Many works reported the encapsulation of iodine in metal–organic frameworks as well as the I2 → I3 – chemical conversion. This transformation has been examined by adsorbing gaseous iodine on a series of UiO-66 materials and the different Hf/Zr metal ratios (0–100% Hf) were evaluated during the evolution of I2 into I3 –. The influence of the hafnium content on the UiO-66 structure was highlighted by PXRD, SEM images, and gas sorption tests. The UiO-66­(Hf) presented smaller lattice parameter (a = 20.7232 Å), higher crystallite size (0.18 ≤ Φ ≤ 3.33 μm), and smaller SSABET (818 m2·g–1) when compared to its parent UiO-66­(Zr)  a = 20.7696 Å, 100 ≤ Φ ≤ 250 nm, and SSABET = 1262 m2·g–1. The effect of replacing Zr atoms by Hf in the physical properties of the UiO-66 was deeply evaluated by a spectroscopic study using UV–vis, FTIR, and Raman characterizations. In this case, the Hf presence reduced the band gap of the UiO-66, from 4.07 eV in UiO-66­(Zr) to 3.98 eV in UiO-66­(Hf). Furthermore, the UiO-66­(Hf) showed a blue shift for several FTIR and Raman bands, indicating a stiffening on the implied interatomic bonds when comparing to UiO-66­(Zr) spectra. Hafnium was found to clearly favor the capture of iodine [285 g·mol–1, against 230 g·mol–1 for UiO-66­(Zr)] and the kinetic evolution of I2 into I3 – after 16 h of I2 filtration. Three iodine species were typically identified by Raman spectroscopy and chemometric analysis. These species are as follows: “free” I2 (206 cm–1), “perturbed” I2 (173 cm–1), and I3 – (115 and 141 cm–1). It was also verified, by FTIR spectroscopy, that the oxo and hydroxyl groups of the inorganic [M6O4(OH)4] (M = Zr, Hf) cluster were perturbed after the adsorption of I2 into UiO-66­(Hf), which was ascribed to the higher acid character of Hf. Finally, with that in mind and considering that the EPR results discard the possibility of a redox phenomenon involving the tetravalent cations (Hf4+ or Zr4+), a mechanism was proposed for the conversion of I2 into I3 – in UiO-66based on an electron donor–acceptor complex between the aromatic ring of the BDC linker and the I2 molecule.

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The mere exposure of trans-stilbene (t-St) to three types of dehydrated medium pore acid zeolites that differ by their pore diameter induces t-St spontaneous ionization in high yield. In situ diffuse reflectance UVÀvisible, EPR, and Raman spectra recorded over several months highlight the exceptional stability of the charge separated states formed in ferrierite (H-FER), H-MFI, and mordenite (H-MOR). The increase in the pore diameter from H-FER to H-MOR induces different behaviors after radical cation formation. t-St •+ is stabilized for months in the narrow pores of H-FER, whereas in the larger pore H-MFI, relatively fast electron abstraction (hole transfer) takes place from the zeolite framework to create charge transfer complexes. Pulsed EPR experiments were performed using t-St and marked [D 12 ]t-St and [ 13 C 2 ]t-St molecules to reveal the structural environment of the unpaired electrons through the assignment of the couplings with 1 H, 2 H, 13 C, 27 Al, and 29 Si nuclei.

In situ FT-Raman scattering spectroscopy was used to monitor the sorption kinetics of 2,2'- and 4,4'-bipyridine in acidic ZSM-5 zeolites. The data processing of all the Raman spectra was applied to extract the characteristic Raman spectra of occluded species and respective Raman contribution generated from many spectral data which resolves spectrum of mixture into pure component spectra without any prior information. The assignment of the extracted spectra was performed according to careful comparison with corresponding spectra extracted from a set of Raman spectra recorded during the protonation of 2,2'- or 4,4'-bipyridine (bpy) in hydrochloric acid aqueous solutions. The data processing of the Raman spectra recorded during the slow sorption of 4,4'-bpy in acidic H(n)ZSM-5 (n = 3, 6) zeolites provides specific Raman spectrum of N,N'-diprotonated dication 4,4'-bpyH(2)(2+) as unique species generated in the void space of acidic ZSM-5 zeolites. No evidence of Lewis acid sites was found during the sorption of 4,4'-bpy by Raman scattering spectroscopy. The data processing of the Raman spectra recorded during the slow sorption of 2,2'-bpy in acidic H(n)ZSM-5 (n = 3, 6) zeolites provides specific Raman spectrum of trans-N-monoprotonated cation 2,2'-bpyH+ as major species generated in the void space of acidic ZSM-5 zeolites at loading corresponding to 1 mol per unit cell. The trans/cis interconversion occurs at higher loading even after the complete uptake of the sorbate and indicates some rearrangement in the void space over a long time. The cations were found to be located in straight channels in the vicinity of the intersection with the zigzag channel of the porous materials with the expected conformations deduced from ab initio calculations. However, the motions of occluded species within the channel of ZSM-5 are hindered but remain in the range of the isotropic limit of a liquid at room temperature.

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A series of Zr-based UiO-n MOF materials (n = 66, 67, 68) have been studied for iodine capture. Gaseous iodine adsorption was collected kinetically from a home-made setup allowing the continuous measurement of iodine content trapped within UiO-n compounds, with organic functionalities (À H, À CH 3 , À Cl, À Br, À (OH) 2 , À NO 2 , À NH 2 , (À NH 2 ) 2 , À CH 2 NH 2 ) by in-situ UV-Vis spectroscopy. This study emphasizes the role of the amino groups attached to the aromatic rings of the ligands connecting the {Zr 6 O 4 (OH) 4 } brick. In particular, the preferential interaction of iodine with lone-pair groups, such as amino functions, has been experimentally observed and is also based on DFT calculations. Indeed, higher iodine contents were systematically measured for amino-functional-ized UiO-66 or UiO-67, compared to the pristine material (up to 1211 mg/g for UiO-67-(NH 2 ) 2 ). However, DFT calculations revealed the highest computed interaction energies for alkylamine groups (À CH 2 NH 2 ) in UiO-67 (À 128.5 kJ/mol for the octahedral cavity), and pointed out the influence of this specific functionality compared with that of an aromatic amine. The encapsulation of iodine within the pore system of UiO-n materials and their amino-derivatives has been analyzed by UV-Vis and Raman spectroscopy. We showed that a systematic conversion of molecular iodine (I 2 ) species into anionic I À ones, stabilized as I À •••I 2 or I 3 À complexes within the MOF cavities, occurs when I 2 @UiO-n samples are left in ambient light.

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The organized internal porous void of dehydrated zeolites provides a suitable environment to promote long-lived photoinduced charge separation. Herein we have conducted time-resolved UV–visible absorption spectroscopy experiments from nanosecond to day time scale following nanosecond UV (266 nm) pulsed laser irradiation of trans-stilbene (t-St) occluded in channels of nonacidic M–FER, M–MFI, and M–MOR zeolites with various pore diameters, with differing framework aluminum content, and with different extraframework cations (M = Na+, K+, Rb+, and Cs+). The cation radical of trans-stilbene (t-St•+) and trapped electron (AlO4 •–) have been generated directly by means of laser-induced electron transfer within the channels of medium pore zeolites. We have highlighted that the general back electron transfer processes include direct charge recombination (CR), hole transfer (HT), and finally electron–hole recombination to re-form the occluded t-St ground state without any isomerization or oligomerization. It was demonstrated once again that zeolites can be active participants as electron acceptors and electron donors. The decays of t-St•+ are the combination of two processes: direct CR and hole transfer. The charge-separated species as t-St•+···AlO4 •– and t-St-AlO4 •+···AlO4 •– moieties are stabilized for approximately 10 h in aluminated medium pore zeolites with small extraframework cation such as Na+. The most remarkable feature of the transient t-St–AlO4 •+ entity formation in M–MFI and M–MOR is the persistent intense color due to the prominent absorption bands in the visible range. The very slow CR rates are explained both by the long distance between the separated charges and by the large difference in free energy between the electron acceptor and electron donor (driving force −ΔG 0), which increases with Al content in the order Cs+ < Rb+ < K+ < Na+. The CR rates are markedly slowed by shifting them deep into the inverted region of the Marcus parabola where −ΔG 0 is larger than the reorganization energy coefficient (λ), which is particularly small under high confinement. The close match between t-St molecular size and zeolite channel diameter is critical to generating long-lived charge separations (hours).