Photocatalytic water splitting with acridine dyes: Guidelines from computational chemistry

The photocatalytic splitting of water into H and OH radicals in hydrogen-bonded chromophore-water complexes has been explored with computational methods for the chromophores acridine orange (AO) and benzacridine (BA). These dyes are strong absorbers within the range of the solar spectrum. It is shown that low-lying charge-transfer excited states exist in the hydrogen-bonded AO-H₂O and BA-H₂O complexes which drive the transfer of a proton from water to the chromophore, which results in AOH-OH or BAH-OH biradicals. The AOH and BAH radicals possess bright ππ∗ excited states with vertical excitation energies near 3.0 eV which are predissociated by a low-lying repulsive πσ∗ state. The conical intersections of the πσ∗ state with the ππ∗ excited states and the ground state provide a mechanism for the photodetachment of the H-atom by a second photon. Our results indicate that AO and BA are promising chromophores for water splitting with visible light.