Photoinduced electron transfers after t-stilbene incorporation in zeolite. Effect of the presence of an electron acceptor on the reactivity

- Coadsorption of t-stilbene (t-St) and of dicyanobenzene (DCB) in MFI zeolite is described.
- In Al-rich ZSM-5, photoinduced electron transfers are mediated by the framework.
- Confinement and internal electrostatic field strongly influence the electron transfers.
- The acceptor characters of NaZSM-5 and of dicyanobenzene are competitive.
- Strong electron density close to DCB is associated with partial electron trapping.

Diffuse reflectance UV-vis absorption and EPR spectroscopies are used to study photoinduced electron transfers in supramolecular assemblies composed of donor (t-stilbene, t-St) and acceptor (dicyanobenzene, DCB) guest molecules incorporated in MFI zeolites. By using Al-free silicalite-1, it clearly appears that the presence of the acceptor has no effect on the reactivity and that no electron transfer occurs to DCB after t-St photoionization and radical cation (t-St●+) formation. On the contrary, we show evidence for the striking slowing down of the recombination reaction in the presence of the acceptor molecule in the channels of MZSM-5 (M = Na+, Cs+). In these Al-rich zeolites, electron transfers are mediated by the framework which does not remain passive after photoexcitation. Indeed, zeolite acts both as an electron acceptor and as an electron donor and strongly influences the reaction mechanisms. The donor character is put forward to explain the indirect recombination of the radical cation to form an electron/hole pair. Concerning the acceptor ability of zeolite, we show that the attracting and stabilizing character of NaZSM-5 competes with the acceptor property of dicyanobenzene and then, does not allow total electron transfer to the guest. Even if the DCB●− radical anion formation does not occur, HYSCORE experiments show strong electron density in the surrounding of DCB indicating partial electron transfer and explaining the trapping of electron and the slowing down of the charge recombination.

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