A novel fluorescent–colorimetric chemosensor 1,8-bis{2-{N-[2′-(8′-hydroxy-9,10-anthraquinon-1-yloxy)ethyl]benzimidazoliumyl}ethoxy}-9′,10′-anthraquinone hexafluorophosphate (3) was prepared and characterized by ¹H and ¹³C NMR spectroscopies; its single crystal structure was also obtained. By fluorescence and UV titrations, ¹H NMR titrations, and HRMS and IR spectra, we explored the cation recognition performance of 3, and the results revealed that 3 was a highly effective chemosensor for Co²⁺. After mixing with Co²⁺, a distinct color change (from orange to red under visible light) was visible to naked eyes accompanied with remarkable fluorescence quenching. A hypochromatic shift (ca. 27 nm) and a new peak at 487 nm were observed in the absorption spectra of 3. Furthermore, 3 was also applied for the detection of Co²⁺ by using a smartphone with the calculated limit of detection down to 0.47 μM.