Effect of the π-bridge on the light absorption and emission in push-pull coumarins and on their supramolecular organization

A family of eight π-extended push-pull coumarins with cross-conjugated (amide) and directly conjugated (p-phenylene, alkyne, alkene) bridges were synthesized through a convergent strategy. Using an experimentally calibrated computational protocol, their UV-Visible light absorption and emission spectra in solution were investigated. Remarkably, amide-, alkyne- and alkene-bridges undergo comparable vertical excitations. The different nature of these bridges manifests during excited-state relaxation and fluorescence. We predict that these molecules can serve as building blocks for p-type semiconductors with low reorganization energies, below 0.2 eV. Since solid-state self-assembly is crucial for this application, we examined the effect of the π-bridge over the supramolecular organization in this family of compounds to determine if stacking prevails in these π-extended coumarin derivatives. Amide and alkyne spacers allow coplanar conformations which crystallize readily; p-phenylene hinders planarity yet allows facile crystallization; alkene-bridged molecules eluded all crystallization attempts. All the crystals obtained feature dense face-to-face π-stacking with 3.5-3.7 Å interlayer distances, expected to facilitate charge transfer processes in the solid state.

Synthesis of 2-chromanone-fused [3.2.0] bicycles through a phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction

A phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction of alkynone is described. 2-Chromanone-fused bicyclo[3.2.0]heptenones were synthesized in moderate to high yields with remarkably high regio- and diastereoselectivities.

Molecular engineering of carbazole–acrylonitrile fluorophores: substituent-dependent optical properties and mechanochromism

Controlling the fluorescence properties of organic molecules in the aggregation state and understanding the structure–activity relationship are important for developing excellent luminophors with tunable solid emission.