A Metal–Organic Tetrahedron as a Redox Vehicle to Encapsulate Organic Dyes for Photocatalytic Proton Reduction

The
design of artificial systems that mimic highly evolved and
finely tuned natural photosynthetic systems is a subject of intensive
research. We report herein a new approach to constructing supramolecular
systems for the photocatalytic generation of hydrogen from water by
encapsulating an organic dye molecule into the pocket of a redox-active
metal–organic polyhedron. The assembled neutral Co4L4 tetrahedron consists of four ligands and four cobalt
ions that connect together in alternating fashion. The cobalt ions
are coordinated by three thiosemicarbazone NS chelators and exhibit
a redox potential suitable for electrochemical proton reduction. The
close proximity between the redox site and the photosensitizer encapsulated
in the pocket enables photoinduced electron transfer from the excited
state of the photosensitizer to the cobalt-based catalytic sites via
a powerful pseudo-intramolecular pathway. The modified supramolecular
system exhibits TON values comparable to the highest values reported
for related cobalt/fluorescein systems. Control experiments based
on a smaller tetrahedral analogue of the vehicle with a filled pocket
and a mononuclear compound resembling the cobalt corner of the tetrahedron
suggest an enzymatic dynamics behavior. The new, well-elucidated reaction
pathways and the increased molarity of the reaction within the confined
space render these supramolecular systems superior to other relevant
systems.