Single-Molecule Reaction Studies Based On Surface-Enhanced Raman Spectroscopy

At present,the research of chemical reaction mainly stays at the macro level,and the study of reaction kinetics,transition states and intermediates at the single molecular level is still lacking.The surface of metal nanostructures can induce the generation of energetic charge carriers(electrons and holes)under light excitation,which is also called "surface plasmons photocatalysis".The process also produces highly reactive adatoms,allowing visible light to drive microscopic chemical reactions effectively.This made it possible for the study of microscopic chemical reactions.Such studies have shown that collisions between gold nanoparticles and gold nanoelectrodes and adsorption on the gold nanoelectrodes can form nanogaps,while "gold nanoelectrode-molecule-gold nanoparticle" molecular junctions are generated when molecules are connected between their nanogaps.When the laser irradiates the molecular junction,the gold atoms on the surface of the gold nanoparticles will break free and form the gold adatoms,which will then generate a "picocavity" with local strong electric field,which will greatly enhance the Raman spectral signal of the molecules to achieve the resolution at the single-molecule level.The generation of gold adatoms makes it easier and clearer to observe the interactions between molecules,the structural changes of intermediates and the chemical reaction paths at the single-molecule level.Based on the above research techniques,the following research works are completed in this paper:1.We used surface-enhanced Raman spectroscopy(SERS)to study the interaction of a series of aromatic molecules with gold atoms on the surface of gold nanoparticles.By means of density functional theory(DFT)calculation,the raman characteristic peaks of gold nanoparticles interacting with molecular functional groups and the corresponding structural changes were verified.2.We used SERS technology to study the dehydration reaction of primary amide in the molecular junction of gold nanoparticle-aromatic primary amide-gold nanoelectrode under visible light irradiation.Through SERS experiment and a series of DFT theoretical calculations,we could track the evolution of SERS spectrum of 4-mertobenzamide molecules,and found that the vibration wavenumber of C-N bond was continuously enhanced to reach the vibration wavenumber of nitrile group.Combining density functional theory and the "picocavity" model,we find that the most obvious Raman transient spectral changes come from the generation of intermediates.Based on this,we identified complex reaction pathways involving multiple proton transfers,carbon-nitrogen double and triple bond intermediates of amide groups.3.We used SERS technology to study the deprotonation and dimerization of 4-mercapbenzenethiol.The formation of gold adatoms was observed during deprotonation of methyl groups.This effectively enhances the characteristic signal of the product so that the dynamic changes in the reaction system can be detected successfully.In addition,we determined the pathway and intermediate structure of 4-mercapbenzenethiol dimerization.On the basis of the above,in order to control the single molecular reaction,we further apply a certain bias to the gold electrode so that a specific direction and size of electric field can be formed in the molecular junction,through which the electric field can change the electron cloud distribution of the molecule and the size of the nanogap,so as to achieve the purpose of modulating the single molecular reaction.Taking 4-mercapbenzenethiol as an example,we show that the deprotonation of methyl group in benzene ring can be modulated by electric field.Density functional theory and local electric field enhancement calculation were combined to verify the complex molecular structure changes under photocatalysis.