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A series of donor-acceptor (D-A) type low-bandgap compounds with different end groups (alkyl, alkoxy, and alkylthio) were designed and synthesized for investigation of the material properties and photodetector performance brought by subtle changes in the end groups. Among all, compound 4 with hexylthio groups exhibits the most red-shifted absorption, strongest molecular stacking, highest mobility, and ideal film morphology. These unique properties make it a promising material for use in small-molecule photodetectors. Photodetector SMPD-4 based on compound 4 exhibits broad response in the spectral region of 300-900 nm and a high specific detectivity of 1.3 x 1013 Jones at 650 nm at -0.1 V, which is the among the best values reported for solution-processed small-molecule photodetectors and even comparable to that of silicon photodetector. This work suggests that end-group engineering is a useful method in tuning the material properties and device performance of organic semiconductors.