Photocatalytic Water Splitting with the Acridine Chromophore: A Computational Study

The hydrogen-bonded acridine-water complex is considered as a model system for the exploration of photochemical reactions which can lead to the splitting of water into H-center dot and OH center dot radicals. The vertical excitation energies of the lowest singlet and triplet excited states of the complex were calculated with the CASSCF/CASPT2 and ADC(2) ab initio electronic-structure methods. In addition to the well-known excited states of the acridine chromophore, excited states of charge-transfer character were identified, in which an electron is transferred from the p orbital of the H2O molecule to the pi* orbital of acridine. The low-energy barriers which separate these reactive charge-transfer states from the spectroscopic states of the acridine-water complex have been characterized by the calculation of two-dimensional relaxed potential-energy surfaces as functions of the H atom-transfer coordinate and the donor (O)-acceptor (N) distance. When populated, these charge-transfer states drive the transfer of a proton from the water molecule to acridine, which results in the acridinyl-hydroxyl biradical. The same computational methods were employed to explore the photochemistry of the (N-hydrogenated) acridinyl radical. The latter possesses low-lying (about 3.0 eV) pi pi* excited states with appreciable oscillator strengths in addition to a low-lying dark pi pi* excited state. The bound potential-energy functions of the pi pi* excited states are predissociated by the potential-energy function of an excited state of pi sigma* character which is repulsive with respect to the NH stretching coordinate. The dissociation threshold of the pi sigma* state is about 2.7 eV and thus below the excitation energies of the bright pi pi* states. The conical intersections of the pi sigma* state with the pi pi* excited states and with the electronic ground state provide a mechanism for the direct and fast photodetachment of the H atom from the acridinyl radical. These computational results indicate that the H2O molecule in the acidine-H2O complex can be dissociated into H-center dot and OH center dot radicals by the absorption of two visible/ultraviolet photons.