Study Of Bridge And Solvent-Mediated Electron Transfer In Dimolybdenum D-B-A Systems

Based on the theory of quantum mechanics,electrons behave wave-particle duality.According to the wave characteristics in electronic motion,relevant theoretical and experimental studies focus on the electron transfer process under the Frank-Conton or Born-Oppenheimer approximation,in which the electron transfer is achieved by coherent coupling between electronic states and assigned to adiabatic reaction.As the coupling strength between electronic states is so weak that the electron transfer cannot be achieved by coherent coupling,the motion of electrons is affected by the motion of the nucleus,depending on the coupling between vibrational states.The electron transfer reaction occupred in such cases should be assigned to non-adiabatic reaction,leading to be invalid of above approximations.In this thesis,a the dimolybdenum quadruple bonded[Mo₂]unit with a well-defined electronic structure（σ²π⁴δ²）was selected as electron donor/acceptor to synthesize a series of Donor-Bridge-Acceptor molecules,which were mainly used to study the photoinduced electron transfer mediated by the bridge as well as the thermarl induced electron transfer by the solvent.The advantage of such molecules is that the optical,electrical,magnetic,and redox properties are single related to high-levelδelectrons,which greatly simplifies the experimental and theoretical research processes.In chapters 2-4,the transient UV-Vis spectroscopy was used to explore the photoinduced electron transfer in a series of bridged dimolybdenum compounds.From the experimental data,we can conclude that two charge-separated states,¹MMCT and ¹MLCT,are formed after excitation.The decay of the two states corresponds toδ*→δandπ*→δelectron transfer process,respectively;Also,the formation and breaking of Mo-Mo bond during theδ*→δelectron transfer support the fact that the Mo-Mo vibration can be treated as the single mode of nuclear tunneling electron transfer.In Chapter 2,three symmetrical dimolybdenum dimers bridged by 4,4′-biphenyldicarboxylate and the partially and fully thiolated derivatives have been synthesized,and their photoinduced electron transfer were studied.The published results demonstrated that the introduction of sulfur atoms along the charge transfer axis effciently enhances the electronic coupling and facilitates the electron transfer between the two dimetal centers.While in the process of photo-induced electron transfer,the electron transfer rate constant ofδ*→δpresent the coordinated atom independent.This indicates that the photoinducedδ*→δelectron transfer and thermally inducedδ→δelectron transfer follow different mechanism.In Chapter 3,the effect of bridge length on the photo-induced electron transfer was studied.Four complexes,[Mo₂（DAniF）₃]₂（μ-O₂C（ph）_n CO₂）（n=1-4）（DAniF=N,N’-dimethoxyphenyl formamidine）,varied in bridge length were studied by the transient spectroscopy technique.Upon an increase in the number of bridging units from one to three,the results show that theδ*→δelectron transfer rate constant decreases significantly as the bridge length increase.While the slight decrease was displayed from three to four,which means that the electron transfer mechanism changed（tunneling to hopping）.InChapter4,wesynthesizedthreecomplexes,[Mo₂（DColF）₃]₂（μ-O₂C-ph-（ph）_n-ph-CO₂）,[Mo₂（DColF）₃]₂（μ-O₂C-ph-Naph-ph-CO₂）and[Mo₂（DColF）₃]₂（μ-O₂C-ph-Anth-ph-CO₂）.（ph=phenylene,Naph=naphthalene,Anth=anthracene,DColF is N,N’-di（p-toluyl）formamidinate）and studied in terms of photoinduced electron transfer.Firstly,the coupling strength between two redox centers of the three compounds was qualitatively analyzed by electrochemical method.It was found that the coupling strength of the redox center increases as the enlargement of the bridge conjugate system.On the other hand,the enlargement of the bridge conjugate system leads to a large torsion angle between redox center and bridge,which diminishes significantly the electronic coupling,as indicated by the absence of intervalence charge transfer（IVCT）absorption for the MV complexes.An IVCT absorption is observed in the Naph bridged complex,while the rest of two complxes not.It obviously indicates that the conformation has a significant effect on the thermally induced electron transfer between[Mo₂]units.Also,the photoinduced electron transfer occurred in the current complexes were exploxed by time-resolved UV-Vis spectroscopic method.The experimental results demonstrated that the photoinducedδ*→δelectron transfer presents the bridge conformation independence.The difference in mechanism of photoinducedδ*→δelectron transfer and thermarlly inducedδ→δelectron transfer also can be confirmed by this study.Through the investigation of photo-induced electron transfer by transient spectroscopy,we can conclude that photoinducedδ*→δelectron transfer in the dimolybdenum D-B-A systems displays coordinated and bridge conformation independence and mechanism transition from tunneling to hopping.This demonstrated that the thermally inducedδ→δelectron transfer electron transfer in the mixed-valence comples is quite different from the photoinduced electron transferδ*→δelectron transfer.In chapter 5,three thienylene（C₄H₂S）bridged Mo₂ dimers,[Mo₂（DAniF）₃]₂（μ-N（H）SCC₄H₂SCN（H）S）,[Mo₂（DAniF）₃]₂（μ-SOCC₄H₂SCNOS）,and[Mo₂（DAniF）₃]₂（μ-S₂CC₄H₂SCS₂）（DAniF=N,N′-di（p-anisyl）formamidinate）,were synthesized and studied in terms of solvent effect on the electronic coupling and thermally induced electron transfer.The results show that as the polarity of the solvent decreases,the energy of the IVCT absorption decreases;however,the non-polar solvents of benzene and tolune shows the opposite trend.The DFT calculation and spectral data show that the solvent molecules benzene and toluene can be inserted into the core of the compound,perpendicular to the plane of the thiophene bridge,forming a CH????interaction with the aromatic center of the thiophene bridge;for hexafluorobenzene,the high electronegativity of fluorine atom makes hexafluorobenzene parallels to thiophene bridge and?????interaction formed.