Kinetics and characterization of photoinduced long-lived electron–hole pair of p-terphenyl occluded in ZSM-5 zeolites. Effects of aluminium content and extraframework cation

Diffuse reflectance UV-visible in combination with FT-Raman spectroscopies demonstrate the total incorporation without any solvent of p-terphenyl (p-TP) as an intact molecule in the medium size channel of non-acidic MnZSM-5 (M = Li+, Na+, K+, Rb+, Cs+ and n = 0, 3.4, 6.6) zeolites. The combined effects of confinement and electrostatic field induced by alkaline ions in the MnZSM-5 zeolites lead only to weak conformational changes in the occluded p-TP after very long organization periods. The interaction between the counterbalancing cation and p-TP occurs through one phenyl group facially coordinated to the cation near the O atoms binding Al atoms. The laser UV photolysis of p-terphenyl occluded as intact molecules in non-acidic MnZSM-5 zeolites generates long-lived charge separated states. The photoionization induces a p-TP˙+–electron pair as a primary phenomenon. The recombination of the p-TP˙+@MnZSM-5˙− radical cation moiety occurs mainly through unusual electron abstraction from the zeolite framework and p-TP@MnZSM-5˙−˙+ electron–hole pair formation which exceeds several days at room temperature in Li6.6ZSM-5. The very long-lived radical pairs are characterized by conventional DRUVv, FT-Raman and CW-EPR spectroscopy. Two-dimensional hyperfine sublevel correlation (2D-HYSCORE) experiments reveal the structural surroundings of the unpaired electrons through the proper assignment of unpaired electron couplings. The subsequent hole transfer from the radical cation of the channels as well as the final electron–hole pair recombination appear to be largely controlled by the aluminium content, the size of the extra framework cation and the associated local electrostatic field. The effects of the counterbalancing cations have been investigated and because the zeolite electron affinity increases on going from Li+ to Cs+, the electron transfer rates increase according to the following order Li+ < Na+ < K+ < Rb+ < Cs+.