Thienothiadiazole-Based NIR-II Dyes with D–A–D Structure for NIR-II Fluorescence Imaging Systems

Fluorescence imaging (FI) in the second near-infrared optical window (NIR-II, 1000–1700 nm) has received increasing focus due to its capacity of high spatiotemporal resolution, rapid real-time imaging, and deep penetration depth. In addition, D–A–D-based organic small molecules have also attracted wide attention due to their designed chemical structure and rapid renal metabolism. However, most of the fluorescent cores were based on benzobisthiadiazole (BBTD) and 6,7-diphenyl-[1,2,5]thiadiazolo[3,4- g ]quinoxaline (TTQ). The design and development of fluorescent core still remain challenging. Therefore, two NIR-II dyes based on the acceptor 4,6-di(2-thienyl)thieno[3,4- c ][1,2,5]thiadiazole (TTDT) were designed and developed with donors tributyl(5-(9,9-dioctyl-9H-fluoren-2-yl)thiophen-2-yl)stannane (TF) and (5-(9,9′-spirobi[fluoren]-2-yl)thiophen-2-yl)tributylstannane (TSF) by the Stille coupling reaction, respectively. Subsequently, the corresponding nanoparticles were prepared, and then TTDT-TF-based nanoparticles with superior photostability and strong NIR-II fluorescence signals were chosen for NIR-II FI. More importantly, the in vivo experiments suggested that TTDT-TF NPs exhibited significant accumulation at tumor sites and high signal-to-background ratio (SBR). The above results indicated that the two D–A–D-type fluorophores based on TTDT have potential for NIR-II FI with superior imaging quality and imaging-guided surgery or therapy.