Binding and static quenching behavior of a terthiophene carboxylate on monodispersed zinc oxide nanocrystals

The synthesis of 3′,4′-dibutyl-2-phenyl-2,2′:5′, 2″-terthiophene-5″-carboxylic acid and its behavior with monodispersed ZnO having diameters from 2.7 to 3.2 nm are reported. The excited state of the dye (E⁰* = -1.61 V vs NHE) was quenched upon binding to ZnO nanocrystals. Adsorption isotherms were measured for the terthiophene dye in ethanol and fit with a Langmuir model, which gave a size-independent K_ads of 2.3 ± 1.0 × 10⁵ M^-1. The maximum number of attached dyes per nanocrystal depended on the diameter and was consistent with each dye occupying 0.5 ± 0.1 nm ² at maximum coverage. Deviation from the Langmuir model observed at low dye concentrations was attributed to a small amount of free zinc ion present in solution that binds the carboxylate ions more strongly than do ZnO nanocrystals. Incorporation of the equilibrium expression between zinc ion and free carboxylate into the model provided a satisfactory fit for both the adsorption isotherm experiments and the complex shape of the Stern-Volmer graphs. Treatment of the terthiophene dye-nanocrystal dyads with increasing concentrations of sodium acetate in ethanol resulted in gradual displacement of the dye.