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

Abstract

After coadsorption of electron-donor (p-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, p-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.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

“…For DHP/Na 6.6 ZSM-5 composites, the CS state’s lifetime ranges from 3850 min at 293 K to 8 min at 353 K, while for DHP/Na 3.4 ZSM-5, the lifetime changes from 65 min at 293 K to 3 min at 353 K. This difference reflects the key role of both the temperature and aluminum content in the DHP •+ electron recombination process . When trans -stilbene ( t -St, Scheme ) is trapped in zeolite cages, it also has a long-lived CS, one that stays for hours. ,, Within the channels of nonacidic-doped ferrierite, mordenite, and inverted mordenite zeolites [M-FER, M-MOR and M-MFI, respectively where (M = Na + , K + , Rb + and Cs + )], a photoinduced intermolecular ET reaction was observed in t -St. , In these composites, the back ET process includes direct charge recombination (CR), hole transfer (HT), and electron–hole recombination to reform the caged t -St (Figure ). …”

“…Thus, NaY is found to be the best zeolite to facilitate charge separation by electron migration in photoexcited species. Another report studied ET processes between the electron-donor p -terphenyl (PTP, Scheme ) and acceptor DCB within Na-ZSM5 zeolites . In the absence of DCB, photoexcitation of PTP in PTP/NaZSM-5 composites generates PTP •+ radical-cations with a lifetime of 21 min that evolve into an electron–hole pair in which the electron is captured by the zeolite framework with a time constant of 653 min (Figure ).…”

“…In PTP-DCB/NaZSM-5 complexes, the radical-cation and electron–hole pair lifetimes are longer (83 and 3300 min, respectively). This long-living state is mainly due to the strong confinement that is imposed by the ZSM-5 nanochannels and the intrinsic strength of its A sites . In addition to that, both the ZSM framework and the encapsulated DCB molecules compete for the electron capture …”

Electronic and Geometrical Structure of Zn+ Ions Stabilized in the Porous Structure of Zn-Loaded Zeolite H-ZSM-5: A Multifrequency CW and Pulse EPR Study

“…On contact with dehydrated zeolites, guest metal atoms, introduced from the vapor phase, can be spontaneously ionized by the zeolite host lattice, leading to the stabilization of extra-lattice cations and excess electrons into the zeolite structure . In a similar way, molecular species can spontaneously ionize, leading to long-lasting charge-separated states. , …”

“…1 In a similar way, molecular species can spontaneously ionize, leading to long-lasting charge-separated states. 2,3 Of the many cationic clusters reported to form in zeolites, the majority have been alkali metal species, 1,4 although heavy metals of group 12 (Zn, Cd, and Hg) have also been investigated in the past. 5−9 Recently, attention has been focused on zinc-modified zeolite materials prepared through the direct sublimation of metallic zinc, where the stabilization of unusual Zn 2 2+10−12 and Zn +13,14 species in the zeolite cages has been reported, along with interesting catalytic properties.…”