Photocatalytic Water Splitting with the Acridine Chromophore: A Computational Study

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• 作者：Xiaojun Liu ; Tolga N. V. Karsili ; Andrzej L. Sobolewski ; Wolfgang Domcke
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：August 20, 2015
• 年：2015
• 卷：119
• 期：33
• 页码：10664-10672
• 全文大小：502K
• ISSN：1520-5207

文摘

The hydrogen-bonded acridine鈥搘ater complex is considered as a model system for the exploration of photochemical reactions which can lead to the splitting of water into H^{鈥?/sup> and OH鈥?/sup> 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 蟺* orbital of acridine. The low-energy barriers which separate these reactive charge-transfer states from the spectroscopic states of the acridine鈥搘ater 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)鈥揳cceptor (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) 蟺蟺* excited states with appreciable oscillator strengths in addition to a low-lying dark 蟺蟺* excited state. The bound potential-energy functions of the 蟺蟺* excited states are predissociated by the potential-energy function of an excited state of 蟺蟽* character which is repulsive with respect to the NH stretching coordinate. The dissociation threshold of the 蟺蟽* state is about 2.7 eV and thus below the excitation energies of the bright 蟺蟺* states. The conical intersections of the 蟺蟽* state with the 蟺蟺* 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鈥揌₂O complex can be dissociated into H鈥?/sup> and OH鈥?/sup> radicals by the absorption of two visible/ultraviolet photons.}