P. Lakshminarasimhan et al., Characterization of persistent intermediates generated upon inclusion of 1,1-diarylethylenes within CaY zeolite: Spectroscopy and product studies, J PHYS CH B, 103(43), 1999, pp. 9247-9254
Inclusion of 1,1-diarylethylenes within activated CaY results in the format
ion of colored samples that are stable for prolonged periods. For example,
diphenylethylene generates a green color within CaY. Two reactive intermedi
ates have been shown to be responsible for this color. The species with the
short wavelength (lambda(max) 410 nm) absorption has been established to b
e the diphenylmethyl cation. The identity of the species with the long wave
length ((lambda(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 th
is absorption for diarylethylene-CaY samples unequivocally eliminates the p
ossibility 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 ((lam
bda(max) 620 nm) absorption within CaY.