*2.3. General Procedure for the Synthesis of PP11–PP20*

Indane-1,3-dione (1*H*-indene-1,3(2*H*)-dione) **EA1** (0.37 g, 2.55 mmol) and the appropriate substituted benzaldehyde (2.55 mmol, 1 eq.) were dissolved in absolute ethanol (50 mL) and a few drops of piperidine were added. The reaction mixture was refluxed and progress of the reaction was followed by TLC. After cooling, a precipitate formed in most of the case. This latter was filtered off, washed several times with ethanol and dried under vacuum. For several chromophores (**PP11**, **PP12**, **PP13** or **PP20**), a purification by column chromatography on SiO2 was required.
