Rational Design of Molecular Fluorophores for Biological Imaging in the NIR-II Window

Researchers report a new family of near-infrared (NIR-II) molecular fluorophores with improved fluorescence characteristics in organic solvents and aqueous solutions. The molecular fluorophore, IR-FE, is constructed using a shielding unit–donor–acceptor–donor–shielding unit (S-D-A-D-S) structure with benzobisthia-diazole (BBTD) as the acceptor, 3,4-ethylenedioxy thiophene (EDOT) as the donor, and dialkyl fluorene as the shielding unit. Compared to the thiophene donor, calculations reveal that EDOT affords a larger backbone distortion and a less delocalized lowest unoccupied molecular orbital (LUMO), and is also able to tune the electrostatic potential distribution. The alkyl chains on fluorene stretch out of the plane of conjugated backbone, which can shield the backbone from aggregation.