Extending the Stokes Shifts of Donor–Acceptor Fluorophores by Regulating the Donor Configuration for In Vivo Three-Photon Fluorescence Imaging

Simple donor–acceptor (D–A) molecules with large Stokes-shifted and bright near-infrared (NIR) emissions are extremely attractive for the high-resolution three-photon fluorescence (3PF) imaging of deep tissues. Herein, we present a new strategy for significantly extending the Stokes shifts of D–A fluorophores, relying on increasing the excited-state structural/electronic relaxation of the donor moiety. The prototype of the D–A structure DPCN, containing the planar donor N,N′-diphenyl-dihydrophenazine (DHP) and the acceptor malononitrile, exhibits normal Stokes-shifted (∼60 nm) NIR emission. By introducing two methyl groups at the ortho sites of the fused DHP ring of DPCN, DMPCN (methyl-decorated DPCN) is developed with an obviously bent donor unit. Compared to DPCN, DMPCN exhibits significantly blue-shifted absorption and keeps NIR luminescence with a large Stokes shift of ∼200 nm, mainly due to the remarkable bent-to-planar transformation of the donor upon photoexcitation. Additionally, the DMPCN NPs offer enhanced luminescence quantum yields (11%) and three-photon excitation cross sections. More importantly, DMPCN NPs exhibit excellent performances for in vivo 3PF imaging of the cerebrovascular tissue and lipid droplets (LDs) in a fatty liver with a depth of 1200 and 50 μm, respectively. This research provides a distinct strategy for extending the Stokes shifts of D–A fluorophores, inspiring the utilization of dynamic NIR fluorophores for in vivo 3PF imaging.