Carbazole-functionalized polyphenylene-decorated solid state emissive D–A–D molecules: Reduced donor–acceptor interaction and enhanced emission in the solid state

A new series of D–A–D molecules (1–3) with carbazole as a donor and 2,5-diaryl-1,3,4-oxadiazole, dibenzothiophene-S,S-dioxide and benzo[1,2,5]thiadiazole as acceptors have been synthesized. Polyphenylene dendrons have also been incorporated to prevent the face to face π–π stacking in aggregates. Unlike traditional D–A systems, which show ACQ problems and large red shifts in the solid state, these derivatives (1–3) exhibit good solid state luminescence behavior without undergoing a red shift or fluorescence quenching. The present results indicate that just the combination of a donor–acceptor system with bulkier dendrons is not enough to generate efficient solid state emissions but the prevention of co-facial interaction between donors and acceptors in aggregates is more important. In the case of derivative 3, restriction of complete molecular planarization and steric hindrance near the acceptor can effectively prevent intermolecular donor–acceptor interactions, thus leading to enhanced emission in aggregates. While in the case of derivative 1, a sterically unhindered acceptor is easily approached by a donor due to close molecular packing in aggregates forming weakly luminescent species