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 I ₀/I reached a steady value of ∼2.5. The peak in the Stern-Volmer plot at a 20/1 ratio was consistent with a maximum in the contribution from concentration quenching at this coverage. On the basis of the appearance of the dye's radical cation spectrum at low dye/NC ratios, ultrafast transient absorption spectroscopy confirmed electron transfer from the singlet excited state of the dye to the CdS NC with a lifetime of 16 ps. At higher dye/NC ratios, the signal from the radical cation was much less dominant, and the decay of the singlet excited state was dominated by the concentration quenching process having a 1 ps lifetime.