A single solvating benzene molecule decouples the mixed-valence complex through intermolecular orbital interactions.

Characterization of covalency of intermolecular interactions in the van der Waals distance limit remains challenging because the interactions between molecules are weak, dynamic, and not measurable. Herein, we approach this issue in a series of supramolecular mixed-valence (MV) donor(D)-bridge(B)-acceptor(A) systems consisting of two bridged Mo2 units with a C6H6 molecule encapsulated, as characterized by the X-ray crystal structures. Comparative analysis of the intervalence charge transfer spectra in benzene and dichloromethane substantiates the strong electronic decoupling effect of the solvating C6H6 molecule that breaks down the dielectric solvation theory. Ab initio and DFT calculations unravel that the intermolecular orbital overlaps between the complex bridge and the C6H6 molecule alter the electronic states of the D-B-A molecule through intermolecular nuclear dynamics. This work exemplifies that site-specific intermolecular interaction can be exploited to control the chemical property of supramolecular systems and to elucidate the functionalities of side-chains in biological systems.

Crossover between the adiabatic and nonadiabatic electron transfer limits in the Landau-Zener model.

The semiclassical models of nonadiabatic transition were proposed first by Landau and Zener in 1932, and have been widely used in the study of electron transfer (ET); however, experimental demonstration of the Landau-Zener formula remains challenging to observe. Herein, employing the Hush-Marcus theory, thermal ET in mixed-valence complexes {[Mo2]-(ph)n-[Mo2]}+ (n = 1-3) has been investigated, spanning the nonadiabatic throughout the adiabatic limit, by analysis of the intervalence transition absorbances. Evidently, the Landau-Zener formula is valid in the adiabatic regime in a broader range of conditions than the theoretical limitation known as the narrow avoided-crossing. The intermediate system is identified with an overall transition probability (κel) of ∼0.5, which is contributed by the single and the first multiple passage. This study shows that in the intermediate regime, the ET kinetic results derived from the adiabatic and nonadiabatic formalisms are nearly identical, in accordance with the Landau-Zener model. The obtained insights help to understand and control the ET processes in biological and chemical systems.

Enhanced charge transport via d(δ)-p(π) conjugation in Mo2-integrated single-molecule junctions.

A trans-dimolybdenum nicotinate (m-Mo2) complex and its isonicotinate isomer (p-Mo2) were synthesized and characterized crystallographically, and their single-molecule charge transport properties were investigated using the STM break junction (STM-BJ) technique. With a quadruply bonded Mo2 complex unit integrated into molecular backbones, the single-molecule conductance for complex molecules was increased by more than one order of magnitude compared with that of the organic π-conjugated analogues 1,4-bis(4-pyridyl)benzene (p-Ph) and 1,4-bis(3-pyridyl)benzene (m-Ph). More interestingly, unlike m-Ph, m-Mo2 with meta connected pyridyl anchors presents larger conductance than that of p-Mo2 with two para connected pyridyl groups. DFT-based transmission calculations revealed that the significant conductance enhancement of Mo2 molecules originates from the largely reduced HOMO-LUMO gap, and the unique d(δ)-p(π) conjugation between the Mo2 unit and the pyridine rings gives rise to a delocalized electronic structure that endows the Mo2 molecules with an unexpected high conductance.

Mediation of Electron Transfer by Quadrupolar Interactions: The Constitutional, Electronic, and Energetic Complementarities in Supramolecular Chemistry.

Studies of intermolecular interactions enhance our knowledge of chemistry across molecular and supramolecular levels. Here, we show that host-guest quadrupolar interaction has a profound influence on the molecular system. With covalently bonded dimolybdenum complex units as the electron donor (D) and acceptor (A) and a thienylene group (C4H2S) as the bridge (B), the mixed-valence D-B-A complexes are shaped with clefts in the middle of the molecule. Interestingly, in aromatic solvents, the D-A electronic coupling constants (Hab) and electron transfer rates (ket) are dramatically reduced. Theoretical computations indicate that an aromatic molecule is encapsulated in the cleft of the D-B-A array; quadrupole-quadrupole interaction between the guest molecule and the C4H2S bridge evokes a charge redistribution, which increases the HOMO-LUMO energy gap, intervening in the through-bond electron transfer. These results demonstrate that a supramolecular system is unified underlying the characteristics of the assembled molecules through constitutional, electronic, and energetic complementarities.

Making Use of the δ Electrons in K4Mo2(SO4)4 for Visible-Light-Induced Photocatalytic Hydrogen Production.

Quadruply bonded dimolybdenum complexes with a σ2π4δ2 electronic configuration for the ground state have rich metal-centered photochemistry. An earlier study showed that stoichiometric or less amount of molecular hydrogen was produced upon irradiation by ultraviolet light (λ = 254 nm) of K4Mo2(SO4)4 in sulfuric acid solution, which was attributed to the reductive capability of the ππ* excited state. To make use of the δ electrons for visible-light-induced photocatalytic hydrogen evolution, a multicomponent heterogeneous photocatalytic system containing K4Mo2(SO4)4 photosensitizer, TiO2 electron relay, and MoS2 cocatalyst is designed and tested. With ascorbic acid added as a sacrificial reagent, irradiation by artificial sunlight (AM 1.5) on the reaction in 5 M H2SO4 has produced 13 400 µmol g-1 of molecular hydrogen (based on the Mo2 complex), which is 30 times higher than the hydrogen yield obtained from the reaction of bare K4Mo2(SO4)4 with H2SO4 under ultraviolet light irradiation. Further improvement of hydrogen evolution is achieved by addition of oxalic acid, along with an electron donor, which gives an additional 50% increase in H2 yield. Spectroscopic analyses indicate that, in this case, a junction between the Mo2 complex and TiO2 is built by the oxalate bridging ligand, which facilitates charge injection and separation from the Mo2 core. This Mo2-TiO2-MoS2 system has achieved a high hydrogen evolution rate up to 4570 µmol g-1 h-1. The efficiency of K4Mo2(SO4)4 as a metal-centered photosensitizer is also proved by parallel experiments with a dye chromophore, fluorescein, which presents comparable H2 yields and hydrogen evolution rates. Most importantly, in this study, detailed analyses illustrate that the photocatalytic cycle with hydrogen gas as an outcome of the reaction is established by involvement of the δδ* excited state generated by visible light irradiation. Therefore, this work shows the potential of quadruply bonded Mo2 complexes as photosensitizers for photocatalytic hydrogen evolution.

Author Correction: Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Author Correction: Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways.

The original version of this Article contained errors in the symbols displayed in the eighteenth sentence of the third paragraph of the 'Determination of Hab and kET data for the Mo2 dimers' section of the Results, and the third sentence of the Discussion. This has been corrected in both the PDF and HTML versions of the Article.

Efficient electron transfer across hydrogen bond interfaces by proton-coupled and -uncoupled pathways.

Thermal electron transfer through hydrogen bonds remains largely unexplored. Here we report the study of electron transfer through amide-amide hydrogen bonded interfaces in mixed-valence complexes with covalently bonded Mo2 units as the electron donor and acceptor. The rate constants for electron transfer through the dual hydrogen bonds across a distance of 12.5 Å are on the order of ∼ 1010 s-1, as determined by optical analysis based on Marcus-Hush theory and simulation of ν(NH) vibrational band broadening, with the electron transfer efficiencies comparable to that of π conjugated bridges. This work demonstrates that electron transfer across a hydrogen bond may proceed via the known proton-coupled pathway, as well as an overlooked proton-uncoupled pathway that does not involve proton transfer. A mechanistic switch between the two pathways can be achieved by manipulation of the strengths of electronic coupling and hydrogen bonding. The knowledge of the non-proton coupled pathway has shed light on charge and energy transport in biological systems.

Electronic Coupling and Electron Transfer between Two Mo2 Units through meta- and para-Phenylene Bridges.

A series of three Mo2 dimers bridged by a meta-phenylene group has been studied in terms of electronic coupling (EC) and electron transfer (ET) in comparison with the para isomers. Optical analyses on the mixed-valence complexes indicate that by replacing a para-phenylene bridge with a meta one, the EC between the two Mo2 centers is dramatically weakened; consequently, the ET rates (ket ) are lowered by two to three orders of magnitude. In the para series, the EC parameters (Hab ) and ET rates (ket ) are greatly affected by O/S atomic alternation of the bridging ligand. However, for the meta analogues, similar EC and ET parameters are obtained, that is, Hab =300-400 cm-1 and ket ≈109  s-1 . These results suggest that through-σ-bond and/or through-space coupling channels become operative as the π conjugation is disabled. DFT calculations reveal that destructive quantum interference features seen for the meta series arise from the cancellation of two π-conjugated coupling pathways.

Understanding the charge transport properties of redox active metal-organic conjugated wires.

Layer-by-layer assembly of the dirhodium complex [Rh2(O2CCH3)4] (Rh2) with linear N,N'-bidentate ligands pyrazine (LS) or 1,2-bis(4-pyridyl)ethene (LL) on a gold substrate has developed two series of redox active molecular wires, (Rh2LS) n @Au and (Rh2LL) n @Au (n = 1-6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current-voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh2LL) n @Au wires exhibit smaller distance attenuation factors (ß) in both the tunneling (ß = 0.044 Å-1) and hopping (ß = 0.003 Å-1) regimes, although in (Rh2LS) n @Au the electronic coupling between the adjacent Rh2 centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh2)-π(L) orbital interactions, and (Rh2LL) n @Au has a smaller energy gap between the filled π*(Rh2) and the empty π*(L) orbitals. Thus, for (Rh2LL) n @Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh2LS) 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.

Mapping Bridge Conformational Effects on Electronic Coupling in Mo2-Mo2 Mixed-Valence Systems.

The large bridging ligand 9,10-anthracenedicarboxylate and its thiolated derivatives have been employed to assemble two dimolybdenum complex units and develop three Mo2 dimers, [Mo2(DAniF)3]2(µ-9,10-O2CC14H8CO2), [Mo2(DAniF)3]2(µ-9,10-OSCC14H8COS), and [Mo2(DAniF)3]2(µ-9,10-S2CC14H8CS2) (DAniF = N, N'-di( p-anisyl)formamidinate), for the study of conformation dependence of the electronic coupling between the two Mo2 centers. These compounds feature a large deviation of the central anthracene ring from the plane defined by the Mo-Mo bond vectors, with the torsion angles (ϕ = 50-76°) increasing as the chelating atoms of the bridging ligand vary from O to S. Consequently, the corresponding mixed-valence complexes do not exhibit characteristic intervalence charge transfer absorptions in the near-IR spectra, in contrast to the phenylene and naphthalene analogues, from which these systems are assigned to the Class I in Robin-Day's scheme. Together with the phenylene and naphthalene series, the nine total mixed-valence complexes in three series complete a transition from the electronically uncoupled Class I to the strongly coupled Class II-III borderline via moderately coupled Class II and permit a systematic mapping of the bridge conformation effects on electronic coupling. Density functional theory calculations show that the HOMO-LUMO energy gap, corresponding to the metal (δ) to ligand (π*) transition energy, and the magnitude of HOMO-HOMO-1 splitting in energy are linearly related to cos2 ϕ. Therefore, our experimental and theoretical results concur to indicate that the coupling strength decreases in the order of the bridging units: phenylene > naphthalene > anthracene, which verifies the through-bond superexchange mechanism for electronic coupling and electron transfer.

Control of the rectifying effect and direction by redox asymmetry in Rh2-based molecular diodes.

Four asymmetrical self-assembled monolayers (SAMs) consisting of two subunits with different Rh2 building blocks present a pronounced rectifying behavior. The rectification ratio (RR) increases on increasing the redox potential difference between the two Rh2 subunits, and the rectifying direction can be reversed by reordering the subunits in the assembly.

Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo-Mo δ Bond.

We have synthesized and characterized the mixed-ligand dimolybdenum paddlewheel complex Na[(DAniF)3Mo2(C3S5)] (Na[1]; DAniF = N,N'-di-p-anisylformamidinate, dmit = 1,3-dithiole-2-thione-4,5-dithiolate), which has a six-membered chelating [Mo2S2C2] ring created by equatorial coordination of the dmit (C3S5) ligand to the Mo2 unit. One-electron oxidation of Na[1] using Cp2FePF6 yields the neutral complex [(DAniF)3Mo2(C3S5)] ([1]), and removal of two electrons from Na[1] using AgBPh4 gives [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). In the crystal structures, [1]- and [1] present dihedral angles of 118.9 and 142.3° between the plane defined by the Mo-Mo bond vector and the dmit ligand, respectively, while DFT calculations show that in [1]+ the Mo-Mo bond and the dmit ligand are coplanar. Complex [1] is paramagnetic with a g value of 1.961 in the EPR spectrum and has a Mo-Mo bond distance of 2.133(1) Å, increased from 2.0963(9) Å for [1]-. Consistently, a broad absorption band is observed for [1] in the near-IR region, which arises from charge transfer from the dmit ligand to the cationic Mo25+ centers. Interestingly, complex [1]+ has an aromatic [Mo2S2C2] core, as evidenced by a large diamagnetic anisotropy, in addition to the coplanarity of the core structure, which shifts downfield the 1H NMR signal of the horizontal methine proton (ArN-(CH)-NAr) but upfield those of the vertical protons, relative to the methine proton resonances for the precursor ([1]-). The magnetic anisotropy (Δχ = χ⊥ - χ∥) for the [Mo2S2C2] ring in [1]+ is -105.5 ppm cgs, calculated from the McConnell equation, which is about 2-fold larger than that for benzene. The aromaticity of the [Mo2S2C2] ring is supported by theoretical studies, including single-point calculations and gauge-including atomic orbital (GIAO) NMR spectroscopic calculations at the density functional theory (DFT) level. DFT calculations also show that the [Mo2S2C2] core in [1]+ possesses a set of three highest occupied and three lowest unoccupied molecular orbitals in π character, corresponding to those of benzene in symmetry, and six π electrons that conform to the Hückel 4n + 2 rule for aromaticity. Therefore, this study shows that an aromatic [Mo2S2C2] core is formed by coupling the δ orbital of the Mo≣Mo bond with the π orbital of the C═C bond through the bridging atoms (S), thus validating the equivalency in bonding functionality between δ and π orbitals.

Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads.

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)3Mo2(µ-O2CC6H4N(C6H4CH3)2] (DAniF = N,N'-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)3Mo2(µ-OSCC6H4N(C6H4CH3)2] ([OS-ph-N]), and [(DAniF)3Mo2(µ-S2CC6H4N(C6H4CH3)2] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ΔE1/2(E1/2(2)(N/N•+) - E1/2(1)(Mo2IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]+, [OS-ph-N]+, and [SS-ph-N]+, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo2) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (Hab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm-1 in CH2Cl2. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo2) charge transfer.

A study of asymmetrical mixed-valent Mo2-Mo2 complexes in the class III regime.

Three novel asymmetrical dimolybdenum dimers, [Mo2(DAniF)3]2(µ-OOCCOS) (DAniF = N,N'-di(p-anisyl)formamidinate) ([OO-OS]), [Mo2(DAniF)3]2(µ-S2CCO2) ([SS-OO]), and [Mo2(DAniF)3]2(µ-SSCCOS) ([SS-OS]), have been synthesized and characterized by either single-crystal X-ray crystallography or 1H NMR spectroscopy. The structural asymmetry for these compounds gives rise to a redox asymmetry, which enlarges the potential separation (ΔE1/2) between the two [Mo2] units. The mixed-valance (MV) species [OO-OS]+, [SS-OO]+ and [SS-OS]+, prepared by one-electron chemical oxidation of the neutral precursors, exhibit an intense and symmetrical intervalence charge transfer (IVCT) absorption band in the near-IR region, along with the high energy metal (δ) to ligand (π*) (ML) and ligand (π) to metal (δ) charge transfer (LMCT) absorptions. The LMCT band, which is absent in the neutral precursors, is reflective of the cationic [Mo2]+ unit in the MV species; therefore, it is evidenced that in the MV complexes optical electron transfer from the electron donor to acceptor occurs, while the thermal process is energetically unfavorable. The C(1)-C(2) bonds (1.44-1.48 Å) that connect the two [Mo2] units are significantly shorter than a C-C single bond, showing that the two Mo2 centers are strongly coupled. For the series, TD-DFT calculations show that the molecular orbitals have an unsymmetrical charge density distribution over the two dimolybdenum sites. For each of the complex systems, the calculated orbital energy gaps, SOMO(δ - δ)-LUMO(bridging ligand π*), HOMO-8(bridging ligand π)-SOMO(δ - δ) and SOMO(δ - δ)-HOMO-1(δ + δ), are in good agreement with the observed MLCT, LMCT and IVCT absorption band energies, respectively. The consistency in energy between the IVCT band and the SOMO(δ - δ)-HOMO-1(δ + δ) gap permits assignment of the MV complexes to Class III in the Robin-Day scheme.

Optically probing the localized to delocalized transition in Mo2-Mo2 mixed-valence systems.

Four thienylene (C4H2S) bridged Mo2 dimers, [Mo2(DAniF)3]2(µ-OOCC4H2SCOO) (DAniF = N,N'-di(p-anisyl)formamidinate), [Mo2(DAniF)3]2(µ-N(H)SCC4H2SCN(H)S), [Mo2(DAniF)3]2(µ-OSCC4H2SCSO) and [Mo2(DAniF)3]2(µ-SSCC4H2SCSS), have been synthesized and studied in terms of electronic coupling. The subtle structural differences between these compounds vary largely the extent of electron delocalization; consequently, a systematic transition from Class II to Class III via Class II-III is achieved, which is probed using spectral parameters of intervalence charge transfer (IVCT) absorption (band energy, intensity and shape) for the mixed-valence complexes. Significantly, the species in Class II-III displays a low energy, half cut-off and solvent-dependent IVCT band, while a high energy, less asymmetrical IVCT band is observed for the complex in Class III. These results give fresh and detailed understanding of the system transition.

Photoinduced δ electron transfer in phenylene bridged Mo2 dimers.

Photoinduced electron transfer (ET) in Mo2 dimers, [Mo2(DAniF)3]2(µ-E2C(ph)CE2) (DAniF = N,N'-di(p-anisyl)formamidinate, E = O or S), has been studied by femtosecond transient spectral techniques. Using a 355 nm laser pulse, the δ electrons on the Mo-Mo quadruple bonds are selectively excited and subsequent charge separation yields diradicals [Mo2]˙--ph-[Mo2]˙+ and [Mo2]-ph˙--[Mo2]˙+. The charge separation (CS) and recombination (CR) rates are derived by fitting the decay kinetic data of the excited states. Surprisingly, it is found that the CR rate constants (kCR-1, ∼1012 s-1) for [Mo2]˙--ph-[Mo2]˙+ are larger than the data (kCR-2) for [Mo2]-ph˙--[Mo2]˙+ by about one order of magnitude, although in the first case, the ET distance is doubled and the electronic coupling between the donor and acceptor is weaker. Optical analyses reveal that the free energy changes (ΔG°) for the two CR processes correspond to the δ → δ* and the metal (δ) to bridging ligand (π*) transition energies, respectively, and thus, the ET kinetics is dominated likely by the driving force (-ΔG°).

Electronic Coupling between Two Covalently Bonded Dimolybdenum Units Bridged by a Naphthalene Group.

Using 2,6-naphthalenedicarboxylate and its thiolated derivatives as bridging ligands, three Mo2 dimers of the type [Mo2(DAniF)3](E2CC10H6CE2)[Mo2(DAniF)3] (DAniF = N,N'-di-p-anisylformamidinate; E = O, S) have been synthesized and characterized by X-ray diffraction. These compounds can be generally formulated as [Mo2]-naph-[Mo2], where the complex unit [Mo2] ([Mo2(DAniF)3(µ-E2C)]) functions as an electron donor (acceptor) and the naphthalene (naph) group is the bridge. The mixed-valence (MV) complexes, generated by one-electron oxidation of the neutral precursors, display weak, very broad intervalence charge-transfer absorption bands in the near-to-mid-IR regions. The electronic coupling matrix elements for the MV complexes, Hab = 390-570 cm(-1), are calculated from the Mulliken-Hush equation, which fall between those for the phenyl (ph) and biphenyl (biph) analogues reported previously. The three series consisting of three complexes with the same [Mo2] units exhibit exponential decay of Hab as the bridge changes from ph to biph via naph, with decay factors of 0.21-0.17 Å(-1). Therefore, it is evidenced that while the extent of the bridge conjugacy varies, the electronic coupling between the two [Mo2] units is dominated by the Mo2···Mo2 separation. The absorption band energies for metal-to-ligand charge transfer are in the middle of those for the ph and biph analogues, which is consistent with variation of the HOMO-LUMO energy gaps for the complex series. These results indicate that the interplay of the bridge length and conjugacy is to affect the enegy for charge transfer crossing the intervening moiety, in accordance with a superechange mechanism.

Tuning the Electronic Coupling and Electron Transfer in Mo2 Donor-Acceptor Systems by Variation of the Bridge Conformation.

Assembling two quadruply bonded dimolybdenum units [Mo2 (DAniF)3 ](+) (DAniF=N,N'-di(p-anisyl)formamidinate) with 1,4-naphthalenedicarboxylate and its thiolated derivatives produced three complexes [{Mo2 (DAniF)3 }2 (µ-1,4-O2 CC10 H6 CO2 )], [{Mo2 (DAniF)3 }2 (µ-1,4-OSCC10 H6 COS)], and [{Mo2 (DAniF)3 }2 (µ-1,4-S2 CC10 H6 CS2 )]. In the X-ray structures, the naphthalene bridge deviates from the plane defined by the two Mo-Mo bond vectors with the torsion angle increasing as the chelating atoms of the bridging ligand vary from O to S. The mixed-valent species exhibit intervalence transition absorption bands with high energy and very low intensity. In comparison with the data for the phenylene analogues, the optically determined electronic coupling matrix elements (Hab =258-345 cm(-1) ) are lowered by a factor of two or more, and the electron-transfer rate constants (ket ≈10(11)  s(-1) ) are reduced by about one order of magnitude. These results show that, when the electron-transporting ability of the bridge and electron-donating (electron-accepting) ability of the donor (acceptor) are both variable, the former plays a dominant role in controlling the intramolecular electron transfer. DFT calculations revealed that increasing the torsion angle enlarges the HOMO-LUMO energy gap by elevating the (bridging) ligand-based LUMO energy. Therefore, our experimental results and theoretical analyses verify the superexchange mechanism for electronic coupling and electron transfer.