Various functional precursors based on polythiophene derivatives are designed to prepare carbon dots (C-dots) with tunable emissions ranging from blue to near-infrared (NIR) at a single excitation wavelength (400 nm). The as-prepared C-dots demonstrate homogeneous size, superior optical properties, excellent water solubility, and low cytotoxicity. Thus, the C-dots are candidates for bio-imaging. A tunable photoluminescence mechanism is proposed to result from variations in the surface state and N content.