High Pressure ESR Studies of Electron Self-Exchange Reactions of Organic Radicals in Solution

Simple electron self-exchange reactions are often used to study the role of the reaction medium on a chemical process, commonly implying the use of various solvents with different physical properties. In principle, similar studies may be conducted using a single solvent, changing its physical properties by application of elevated pressures, but so far only little information is available on pressure dependent exchange reactions. In this work, we have used a recently constructed high pressure apparatus for use with electron spin resonance (ESR) spectroscopy to investigate simple electron self-exchange reactions involving 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetracyanoethylene (TCNE) and their respective radical anions as well as TMPPD and its radical cation in three different solvents. The self-exchange was observed by ESR line broadening experiments, yielding rate constants and volumes of activation. The experimental results were compared to theoretical calculations based on Marcus theory and taking into account solvent dynamic effects. The use of elevated pressures has enabled the study of solvent effects without commonly encountered problems like solubility issues or chemical reactions between solvent and solute which sometimes limit the range of useable solvents

Effect of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino Acids

The kinetics of the photooxidation of aromatic amino acids histidine (His), tyrosine (Tyr), and tryptophan (Trp) by 3,3′,4,4′-benzophenonetetracarboxylic acid (TCBP) has been investigated in aqueous solutions using time-resolved laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization. The pH dependence of quenching rate constants is measured within a large pH range. The chemical reactivities of free His, Trp, and Tyr and of their acetylated derivatives, <i>N</i>-AcHis, <i>N</i>-AcTyr, and <i>N</i>-AcTrp, toward TCBP triplets are compared to reveal the influence of amino group charge on the oxidation of aromatic amino acids. The bimolecular rate constants of quenching reactions between the triplet-excited TCBP in the fully deprotonated state and tryptophan, histidine, and tyrosine with a positively charged amino group are <i>k</i>_q = 2.2 × 10⁹ M^–1 s^–1 (4.9 < pH < 9.4), <i>k</i>_q = 1.6 × 10⁹ M^–1 s^–1 (6.0 < pH < 9.2), and <i>k</i>_q = 1.5 × 10⁹ M^–1 s^–1 (4.9 < pH < 9.0), respectively. Tryptophan, histidine, and tyrosine with a neutral amino group quench the TCBP triplets with the corresponding rate constants <i>k</i>_q = 8.0 × 10⁸ M^–1 s^–1 (pH > 9.4), <i>k</i>_q = 3.0 × 10⁸ M^–1 s^–1 (pH > 9.2), and <i>k</i>_q = (4.0–10.0) × 10⁸ M^–1 s^–1 (9.0 < pH < 10.1) that are close to those for the N-acetylated derivatives. Thus, it has been established that the presence of charged amino group changes oxidation rates by a significant factor; i.e., His with a positively charged amino group quenches the TCBP triplets 5 times more effectively than <i>N</i>-AcHis and His with a neutral amino group. The efficiency of quenching reaction between the TCBP triplets and Tyr and Trp with a positively charged amino group is about 3 times as high as that of both Tyr and Trp with a neutral amino group, <i>N</i>-AcTyr and <i>N</i>-AcTrp

Exciplexes versus Loose Ion Pairs: How Does the Driving Force Impact the Initial Product Ratio of Photoinduced Charge Separation Reactions?

Many donor–acceptor systems can undergo a photoinduced charge separation reaction, yielding loose ion pairs (LIPs). LIPs can be formed either directly via (distant) electron transfer (ET) or indirectly via the dissociation of an initially formed exciplex or tight ion pair. Establishing the prevalence of one of the reaction pathways is challenging because differentiating initially formed exciplexes from LIPs is difficult due to similar spectroscopic footprints. Hence, no comprehensive reaction model has been established for moderately polar solvents. Here, we employ an approach based on the time-resolved magnetic field effect (MFE) of the delayed exciplex luminescence to distinguish the two reaction channels. We focus on the effects of the driving force of ET and the solvent permittivity. We show that, surprisingly, the exciplex channel is significant even for an exergonic ET system with a free energy of ET of −0.58 eV and for the most polar solutions studied (butyronitrile). Our findings demonstrate that exciplexes play a crucial role even in polar solvents and at moderate driving forces, contrary to what is usually assumed

Electron Spin Relaxation of C₆₀ Monoanion in Liquid Solution: Applicability of Kivelson–Orbach Mechanism

We report the results of our investigation on the electron spin relaxation mechanism of the monoanion of C₆₀ fullerene in liquid solution. The solvent chosen was carbon disulfide, which is rather uncommon in EPR spectroscopy but proved very useful here because of its liquid state over a wide temperature range. The conditions for exclusive formation of the monoanion of C₆₀ in CS₂ were first determined using electrochemical measurements. Using these results, only the monoanion of C₆₀ was prepared by chemical reduction using Hg₂I₂/Hg as the reducing agent. The EPR line width was measured over a wide temperature range of 120–290 K. The line widths show weak dependence on temperature, changing by a factor of only about 2, over this temperature range. We show that the observed temperature dependence does not obey the Kivelson–Orbach mechanism of electron spin relaxation in liquids, applicable for radicals with low-lying, thermally accessible excited electronic states. The observed temperature dependence can be empirically fitted to an Arrhenius type of exponential function, from which an activation energy of 74 ± 3 cm^–1 is obtained. From the qualitative similarities in the characteristics of the spin relaxation rates of C₆₀ monoanion radical and the cyclohexane type of cation radicals reported in the literature, we propose that a pseudorotation-induced electron spin relaxation process could be operating in the C₆₀ monoanion radical in liquid solution. The low activation energy of 74 cm^–1 observed here is consistent with the pseudorotation barrier of C₆₀ monoanion, estimated from reported Jahn–Teller energy levels

Electron Spin–Lattice Relaxation Mechanisms of Nitroxyl Radicals in Ionic Liquids and Conventional Organic Liquids: Temperature Dependence of a Thermally Activated Process

During the past two decades, several studies have established a significant role played by a thermally activated process in the electron spin relaxation of nitroxyl free radicals in liquid solutions. Its role has been used to explain the spin relaxation behavior of these radicals in a wide range of viscosities and microwave frequencies. However, no temperature dependence of this process has been reported. In this work, our main aim was to investigate the temperature dependence of this process in neat solvents. Electron spin–lattice relaxation times of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-hydroxy-TEMPO (TEMPOL), in X-band microwave frequency, were measured by the pulse saturation recovery technique in three room-temperature ionic liquids ([bmim]­[BF4], [emim]­[BF4], and [bmim]­[PF6]), di-isononyl phthalate, and <i>sec</i>-butyl benzene. The ionic liquids provided a wide range of viscosity in a modest range of temperature. An auxiliary aim was to examine whether the dynamics of a probe molecule dissolved in ionic liquids was different from that in conventional molecular liquids, as claimed in several reports on fluorescence dynamics in ionic liquids. This was the reason for the inclusion of di-isononyl phthalate, whose viscosities are similar to that of the ionic liquids in similar temperatures, and <i>sec</i>-butyl benzene. Rotational correlation times of the nitroxyl radicals were determined from the hyperfine dependence of the electron paramagnetic resonance (EPR) line widths. Observation of highly well-resolved proton hyperfine lines, riding over the nitrogen hyperfine lines, in the low viscosity regime in all the solvents, gave more accurate values of the rotational correlation times than the values generally measured in the absence of these hyperfine lines and reported in the literature. The measured rotational correlation times obeyed a modified Stokes–Einstein–Debye relation of temperature dependence in all solvents. By separating the contributions of <i>g</i>-anisotropy, <i>A</i>-anisotropy and spin-rotation interactions from the observed electron spin–lattice relaxation rates, the contribution of the thermally activated process was obtained and compared with its expression for the temperature dependence. Consistent values of various fitted parameters, used in the expression of the thermal process, have been found, and the applicability of the expression of the thermally activated process to describe the temperature dependence in liquid solutions has been vindicated. Moderate solvent dependence of the thermally activated process has also been observed. The rotational correlation times and the spin–lattice relaxation processes of nitroxyls in ionic liquids and in conventional organic liquids are shown to be explicable on a similar footing, requiring no special treatment for ionic liquids

Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems

Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis­(triarylamine)­paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ τ_L ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526)

Rotational and Translational Diffusion of Spin Probes in Room-Temperature Ionic Liquids

We have studied the rotational and translational diffusion of the spin probe 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPOL) in five imidazolium-based room-temperature ionic liquids (RTILs) and glycerol by means of X-band electron paramagnetic resonance (EPR) spectroscopy. Rotational correlation times and rate constants of intermolecular spin exchange have been determined by analysis of the EPR line shape at various temperatures and spin probe concentrations. The model of isotropic rotational diffusion cannot account for all spectral features of TEMPOL in all RTILs. In highly viscous RTILs, the rotational mobility of TEMPOL differs for different molecular axes. The translational diffusion coefficients have been calculated from spin exchange rate constants. To this end, line shape contributions stemming from Heisenberg exchange and from the electron–electron dipolar interaction have been separated based on their distinct temperature dependences. While the Debye–Stokes–Einstein law is found to apply for the rotational correlation times in all solvents studied, the dependence of the translational diffusion coefficients on the Stokes parameter <i>T</i>/<i>η</i> is nonlinear; i.e., deviations from the Stokes–Einstein law are observed. The effective activation energies of rotational diffusion are significantly larger than the corresponding values for translational motion. Effects of the identity of the RTIL cations and anions on the activation energies are discussed

Hole Transfer Processes in <i>meta-</i> and <i>para-</i>Conjugated Mixed Valence Compounds: Unforeseen Effects of Bridge Substituents and Solvent Dynamics

To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in <i>para</i>- or in <i>meta</i>-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at <i>para</i>- and <i>meta</i>-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the <i>meta</i>-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the <i>para</i>-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the <i>para</i>-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO₂, <i>o</i>DCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for <i>meta</i>-conjugated compounds just as for the <i>para</i>-series. This effect demonstrates that the <i>para-meta</i> paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of <i>meta-</i>topologies for optoelectronic applications

Visible Light Mediated Cyclization of Tertiary Anilines with Maleimides Using Nickel(II) Oxide Surface-Modified Titanium Dioxide Catalyst

Surface-modified titanium dioxides by highly dispersed NiO particles have an extended absorption in the visible light region and a reduced hole–electron pair recombination than unmodified TiO₂. They have now been successfully applied as highly active heterogeneous photocatalysts in the visible light mediated direct cyclization of tertiary anilines with maleimides to give tetrahydroquinoline products in moderate to high yields at ambient temperature. In contrast with unmodified titanium dioxide catalysts that are conventionally used in a stoichiometric amount in combination with UVA light, only a catalytic amount (1 mol %) of the surface-modified TiO₂ catalyst is needed along with visible light to efficiently catalyze the reaction. Compared with transition-metal complexes such as Ru­(bpy)₃Cl₂ or Ir­(ppy)₂(dtbbpy)­PF₆, advantages of these surface-modified titanium dioxides as photocatalyst include high catalytic activity, low cost, ease of recovering, and being able to be used for at least nine times without significant decay of catalytic activity