Validating a Density Functional Theory Approach for Predicting the Redox Potentials Associated with Charge Carriers and Excitons in Polymeric Photocatalysts

We
compare, for a range of conjugated polymers relevant to water-splitting
photocatalysis, the predictions for the redox potentials associated
with charge carriers and excitons by a total-energy ΔDFT approach
to those measured experimentally. For solid-state potentials, of the
different classes of potentials available experimentally for conjugated
polymers, the class measured under conditions which are the most similar
to those during water splitting, we find a good fit between the ionization
potentials predicted using ΔB3LYP and those measured experimentally
using photoemission spectroscopy (PES). We also observe a reasonable
fit to the more limited data sets of excited-state ionization potentials,
obtained from two-photon PES, and electron affinities, measured by
inverse PES, respectively. Through a comparison of solid-state potentials
with gas phase and solution potentials for a range of oligomers, we
demonstrate how the quality of the fit to experimental solid-state
data is probably the result of benign error cancellation. We discuss
that the good fit for solid-state potentials in vacuum suggests that
a similar accuracy can be expected for calculations on solid-state
polymers interfaced with water. We also analyze the quality of approximating
the ΔB3LYP potentials by orbital energies. Finally, we discuss
what a comparison between experimental and predicted potentials teaches
us about conjugated polymers as photocatalysts, focusing specifically
on the large exciton-binding energy in these systems and the mechanism
of free charge carrier generation.