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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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