11 research outputs found
Isomerization between 2-(2,4-Dihydroxystyryl)-1-benzopyrylium and 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium
2-Phenyl-1-benzopyrylium (flavylium) and 2-styryl-1-benzopyrylium
(styrylflavylium) cations establish in aqueous solution a series of
equilibria defining chemical reaction networks responsive to several
stimuli (pH, light, redox potential). Control over the mole fraction
distribution of species by applying the appropiate stimuli defines
a horizontal approach to supramolecular chemistry, in agreement with
the customary bottom-up approach toward complex systems. In this work,
we designed an asymmetric styrylchalcone able to cyclize in two different
ways, producing two isomeric styrylflavylium cations whose chemical
reaction networks are thus interconnected. The chemical reaction networks
of 2-(2,4-dihydroxystyryl)-1-benzopyrylium (AH<sup>+</sup>) and 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium
(AH<sup>+</sup><sub>iso</sub>) comprise the usual species observed
in flavylium-derived networks, in this case, the styryl derivatives
of quinoidal bases, hemiketals, and chalcones. The thermodynamics
and kinetics of the crossed networks were characterized by the use
of UVāvis absorption and NMR spectroscopy as well as time-resolved
pH jumps followed by stopped-flow. The two styrylflavylium cations
are connected (isomerize) through two alternative intermediates, the
asymmetric <i>trans</i>-styrylchalcone (Ct) and a spiropyran-type
intermediate (SP). At pH = 1, AH<sup>+</sup> slowly evolves (<i>k</i><sub>obs</sub> ā 10<sup>ā5</sup> s<sup>ā1</sup>) to a mixture containing 62% AH<sup>+</sup><sub>iso</sub> through
the Ct intermediate, while at pH = 5, the SP intermediate is involved.
The observed rate constants for the conversion of the styrylflavylim
cations into equilibrium mixtures containing essentially Ct follow
a pH-dependent bell-shaped curve in both networks. While at pH = 1
in the dark, AH<sup>+</sup> evolves to an equilibrium mixture containing
predominantly AH<sup>+</sup><sub>iso</sub>, irradiation at Ī»
> 435 nm induces the opposite conversion
A Multistate Molecular Switch Based on the 6,8-Rearrangement in Bromo-apigeninidin Operated with pH and HostāGuest Inputs
The equilibrium between 6- and 8-bromo-apigeninidin
is quantitatively
displaced toward the formation of the former in the presence of cucurbit[7]Āuril
because of the selective recognition of the 6-bromo isomer by the
host. This phenomenon permits us to conceive a unidirectional multistate
switch addressed with hostāguest inputs and enables the reversible
activation and deactivation of the 6-/8-bromo-apigeninidin dynamic
molecular multistate through coupled hostāguest and pH inputs
Rationalizing the Color in Heavenly Blue Anthocyanin: A Complete Kinetic and Thermodynamic Study
All
equilibrium and rate constants of heavenly blue anthocyanin
(HBA <b>1</b>) as well as the derivatives with two (HBA <b>2</b>) or none (HBA <b>3</b>) acylated units were determined.
The three acylated units of the sugar in position 3 of the peonidin
chromophore of HBA <b>1</b> are essential to confer the peculiar
stability of its purple and blue colors. The sugars generate an efficient
protective environment around position 2 (and 4) of the flavylium
cation, through an intramolecular sandwich-type stacking that retards
35-fold the hydration reaction (<i>k</i><sub>h</sub>) and
increases 8.8-fold the dehydration reaction (<i>k</i><sub>āh</sub>), when compared with the peonidin chromophore HBA <b>3</b>. The conjugation of these two rates lowers 308-fold the
hydration equilibrium constant (<i>K</i><sub>h</sub>), corresponding
to a raise of the energy level of the hemiketal by 14.2 kJ mol<sup>ā1</sup>. Conversely, the p<i>K</i><sub>a</sub> of
the quinoidal base in HBA <b>1</b> is only slightly stabilized
in comparison with that of HBA <b>2</b> and HBA <b>3</b>. The energy level of hemiketal increases with the number of acylated
units, but the inversion of energies between hemiketal and quinoidal
base takes place exclusively for HBA <b>1</b> (three acylated
units), permitting in moderately acidic solutions the stabilization
of the purple quinoidal base. Identical inversion of energy was observed
for the corresponding ionized species, allowing the stabilization
of the blue ionized quinoidal base in slightly basic solutions. At
pH values higher than 8, the hydroxyl groups of the hydroxycinnamic
acid units start to deprotonate disrupting the intramolecular sandwich-type
stacking and the more or less slow degradation of the anthocyanin
is observed
Formation of Photoluminescent Lead Bromide Nanoparticles on Aluminoborosilicate Glass
A multicomponent aluminoborosilicate
photoluminescent glass was
synthesized by introducing PbĀ(II) and NaBr in its composition. The
room-temperature photoluminescence is due to the existence of 4 nm
nanocrystals, shown using TEM imaging and XRD analysis, which are
assigned to PbBr<sub>2</sub> nanocrystals. The glasses display a broad
emission band with a peak at 2.85 eV by exciting at 3.35 eV, with
an anisotropy equal to 0.19 at room temperature. At 77 K, the emission
intensity increases 1 order of magnitude and a vibronic structure
appears, indicating an electronāphonon coupling with the glass
matrix. Time-resolved luminescence measurements of these nanocrystals
reveal mixed-order kinetics, with second-order recombination of self-trapped
electron centers and a first-order temperature-dependent nonradiative
rate constant connected with pathways due to confinement of self-trapped
centers
Emptying the Ī²āCyclodextrin Cavity by Light: Photochemical Removal of the <i>trans</i>-Chalcone of 4ā²,7-Dihydroxyflavylium
The interaction between
the network of chemical reactions of the
compound 4ā²,7-dihydroxyflavylium and Ī²-cyclodextrin was
studied by means of pH jumps, followed by UVāvis absorption,
flash photolysis, stopped flow, and NMR. The <i>trans</i>-chalcone is the network species exhibiting the strongest interaction
with the host. In moderately acidic medium, 95% of the <i>trans</i>-chalcone, 2.5 Ć 10<sup>ā5</sup> M, in the presence of
Ī²-cyclodextrin, 9 Ć 10<sup>ā3</sup> M, is expected
to fill the host cavity (association constant 2.2 Ć 10<sup>3</sup> M<sup>ā1</sup>). In contrast, flavylium cation does not interact
(association constant ā 0). Irradiation of the <i>trans</i>-chalcone in the presence of Ī²-cyclodextrin 9 mM leads to the
flavylium cation appearance. Light is thus capable of removing the <i>trans</i>-chalcone from the Ī²-cyclodextrin, leaving the
cavity empty. The system is reversible and <i>trans</i>-chalcone
goes back to the initial state upon switching off the light due to
the thermodynamic favorable conversion of flavylium cation to <i>trans</i>-chalcone in the presence of Ī²-cyclodextrin
Photoluminescent Nanocrystals in a Multicomponent Aluminoborosilicate Glass
In
this study, stable and nonexpensive aluminoborosilicate glasses
with different photoluminescence colors were synthesized by doping
with PbĀ(II), BaĀ(II) and sodium halides. While glasses with NaF and
NaCl exhibit no (or very low) luminescence, glasses doped with NaBr
and NaI display room-temperature photoluminescence at 435 and 530
nm, respectively. The observed room-temperature photoluminescence
is attributed to nanocrystals whose presence is revealed by transmission
electron microscopy. The crystalline nature of the particles, which
are pointed out as barium-lead halides, is also revealed by anisotropy
measurements for Br and I doped samples. Time-resolved luminescence
measurements show a second-order kinetics component combined with
a first-order nonradiative rate constant. The photoluminescence properties
here described are important for
the future design of new optical materials or devices based on lead
halide nanocrystals
Impact of Lignosulfonates on the Thermodynamic and Kinetic Parameters of Malvidin-3ā<i>O</i>āglucoside in Aqueous Solutions
The
interaction of malvidin-3-<i>O</i>-glucoside (<b>1</b>) and a lignosulfonate was studied by UVāvisible spectroscopy,
and the results obtained showed the formation of a complex between
the negatively charged lignosulfonate and the flavylium cation form
(AH<sup>+</sup>) of this anthocyanin at pH 1. The thermodynamic and
kinetic parameters of <b>1</b> in the presence of a lignosulfonate
were determined by UVāvisible spectroscopy and stopped-flow
techniques. The main differences were observed in the flavylium cation
(AH<sup>+</sup>)/quinoidal base (<b>A</b>) equilibrium, the
AH<sup><b>+</b></sup> form being more stabilized than <b>A</b> (p<i>K</i><sub>a1</sub> = 4.4 Ā± 0.1) compared
with <b>1</b> in the absence of the lignosulfonate (p<i>K</i><sub>a1</sub> = 3.9 Ā± 0.1). Furthermore, comparing
the hydration (<i>k</i><sub>h</sub> = 0.028 s<sup>ā1</sup>) and dehydration (<i>k</i><sub>āh</sub> = 40 M<sup>ā1</sup> s<sup>ā1</sup>) processes of <b>1</b> in the presence of the lignosulfonate with the processes of <b>1</b> (<i>k</i><sub>h</sub> = 0.12 s<sup>ā1</sup> and <i>k</i><sub>āh</sub> = 35 M<sup>ā1</sup> s<sup>ā1</sup>) show that the hydration process is slower
while the dehydration process is practically unaffected in the presence
of the lignosulfonate
Additional file 1 of Iron-gall inks: a review of their degradation mechanisms and conservation treatments
Additional file 1. Cysteine-based interventions to protect metals from corrosion
Impact of Water on the CisāTrans Photoisomerization of Hydroxychalcones
The
photochromism of a 2-hydroxychalcone has been studied in CH<sub>3</sub>CN and H<sub>2</sub>O/CH<sub>3</sub>OH (1/1, v/v), as well
as in analogous deuterated solvents using steady-state (UVāvis
absorption, <sup>1</sup>H and <sup>13</sup>C NMR) and time-resolved
(ultrafast transient absorption and nanosecond flow flash photolysis)
spectroscopies. Whereas the irradiation of <i>trans</i>-chalcone
(<b>Ct</b>) under neutral pH conditions leads to the formation
of the same final chromene derivative (<b>B</b>) in both media,
two distinct photochemical mechanisms are proposed in agreement with
thermodynamic and kinetic properties of the chemical reaction network
at the ground state. Following light excitation, the first steps are
identical in acetonitrile and aqueous solution: the FranckāCondon
excited state rapidly populates the <i>trans</i>-chalcone
singlet excited state <sup>1</sup><b>Ct</b>* (LE), which evolves
into a twisted state <sup>1</sup><b>P</b>*. This excited state
is directly responsible for the photochemistry in acetonitrile in
the nanosecond time scale (16 ns) leading to the formation of <i>cis</i>-chalcone (<b>Cc</b>) through a simple isomerization
process. The resulting <i>cis</i>-chalcone evolves into
the chromene <b>B</b> through a tautomerization process in the
ground state (Ļ = 10 ms). Unlike in acetonitrile, in H<sub>2</sub>O/CH<sub>3</sub>OH (1/1, v/v), the <b>P</b>* state becomes
unstable and evolves into a new state attributed to the tautomer <sup>1</sup><b>Q</b>*. This state directly evolves into <b>B</b> in one photochemical step through a consecutive ultrafast tautomerization
process followed by electrocyclization. This last case represents
a new hypothesis in the photochromism of 2-hydroxychalcone derivatives
2,2ā²-Spirobis[chromene] Derivatives Chemistry and Their Relation with the Multistate System of Anthocyanins
The chemistry of 2,2ā²-spirobisĀ[chromene]
derivatives is
intimately related to the one of anthocyanins and similar compounds.
The 2,2ā²-spirobisĀ[chromene] species plays a central role in
the network of chemical reactions connecting two different flavylium-based
multistate systems. In the present work, a new asymmetric 2,2ā²-spirobisĀ[chromene]
intermediate possessing a constrained propylenic bridge between carbons
3 and 3ā² was isolated and its role as a pivot in the anthocyanins-type
multistate of chemical reactions was investigated by the conjugation
of absorption spectroscopy, stopped-flow, NMR, and X-ray crystallography.
It was confirmed that the propylenic bridge is essential to stabilize
the spirobisĀ[chromene] species. Furthermore, under acidic conditions,
two <i>cis</i>ā<i>trans</i> styrylflavylium
isomers were identified, which could be interconverted directly into
one another with light. This is the first report of styrylflavylium
cations with photoisomerization on the styryl moiety