Research On The Important Role Of Excited Hydrogen Bonds In Artificial Photosynthesis System

Inspired by natural photosynthesis,the concept of artificial photosynthesis was proposed to convert solar energy into usable chemical energy without producing greenhouse gases and pollutants in the process.However,there are still a lot of problems in this process,the most important problem is the low energy conversion rate.The main reason lies in the insufficient research on the reaction mechanism.Artificial photosynthesis is mainly determined by three reaction processes:light-harvesting processes,charge generation,and separation processes,and catalytic reaction processes.The total efficiency is determined by the thermodynamic and kinetic balance of the above process,but it is difficult to determine where the bottleneck of the reaction occurs because the mechanism is not clear.Moreover,the photocatalytic reaction is carried out in the excited state,not the ground state,but previous research focuses on the ground state properties of the photocatalyst.Only by studying the photophysical and photochemical processes in the excited state can we further understand the mechanism of artificial photosynthesis.More importantly,artificial photosynthesis systems are all carried out in aqueous solutions.There are a large number of hydrogen bonds in the system that connect reactants and catalysts,and hydrogen bonds often play an important role in the transfer of energy,electrons,and hydrogen.In summary,it is extremely important to study the behavior of hydrogen bonds in excited states.In this paper,molecular photochemistry theory and photocatalysis theory are combined,two representative artificial photosynthesis systems of g-C₃N₄ and homogeneous catalyst sodium iron chlorophyll are selected,and time-dependent perturbation theory and Fermi golden rule are used to study the photophysical process.Time-dependent density functional theory studies the photochemical process and combines experiments to discuss the important role of excited state hydrogen bonds in artificial photosynthesis.（1）Select g-C₃N₄ as the photocatalyst and successfully photodecompose water to produce hydrogen by adding photosensitizer.In theoretical calculations,the periodic structure is intercepted by structural primitives to make it possible to calculate the excited state,and the structural primitives and H₂O are used to construct hydrogen-bonded complexes,and the overall photophysical and photochemical processes of the hydrogen-bonded complexes are studied.Through calculation results,it is found that the excited state hydrogen bond promotes energy,hydrogen,and electron transfer in the electron-hole recombination and photochemical process.At the same time,it is obtained that the bottleneck of g-C₃N₄ photolysis of water for hydrogen production is in the photogenerated electron-hole recombination part.Its main recombination occurs in the ISC（S1-T1）process,and this process is mainly proportional to H_SO,so it is predicted that enhancing this process can enhance the catalytic activity.（2）Using g-C₃N₄ as the carrier successfully loaded Fe single atoms,and under the same conditions,the photocatalytic hydrogen production rate was increased by 100 times.To explore the role of Fe single atoms in the reaction,this article discusses through experiments combined with theoretical calculations.In experiments,through PL and UV-vis,it is found that the fluorescence emission ability is reduced after Fe single atom is loaded,and the light absorption is enhanced,which is beneficial to the photoreaction.Theoretical calculation results show that,first,the addition of Fe single atoms enhances the electronegativity of the N atoms coordinated with Fe atoms through the MLCT effect,second,enhances the hydrogen transfer induced by the excited state hydrogen bonds during the reaction;second,Fe single atoms The addition of atoms increases the H_SO of the ISC（S₁-T₁）and enhances the electron-hole recombination ability.The above conclusions not only verify the predicted bottleneck but also successfully break the bottleneck by loading a single atom.（3）The homogeneous catalyst sodium iron chlorophyll（Fe Chl Na₃）reduces CO₂ to CO by adjusting p H without sacrificial agent and photosensitizer.The photophysical and photochemical processes of Fe Chl Na₃ and CO₂ and H₂O hydrogen bond complexes were constructed computationally.The theoretical calculation structure shows that the excited state hydrogen bond activates linear CO₂ into curved HCO₂^-in the process of ISC（S₁-T₁）,and activates H₂O to make it have free radical properties in the process of S0-S1.Both are conducive to CO₂ light reduction.At the same time,comparing the presence or absence of hydrogen bonds,it is found that excited state hydrogen bonds increase the lifetime of the triplet state,making it more compatible with the photochemical kinetic process,and effectively enhancing the reactivity.This paper combines theoretical calculations and experiments to study the mechanism of artificial photosynthesis of two typical systems and draws the following common conclusions:the recombination of the excited state hydrogen bond in the electron-hole,the activation of the reactant molecule,energy,electron,and the main steps such as hydrogen transfer plays a very important role.