| In order to further investigate the effect of cross-conjugated bridges on intramolecular electronic coupling and electron transfer in Donor-Bridge-Accept system,a series of dimolybdenum dimers were synthesized.Which were assembled by an electron donor(D)and an acceptor(A)spaced by various cross-congugated bridges(B).The electrochemistry,spectroscopy and density functional theory(DFT)calculation are used to study intramolecular electron transfer of these system.Then,the experimental and calculational results of cross-conjugated system were compared to the linear-conjugated systems,quantitatively explain the effect of the cross-conjugated bridges on the electronic coupling and electron transfer in the D-B-A system.In theory,the mechanism of the cross-conjugated bridge was expounded.And at the molecular level,a deep understanding of the decoherence phenomenon has been established from the principle of symmetry of electronic transition.In the second chapter of this thesis,two cross-conjugated biphenyl bridged dimolybdenumdimers,[Mo2(DAniF)3]2(?-3,3'-O2CC6H4C6H4CO2)and[Mo2(DAniF)3]2(?-3,4'-O2CC6H4C6H4CO2)were synthesized and characterized.The mixed-valent compounds were obtained by chemical oxidation,and the near-infrared spectrum of the D-B-A cations exhibited a characteristic intervalence charge-transfer(IVCT)band.According to the Hush formula,the electron coupling matrix element Hab(160 cm-1)was nearly 80 cm-1 lower than the corresponding linear conjugated molecule[Mo2(DAniF)3]2(?-4,4'-O2CC6H4C6H4CO2).Based on the semi-classical theory of electron transfer,we got the thermal electron transfer rate ket(3.5×108 s-1),which was about twice as slow as the electron transfer rate of the corresponding linear conjugated molecule.In the third chapter of this thesis,three cross-conjugated phenyl bridged dimolybdenum dimers,[Mo2(DAniF)3]2(?-1,3-O2CC6H4CO2),[Mo2(DAniF)3]2(?-1,3-OSCC6H4COS)and[Mo2(DAniF)3]2(?-1,3-SSCC6H4CSS)were designed and synthesized.Their corresponding mixed valence compounds were obtained by chemical oxidation method,and the near-infrared spectrum of the three mixed compounds exhibited a characteristic IVCT band.From the Hush formula,the electron coupling matrix element was obtained,Hab=300400 cm-1.Based on the semi-classical theory of electron transfer,the thermal electron transfer rate ket(109 s-1)was obtained.In comparing with the linear conjugated phenyl bridging molecule[Mo2(DAniF)3]2(?-1,4-O2CC6H4CO2),[Mo2(DAniF)3]2(?-1,4-OSCC6H4COS)and[Mo2(DAniF)3]2(?-1,4-SSCC6H4CSS),Hab were reduced by 260460 cm-1 and the ket were lowered by two to three orders of magnitude.In addition,the electronic coupling parameter Hab of the cross-conjugated benzene bridged system shifted slightly(from 300 to 400 cm-1)when S atoms were added into the chelating unit,and the change of the ket(109 s-1)did not change significantly.In contrast,in the linear conjugated benzene bridged system,the Hab increases with the number of S atoms in the chelation unit,and the electron transfer rate also increased by 12 orders of magnitude.Density functional theory analysis was carried out on the five pairs of isomer compounds,cross-conjugated and linear conjugated bridged dimolybdenum dimers.The calculation results showed that there were two metal to ligand electronic transitions paths in the five cross-conjugated molecules,which occur through different symmetry channels.The overall coupling effect caused by the interaction between the metal and the ligand is destroyed by the two paths with different symmetry,which has a destructive quantum interference effect on the electronic coupling and electron transfer of the system.However,in the linear conjugated molecules,there are two metal to ligand transition pathways that are parallel in terms of symmetry relationship and polarization direction so that the orbital overlap is higher.The overall electronic coupling effect on the system exhibits constructive quantum interference effect.Therefore,the intramolecular electron coupling and electron transfer rate of the cross-conjugated system are greatly reduced in comparision the linear conjugated molecule due to the existence of destructive quantum interference effect in the cross-conjugated bridges.Additionally,combined with the spectral analysis and DFT calculations,these results suggest that through?-bond and/or through-space coupling channels become operative as the?-conjugation electron coupling path is blocked. |
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