| Reaction kinetics can reveal the internal mechanism of the chemical reaction,and also effectively regulate the reaction process,which is of great significance for organic synthesis and chemical production.However,due to the difficulty to control the reaction rate,the complexities to uncover the reaction mechanism under different environments,it is urgent to develop a new characterization method for reaction kinetics investigation.Traditional characterization methods to investigate those processes of mechanism can only provide an average state for the whole population of molecules,which is difficult to detect the interaction mechanism between different molecules and their intermediate states during the functioning process.Single-molecule technique is a kind of method to reveal the details of chemical reaction process from single-molecule level.It can directly observe the reaction trajectory and the intermediate state of a single molecule.Thus,the hidden information between the molecular structure and function can be discovered,then the manipulation and regulation of a single molecule can also be realized.This thesis firstly adopts the technique of mechanically controllable break junction(MCBJ)to in situ study the charge transport properties of the radical triggering reaction.the single-molecule electrical measurement is verified to be able to detect the reactant and product states during the whole chemical reaction process.Furthermore,the reaction kinetics and mechanism of the whole chemical reaction process were able to be studied.The main research contents and results of this thesis are as follows:1.It is demonstrated that the single-molecule electrical property measurement technique can be used to in situ monitor the constant-state changes of chemical reactions.A novel radical of phenothiazine(PTZ)derivative was triggered in situ.For the first time,the conduction-distance information of this pristine phenothiazinc(PTZ)derivative and its corresponded radical state were obtained,and it was found that the single-molecule conductance was increased by more than 200 times upon the formation of the radical state.Moreover,the radical state could stably exist at room temperature for at least two months.2.The kinetic characterization of chemical reaction at single-molecule scale was realized through the single-molecule electrical measurement technique.In the experiment,a classical two-step cascade reaction system of tetrazine and dihydrofuran was used as the model system.Through the statistical analysis of single-molecule events,the dynamic processes of the Diels-Alder reaction and the subsequent aromatization reaction were quantitatively characterized,the reaction products of each reaction were thoroughly analyzed.3.The reaction kinetics and rate constants were obtained through the conductance data processing and analysis.Combining with the macroscopic spectroscopic characterization,it was found that the kinetic process of this two-step chemical reaction in the confined space of the single molecule junction was significantly different from the macro-scale in bulk phase.The reaction rate of the first Diels-Alder reaction was consistent with the macroscopic result,while the reaction rate constant of the second aromatization reaction increased by 10 times in the nanogap than that in NMR bottle4.Combined with theoretical calculations,the catalytic effect of th e applied electric field was further explored.It is proved that the selective catalytic effect of electric fields on chemical reaction is based on the coincidence degree of reaction-axis and the oriented electric field.The results show that the external electric field can be used to regulate the reaction rate and selectivity of chemical reactions,providing a new opportunity for efficient chemical synthesis and even green chemistry in the future. |
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