Solar energy can be used to fulfil our increasing global energy demands. Since a large amount of solar
energy reaches the earth in the form of light, this type of energy can be used to trigger a system which
converts the solar energy to a fuel. This system includes a photosensitizer (PS), which can use this
energy to excite an electron to an excited state, a proton reduction catalyst (PRC), which can use the
high energy electrons to reduce protons and form H2 gas, and a sacrificial electron donor (SED), which
can donate electrons to the photosensitizer.
Reversible proton reduction in biological systems is performed by the hydrogenase enzyme. Mimics of
the active site of this enzyme, such as Fe2(bdt)(CO)6, can be employed for the use of artificial solar cells
as the PRC. To study the behaviour of the PRC and the PS in an aqueous behaviour, micelles (SDS) are
used to dissolve the PRC.
A frequently used PS is Ru(bpy)3Cl2. The combination of Ru(bpy)3Cl2 and Fe2(bdt)(CO)6 in micellar
solution with ascorbic acid as SED and 455 nm light has shown photocatalytic H2 production. However,
there is overlap between the absorption wavelengths of this PS and the PRC, making the catalyst prone
to light-induced decomposition. To prevent light-induced decomposition of the PRC a different PS has
to be employed. Two zinc porphyrin photosensitizers (PS 1 and PS 3) have been investigated, which
are known to absorb at higher wavelengths.
Electrochemical and spectrochemical experiments have been performed to determine the Gibbs free
energy change of the photoinduced electron transfer from PS 1 and PS 3 to PRC. Furthermore, emission
quenching experiments have been performed to look at the compatibility of the PS and the PRC. The
calculated Gibbs free energy changes for electron transfer from PS 1 and PS 3 to the PRC are negative
and therefore these processes are thermodynamically feasible. However, the spectrochemical
experiments showed only quenching by PRC of PS 1. There was no quenching observed for PS 3.
For the electron transfer between the SED and the PS, the Gibbs free energy change has been
calculated and reductive quenching has been examined. Again the Gibbs free energy change is negative
and so the process is feasible. However, for both PS 1 and PS 3 no reductive quenching of the excited
state was measured in the spectrochemical experiments. To test the systems with PRC and PS 1 and
PS 3 in practice, the photocatalytic produced H2 was measured with GC experiments. Comparing these
results with the results obtained for the Ru(bpy)3Cl2 photosensitizer, it is suspected that the electron
flow from the SED to the PS is poor and could be limiting. This problem could be solved by using
electron shuttles, different SEDs, or water soluble photosensitizers which work in a similar fashion as
Ru(bpy)32+.