Electric Field-Assisted Photochemical Water Splitting Should Operate with 287 nm Light

The major photoreaction of water is the homolytic splitting of one O–H bond starting from the 11B1 excited state (λmax = 167 nm). This reaction produces H• and •OH radicals. The combination of two H• atoms leads to the potential energy carrier dihydrogen. However, the energy required to obtain the photoreactive 11B1 electronic state is about 7.4 eV, which cannot be effectively provided by solar radiation. The sun light spectrum on earth comprises the visible and ultraviolet region, but shows vanishing intensity near 7 eV (177.1 nm). This work provides theoretical evidence that the photoreactive 11B1 state of water can be shifted into the ultraviolet (UV‐B) light region (≈287 nm) by including explicitly an electric field in the calculations of the water absorption spectrum. To accomplish such bathochromic shift, a large field strength of 3.08 VÅ−1 is required. The field‐dependent excitation energies were calculated by applying the symmetry‐adapted cluster configuration interaction (SAC‐CI) procedure. Based on this theoretical analysis, we propose that photochemical water splitting can be accomplished by means of 287 nm light provided the water molecule is favorably oriented by an external electric field and is subsequently activated by a reversal of the field orientation. MOs characterizing the electronic structure of the Frank‐Condon 11B1 and 11A2 states of water influenced by an external electric field. The field impact should induce homolytic water splitting.