Multistate/multifunctional molecular-level systems: Light and pH switching between the various forms of a synthetic flavylium salt

Article abstract of DOI:10.1002/(SICI)1521-3765(19980710)4:7<1184::AID-CHEM1184>3.0.CO;2-6

The photochromic properties of the 4'-hydroxyflavylium ion (AH⁺) have been investigated in aqueous solution. This system can be interconverted between as many as ten different forms by light excitation and/or pH changes. In neutral or moderately acidic solution the thermodynamically stable form of this compound is trans-2,4'-dihydroxychalcone (Ct). Light excitation of Ct in acidic or neutral solution gives rise to cis-chalcone Cc (Φ=0.04 at 365 nm), which undergoes slow equilibration with three other forms, namely hemiacetal B, flavylium cation AH⁺, and quinoidal base A. The relative amounts of the photoproducts depend on pH. At pH < 3 the only product is the colored AH⁺. In neutral and moderately acidic solutions, the four photoproducts revert back to Ct by a slow thermal reaction (k = 4.0 x 10^-5 s^-1, half-life 27 hours at 25°C, pH = 3), whose rate can be accelerated by increasing the temperature (k= 1.8 x 10^-2 s^-1 at 75°C, pH=3), and by exploiting the photochemical conversion of Cc to Ct (Φ > 0.16 at 313 nm). At pH < 1, AH⁺ is the thermodynamically stable form of the system. A pH jump from 1 to 12 causes the complete conversion of AH⁺ to the Cc^2- dianion. This species is relatively stable (half-life 400 hours at pH 12 and 25°C). Its thermal and photochemical reactions (Φ = 0.17 at 313 nm) lead to Ct^2-, which is the thermodynamically stable form of the system in basic solution. Ct^2- does not undergo any photochemical reaction. All the observed processes are fully reversible and accompanied by large changes in the absorption and emission spectra. The flavylium salt investigated represents a multistate/multifunctional molecular-level system. It has properties required by optical memory devices with multiple storage in two different memory levels and nondestructive readout capacity through a write - lock - read - unlock - erase cycle. Its light- and/or pH-induced transformations can be taken as a basis for simple logic operations and create an intricate network of chemical processes. The photochromic properties of the 4′-hydroxyflavylium ion (AH⁺) have been investigated in aqueous solution. This system has been interconverted between several forms by light excitation and/or pH changes. Different processes have been observed which are accompanied by large changes in the absorption and emission spectra.

Full text of DOI:10.1002/(SICI)1521-3765(19980710)4:7<1184::AID-CHEM1184>3.0.CO;2-6

FULL PAPER
Multistate/Multifunctional Molecular-Level Systems: Light and pH Switching
between the Various Forms of a Synthetic Flavylium Salt
Fernando Pina,* Ana Roque, Maria Joao Melo, Mauro Maestri,* Livia Belladelli,
Ä
Vincenzo Balzani*
Abstract: The photochromic properties
of the 4'-hydroxyflavylium ion (AH )
back to Ct by a slow thermal reaction
thermodynamically stable form of the
system in basic solution. Ct2 does not
undergo any photochemical reaction.
All the observed processes are fully
reversible and accompanied by large
changes in the absorption and emission
spectra. The flavylium salt investigated
represents a multistate/multifunctional
molecular-level system. It has properties
required by optical memory devices with
multiple storage in two different mem-
ory levels and nondestructive readout
capacity through a write ± lock ± read ±
unlock ± erase cycle. Its light- and/or
pH-induced transformations can be tak-
en as a basis for simple logic operations
and create an intricate network of
chemical processes.
‡
1
(k ˆ 4.0  10 5 s , half-life 27 hours at
have been investigated in aqueous sol-
ution. This system can be interconverted
between as many as ten different forms
by light excitation and/or pH changes. In
neutral or moderately acidic solution the
thermodynamically stable form of this
compound is trans-2,4'-dihydroxychal-
cone (Ct). Light excitation of Ct in
acidic or neutral solution gives rise to
cis-chalcone Cc (F ˆ 0.04 at 365 nm),
which undergoes slow equilibration with
three other forms, namely hemiacetal B,
258C, pH ˆ 3), whose rate can be accel-
erated by increasing the temperature
1
(k ˆ 1.8  10 2 s at 758C, pH ˆ 3), and
by exploiting the photochemical conver-
sion of Cc to Ct (F > 0.16 at 313 nm). At
‡
pH < 1, AH is the thermodynamically
stable form of the system. A pH jump
from 1 to 12 causes the complete con-
‡
version of AH to the Cc2 dianion. This
species is relatively stable (half-life
400 hours at pH 12 and 258C). Its ther-
mal and photochemical reactions (F ˆ
0.17 at 313 nm) lead to Ct2 , which is the
‡
flavylium cation AH , and quinoidal
base A. The relative amounts of the
photoproducts depend on pH. At pH <
Keywords: flavylium salts
´ infor-
‡
3 the only product is the colored AH .
mation storage ´ logic gates ´ mo-
lecular devices ´ photochromism
In neutral and moderately acidic solu-
tions, the four photoproducts revert
Molecules that can exist in two forms interconvertible by an
external input are rather common. Typical examples are
photochromic compounds, where the input causing the
switching between the two (stable) species is light.[10] From a
logic viewpoint,[5, 8, 11, 12] such simple systems perform YES/
NO logic functions. Systems capable of existing in more than
two forms (multistate) and that can be interconverted by more
than one type of external stimulus (multifunctional), on the
other hand, are less common.[13, 14] Such multistate/multifunc-
tional systems can behave as complex logic devices. Chemical
systems capable of performing AND,[11] OR,[11] XOR,[9a] and
XNOR[9b] logic operations have been recently reported.
Like anthocyanins, which are one of the most important
sources of color in flowers and fruits, synthetic flavylium salts
undergo structural transformations in aqueous solution that
follow a previously discussed general scheme.[15±19] Some of
the structural transformations of the 4'-hydroxyflavylium ion
are illustrated in Scheme 1. Several reports concerning the
thermodynamic as well as the kinetic aspects of the thermal
reactions of flavylium-type compounds have appeared in the
literature.[15±19] Much less attention has been paid to the
Introduction
Much effort is currently being devoted to the design of
molecular-level switching devices. By reducing the switching
elements to molecular size, the memory density of computers
could be increased by several orders of magnitude and the
power input decreased significantly.[1] Although computation
at the molecular level appears to be far off,[2] the design and
construction of molecular-level systems capable of existing in
different forms interconvertible by external stimuli is of great
interest in both basic and applied research.[3±9]
[*] Prof. F. Pina, A. Roque, Dr. M. J. Melo
Departamento de Química, Universidade Nova de Lisboa
2825 Monte de Caparica (Portugal)
E-mail: fjp@dq.fct.unl.pt
Prof. V. Balzani, Prof. M. Maestri, L. Belladelli
Dipartimento di Chimica G. Ciamician, Universita di Bologna
Á
40126 Bologna (Italy)
Fax : (‡ 39)51-259456
E-mail: vbalzani@ciam.unibo.it
E-mail: mmaestri@ciam.unibo.it
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1184±1191
Photochemical experiments: In continuous irradiation
experiments, light excitation was performed by
a
medium-pressure mercury lamp. Interference filters
(Oriel) were used to select a narrow spectral range with
lmax 365 and 313 nm. The irradiated solution was
contained in a 3 mL spectrophotometric cell. The
intensity of the incident light (2.4 Â 10 7 einstein/min
at 365 nm and 1.2 Â 10 7 einstein/min at 313 nm) was
measured with
a
ferrioxalate actinometer.[24] The
estimated uncertainty in the quantum yields of the
photochemical reactions is 10%. Flash photolysis
experiments were performed according to the proce-
dure described elsewhere.[22c]
Results and Discussion
Synthetic flavylium salts in aqueous solution
undergo a series of complex structural trans-
formations that can be driven by pH changes
and light excitation. These complex trans-
formations follow a general scheme previ-
ously discussed for the compounds which
carry a methyl or methoxy group in the 4'
position.[15±18] In the case of the 4'-hydroxy-
flavylium ion, the scheme is further compli-
cated by the presence of the hydroxyl sub-
stituent and the consequent formation of a
quinoidal base. Altogether, as many as ten
‡
Scheme 1. Structural transformations of the 4'-hydroxyflavylium ion (AH ). For the sake of
simplicity, the monoanionic forms B , Cc , and Ct , which play a marginal role in the
experiments dealt with in this paper, are not represented.
different forms are involved and connected
photochemical and photophysical behavior of these com-
pounds except for an interesting paper by von Sperling et al.
on the trans ± cis photoisomerization of some flavylium ions[20]
and the excited-state twisting of flavylium ions and its
consequences on their fluorescent behavior.[21] We and others
have recently undertaken a systematic investigation in this
field.[22] In a previous paper[13i] we illustrated the possibility of
using a photochromic flavylium ion as the basis for an optical
memory device with multiple storage and nondestructive
readout capacity through a write ± lock ± read ± unlock ± erase
cycle. Here we report the results of an investigation on the
photochromic behavior of another flavylium compound,
namely the 4'-hydroxyflavylium ion, which can exist in four
different thermodynamically stable forms depending on the
pH of the solution and can be interconverted between as
many as ten forms by using light excitation or changes in pH.
by a network of processes. As far as the present study is
concerned, the species playing an important role are the seven
shown in Scheme 1. The three species not directly involved in
our discussion, and therefore not shown in the scheme, are the
monoanionic Ct , Cc , and B forms. As described below, the
‡
thermodynamically stable species are AH , Ct, Ct , and Ct2 ,
depending on the pH of the solution. The other species can be
transiently obtained by pH jump or flash photolysis. Their
half-lives vary from submilliseconds to months, depending on
the specific species and the experimental conditions.
Thermal reactions: The thermal reactions of the 4'-hydroxy-
flavylium ion have been investigated in great detail by
McClelland and McGall.[19] We re-examined several aspects
of the thermal behavior of this system; our results agree
substantially with those reported by those authors, except for
the values of a pKa and a molar absorption coefficient (vide
infra).
In strongly acidic solution (pH ˆ 1), the 4'-hydroxyflavy-
‡
lium ion (AH ) is the thermodynamically stable form of the
Experimental Section
system (Scheme 1). This species is yellow, its absorption
‡
Materials: Synthetic 4'-hydroxyflavylium (AH ) perchlorate was prepared
spectrum showing an intense band with lmax ˆ 435 nm (e ˆ
according to a published procedure.[23] A useful procedure for preparing a
1
41000 m 1 cm ). In moderately acidic or neutral solution
stock solution of Ct is to dissolve a solid sample of 4'-hydroxyflavylium in
(pH 3 ± 7) the thermodynamically stable form is the neutral,
colorless trans-2,4'-dihydroxychalcone (Ct), which exhibits a
moderately intense broad band with lmax ˆ 350 nm (e ˆ
‡
NaOH (0.01n). Under these conditions, AH is completely converted into
Ct2 within a few days at 608C. Solutions of Ct at the desired pH are then
obtained by addition of the necessary amount of HClO4 and a buffer
(citrate or phosphate buffer, 0.01m). A solution of Ct at ca. pH 7 was also
1
21000 m 1 cm ). In basic solution (pH ˆ 12) the thermody-
‡
obtained by dissolving AH perchlorate at this pH value and keeping the
namically stable form is the pale yellow trans-2,4'-dihydroxy-
‡
solution at room temperature for 7 days. The transformation of AH into
1
chalcone dianion (Ct2 ) (lmax ˆ 428 nm, e ˆ 22600 m 1 cm ).
Ct was followed by UV/Vis absorption spectroscopy. The molar fraction
distribution of the various species as a function of pH was calculated as
reported in refs. [22a,b].
‡
The absorption spectra of AH , Ct, and Ct2 are shown in
Figure 1a. Since we will discuss the behavior of the system at
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FULL PAPER
pH 1 ± 7 and 12, the spec-
trum of Ct , the most
stable form around pH ˆ
8, is not relevant to our
discussion.
after the pH jump) is also shown in Figure 1b. As is always the
case for aromatic derivatives of ethylene,[26] the absorption
spectrum of Cc must be less intense and slightly blue-shifted
relative to that of Ct. Compound B has a structure similar to
Cc and is therefore expected to exhibit a similar spectrum.
On a much longer time scale (half-life 10 hours at pH 7) the
system undergoes a third process, leading to Ct, the thermo-
dynamically stable product at this pH value. Figure 2, which
displays the absorbance changes at 500 nm for a solution at
558C, clearly shows two consecutive decay processes follow-
ing the immediate formation of A.
Upon addition of base
‡
to a solution of AH at
‡
pH 1, AH is converted
into Ct, Ct , or Ct2 . As is
apparent from the formu-
lae shown in Scheme 1,
such a conversion can on-
ly occur through a com-
plex reaction sequence in-
volving the formation of
‡
Upon a pH jump from 1 to 4, the fast deprotonation of AH
to give the quinoidal base A is incomplete and leaves a
mixture of both compounds, which, on the minute time scale,
equilibrates with B and Cc. This process is then followed by
the slow reaction leading to the thermodynamically stable
product at pH 4, Ct. Similar behavior is observed throughout
the acidic pH range. Figure 3 shows the absorption spectra
obtained 30 minutes after a pH jump from 1 to a series of final
pH values. These spectra are due to the presence of the four
intermediate
species,
whose spectra and rate
constants for formation/
disappearance can be ob-
tained by pH-jump, tem-
Figure 1. Absorption spectra of the
species derived from 4'-hydroxyflavy-
lium ion in aqueous solution at 258C:
‡
‡
a) AH , Ct, and Ct2 ; b) A, AH , Cc2-,
perature-jump,
flash photolysis techni-
ques.
and/or
and B and Cc mixture at pH ˆ 7.
‡
Acidic and neutral solutions: When an aqueous AH solution
is subjected to a pH jump from pH 1 to pH 7, the first spectral
change observed is a decrease in absorbance at 435 nm and an
increase at 500 nm. This process, assigned[19] to deprotonation
to form quinoidal base A (Scheme 1), is so rapid that its rate
cannot be measured by stopped-flow analysis. It can, however,
be measured by the technique of temperature jump.[19] The pK
‡
‡
of the equilibrium AH > A ‡ H , hereafter indicated by
pKI, is 5.53.[19] Therefore at pH 7 practically 100% of the
flavylium cation is converted to the quinoidal base in the first
step. The immediate formation of A is accompanied by a
slower process, readily observable on the stopped-flow time
scale (Figure 2). This process is attributed[19] to the equilibra-
Figure 3. Absorption spectra recorded after 30 min for 4'-hydroxyflavy-
‡
lium (AH ) aqueous solution upon a series of pH jumps from pH ˆ 1 to
‡
different pH values. The inset shows the molar fraction of AH as a
‡
function of pH.
tion of A with species B and Cc via AH . Extrapolation of the
absorbance changes to t ˆ 0 provides the spectrum of A shown
1
in Figure 1b (lmax ˆ 500 nm, e ˆ 44000m 1 cm ).[25] The spec-
‡
species AH , A, B, and Cc, whose relative amounts depend on
trum of the equilibrated mixture of B and Cc at pH 7 (30 min
the final pH of the solution. The inset of Figure 3 shows the
molar fraction of AH as a function of pH, calculated from
‡
the absorbance at 435 nm. A best-fitting procedure on this
plot gives a pK, hereafter indicated by pKII, of 4.4 for the
‡
dissociation equilibrium of AH to give A, B, and Cc. This
value is somewhat different from that (5.0) reported by
McClelland.[19]
‡
The percentage of AH present at the final (thermody-
namic) equilibrium allowed us to calculate the equilibrium
‡
constant between AH and Ct (pKIII ˆ 1.9 at 608C). As shown
in Figure 4, the measurements performed in the pH range 4 ± 7
showed a linear dependence of the rate constant of the
reaction leading to Ct on the molar fraction of Cc. This
confirms that the rate-determining step of the conversion of
‡
AH after the pH jump is the isomerization reaction Cc !Ct.
All the observed reactions follow first-order (or pseudo-
Figure 2. Decay of the absorbance at 500 nm after a pH jump of 4'-
‡
first-order) kinetics. At pH ˆ 4 activation energies of 29 and
hydroxyflavylium (AH ) from pH ˆ 1 to pH ˆ 7 in aqueous solution at
1
558C. The inset shows the first part of the decay for a solution at 258C.
129 kJmol were obtained from the Arrhenius plots for the
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Multistate, Multifunctional System
1184±1191
Figure 4. Plot of the first-order rate constant of the reaction leading from
‡
AH /A to Ct as a function of the molar fraction distribution of Cc.
‡
equilibration of AH with A, B, and Cc and the trans-
‡
formation of AH into Ct, respectively. The experiments at
pH < 3 were carried out only at high temperature because of
the relative inertness of the flavylium cation.
In conclusion, starting from a solution at pH ˆ 1, where the
Figure 6. Energy-level diagram for the species involved in the pH-jump
and flash-photolysis experiments. pH values are indicated in brackets. For
the sake of simplicity the species Cc and B are represented by the same
energy level. The ratio between the concentrations of Cc and B is ꢀ 1.[20]
‡
only species present is AH , and jumping to less acidic or
neutral media, complex behavior is observed due to the
presence of three equilibria [more properly, two pseudoequi-
libria and one real equilibrium, Eqs. (1), (2), and (3)] which
are well separated in time allowing the determination of their
pK values.
levels of the species involved (including those pertinent to the
experiments in basic solution, vide infra) are indicated.
The technique of pH jump can also be applied in the reverse
direction.[16, 18, 19] Starting from a solution at pH ˆ 7 at its
pseudoequilibrium (i.e., when the species present are A, B,
and Cc) a reverse pH jump to pH ˆ 1 shows a very fast process
due to the protonation of the quinoidal base A, a fast process
‡
‡
AH > A ‡ H
pKI ˆ 5.53 at 258C
pKII ˆ 4.4 at 258C
pKIII ˆ 1.9 at 608C
(1)
(2)
(3)
‡
‡
AH > A ‡ B ‡ Cc ‡ H
‡
‡
‡
AH > Ct ‡ H
corresponding to the formation of AH from B, and a slower
‡
process in which more AH appears from the conversion of
Making the reasonable assumption that the value of pKIII is
Cc through B.
not appreciably temperature-dependent, we calculated the
‡
molar fractions of the five species AH , A, Ct, Cc, and B in
Basic solutions: McClelland and McGall[19] have shown that
the pH range 0 ± 7 under the pseudoequilibrium and equili-
‡
upon a pH jump in an aqueous solution of AH from pH ˆ 1
brium situations at 258C from the above pK values (Figure 5).
to pH ˆ 12, the behavior is much simpler (Figure 6) because
‡
The thermodynamically stable species are AH at pH < 1 and
‡
the deprotonation of AH and subsequent disappearance of
Ct at pH > 3, while A, Cc, and B (always present in a 1:2:2
ratio)[19] are transient species.
The behavior of the system under the above experimental
conditions is illustrated in Figure 6, where the relative energy
A are very fast processes. Only one intermediate state is
observed (half-life 400 hours at pH ˆ 12), consisting of mono-
or di-ionized species of cis-chalcone (pKa 7.9 and 9.7, respec-
tively).[19] The thermodynamically stable products are the
mono- and dianionic trans-chalcones, which we have found to
have pKa 7.6 and 8.3, respectively. The absorption spectra of
the relevant basic forms, the colored trans-2,4'-dihydroxy-
chalcone dianion (Ct2 ) and the colorless cis-2,4'-dihydrox-
1
ychalcone dianion (Cc2 ) (lmax ˆ 347 nm, e ˆ 19000 m 1 cm )
are shown in Figure 1.
‡
Photochemical reactions: We found that AH and Ct2 are not
photosensitive, whereas Ct shows interesting photochemical
behavior. The transient species Cc and Cc2 are also photo-
sensitive.
Figure 5. Molar fraction distribution in aqueous solution at 258C as a
function of pH. Solid lines refer to the species obtained at the thermody-
namic equilibrium. Thick dotted lines refer to the pseudoequilibrium
Continuous irradiation: Continuous irradiation of Ct with
365 nm light causes spectral variations at all pH values in the
range 1 ± 7. Figure 7 shows the absorbance changes obtained at
pH 4 and 7. These spectral changes are exactly the reverse of
those observed in the thermal reactions following the pH
‡
reached in the minute time scale after a pH jump on AH solution or flash
photolysis of Ct solutions. Dashed lines refer to the pseudoequilibrium
‡
reached in the submillisecond time scale after a pH jump on AH solutions.
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F. Pina, M. Maestri, V. Balzani
FULL PAPER
‡
jumps of AH solutions
from pH ˆ 1 to the same
pH of the irradiated solu-
tion.
As shown by the flash
photolysis experiments de-
scribed below, the primary
product of the photochem-
ical reaction of Ct is Cc
(Scheme 1), which then
undergoes the previously
discussed thermal reac-
tions leading on a short
time scale (minutes) to its
pseudoequilibrium with B,
Figure 8. Spectral changes caused by continuous irradiation with 313 nm
light of a 2.0  10 5 m aqueous solution of Cc2 at pH ˆ 12.
solution (or of a Ct solution after pH jump to pH ˆ 12) does
not cause any spectral changes.
‡
We found that AH and Ct2 show fluorescence emission
‡
AH , and A. The relative
(Figure 9) with maxima at 515 and 615 nm, respectively, and
amounts of the photoprod-
ucts Cc, B, AH , and A are
4
‡
comparable quantum yields (5 Â 10 ). This is an important
‡
result since AH and Ct2 , which absorb in the same spectral
determined by the pH of
the solution (Figure 5, dot-
ted lines). At pH < 3, the
Figure 7. Spectral changes observed
by continuous irradiation with
365 nm light of Ct aqueous solutions.
a) pH ˆ 4 and 2.4  10 5 m; b) pH ˆ 7
and 2.9 Â 10 5 m.
region, can be distinguished by fluorescence measurements
‡
(vide infra). The reason why AH exhibits a Stokes shift much
smaller than Ct2 is probably related to the rigidity of the
‡
only photoproduct is AH .
‡
former structure.
At pH ˆ 4, AH is still the
main photoproduct, but
small amounts of A, B,
‡
and Cc are also formed. At pH 5 and 6 a mixture of AH ,
‡
A, B, and Cc is obtained, while at pH ˆ 7 no AH is present
in the mixture. As in the thermal reactions described
above, the photochemical products revert to Ct on a long
time scale (hours). At pH < 3, when the thermal back reac-
‡
tion is negligibly slow and the only photoproduct is AH ,
‡
the quantum yield of AH formation is 0.04. This, of
course, is also the quantum yield of the photoisomerization
Ct !Cc.
As stated above, a pH jump experiment from 1 to 7 on an
AH solution at pH ˆ 7 after 30 minutes leads to an equili-
‡
Figure 9. Fluorescence emission spectra of AH (pH ˆ 1) and Ct2 (pH ˆ
‡
12) aqueous solutions at 258C (lexc ˆ 435 nm).
brated mixture of A, B, and Cc that can be maintained for a
time period sufficiently long to allow the study of its photo-
chemical behavior. Continuous irradiation of this equilibrated
mixture at 313 nm causes an absorbance increase at 350 nm,
indicating the formation of Ct. The apparent photochemical
quantum yield for this reaction, based on the total light
absorbed by the mixture, is ca. 0.16. From the formulae of the
three components of the mixture (Scheme 1), it is clear
that the only species responsible for the photoreaction leading
to Ct is Cc. Since at pH ˆ 7 Cc constitutes about 40% of
the mixture and presumably absorbs a comparable frac-
tion of the total absorbed light, it follows that the quantum
yield of the cis !trans photoisomerization is probably close
to 0.4.
Flash photolysis: Previous investigations[13i, 22d] have revealed
that important kinetic information on the conversion of the
various forms of flavylium ions can be obtained by means of a
simple flash photolysis apparatus.[22c]
Flash photolysis experiments were performed on aqueous
solutions of Ct at 258C and pH ˆ 4.0 and 7.0. After flash
excitation, absorbance vs. time traces like those depicted in
the insets of Figures 10 and 11 were recorded at several
wavelengths in the spectral region 250 ± 500 nm. From these
traces, the difference spectra at pH ˆ 4.0 and 7.0 shown in
Figures 10 and 11 were obtained. At pH ˆ 4, the absorbance
changes occurred during the flash (Figure 10, bleaching at
350 nm and no absorption at 435 nm for the zero-time
extrapolation) and are consistent with the formation of Cc.
‡
A pH jump from 1 to 12 on a solution containing AH was
found to give an absorption spectrum (Figure 8) identical to
that reported by McClelland and McGall for Cc2 .[19] Con-
tinuous irradiation of this solution at 313 (or 365) nm caused
the spectral changes shown in Figure 8. The final spectrum is
identical to the spectrum of Ct2 obtained by a pH jump to 12
on a solution of Ct. The initial quantum yield of the
Cc2 !Ct2 photochemical conversion is 0.17 at 313 nm.
Irradiation at 436, 365, or 313 nm of the final irradiated
1
A subsequent process is observed (rate constant 0.22 s ) that
causes an increase in absorption at l > 400 nm as expected for
the formation of AH and A (Figure 1). This process is
‡
attributed to the conversion Cc !B, known from the pH jump
experiments to be the rate-determining step (rate constant
0.27 s )[19] of the process Cc !AH ‡ A. On a longer time
1
‡
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Multistate, Multifunctional System
1184±1191
‡
of Cc to form B, AH , and A. In other words, light excitation
of trans-chalcone converts the system from the thermody-
namic equilibrium indicated by the full lines in Figure 5 to the
pseudoequilibrium situation indicated by the dotted lines.
This pseudoequilibrium is maintained for a length of time
which depends on temperature, pH, and light excitation of the
Cc form. At 258C and pH 7 in the absence of light, the
recovery of Ct occurs with a half-life of 22 hours. However, as
can be understood from Figure 6, the products of the Ct !Cc
‡
photoreaction can be a) frozen indefinitely as AH by
lowering the pH to 1, b) rapidly back-converted to Ct by
increasing the temperature and/or by irradiation with 313 nm
light, c) temporarily converted to Cc2 by increasing the pH to
12. In turn, Cc2 can be thermally or photochemically
converted to the stable Ct2 form, which can be converted
back to the starting Ct species by lowering the pH to 4.
Figure 10. Difference spectra obtained after flash excitation of a aqueous
5
~
*
solution of Ct (6  10 m) at pH ˆ 4. The delay times are 1 s ( ), 2 s ( ),
*
Properties of the network of chemical processes: As men-
tioned in the introduction, molecular or supramolecular
systems capable of existing in different forms (multistate)
that can be interconverted by different external stimuli
(multifunctional) are interesting for both basic and practical
reasons. The system examined in this paper can exist as four
different thermodynamically stable species depending on the
pH of the solution and can be interconverted between a
number of different forms by two different inputs, namely
light (three reactions can be light-driven) or changes in pH. In
the following discussion on the interconversion processes, the
transient products B and A have been neglected for the sake
of simplicity since they are always in equilibrium with Cc.
and 5 s ( ). The unbroken line is the final difference spectrum obtained
after continuous irradiation. The inset shows the absorbance vs. time trace
‡
obtained at l ˆ 435 nm (where Ct shows a minimum and AH a maximum).
A write ± lock ± read ± unlock ± erase cycle: It is well known
that photochromic systems represent potential molecular-
level memory devices.[10, 13, 14] A number of problems, how-
ever, must be solved for practical applications. A particular
challenge is to find systems with multiple storage and
nondestructive readout capacity, that is, where the record
can be erased when necessary, but is not destroyed by the
readout. The 4'-hydroxyflavy-
Figure 11. Difference spectrum obtained 1 s after flash excitation of a
solution of Ct (6  10 5 m) at pH ˆ 7. No further spectral change was
observed in the second time scale. The unbroken line is the final difference
spectrum obtained after continuous irradiation. The inset shows the
absorbance vs. time traces obtained at 350 nm (where Ct shows a maximum
lium ion investigated in this
paper can be taken as the basis
for an optical memory system
‡
and AH a minimum).
with multiple storage and non-
scale, back reaction to Ct takes place, as in the pH jump
experiments.
destructive readout capacity
through a write ± lock ± read ±
In the flash experiments at pH 7.0 (Figure 11) a fast process
exhibiting the same characteristics discussed above takes
place within the time scale of the flash. This process is again
assigned to the photochemical conversion of Ct to Cc. A
unlock ± erase cycle similar to
that previously described for
another flavylium deriva-
tive.[13i] This behavior can be
described with reference to
Figures 6 and 12:
a) At pH 4 ± 7, the stable or
kinetically inert (depend-
Figure 12. Write ± lock ± read ±
unlock ± erase cycle starting
from the Ct form.
1
second, much slower, process (rate constant 0.0026 s ), not
discernible in Figure 11, follows. The spectral changes indicate
that this second process leads to the quinoidal base A. These
results are again in full agreement with those obtained by
McClelland from pH jump experiments[19] which showed that,
at pH 7, the rate-determining step for formation of A from Cc
ing on pH) colorless species Ct can be photochemically
converted (365 nm light) into the thermodynamically
unstable, but relatively inert, form Cc (write);
‡
1
is the reaction B !AH (rate constant 0.0025 s ).
In conclusion, light excitation of Ct causes a trans !cis
photoisomerization that is followed by thermal equilibration
b) by a second stimulus (addition of acid), Cc can be
converted into the kinetically inert or thermodynamically
Chem. Eur. J. 1998, 4, No. 7
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F. Pina, M. Maestri, V. Balzani
FULL PAPER
‡
stable (depending on pH) AH form (lock); if the initial
Oscillating absorbance pat-
terns: Another feature of
the investigated system
should be emphasized.
‡
pH is 1, Cc autolocks as AH ;
‡
c) AH is photochemically inactive and shows an absorption
spectrum clearly distinct from that of Ct (Figure 1), so that
it can be optically detected (read);
d) by addition of base, AH can be reconverted into Cc
‡
Starting from AH , alterna-
‡
tion of pH jump and light
excitation causes oscillation
patterns of absorbance at
different wavelengths, as
shown in Figure 15. Such
patterns may be of interest
for signal generation pur-
poses.
(unlock);
e) Cc can be reconverted to the initial Ct form by a thermal or
a photochemical reaction (erase).
Reading without writing in a write ± lock ± read ± unlock ± erase
cycle: A generally overlooked difficulty with photochromic
systems is that the starting form (Ct in the above discussion) is
the photoreactive one, so that
Logic operations: Multi-
state/multifunctional molec-
ular-level systems can be
taken as bases for logic
operations.[5, 8, 9, 11, 12] In the
present system light excita-
tion and pH jumps can be
taken as inputs, and absorb-
ance or fluorescence as out-
puts. Starting from the non-
emitting Ct and taking the emission of AH at 515 nm as
output signal, a jump to pH ˆ 1 or light excitation alone
cannot generate the output, whereas when these two imputs
are applied in series, the output is obtained (AND logic
function).
it cannot be read by absorp-
tion spectroscopy without
writing. In our system, this
difficulty can be overcome
‡
starting from AH , which is
Figure 15. Absorbance oscillations
caused by alternate pH jump and
the thermodynamically sta-
ble form at pH ˆ 1 (Figures 6
and 13). Since this form is not
photosensitive, it can be read
by light excitation (that is, by
recording its absorption spec-
trum) without writing. It can
then be unlocked by a pH
jump, which yields the meta-
stable Cc2 form. At this
stage, one can write the opti-
cal information obtaining the
light excitation of
a
3.3 Â 10 5 m
aqueous solution starting from
‡
AH at pH ˆ 1.
‡
Figure 13. Read ± write ± lock ± r-
ead ± unlock ± erase cycle starting
from the AH form.
‡
Multiple reaction patterns: Some of the species studied in this
work are interconnected by multiple reaction patterns. For
example, in order to go from AH to Ct three different routes
can be chosen, as represented pictorially in Figure 16: i) jump
to pH 12 with formation of Cc2 , followed by excitation with
stable (locked) Ct2 form that can then be read. When
necessary, the information stored into Ct2 can be unlocked by
a pH jump yielding Ct and can then be erased by light
excitation. The same performance can be obtained starting
from Ct2 .
‡
Permanent and temporary memories: The human brain
contains shallow and deep memory forms.[27] The network of
processes interconverting the various species (Scheme 1 and
Figure 6) allows the presence of different levels of memory.
‡
Once the permanent (deep) AH form of memory has been
obtained (write and lock, Figure 14), a jump to pH 12 leads to
the formation of a temporary (shallow) memory state, Cc2 ,
whose spontaneous slow erasure to give the deep Ct2
memory can be accelerated by light. Reset can then be
accomplished by a back pH jump to pH 4.
Figure 16. The network of processes caused by pH jumps and light
‡
excitations interconnecting the AH and Ct forms of the investigated
system.
313 nm light to obtain Ct2 , and by a jump to pH ˆ 6; ii) jump
to pH ˆ 6 to form Cc; at this stage, one can choose between
two sub-routes, namely iia) light excitation with 313 nm light,
or iib) jump to pH 12 to merge into the preceding path which
goes via Cc2 and Ct2 . Once Ct has been obtained, one can go
Figure 14. A write ± lock ± read ± unlock ± erase cycle with two memory
levels.
‡
back to AH by two different routes: iii) light excitation at
1190
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Chem. Eur. J. 1998, 4, No. 7

Multistate, Multifunctional System
1184±1191
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‡
Ct at pH 6, one obtains the same result (AH ) regardless of
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‡
For example (not shown in Figure 16), starting from AH at
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Conclusions
We have described the properties of a chemical system that
can exist in several forms (multistate) that can be intercon-
verted by more than one type of external stimulus (multi-
functional). Such a system exhibits properties required by
optical memory devices, with multiple storage in two different
memory levels and nondestructive readout capacity through a
write ± lock ± read ± unlock ± erase cycle. Its light- and/or pH-
induced transformations, which are accompanied by changes
in the absorption and emission spectra, can be taken as a basis
for simple logic operations and create an intricate network of
chemical processes.
In the brain, neurons store, exchange, and retrieve infor-
mation by extremely complicated chemical processes. Simple
multistate/multifunctional systems may play the role of
models for an initial understanding of the chemical basis of
complex biological processes. It is by no means clear whether
wet artificial systems can find real applications, for example in
molecular-scale computers.[2] We point out, however, that the
design and construction of molecular or supramolecular
species capable of existing in different forms that can be
interconverted by external stimuli introduces new concepts in
the field of chemistry and stimulates the ingenuity of
researchers engaged in the bottom-up approach to nano-
technology.
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Acknowledgements: This work was supported in Portugal by JNICTand in
Italy by MURST and the University of Bologna (Funds for Selected
Research Topics).
Received: January 5, 1998 [F950]
[23] C. Michaelis, R. Wizinger, Helv. Chim. Acta 1951, 34, 1761.
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