Photocatalytic Water Splitting with the Acridine Chromophore: A Computational Study

Abstract

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(•) and OH(•) 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 π* 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) ππ* excited states with appreciable oscillator strengths in addition to a low-lying dark ππ* excited state. The bound potential-energy functions of the ππ* excited states are predissociated by the potential-energy function of an excited state of πσ* character which is repulsive with respect to the NH stretching coordinate. The dissociation threshold of the πσ* state is about 2.7 eV and thus below the excitation energies of the bright ππ* states. The conical intersections of the πσ* state with the ππ* 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(•) and OH(•) radicals by the absorption of two visible/ultraviolet photons.

Solvation effects alter the photochemistry of 2-thiocytosine

“…In fact, this phenomenon is closely related to UV-induced water splitting reactions observed for pyridine [66,67], acridine [68] and heptazine [69]. Therefore, we suggest that further studies of thionated compounds with similar photochemical properties might be an interesting further direction for photochemical water splitting, particularly since, the aforementioned nitrogenous heterocycles exhibit predominantly singlet and not triplet reactive photochemistry.…”

Photoinduced water splitting in pyridine water clusters

“…It seems that only hot PyH radicals can lose hydrogen atoms. Other aromatic molecules 42,43 are expected to lead to similar water splitting reactions and will be tested in the near future. Fig.…”

Photoinduced water splitting via benzoquinone and semiquinone sensitisation

“…2,4 However, judicious substitution with π-donating groups (such as NH 2 or OMe) has previously been shown to greatly reduce the bond dissociation energy of phenol 47 -which, to first order, should translate to BQ and anthraquinones, thus lowering the threshold for photoreaction (2). This fact is related, of course, to the exceptionally high oxidation potential of photoexcited BQ and implies that near-UV photons are required to drive the photodissociation of BQH· or BQH 2 .…”

“…The UV mediated photosynthesis of H 2 O and CO 2 [2][3][4] BQ is a photoactive chromophore that exhibits a large redox potential. It is therefore widely used as a hydrogen-accepting reagent in organic syntheses and serves as a dehydration agent.…”