Investigation Of The Properties Of DPP Derivatives In Single-Molecule

Diketopyrrolopyrrole(DPP)derivatives have been intensively reported to study single-molecule device performance by changing the conduction channel.However,previous studies lacked in-depth exploration of the DPP system.In this thesis,three states of DPP derivatives are designed and synthesized with different properties,and the molecular charge transport properties of these DPP derivatives are researched by Scanning Tunnelling Microscopy Break Junction(STM-BJ)technique.The thesis includes two parts.In the first part,four closed-shell DPP derivatives(DPPS,DPPO,DPPSS,DPPOS)with pyridine as anchor group and different heteroatoms are designed and synthesized to study the effect of heteroatoms on the electrical transport properties of single molecules.The conductance of DPPS measured by STM-BJ technology is 10^-4.01 G₀,and that of DPPO is 10^-4.29 G₀.The conductance of DPPO is smaller than DPPS.From the Ultraviolet-visible(UV-vis)absorption spectra and electrochemical cyclic voltammetry curves,it can be observed that the HOMO-LUMO energy gaps of DPPS and DPPO are1.82 e V and 1.97 e V respectively,and the energy gap of DPPO is larger.According to the Landauer–Buttiker theorem,the increase of HOMO-LUMO energy gap of small molecules(molecular length<3 nm)leads to the decrease of conductance value.Therefore,the HOMO-LUMO energy gap of DPPO is larger than that of DPPS,so that the conductance value of DPPO is lower than that of DPPS.In addition,DPPSS and DPPOS,introducing thiophenes on DPPS and DPPO,are as the control group for further study,which increase the conjugation length of the molecules.The conductance value of DPPSS is 10^-4.87 G₀,that of DPPOS is 10^-5.06 G₀.The conductance value of DPPOS is smaller than DPPSS.As shown in the UV-vis absorption spectra and electrochemical cyclic voltammetry curves,HOMO-LUMO energy gaps of DPPSS and DPPOS are 1.79e V and 1.83 e V respectively.The DPPOS with five-membered furan ring has larger energy gap and smaller conductance value,which is consistent with the experimental conclusions of DPPS and DPPO.Therefore,the above results show that in DPP derivatives with pyridine as the anchor group,replacing thiophene with furan will increase the HOMO-LUMO energy gap of the molecule and reduce the conductance value.In the second part,DPP derivatives with large polarity and radical state are designed and synthesized,and their optoelectronic properties are studied and simulated by density functional theory.Finally,their electrical conductivity properties are studied by STM-BJ.It shows that the designed large-polarity DPP derivative molecule is long,and the metal electrode is difficult to stretch it up.The molecule also has multiple anchor points,resulting in metal electrodes being anchored to different sites of molecules,so it is challenging to construct single-molecule junctions,which makes the measured conductivity inaccurate.In addition,the introduction of electron withdrawing group DPP can enhance the stability of diradical compounds.However,our work shows that the chemical stability of DPP derivatives in the free radical state is poor,which is not enough to support the study of their properties in single-molecule devices.In summary,this thesis studies the charge transport properties of DPP derivatives in single molecular junctions,and realizes the fine-tuning of the conductance properties.