Thermal and photochemical properties of 4′,7-dihydroxyflavylium in water-ionic liquid biphasic systems: A write-read-erase molecular switch

Article abstract of DOI:10.1002/anie.200353131

Switching colors: 4′,7-Dihydroxyflavylium (AH⁺) in water-ionic liquid biphasic systems can be used as a write-read-erase system. In acid media, the chalcone (Ct) form is soluble in ionic liquids and is thermally metastable, but reacts photochemically (write) to give the yellow flavylium salt, which can be optically read without being erased. The system is prepared for a new cycle by two consecutive pH jumps .

Full text of DOI:10.1002/anie.200353131

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Molecular Switches
because: 1) it is stable over the whole pH range, 2) it exhibits
a variety of colored species, which are easily identified and
therefore a useful sensor to the “naked eye” in the ionic
liquids, and 3) its thermal and photochemical behavior was
previously characterized in water.[5–7]
Thermal and Photochemical Properties of
4’,7-Dihydroxyflavylium in Water–Ionic Liquid
Biphasic Systems: A Write–Read–Erase
Molecular Switch**
In acidic water, five species of this compound can be
distinguised: the flavylium cation (AH+); the quinoidal
neutral base (A) formed upon deprotonation of AH+; the
hemiketal (B) obtained by hydration of AH+; the cis-chalcone
(Cc) resulting from tautomerization of B; and the trans-
chalcone (Ct) owing to the isomerization of Cc. However, B
and Cc are short-lived, transient species whose absorption
spectra have not been observed experimentally.[5–7] In basic
water, ionized quinoidal base (Aꢀ) and ionized trans-chal-
cones (Ctꢀ and Ct2ꢀ), obtained by deprotonation of the
hydroxy groups, are found in equilibrium or as transient
species.
Fernando Pina,* Jo¼o C. Lima, A. Jorge Parola, and
Carlos A. M. Afonso
In the framework of supramolecular chemistry, synthetic
flavylium salts can be envisaged as molecular systems that
exist in different forms (multistate), which can be intercon-
verted by different external stimuli (multifunctional).[1]
Herein we explore a new dimension brought by ionic liquids
to flavylium chemistry.
Room-temperature ionic liquids are thermally stable salts
constituted by an organic cation and either an organic or
inorganic anion. These liquids, unlike traditional inorganic
molten salts, present a high degree of asymmetry that inhibits
crystallization at room temperatures. They are air- and water-
stable, exhibit a nonmeasurable vapor pressure, and solvate a
variety of organic and inorganic compounds. For these
reasons ionic liquids are emerging as alternative green
solvents, that is, as reaction media in many transformations
including catalysis,[2] biocatalysis,[3] and separation pro-
cesses.[4]
The pH-dependent mole-fraction distribution of the
species reported in Scheme 1 are represented in Figure 1, at
equilibrium (red solid lines, bottom color bars), or as transient
forms obtained immediately upon a pH stimulus (blue dotted
lines, top color bars). From these data, it can be concluded
that equilibrated solutions of 4’,7-dihydroxyflavylium are
yellow at pH < 2, slightly orange-brown at pH 4–8, pale
brown-pink at around pH 9, and orange at pH > 11. On the
other hand, immediately upon a “direct” pH jump from pH 1
(yellow AH+) to pH 6 or pH 10, A (orange-brown) and Aꢀ
(pink), respectively, are formed (Figure 1a). In the case of
“reverse” pH jumps, from ꢁ pH 12 (orange Ct2ꢀ), the
transient species Ctꢀ (pale yellow) and Ct (colorless) can be
detected (Figure 1b). In all pH jumps, the final equilibrium is
In Scheme 1 the complex network of chemical reactions in
which the compound 4’,7-dihydroxyflavylium is involved is
represented.[5] We selected this particular flavylium salt,
Scheme 1. Network of reactions involving 4’,7-dihydroxyflavylium in water.
[*] Prof. F. Pina, Dr. J. C. Lima, Dr. A. J. Parola, Dr. C. A. M. Afonso
REQUIMTE/CQFB
reached after several minutes, depending on pH and temper-
ature.
Departamento de Química, Faculdade de CiÞncias e Tecnologia
Universidade Nova de Lisboa
2829-516 Monte de Caparica (Portugal)
Fax: (+351)21-294-8550
The photochemistry of 4’,7-dihydroxyflavylium in water is
based on the efficient trans–cis photoisomerization of the
chalcone.[6–7] In a typical photochemical experiment, a
solution previously equilibrated in the pH range 3–6 and
containing ꢁ 90% Ct and ꢁ 10% A is irradiated. Depending
on the specific pH value, the formation of AH+ (yellow) or A
(orange-brown) is observed. However, these species sponta-
neously revert back to the initial equilibrium concentrations;
E-mail: fjp@dq.fct.unl.pt
[**] We thank the Fundaç¼o para a CiÞncia e Tecnologia and FEDER for
projects POCTI/QUI/47357/2002 and POCTI/EQU/35437/1999.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200353131
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Communications
When aqueous solutions of 4’,7-dihydroxyflavylium are
combined with the ionic liquid 1-n-butyl-3-methylimidazo-
lium hexaflurophosphate ([bmim][PF6]), the two solutions are
not miscible, and several interesting phenomena take place.
Figure 2 describes one sequence of steps that allows a
Figure 1. Mole-fraction distribution of the several species of 4’,7-dihy-
droxyflavylium in water, and color bars representing the corresponding
color of the solution; a) pH jumps from pH 1 (yellow AH+) to less
acidic or basic pH values lead to transient mole-fraction distributions
immediately after the jump (blue dotted lines and top color bar) that
evolve in time to reach a thermodynamic equilibrium (red solid lines
and bottom color bar); b) “reverse” pH jumps from pH 12 (orange
Ct2ꢀ) to less basic or acidic solutions lead to transient mole-fraction
distributions immediately after the jump (blue dotted lines and top
color bar) that evolve in time to reach a thermodynamic equilibrium
(red solid lines and bottom color bar).
Figure 2. a) 1-n-Butyl-3-methylimidazolium hexafluorophosphate,
[bmim][PF6] (lower phase), 4’,7-dihydroxyflavylium perchlorate in
0.01m HCl, AH+ species (upper phase); b) after moderate shaking, A
species (lower phase); c) after vigorous shaking, AH+ species (lower
phase); d) after making the aqueous phase alkaline (pH 12) with mini-
mum shaking, Aꢀ (lower phase), Ct2ꢀ (upper phase); e) after moderate
shaking; f) after vigorous shaking, Ct2ꢀ species (upper phase); g) after
reacidification of the aqueous phase and vigorous shaking, Ct species
(lower phase); h) after irradiation of the ionic-liquid phase at 366 nm,
AH+ species.
the process takes only several minutes. More information on
the kinetics of the reaction network can be achieved by flash
photolysis.[6,7] When the Ct species is submitted to a pulse of
light, its absorption is bleached during the lifetime of the
flash, but partial recovery of the absorbance occurs on a
timescale of seconds. These results clearly indicate the lack of
thermal barrier between the two chalcone species. On the
other hand, the absorption of AH+ (or A) species increases,
indicating that not all the photoproducts (Cc) revert back to
Ct, but some go forward to form AH+ (or A) (Scheme 1). The
process is complete when photochemically produced AH+ (or
A) reverts spontaneously to Ct (timescale of minutes at room
temperature at pH 3.5–5.5; see Figure 3S in the Supporting
Information).
In a typical “optical memory”, the write step consists of
the use of light to irradiate the Ct species, which should give
the AH+ (or A) form, whose color formation is the output of
the system, its signal. To read this signal, a wavelength at
which Ct does not absorb must be used, which can easily be
selected as the flavylium species exhibits a red-shifted
absorption band relative to the chalcones and is photochemi-
cally stable. The existence of a cis–trans thermal barrier that
prevents the back reaction is required for the operation of
such an “optical memory”, otherwise the written information
is erased before it can be read. The erasing of the “memory”
and preparation for a new cycle can be achieved by a pH jump
to basic medium, leading to Ct2ꢀ, followed by a “reverse” pH
jump to regain the Ct species. Some synthetic flavylium salts,
namely those with a single substituent such as an hydroxy or
methoxy group in the 4’ position, exhibit these properties in
water and have been used as models for “optical memories”.[1]
The lack of the cis–trans isomerization barrier of 4’,7-
dihydroxyflavylium in aqueous solutions immediately
excludes this compound for these purposes.
description of a write–read–erase cycle and provides informa-
tion on the ionic-liquid properties. In a typical experiment, an
aqueous solution of 4’,7-dihydroxyflavylium perchlorate (2 ”
10ꢀ5 m, 2 mL) at pH 1.0 was added to the ionic liquid (2 mL).
Moderate shaking leads to the dissolution of all the flavylium
cation in the ionic liquid in its orange-brown quinoidal base
form, A. However, after shaking vigorously for approximately
1 minute, the bottom ionic-liquid solution becomes yellow,
indicating that A was protonated to form the AH+ species.
The flavylium cation, AH+, dissolved in the ionic liquid is
the starting and finishing point of the proposed cycle.
Figures 2d,e, and f show three stages obtained upon a jump
from pH 1 to pH 12 in the aqueous phase. The final
equilibrium in this step is obtained with the Ct2ꢀ species
completely dissolved in the aqueous phase (Figure 2 f). The
next step of the cycle consists of a pH jump in the aqueous
phase back to pH 1, followed by vigorous mixing of the two
phases. This procedure allows the Ct species formed in the
aqueous phase to dissolve completely in the ionic liquid. The
equilibrium species at this pH value (AH+) is formed at a slow
rate, (after 1 hour at 408C, less than 2% of flavylium cation
was formed) showing that, in contrast to the behavior in
water, the Ct species in the ionic liquid exhibits a very large
kinetic barrier that prevents rapid cis–trans isomerization, a
requirement for a “memory” model.
It is known that ionic liquids based on the imidazolium
cation (im) have an organized layer-by-layer structure and are
not an ensemble of statistical aggregates of anions and
cations.[8] In line with reported strong interactions between
ionic liquids and solutes capable of p–p, n–p, or hydrogen
bonding,[4c,9] a strong interaction of the Ct form with the ionic
liquid is expected. The large kinetic barrier of cis–trans
isomerization in the ionic liquid can be explained by this more
rigid and stable distribution of Ct in the ionic liquid that needs
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Chemie
to be disrupted during the thermal isomerization owing to the
formation of less-planar intermediates, as well as by the large
viscosity of the ionic liquid.
The spectral variations that occur upon steady-state
irradiation at 365 nm of the Ct species dissolved in the ionic
liquid are qualitatively similar to those previously reported
for the 4’,7-dihydroxyflavylium in pure water (see Supporting
Information).[6,7] Flash photolysis measurements are reported
in Figure 3. Two processes that follow the light pulse were
The advantage of the present cycle is that no significant
mass variation occurs in the ionic liquid. Moreover, the top
aqueous solutions or even the synthetic flavylium salt can be
removed from the ionic liquid and substituted if necessary.
Ionic liquids extend the possibility of using synthetic flavy-
lium salts that lack a Ct!Cc thermal barrier, e.g. 4’,7-
dihydroxyflavylium, as models for “optical memories”.
Received: October 21, 2003 [Z53131]
Keywords: ionic liquids · molecular switches · optical memory ·
.
photochemistry
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Figure 3. Flash photolysis of the Ct species in the ionic liquid, mea-
sured at: a) 360 nm (Ct absorption); b) 475 nm (AH+/A absorption).
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identified. The first corresponds to the bleaching of the
solution at 360 nm, which occurs during the lifetime of the
flash. This process can be assigned to the cis–trans isomer-
ization. The second process is the formation of AH+ from Cc
(k = 0.5 sꢀ1) clearly shown by the increasing absorption at
475 nm. According to the traces in Figure 3, neither recovery
of the Ct absorption (Figure 3a) nor a decrease in AH+
absorption (Figure 3b) was observed, confirming the exis-
tence of the thermal barrier.[10]
In conclusion, the following write–read–erase cycle can be
proposed:[11] 1) the write step consists of the irradiation of the
Ct species dissolved in the ionic liquid, with formation of
flavylium cation, AH+; 2) the information thus obtained can
be read at a wavelength at which the AH+ (or A) absorbs but
the Ct species does not, e.g. 475 nm; 3) to erase the system and
prepare it for a new cycle, a sequence of operations need to be
carried out: a jump to pH 12 and vigorous shaking to extract
the compound from the ionic liquid into water in its Ct2ꢀ
form, followed by a second jump back to pH 1.0 and vigorous
shaking to dissolve the Ct species in the ionic liquid
(Scheme 2).
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[10]Compare with Figure 3S in the Supporting Information.
[11]An identical cycle was performed with [bmim[]NTf 2](Tf = tri-
fluoromethanesulfonyl). In this case, the ionic-liquid phase is
more turbid, thus making spectrophotometric measurements
difficult.
Scheme 2. Write–read–erase cycle of 4’,7-dihydroxyflavylium in a bipha-
sic water/[bmim][PF6] system.
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