Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

Abstract

The locations of Brønsted acid sites (BAS) in the channels of medium-pore zeolites have a significant effect on the spontaneous ionization of para-terphenyl (PP(3)) insofar as spatial constraints determine the stability of transition states and charge-transfer complexes relevant to charge separation. The ionization rates and ionization yield values demonstrate that a strong synergy exists between the H(+) polarization energy and spatial constraints imposed by the channel topology. Spectroscopic and modeling results show that PP(3) incorporation, charge separation, charge transfer and charge recombination differ dramatically among zeolites with respect to channel structure (H-FER, H-MFI, H-MOR) and BAS density in the channel. Compartmentalization of ejected electrons away from the initial site of ionization decreases dramatically the propensity for charge recombination. The main mode of PP(3)(.+) decay is hole transfer to form AlO(4)H(.+) ⋅⋅⋅PP(3) charge-transfer complexes characterized by intense absorption in the visible range. According to the nonadiabatic electron-transfer theory, the small reorganization energy in constrained channels explains the slow hole-transfer rate.

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

“…Moreover, it might be interesting to compare the present results with the data previously observed with tstilbene 60 for which the reaction mechanism of electron hole pair formation from radical cations was found to be reversed by excitation at the maximum of absorption of the CTC.Consequently, in the present case the excitation of the sample at 532 nm is also expected to induce partial reformation of radical cation. Thus, the presence of a broad band in the 1600-1620 region might also include a contribution of the radical cation reformed upon excitation.This species is indeed expected around 1614 cm -1 according to the resonance Raman experiments reported above.Thus, contrary to what was observed for other parent molecules (biphenyl, p-terphenyl)19,28,39 and other polyaromatics (naphthalene, tetracene)21,29 , the FT Raman spectra allow the observation of the radical cation. This result gives evidence of the high content of radicals stabilized in the zeolite framework and has to be associated with the very slow kinetic of the reaction.…”

“…Nevertheless, the evolution observed here after mixing p-QPh and H-ZSM-5 is very similar in terms of spectral features and of spectral concentrations to our previous works performed with other polyaromatic molecules in various channel type zeolites. 21,22,23,24,28,29,39,40,41 For all these studies, we have shown that the initial radical cation formed after ionization evolved more or less rapidly toward very long lived charge separated states due to electron transfer from zeolite to the radical cation. Even if the development of the DRUVv bands clearly shows that this second species develops at the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 expense of the radical cation, the assignment has to be done by coupling the UV-visible data with the Raman scattering and EPR experiments (see below) which demonstrate also the formation of two different species all along the time evolution.…”

“…To our knowledge, this spectrum does not correspond to any previously reported spectrum for p-QPh related compounds. Based on our own experience in such systems and by analogy with previous works related to electron donor ability of ZSM-5 zeolite framework towards electron deficient radical cations of biphenyl, p-terphenyl, naphthalene, tetracene, and t-stilbene, 21,22,23,24,28,29,39,40,41 this spectrum is tentatively assigned to a charge transfer complex (AlO 4 •+ -p-QPh) associated with the formation of an electronhole pair due to subsequent electron transfer occurring after the radical formation according to equation 3.…”

“…For this study, we have chosen p-quaterphenyl (p-QPh) as probe molecule because i) it belongs to the poly-p-phenylene oligomer family, the molecules of which display interesting opto-electronic properties as they can be active layers in organic light emitting devices and field effect transistors; 42,43 they also gain high electrical conductivity upon doping with acceptors and donors; 44,45 ii) it presents a relatively low ionization potential value in gas phase (I.P.= 8.08 eV) 46 and iii) it has the suitable dimensions to penetrate into the pores of dehydrated medium pore M-ZSM-5. Note that we have already reported the sorption and ionization reactions of biphenyl 19,40,41 and of p-terphenyl 28,39 in ZSM-5, corresponding to the two first molecules of poly p-phenylene family, but the higher 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 …”

Spontaneous Ionization of N‐Alkylphenothiazine Molecules Adsorbed in Channel‐Type Zeolites: Effects of Alkyl Chain Length and Confinement on Electron Transfer

“…[7,14] In particular, the remarkable properties of the micropore volume can lead to spontaneous ionization of an electron-donor molecule incorporated in the zeolite and to stabilization of charge-separated states for very long times. [15][16][17] The mechanism of spontaneous charge separation has been well characterized and reported even in the case of polyaromatic molecules with relatively low ionization potential occluded in channel-type zeolites.…”