Electron Transfers in Donor–Acceptor Supramolecular Systems: Highlighting the Dual Donor and Acceptor Role of ZSM‑5 Zeolite

After coadsorption of electron-donor (<i>p</i>-terphenyl, PTP) and electron-acceptor (1,4-dicyanobenzene, DCB) molecules within the channels of silicalite-1 and MZSM-5 (M = Na⁺, H⁺) zeolites, photoinduced or spontaneous electron transfers were investigated. In aluminum-free silicalite-1, the reaction mechanisms after PTP ionization are similar in the presence and in the absence of the acceptor molecule. Photoionization leads to a PTP^•+ radical cation, which recombines directly. In NaZSM-5, <i>p</i>-terphenyl photoexcitation induces PTP^•+ formation evolving to an electron–hole pair through capture of another electron of zeolite. This behavior is observed with and without DCB. However, when DCB is coadsorbed with PTP, recombination decays for PTP^•+ and for the electron–hole pair are significantly slower. Pulsed EPR experiments show strong electron density close to DCB, through a coupling of unpaired electrons with ¹⁴N nuclei. Nevertheless, the electron transfer remains insufficient to allow DCB^•– radical anion formation. High confinement within ZSM-5 and intrinsic strength of zeolite acceptor sites might be put forward to explain the nonformation of the anion. The acceptor properties of DCB and of the zeolite might then be competitive. The zeolite electron acceptor character is even more marked when PTP is coadsorbed with DCB in acidic HZSM-5. Ionization occurs spontaneously, and transient species are stabilized for months. No electronic coupling with nitrogen atoms of DCB could be observed, indicating no partial transfer to the acceptor molecule and electron trapping in acidic zeolite

Influence of Confinement Effect on Electron Transfers Induced by <i>t-</i>Stilbene Sorption in Medium Pore Acidic Zeolites

The mere exposure of <i>trans</i>-stilbene (<i>t</i>-St) to three types of dehydrated medium pore acid zeolites that differ by their pore diameter induces <i>t</i>-St spontaneous ionization in high yield. In situ diffuse reflectance UV–visible, 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. <i>t-</i>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 <i>t</i>-St and marked [D₁₂]<i>t-</i>St and [¹³C₂]<i>t-</i>St molecules to reveal the structural environment of the unpaired electrons through the assignment of the couplings with ¹H, ²H, ¹³C, ²⁷Al, and ²⁹Si nuclei

Chemical Control of Photoinduced Charges under Confinement in Zeolites

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 <i>trans</i>-stilbene (<i>t</i>-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 <i>trans</i>-stilbene (<i>t</i>-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 <i>t</i>-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 <i>t</i>-St^•+ are the combination of two processes: direct CR and hole transfer. The charge-separated species as <i>t</i>-St^•+···AlO₄^•– and <i>t</i>-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 <i>t</i>-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 −Δ<i>G</i>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 −Δ<i>G</i>⁰ is larger than the reorganization energy coefficient (λ), which is particularly small under high confinement. The close match between <i>t</i>-St molecular size and zeolite channel diameter is critical to generating long-lived charge separations (hours)