π-Conjugated Indole Dyads with Strong Blue Emission Made Possible by Stille Cross-Coupling and Double Fischer Indole Cyclisation

Small fluorescent π-conjugated indolyl-based molecules 4–6, 23 are prepared through direct Fischer synthesis or/and Stille cross-coupling method in appreciable yields. Our synthetic results have shown the benefits using Stille approach when Fischer double cyclisation method to access the bisindole dyads 4–6, 23 is not efficient. The synthetic routes to these materials have been designed to investigate the substrate requirements for the respective cyclisation and CC-coupling reactions and to evaluate their wider synthetic applicability. Comparative analysis of the different substituents and the different π-bridging units e. g. pyridine, thiophene and thiazole on the electronic and photophysical properties of the final compounds 4–6, 23 has been carried out. The structure−property relationship of the final bisindole dyads has been investigated via photophysical characterisation, and computational modelling. The obtained compounds absorb near-UV and visible (blue) light, with the spectral range dependent on the nature of the π-bridging units, and are capable of bright blue emission.