The occurrence of
anthocyanin (ACN) and metal (Me) complexes has
been widely supported by many research works while the possibility
that ACNs bind to metalloids (Mds) is yet to be proven. Here, metalloids
(H
3
BO
3
for B; GeO
2
for Ge) were added
to cyanidin-based solutions at pH 5, 6, and 7 and ACN–Md stoichiometric
ratios of 1:1, 1:10, 1:100, and 1:500, and UV–vis transmittance
spectroscopy as well as density functional theory (DFT) calculations
were performed to test this hypothesis. Ge and B addition caused bathochromic
and hyperchromic shifts on ACN UV–vis spectra, particularly
pronounced at pH 5 and a 1:500 (ACN:Md) ratio. ACN–Me complexation
reactions have been evaluated where Ge showed a higher capability
to bind to ACNs than B. Among the complexes envisioned, those labeled
as
b1
,
b2
, and
b3
feature UV–vis
spectra compatible with experiments. The combination of experimental
and computational data offers for the first time evidence of the formation
of ACN–Md complexes.
In this work, the electronic and optical properties of hybrid boron-nitrogen-carbon structures (h-BNCs) with embedded graphene nanodisks are investigated. Their molecular affinity is explored using pyridine as model system and comparing the results with the corresponding isolated graphene nanodisks. Time-dependent density functional theory (TDDFT) analysis of the electronic excited states was performed in the complexes in order to characterize possible surface and charge transfer resonances in the UV region. Static and dynamic (hyper)polarizabilities were calculated with coupled-perturbed Kohn-Sham theory (CPKS) and the linear and nonlinear optical responses of the complexes were analyzed in detail using laser excitation wavelengths available for (Hyper)Raman experiments and near-to-resonance excitation wavelengths. Enhancement factors around 103 and 108 were found for the polarizability and first order hyperpolarizability, respectively. The quantum chemical simulations performed in this work point out that nanographenes embedded within hybrid h-BNC structures may serve as good platforms for enhancing the (Hyper)Raman activity of organic molecules immobilized on their surfaces and for being employed as substrates in surface enhanced (Hyper)Raman scattering (SERS and SEHRS). Besides the better selectivity and improved signal-to-noise ratio of pristine graphene with respect to metallic surfaces, the confinement of the optical response in these hybrid h-BNC systems leads to strong localized surface resonances in the UV region. Matching these resonances with laser excitation wavelengths would solve the problem of the small enhancement factors reported in Raman experiments using pristine graphene. This may be achieved by tuning the size/shape of the embedded nanographene structure.
Herein,
it is shown how anion recognition in highly polar solvents
by neutral metal-free receptors is feasible when multiple hydrogen
bonding and anion−π interactions are suitably combined.
A neutral aromatic molecular tweezer functionalized with azo groups
is shown to merge these two kinds of interactions in a unique system
and its efficiency as an anion catcher in water is evaluated using
first-principles quantum methods. Theoretical calculations unequivocally
prove the high thermodynamic stability in water of a model anion,
bromide, captured within the tweezer’s cavity. Thus, static
calculations indicate anion–tweezer interaction energies within
the range of covalent or ionic bonds and stability constants in water
of more than 10 orders of magnitude. First-principles molecular dynamics
calculations also corroborate the stability through the time of the
anion–tweezer complex in water. It shows that the anion is
always found within the tweezer’s cavity due to the combination
of the tweezer–anion interactions plus a hydrogen bond between
the anion and a water molecule that is inside the tweezer’s
cavity.
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