  Effective photoinduced charge transfer makes molecular bimetallic assemblies
attractive for applications as active light induced proton reduction systems.
For a more sustainable future, development of competitive base metal dyads is
mandatory. However, the electron transfer mechanisms from the photosensitizer
to the proton reduction catalyst in base metal dyads remain so far unexplored.
We study a Fe-Co dyad that exhibits photocatalytic H2 production activity using
femtosecond X-ray emission spectroscopy, complemented by ultrafast optical
spectroscopy and theoretical time-dependent DFT calculations, to understand the
electronic and structural dynamics after photoexcitation and during the
subsequent charge transfer process from the FeII photosensitizer to the
cobaloxime catalyst. Using this novel approach, the simultaneous measurement of
the transient Kalpha X-ray emission at the iron and cobalt K-edges in a
two-colour experiment is enabled making it possible to correlate the excited
state dynamics to the electron transfer processes. The methodology, therefore,
provides a clear and direct spectroscopic evidence of the Fe->Co electron
transfer responsible for the proton reduction activity.
