Polyenes, ionization spectroscopy

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Keywords ZSM-5, diphenyl polyene, ionization, charge separation, spectroscopy. 

We mentioned in Section III.A that one of the unique features of radical ion optical spectroscopy is that it allows one to measure excited-state energies of a molecule at two different geometries, namely that of the neutral species (If in PE spectra) and that of the relaxed radical cation (Xmax of the EA bands). In many cases this feature is of little relevance because either the geometry changes upon ionization are too small to lead to noticeable effects (e.g. in aromatic hydrocarbons), or because such effects are obscured, due to the invisibility of the states in one or other of the two experiments (i.e. strong cr-ionizations in the PE spectrum) or because of the near-cancellation of opposing effects (as in the case of linear conjugated polyene radical cations). 

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