Flavylium based dual photochromism: Addressing cis - trans isomerization and ring opening-closure by different light inputs

Article abstract of DOI:10.1039/c5cc01677k

The multistate system of 4′,7-dihydroxy-3-methoxyflavylium is constituted by a multiequilibrium involving trans-chalcone, cis-chalcone, hemiketal, flavylium cation and quinoidal base. This system possesses two independently addressable inter-connected photochromic systems based on the cis - trans isomerization and ring opening-closure of the hemiketal.

Full text of DOI:10.1039/c5cc01677k

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Flavylium based dual photochromism: addressing
cis–trans isomerization and ring opening-closure
by different light inputs†
Cite this:DOI: 10.1039/c5cc01677k
Received 25th February 2015,
Accepted 19th March 2015
Sandra Gago, Nuno Basılio, Artur J. Moro and Fernando Pina*
´
DOI: 10.1039/c5cc01677k
www.rsc.org/chemcomm
The multistate system of 40,7-dihydroxy-3-methoxyflavylium is consti-
tuted by a multiequilibrium involving trans-chalcone, cis-chalcone,
hemiketal, flavylium cation and quinoidal base. This system possesses
two independently addressable inter-connected photochromic systems
based on the cis–trans isomerization and ring opening-closure of the
hemiketal.
Photochromic molecules are better known by the possibility of
achieving reversible changes in the colour upon light absorption.
However, they may also find many other applications such as
switches, optical memory devices, sensors, or affecting the rheological
properties of the materials containing these molecules.1–6 The
two main mechanisms to achieve photochromism are the cis–
trans isomerization and the ring-opening closure, exemplified,
Scheme 1 Multistate system of 40,7-dihydroxy-3-methoxyflavylium in
acid to neutral medium.
The mole fraction distribution of the conjugate base, CB,
respectively, by azobenzenes and stilbenes on one side and is dramatically dependent on the substitution pattern of the
diarylethenes and spiropyrans on the other.
2-phenylbenzopyrylium core. For example, 40,7-dihydroxyflavylium
In this communication we describe a dual photochromic possesses 90% of Ct and 10% of A, while anthocyanins have 70% of
system based on cis–trans isomerization and ring opening-closure of B and 4-methyl-7-hydroxyflavylium has almost 100% of A.8
40,7-dihydroxy-3-methoxyflavylium, a compound bio-inspired in
The rate and equilibrium constants of the reactions shown
anthocyanins, the dyes responsible for most of the red and blue in Scheme 1 can be conveniently studied by direct pH jumps
colours of flowers and fruits.7 Similarly to anthocyanins five species defined as the addition of a base to equilibrated solutions
of this compound are reversibly interconverted by pH changes, sufficiently acidic to have AH+ as the stable species, as well as by
Scheme 1.8 The flavylium cation is the stable species in highly acidic reversed pH jumps carried out by addition of acid to equilibrated
solutions. Upon raising the pH the multistate system reaches a solutions of CB. The chemical reactions reported in Scheme 1
different equilibrium involving the other species of Scheme 1. The exhibit different rates. Proton transfer is by far the fastest
complex multistate system can be simplified considering a single process and takes place during the mixing time of the stopped
acid base reaction involving the species AH+ and its conjugate base flow apparatus (circa 6 ms). Consequently, after a direct pH jump
CB defined as the sum of the other species A, B, Cc and Ct, eqn (1) species A is immediately formed. Brouillard and Dubois7 dis-
covered that A is stable and its disappearance in moderately
AH+ + H2O " CB + H3O+ Ka = Ka + Kh + KhKt + KhKtKi with
0
acidic solutions only occurs via the hydration of the flavylium
cation (AH+) to give hemiketal (B). The hydration is pH depen-
dent and its rate decreases upon increasing the pH, ranging
from several seconds to minutes. The tautomerization reaction
(see Scheme 1) takes place in sub-seconds and can be better
studied by reverse pH jumps. Finally the isomerization occurs in
several hours or seconds according to the nature and position of
the flavylium substituents. The rates of the kinetic processes of
40,7-dihydroxy-3-methoxyflavylium are represented in Fig. 1A.
[CB] = [A] + [B] + [Cc] + [Ct] (1)
´
ˆ
LAQV, REQUIMTE, Departamento de Quımica, Faculdade de Ciencias e Tecnologia,
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. E-mail: fp@fct.unl.pt;
Fax: +35 12948550; Tel: +35 12948355
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cc01677k
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Fig. 1 (A) Rate constants of the several distinct kinetic processes. The
acidic and basic catalytic effects on the tautomerization were neglected;
(B) mole fraction distribution at the equilibrium of 40,7-dihydroxy-3-
methoxyflavylium.
Scheme 3 Energy level diagram of 40,7-dihydroxy-3-methoxyflavylium at
the equilibrium. Arrows of the excitation energy are not in scale.
Table 1 Equilibrium constants of compound 40,7-dihydroxy-3-methoxy-
The flavylium photochromic systems described up to now
have been based on the cis–trans isomerization.8 In most cases
the mole fraction of B and Cc is very small and their energy level
is higher than that of Ct and A, Scheme 2. For example,
irradiation of Ct of 30,40,7-trihydroxyflavylium at pH = 4.1,
Scheme 2, leads to Cc, which due to the relatively low cis–trans
isomerization barrier, can spontaneously revert backward to Ct
or forward to the flavylium cation/quinoidal base.9 The compe-
tition backward/forward is dependent on the isomerization
barrier and pH. When the barrier is very high, the backward
reaction is negligible and models for optical memory devices
capable of writing, reading and erasing can be conceived.8,10
The behaviour of 40,7-dihydroxy-3-methoxyflavylium is different,
due to the relative stability of species B, see Scheme 3. In Fig. 2 the
spectral variations accompanying the light irradiation at 275 nm
and 365 nm of this compound, at pH = 3.5, are shown. The most
interesting feature of the photochemistry is the different behaviour
at these two wavelengths: while at 275 nm Ct is formed essentially
from the disappearance of B and some AH+, at 365, Ct disappears
to give B and AH+. Considering that the contribution of Cc to the
light absorption may be neglected (its mole fraction is only 3%) the
appearance of Ct upon irradiation at 275 nm can only be explained
from the excited state of B. Hemiketal, B, is a chromene and these
species are known to open their ring upon light irradiation.4,11,12
flavyliuma
pKa0
pKa^
pKa
Kh/ Mꢀ1
Kt
Ki
2.2
2.37
4.6
4 ꢁ 10ꢀ3
0.05
10.4
a
Estimated error 10%.
Additional information on the system can be achieved by NMR,
see the ESI.†
In previous work we reported a step by step procedure to
obtain the rate and equilibrium constants of the flavylium
based systems.8 In the ESI† the complete set of experiments
regarding direct and reverse pH jumps and the calculations of
the rate and equilibrium constants here presented in Tables 1
and 2 and Fig. 1A are described. In particular using the data of
Table 1, it is possible to represent the pH dependent mole
fraction distribution of the multistate system species, Fig. 1B.
Another convenient way to represent the flavylium multistate
system is the use of an energy level diagram, where for each
chemical reaction the standard Gibbs free energy DG0 is defined
by the relation DG0 = ꢀRT ln K where R is the gas constant, T the
absolute temperature and K the equilibrium constant, as shown
in Schemes 2 and 3.7,11
Table 2 Rate constants of compound 40,7-dihydroxy-3-methoxyflavyliuma
kh (sꢀ1
)
k
ꢀh
(Mꢀ1 sꢀ1
)
kt (sꢀ1
)
k
(sꢀ1
)
ki (sꢀ1
)
k
(sꢀ1
)
ꢀi
ꢀt
0.3
75
1.25 ꢁ 10ꢀ2 0.25
2.3 ꢁ 10ꢀ4 2.2 ꢁ 10ꢀ5
a
Estimated error 15%.
Fig. 2 (A) Spectral modifications upon irradiation of equilibrated solutions
of compound 40,7-dihydroxy-3-methoxyflavylium, 9.2 ꢁ 10ꢀ5 M pH = 3.5
at 275 nm and (B) irradiation of the solution at 365 nm after reaching the
photo-stationary state at 275 nm.
Scheme 2 Energy level diagram of compound 30,40,7-trihydroxyflavylium
at the equilibrium. Arrows of the excitation energy are not in scale.
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The results described here can be rationalized by means of
the energy level diagram of the compound 40,7-dihydroxy-3-
methoxyflavylium, Scheme 3.
Irradiation at 365 nm pumps Ct to Cc, which spontaneously
gives B (blue arrows). Differently, irradiation at 275 nm excites
species B, which forms directly Ct (red arrows). The disappearance
of B drags also the disappearance of some AH+. Conversely the
appearance of B equilibrates with a small fraction of AH+. Cc
formed from the excited state of B reverts back to the ground state
of B in sub-seconds and is not used to form Ct (grey arrows).
As far as we know this is the first clear demonstration of a photo-
chromic system based on the cis–trans isomerization connected with
ring opening-closure, both addressed using light of different wave-
lengths. The dual photochromic system is also pH dependent and as
a consequence the system is able to respond to three stimuli. This
example together with many others reported in the literature during
the last few years show the potentialities of this family of compounds,
bio-inspired in anthocyanins, as versatile photochromic systems.
Fig. 3 (A) Quantum yields for the disappearance (open circles) and
appearance (black circles) of Ct, 5.4 ꢁ 10ꢀ5 M at pH = 6.0; (B) nanosecond
laser flash photolysis at an excitation wavelength of 266 nm pH = 6.0 trace
obtained at 310 nm; inset-representation of the amplitude immediately
after the flash fitted with the Ct absorption.
According to the literature there are two pathways for the
photochemical reaction of a chromene: to produce Cc and/or
Ct. The formation of both chalcones through a conical inter-
section resulting from the excitation of the hemiketal present
in anthocyanins was recently reported by Macanita and co-
workers.11 The photochemical ring opening leads to the ground
state formation of both isomers, i.e., all the Ct is photochemi-
cally formed from the reaction of the excited state of B and not
by the ground state isomerization of Cc, which takes hours. The
photoproduct is thermally reversible, defining a new photo-
chromic process in the flavylium multistate system.
In Fig. 2B the photo-stationary state obtained in Fig. 2A was
irradiated at 365 nm, a wavelength at which Ct absorbs. The fact
that the absorption of Ct at the equilibrium is reached during the
first stages of the irradiation at this wavelength, but continues to
decrease up to a new photo-stationary state is worthy of note. In
other words the absorption of Ct can oscillate around the equili-
brium absorption by means of two different light inputs.
More evidence for the new photochromic system based on
irradiation of B was achieved by representing the quantum
yields of the disappearance and formation of Ct, as a function
of the irradiation wavelength, Fig. 3A. At an irradiation wave-
length higher than 320 nm only Ct absorbs and the quantum
yield for the formation of B is constant within the error as
expected. On the other hand below 320 nm the quantum
yield is roughly proportional to the absorption of B minus a
fraction of the absorption of Ct. Further experimental evidence
was obtained by the traces of nano-second flash photolysis
under a pulse laser at 266 nm. Representation of the initial
amplitude of these traces as a function of the monitoring
wavelength can be fitted with the absorption spectrum of Ct,
supporting that the last species is formed directly from the
excited state of B.
˜
ˆ
This work was supported by Fundaçao para a Ciencia e
Tecnologia, Grant PEst-C/EQB/LA0006/2013; N.B. and A.M. are
thankful for postdoctoral grants SFRH/BPD/84805/2012 and
SFRH/BPD/69210/2010 respectively.
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