A series of all-donor type alternating (D1-D2) monodisperse oligomers based on cyclopentadithiophene (CPDT) and 1,4-difluorobenzene (DFB) with gradually increasing chain lengths containing 3 to 15 monomers were successfully synthesized via one-pot C-H direct arylation reaction.As confirmed by various structural characterizations,all these long-chain oligomers are monodispersed with defined structures.The length-dependent optical and electrochemical properties with the evolution from the shortest oligomer (O1) to the longest oligomer (O7) and their parent polymer P1 have been studied in details.By simply tuning the mixing ratio of discrete oligomers,the photoluminescence (PL) in a wide range of color emissions extending to near-white can be facilely modulated,illustrating the potential of these monodisperse conjugated oligomers for light emission application.The atom-,step-and pot-economic synthetic strategy here developed will open the door toward efficient and controllable synthesis of π-conjugated oligomers for accurate structure-property relationship study and optic & electronic device applications.