Photocatalytic water splitting with acridine dyes: Guidelines from computational chemistry

- Photoexcited acridine dyes are able to abstract a hydrogen atom from water.
- Photodetachment of the hydrogen atom from the radicals regenerates the catalyzer.
- The reaction mechanisms were characterized with ab initio electronic-structure calculations.
- The chromophores and radicals absorb within the range of the solar spectrum.

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 AOH2O and BAH2O complexes which drive the transfer of a proton from water to the chromophore, which results in AOHOH or BAHOH 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.

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