Tunable benzothiadiazole-based donor–acceptor materials for two-photon excited fluorescence†
Here we describe the synthesis of eight benzothiadiazole-based D–π–A–π–D materials and evaluate their potential as imaging agents for two-photon excited fluorescence (2PEF). These compounds exhibit fluorescent emission with λmax ranging from 529 to 674 nm in toluene and quantum yields as high as 99%. Emission from the aggregated state is also examined with all species exhibiting emission from the aggregated state. Finally, three of these compounds are encapsulated within an amphiphilic water-soluble polymer and explored as imaging agents. The use of benzothiadiazole functionalized with two locked-planarity hexamethylazatriangulene (HMAT) donors was found to give red fluorescence with the highest two-photon cross-section σ2 of 230 GM, while also exhibiting the highest photostability of the series.