Theoretical Study On Excited-state Hydrogen Bonding In Different Systems

Hydrogen bond is the most common,basic and important interaction in aggregates.Because of its strong orientation and various forms,it is called"the universal interaction in the aggregate",also known as"weak interaction,strong effect".Hydrogen bond not only dominates the properties of ground state,but also the properties of excited state.However,little is known about the behavior of excited state hydrogen bonds.The behavior of excited state hydrogen bonds can affect not only the luminescence properties of the reaction system,but also the photophysical and photochemical processes of the system.Therefore,the mechanism of excited state hydrogen bond in different reaction systems has become an urgent scientific problem to be solved.HHTP-DPB COF and NH₃ are combined by hydrogen bond.Hydrogen bond can affect the luminescence mechanism of COF.The behavior of hydrogen bond in excited state is described by comparing the length of hydrogen bond between excited state and ground state,infrared vibration frequency of characteristic bond,hydrogen bond energy,¹H-NMR and noncovalent interaction.It is found that the enhanced hydrogen bonding in the excited state leads to the enhanced electronic coupling between NH₃ and HHTP-DPB COF by energy gap rule analysis,which enhances the internal conversion rate from the excited state to the ground state,increases the non-radiative transition rate,makes the fluorescence conversion rate dcreased from 1.04×10⁸ s^-11 to 8.57×10⁷ s^-1,and leads to the weakening of fluorescence,and then detects NH₃ by the change of fluorescence intensity.In the system of photocatalytic reduction of CO₂,there are abundant hydrogen bonds between the catalyst,CO₂ and H₂O.The mechanism of excited state hydrogen bond in the system was studied by modeling the hydrogen bond complex of catalyst with CO₂ and H₂O.In the system of photocatalytic reduction of CO₂ by TiO₂,based on the hydrogen bond complex model,the complete photophysical and photochemical reaction mechanism was calculated by DFT/TDDFT method.By comparing the rate coefficients of each steps,it is found that the rate-controlling step of the reaction is the breaking of the C=O bond and the activation energy of this step needs 40.1 kcal/mol which reaction rates is 2.72×10^-1717 s^-1.The hydrogen bond-induced electron transfer and proton transfer in the reaction were analyzed by combining charge and spin density.Theoretically predict that the doped metal Mg in TiO₂ can improve the rate-controlling step which reduces the activation energy by 5.5kcal/mol.In validation experiments,it is found that CO yield increased by 3.6 times compared with that of pure TiO₂ with the addition of Mg.In the system of g-C₃N₄ photocatalytic reduction of CO₂,the photophysical and photochemical processes of the system were investigated by theoretical and experimental methods,and the excited state hydrogen bond induced electron transfer and proton transfer were studied.It is found that the C=O bond breaking in photochemical reaction is the rate-controlling step of the reaction and the activation energy of this step needs 38.7 kcal/mol.The modification of g-C₃N₄ catalyst by Cr was selected in theory,and Cr was added in the experiment greatly increased the activity of the reaction which reduces the activation energy by 5.9kcal/mol,and makes the CO yield increased by 2 times compared with that of g-C₃N₄.