Photoinduced water splitting with oxotitanium porphyrin: A computational study

The photochemistry of the hydrogen-bonded oxotitanium porphyrin-water complex (TiOP-H₂O) has been explored with electronic-structure calculations. It is shown that intramolecular charge-transfer processes, which are initiated by the excitation of the Soret band of TiOP, accumulate electronic charge on the oxygen atom of TiOP, which in turn abstracts a hydrogen atom from water by an exoenergetic and essentially barrierless hydrogen-transfer reaction, resulting in the TiPOH-OH biradical. About 75% of the absorbed photon energy is thus stored as chemical energy in two ground-state radicals. Absorption of a second photon by TiPOH can result in the detachment of the H radical and recovery of the photocatalyzer TiOP. Again, about 75% of the photon energy is stored in the dissociation energy of TiPOH. Overall, a water molecule is decomposed into H and OH radicals by the absorption of two visible photons. Exoenergetic radical recombination reactions can yield molecular hydrogen, molecular oxygen or hydrogen peroxide as closed-shell products.