Π-radical Anion Research Articles

Bare Molecular Anions of Unsaturated Hydrocarbons: Density Functional Charge and Spin Distributions Based on Their Single Crystal Structures

Bare π-hydrocarbon radical anions M•, dianions M− as well as even a radical trianion M• can be crystallized as alkali metal salts, [M(•)−n][Me+solv]n if the solvent-shared contact ion multiples, [M(•)−n..(Me+)n]solv, present in aprotic solution after the reduction at an alkali metal mirror can be separated by advantageous cation solvation. Altogether 22 π-hydrocarbon anions M(•)−n comprising polycyclic cata- as well as peri-annelated six-membered ring compounds, polyphenyls, those with nonalternant polycyclic skeletal topology, open chain and organosilicon derivatives have been characterized by low-temperature X-ray crystallographic analyses. Based on the structural data determined, their charge and spin populations have been approximated by density functional calculations including natural bond orbital analyses (DFT/NBO). Although in general the total (σ + π) charges at individual C centers of the hydrocarbon skeletons cannot be correlated neither with the structural data nor with the spin distribution, numerous interesting details as well as some useful rules are provided. They concern, for instance, central C-C bond elongation and peripheral C-C bond shortening, preferred charge localization patterns, effects of conformational changes in the π-radical anions crystallized or Jahn-Teller distortions of formally degenerate molecular radical anion states.

Electron Transfer Studies in C78 (C2v‘), C76 (D2), C70 (D5h), and C60 (Ih) Surfactant Aqueous Solutions

C60 (Ih) and several higher fullerenes, namely, C70 (D5h), C76 (D2), and C78 (C2v‘), have been solubilized via capping with suitable surfactants. Excited states and reduced states of these surfactant systems were probed by photolytic and radiolytic techniques. Flash photolytic irradiation at 355 nm of surfactant solutions of C60, C70, C76, and C78 revealed the rapid grow-in of transient (*S1 → *Sn) absorptions with λmax at 920, 660, 650, and 890 nm, respectively. The associated intersystem crossing rates to the energetically lower lying excited triplet states are typically on the order of 9.0 × 108 s-1. Reductive quenching of the excited triplet states by diazabicyclooctane (DABCO) occurs with (0.8−7.4) × 107 M-1 s-1 and yields the fullerene π-radical anions. Direct reduction of surfactant capped fullerene monomers have been studied by means of time-resolved pulse radiolysis with measurements being conducted in the near-IR region. Radiolytic reduction of (C76)surfactant and (C78)surfactant resulted in the...

Resonance Raman Spectroscopic Characterization of the π-Anion and π-Cation Radicals of Zinc(II) Octaethylchlorin

Cyclic voltammetry (CV), UV−vis spectroscopy, and resonance Raman (RR) spectroscopy are utilized to characterize the π-anion and π-cation radicals of ZnOEC (OEC = octaethylchlorin). Features in the RR spectra are assigned with the aid of data from the meso−d4 isotopomer of ZnOEC and ZnEtI (EtI = etiochlorin I). The vibrational frequency shift patterns of the radicals are consistent with expected bonding changes on placing an electron in the LUMO or a hole in the HOMO. The frontier orbitals are similar in ZnOEC and ZnOEP (OEP = octaethylporphyrin), but the anion spectra differ because of the consequences of the Jahn−Teller effect in ZnOEP-. The cation and anion vibrational shifts do not add to give the shifts previously reported for the triplet state, indicating additional distortion upon triplet excitation.

New Metalloporphyrins with Extremely Altered Redox Properties: Synthesis, Structure, and Facile Reduction to Air-Stable π-Anion Radicals of Zinc and Nickel β-Heptanitroporphyrins

New Metalloporphyrins with Extremely Altered Redox Properties: Synthesis, Structure, and Facile Reduction to Air-Stable π-Anion Radicals of Zinc and Nickel β-Heptanitroporphyrins

Through bond mechanism versus exciplex formation in the photochemistry of fullerene / ferrocene donor-bridge-acceptor dyads

A systematic fluorescence and flash photolytic investigation of a series of covalently linked fullerene / ferrocene based donor-bridge-acceptor dyads is reported as a function of the nature of the bridge between the donor site and acceptor site. The fluorescence of the investigated dyads 2 (Φrel = 0.17 × 10−4, 3 (Φrel = 0.78 × 10−4), 4 (Φrel = 1.5 × 10−4), 5 (Φrel = 0.7 × 10−4), and 6 (Φrel = 2.9 × 10−4) were substantially quenched, relative to N-methyl fulleropyrrolidine (1) (Φrel = 6.0 × 10−4). Photolysis of N-methyl fulleropyrrolidine (1) in toluene revealed formation of the excited singlet state which was followed by a rapid intersystem crossing to the excited triplet state. On the other hand, the fate of the excited singlet state of 2, 3, 4, 5, and 6 was found to be governed by rapid intramolecular quenching, with rate constants of 28×109 s−1, 6.9×109 s−1, and 3.4×109 s−1, 14×109 s−1, 2.3×109 s−1 respectively. The electron transfer process and the charge separation were confirmed by monitoring the characteristic π-radical anion bands at λmax = 400 and 1055 nm in degassed benzonitrile with τ1/2 = 1.8 μs (3) and 2.5 μs (4).

Capped Fullerenes: Stabilization of Water-Soluble Fullerene Monomers As Studied by Flash Photolysis and Pulse Radiolysis

Functionalization of C60 with hydrophilic groups promotes the water solubility of the fullerene core which leads in aqueous solution to the irreversible formation of clusters. Capping the surface of water-soluble fullerene derivatives with surfactants (cetyltrimethylammonium chloride or Triton X-100) was found to exclude formation of fullerene clusters and, in turn, to stabilize fullerene monomers. Flash photolytic and pulse radiolytic techniques were employed to generated excited and reduced states of fullerene monomers and fullerene clusters 1−3. Fullerene aggregation was found to have no significant effects on the physicochemical properties related to the generation and lifetime of the excited singlet state. Nanosecond studies of the processes following photoexcitation revealed noticeable differences between monomeric and colloidal derivatives 1−3 for the absorption maximum of the corresponding *T1 → *Tn absorptions. Clustering was found to invoke also remarkable changes for the decay of the fullerene's excited triplet state. Similarly, λmax of the π-radical anions 1−3 is very sensitive to the environmental parameters of the accommodating assembly. In surfactant media, the maxima ([(C60•-)(C4H10N+)]surfactant 1010 nm) were generally blue-shifted relative to the analogous γ-CD complexes ([(C60•-)(C4H10N+)]/γ-CD 1030 nm). On the basis of comparison with fullerene clusters and γ-CD-incorporated complexes, evidence was found that suggests the presence of fullerene monomers in a core(fullerene)−shell(surfactant) type structure.

A Resonance Raman Spectroelectrochemical Study of the Zn(II) Tetraphenylchlorin Anion

Zinc(II) tetraphenylchlorin (ZnTPC) and its π-anion radical have been studied by UV−vis absorption and resonance Raman (RR) spectroscopies. Analyses of the RR spectra were aided by data for the pyrrole-d8, meso-C13, and phenyl-d20 isotopomers. Upon formation of the anion, significant frequency shifts are observed in skeletal vibrational modes, while substituent modes remain essentially unshifted. Both frequency upshifts (ν37, ν10, and ν19) and downshifts (ν2, ν11, and ν3) occur for the CαCm and CβCβ stretching modes in the π-anion. Modes involving significant CαCβ stretching motion, ν4 and ν41, upshift in the anion spectrum. The pattern of vibrational frequency shifts for the anion of ZnTPC is similar to what is observed for the anion of vanadyl tetraphenylporphyrin ((VO)TPP) despite the reduction of a pyrrole ring in TPC. This resemblance is due to the Jahn−Teller distortion in (VO)TPP-, which gives its singly occupied molecular orbital the same bonding pattern as in ZnTPC-.

Radiolytic Reduction of Tetrapropylporphycene and Its Iron, Cobalt, Nickel, Copper, and Tin Complexes

One-electron and multielectron reductions of 2,7,12,17-tetrapropylporphycene (H2TPrPc) and its Fe, Co, Ni, Cu, and Sn complexes in 2-PrOH solutions have been studied by radiolytic techniques. Formation and decay of intermediates formed upon one-electron reduction have been followed by kinetic spectrophotometric pulse radiolysis, and the absorption spectra of stable reduction products have been recorded following γ-radiolysis. H2TPrPc and SnIVTPrPc are reduced to the π-radical anions and then to the dianions, which are stable in alkaline 2-PrOH. In neutral or acidic solutions, the π-radical anions undergo proton-enhanced disproportionation and the dianions undergo protonation. With the transition metal complexes, redox reactions of FeIII/FeII and CoIII/CoII have been observed. The porphycenes of FeII, CoII, NiII, and CuII are reduced in most cases to form the π-radical anions, MIIPc•-. The exception is the case of the CoIIPc, which forms CoIIPc•- in alkaline 2-PrOH but yields a short-lived CoIPc in neutral 2-PrOH. CuIIPc•- has some CuI character, resulting in rapid demetallation in acidic solutions. The radical anions MIIPc•- also undergo proton-enhanced disproportionation to yield MIIPcH2.

Electron Delocalization in Ruthenium(II) and Osmium(II) 2,2‘-Bipyridyl Complexes Formed from Ethynyl-Bridged Ditopic Ligands

Photophysical and electrochemical properties have been recorded for a series of mono- and binuclear ruthenium(II) and osmium(II) 2,2‘-bipyridyl complexes that contain an ethynyl-bridged ditopic ligand. In particular, the electrochemical properties are indicative of electron delocalization over an extended π*-orbital in the π-radical anions. The site of attachment of the ethynyl substituent to the 2,2‘-bipyridyl ring affects the various properties, especially absorption and emission spectral maxima. In most cases, the rates of nonradiative deactivation of the lowest-energy triplet excited states are slower than expected for a corresponding complex not possessing a conjugated substituent. This effect is rationalized in terms of electron delocalization over part of the ditopic ligand within the triplet state and its significance depends markedly on the triplet energy of the complex in question. The lowest-energy triplet mixes to some extent with an upper-lying triplet that is more strongly coupled to the ground state. According to the nature of the metal complex, this higher-energy triplet might originate from (i) charge transfer from metal center to parent ligand, (ii) a π,π* state localized on the ditopic ligand, or (iii) a metal-centered excited state. For the OsII complexes at 77 K electron delocalization over an extended π*-orbital is accompanied by a reduction in the amount of nuclear displacement between triplet and ground states and by a smaller vibronic coupling matrix element relative to the parent complex. These two factors combine, within the framework of the energy-gap law, to decrease the rate at which electronic energy can be dissipated among medium-frequency vibrational (i.e., −CC− and −CN−) modes. This realization permits a quantitative explanation of the measured rate constants for nonradiative decay of the triplet excited states of these ethynyl-substituted metal complexes.

Switchable photoreduction pathways of antimony(V) tetraphenylporphyrin. A potential multielectron transfer photosensitizer

Photoreduction of [SbV(tpp)(OH)2]+(tpp = dianion of 5,10,15,20-tetraphenylporphyrin) yields either the metastable π-radical anion [SbV(tpp˙–)(OH)2] or the two-electron reduced[SbIII(tpp)]+ complex; product formation can be channelled in the desired direction by an appropriate choice of the reaction medium and dioxygen regenerates the starting compound.

Reduction and Alkylation of Rhodium Porphyrins in Alcohol Solutions. Radiation Chemical and Photochemical Studies

Radiolytic reduction of chlororhodium(III) tetramesitylporphyrin (ClRhIIIP) in alcohol solutions forms a transient RhIIP, which reacts to yield different products under different conditions. In alkaline 2-propanol the product is RhIP-, in weakly acidic conditions HRhIIIP is formed, and under strongly acidic conditions the main products of radiolysis are the alkylated rhodium complexes, R−RhIIIP. The latter products are formed by reaction of RhIIP with alkyl radicals (R•) that are produced in the irradiated solvent (R• = •CH3 and (CH3)2C•OH in 2-propanol). UV photolysis of ClRhIIIP in acetone/2-propanol solutions led to formation of HO(CH3)2C−RhIIIP. One-electron reduction of CH3−RhIIIP occurs at the porphyrin ligand to produce a transient π-radical anion, CH3−RhIIIP•-. In alkaline solution, this transient eliminates •CH3 to form the stable RhIP-, but in neutral or acidic solutions, it undergoes disproportionation, promoted by protonation on the macrocycle, to form CH3−RhIII−chlorin and then CH3−RhIII−isob...

1H NMR spectra and electronic structure of reduced iron porphyrins: Fe(II), Fe(I) and Fe(0) porphyrins

Fe(II), Fe(I) and Fe(0) porphyrins have been generated by stepwise reduction with a sodium mirror in vacuum and their 1H NMR spectra have been recorded and analyzed. Fe(II) porphyrins (Fe(II)P) have been examined in three spin states, an S = 0 state in pyridine- d 8, an S = 1 state in benzene- d 6 and an S = 2 state in tetrahydrofuran- d 8 (THF- d 8). The analysis of isotropic shifts for low-spin Fe(II)P ( S = 0) has indicated that no charge transfer has been observed. The ground state configuration is (d xy) 2 (d xz,d yx) 4. The contact shifts for intermediate-spin Fe(II)P ( S = 1) reflect P → Fe π charge transfer. The proposed electron configuration is (d xy) 2 (d z2) 2 (d xz, d yz) 2, which agrees with Mössbauer data. The pattern of contact shifts for high-spin Fe(II)P ( S = 2) is consistent with σ spin transfer, which suggests that the d x2-y2 orbital possesses an unpaired spin. The electron configuration is (d xy) 2 (d xz, d yz 2 (d z2) 1 (d x2-y2) 1. Our results for Fe(II)P ( S = 1,2) agree with the literature 1H NMR data . In the case of Fe(I)P in THF, the separation of isotropic shifts into the dipolar and contact contributions has shown the dominance of the latter. The observed shifts indicate negative π spin density on pyrrole and meso carbon atoms of the ligand, which seems to be due to a strong π-π spin polarization effect. When this fact is taken into account the pattern of contact shifts is consistent with π spin transmission involving both P → Fe π charge transfer out of the ligand-filled molecular (3e(π)) orbital and Fe → P π * charge transfer into the ligand highest unoccupied (4e(π *)) molecular orbital. The occurrence of the unpaired spin in this molecular orbital is consistent with π-radical anion formulation which was found by X-ray crystallography. An S = 1/2 spin state determined by magnetic moment measurements agrees with the most probable electron configuration (d xy) 2 (d xz, d yz 3 (d z2) 2. In the case of Fe(0)P, the isotropic shifts were found to be small, providing evidence of some spin transfer. The ground state configuration is (d xy) 2 (d xz, d yz) 4 (d z2) 2.

“Supersilyl” Compounds (R3Si)3SiSi(SiR3)3 and (R3Si)3SiC6H4Si(SiR3)3: Structures and Properties

Remarkable properties characterize organosilicon compounds like 1 and 2: some of their structures are distorted by steric overcrowding and their first ionization energies drop as a result of additional radical cation stabilization to unexpectedly low values. In the π-radical anion of 2 considerable spin delocalization in the SiSi3 frameworks is detected by ESR/ENDOR spectroscopy.

Synthesis and electrochemical characterization of ruthenium porphyrins containing a bound PF3 axial ligand

The first synthesis and electrochemistry of metalloporphyrins containing a bound PF3 axial ligand has been achieved. The investigated compounds are [Ru(por)(PF3)] where por is the dianion of 5,10,15,20-tetraphenyl-, -tetra(p-bromophenyl)-, -tetra(p-methoxyphenyl)-, 2,7,12,17-tetraethyl-3,8,13,18-tetramethyl- or 2,3,7,8,12,13,17,18-octaethyl-porphyrin. Each [Ru(por)(PF3)] species was investigated with respect to its spectroscopic and electrochemical properties and the resulting data compared with those for [Ru(por)(CO)] having the same porphyrin ring. A number of similarities exist between the carbonyl and PF3 derivatives in methylene chloride but major differences can be observed in other non-aqueous solutions. The first reduction of each complex is reversible in tetrahydrofuran (thf) and leads to a porphyrin π-anion radical rather than a ruthenium(I) species as identified by UV/VIS spectroelectrochemistry. Each investigated complex also undergoes two reversible oxidations in dichloromethane, the first of which leads to a porphyrin π-cation radical. The [Ru(por)(PF3)] derivatives appear to be more stable than the [Ru(por)(CO)] analogues in thf or CH2Cl2, but an electrochemically initiated conversion of [RuII(por)(PF3)(py)] into [RuIII(por)(py)2]+ can be readily accomplished in pyridine (py) or CH2Cl2–pyridine mixtures. This type of reaction has never been seen upon oxidation of a ruthenium(II) porphyrin and was monitored by cyclic voltammetry and UV/VIS spectroelectrochemistry.

Steric and inductive effects on the basicity of porphyrins and on the site of protonation of porphyrin dianions: radiolytic reduction of porphyrins and metalloporphyrins to chlorins or phlorins

A series of sulfonated, water-soluble, phenyl-substituted porphyrins has been prepared, containing halogen or alkyl groups in the ortho, di-ortho or para positions. While the para-substituted compounds exhibited monomer–dimer behaviour, all the ortho and di-ortho substituted porphyrins were monomeric in aqueous solution at 0.1 mol dm–3 ionic strength. The proton basicities varied over a 105 range along the series, from the strongly basic tetra(4-methoxyphenyl) species to the weakly basic and less deformable sulfonated tetrakis(2,6-dichlorophenyl) porphyrin. Certain of these porphyrins and related metalloporphyrins were reduced by radiolytic methods in aqueous solutions. Pulse radiolysis studies provided the spectra of the short-lived π-radical anions and γ-radiolysis led to formation of stable chlorins or phlorins, the products of two-electron reduction and protonation at the β-pyrrole or at the meso position, respectively. Whereas H2TPPS4[tetrakis(4-sulfonatophenyl)porphyrin] yields phlorin at all pH values, ZnII-, AlIII-, lnIII- and SnIV-TPPS4 form phlorins at high pH but mostly chlorins at lower pH. The ratio of phlorin to chlorin production is enhanced by increased pH and by increased metal electronegativity. 2,6-Disubstitution at the phenyl rings diminishes the likelihood of phlorin formation while N-methyl substitution at one of the central nitrogens of ZnTPPS4 enhances phlorin formation. These and other results indicate that electron withdrawal from the porphyrin π-system enhances the ratio of phlorin/chlorin production. On the other hand, steric crowding around the meso position retards protonation at this site by preventing the geometric reorientation necessary for phlorin formation.

Mössbauer, magnetic susceptibility, radiolytic and photochemical studies of europium and lutetium porphyrins

The chemistry of the lanthanide complexes of europium and lutetium with several porphyrin molecules were probed in both the solid state and in solution. Mössbauer and variable temperature magnetic susceptibility studies indicated that the trivalent state of europium was formed under the synthetic conditions. Electron addition to both europium(III) and lutetium(III) porphyrins by photochemical and radiolytic techniques resulted in reduction of the porphyrin ring rather than the coordinated trivalent metal ion, and the resulting π-radical anion disproportionated into the initial metalloporphyrin and the ring reduced metalloporphyrin di-anion. The reduction potentials in solution for mono- and di-anion formation of the lutetium porphyrin were measured by electrochemical procedures. While the lutetium(III) porphyrin is stable for years at pH 7, the metal is rapidly removed from the metalloporphyrin by protons in the di-anion state. The photophysics of the lutetium(III) porphyrin in solution are dominated by the triplet state, and the reactivity of this triplet with dioxygen, NADH and persulfate is reported.

Reduction reactions of water soluble cyano-cobalt(III)-porphyrins: Metal versus ligand centered processes

Reduction reactions of dicyano-cobalt(III)-porphyrins [potential in vivo cyanide scavenger drugs] were studied by radiolytic and electrochemical methods using the water soluble tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and tetrakis(N-methyl-4-pyridyl)porphyrin (TMPyP). For [(CN) 2Co IIITPPS] −, reduction occurs stepwise to the Co II, Co I, and finally to the phlorin anion. This behavior is similar to that of the cobalt porphyrins in the absence of cyanide, except that the cyanide ligand shifts the reduction potentials to much more negative values. On the other hand, under radiolytic conditions, [(CN) 2Co IIITMPyP] − is reduced on the porphyrin macrocycle by one electron to give the Co III π-radical anion, which disproportionates into the initial complex and the two-electron ring reduced Co III phlorin. The radical anion is also formed by intramolecular electron transfer subsequent to the reaction of Co IITMPyP and cyanide. The results are compared with the chemistry of Vitamin B-12.

Depression of Disproportionation of π-Radical Anion of Gold Porphyrin Electrostatically Fixed in a Polymer Matrix

Abstract The electrochemical and photochemical reduction of the fully methylated derivative of gold meso-tetrakis-(4-pyridyl)porphine (AuTMPyP) in homogeneous solution gives not only a π-radical anion but also its successive product, phlorin, by disproportionation. The electrostatic fixation of AuTMPyP in a Nafion matrix inhibits the latter undesired reaction against effective charge separation, which is explained by the diffusion constant lower by 8 order of magnitude during electrolytic reduction of AuTMPyP compared to that in homogeneous aqueous solution.

Electronic reduction of haloaromatic compounds. A theoretical study

A series of haloaromatic molecules with a wide range of standard reduction potentials (E°) and bond-cleavage rate constants (k) has been studied using the MNDO semiempirical procedure. It has been found that the standard potential correlates better with the difference in energy between the radical anion and the neutral geometry when the geometry of the former is kept unchanged, in agreement with the Franck–Condon principle. A reaction co-ordinate study of the bond cleavage of the radical anion for the chloro derivatives shows the existence of two radical anions (π and σ) that differ in the orbital occupied by the unpaired electron. Whereas the π-radical anion is the most stable, the bond-breaking process takes place through the σ-structure. The study of the energies as well as of different molecular parameters related to the C–Cl bond cleavage allows us to explain qualitatively the experimental ordering of rate constants when different substituent groups are considered. However, the theoretical predictions do not account for the kinetic differences when only small structural changes are present. The significance of the solvent effect in these cases is pointed out.

Photochemistry of metalloporphyrins in aqueous solutions

The fluorescence spectra of Ga(III), In(III), Ge(IV) and Sn(IV) tetramethyl pyridine porphyrin showed a small stokes shift ca 400 ± 100 cm −1 attributed to the similarity between their ground and excited states. The one and two electron reduction products of the metalloporphyrins were studied by steady-state method. The nature of the product was dependent upon the pH of the reaction medium. Chlorin and phlorin were obtained at low and high pH's, respectively. Sn TMPyP was the only metalloporphyrin which yielded the Π-radical anion whose stability is due to the high electronegativity of Sn(IV).