Ultrafast Studies of Excess Electrons in Liquid Acetonitrile: Revisiting the Solvated Electron/Solvent Dimer Anion Equilibrium

Photoionization of 3-Methylindole Embedded in Sodium Dodecyl Sulfate and Dodecyltrimethylammonium Chloride Micelles: Migration of Electrons Generated in Micelle Cores and Their Solvation in Outside Water

Electrons were generated in the core of micelles formed by negatively charged sodium dodecyl sulfate (SDS) or positively charged dodecyltrimethylammonium chloride (DTAC) by photoionization of 3-methylindole embedded in the core. The electrons were hydrated after they moved out of the core to the outer aqueous phase. These processes were monitored with femtosecond time-resolved absorption spectroscopy. The migration of electrons from the micelle core to the outer aqueous phase was faster than the instrumental response time of 200 fs. Hot electrons in the aqueous phase were produced in ≤320 fs. There was no significant difference observed for the micellar solutions of negatively charged SDS and positively charged DTAC, or for water. The geminate recombination between the electrons and the radical cations was hindered to a large extent once the electrons hydrated at the outer aqueous phase were separated from the radical cations remaining in the micelle core.

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

The ultrafast photochemical reaction of quinone methide (QM) formation from adamantylphenol was monitored in real time using femtosecond transient absorption spectroscopy and fluorescence upconversion in solution at room temperature. Experiments were complemented by theoretical studies simulating the reaction pathway and elucidating its mechanism. Excitation with sub-20 fs UV pulses and broadband probing revealed ultrafast formation of the long-lived QM intermediate directly in the ground state, occurring with a time constant of around 100 fs. UV-vis transient absorption data covering temporal dynamics from femtoseconds to hundreds of milliseconds revealed persistence of the absorption band assigned to QM and partially overlapped with other contributions tentatively assigned to triplet excited states of the adamantyl derivative and the phenoxyl radical that are clearly distinguished by their evolution on different time scales. Our data, together with the computations, provide evidence of a non-adiabatic photodehydration reaction, which leads to the formation of QM in the ground state via a conical intersection, circumventing the generation of a transient QM excited state.

Photochemical formation of quinone methide from adamantylphenol was investigated computationally and experimentally, showing evidence of ultrafast non-adiabatic dehydration via a conical intersection.

Femtosecond Dynamics of Stepwise Two-Photon Ionization in Solutions as Revealed by Pump-Repump-Probe Detection with Burst Mode of Photoexcitation

Pump-repump-probe spectroscopy with a burst mode of photoexcitation was applied to the direct observation of photoionization dynamics of perylene in the solution phase. The irradiation of the pump pulse train...

Dynamics of electron ejection on photoionization of trans-stilbene and biphenyl in acetonitrile as observed with femtosecond time-resolved near-IR absorption spectroscopy

Photoionization in solution is a basic but complex phenomenon involving a solute, an ejected electron and surrounding solvent molecules. It may seem obvious that an electron is released immediately after...

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L₃-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (³MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the ³MC state. A quarter of the ³MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.

Solvated Electron in Acetonitrile: Radiation Yield, Absorption Spectrum, and Equilibrium between Cavity- and Solvent-Localized States

The equilibrium between a solvent cavity-localized electron, e_cav^-, and a dimeric solvent anion, (CH₃CN)₂^•-, which are the two lowest energy states of the solvated electron in acetonitrile, has been investigated by pulse radiolysis at 233-353 K. The enthalpy and entropy for the e_cav^- to (CH₃CN)₂^•- conversion amount to -11.2 ± 0.3 kcal/mol and -39.3 ± 1.2 cal/(mol K), corresponding to a 0.44 ± 0.35 equilibrium constant at 25 °C. The radiation yield of the solvated electron has been quantified using a Co(II) macrocycle that scavenges electrons with a 1.55 × 10¹¹ M^-1 s^-1 rate constant. The apparent yield increases without saturation over the attainable scavenger concentration range, reaching 2.8 per 100 eV; this value represents the lower limit for the acetonitrile ionization yield in pulse radiolysis. The apparent molar absorption coefficient of (20.8 ± 1.5) × 10³ M^-1 cm^-1 at 1450 nm and 20 °C for the solvated electron and individual vis-near-infrared (NIR) absorption spectra of e_cav^- and (CH₃CN)₂^•- are derived from the data. Variances with previous reports are thoroughly discussed. Collectively, these results resolve several controversies concerning the solvated electron properties in acetonitrile and furnish requisite data for quantitative pulse radiolysis investigations in this commonly used solvent.

Ab Initio Simulations of Poorly and Well Equilibrated (CH3CN)n- Cluster Anions: Assigning Experimental Photoelectron Peaks to Surface-Bound Electrons and Solvated Monomer and Dimer Anions

Excess electrons in liquid acetonitrile are of particular interest because they exist in two different forms in equilibrium: they can be present as traditional solvated electrons in a cavity, and they can form some type of solvated molecular anion. Studies of small acetonitrile cluster anions in the gas phase show two isomers with distinct vertical detachment energies, and it is tempting to presume that the two gas-phase cluster anion isomers are precursors of the two excess electron species present in bulk solution. In this paper, we perform DFT-based ab initio molecular dynamics simulations of acetonitrile cluster anions to understand the electronic species that are present and why they have different binding energies. Using a long-range-corrected density functional that was optimally tuned to describe acetonitrile cluster anion structures, we have theoretically explored the chemistry of (CH₃CN)_n^- cluster anions with sizes n = 5, 7, and 10. Because the temperature of the experimental cluster anions is not known, we performed two sets of simulations that investigated how the way in which the cluster anions are prepared affects the excess electron binding motif: one set of simulations simply attached excess electrons to neutral (CH₃CN)_n clusters, providing little opportunity for the clusters to relax in the presence of the excess electron, while the other set allowed the cluster anions to thermally equilibrate near room temperature. We find that both sets of simulations show three distinct electron binding motifs: electrons can attach to the surface of the cluster (dipole-bound) or be present either as solvated monomer anions, CH₃CN^-, or as solvated molecular dimer anions, (CH₃CN)₂^-. All three species have higher binding energies at larger cluster sizes. Thermal equilibration strongly favors the formation of the valence-bound molecular anions relative to surface-bound excess electrons, and the dimer anion becomes more stable than the monomer anion and surface-bound species as the cluster size increases. The calculated photoelectron spectra from our simulations in which there was poor thermal equilibration are in good agreement with experiment, suggesting assignment of the two experimental cluster anion isomers as the surface-bound electron and the solvated molecular dimer anion. The simulations also suggest that the shoulder seen experimentally on the low-energy isomer's detachment peak is not part of a vibronic progression but instead results from molecular monomer anions. Nowhere in the size range that we explore do we see evidence for a nonvalence, cavity-bound interior-solvated electron, indicating that this species is likely only accessible at larger sizes with good thermal equilibration.

Ultrafast capture of electrons ejected by photoionization leading to the formation of a charge-separated state at a high energy level

Electron transfer reactions driven by two-photon ionization in the higher excited state were investigated via transient absorption spectroscopy, with the aim to develop a method for creating the charge-separated (CS) state with a large formation rate, high energy level, and long lifetime. In the proof-in-principle experiments using pyrene and biphenyl as a model system, femtosecond transient absorption spectroscopy revealed that intense irradiation of an ultraviolet laser pulse at 355 nm efficiently pumps up pyrene into a higher excited state via a stepwise two-photon absorption, and then an ionization process takes place. An electron ejected from pyrene is directly captured by biphenyl with a time constant of 200 fs without the diffusion process of the electron in solution. The energy level of the CS state (Py⁺-Bp^-) thus formed was estimated to be higher than that of the S₁ state of pyrene by 0.53 eV. In addition, the subsequent ionic dissociation without a remarkable geminate recombination in the sub-nanosecond to nanosecond time region effectively avoids the quantity loss of the CS state. By applying the two-photon excitation method, we experimentally achieved ultrafast formation of the long-lived CS state at a high energy beyond the traditional framework of electron transfer reactions.

Electronic Structures, Spectroscopy, and Electrochemistry of [M(diimine)(CN-BR3)4]2- (M = Fe, Ru; R = Ph, C6F5) Complexes

Complexes with the formula [M(diimine)(CN-BR₃)₄]^2-, where diimine = bipyridine (bpy), phenanthroline (phen), 3,5-trifluoromethylbipyridine (flpy), R = Ph, C₆F₅, and M = Fe^II, Ru^II, were synthesized and characterized by X-ray crystal structure analysis, UV-visible spectroscopy, IR spectroscopy, and voltammetry. Three highly soluble complexes, [Fe^II(bpy)(CN-B(C₆F₅)₃)₄]^2-, [Ru^II(bpy)(CN-B(C₆F₅)₃)₄]^2-, and [Ru^II(flpy)(CN-B(C₆F₅)₃)₄]^2-, exhibit electrochemically reversible redox reactions, with large potential differences between the bpy^0/- or flpy^0/- and M^III/II couples of 3.27, 3.52, and 3.19 V, respectively. CASSCF+NEVPT2 calculations accurately reproduce the effects of borane coordination on the electronic structures and spectra of cyanometallates.

Ultrafast transient absorption spectroscopy of doped P3HT films: distinguishing free and trapped polarons

It is generally presumed that the vast majority of carriers created by chemical doping of semiconducting polymer films are coulombically trapped by the counteranion, with only a small fraction that are free and responsible for the increased conductivity essential for organic electronic applications. At higher doping levels, it is also possible for bipolarons to form, which are expected to be less conductive than single polarons. Unfortunately, there is no simple way to distinguish free polarons, trapped polarons and bipolarons using steady-state spectroscopy. Thus, in this work, we use ultrafast transient absorption spectroscopy to study the dynamics of polarons in 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TNCQ)-doped films of poly(3-hexylthiophene-2,5-diyl) (P3HT) as a function of dopant concentration and excitation wavelength. When exciting on the red side of the polaron P1 transition, our transient absorption spectra and kinetics match well with what is expected for free 2-D-delocalized polarons; the measurements are not consistent with a recent theory of doped conjugated polymer electronic structure that suggests that the half-filled state lies deeper in the conduction band rather than in the bandgap. As we tune the excitation wavelength to the blue, our measurements reveal an increasing amount of slower transient kinetics that are consistent with the presence of coulombically-trapped polarons rather than bipolarons. Taking advantage of their distinct ultrafast relaxation kinetics as a type of action spectroscopy, we are able to extract the steady-state absorption spectra of free and trapped polarons as a function of dopant concentration. By comparing the results to theoretical models, we determine that in F4TCNQ-doped P3HT films, trapped polarons sit ∼0.4 nm away from the anion while free polarons reside between 0.7 and 0.9 nm from the counteranion. Perhaps counterintuitively, the ratio of trapped to free polarons increases at higher doping levels, an observation that is consistent with a plateau in the concentration-dependent conductivity of F4TCNQ-doped P3HT films.

Radical Ions of 3-Styryl-quinoxalin-2-one Derivatives Studied by Pulse Radiolysis in Organic Solvents

The absorption-spectral and kinetic behaviors of radical ions and neutral hydrogenated radicals of seven 3-styryl-quinoxalin-2(1 H)-one (3-SQ) derivatives, one without substituents in the styryl moiety, four others with electron-donating (R = -CH₃, -OCH₃, and -N(CH₃)₂) or electron-withdrawing (R = -OCF₃) substituents in the para position in their benzene ring, and remaining two with double methoxy substituents (-OCH₃), however, at different positions (meta/para and ortho/meta) have been studied by UV-vis spectrophotometric pulse radiolysis in neat acetonitrile saturated with argon (Ar) and oxygen (O₂) and in 2-propanol saturated with Ar, at room temperature. In acetonitrile solutions, the radical anions (4R-SQ^•-) are characterized by two absorption maxima located at λ_max = 470-490 nm and λ_max = 510-540 nm, with the respective molar absorption coefficients ε_470-490 = 8500-13 100 M^-1 cm^-1 and ε_510-540 = 6100-10 300 M^-1 cm^-1, depending on the substituent (R). All 4R-SQ^•- decay in acetonitrile via first-order kinetics, with the rate constants in the range (1.2-1.5) × 10⁶ s^-1. In 2-propanol solutions, they decay predominantly through protonation by the solvent, forming neutral hydrogenated radicals (4R-SQH^•), which are characterized by weak absorption bands with λ_max = 480-490 nm. Being oxygen-insensitive, the radical cations (4R-SQ^•+) are characterized by a strong absorption with λ_max = 450-630 nm, depending on the substituent (R). They are formed in a charge-transfer reaction between a radical cation derived from acetonitrile (ACN^•+) and substituted 3-styryl-quinoxalin-2-one derivatives (4R-SQ) with a pseudo-first-order rate constant k = (2.7-4.7) × 10⁵ s^-1 measured in solutions containing 0.1 mM 4R-3-SQ. The Hammett equation plot gave a very small negative slope (ρ = -0.08), indicating a very weak influence of the substituents in the benzene ring on the rate of charge-transfer reaction. The decay of 4R-SQ^•+ in Ar-saturated acetonitrile solutions occurs with a pseudo-first-order rate constant k = (1.6-6.2) × 10⁴ s^-1 and, in principle, is not affected by the presence of O₂, suggesting charge-spin delocalization over the whole 3-SQ molecule. Most of the radiolytically generated transient spectra are reasonably well-reproduced by semiempirical PM3-ZINDO/S (for 4R-SQ^•-) and density functional theory quantum mechanics calculations employing M06-2x hybrid functional together with the def2-TZVP basis set (for 4R-SQ^•+).

Radiolytic formation of the carbon dioxide radical anion in acetonitrile revealed by transient IR spectroscopy

First IR detection of CO₂˙⁻ in acetonitrile, produced by radiation-induced CO₂ reduction and oxidation of formate.

Electron Solvation in Liquid Ammonia: Lithium, Sodium, Magnesium, and Calcium as Electron Sources

A free electron in solution, known as a solvated electron, is the smallest possible anion. Alkali and alkaline earth atoms serve as electron donors in solvents that mediate outer-sphere electron transfer. We report herein ab initio molecular dynamics simulations of lithium, sodium, magnesium, and calcium in liquid ammonia at 250 K. By analyzing the electronic properties and the ionic and solvation structures and dynamics, we systematically characterize these metals as electron donors and ammonia molecules as electron acceptors. We show that the solvated metal strongly modifies the properties of its solvation shells and that the observed effect is metal-specific. Specifically, the radius and charge exhibit major impacts. The single solvated electron present in the alkali metal systems is distributed more uniformly among the solvent molecules of each metal's two solvation shells. In contrast, alkaline earth metals favor a less uniform distribution of the electron density. Alkali and alkaline earth atoms are coordinated by four and six NH3 molecules, respectively. The smaller atoms, Li and Mg, are stronger electron donors than Na and Ca. This result is surprising, as smaller atoms in a column of the periodic table have higher ionization potentials. However, it can be explained by stronger electron donor-acceptor interactions between the smaller atoms and the solvent molecules. The structure of the first solvation shell is sharpest for Mg, which has a large charge and a small radius. Solvation is weakest for Na, which has a small charge and a large radius. Weak solvation leads to rapid dynamics, as reflected in the diffusion coefficients of NH3 molecules of the first two solvation shells and the Na atom. The properties of the solvated electrons established in the present study are important for radiation chemistry, synthetic chemistry, condensed-matter charge transfer, and energy sources.

Picosecond Pulse Radiolysis of Propylene Carbonate as a Solute in Water and as a Solvent

The ester propylene carbonate (PC) is a solvent with a high static dielectric constant where the charges generated by ionizing radiation are expected to be long-lived at room temperature. Time-resolved optical absorption spectroscopy after picosecond electron pulses reveals the formation of a UV band, within less than two nanoseconds, that is assigned to the radical anion PC(-•), arising from a fast attachment reaction of electrons onto PC. Assignment and reactivity of PC(-•) in neat solvent and solutions are discussed in relation with data obtained in solutions of PC in water under reducing or oxidizing conditions and in solutions in PC of aromatic scavengers with various reduction potentials. The fate of the electrons and the ionization yield in PC are compared with those of other solvents.

Ultrafast primary processes of the stable neutral organic radical, 1,3,5-triphenylverdazyl, in liquid solution

Femtosecond pump–probe spectroscopy reveals ultrafast photochemical processes of a stable neutral organic radical in solution.

Crucial role of solvent-impacted molecular anionic resonances in controlling protonation modes in the acetonitrile-water anionic cluster revealed by ab initio molecular dynamics simulations

We present an ab initio molecular dynamics simulation study of a CH3CN-(H2O)40 cluster with an excess electron (EE) injected vertically in this work. Instead of surface bound or internally solvated electron, a hydrated CH3CN(-) is first formed as the CN transient after geometrical relaxation. The driving forces for the formation of CH3CN(-) are bending vibration of ∠CCN angle, which initiates transfer of an extra charge to the CH3CN LUMO, and hydration effect of the immediate water molecules, which plays a stabilizing role. Solvent thermal fluctuation can lead to different resonances (the quasi-C2-resonance versus quasi-N-resonance) from the CN transient and further cause the hydrated CH3CN(-) system to evolve via two distinctly different pathways featuring spontaneous proton transfer to the central C and N sites, producing two different protonation products, respectively. The solvent thermal fluctuation induced formation of hydrogen bonding with the corresponding sites (C2 versus N) is responsible for the quasi-resonances and interconversion between three resonant structures and further proton transfers featuring spontaneous transfer of a proton to C2 or to N from its interacting water molecule. The duration of CH3CN(-) for either of the two proton transfer processes is less than 200 fs. On the basis of experimental ESR results in which only the CH3CHN radical was found and present theoretical calculations, it is suggested that the trans-CH3CNH radical can be further converted to the CH3CHN radical via a water-mediated hydrogen atom transfer path.