  The photocatalytic splitting of water into molecular hydrogen and molecular
oxygen with sunlight is the dream reaction for solar energy conversion. Since
decades, transition-metal-oxide semiconductors and supramolecular
organometallic structures have been extensively explored as photocatalysts for
solar water splitting. More recently, polymeric carbon nitride materials
consisting of triazine or heptazine building blocks have attracted considerable
attention as hydrogen-evolution photocatalysts. The mechanism of hydrogen
evolution with polymeric carbon nitrides is discussed throughout the current
literature in terms of the familiar concepts developed for photoelectrochemical
water splitting with semiconductors since the 1970s. We discuss in this
perspective an alternative mechanistic paradigm for photoinduced water
splitting with carbon nitrides, which focusses on the specific features of the
photochemistry of aromatic N-heterocycles in aqueous environments. It is shown
that a water molecule which is hydrogen-bonded to an N-heterocycle can be
decomposed into hydrogen and hydroxyl radicals by two simple sequential
photochemical reactions. This concept is illustrated by first-principles
calculations of excited-state reaction paths and their energy profiles for
hydrogen-bonded complexes of pyridine, triazine and heptazine with a water
molecule.
