Tuning The Electronic Coupling And Electron Transfer In Mo₂ Donor-Acceptor Systems By Variation Of The Bridge Conformation

Previous researches showed that introducing sulfur atoms into [Mo₂] unit as the coordination atoms and using a big fused ring as bridge in organic D-B-A molecular can enhance the intramolecular electronic coupling and electron transfer. In this paper,we assembled two quadruply bonded dimolybdenum units [Mo₂（DAni F）3]+（DAni F=N,N￠-di（p-ani syl） formamidinate） with 1,4-naphthalenedicarboxylate and its thiolated derivatives to produce three D-B-A complexes [Mo₂（DAni F）3]2（m-1,4-O2CC10H6CO2）, [Mo₂（DAni F）3]2（ m- 1,4- OSCC10H6COS） and [Mo₂（DAni F）3]2（ m-1,4-S2CC10H6CS2）, abbreviated as [O2-（1, 4-naph）-O2], [OS-（1, 4-naph）-OS] and[S2-（1, 4-naph）-S2]. X-ray single crystal diffraction was carried out to conform the exact structure of the compounds. According to the X-ray structure results we found the naphthalene ring is deviated from the plane defined by the two [Mo₂] units. We use a torsion angle to evaluate the deviation level. The torsion angle varies from26.3 ° to 65.8 ° as the coordinated atoms were gradually changed to surfur.Electrochemical test of the nuetral compounds show two successive reversible redox peaks in Cyclic voltamemetry and differential pulse graph. Three mixed valence compounds [O2-（1, 4-naph）-O2]+, [OS-（1, 4-naph）-OS]+ and [S2-（1, 4-naph）-S2]+ were obtained by chemical oxidation and characterized by ultraviolet-visiblenear infrared spectrum. All the three mixed-valence species exhibit a tipical intervalence charge transition（IVCT） absorption peak on the near infrared area with relative high energy and low intensity. When compared its optical behavior with the phenylene analogues, the electronic coupling matrix elements Hab, calculated from Hush model, surprisingly raged in a small scope（Hab=258 cm- 1-345 cm- 1） and lowered by a factor of two or more. Unexpectedly, the maximum electronic coupling matrix element is given by the partly thiolated complex [OS-（1, 4-naph）-OS]+ rather than the tetrathiolated compound. We also had an insight into the dynamics of the charge transfer process by using the optical spectrum data. The electron-transfer rate constants ket of the three complexes are all located at 1011 s-1, reducing by about one order of magnitude when compared to the series of phenylene compounds. DFT calculation is carried out to conform the abnormal phenomena. The DFT calculation indicated that larger torsion angle raising the energy of bridging ligand-based LUMO molecular orbital while decreasing the energy of matal-based HOMO orbital. Thus,enlarging the energy gap of the HOMO- LUMO cause a decrease of the metal to ligand charge transfer（MLCT） ability. And the energy gap of HOMO-HOMO-1 of the three naphthalene complexes varied in a small range which is in line with the coupling matrix elements（calculated from the spectrum data）.All the experimental data and theory calculation are in good consistency. Therefore we can concluded that the intramolecular electronic communication in [Mo₂] dimer system is mainly tuned by two factors. One is the electron-transporting ability of the bridge and the other is the electron-donating（electron-accepting） ability of the donor（acceptor）. In the naphthalene series the former plays a key role in controlling the intramolecular charge transfer while in the phenylene the latter does.Our experimental results and theoretical analyses verify the superexchange mechanism for electronic coupling and electron transfer.