Electron transfer in oxidized biferrocene, biferrocenylene, and [1.1]ferrocenophane systems

Thermal Properties and Solvent Polarities of Mixed-Valence Ionic Liquids Containing Cationic Biferrocenylene Derivatives

Ionic liquids (ILs) containing cationic mixed-valence biferrocenylene derivatives were synthesized with an octanoyl or octyl substituent in each cation. Their melting points ranged between 25 and 39 °C, and the octanoyl derivatives exhibited higher melting points than the octyl derivatives. In addition, each IL exhibited a glass transition in the temperature ranging from -66 to -45 °C after melting. Their melting points were ∼10 °C higher than those of mononuclear octamethylferrocenium salts bearing the same substituents. The solvent polarity (E_T^N) and Kamlet-Taft parameters (π*, α, and β) of these dinuclear and mononuclear ILs were then examined. The dinuclear ILs bearing octanoyl substituents exhibited significant increases in E_T^N and π* and a decrease in α with the decreasing temperature, whereas the other ILs exhibited a significantly less pronounced temperature dependence. Finally, the intervalence charge-transfer (or charge-resonance) bands of the octanoyl dinuclear ILs exhibited red shifts with the decreasing temperature, which can be regarded as self-thermosolvatochromism.

Anion-Ordering Phase Transitions in Biferrocenium and Ferrocenium Salts with the Bis(trifluoromethanesulfonyl)amide Anion

The bis(trifluoromethanesulfonyl)amide anion (Tf₂N), which is a common component of ionic liquids, often exhibits disorder in the solid state. In this study, the phase transitions and crystal structures of the Tf₂N salts of 1,1‴-dineopentyl-1',1″-biferrocene (=npBifc), 1',1″-biferrocene (=Bifc), ferrocene, and cobaltocene (1-4, respectively) were compared. All the salts exhibited phase transitions at low temperatures, which are accompanied by anion ordering, though the ordering was not complete in 2 and 3. X-ray crystallographic investigation revealed that the cations and anions in 1 and 2 adopted alternating arrangements and segregated columnar arrangements, respectively. The cation in 1 exhibited a symmetrical, average-valence structure in the room-temperature phase owing to rapid valence tautomerization, whereas the cation exhibited an unsymmetrical structure in the low-temperature phase. The cation in 2 exhibited an unsymmetrical, trapped-valence structure in both phases. The cation valence states in these salts were accounted for by the electrostatic interactions between the cations and anions. The crystal structures and phase behavior of the ferrocenium salt 3 were very different from those of 4.

A biferrocenium salt containing paramagnetic tetracyanoquinodimethane hexamers: charge disproportionation via donor-acceptor interactions

[Dineopentyl-biferrocene]2[Cl1TCNQ]7, which has an unprecedented high donor-acceptor ratio of 2 : 7, contains a linear paramagnetic hexamer of Cl1TCNQ. Both the donor and acceptor molecules exhibit charge disproportionation in the crystal through mutual electrostatic interactions.

Strain-free [N]ferrocenylenes and cyclo[10]ferrocenylene

Calculated attraction: Trans-linear and cis-angular [N]ferrocenylenes, as well as cyclo[10]ferrocenylene, higher homologues of biferrocenylene, and analogues of [N]phenylenes, are demonstrated as strain-free.

Insight into the electronic structure, optical properties, and redox behavior of the hybrid phthalocyaninoclathrochelates from experimental and density functional theory approaches

An insight into the electronic structure of several hafnium(IV), zirconium(IV), and lutetium(III) phthalocyaninoclathrochelates has been discussed on the basis of experimental UV-vis, MCD, electro- and spectroelectrochemical data as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. On the basis of UV-vis and MCD spectroscopy as well as theoretical predictions, it was concluded that the electronic structure of the phthalocyninoclathrochelates can be described in the first approximation as a superposition of the weakly interacting phthalocyanine and clathrochelate substituents. Spectroelectrochemical data and DFT calculations clearly confirm that the highest occupied molecular orbital (HOMO) in all tested complexes is localized on the phthalocyanine ligand. X-ray crystallography on zirconium(IV) and earlier reported hafnium(IV) phthalocyaninoclathrochelate complexes revealed a slightly distorted phthalocyanine conformation with seven-coordinated metal center positioned ∼1 Å above macrocyclic cavity. The geometry of the encapsulated iron(II) ion in the clathrochelate fragment was found to be between trigonal-prismatic and trigonal-antiprismatic.

Single electron tunneling in large scale nanojunction arrays with bisferrocene-nanoparticle hybrids

We report on the fabrication and single electron tunneling behaviour of large scale arrays of nanogap electrodes bridged by bisferrocene-gold nanoparticle hybrids (BFc-AuNP). Coulomb staircase was observed in the low temperature current-voltage curves measured on the junctions with asymmetric tunnel barriers. On the other hand, junctions with symmetric tunneling barrier exhibited mere nonlinear current voltage characteristics without discrete staircase. The experimental results agreed well with simulations based on the orthodox theory. The junction resistance showed thermally activated conduction behaviour at higher temperature. The overall voltage and temperature dependent results show that the transport behaviour of the large arrays of single particle devices obtained by a facile optical lithography and chemical etching process corresponds with the behaviour of single particle devices fabricated by other techniques like e-beam lithography and mechanical breaking methods.

Electron-transfer processes in metal-free tetraferrocenylporphyrin. Understanding internal interactions to access mixed-valence States potentially useful for quantum cellular automata

Redox properties of H(2)TFcP [TFcP(2-) = 5,10,15,20-tetraferrocenylporphyrin(2-)] were investigated using cyclic voltammetry, differential pulse voltammetry, and square-wave voltammetry methods in a large variety of solvents and electrolytes. When DMF, THF, and MeCN were used with TBAP as the supporting electrolyte, the first oxidation wave was assigned to a single four-electron oxidation process reflecting simultaneous oxidation of all iron(II) centers into iron(III) centers in H(2)TFcP. When an o-DCB (1,2-dichlorobenzene)/TBAP combination was used in electrochemical experiments, four ferrocene substituents underwent two very diffuse, "two-electron" stepwise oxidations. The use of a weakly coordinating TFAB ([NBu(4)][B(C(6)F(5))(4)]) electrolyte in o-DCB or DCM results in four single-electron oxidation processes for ferrocene substituents in which the first and second single-electron waves have a relatively large separation, while the second, third, and fourth oxidation processes are more closely spaced; similar results were observed when a DCM/TBAP system and an imidazolium cation-based ionic liquid ((bmim)Tf(2)N = N-butyl-N'-methylimidazolium bis(trifluoromethanesulfonyl)imide) were used. Spectroelectrochemical oxidation of H(2)TFcP in o-DCB or DCM with TFAB as the supporting electrolyte allowed for characterization of the mixed-valence [H(2)TFcP](+), [H(2)TFcP](2+), and [H(2)TFcP](3+) compounds by UV-vis spectroscopy in addition to the "all-Fe(III)" [H(2)TFcP](4+). The chemical oxidation of H(2)TFcP was tested using a variety of oxidants which resulted in formation of mixed-valence [H(2)TFcP](+) and [H(2)TFcP](2+) as well as [H(2)TFcP](4+), which were characterized by UV-vis-NIR, MCD, IR, Mossbauer, and XPS spectroscopy. The intervalence-charge-transfer bands observed in the near-IR region in [H(2)TFcP](+) and [H(2)TFcP](2+) complexes were analyzed using Hush formalism and found to be of class II (in Robin-Day classification) character with localized ferrous and ferric centers. Class II behavior of [H(2)TFcP](+) and [H(2)TFcP](2+) complexes was further confirmed by Mossbauer, IR, and XPS data.

Exploring the ground and excited state potential energy landscapes of the mixed-valence biferrocenium complex

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to explore the potential energy landscapes in the class II (in Robin and Day classification) mixed-valence biferrocenium mono-cation (BF(+)) in an effort to evaluate factors affecting optical and thermal intramolecular electron transfer rates. Both energy- and spectroscopy-based benchmarks were used to explore the adiabatic potential energy surfaces (PESs) of the mixed-valence BF(+) cation along with the optimization of appropriate ground-, excited-, and transition-state geometries. The calculation of Mossbauer isomer shifts and quadrupole splittings, UV-vis excitation energies, and the electronic coupling matrix element, H(ab), corroborate the PES analyses. The adiabatic electron transfer pathway is also analyzed with respect to several possible vibronic coordinates. The degree of the electronic coupling between iron sites, the value of H(ab), and the nature of the electron transfer pathway correlate with the amount of Hartree-Fock exchange involved in the DFT calculation with hybrid (approximately 20% of Hartree-Fock exchange) methods providing the best agreement between theory and experiment. DFT (B3LYP) predicted values of H(ab) (839, 1085, and 1265 cm(-1)) depend on the computational method and are in good agreement with experimental data.

Electronic structure and absorption spectra of biferrocenyl and bisfulvalenide diiron radical cations: detection and assignment of new low-energy transitions

UV-visible/near-IR (NIR)/mid-IR (MIR) solution, solid-state, and matrix-isolation electronic absorption spectra of the Fe(II)-Fe(III) mixed-valent homobimetallic compounds biferrocenyl triiodide (1) and 1',1'''-diethylbiferrocenyltriiodide (2) reveal the presence of a low-energy transition in the MIR region that has not been reported before. The new absorption feature and the known NIR band are both assigned to intervalence charge-transfer (IVCT) transitions. To obtain insight into the electronic structures of 1 and 2, DFT calculations with the BP86 functionals and different basis sets have been performed. Based on the molecular orbital scheme of cation 1, one band corresponds to the transition between the highest occupied d(x(2)-y(2)) orbitals on the two iron centers, whereas the other one is assigned to a transition from a lower-lying d(z(2)) orbital to the d(x(2)-y(2)) orbital. For comparison, the doubly bridged bisfulvalenide diiron cation (3) has been investigated by optical absorption spectroscopy and DFT calculations. The experimental and theoretical results are discussed with respect to the degree of electron localization/delocalization in these systems.

Synthesis and cyclic voltammetric studies of diiron complexes, ER(2)[(eta-C(5)H(4))Fe(L(2))Me](2) (E = C, Si, Ge, Sn; R = H, alkyl; L(2) = diphosphine] and (eta-C(5)H(5))Fe(L(2))ER(2)Fc [Fc = (eta-C(5)H(4))Fe(eta-C(5)H(5))]

The cyclic voltammetric studies on ER(2)[(eta(5)-C(5)H(4))Fe(L(2))Me](2) (L(2) = dppe; ER(2) = CH(2) (1a), SiMe(2) (2a), GeMe(2) (3a), SnMe(2) (4a) revealed two well resolved reversible waves [(1)E(1/2) = -0.33 V, (2)E(1/2) = -0.20 V (for 1a); (1)E(1/2) = -0.35 V, (2)E(1/2) = -0.21 V (for 2a);(1)E(1/2) = -0.36 V, (2)E(1/2) = -0.23 V (for 3a);(1)E(1/2) = -0.36 V, (2)E(1/2) = -0.22 V (for 4a)] in CH(2)Cl(2) suggesting electronic communication between two iron centers which is seen for the first time in this family of organometallic complexes. The resolution between two reversible waves increases in the order of 1a < 2a < 3a < 4a; however, coordinating solvents such as pyridine, PhCN, DMSO and DMF decreased these interactions attributable to the stabilization of cationic species formed after the first oxidation. UV/Vis spectroelectrochemistry of 1a-4a revealed two distinct absorbance patterns for both redox processes and reflected the stepwise oxidation. Homobimetallic complexes containing ferrocenyl groups, (eta(5)-C(5)H(5))Fe(L(2))ER(2)Fc [ER(2) = none, L(2) = cis-dppen (5a), ER(2) = SiMe(2), L(2) = cis-dppen (6a), dppm (6b); ER(2) = GeMe(2), L(2) = cis-dppen (7a), dppm (7b); ER(2) = Sn(t)Bu(2), L(2) = dmpe (8a); Fc = (eta(5)-C(5)H(4))Fe(eta(5)4-C(5)H(5))] were prepared and studied in terms of electrochemistry. The cyclic voltammogram of 5a exhibited two well resolved one electron reversible waves at (1)E(1/2) = -0.21 V and (2)E(1/2) = 0.58 V corresponding to oxidation of the Fe(P-P) and Fc iron atoms respectively. Other complexes in this series (6a/6b, 7a/7b, 8a) containing direct Fe-E-Fc (E = Si, Ge and Sn) bridging units were not stable under electrochemical conditions and rupture of the Fe-E bonds was observed.

Examination of the mixed-valence state of the doubly boron-bridged diferrocene cation [(FeCp)2{mu-C10H6(BPh)2}]+

A series of mixed-valent (MV) complexes [(FeCp)2(mu-C10H6(BPh)2)]+X ([1+]X; X=I 5, PF6, SbF6, B(C6F5)4) were prepared by oxidation of diboradiferrocene [(FeCp)2(mu-C10H6(BPh)2)] (1) with I 2, AgPF6, and AgSbF6, respectively, and through anion exchange of the I 5(-) salt with [Li(Et2O)x][B(C6F5)4] in the case of X=B(C6F5)4. The MV state of the cation was investigated in solution by multinuclear NMR spectroscopy, CV, and UV/Vis-NIR absorption spectroscopy, and in the solid state by IR spectroscopy, single-crystal X-ray crystallography, and Mössbauer spectroscopy. The cyclic voltammogram of 1 shows two distinct redox waves with a large redox splitting of Delta E=510 mV in CH2Cl2 and the NIR spectrum for the mono-oxidized species displays an intervalence charge-transfer band at around 1500 to 1700 nm depending on the specific counterion present. The X-ray crystal structures of [1+]X show inversion-symmetric cations with X=I 5 and B(C6F5)4 and unsymmetric valence-trapped structures composed of one ferrocene and one ferrocenium moiety with X=PF6 and SbF6. Mössbauer data for X=PF6 are consistent with valence trapping at all temperatures between 90 and 343 K. In comparison, fast electron transfer is evident on the Mössbauer timescale for X=I 5 and temperature-dependent behavior is observed for X=B(C6F5)4. The anion dependence of the X-ray structural and Mössbauer data is discussed in the context of crystal symmetry and the possibility of static and dynamic disorder effects is considered.

Rapid-Scan Time-Resolved FT-IR Spectroelectrochemistry Studies on the Electrochemical Redox Process

The electron transfer to or from molecules containing multiple redox centers has been extensively investigated. Rapid scan time-resolved FT-IR-RAS spectroelectrochemistry was used to investigate the electron-transfer mechanism in this report. The electron transfer of two typical compounds, 1,4-benzoquinone and 1,4-bis(2-ferrocenylvinyl)benzene, was examined with this method. Although the two compounds show two-electron transfer in the redox process, 1,4-benzoquinone exhibits two single electron waves while 1,4-bis(2-ferrocenylvinyl)benzene exhibits a single wave in cyclic voltammetric experiments. The IR absorption of the intermediate, BQ*- and p-(Fc-CH=CH)+2-benzene, at 1506 and 1589 cm(-1), respectively, appeared and disappeared on the experimental time scale in the oxidation and reduction process was observed. In the oxidation process of the p-(Fc-CH=CH)2-benzene molecule, one Fc was oxidated to Fc+ first and the electron-withdrawing ability of Fc+ was stronger than that of Fc, which resulted in the D-pi-A structure and the band at 1589 cm(-1) becoming visible. Then as the oxidation continues, the other Fc was oxidated to Fc+ too, which resulted in the reforming of the symmetry of the benzene ring A-pi-A, so the band at 1589 cm(-1) disappeared. Similar phenomenon can be elucidated in the reduction process but the configuration type changed from A-pi-A to D-pi-A and finally to D-pi-D. Hence, not only 1,4-benzoquinone but also 1,4-bis(2-ferrocenylvinyl)benzene show two consecutive one-electron processes. In addition, it is observed that the existing time of the electrochemical reaction intermediate (BQ*- and p-(Fc-CH=CH)+2-benzene) is prolonged at low temperatures due to slow reaction kinetics.

Solvent Effect on Intramolecular Electron Transfer Rates of Mixed-Valence Biferrocene Monocation Derivatives

Intramolecular electron transfer (ET) rates in various solvents of mixed-valence biferrocene monocation (Fe(II), Fe(III)) and the 1',1' ''-diiodo and 1',1' ''-diethyl derivatives (respectively abbreviated as BFC(+), I(2)BFC(+), and Et(2)BFC(+)) were determined by means of the spin-lattice relaxation times of the protons, taking into account the local magnetic field fluctuation caused by the electron hopping between the two ferrocene units. We also determined the ET rates of a mixed-valence diferrocenylacetylene monocation (DFA(+)) in order to examine the effect of the insertion of an acetylene bridge between the two ferrocene units. The insertion of the bridge decreased the ET rate, while the effect of substitution on the cyclopentadienyl rings on the rate was minor. The observed rates for each mixed-valence monocation in various solvents did not correlate with the reorganization energies, but we did find a significant contribution of the solvent dynamics. The observed rates were considerably higher than those expected on the basis of the Sumi-Marcus-Nalder model in which the solvents were regarded as dielectric continua. The slope of the logarithm plot of the pre-exponential factors in various solvents for each mixed-valence monocation versus the inverse of the longitudinal dielectric relaxation times of the solvents was significantly smaller than unity, and the slope for DFA(+) was larger than those for BFC(+), I(2)BFC(+), and Et(2)BFC(+). These results were ascribed to a partial contribution of the dielectric friction to the dynamics along the solvent coordinate; the extent of the contribution decreased with a reduction in the ET distance. For the dynamics along the solvent coordinate of the ET reactions in methanol, the observed rates indicated an important contribution by the minor dielectric relaxation components with faster relaxation times, rather than the major component with an extraordinarily long relaxation time.

Optical control over photoconductivity in polyferrocenylsilane films

We report the study and elucidate the origin of the photoconductivity of polyferrocenylsilanes achieved through photooxidation performed by ultraviolet irradiation in the presence of chloroform. The persistence over months of the changes in the optoelectronic properties allowed more detailed studies of the charge photogeneration process. The photocurrent spectrum mimics that of the absorption indicating that the photooxidized material is not a mechanical mixture of oxidized and unoxidized polymer units. Photomodulation spectroscopy revealed the existence of long-lived photoexcited states with a lifetime in the millisecond range. They have been interpreted as trapped excitons at the oxidized sites where the polymer is deformed due to the presence of the chloroform derived counter ions. Because of the relatively long lifetime of the trapped excitons they can dissociate and the formed charge carriers can be separated in an externally applied electric field. The effect of the polymer chain deformation around the oxidized unit extends over the neighboring polymer units. In light of the potential applications of this class of polymers in various electronic and photonic devices, the clarification of such a basic process as the photocurrent generation will be a key factor for further technological development.

Effect of asymmetry on the electronic delocalization in diiron and iron-cobalt mixed valence metallocenic compounds

In this work, we report the synthesis and a study on the degree of electronic delocalization in the asymmetric mixed valence complexes [CpFe(C(8)H(6))Fe(C(8)H(7))](+), 3a(+), and [CpCo(C(8)H(6))Fe(C(8)H(7))](+), 3b(+), (Cp = C(5)Me(5), C(8)H(6) = pentalenyde, C(8)H(7) = hydropentalenyde, and = ((3,5(CF(3))(2)C(6)H(3))(4)B(-))). Electrochemical methods, (57)Fe Mössbauer spectroscopy, electronic spectroscopy, and electron paramagnetic resonance were used for this purpose. Although the anti conformation of the complexes precludes any metal-metal interaction, all the techniques employed show that 3a(+) is a electronic delocalized system, while 3b(+) behaves as two individual metallic centers with localized electron density.