  Hydrogen fuel can contribute as a masterpiece in conceiving a robust
carbon-free economic puzzle if cleaner methods to produce hydrogen become
technically efficient and economically viable. Organic photocatalytic materials
such as conjugated microporous materials (CMPs) are potential attractive
candidates for water splitting as their energy levels and optical bandgap as
well as porosity are tunable through chemical synthesis. The performances of
CMPs depend also on the mass transfer of reactants, intermediates and products.
Here, we study the mass transfer of water (H2O and D2O), and of triethylamine
used as a hole scavenger for hydrogen evolution, by means of neutron
spectroscopy. We find that the stiffness of the nodes of the CMPs is correlated
with an increase in trapped water, reflected by motions too slow to be
quantified by quasi-elastic neutron scattering (QENS). Our study highlights
that the addition of the polar sulfone group results in additional interactions
between water and the CMP, as evidenced by inelastic neutron scattering (INS),
leading to changes in the translational diffusion of water, as determined from
the QENS measurements. No changes in triethylamine motions could be observed
within CMPs from the present investigations.
