Spontaneous Charge Separation and Recombination Induced by Tetracene Incorporation in Pores of Acidic H<i>_n</i>ZSM-5 Zeolites

Abstract

In situ CW-EPR and diffuse reflectance UV−visible spectroscopy were used to monitor the spontaneous incorporation of ACENE-4 or tetracene (C 18 H 12 ) in the medium-pore M n ZSM-5 zeolites [M n (AlO 2 ) n (SiO 2 ) 96 - n; M Na +, H +; n = 3.4, 6.6] by direct exposure under dry and inert atmosphere of solid ACENE-4 to dehydrated porous materials without any solvent. The sorption of the large ACENE-4 molecule with relatively low ionization potential (6.97 eV) occurs in Brønsted acidic H n ZSM-5 zeolites according to a complex and slow reaction sequence including protonation, charge separation, hole transfer, and charge recombination while ACENE-4 is incorporated as an intact molecule in nonacidic Na n ZSM-5. After a long organization period, ACENE-4 lies in the straight channel in front of sodium cation in close proximity of Al framework atom as simulated by Monte Carlo calculations. The multivariate curve resolution (MCR) analysis of the huge DRUVv spectra set recorded during the ACENE-4 sorption course in acidic H n ZSM-5 resolved successfully the specific absorption spectra and respective concentrations of all species as function of time. HACENE-4 + @H n - 1 ZSM-5 - protonated species and ACENE-4 •+ @H n ZSM-5 •- radical pair are generated in the first steps of sorption while a long-lived ACENE-4@H n ZSM-5 •-•+ electron−hole pair is formed through hole transfer and recombines slowly to ACENE-4@H n ZSM-5 without any protonation. Two-dimensional hyperfine-sublevel correlation (2D-HYSCORE) experiments reveal the structural surroundings of the unpaired electrons through the proper assignment of unpaired electron couplings with 1 H, 29 Si, and 27 Al nuclei. The tight fit between the rod shape ACENE-4 and the pore size of ZSM-5 zeolites combined with the efficient polarizing effect of proton and aluminum electron trapping sites are the most important factors responsible for the stabilization of the electron−hole moiety and hinder the charge recombination efficiently.