  We present a conspectus of recent joint spectroscopic and computational
studies which provided novel insight into the photochemistry of hydrogen-bonded
complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules
(water and phenol). It was found that a functionalized derivative of Hz,
tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a
homogeneous photochemical reaction. This allows the exploration of the basic
mechanisms of the proton-coupled electron-transfer (PCET) process involved in
the water photooxidation reaction in well-defined complexes of chemically
tunable molecular chromophores with chemically tunable substrate molecules. The
unique properties of the excited electronic states of the Hz molecule and
derivatives thereof are highlighted. The potential energy landscape relevant
for the PCET reaction has been characterized by judicious computational
studies. These data provided the basis for the demonstration of rational laser
control of PCET reactions in TAHz-phenol complexes by pump-push-probe
spectroscopy, which sheds light on the branching mechanisms occurring by the
interaction of nonreactive locally excited states of the chromophore with
reactive intermolecular charge-transfer states. Extrapolating from these
results, we propose a general scenario which unravels the complex photoinduced
water-splitting reaction into simple sequential light-driven one-electron redox
reactions followed by simple dark radical-radical recombination reactions.
