π-Expanded benzothiazole dyes with excited-state intramolecular proton-transfer process: Synthesis, photophysical properties, imaging in cells and zebrafishs

Abstract In this paper, we have designed and synthesized HBT-fused aryl-imidazole derivatives HBTA, HBTB, HBTC and HBTD through π-expanded system, which have large absorption extinction coefficients, relatively high quantum yields, large Stokes shifts. The dyes HBTB, HBTC and HBTD still retained excited state intramolecular proton-transfer property (E* and K* emission). They also exhibited good ESIPT properties in serum. The experimental results and theoretical calculations verified that the nonplanar and nitrogen heterocyclic structure of π-expanded rings was beneficial for the k* emission under the same conditions. Moreover, The HBTA, HBTB, HBTC and HBTD were successfully applied for the imaging in living cells and zebrafish.