Sorption of 3-hydroxyflavone within channel type zeolites: the effect of confinement on copper(ii) complexation.

The confinement effect on the complexation process of Cu(ii) by 3-hydroxyflavone (3HF) was investigated by studying 3HF incorporation in channel-type copper-containing ZSM-5 and mordenite (MOR) zeolites characterized by different pore diameters. Complementary electronic and vibrational spectroscopy techniques point out two distinct behaviors upon 3HF sorption and subsequent complexation depending on the channel diameter in CuZSM-5 and CuMOR. To determine the influence of the internal environment on the interaction between the copper cation and the guest molecule, and to predict the structure of the complexes formed within the narrow-pore ZSM-5 and in the larger pore mordenite, the vibrational spectra of the complexes were calculated using quantum chemical calculations at the DFT level. From the calculations, it is derived that the Cu(3HF)+ chelate is formed in CuMOR indicating a weak interaction with the pore walls. In contrast, due to high confinement in CuZSM-5, interactions between copper cations and the narrower pore walls are assumed to take place in addition to 3HF metal complexation. To emphasize the fact that zeolites act as a solid solvent, 3HF complexation was also investigated in methanol solution. In such liquid media, a stable complex Cu(3HF)2 of 1 : 2 stoichiometry resulting in a double chelation with the metal cation was found to coexist with a minor species [Cu(3HF)(MeOH)2]+ of 1 : 1 stoichiometry. These two complexes show striking analogy with those observed in CuZSM-5 and CuMOR, respectively. Thus, it appears clearly that zeolites can constitute an ideal tool to control and orientate molecular reactivity for the guest in the isolated state.

Spectroscopic evidence of 3-hydroxyflavone sorption within MFI type zeolites: ESIPT and metal complexation.

Due to its chemical and photochemical properties and potential applications in numerous domains as a molecular probe, 3-hydroxyflavone (3HF) is a molecule of high interest. In particular, the processes of intramolecular proton transfer in the excited state and metallic complexation are known to be dependent on the chemical environment. In this context, the particular properties of zeolites make these microporous materials an environment adapted to study the reactivity of isolated molecules adsorbed in their porous void space. Thus, this report investigates the incorporation without any solvent of 3HF into the internal volume of various channel-type MFI zeolites. Using complementary techniques (diffuse reflectance UV-vis absorption, Raman scattering, FTIR, fluorescence emission and molecular modelling), very different spectral behaviours are observed in totally dealuminated silicalite-1 and in Al rich MZSM-5 (M = H(+), Na(+), Zn(2+)). In silicalite-1, the non-polar and non-protic internal micro-environment does not induce any valuable interaction between 3HF and the channel walls. Therefore, the molecule shows easy tautomer formation upon excitation. Within HZSM-5, 3HF is adsorbed in close proximity of the acid proton of the zeolite which inhibits the intramolecular proton transfer and then, only the normal form is observed at the excited state. For NaZSM-5, the spectral data show an intermediary behaviour due to the aprotic but polar environment, in agreement with 3HF sorption in close proximity of the Na(+) extra framework cation. After mixing 3HF and ZnZSM-5, the spectral features clearly indicate metallic complexation of the guest molecule. The zeolite dependent reactivity reported here demonstrates the adsorption of the guest within the internal volume because the charge balancing cations which clearly control the reaction are principally located in the zeolite channels. The 3HF incorporation into the internal volume is proved by the decrease of the microporous volume observed by nitrogen adsorption-desorption isotherm measurements. The experimental data are confirmed by Monte Carlo molecular modelling which also predicts 3HF sorption in the zeolite channels in the proximity of charge compensating cations. Consequently, as the molecule dimensions are assumed to be slightly larger than the channel size, the flexibility of the molecule and the lattice deformation have to be considered to allow 3HF penetration into the zeolite void space.

Surface-enhanced vibrational spectroscopy of B vitamins: what is the effect of SERS-active metals used?

Surface-enhanced Raman scattering (SERS) spectroscopy and surface-enhanced infrared absorption (SEIRA) spectroscopy are analytical tools suitable for the detection of small amounts of various analytes adsorbed on metal surfaces. During recent years, these two spectroscopic methods have become increasingly important in the investigation of adsorption of biomolecules and pharmaceuticals on nanostructured metal surfaces. In this work, the adsorption of B-group vitamins pyridoxine, nicotinic acid, folic acid and riboflavin at electrochemically prepared gold and silver substrates was investigated using Fourier transform SERS spectroscopy at an excitation wavelength of 1,064 nm. Gold and silver substrates were prepared by cathodic reduction on massive platinum targets. In the case of gold substrates, oxidation-reduction cycles were applied to increase the enhancement factor of the gold surface. The SERS spectra of riboflavin, nicotinic acid, folic acid and pyridoxine adsorbed on silver substrates differ significantly from SERS spectra of these B-group vitamins adsorbed on gold substrates. The analysis of near-infrared-excited SERS spectra reveals that each of B-group vitamin investigated interacts with the gold surface via a different mechanism of adsorption to that with the silver surface. In the case of riboflavin adsorbed on silver substrate, the interpretation of surface-enhanced infrared absorption (SEIRA) spectra was also helpful in investigation of the adsorption mechanism.