| Molecular electronics is a frontier subject involving the intersection of organic electronics and nanoelectronics.The goal when it appeared was to build molecular-level electronic devices and form molecular-level computers.With the fast development of theoretical researches and measurement techniques in molecular electronics,many singlemolecule junction systems with unique charge transport properties have been reported.Molecule contains unpaired electron（s）,which shows very different properties to closed shell molecules in the fields of optics,electronics and magnetism,etc.The application of radicals such as light-emitting diodes,field effect transistors and solar cells has been extensively studied.However,there are few reports on introducing radicals into singlemolecule junctions to study the effect of unpaired electrons on the electronic transport properties.This is mainly due to the active chemical properties of radicals,which makes it difficult for constructing stable single-molecule junctions.In this thesis,two types of radicals are successfully synthesized for the study of single-molecule conductance,the stable single-molecule junctions containing unpaired electrons are constructed,and the influence of unpaired electrons on the electronic transport properties of single-molecule junctions is investigated.Firstly,through the study of fluorene-based radicals,the question "how do unpaired electrons affect electronic transport in single-molecule junctions " is answered.Two groups of fluorene derivatives and corresponding radicals（36-FO,36-FOH and 36-FR,and 27-FO,27-FOH and 27-FR）are designed and synthesized.Their single-molecule conductance is measured by using the scanning tunnelling microscopy break-junction technique（STM-BJ）,and it is found that the conductance of the radical 36-FR is more than 23 times higher than that of the corresponding non-radical 36-FO and 36-FOH.However,radical 27-FR cannot be obtained in a stable form,and theoretical calculations show that its conductance is only about 3 times higher than that of 27-FO and 27-FOH.Therefore,the effects of unpaired electrons on the electronic transport properties of the two radical single-molecule junctions are different.Combined with theoretical research,the unpaired electron of 36-FR has a greater spin delocalization,the coupling between the molecular orbitals and the electrodes is stronger,and the unpaired electron has a significant effect on optimizing the charge transport path of the single-molecule junction.The combination of these reasons led to a huge increase in the conductance value.In contrast,the unpaired electron of 27-FR is mainly localized on the fluorene core,and the optimization of the charge transport path is limited,so the improvement of singlemolecule conductance is also limited.Secondly,the potential application of radicals in single-molecule devices is discussed.Utilizing the characteristic of HABI molecules to generate TPIR radicals under ultraviolet light,two groups of TPI derivatives and radicals（TPI-45 S,HABI-45 S and TPIR-45 S,and TPI-24 S,HABI-24 S and TPIR-24S）are designed and constructed in single-molecule junctions.In the experiment,it is found that the single-molecule junction conductance of TPIR-45 S and TPIR-24 S generated after illumination is 3.89 and 6.61 times higher than that of TPI and HABI in the non-radical state,respectively.It is proved that TPIR has the potential to construct photoresponsive single-molecule electronic devices.In conclusion,this thesis explores the role of unpaired electrons in the charge transport process of single-molecule junctions and its potential applications,which enriches the research field of single-molecule devices. |
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