Thermo-responsive fluorescence of AIE-active poly(N-isopropylacrylamides) labeled with highly twisted bis(N,N-dialkylamino)arenes

Abstract

Highly twisted bis(N,N-dialkylamino)arenes, which represent a new class of viscosity-sensitive fluorophores with aggregation-induced emission (AIE) luminogens, were introduced as co-monomers and cross-linkers into poly(N-isopropylacrylamides) (PNIPAMs). Despite the excellent performance of these bis(N,N-dialkylamino)arenes as fluorophores that are sensitive to the steric environment, synthetic methods to endow them with other reactive groups have not yet been reported. This study presents short synthetic pathways to 9,10-bis(N,N-dialkylamino)anthracenes (BDAAs) and 1,4-bis(N,N-dialkylamino)-2,3-dimethylnaphthalenes (DMe-BDANs) with hydroxyl groups at their alkyl chains. These hydroxyl groups were acylated to afford methacrylate moieties, which were subsequently used for the co-polymerization with N-isopropylacrylamide. At T = 20 °C, the resulting BDAA-containing PNIPAMs exhibited faint fluorescence (Φ_fl ≈ 0.05) in THF, whereas DMe-BDAN-containing PNIPAMs showed substantially stronger fluorescence in THF (Φ_fl ≈ 0.32) and water (Φ_fl ≈ 0.55) relative to the corresponding monomers. Moreover, BDAA-containing PNIPAMs featured a sharp increase of fluorescence intensity and quantum yield at T = 27–35 °C, while DMe-BDAN-containing PNIPAMs exhibited a continuous decrease of fluorescence intensity and Φ_fl with increasing temperature.