Electron Transfer to Colloidal ZnO Nanocrystals from the Excited State of a Perylene Chromophore Formed by Direct Excitation or Triplet–Triplet Annihilation Upconversion

Abstract

Upon binding 4-(perylen-3-yl)benzoic acid 1, to oleate-capped ZnO nanocrystals (NCs), significant broadening of the perylene absorption spectrum and fluorescence quenching were observed. In contrast, no broadening of the absorption spectrum or fluorescence quenching was observed when 1 was bound to oleylamine-capped ZrO2 NCs. Measurement of emission quenching by ZnO NCs as a function of dye concentration quantified binding and established that approximately 150 dyes attached to the surface of 3.7 nm diameter ZnO NCs, which correlated roughly to the number of zinc ions on the NC surface. A combination of steady-state spectroscopy, spectroelectrochemistry, and ultrafast pump-probe spectroscopy established that fluorescence quenching occurred via a fast excited-state electron transfer mechanism with a lifetime of less than 100 fs. The singlet excited state of 1 was also generated in solution and on the surface of ZrO2 NCs via triplet–triplet annihilation upconversion (TTA-UC) using a bodipy sensitizer that was present in solution or bound to the ZrO2 surface. Out of a possible maximum of 0.5, the TTA-UC quantum yields in CH2Cl2 were 0.0037 with and 0.0034 without ZrO2 NCs. Following selective excitation of the bodipy dye, dispersions containing both dyes and ZnO NCs showed no upconverted fluorescence from the perylene, demonstrating the ability of ZnO NCs to harvest two low-energy photons through TTA-UC.