Authors: Giane Benvinda Damas, Cleber Fabiano N. Marchiori, and Carlos Moyses Araujo
Polymeric materials containing an extended π-conjugated backbone have shown a wide range of applicability including photocatalytic activity for hydrogen evolution reaction (HER). The latter requires highly efficient materials with optimal light absorption and thermodynamic driving force for charge transfer processes, properties that are tailored by linking chemical units with distinct electron-affinity to form donor-acceptor architecture. Here, this concept is explored by means of ab initio theory in benzothiadiazole-based polymers with varying electron-rich moieties, viz. fluorene (PFO), cyclopentadithiophene (CPT), methoxybenzodithiophene (O-BzT), thiophenebenzodithiophene (T-BzT), thiophene- (T, VT) and thienethiophene (TT, VTT)-based units. All materials exhibit a red-shifted absorption spectrum with respect to the reference polymer (PFO-DT-BT), while keeping the catalytic power for hydrogen production almost unchanged. In particular, a displacement of ∆λ=167 nm in the first absorption maximum has been achieved upon combination of chemical units with high donating character in CPT-VTT-BT. Furthermore, the exciton binding energies (Eb) have been systematically investigated to unveil the effects of geometry relaxation, environment polarity and finite temperature contributions to the free energy. For instance, we show a significant change in Eb when going from the gas phase (Eb=1.43-1.85 eV) to the solvent environment (Eb=0.29-0.54 eV in 1-bromooctane with ε=5.02). Furthermore, we have found a linear correlation between the lowering of exciton binding energies and the increasing of the ratio between donor and acceptor contributions to the HOMO orbital. This is a consequence of increased donating ability and enhanced spatial separation of electron-hole pairs, which weakens their interaction. Finally, our findings reveal that the donor unit plays a crucial role on key properties that govern the photocatalytic activity of donor-acceptor polymers contributing to the development of a practical guideline to design more efficient photocatalysts for HER. This goes through a proper combination of electron-rich moieties to tune the optical gap, favor thermodynamic driving force for charge transfer and lower exciton binding energies.
J. Phys. Chem. C, 2019, 123, 42, 25531-25542