Strong supramolecular binding of Li(+)@C60 with sulfonated meso-tetraphenylporphyrins and long-lived photoinduced charge separation

A supramolecular binding occurred between lithium ion encapsulated [60]fullerene (Li(+)@C(60)) and sulfonated tetraphenylporphyrins ([MTPPS](4-) M = H(2) and Zn) in a benzonitrile solution. Photoexcitation of Li(+)@C(60)/[MTPPS](4-) results in formation of a long-lived charge-separated state by photoinduced electron transfer.     

Supramolecular electron transfer by anion binding

Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation. This difference in rates ensures that the linkage between electron donor and acceptor moieties is maintained over the course of most forward and back electron-transfer processes. A particular example of this principle is illustrated by electron-transfer ensembles based on tetrathiafulvalene calix[4]pyrroles (TTF-C4Ps). In these ensembles, the TTF-C4Ps act as donors, transferring electrons to various electron acceptors after anion binding. Competition with non-redox active substrates is also observed. Anion binding to the pyrrole amine groups of an oxoporphyrinogen unit within various supramolecular complexes formed with fullerenes also results in acceleration of the photoinduced electron-transfer process but deceleration of the back electron transfer; again, this is ascribed to favourable structural and electronic changes. Anion binding also plays a role in stabilizing supramolecular complexes between sulphonated tetraphenylporphyrin anions ([MTPPS](4-): M = H(2) and Zn) and a lithium ion encapsulated C(60) (Li(+)@C(60)); the resulting ensemble produces long-lived charge-separated states upon photoexcitation of the porphyrins.     

Rock-salt-type crystal of thermally contracted c(60) with encapsulated lithium cation

Rock solid: Fullerene-encapsulated Li(+) (Li(+) @C(60) ) is an alkaline cation owing to the spherical shape and positive charge. Li(+) @C(60) crystallizes as a rock-salt-type crystal in the presence of PF(6) (-) . The orientations of C(60) and PF(6) (-) (orange) are perfectly ordered below 370?K, and Li(+) (purple) hops within the cage. At temperatures below 100?K two Li(+) units are localized at two polar positions within each C(60) .     

On the viability of small endohedral hydrocarbon cage complexes: X@C4H4, X@C8H8, X@C8H14, X@C10H16, X@C12H12, AND X@C16H16

Small hydrocarbon complexes (X@cage) incorporating cage-centered endohedral atoms and ions (X = H(+), H, He, Ne, Ar, Li(0,+), Be(0,+,2+), Na(0,+), Mg(0,+,2+)) have been studied at the B3LYP/6-31G(d) hybrid HF/DFT level of theory. No tetrahedrane (C(4)H(4), T(d)()) endohedral complexes are minima, not even with the very small hydrogen atom or beryllium dication. Cubane (C(8)H(8), O(h)()) and bicyclo[2.2.2]octane (C(8)H(14), D(3)(h)()) minima are limited to encapsulating species smaller than Ne and Na(+). Despite its intermediate size, adamantane (C(10)H(16), T(d)()) can enclose a wide variety of endohedral atoms and ions including H, He, Ne, Li(0,+), Be(0,+,2+), Na(0,+), and Mg(2+). In contrast, the truncated tetrahedrane (C(12)H(12), T(d)()) encapsulates fewer species, while the D(4)(d)() symmetric C(16)H(16) hydrocarbon cage (see Table of Contents graphic) encapsulates all but the larger Be, Mg, and Mg(+) species. The host cages have more compact geometries when metal atoms, rather than cations, are inside. This is due to electron donation from the endohedral metals into C-C bonding and C-H antibonding cage molecular orbitals. The relative stabilities of endohedral minima are evaluated by comparing their energies (E(endo)) to the sum of their isolated components (E(inc) = E(endo) - E(cage) - E(x)) and to their exohedral isomer energies (E(isom) = E(endo) - E(exo)). Although exohedral binding is preferred to endohedral encapsulation without exception (i.e., E(isom) is always exothermic), Be(2+)@C(10)H(16) (T(d)(); -235.5 kcal/mol), Li(+)@C(12)H(12) (T(d)(); 50.2 kcal/mol), Be(2+)@C(12)H(12) (T(d)(); -181.2 kcal/mol), Mg(2+)@C(12)H(12) (T(d)(); -45.0 kcal/mol), Li(+)@C(16)H(16) (D(4)(d)(); 13.3 kcal/mol), Be(+)@C(16)H(16) (C(4)(v)(); 31.8 kcal/mol), Be(2+)@C(16)H(16) (D(4)(d)(); -239.2 kcal/mol), and Mg(2+)@C(16)H(16) (D(4)(d)(); -37.7 kcal/mol) are relatively stable as compared to experimentally known He@C(20)H(20) (I(h)()), which has an E(inc) = 37.9 kcal/mol and E(isom) = -35.4 kcal/mol. Overall, endohedral cage complexes with low parent cage strain energies, large cage internal cavity volumes, and a small, highly charged guest species are the most viable synthetic targets.     

Elongation of lifetime of the charge-separated state of ferrocene-naphthalenediimide-[60]fullerene triad via stepwise electron transfer

Photoinduced electron-transfer processes of a newly synthesized rodlike covalently linked ferrocene-naphthalenediimide-[60]fullerene (Fc-NDI-C(60)) triad in which Fc is an electron donor and NDI and C(60) are electron acceptors with similar first one-electron reduction potentials have been studied in benzonitrile. In the examined Fc-NDI-C(60) triad, NDI with high molar absorptivity is considered to be the chromophore unit for photoexcitation. Although the free-energy calculations verify that photoinduced charge-separation processes via singlet- and triplet-excited states of NDI are feasible, transient absorption spectra observed upon femtosecond laser excitation of NDI at 390 nm revealed fast and efficient electron transfer from Fc to the singlet-excited state of NDI ((1)NDI*) to produce Fc(+)-NDI(?-)-C(60). Interestingly, this initial charge-separated state is followed by a stepwise electron transfer yielding Fc(+)-NDI-C(60)(?-). As a result of this sequential electron-transfer process, the lifetime of the charge-separated state (τ(CS)) is elongated to 935 ps, while Fc(+)-NDI(?-) has a lifetime of only 11 ps.     

Hydrated magnesium cations Mg+(H2O)n, n ≈ 20-60, exhibit chemistry of the hydrated electron in reactions with O2 and CO2

Ion-molecule reactions of Mg(+)(H(2)O)(n), n ≈ 20-60, with O(2) and CO(2) are studied by Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. O(2) and CO(2) are taken up by the clusters. Both reactions correspond to the chemistry of hydrated electrons (H(2)O)(n)(-). Density functional theory calculations predicted that the solvation structures of Mg(+)(H(2)O)(16) contain a hydrated electron that is solvated remotely from a hexa-coordinated Mg(2+). Ion-molecule reactions between Mg(+)(H(2)O)(16) and O(2) or CO(2) are calculated to be highly exothermic. Initially, a solvent-separated ion pair is formed, with the hexa-coordinated Mg(2+) ionic core being well separated from the O(2)(?-) or CO(2)(?-). Rearrangements of the solvation structure are possible and produce a contact-ion pair in which one water molecule in the first solvation shell of Mg(2+) is replaced by O(2)(?-) or CO(2)(?-).     

Synthesis and charge-transfer chemistry of La2@I(h)-C80/Sc3N@I(h)-C80-zinc porphyrin conjugates: impact of endohedral cluster

Two stable electron donor-acceptor conjugates, that is, 3 and 5b, employing La(2)@I(h)-C(80) and Sc(3)N@I(h)-C(80), on one hand, and zinc tetraphenylporphyrin, on the other hand, have been prepared via [1+2] cycloaddition reactions of a diazo precursor. Combined studies of crystallography and NMR suggest a common (6,6)-open addition pattern of 3 and 5b. Still, subtly different conformations, that is, a restricted and a comparatively more flexible topography, emerge for 3 and 5b, respectively. In line with this aforementioned difference are the electrochemical assays, which imply appreciably stronger I(h)-C(80)/ZnP interactions in 3 when compared to those in 5b. Density functional calculations reveal significant attractions between the two entities of these conjugates, as well as their separately localized HOMOs and LUMOs. The geometrical conformations and LUMO distributions of 3 and 5b, at our applied computational level, are slightly varied with their different endohedral clusters. The clusters also exert different impact on the excited state reactivity of the conjugates. For example, 3 undergoes, upon photoexcitation, a fast charge separation process and yields a radical ion pair, whose nature, namely, (La(2)@C(80))(?-)-(ZnP)(?+)) versus (La(2)@C(80))(?+)-(ZnP)(?-)), varies with solvent polarity. 5b, on the other hand, afforded the same (Sc(3)N@C(80))(?-)-(ZnP)(?+)) radical ion pair regardless of the solvent.     

Evolution of lone pair of excess electrons inside molecular cages with the deformation of the cage in e2@C60F60 systems

For unusual e(2)@C(60)F(60)(I(h), D(6h), and D(5d)) cage structures with two excess electrons, it is reported that not only the lone pair in singlet state but also two single excess electrons in triplet state can be encapsulated inside the C(60)F(60) cages to form single molecular solvated dielectrons. The interesting relationship between the shape of the cage and the spin state of the system has revealed that ground states are singlet state for spherical shaped e(2)@C(60)F(60)(I(h)) and triplet states for short capsular shaped e(2)@C(60)F(60)(D(6h)) and long capsular shaped e(2)@C(60)F(60)(D(5d)), which shows a spin evolution from the singlet to triplet state with the deformation of the cage from spherical to capsular shape. For these excess electron systems, the three ground state structures have large vertical electron detachment energies (VDEs (I) of 1.720-2.283 eV and VDEs (II) of 3.959-5.288 eV), which shows their stabilities and suggests that the large C(60)F(60) cage is the efficient container of excess electrons.     

From the tetra(amino) phosphonium cation, [P(NHPh)4]+, to the tetra(imino) phosphate trianion, [P(NPh)4]3-, two-faced ligands that bind anions and cations

The tetraanilino phosphonium cation, [P(N(H)Ph)4]+, 1+, is sequentially deprotonated by Bu(n)Li in thf. The deprotonation reaction of the chloride derivative, Cl, was monitored by (31)P NMR, which revealed the successive formation of the neutral [P(N(H)Ph)3(NPh)], 2, the monoanionic [P(N(H)Ph)2(NPh)2]-, 3-, the dianionic [P(N(H)Ph)(NPh)3]2-, 4(2-), and finally the trianionic species [P(NPh)(4)](3-), (3-). Considering the isoelectronic relationship of oxo, =O, and imino groups, =NR, as well as hydroxy, -OH, and amino groups, -N(H)R, the neutral complex corresponds to phosphoric acid, H3PO4, whereas the anions 3-, 4(2-) and 5(3-) are analogues of dihydrogen phosphate, H2PO4-, monohydrogenphosphate, HPO4(2-), and orthophosphate ions, PO4(3-), respectively. Solid state structures were obtained of 1Cl, 2LiCl(thf)(2), 3Li(thf)(3.5), 3Li(2)Cl(thf)(4.25), 3Li(2)Cl(thf)(6) and 5Li(4)Cl(thf)(4). All systems provide two separate N-P-N chelation sites at opposite ligand faces, either consisting of the di(amino) arrangement P(NH)(2), acting as a double H-bond donor, the di(imino) arrangement PN(2), donating two electron pairs, or the mixed amino imino arrangement P(N)(NH), which supplies both electron pair and H-donor site. Interesting in this aspect is the mixed amino imino derivative 3- which has the ability to chelate a Lewis acid, such as a metal ion, at one face and a Lewis base, such as an anionic or neutral donor at the opposite ligand face. The formation of 1-D aggregates and the entrapment of lithium chloride are key characteristics of the supramolecular structures of the discussed complexes.     

Demonstration of a chemical transformation inside a fullerene. The reversible conversion of the allotropes of H2@C60

The interconversion of the two allotropes of the hydrogen molecule (para-H2 and ortho-H2) incarcerated inside the fullerene C60 is reported (oH2@C60 and pH2@C60, respectively). For conversion, oH2@C60 was adsorbed at the external surface of the zeolite NaY and immersed into liquid oxygen at 77 K. Equilibrium was reached in less than 0.5 h. Rapid removal of oxygen provides a sample of enriched pH2@C60 that is stable for many days in the absence of paramagnetic catalysts (half-life approximately 15 days). Enriched pH2@C60 is nonvolatile and soluble in organic solvents. At room temperature in the presence of a paramagnetic catalyst (dissolved O2 or the nitroxide Tempo) a slow back conversion into oH2@C60 was observed by 1H NMR. A bimolecular rate constant for conversion of pH2@C60 to oH2@C60 using Tempo of kTempo approximately 4 x 10-5 M-1 s-1 was observed, which is approximately 3 orders of magnitudes slower than that for dissolved pH2 in organic solvents which is not protected by the C60 shell.     

Electron localization function study on intramolecular electron transfer in the QTTFQ and DBTTFI radical anions

The unsymmetrical distribution of the unpaired electron in the ground state of the DBTTFI(?-) radical anion (bi(6-n-butyl-5,7-dioxo-6,7-dihydro-5H-[1,3]dithiolo[4,5-f]isoindole-2-ylidene) is theoretically predicted using the M06-2X/6-31+G(d,p) level of calculations. The results are additionally confirmed by single point calculations at B3LYP/aug-cc-pVTZ, LC-ωPBE/aug-cc-pVTZ, and M06-2X/aug-cc-pVTZ levels. DBTTFI, containing the TTF (tetrathiafulvalene) fragment, may be used in the construction of organic microelectronic devices, similarly to the radical anion of QTTFQ. The unsymmetrical distribution of spin density in (QTTFQ)(?-) has been confirmed using M06-2X/aug-cc-pVTZ calculations, with subsequent study using topological analysis of electron localization function (ELF). The reorganization of the chemical bonds during intramolecular electron transfer in (QTTFQ)(?-) and (DBTTFI)(?-) has been analyzed using bonding evolution theory (BET). The reaction path has been simulated by the IRC procedure, and the evolution of valence basins has been described using catastrophe theory. The simple mechanisms: (QTTFQ)(?-): η-1-3-CC(+)-0: (-?)(QTTFQ) and (DBTTFI)(?-): η-1-3-[F](4)[F(+)](4)-0: (-?)(DBTTFI), each consisting of three steps, have been observed. Two cusp or 4-fold catastrophes occur immediately after the TS. Our study shows that potential future microelectronic devices, constructed on the basis of the (QTTFQ)(?-) and (DBTTFI)(?-) systems, should exploit the properties of the C═C bond.     

双噻吩基四硫富瓦烯富勒烯C60几何构型与电子结构的理论研究

利用半经验AM1法研究双噻吩基四硫富瓦烯富勒烯-C60(BTTTF-C60)和四硫富瓦烯-C60(TTF-C60)的几何构型、电子结构和前线轨道.计算结果显示,两化合物的TTF面发生弯曲,形成独特的空间构型,电子结构的分析表明其原因是由C60与TTF或BTTTF的相互作用引起的.C60的LUMO能与BTTTF的HOMO能接近,易发生D-A反应,形成BTTTF-C60.BTTTF-C60和TTF-C60的LUMO能仍较低.LUMO分布集中在C60部分, 表明BTTTF-C60的C60母体仍可接受电子.另外对两分子的电荷分布、 HOMO及LUMO的分析比较,表明所设计的BTTTF-C60分子可能产生与TTF-C60分子类似的电荷分离态.     

An efficient fluorescence sensor for superoxide with an acridinium ion-linked porphyrin triad

Addition of potassium superoxide with 18-crown-6 ether (KO(2)(?-)-18-crown-6) to a toluene solution of an acridinium ion-linked porphyrin triad (Acr(+)-H(2)P-Acr(+)) resulted in a remarkable enhancement of the fluorescence intensity. Thus, Acr(+)-H(2)P-Acr(+) acts as an efficient fluorescence sensor for superoxide. Electron transfer from KO(2)(?-)-18-crown-6 to the Acr(+) moiety to produce the two-electron-reduced species (Acr(?)-H(2)P-Acr(?)) results in inhibition of the fluorescence quenching via photoinduced electron transfer, as revealed by laser flash photolysis measurements.     

Li+ ionic conductivities and diffusion mechanisms in Li-based imides and lithium amide

In this study, both experimental ionic conductivity measurements and the first-principles simulations are employed to investigate the Li(+) ionic diffusion properties in lithium-based imides (Li(2)NH, Li(2)Mg(NH)(2) and Li(2)Ca(NH)(2)) and lithium amide (LiNH(2)). The experimental results show that Li(+) ions present superionic conductivity in Li(2)NH (2.54 × 10(-4) S cm(-1)) and moderate ionic conductivity in Li(2)Ca(NH)(2) (6.40 × 10(-6) S cm(-1)) at room temperature; while conduction of Li(+) ions is hardly detectable in Li(2)Mg(NH)(2) and LiNH(2) at room temperature. The simulation results indicate that Li(+) ion diffusion in Li(2)NH may be mediated by Frenkel pair defects or charged vacancies, and the diffusion pathway is more likely via a series of intermediate jumps between octahedral and tetrahedral sites along the [001] direction. The calculated activation energy and pre-exponential factor for Li(+) ion conduction in Li(2)NH are well comparable with the experimentally determined values, showing the consistency of experimental and theoretical investigations. The calculation of the defect formation energy in LiNH(2) reveals that Li defects are difficult to create to mediate the Li(+) ion diffusion, resulting in the poor Li(+) ion conduction in LiNH(2) at room temperature.     

Is This a Chemical Bond? A Theoretical Study of Ng2@C60 (Ng=He,Ne, Ar,Kr, Xe)

Quantum-chemical calculations using DFT (BP86) and ab initio methods (MP2, SCS-MP2) have been carried out for the endohedral fullerenes Ng2@C60 (Ng=He-Xe). The nature of the interactions has been analyzed with charge- and energy-partitioning methods and with the topological analysis of the electron density (Atoms-in-Molecules (AIM)). The calculations predict that the equilibrium geometries of Ng2@C60 have D3d symmetry when Ng=Ne, Ar, Kr, while the energy-minimum structure of Xe2@C60 has D5d symmetry. The precession movement of He2 in He2@C60 has practically no barrier. The Ng--Ng distances in Ng2@C60 are much shorter than in free Ng2. All compounds Ng2@C60 are thermodynamically unstable towards loss of the noble gas atoms. The heavier species Ar2@C60, Kr2@C60, and Xe2@C60 are high energy compounds which are at the BSSE corrected SCS-MP2/TZVPP level in the range 96.7-305.5 kcal mol(-1) less stable than free C60+2 Ng. The AIM method reveals that there is always an Ng--Ng bond path in Ng2@C60. There are six Ng--C bond paths in (D3d) Ar2@C60, Kr2@C60, and Xe2@C60, whereas the lighter D3d homologues He2@C60 and Ne2@C60 have only three Ng--C2 paths. The calculated charge distribution and the orbital analysis clearly show that the bonding situation in Xe2@C60 significantly differs from those of the lighter homologues. The atomic partial charge of the [Xe2] moiety is +1.06, whereas the charges of the lighter dimers [Ng2] are close to zero. The a2u HOMO of (D3d) Xe2@C60 in the 1A1g state shows a large mixing of the highest lying occupied sigma* orbital of [Xe2] and the orbitals of the C60 cage. There is only a small gap between the a2u HOMO of Xe2@C60 and the eu LUMO and the a2u LUMO+1. The calculations show that there are several triplet states which are close in energy to each other and to the 1A1g state. The bonding analysis suggests that the interacting species in Xe2@C60 are the charged species Xe2q+ and C60q-, where 1相似文献   

Sequential charge separation in two axially linked phenothiazine-aluminum(III) porphyrin-fullerene triads

New supramolecular triads (PTZpy→AlPor-C(60), TPTZpy→AlPor-C(60)), containing aluminum(III) porphyrin (AlPor), fullerene (C(60)), and phenothiazine (phenothiazine = PTZ, 2-methylthiophenothaizine = TPTZ) have been constructed. In these triads the fullerene and phenothiazine units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. The ground- and excited-state properties of the triads and reference dyads are studied using steady-state and time-resolved spectroscopic techniques. The time-resolved data show that photoexcitation results in charge separation from the excited singlet state of the porphyrin to the C(60) unit, generating (Donor)py→AlPor(?+)-C(60)(?-), Donor = PTZ and TPTZ. A subsequent hole shift from the porphyrin to phenothiazine generates the charge-separated state (Donor)(?+)py→AlPor-C(60)(?-). The lifetime of the charge separation exhibits a modest increase from 39 ns in the absence of the donor to 100 ns in PTZpy→AlPor-C(60) and 83 ns in TPTZpy→AlPor-C(60). These lifetimes are discussed in terms of the electronic coupling between phenothiazine, the porphyrin, and C(60).     

Sc2@C70 rather than Sc2C2@C68: density functional theory characterization of metallofullerene Sc2C70

Detailed study on Sc(2)C(70) series has been performed based on fully screening for C(70) tetra- and hexa- anions. With a combined methodology of quantum chemistry and statistical mechanics, our calculation results reveal that the Sc(2)C(70), which was proposed as the first metal-carbide endohedral metallofullerene with a non-isolated pentagon rule (non-IPR) cage (Sc(2)C(2)@C(68):6073_C(2v)), is in fact a C(70) non-IPR metallofullerene structure (Sc(2)@C(70):7854_C(2v)) with three pair of pentagon adjacency thanks to its significant thermodynamic and kinetic stability. According to the natural bond analysis and orbital interaction diagram, each scandium atom should only transfer two 4s electrons to the carbon cages and the valence state of Sc(2)@C(70) is (Sc(2+))(2)@C(70) (4-). In addition, the simulation of UV-Vis-NIR spectrum for Sc(2)@C(70):7854_C(2v) shows good accordance to the experimental spectrum.     

Role of encapsulation of Na+ and F- ions on the Diels-Alder reactivity of C32

The density functional theory (DFT)-based Becke's three parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) calculations have been performed to understand the role of encapsulation of Na(+) and F(-) ions on the Diels-Alder reactivity of C(32). In this context, C(32) has been taken as the dienophile and cis-1,3-butadiene has been considered as diene. Results obtained from the calculations on the Na(+)@C(32) and F(-)@C(32) have also been compared with that of C(32). It is found from the results that encapsulated Na(+) ion acts as a catalyst, whereas the encapsulated F(-) does not accelerate the reaction between C(32) and cis-1,3-butadiene. Thus, the reactivity of F(-)@C(32) is less than that of free C(32) and Na(+)@C(32). Formation of adduct involving [5,5]-B bond is preferred over other bonds. The energy decomposition analysis has been applied to understand the role of confinement on an encaged ion. The part played by the charge transfer interaction is evident from the NBO analysis. The frontier orbital analysis points out that the reaction is driven by the normal electron demand.     

Dramatic effect of the metal cation in dealkylation reactions of phosphinic esters promoted by complexes of polyether ligands with metal iodides

Metal ion electrophilic catalysis has been revealed in dealkylation reactions of phosphinic esters 1-4 promoted by complexes of polyether ligands 5-7 with metal iodides MI(n) (M[n+] = Li(+), Na(+), K(+), Rb(+), Ca(2+), Sr(2+), Ba(2+)) in low polarity solvents (chlorobenzene, 1,2-dichlorobenzene, and toluene) at 60 degrees C. The catalytic effect increases with increasing the Lewis acid character of the cation, in the order Rb(+)< K(+)< Na(+)< Li(+) and Ba(2+)< Sr(2+)< Ca(2+). The results are interpreted in terms of a transition state where the complexed cation (M[n+] subset Lig) assists the departure of the leaving group Ph(2)P(O)O(-) and, at the same time, favors the attack at carbon of the nucleophile I(-) ("push-pull" mechanism). The rate sequence found for 1-4 (Me > Et > i-Pr and t-Bu) shows that this reaction can be utilized for the selective dealkylation of these substrates.     

Electrochemically induced chemically reversible proton-coupled electron transfer reactions of riboflavin (vitamin B2)

The electrochemical behavior of the naturally occurring vitamin B(2), riboflavin (Fl(ox)), was examined in detail in dimethyl sulfoxide solutions using variable scan rate cyclic voltammetry (ν = 0.1 - 20 V s(-1)) and has been found to undergo a series of proton-coupled electron transfer reactions. At a scan rate of 0.1 V s(-1), riboflavin is initially reduced by one electron to form the radical anion (Fl(rad)(?-)) at E(0)(f) = -1.22 V versus Fc/Fc(+) (E(0)(f) = formal reduction potential and Fc = ferrocene). Fl(rad)(?-) undergoes a homogeneous proton transfer reaction with the starting material (Fl(ox)) to produce Fl(rad)H(?) and Fl(ox)(-), which are both able to undergo further reduction at the electrode surface to form Fl(red)H(-) (E(0)(f) = -1.05 V vs Fc/Fc(+)) and Fl(rad)(?2-) (E(0)(f) = -1.62 V vs Fc/Fc(+)), respectively. At faster voltammetric scan rates, the homogeneous reaction between Fl(rad)(?-) and Fl(ox) begins to be outrun, which leads to the detection of a voltammetric peak at more negative potentials associated with the one-electron reduction of Fl(rad)(?-) to form Fl(red)(2-) (E(0)(f) = -1.98 V vs Fc/Fc(+)). The variable scan rate voltammetric data were modeled quantitatively using digital simulation techniques based on an interconnecting "scheme of squares" mechanism, which enabled the four formal potentials as well as the equilibrium and rate constants associated with four homogeneous reactions to be determined. Extended time-scale controlled potential electrolysis (t > hours) and spectroscopic (EPR and in situ UV-vis) experiments confirmed that the chemical reactions were completely chemically reversible.