Characterization of persistent intermediates generated upon inclusion of 1,1-diarylethylenes within CaY zeolite: Spectroscopy and product studies

Inclusion of 1,1-diarylethylenes within activated CaY results in the formation of colored samples that are stable for prolonged periods. For example, diphenylethylene generates a green color within CaY. Two reactive intermediates have been shown to be responsible for this color. The species with the short wavelength (λ_max 410 nm) absorption has been established to be the diphenylmethyl cation. The identity of the species with the long wavelength ((λ_max 620 nm) absorption has been debated for three decades. We establish that the diarylethylene radical cation is not responsible for the long wavelength absorption. Laser flash photolysis studies show that a number of diarylethylene radical cations have a characteristic absorption in the 400 nm region in both solution and in NaY zeolite. The absence of this absorption for diarylethylene-CaY samples unequivocally eliminates the possibility that these radical cations are responsible for color formation. This is substantiated by the fact that a number of 2-methyl-substituted 1,1-diarylethylenes generate only the short wavelength cation absorption upon inclusion in CaY; radical cation formation would be expected to be equally facile for the methyl-substituted derivatives. Our results suggest that an allylic dimer cation is the most likely source of the long wavelength ((λ_max 620 nm) absorption within CaY.