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).
Diffuse reflectance UV-visible absorption and Raman scattering experimental data show evidence of the phenothiazine (PTZ) sorption and spontaneous ionization in the straight channels of three medium pore acid zeolites with various topologies (ferrierite (H-FER), H-ZSM-5 and mordenite (H-MOR)) but analogous Si/Al contents. The spectral data highlight the combined effects of confinement and local electrostatic field on the sorption and charge separation kinetics. The PTZ incorporation and ionization appeared to be quicker in the larger pore H-MOR than in H-ZSM-5 and in H-FER. However, sorption and ionization are almost complete in the three zeolites after about one year. The low ionization potential value of PTZ (I.P. = 6.73 eV) induced quasi instantaneous formation of the radical cation PTZ N+ in high yield within the internal space of each channel structure. Nevertheless, the higher confinement effect and higher polarizing effect offered by the 10-membered rings (10-MR) channels of H-FER favoured the PTZ second ionization to form the dication PTZ 2+ . The very long lifetimes of these charge separated states are probably due to the restricted mobility of PTZ in the narrow channels and to the compartmentalization of the trapped electron away from the initial site of PTZ ionization. However, a very slow charge recombination process is observed within the three zeolite morphologies after about one year. This reaction is only partial in the narrower pores of H-FER and H-ZSM-5 whereas the faster diffusion process within the larger pore H-MOR induces quasi total cation disappearance after 2 years. Therefore, the reaction mechanism indicates clearly that PTZ N+ and PTZ 2+ are only intermediates and that the thermodynamically stable end product is the occluded PTZ molecule.
In situ diffuse reflectance UV-visible, Raman scattering and EPR experiments, carried out as a function of time after phenothiazine (PTZ) direct exposure to thermally activated acid H(n)ZSM-5 zeolite without any solvent, provide evidence of phenothiazine sorption and simultaneous spontaneous ionization. For comparison, phenothiazine behavior was investigated within non acidic zeolite and showed that most of the phenothiazine molecules were occluded as intact molecules and that ionization was very weak. The multivariate curve resolution analysis of the diffuse reflectance UV-visible spectra set recorded during the sorption process resolve the absorption spectra and respective concentrations of individual species involved in the sorption course. When PTZ entered through the acidic zeolite channels, PTZ (+)@H(n)ZSM-5 (-) radical pair is generated by the polarization energy. Subsequent PTZ(2+) formation was fast but only partial. After months, equilibrium including PTZ (+), PTZ(2+) and occluded PTZ was reached within acid H(n)ZSM-5 but depends on the Si/Al ratio: the higher the Al content, the easier the spontaneous ionization. The close match between PTZ and the pore size of zeolites combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of PTZ (+) and PTZ(2+) and hinder efficiently the charge recombination. No evidence of Brønsted acid sites of H(n)ZSM-5 was found during the sorption of phenothiazine through generation of protonated species.
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