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Excited State Electron Transfer from Donor−π System–Acceptor Dyes to ZnO Nanocrystals
journal contribution
posted on 2020-07-02, 02:29 authored by Rachel Swedin, Deepak Badgurjar, Andrew Healy, Robin Harkins, Amanda Oehrlein, Lindsey Greenlund, Mohammed Alshebber, Nathan Ripp, Nathaniel T. Anderson, Benjamin R. Honzay, Ted M. Pappenfus, Daron E. Janzen, David A. Blank, Wayne L. GladfelterMonodispered 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.
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state electron transfer lifetimesZnO NC diameterselectron transfer lifetimesZnO Nanocrystals Monodispered ZnO n...dyeZnO NC diametercrystal X-ray crystallographydonor nitrogen electronsCH 2 Cl 2DFTZnO NCsstate electron transferZnO nanocrystal fluorescence quench...Excited State Electron Transferquasi-reversible one-electron oxidation
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