Promoting the near-infrared-II fluorescence of diketopyrrolopyrrole-based dye for in vivo imaging via donor engineering

Small-molecule dyes for fluorescence imaging in the second near-infrared region (NIR-II, 900‒1880 nm) hold great promise in clinical applications. Constructing donor-acceptor-donor (D-A-D) architecture has been recognized to be a feasible strategy to achieve the NIR-II fluorescence. However, the development of NIR-II dyes via such a scheme is hampered by the lack of high-performance electron-acceptors and donors. Diketopyrrolopyrrole (DPP), as a classic organic optoelectronic material, enjoys strong light absorption, high fluorescence quantum yield (QY), and facile derivatization. Nevertheless, its application in the NIR-II imaging field has been hindered by its limited electron-withdrawing ability and the aggregation-caused quenching (ACQ) effect resulted from the planar structure of DPP. Herein, with DPP as an electron-acceptor and through the donor engineering, we have successfully designed and synthesized a DPP-based dye named as T-27, in which the strong D-A interaction confers excellent NIR absorption and high-brightness NIR-II fluorescence tail emission. By strategically introducing long alkyl chains on the donor unit to increase intermolecular spacing and reduce the influence of solvent molecules, T-27 exhibits an improved anti-ACQ effect in the aqueous solutions. After being encapsulated into DSPE-PEG2000, T-27 nanoparticles (NPs) show a relative NIR-II fluorescence QY of 3.4% in water, representing the highest value among the DPP-based NIR-II dyes reported to date. The outstanding photophysical properties of T-27 NPs enable the multi-mode NIR-IIa bioimaging under the 808 nm-excitation. As such, the T-27 NPs can distinguish mouse femoral vein and artery, and achieve cerebral vascular microscopic imaging with a penetrating depth of 800 μm, demonstrating the capability for high-resolution deep-tissue imaging.