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It is challenging to develop molecular fluorophores in the second near-infrared (NIR-Ⅱ) window with long wavelength emission and high brightness,which can improve the performance of biological imaging.Herein,we report a molecular engineering approach to afford NIR-Ⅱ fluorophores with these merits based on fused-ring acceptor (FRA) molecules.Dioctyl 3,4-propylenedioxy thiophene (PDOT-C8) is utilized as the bridging donor to replace 3-ethylhexyloxy thiophene (3-EHOT),leading to more than 20 times enhancement of brightness.The nanofluorophores (NFs) based on the optimized CPTIC-4F molecule exhibit an emission peak of 1,110 nm with a fluorescence quantum yield (QY) of 0.39％ (QY of IR-26 is 0.050％ in dichloroethane as reference) and peak absorption coefficient of 14.5 × 104 M-1·cm-1 in aqueous solutions,which are significantly higher than those of 3-EHOT based COTIC-4F NFs.It is found that PDOT-C8 can weaken intermolecular aggregation,enhance protection of molecular backbone from water,and decrease backbone distortion,beneficial for the high brightness.Compared with indocyanine green with same injection dose,CPTIC-4F NFs show 10 times higher signal-to-background ratio for whole body vessels imaging at 1,300 nm long pass filters.