Excited-State Quenching Mechanism of a Terthiophene Acid Dye Bound to Monodisperse CdS Nanocrystals: Electron Transfer versus Concentration Quenching

Oleate-capped CdS nanocrystals (NCs) dispersed in dichloromethane were found to quench the excited-state fluorescence of the terthiophene derivative 3′,4′-dibutyl-5″-phenyl-[2,2′:5′,2″-terthiophene]-5-carboxylic acid (1-CO2H). Infrared and 1H NMR spectroscopies provided evidence that 1-CO2H substitutes for oleate on the surface of the CdS NCs. Upon binding, the fluorescence of 1-CO2H is quenched, and the 1H NMR lines from 1-CO2H are broadened. The importance of the carboxylate group in binding to the CdS NC was further established by examining the behavior of a similar fluorophore where the carboxylic acid group was replaced with a bromo substituent (1-Br). The CdS NCs had no influence on the fluorescence intensity or NMR line shapes of 1-Br. For 1-CO2H, Stern–Volmer plots indicated a nearly linear increase in I0/I as the CdS NCs’ concentration was increased, but as the dye/NC ratio reached ∼20/1, I0/I reached a maximum of ∼8 and began to decrease. By a dye/NC ratio of 2/1, the I0/I reached a steady value...