Bis-, tris-, and tetrakis(squaraines) linked by stilbenoid scaffolds

The oligosquaraines 1-5, with stilbenoid scaffolds, were prepared by multistep syntheses in which the final steps consisted of condensation reactions between the semisquaric acid 21 and multiple resorcinols 12b-15b and 17b. The target compounds exhibit intense (epsilon > 250000 L(.)mol(-1.)cm(-1)) and sharp absorption bands with maxima between 687 to 778 nm, depending on the conjugation in the stilbenoid scaffold. Comparison with the monosquaraine 6 as a model compound reveals intramolecular interactions between different donor-acceptor-donor moieties, which give rise to increased absorption intensities.