Excited State Electron Transfer from Donor−π System–Acceptor Dyes to ZnO Nanocrystals

Monodispered ZnO nanocrystals (NCs) were found to quench the fluorescence of two donor−π system–acceptor (D−π–A) dyes; (E)-2-cyano-3-[5-[4-(diethylamino)phenyl]thiophen-2-yl]-2-propenoic acid, 1, and its furan analogue, 2. Parameters based on single crystal X-ray crystallography and DFT calculations confirmed planar structures for both dyes and delocalization of the donor nitrogen electrons into the π-system. Both dyes exhibited a quasi-reversible one-electron oxidation with Eo values of 0.39 and 0.35 V for 1 and 2, respectively, versus the ferrocene/ferrocenium redox couple, and spectroelectrochemical measurements revealed the absorption spectra of the oxidized products. In stark contrast to earlier reports of related dyes that did not bear a strong donor substituent, the ZnO nanocrystal fluorescence quenching efficiency was nearly quantitative. Adsorption isotherms revealed equilibrium binding constants 2.5(0.5) × 105 and 8(1) × 105 M−1 for 1 and 2, respectively, and large values for the maximum number of dyes per nanocrystal. Ultrafast fast pump–probe measurements of 1 and 2 in CH2Cl2 revealed formation of singlet excited states that decayed with lifetimes of 1660(30) and 1600(100) ps, respectively. Addition of an equimolar amount of ZnO NCs caused the singlet excited state of each dye to disappear with concurrent formation of the spectral signatures for the corresponding oxidized products, thus allowing the assignment of the process to an excited state electron transfer to the ZnO NCs. Electron transfer lifetimes for 1 ranged from 14.8(4) to 18.2(6) ps as the ZnO NC diameter decreased from 5.0 to 3.2 nm, while for 2 the lifetimes ranged from 11.1(3) to 9.5(3) ps for a similar change in ZnO NC diameters. The weak dependence of the excited state electron transfer lifetimes on the diameter of the NCs is consistent with a reaction dominated by changes in their size-dependent density of states.