Long-Lived Spin-Correlated Pairs Generated by Photolysis of Naphthalene Occluded in Non-Brønsted Acidic ZSM-5 Zeolites

Abstract

Long-lived spin-correlated pairs were generated by laser irradiation of naphthalene (NAP) occluded as intact molecule within non-Brønsted acidic MnZSM-5 zeolites with MnSiO(2))(96-n)(AlO(2)n formula per unit cell. The laser UV photoionization generates primary NAP.+-electron pair as a fast phenomenon. These charge carriers exhibit lifetimes that extend over less than 1 h at room temperature and disappear according to two parallel competitive ways: direct charge recombination and electron transfer. This subsequent electron transfer takes place between the electron-deficient radical cation (NAP.+) and the electron-donor oxygen atom of zeolite framework. The aluminum rich MnZSM-5 zeolites (n = 3.4, 6.6) hinder efficiently the charge recombination and promote the electron transfer to generate a very long electron-hole pair which exceeds several weeks at room temperature in NAP@Li(6.6)ZSM-5. The electron-hole pair exhibits broad visible absorption bands at 482 and 525 nm. The electron-hole distance, 1.3 nm, was deduced from the dipolar interaction term (D) value. The spin density of trapped electron appears spread over (27)Al, (29)Si, (7)Li, and (1)H nuclei as deduced by two-dimensional approach of hyperfine sublevel correlation (HYSCORE). The very low recombination rate by tunneling effect was found to be in agreement with the very low value (J approximately 0) of the magnetic exchange. The combined effects of tight fit between NAP size and straight-channel dimension, the high aluminum content of the framework, and the highly polarizing cation Li(+) trapped efficiently the ejected electron in the conduction band and the hole in the valence band of the porous materials.