Convergence of absorption and fluorescence in cross-conjugated oligomers consisting of chalcone building blocks

The absorption and fluorescence spectra of two series of chalcone oligomers (1a-d) and (2a-d) are reported. Due to small or vanishing orbital coeffici ents (HMO calculation) within the cross-conjugated systems, the bathochromi c shifts caused by the extension of the chains are modest. The effective co njugation lengths amount to 6 and 14 chalcone building blocks in the absorp tions of the series 1 and 2. The convergence of the fluorescence bands is a lready reached in Id and 2d with 4 and 8 enone units, respectively. Besides the uniform or alternate arrangement of the chalcone building blocks, the positions of the propoxy substituents show a considerable influence on the electronic transitions provided that push-pull effects are operating.