Generation Of Radical Ions Research Articles

On the preparation and spectroelectrochemical characterization of certain 2,5‐bis(het)aryl substituted thiophenes

AbstractIn this work, a series of novel 2,5‐bis(het)aryl and 2,5‐bis‐thienyl substituted thiophenes have been synthesized and characterized by ultraviolet‐visible‐near infrared (UV‐Vis‐NIR) absorption and fluorescence spectroscopy as well as cyclic voltammetry. From the electron paramagnetic resonance (EPR)/UV‐Vis‐NIR spectroelectrochemical data, information about the optical and magnetic properties of the charged species of these compounds have been provided. The spin distributions in the electrochemically generated radical ions were estimated experimentally and compared with theoretical data.

Symmetry-Breaking Charge Transfer in Metal-Organic Frameworks.

High quantum-yield charge carrier generation from the initially prepared excitons defines a key step in the light-harvesting and conversion scheme. Photoinduced charge transfer in molecular electron donor-acceptor assemblies is driven by a sizable ΔG0, which compromises the potential of the generated carriers. Reminiscent of the special pair at the reaction center of the natural light-harvesting complex, symmetry-breaking charge transfer (SBCT) within a pair of identical struts of metal-organic framework (MOF) will facilitate the efficient generation of long-lived charge carriers with maximized potentials without incorporating any foreign redox species. We report SBCT in pyrene-based zirconium metal-organic framework (MOF) NU-1000 that leads to efficient generation of radical ions in a polar solvent and bound CT states in a low-polar solvent. The probe unveils the role of the low-lying non-Franck-Condon excitonic states as intermediates in the formation of the SBCT state from the initially prepared Franck-Condon S1 states. Ultrafast and transient spectroscopy─probed over 200 fs-30 μs time scale─evinces a kSBCT = (110 ps)-1 in polar media (εs = 37.5) forming solvated radical ions with recombination rate kCR = (∼45 ns)-1. A slower rate with kSBCT = (203 ps)-1 was recorded in low-polar (εs = 7.0) solvent manifesting a bound [TBAPy•+ TBAPy•-] state with kCR ≈ (17 μs)-1. This discovery, along with other unique photophysical features relevant to light harvesting, should define a MOF-based platform for developing heterogeneous artificial photon energy conversion systems.

EPR of Type I photosynthetic reaction centers.

Light-induced reactions in photosynthetic reaction centers are initiated by the absorption of a photon, which results in the transfer of a single electron and the generation of radical ions in the donor and acceptor molecules involved in the charge-separated state. Electron paramagnetic resonance (EPR) spectroscopy is the ideal method for the study of such reactions. In addition to measuring spectra of the electron transfer cofactors in continuous light, reactions can be initiated by brief flashes of light, thereby allowing the kinetics of forward electron transfer as well as recombination reactions to be obtained. Because the donor and acceptor pairs are so closely spaced and because light induced charge separation is so rapid, the donor and early acceptors are in a quantum mechanically spin entangled state, which confers properties such as increased sensitivity, the ability to measure reactions on the nanosecond timescale, and the determination of bond angles between cofactors. Additionally, distances between pairs of cofactors can be measured by detecting the modulation of a phase shifted "out-of-phase" electron spin echo signal. In this methods article, we will describe how continuous wave EPR, time resolved EPR, and pulsed EPR can be used to measure these properties in Type I photosynthetic reaction centers. Methods of analysis are described for the bound electron transfer cofactors in the heterodimeric Photosystem I reaction center of plants and cyanobacteria and in the homodimeric reaction centers found in phototrophic members of the phyla Bacillota, Chlorobiota, and Acidobacteriota.

Laser flash photolytic generation of radical ions of carotenoids in organic solvents. Studies of their subsequent fates, including formation of stable carotenoid sigma dimer radical anion (CAR)2[rad

In this study, time-resolved laser flash photolysis technique (LFP) was used to generate and study extraordinary stable transients formed following the decay of radical anions of canthaxanthin (CAN) and astaxanthin (ASTA). The radical anions were obtained via one-electron reduction of CAN and ASTA by acetone ketyl radical (AC−) in various solvents. At room temperature, the decay of CAN− (λmax = 870 nm) and ASTA− (λmax = 1000 nm), in acetonitrile (ACN), led to the formation of indefinitely stable colored transients (blue-green for CAN and navy blue for ASTA) absorbing at shorter wavelengths (λmax = 690 nm and 720 nm, respectively) relative to their corresponding radical anions. Rapid quenching by oxygen was observed indicating the radical nature of these transients, which was further confirmed by EPR spectroscopy. By employing EPR spectroscopy, LFP technique, and UV–visible absorption measurements, these extraordinary stable transients were identified as sigma-dimer radical anions ((CAR)2−) formed via the dimerization of the parent carotenoid (CAR) with its radical anion (CAR−). The suggested structures of these dimers were postulated using DFT calculations, which indicated that the unpaired electron is delocalized over the entire molecule. This result is extremely important because of the paucity of reports on dimer radical anions in solution.

Luminescence properties and ROS generation of magnetic porous silicon nanoparticles

Magnetite‒Porous Silicon 100−150 nm size nanoparticles (MPSi) were obtained combining luminescent and magnetic properties from silicon and magnetite, respectively.MPSi hybrids were characterized by high-resolution transmission electron microscopy, atomic and magnetic force microscopy and X-ray photoelectron spectroscopy.The presence of magnetite quenches statically visible luminescence of Porous Silicon toluene suspensions. Whereas MPSi, maintain the luminescence in the 300−450 nm spectral region.Particles retained the capacity for singlet oxygen and superoxide radical ion generation (Reactive Oxygen Species, ROS). However quantum yield singlet oxygen generation is much lower than the PSi analogues and superoxide radical ion concentration dismiss when magnetite is incorporated in the PSi matrix.Silanization of Porous Silicon and MPSi yield nanoparticles with − SH terminal groups with unique luminescence properties.

Frontispiece: Appending Diverse π-Extended Acceptors with Tetrathiafulvalene/Dithiafulvalene Donors: Multistate Redox Properties, Radical Ion Generation, and Mid-IR-Absorbing Mixed-Valence States

Frontispiece: Appending Diverse π-Extended Acceptors with Tetrathiafulvalene/Dithiafulvalene Donors: Multistate Redox Properties, Radical Ion Generation, and Mid-IR-Absorbing Mixed-Valence States

Appending Diverse π-Extended Acceptors with Tetrathiafulvalene/Dithiafulvalene Donors: Multistate Redox Properties, Radical Ion Generation, and Mid-IR-Absorbing Mixed-Valence States.

Three classes of donor-acceptor (D-A) π-extended chromophores (1-12) were synthesized through a phosphite-mediated cross-coupling reaction, in which the anhydride- or imide-based π-As and number of tetrathiafulvalene (TTF)/dithiafulvalene (DTF) Ds were systematically changed. Large π rings, such as benzoperylene and coronene, were integrated into the TTF/DTF unit, for the first time, to overcome their high insolubility. The anhydride and imide groups in the π acceptors can significantly alter the frontier orbitals and influence the optoelectronic properties. The D moieties allow the formation of radical cations (D.+ ) and the π-extended A moieties aid the formation of radical anions (A.- ) by oxidation/reduction under ambient conditions. The molecules revealed UV/Vis/near-IR absorption, fluorescence extending into the near-IR region, and amphoteric electrochemical properties. Chromophores 10 and 12 show solvatochromism in a wide range of solvents. The π-As with anhydride functionality allow easier electron uptake, relative to the imide groups, whereas the increasing number of D TTF/DTF units make them easy to oxidize. Interestingly, the trans-TTF-fused molecules (1, 6, and 11) exhibited a mixed-valence state in the mid-IR region (ν˜ =5130-4000 cm-1 ). Moderate electron coupling between the redox centers is inferred to the compounds being of Robin-Day class II. The multistate redox activity along with panchromism and near-/mid-IR optical absorption of these systems can be attractive towards advanced switchable materials.

Low-Dimensional Carbon Allotropes: Ground- and Excited-State Charge Transfer with NIR-Absorbing Heptamethine Cyanine

Summary At the focal point of our studies are mutual interactions between an anionic heptamethine cyanine and individualized single-walled carbon nanotubes (SWCNTs) or exfoliated nanographene (NG). Here, we report the distinct near-infrared absorption of the heptamethine cyanine, which assists in visualizing the electronic interactions in the ground state with ease and precision. In statistical Raman assays, we conclude from downshifted 2D- and G-modes for SWCNTs, as well as upshifted 2D- and downshifted G-modes for NG, that these electronic interactions result in stable n-doping of SWCNTs and NG. Key to this shift of charge density is the electron-donating character of the heptamethine cyanine. In the excited state, a complete but metastable transfer of charges is accompanied by a radical-ion-pair state that lives for several nanoseconds. Such a time domain has not yet been realized, for example, in NG to date.

Single-layer graphene as a stable and transparent electrode for nonaqueous radical annihilation electrogenerated chemiluminescence.

We explored the use of single-layer graphene (SLG) obtained by chemical vapor deposition, and transferred to a glass substrate, as a transparent electrode material for use in coupled electrochemical and spectroscopic experiments in nonaqueous media through electrogenerated chemiluminescence (ECL). SLG was used with classical ECL luminophores, rubrene and 9,10-diphenylanthracene, in an inert environment to generate stable electrochemical responses and measure light emission through it. As an electrode material, SLG displayed excellent stability during electrochemical potential stepping and voltammetry in a window that spanned at least from ca. -2.4 to +1.8 V versus SCE in acetonitrile and acetonitrile/benzene. Although the peak splitting between forward and reverse sweeps in voltammetry was larger in comparison to metal electrodes due to in-plane resistance, SLG displayed sufficiently facile electron transfer properties to yield stable voltammetric cycling and ECL. SLG electrodes patterned with poly tetrafluoroethylene permitted the stable generation of radical ions on an SLG microelectrode to be studied through scanning electrochemical microscopy in the generation/collection mode. SLG was able to stably collect radical ions produced by a 50 μm gold tip with up to 96% collection efficiency. The transparency of graphene was used to obtain accurate spectral responses in ECL. While inner filter effects are known to cause a shift in peak emission wavelength of spectroelectrochemical studies, the use of SLG electrodes with detection through the graphene window reduced apparent peak shifts by up to 10 nm in peak wavelength. This work introduces SLG as a virtually transparent, electrochemically active, and chemically stable platform for studying ECL in the radical annihilation mode, where large electrode polarizations could compromise the chemical stability of other existing transparent electrodes.

Comparison of the Photocatalytic and Photoelectrocatalytic Methyl Orange Color Removal over TiO(2-x-y)NxFy

Photocatalysis and photoelectrocatalysis are environmentally friendly methodologies for remediation pollutants present in waste waters, these methodologies employ semiconductor materials capable to generate electron hole pairs when they are illuminated [1]. Photogenerated charge carriers together with oxidizing transient species, mainly the hydroxyl radical (OH*), can produce deep changes in the chemical structure of the contaminants [2]. One of the most commonly used semiconductor for these purposes is the TiO2, however it is only active under UV light, therefore, many approaches have been conducted towards TiO2 modification with different metallic and nonmetallic elements in order to reduce the gap between the bands of the oxide, allowing to use visible light for this process [3,4]. Photocatalytic (PC) and photoelectrocatalytic (PEC) methyl orange (MO) color removal may involve two mechanisms simultaneously [5]. The first path is the oxidative radicals’ formation such OH* which carry the pollutant oxidation in the solution [6], and the second way refers to the direct interaction of photogenerated holes with the dye adsorbed on the semiconductor surface [7,8]. The dominant mechanism will depend on the kind of processes applied (PC or PEC), and the modification of the TiO2 that can induce defects such oxygen vacancies in the oxide lattice, or alter the surface properties of the oxide [9].TiO(2-x-y)-Nx-Fy powders and thin films were prepared by combining sol gel method and hydrothermal treatment. The obtained materials were characterized by X-ray diffraction (XRD), Raman and infrared (FTIR) spectroscopies, diffuse reflectance (UV-VIS) spectrophotometry and Zeta potential measurements. Moreover, the supported films on AISI 304 stainless steel foils were characterized by scanning electron microscopy (SEM) and contact angle measurements. Photocatalytic and photoelectrocatalytic test were carried in a 5 ppm MO solution at a pH of 6.8.The results showed that the PC Methyl orange degradation rate over fluoride modified photocatalysts (TiO2-yFy and TiO(2-x-y)NxFy) was comparatively lower than over pristine TiO2 and TiO2-xNx. However, the trend reversed when the MO degradation was carried in a photoelectrocatalytic cell (see Fig. 1). The material characterization showed that fluoride modification alter not only the crystallinity (XRD) and structure (UV-Vis and Raman) of the TiO2, but also its surface properties (Isoelectric point and wettability), favoring the generation of radical ions in the solution. When the degradation is carried in suspension (photocatalysis) there is a high surface area available for the oxidation process favoring the direct charge transfer between the dye and the semiconductor. On the other hand, when the process is carried over a thin film (photoelectrocatalysis) there is a low contact area causing limitation in the interfacial charge transfer between the pollutant and the oxide, then the generation of radical species that travel to the bulk of the solution to carried the MO discoloration, gain relevance in the process [10]. References I. Sirés, E. Brillas, Environment International 40, 212, 2012. Yu, J. G.; Yu, J. C.; Cheng, B.; Hark, S. K.; Lu, K. J. Solid State Chem., 2003, 174: 372. V. I. Shapovalov. Glass Physics and Chemistry, 2010 , Vol. 36, No. 2, pp. 121–157. D. Wu, M. Long, W. Cai, Ch. Chen, Y. Wu, J. Alloy Compd. 502, 289, 2010. R. Asahi, T. Morikawa, K. Ohwaki, Aoki, Y. Taga, Science, 293, 269, 2001. C. Minero; G. Mariella; V. Maurino; D. Vione; and E. Pelizzetti. Langmuir 2000, 16, 8964-8972. J. Zhu; Z. Deng, F; Chen, J; Zhang, H. Chen; M. Anpo; J. Huang; L. Zhang. Appl. Catal. B: Environ. 62, 2006, 329–335. L. Gomathi Devi; R. Kavitha. Electrochimica Acta 129, 2014, 137–141. A.E. Giannakas; E. Seristatidou; Y. Deligiannakis; I. Konstantinou. Appl. Cat. B: Environ. 132– 133, 2013, 460– 468. E. Peralta; G. Roa; J.A. Hernandez-Servin; R. Romero; P. Baldera; R. Natividad. Electrochimica Acta 129, 2014, 137–141.

Optical nonlinearity and electric conductivity origin study on sucrose crystal by using IR, Raman, INS, NMR, and EPR spectroscopies

The supposed importance of hydrogen bonds toward the origin of second harmonic generation (SHG) and electric conductivity in crystalline sucrose was investigated by IR (4000–10cm−1), INS (2000–10cm−1, at 35K), polarized Raman (3600–50cm−1) spectra, and 1H NMR second moment line records in the temperature range 450–80K. The temperature dependence of NIR (7000–5500cm−1) polarized spectra gave information about –CH2 motions complementary to NMR results concerning –CH2OH group rearrangements. The EPR spectra were applied to study the generation of radical ions by exposure to NIR radiation. Density functional theory quantum chemical calculations were performed to reproduce the vibrational spectra in order to complete as far as possible the assignments of bands observed by us and in the literature in sucrose crystals, and to throw more light on the possible reasons of sucrose electric conductivity and optical nonlinearity by the knowledge of theoretical values of dipole moments, polarizabilities, first order hyperpolarizabilities of sucrose molecule and clusters as well as ionization energy and electron affinity. The proton transfer in one specific hydrogen bond parallel to the helical axis b is proposed to be the most important in SHG and conductivity origin.

Radical-driven peptide backbone dissociation tandem mass spectrometry.

In recent years, a number of novel tandem mass spectrometry approaches utilizing radical-driven peptide gas-phase fragmentation chemistry have been developed. These approaches show a peptide fragmentation pattern quite different from that of collision-induced dissociation (CID). The peptide fragmentation features of these approaches share some in common with electron capture dissociation (ECD) or electron transfer dissociation (ETD) without the use of sophisticated equipment such as a Fourier-transform mass spectrometer. For example, Siu and coworkers showed that CID of transition metal (ligand)-peptide ternary complexes led to the formation of peptide radical ions through dissociative electron transfer (Chu et al., 2000. J Phys Chem B 104:3393-3397). The subsequent collisional activation of the generated radical ions resulted in a number of characteristic product ions, including a, c, x, z-type fragments and notable side-chain losses. Another example is the free radical initiated peptide sequencing (FRIPS) approach, in which Porter et al. and Beauchamp et al. independently introduced a free radical initiator to the primary amine group of the lysine side chain or N-terminus of peptides (Masterson et al., 2004. J Am Chem Soc 126:720-721; Hodyss et al., 2005 J Am Chem Soc 127: 12436-12437). Photodetachment of gaseous multiply charged peptide anions (Joly et al., 2008. J Am Chem Soc 130:13832-13833) and UV photodissociation of photolabile radical precursors including a C-I bond (Ly & Julian, 2008. J Am Chem Soc 130:351-358; Ly & Julian, 2009. J Am Soc Mass Spectrom 20:1148-1158) also provide another route to generate radical ions. In this review, we provide a brief summary of recent results obtained through the radical-driven peptide backbone dissociation tandem mass spectrometry approach.

Donor-Linked Di(perylene bisimide)s: Arrays Exhibiting Fast Electron Transfer for Photosynthesis Mimics

The first example of donor-linked di(perylene bisimide)s is reported. UV-vis absorption spectra of these newly synthesized dyads showed intense absorption across the entire visible region, demonstrating their excellent light-harvesting activities. The severe fluorescence quenching event probed by steady-state fluorescence spectroscopy and the free-energy calculations suggested the possibility of electron transfer (ET) in these arrays upon photoexcitation. Further femtosecond transient absorption spectra clarified that the fluorescence quenching was due to fast intramolecular ET. The rate of the charge separation (CS) was found to be as high as 10(12) s(-1) in CH(2)Cl(2). It was suggested that the large ET driving forces, strong donor-acceptor electronic coupling, and relatively small reorganization energy of diPBI accounted for the rapid ET process in a synergic manner. The fate of the generated radical ion pair depended on the solvent used. Rapid charge recombination to ground state occurred for the dyads in polar CH(2)Cl(2) and for diPBI-TPA in nonpolar toluene. However, sufficient (3)diPBI* population was attained via efficient spin-orbit coupled intersystem crossing from the charge-separated state for diPBI-PdTPP in toluene. These photophysical properties are interpreted as the cooperation between thermodynamic feasibility and kinetic manipulation.

Scanning Electrochemical Microscopy Study of Ion Annihilation Electrogenerated Chemiluminescence of Rubrene and [Ru(bpy)3]2+

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.

Control over Charge Transfer through Molecular Wires by Temperature and Chemical Structure Modifications

A series of electron donor-acceptor arrays containing π-conjugated oligofluorenes (oFL) of variable length between a zinc porphyrin (ZnP) as electron donor and fullerene (C(60)) as electron acceptor have been prepared by following a convergent synthesis. The electronic interactions between the electroactive species were determined by cyclic voltammetry, UV-visible, fluorescence, and femto/nanosecond transient absorption spectroscopy. Our studies clearly confirm that, although the C(60) units are connected to the ZnP donor through π-conjugated oFL frameworks, no significant electronic interactions prevail in the ground state. Theoretical calculations predict that a long-range electron transfer occurs primarily due to a maximized π-conjugated pathway from the donor to the acceptor. Photoexcitation of ZnP-oFL(n)-C(60) results in transient absorption maxima at 715 and 1010 nm, which are unambiguously attributed to the photolytically generated radical ion pair state, [ZnP(•+)-oFL(n)-C(60)(•-)], with lifetimes in the microsecond time regime. Temperature-dependent photophysical experiments have shown that the charge-transfer mechanism is controllable by temperature. Both charge separation and charge recombination processes give rise to a molecular wire behavior of the oFL moiety with an attenuation factor (β) of 0.097 Å(-1). The correlation β to the connection pattern between the ZnP donor and the oFL linker revealed that even small alterations of the linker π-electron system break the homogeneous π-conjugation pattern, leading to higher values of β.

Photochemical Charge Separation in Supramolecular Phthalocyanine−Multifullerene Conjugates Assembled by Crown Ether-Alkyl Ammonium Cation Interactions

Self-assembled phthalocyanine-multifullerene donor-acceptor conjugates have been formed by crown ether-ammonium cation dipole-ion binding strategy to probe the photochemical charge separation. To achieve this, phthalocyanine is functionalized to possess four 18-crown-6 moieties on the macrocycle periphery, whereas fullerene is functionalized to possess an alkyl ammonium cation of short and long chain lengths. Stable donor-acceptor conjugates accommodating multifullerene entities have been obtained by the crown ether-ammonium cation inclusion complexation. From the efficient fluorescence quenching of the zinc phthalocyanine by the bound fullerene entities, the rate constants of charge separation are evaluated to be slightly larger for closely held via shorter alkyl chain length fullerene, which are also larger compared to the earlier reported analogous zinc porphyrin-multifullerene conjugate. Nanosecond transient absorption studies yielded spectral signatures corresponding to both the phthalocyanine radical cation and fullerene radical anion at the same time, providing evidence of light-induced electron transfer within the conjugates. The evaluated lifetimes of the radical ion pairs in the present phthalocyanine-fullerene conjugates are found to be hundreds of nanoseconds and are much longer compared to the earlier reported conjugate of zinc porphyrin analogue, revealing higher possible usage of the generated radical ion pairs.

Investigation of the catalytic activity for ozonation on the surface of NiO nanoparticles

In our density functional theory calculations, we illustrated the atomistic details of NiO surface and presented the mechanism of ozone decomposition, peroxide radical ion generation, and hydroxyl radical generation on the surfaces. And the electronic properties were further investigated to discuss the electronic structure and adsorption characteristics of the catalytic ozonation systems. Furthermore, the hydroxyl radicals were truly identified by spin-trapping/electron paramagnetic resonance (EPR) technique. Results suggest that the presence of NiO nanoparticles promote the decomposition of ozone, the generation of superoxide ions, and the generation of hydroxyl groups favoring the chain reaction for the generation of hydroxyl radicals, promoting the degradation of organic pollutants.

Trans-2 Addition Pattern to Power Charge Transfer in Dendronized Metalloporphyrin C60 Conjugates

Coordinating different transition metals--manganese(III), iron(III), nickel(II), and copper(II)--by a dendronized porphyrin afforded a new family of redox-active metalloporphyrins to which C(60) was attached as a ground-state electron acceptor. Such a strategy introduced an additional center of redoxactivity, that is, a change of the oxidation state of the metal. Cyclic voltammetry and absorption/fluorescence measurements provided support for mutual interactions between the redox-active constituents in the ground state. In particular, slightly anodic shifted reduction potentials/cathodic shifted oxidation potentials and the occurrence of new charge transfer features in the 700-900 nm range prompt to sizable electronic coupling in the range of 300 cm(-1). Photophysical means--steady-state/time-resolved fluorescence and transient absorption measurements--shed light on the excited-state interactions. To this end, we have added pulse radiolytic investigations to characterize the radical cation (i.e., metalloporphyrins) and radical anion (i.e., fullerene) characteristics. Pi-pi stacking of the excited state electron donor and the electron acceptor is key to overcome the intrinsically fast deactivation of the excited states in these metalloporphyrins and to power an exothermic charge transfer. The lifetimes of the rapidly and efficiently generated radical ion pair states, which range from 15 to >3000 ps, revealed several important trends. First, they were found to depend on the solvent polarity. Second, the nature of the transition metal plays a similarly decisive role. It is important that the product of charge recombination, namely tripmultiplet excited states versus ground state, had a great impact. Finally, a correlation between the charge transfer rate (i.e., charge separation and charge recombination) and the free energy change for the underlying reaction reveals a parabolic dependence with parameters of the reorganization energy (0.84 eV) and electronic coupling (70 cm(-1)) closely resembling that seen for the zinc(II) and free base analogues.

Spectroscopic Characterization of Photolytically Generated Radical Ion Pairs in Single-Wall Carbon Nanotubes Bearing Surface-Immobilized Tetrathiafulvalenes

We succeeded in establishing for the first time a conclusive spectroscopic signature for reduced single-wall carbon nanotubes (SWNT), which evolves from electron donor-acceptor interactions between SWNT and electron-donating pi-extended tetrathiafulvalene (exTTF). In particular, pi-pi interactions were employed to anchor the electron donor to the surface of SWNT. New conduction band electrons, injected from photoexcited exTTF, shift the transitions that are associated with the van Hove singularities to lower energies.

Radical Ion Generation Processes of Organic Compounds in Electrospray Ionization Mass Spectrometry

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.