The Study Of Single-molecule Thermoelectric Property Based On The Singular Superposition Effect In Double-channel Molecules And The Radical Trigger Reaction

Organic molecules with diverse molecular orbitals and multiple energy levels have become good candidates for molecular electronic devices and thermoelectric materials.The electrical transport and thermoelectric properties measurements of molecular junctions are of great significance to the study of charge transport mechanisms through the molecular junction and also provide new candidate to promote the miniaturization of integrated circuit industry and the efficient and stable development of thermoelectric conversion.With the development and expansion of precise measuring instruments such as scanning tunneling break junctions(STM-BJ),the thermoelectric properties of many single-molecule junctions and self-assembled monolayer devices have been extensively studied,and the change of thermoelectric properties based on the contact effect of molecules and electrodes,length dependence,internal functional group substitution and external environmental regulation have been developed preliminarily.Furthermore,the thermoelectric transport properties of molecules with multi-channel structure do not follow the traditional Kirchhoff’s law.The conclusions of the research are not systematic,and the research on thermoelectricity still lacks.On the other hand,single-molecule radicals have been found to significantly and stably enhance the electrical transport properties by changing the molecular spin characteristics,providing an opportunity for the development of thermoelectric devices with excellent performance.However,its influence on the thermopower has not been revealed.How to make full use of molecular radicals and regulate them through the external environment has become a novel and valuable issue.1.Here,we studied the thermoelectric properties of flexible double-channel alkyl chain molecules by using the STM-BJ technique based on the measurement of conductance and thermopower and found the singular superposition effect of alkyl chain molecules.On the one hand,in the symmetric two-channel alkyl ring consisting of two carbon chains with same length,its conductance is significantly lower than that of the original individual alkane chain,but its thermopower is higher than that of the chain molecule.On the other hand,as one of the carbon chains lengthens to form an asymmetric double-channel alkyl ring,its conductance increases and thermopower decreases,and the value is between that of the two individual chain molecules.Theoretical calculation shows that the two-channel alkyl ring exhibit σ destructive quantum interference effect,resulting in a sudden drop of the electronic transmission function,causing the decrease conductance and increase thermopower.While one of the chains extends,this effect will be alleviated,weakening change in thermoelectric properties,which indicates that by adjusting the two-channel molecular ring length can regulate the thermoelectric properties stably.2.The regulation of the thermoelectric properties of acridine derivatives by radical trigger reaction and gate voltage was studied by the same method combined with macroscopic characterization.Radical cations of acridine derivatives were obtained by using trifluoroacetic acid or silver hexafluoroantimonate oxidants.It was found that the conductance increased significantly and the thermopower decreased slightly during the process.The existence of radicals and their high stability were verified by UV-Vis spectroscopy and ESR spectroscopy.It is proved that the acridine derivative radical has strong spin properties,which results in a resonant peak near the Fermi level in the transmission spectrum and reduces the gap between the frontier orbitals and Fermi level,thus changing the thermoelectric properties of the molecular junction.In addition,the thermoelectric properties of acridine derivatives can be effectively regulated by gate voltage,and the dominant transporting orbital of acridine derivatives can be changed from the highest occupied molecular orbital to the lowest unoccupied molecular orbital.It is found that the regulation effect of gate voltage on radical is more obvious,which opens up a new way for the effective regulation of thermoelectric conversion.