Comparison between Spontaneous and Photoinduced Ionization Mechanisms for p-Quaterphenyl in M-ZSM-5 (M = H⁺, Na⁺) Zeolites

Abstract

The electron transfers following the initial ionization of a probe molecule (pquaterphenyl) adsorbed in the channels of M-ZSM-5 (M = H + , Na + ) zeolites are investigated using various complementary spectroscopic techniques. Under the same reaction conditions, ionization occurs spontaneously during molecule diffusion in the acid H-ZSM-5 whereas charge separation needs to be photoinduced within the pores of Na-ZSM-5. The electron transfer processes are found to be identical for both the cases in terms of transient species formed before the final charge recombination. The initial ionization leads to the formation of a radical cation which evolves gradually toward an electron/hole pair associated with a charge transfer complex. However, the reaction kinetics depend dramatically on the ionization way.As spontaneous ionization is associated with the sorption process, it is closely correlated with the molecule diffusion and thus, is very slow due to the bulky size of the molecule. In this case, radical cations and subsequent charge transfer complexes are stabilized for months in high yield and are clearly characterized by diffuse reflectance UV-visible absorption, EPR and by Raman scattering in resonance and in off-resonance conditions. After photoionization, the evolution is followed as a function of time by using time resolved UV-visible absorption spectroscopy on a large scale of time extending from a few microseconds to several days.

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

“…The reaction mechanisms observed after (photo)ionization of PTP are similar to those described above for the biphenyl 39 and for p-quaterphenyl, 60 two other molecules belonging to the family of poly(p-phenylene) but also for many other polyaromatic molecules such as naphthalene 43 or t-stilbene. 44,52 The electron transfer from the zeolite to PTP •+ and the formation of the electron−hole pair are closely associated with the oxidizing power of the radical cation [E ox (PTP •+ ) = 1.78 V/ SCE].…”

“…The reaction mechanisms observed after (photo)­ionization of PTP are similar to those described above for the biphenyl and for p-quaterphenyl, two other molecules belonging to the family of poly­(p-phenylene) but also for many other polyaromatic molecules such as naphthalene or t-stilbene. , The electron transfer from the zeolite to PTP •+ and the formation of the electron–hole pair are closely associated with the oxidizing power of the radical cation [ E ox (PTP •+ ) = 1.78 V/SCE]. This is consistent with the results previously obtained with, for example, biphenyl E ox (BP •+ ) = 1.9 V/SCE) and t-stilbene [ E ox (t-St •+ ) = 1.75 V/SCE], whereas this transfer reaction is not observed in the case of anthracene whose potential is significantly lower [ E ox (Ant •+ ) = 1.1 V/SCE] and where the stabilized form is the radical cation .…”

“…In the present work, we study the electron transfers occurring in the porous void of silicalite-1 and of MZSM-5 (M = Na + , H + ) type zeolites between an electron donor molecule ( p -terphenyl) and an acceptor (dicyanobenzene) either after photoexcitation of the sample or simple mixing. The p -terphenyl (PTP) donor molecule belongs to the poly­(p-phenylene) molecule family which is well-known for interesting optoelectronic properties. − The behavior of p -terphenyl , as well as of biphenyl and of p -quaterphenyl parent molecules upon sorption in MFI type zeolites have been already reported in previous studies. Dicyanobenzene (DCB) is an organic molecule known as being a good electron acceptor. , This molecule has two cyano groups that allow them to attract electrons by a mesomeric effect.…”

“…In a less polarizing environment, the guest is occluded as a neutral molecule in close proximity of the charge-balancing cation and ionization has to be photoinduced. − Note that the transient species created within the internal volume after spontaneous or photoionization are identical. Nevertheless, the lifetimes were found to be much longer after spontaneous ionization. ,,,, In contrast, even if this property is less-documented, the electron-donating ability of zeolite was firmly demonstrated upon photoexcitation of strong electron acceptors like 1,2,4,5-tetracyanobenzene (TCNB) , or methylviologen dication (MV 2+ ) , incorporated in zeolite which lead to the formation of the TCNB radical anion and of MV + , respectively. The 3-coordinated aluminum sites on the framework as well as the charge-balancing cations act as Lewis acids, whereas the framework oxygens, especially the oxygens adjacent to Al are considered to be the basic sites.…”

Electron transfers in a TiO2-containing MOR zeolite: synthesis of the nanoassemblies and application using a probe chromophore molecule

“…The mechanism of formation of this moiety depends on the oxidizing power of the radical and on the intrazeolite confinement as was already widely reported and discussed for this molecule and for other polyaromatics in channel type zeolites. 95 In the case of t-St in the TiO 2 -MOR samples, the different steps involved in the photoreactivity mechanism can be written as follows:…”