2, occurs within the flash and is too fast
to be monitored with our apparatus. It may be assigned to
the reaction of Ct to give Cc in equilibrium with B2. The
second process ends in a few seconds and can be assigned
to the formation of AHϩ from the equilibrated Cc/B2 mix-
ture. The third process (not shown in Figure 9), falls into
the time domain of hours and leads back to Ct since it
completely restores the absorbance values observed before
irradiation. The decay trace taken at 394 nm (Figure 9a)
shows that the formation of AHϩ at pH ϭ 2.0 occurs via a
first order process, whose rate constant was found to be pH
dependent. The rate constant value obtained at pH ϭ 2.0,
2.4 sϪ1, is in excellent agreement with the rate constant
found by McClelland and Gedge[5] upon the assumption
that the rate determining step of the process leading from
Cc to AHϩ is the tautomerization reaction that transform
Cc into B2.
Figure 10. Differential time-resolved spectra of dark equilibrated
aqueous solutions of Ct (5.0 ϫ 10Ϫ5 ) at 0.5 (᭹), 1.0 (ᮀ), 3.0 (᭡)
seconds after the flash. a) pH ϭ 2.0. b) pH ϭ 5.2. Full lines are
the final differential spectra obtained in continuous irradiation.
Write؊Lock؊Read؊Unlock؊Erase Cycles
It has been previously shown[7Ϫ10] that substituted flavyl-
ium ions can be taken as a basis for an optical memory
system with multiple storage and nondestructive readout
capacity through a writeϪlockϪreadϪunlockϪerase cycle.
Such a cycle, schematized in the shaded area of Figure 11,
combines two reversible processes that can be driven by me-
ans of two different stimuli (dual-mode systems). Photoex-
citation (write) converts the stable Ct species into the Cc/B2
mixture. A pH change (locks) converts the Cc/B2 mixture to
the kinetically inert AHϩ form, that can be read without
being erased. An opposite pH change (unlock) reconverts
AHϩ into the Cc/B2 mixture, that can be driven back (erase)
by a photochemical or thermal reaction to Ct. Working at
a suitable pH value (vide infra), locking would occur spon-
taneously, thus simplifying the operation of the system.
Figure 9. Spectral variations at 394 nm obtained under flash irradi-
ation of dark equilibrated aqueous solutions of Ct (8.0 ϫ 10Ϫ5 )
as a function of time. (a) pH ϭ 2.0; (b) pH ϭ 5.2. The right hand
scale in (a) refers to the kinetic treatment of the decay as a first
order process
Differential time-resolved spectra were obtained from
traces as those reported in Figure 9 by plotting the differ-
ence between the initial absorbance and the absorbance at
different delay times. These results show that the Cc formed
by light excitation is an unstable species in acid medium
(Figure 10a) since it disappears by a fast pH-controlled re-
action to give AHϩ, whereas it is relatively inert at pH>5
(Figure 10b) since it can only give a slow back reaction
yielding trans-chalcone.
Fluorescence Spectra: An equilibrated solution at pH ϭ
1.0, where the predominant species is AHϩ, excited at
394 nm yields an emission band with maximum at 446 nm,
similar to that previously observed for substituted flavylium
ions.[8] Excitation at 300 nm of an equilibrated solution at
pH ϭ 5.0, where the stable species is Ct, gave a weak emis-
sion with maximum at 443 nm. The large Stokes shift with
respect to the absorption band suggest that the geometry of
the excited state is different from that of the ground state.
Finally, a weak emission band with maximum at 625 nm
was observed on excitation at 430 nm of an equilibrated
solution at pH ϭ 11.0, where the stable species is the CtϪ
anion.
Figure 11. Write-lock-read-unlock-erase cycles for the flavylium
ions. For more detail, see text
The thermal and photochemical behaviour of the unsub-
stituted flavylium cation examined in this paper suggests
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Photo- and pH-Induced Transformations of Flavylium Cation
FULL PAPER
that also this compound can be used in a cycle like that this cycle auto-locking (and auto-unlocking) cannot occur
described above. In order to check the reversibility of this so that two pH changes per cycle are needed.
system, a series of light/dark cycles has been performed on
a 5.0 ϫ 10Ϫ5  aqueous solution of Ct at 60°C, at the autol-
ocking pH ϭ 2.2. The results obtained are reported in Fig-
ure 12. As it can be seen, the reversibility of the system is
Substituent Effects
not fully satisfactory.
The thermodynamic and kinetic constants for the struc-
tural transformations of the flavylium compounds investi-
gated in this and previous papers are collected in Table 1.
Inspection of these data shows that the substituents in the
4Ј or 7 positions have a noticeable effect on some of the
constants.
It is interesting to compare the properties of the four
compounds from the viewpoint of the writeϪlockϪrea-
dϪunlockϪerase cycle initiated by light excitation of Ct. In
a previous paper[9] we have shown that substitution in the
7-position lowers the kinetic barrier of the cisǞtrans ther-
mal isomerization (compare the ki values in Table 1). This
causes autoerasing of the written information and therefore
the 7-substituted compound is unsuitable for memory-stor-
age purposes. In the following, only the two 4Ј-flavylium
derivatives and the unsubstituted flavylium ion will be con-
sidered.
Figure 12. Changes in the absorbance at 394 nm (corresponding to
the maximum of the AHϩ form) observed for a 5.0 ϫ 10Ϫ5  aqu-
eous solutions of Ct at pH ϭ 2.2 and 60°C under 313 nm light
excitation (dashed lines) followed by dark periods (full lines). He-
avy dashed lines correspond to overnight dark periods.
As mentioned above, in such a writeϪlockϪreadϪunlockϪ
erase cycle “autolocking” takes place when irradiation of
A different cycle can be designed involving the anionic Ct is performed at a sufficiently low pH value (see also Fig-
species present in basic media (Figure 11). This cycle starts ure 5). If the chosen pH value is too low, however, the start-
at pH ϭ 11 with the CtϪ form that, being not photosensi- ing Ct form is already in equilibrium with a substantial
tive, can be read without writing. Then, two different paths amount of the AHϩ form where the written information
can be followed. The first one begins with a pH jump to has to be stored. The best compromise between these two
pH ϭ 6 which leads to Ct and goes on as described above opposite requirements is that offered by the pH value of
(shaded area in Figure 11). The second path starts with a the crossing point between the curves describing the molar
pH jump from 11 to 0, leading to Ct that can be photoch- fractions of Ct at the final equilibrium conditions and of
emically written (and locked because of the low pH) to AHϩ at the pseudoequilibrium conditions (see Figure 4).
AHϩ. In this form, the information can be stored perma- In the case of unsubstituted-, 4Ј-hydroxy-, and 4Ј-methoxy-
nently and read without erasing since AHϩ is thermally and flavylium ions such a crossing occurs at pH 2.2, 3.2, and
photochemically stable. When necessary, AHϩ can be un- 3.0, respectively. Therefore the comparison among the
locked by a pH jump to 11 and thermally erased to give properties of the three compounds will be made considering
back CtϪ. An advantage of this cycle lies in the possibility the properties of each system at the pH of that crossing
of reading the system in both the initial (nonwritten) and point, which in the following will be called autolocking
final (written) states without writing or erasing. Moreover, pH value.
CtϪ is more stable than Ct, so that durability of the system
As far as the writing function is concerned, its effective-
could increase. A disadvantage is given by the fact that in ness depends on: i) light absorption by Ct; ii) quantum yield
Table 1. Thermodynamic and kinetic constants for some structural transformations of synthetic flavylium compounds[a]
[b]
[d]
[e]
Substituent
7-OH
none[c]
4Ј-OH
4Ј-OMe
KЈa
Ka
Kh
Kt
2.0 ϫ 10Ϫ3
2.8 ϫ 10Ϫ4
8.0 ϫ 10Ϫ6
-
2.3 ϫ 10Ϫ2
1.26 ϫ 10Ϫ2 [f]
3.16 ϫ 10Ϫ6
3.6 ϫ 10Ϫ6
1
8.0 ϫ 10Ϫ2
9.8 ϫ 10Ϫ4
0.06
3.4 ϫ 10Ϫ5
0.50
Ki
500
400
3500
ca 100
kh
0.48 sϪ1 [a]
3 ϫ 104 sϪ1Ϫ1 [g]
0.57 sϪ1 [g]
8.3 ϫ 10Ϫ4 sϪ1
4.6 sϪ1
8.9 ϫ 10Ϫ2 sϪ1
2.5 ϫ 104 sϪ1Ϫ1 [g]
3.7 ϫ 10Ϫ5 sϪ1
< 10Ϫ7 sϪ1
0.47 sϪ1
kϪh
ki
4.7 ϫ 103 sϪ1Ϫ1
4.1 ϫ 10Ϫ4 sϪ1
1.1 ϫ 10Ϫ6 sϪ1
1.38 ϫ 104 sϪ1Ϫ1
5.8 ϫ 10Ϫ5 sϪ1
< 10Ϫ6 sϪ1
kϪi
[a]
[b]
[c]
[d]
Measured by means of pH jump techniques at 25°C, unless otherwise noted. Ϫ Ref.[9]
Ϫ
Ref.[5] and this work. Ϫ Refs.[6,8]
Ϫ
[e]
[f]
[g]
Refs.[5,7]
Ϫ
At 60 °C. Ϫ Measured by flash photolysis.
Eur. J. Org. Chem. 1999, 3199Ϫ3207
3205

F. Pina, M. J. Melo, M. Maestri, P. Passaniti, N. Camaioni, V. Balzani
FULL PAPER
of the transǞcis photoisomerization; iii) rate of the locking by pH jumps and continuous and flash photolysis exci-
process. Concerning the first point, the best wavelength for tation. The rate constants and equilibrium constants of the
writing is that corresponding to the maximum difference various processes have been obtained and compared with
between the molar absorption coefficients of the Ct and those previously reported for some flavylium derivatives. It
AHϩ species. Comparison of the absorption spectra (Figure is shown that substituents in the 4Ј or 7 positions have a
1; Figure 1 of ref.[8]; Figure 2 of ref.[7]) shows that the best strong influence in determining the thermal and photoch-
region for writing is around 310 nm for each compound and emical reactivity.
that the difference between the molar absorption coef-
WriteϪlockϪreadϪunlockϪerase cycles starting from the
ficients of Ct and AHϩ is about 15000 Ϫ1 cmϪ1 in all cases trans-chalcone (Ct) form and based on pH jumps and light
(ca. 12000 Ϫ1 cmϪ1 for the title compound). The quantum excitation can be designed for each flavylium compound.
yield of the transǞcis photoisomerization is very similar The cycle based on the 7-OH derivative undergoes autoer-
for the three compounds. As far as the locking process is asing. The cycle based on the unsubstituted flavylium ion
concerned, its rate depends on the rates of two consecutive shows some advantages compared with the analogous
steps, ring closure and dehydration (Scheme 1, Figure 5). cycles based on 4Ј derivatives. For the unsubstituted flavyl-
The rate determining step is the ring closure and its rate ium ion, a series of light/dark cycles has shown that the
constants, at the respective autolocking pH values, are 2.4 reversibility of the system is not fully satisfactory
s
Ϫ1, 0.22 sϪ1, and 0.63 sϪ1 for unsubstituted-, 4Ј-hydroxy-,
and 4Ј-methoxyflavylium, respectively (Table 1).
Experimental Section
The best wavelength for the reading function is that cor-
responding to the highest difference between the molar ab-
sorption coefficients of AHϩ and Ct. Such a wavelength is
394 nm (∆ε ϭ 30800) for the unsubtituted flavylium ion,
426 nm (∆ε ϭ 39000) for the 4Ј-hydroxy derivative, and
435 nm (∆ε ϭ 41000) for the 4Ј-methoxy derivative.
The erase function is performed in all cases at high tem-
perature (60°C) in order to overcome the activation energy
for the cisǞtrans isomeritation. The rate constants for AHϩ
disappearance, at the respective autolocking pH values, are
Flavylium perchlorate was prepared according to a published pro-
cedure.[17] All other chemicals were of analytical grade. The experi-
ments were carried out in water at 25°C. The pH of the solutions
was adjusted by addition of HCl (pH < 2) or buffer,[18] and meas-
ured by a Metrohom 713 pH meter. For 1H-NMR spectroscopy
experiments, the flavylium salt was dissolved in DCl ca. 0.5 .
When required, the pH was changed by addition of small aliquots
of NaOD 1.0 or 0.1 . pH measurements were made in the NMR
tube using an Ingold glass electrode. The reported pH values, de-
noted with an asterisk, are direct readings without correction for
the isotope effect.[19]
5.3 ϫ 10Ϫ4 Ϫ1, 1.1 ϫ 10Ϫ4 sϪ1 and 2.0 ϫ 10Ϫ4 sϪ1 for un-
s
substituted-, 4Ј-hydroxy-, and 4Ј-methoxy-flavylium,
respectively.
In conclusion, the unsubstituted flavylium cation, com-
pared to the 4Ј-derivatives, exhibits higher locking and eras-
ing rates, whereas the other properties are not substan-
tially different.
NMR spectroscopy[14,15a] and flash photolysis[16a,b] experiments
were performed as previously described. Absorption and emission
spectra were recorded by a PerkinϪElmer lambda 6 spectrophoto-
meter and a PerkinϪElmer LS 50 spectrofluorimeter. Photoexcit-
ation in continuous irradiation experiments was performed by
using a medium pressure mercury lamp equipped with interference
filters (Oriel) to isolate a narrow excitation band. The incident light
intensity was measured by ferrioxalate actinometry.[20]
Conclusions
The NMR spectrum at pH ϭ 1.0 was in perfect agreement with
the proposed structure, proving the purity of the flavylium salt. A
spectrum taken several minutes after a pH jump from 1 to 6
(pseudo-equilibrium) did not allow to distinguish the several forms
probably because they are in fast equilibrium. The NMR spectrum
taken 2 days after a pH jump from 1 to 6 (final equilibrium) was
compatible with that of the trans-chalcone form, but did not permit
(a straightforward assignment) an unequivocal identification of the
several peaks due to their notable overlap; there are two multiplets
which can be attributed to hydrogens 4, 2Ј, 6Ј and 3, 5, 4Ј respec-
tively.
We have investigated the structural transformations of
the unsubstituted flavylium ion in aqueous solutions caused
Acknowledgments
This work was supported in Portugal by Centro de Quimica Fina
e Biotecnologia, Programa Plurianual, and in Italy by MURST
(Progetto Dispositivi Supramolecolari) and by a joint program be-
tween CNR (Italy) and ICCTI (Portugal) n. 423/CNR.
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[O99034]
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3207