Synthesis and Optical Properties of Push–Push–Pull Chromophores Based on Imidazo[5,1,2-cd]indolizines and Naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines

Four series of new push–push–pull chromophores based on two types of heterocyclic cores, namely imidazo[5,1,2-cd]indolizines and naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines, and two types of linear π spacers, namely diphenylacetylene and biphenyl, were prepared. The two isomeric heterocyclic compounds were prepared by palladium-catalyzed double C–H functionalization reactions. Further palladium-catalyzed C–C coupling reactions allow the construction of diphenylacetylene and biphenyl spacers on the chromophores. Different electron-donors including NMe₂, NPh₂, and OMe and CN acceptors were incorporated into the chromophores. The corresponding donor- and acceptor-free compounds were also prepared for comparative studies. In the case of chromophores with strong NMe₂ or NPh₂ donors, a strong intramolecular charge-transfer (ICT) process occurs leading to fluorescence emission quenching in polar solvents. The use of a slightly weaker OMe donor together with the naphtho[1′,2′:4,5]imidazo[1,2-a]pyridine core allows to obtain an intense blue fluorescence with high quantum yield of up to 0.825. DFT and TD-DFT calculations offer satisfactory explanations to the observed photophysical data.