Controllable Surface Assembly And Charge Transport Properties Of Organic Ligand Bridged Dirhodium Oligmers

Molecular electronics has attracted great interest due to its potential application as a complement to silicon-based electronics.Understanding and controlling charge transport through functional molecules?or molecular assemblies?that link two electrodes are of primary importance to the development of molecular scale devices.Earlier studies have shown that the unique electronic structures of covalently bonded dimetal complexes facilitate the intramolecular electron transfer analysis.Taking this advantage,we plan to fabricate a series of metal-organic hybrid molecular wires on gold surfaces by stepwise coordination methods.Their electronic properties will be characterized by electronic spectroscopy,electrochemical analysis and so on.Furthermore,the charge transport behavior will be investigated by conductive probe atomic force microscopy.Quantitative qualification includingdistance attenuation factor and rectification ratio will be measured.We expect to discover the effects of electronic structures and electrochemical properties on the charge transport behavior,especially those for long-range electron transport ability of these metal-organic hybrid molecular wires and set up a quantitative relationship.In the meantime,study the charge transport mechanism through these molecular wires.The knowledge gained from the proposed project would provide deep understanding on regulation of electron transfer properties,which is critical for the future development of molecular electronic devices with desired properties.Through axial coordination,two series of redox active molecular wires were fabricated by layer-by-layer assembly of dirhodium complex Rh₂?O₂CCH₃?₄?Rh₂?withN,N?-bidentate ligands pyrazine?L_S?or 1,2-bis?4-pyridyl?ethene?L_L?on gold surfaces,called?Rh₂L_S?_n@Au and?Rh₂L_L?_n@Au?n=1-6?,respectively.The number of Rh₂L units n varied systematically by controlling the assembling cycles.The molecular wires were characterized by UV-Vis spectroscopy,cyclic voltammetry and atomic force microscopy.The electrical characters of these molecular wires were investigated using conductive probe atomic microscopy.Analyses of the current-voltage characteristics indicates that in both series,a mechanistic transition for charge transport from super-exchange tunneling to hopping on wires with n=4,disregarding the length and nature of the molecular wires.Surprisingly,?Rh₂L_L?_n@Au exhibit weaker length dependence of electrical resistance in both tunneling(?=0.044?^-1)and hopping(?=0.003?^-1)regimes,although in?Rh₂L_S?_nwith shorter bridging ligand the metal-metal interactions are much stronger.DFT calculations reveal that these wires have a?-conjugated molecular backbone established through??Rh₂?–??L?orbital interactions,and?Rh₂L_L?_n@Au has a smaller energy gap between the filled?*?Rh₂?and the empty?*?L?orbitals.Thus,for?Rh₂L_L?_n@Au,electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap,while in?Rh₂L_S?_n@Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion.On this basis,we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter?n?4?and longer?n>4?wires,respectively,and the Rh₂L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.Fabrications of Rh₂?O₂CR?₄?R=CH₃?H?,C?CH₃?₃?C?and CF₃?F??with pyrazine generated a series of four Rh₂ tetramers immobilized on gold substrate.Three asymmetrical molecular systems,namely,2H2C,2H2F and 2C2F,were developed by two different Rh₂ complexes so that the molecular assemblyis created with two subunits corresponding the donor?D?and acceptor?A?in a D-B?bridge?-A motif.Electric conductance measurements exhibit pronounced rectifying effects for2H2C,2H2F and 2C2F,but symmetrical current-voltage characteristics for 4H.All the rectifying systems possess are dox potential separation between the donor and acceptor,-?35?E_p=-?E_p?D?-E_p?A??>0.2C2F and 2H2F have the largest and smallest-?35?E_p values,respectively,and present the rectification ratios?RR?in the same order,thus,showing a correlation between the RR value and redox asymmetry?-?35?E_p?.DFT calculations produced an array of?conjugated HOMOs featuring electron localization and energy gradient,which supports the D?A electron transfer via an electron hopping?or resonant?pathway.