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 (E 0 * ) -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 × 10 5 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.

Nearly monodispersed, spherical ZnO nanocrystals were synthesized from the reaction of an amide precursor, [Zn(N(i)Bu(2))(2)](2), with hexylamine followed by reactions of the as-formed solution in a moist air flow. Extensive experiments were conducted to optimize the synthesis and to characterize the nanocrystals. The room temperature reactions led to 3.3-5.3 nm nanocrystals with the sizes increasing in direct proportion to the relative humidity. Purification afforded high yields of free-flowing nanocrystals that were dispersible in nonpolar solvents. The overall synthesis requires several days, but it results in multigram quantities of stable, redispersible nanocrystals. The nanocrystals were characterized using elemental analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), solution and solid-state NMR, IR, UV-vis absorption, and photoluminescence spectroscopies. In addition to providing H(2)O to serve as the source of oxygen in the ZnO, the air flow adds CO(2) that converts the alkylamine into an alkylammonium alkylcarbamate, which serves as the surfactant. Elemental analysis, TGA, and XPS results established that the total number of N-hexyl fragments on a 3.7 nm nanocrystal was 200, where they exist as an equal number of anionic carbamates and cationic ammonium ions. The addition of pure hexylammonium hexylcarbamate to ZnO nanocrystals prepared by literature methods resulted in the formation of a product that was similar to the ZnO formed using [Zn(N(i)Bu(2))(2)](2). Larger nanocrystals up to 7.3 nm were also obtained by heating smaller nanocrystals in a mixture of hexylamine and toluene at 119 degrees C.

Photophysical data for solution phase mixtures of a new terthiophene based organic dye, 3′,4′-dibutyl-2-phenyl-2,2′:5′,2′′-terthiophene-5′′-carboxylic acid, and size selected, well-dispersed zinc oxide nanocrystals are reported. Time-resolved fluorescence and time- and frequency-resolved pump−probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals (NCs) in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ± 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern−Volmer model. This observation is in agreement with the static quenching studies presented in the companion manuscript (DOI: 10.1021/jp1080143). By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model proposed in the companion work that incorporates competitive binding between ZnO NC s and free Zn2+ cations in solution.