Donor-substituted alkynes and electron-accepting olefins undergo a formal [2+2] cycloaddition, followed by retro-electrocyclization, under formation of non-planar D-π-A chromophores. The transformations are typical "click-chemistry"-type reactions: high-yielding, atom-economic, and in most cases very rapid. Despite their pronounced non-planarity, the resulting chromophores feature efficient intramolecular CT interactions, show high third-order optical nonlinearities. We also achieved multiple cascades of the click reaction yielding chromophores with strong optical absorptions spanning the entire visible light wavelength.
The new chromophores remain potent electron acceptors and can even be classified as "super acceptors". Lately, we demonstrated that efficient push-pull conjugation also occurs in homoconjugated and spiroconjugated systems. We conjugated these push-pull chromophores to fullerenes and observed unprecedented rearrangements on the carbon sphere. Axial chirality is enforced in push-pull substituted buta-1,3-dienes, conjugated to C60, by a single alkyl group on the surface.
In the second part, synthesis and characterization of unprecedented chiral, alleno-acetylenic chromophores with all-carbon backbones, composed of 1,3-diethynylallene repeat units, is presented. Optically active alleno-acetylenic macrocycles were obtained featuring exceptionally intense Cotton effects. Unprecedented amplification of chirality is observed in monodisperse, enantiomerically pure alleno-acetylenic oligomers. Despite the steric protection of the delicate allene moiety, intramolecular transformations can be established, such as Diels-Alder cycloadditions and allenyl-Cope rearrangements.