Effects of fused rings linked to the 2,5-position of pyrrole derivatives with near-infrared emission on their aggregation-enhanced emission properties

Fluorescent materials with near-infrared (NIR) emission are important kinds of functional dyes for bioimaging and medical diagnosis. In this work, a type of pyrrole derivative with NIR emission was designed and synthesized through the introduction of different fused rings at the 2,5-position of pyrrole (MAP) and a furanylidene (FE) group at the 3-position of pyrrole (MAP-FE), which constructed a typical donor-π bridge-acceptor (D-π-A) structure. All MAP-FEs have shown aggregation-enhanced emission (AEE) properties. Different fused rings or different positions of substitution on the same fused ring had significant effects on the NIR photoluminescence (PL) properties. In addition, by comparison with the change in the radiative and nonradiative rates of the precursor MAPs and MAP-FEs in solution and the solid state, it can be concluded that the rings fused to the 2,5-position of pyrrole have a two-sided effect on the PL properties of organic dyes: one effect is an improvement in the quantum yield, and the other effect causes a π-π stacking interaction that is unfavorable for emission. The introduction of FE groups as acceptors in MAP-FEs not only redshifted emission wavelengths but also effectively avoided π-π stacking in the aggregated state, which are beneficial for PL properties. These results could provide general guidance for designing NIR dyes with AEE characteristics when using fused rings to improve their PL properties.