Stilbenoid dendrimers possessing a methanol or methane core are
prepared using Wittig-Horner reactions. The dendritic tris(stilbenyl)carbinols 3a-c are
converted into the corresponding methylium salts 3′a-c on treatment with trifluoroacetic or
tetrafluoroboric acid. The extended cross-conjugation
in the dendritic methylium ion 3′c results
in a long-wavelength absorption (λmax = 697 nm)
whereas the conjugated model systems 3′a and 3′b are red-shifted (λmax = 772
nm and 748 nm, respectively). Charge delocalization in the cations
is evident from the low-field shifts of the ¹³C NMR
signals. In contrast to the dark-blue dyes 3′a-c, the tris(stilbenyl)carbinols and tetrakis(stilbenyl)methane
derivatives 3a-c and 7 exhibit long-wavelength absorption bands
in the UV region.
stilbene - charge delocalization - Wittig-Horner
reaction - olefination - dendrimers - dyes
