Bathochromic or hypsochromic effects via the extension of conjugation: a study of stilbenoid squaraines

In contrast to normal conjugated oligomers, the stilbenoid squaraines 3a–d do not show a convergence of VIS/NIR absorption due to the extension of conjugation; the observed bathochromic effect in the beginning of the series is followed by a hypsochromic effect; this result seems to be characteristic for stilbenoid compounds with donor–acceptor substitution.