The influence of polar solvents on ions and ion pairs in the anionic polymerization of styrene

In the rather polar organic solvents dimethoxyethane, tetrahydrofuran and 3-methyltetrahydrofuran, the behaviour of contact ion pairs, solvent-separated ion pairs and free carbanions of polystyryl sodium has been investigated by kinetic and conductivity measurements. Both the equilibrium between the two kinds of ion pairs and the dissociation of solvent-separated ion pairs to free anions are followed over a wide range of temperature. Thereby, conditions can be found under which the polymerization takes place almost exclusively via one of the two types of ion pairs.The thermodynamic parameters of the equilibria and the Arrhenius parameters of the propagation rate constants of the different kinds of propagating chain ends are reported. The equilibria between these species are strongly influenced by the solvent whereas the individual propagation rate constants are scarcely affected by the solvent. The “effective charge distance” in the solvent-separated ion pair can be estimated from the corresponding dissociation constant. The mobility of the polymer carbanion is discussed.     

Energy gap law of electron transfer in nonpolar solvents

We investigate the energy gap law of electron transfer in nonpolar solvents for charge separation and charge recombination reactions. In polar solvents, the reaction coordinate is given in terms of the electrostatic potentials from solvent permanent dipoles at solutes. In nonpolar solvents, the energy fluctuation due to solvent polarization is absent, but the energy of the ion pair state changes significantly with the distance between the ions as a result of the unscreened strong Coulomb potential. The electron transfer occurs when the final state energy coincides with the initial state energy. For charge separation reactions, the initial state is a neutral pair state, and its energy changes little with the distance between the reactants, whereas the final state is an ion pair state and its energy changes significantly with the mutual distance; for charge recombination reactions, vice versa. We show that the energy gap law of electron-transfer rates in nonpolar solvents significantly depends on the type of electron transfer.     

How far do electrons move? A semiempirical investigation of thermal electron-transfer distances in cationic bis(hydrazine) and bis(hydrazyl) mixed-valence compounds.

A computational approach for estimating thermal electron-transfer reaction distances in symmetrical mixed-valence compounds is described and applied to a series of bis(hydrazine) and bis(hydrazyl) radical cations and derivatives, some of which have been investigated experimentally by Nelsen and co-workers. Ground-state semiempirical charge distributions are obtained by using optimized reactant geometries. Advantage is then taken of the approximate C(2) symmetry, or the approximate mirror symmetry, of each of the targeted compounds, and the inherent degeneracy of the corresponding electron-transfer reactions, such that the change in dipole moment (Delta-mu) upon charge transfer can be estimated from an appropriately distance-weighted sum of charge differences between approximately symmetry-equivalent atoms found on the donor and acceptor sides of the molecule. Delta-mu can then be related directly to the effective one-electron-transfer distance. We find that calculated adiabatic electron-transfer distances can differ appreciably from the geometric donor-site/acceptor-site separation distances. Furthermore, for a fixed geometric separation distance, the effective electron-transfer distance can vary considerably, depending on chemical substituent composition and/or isomeric configuration. Further advantage is taken of the approximate donor-site/acceptor-site symmetry, in the context of a Newton-Cave type analysis, to establish the relative importance of electronic delocalization effects versus self-polarization and inductive effects in diminishing or enhancing effective one-electron-transfer distances.     

Radical heterolysis reactions. Dynamics of formation, collapse, and solvation of ion pairs determined by a competition kinetic method

The kinetics of radical heterolysis reactions, including rate constants for radical cation-anion contact ion pair formation, collapse of the contact pair back to the parent radical, and separation of the contact pair to a solvent-separated ion pair or free ions were obtained in several solvents for a beta-mesyloxy radical. Rate constants were determined from indirect kinetic studies using thiophenol as both a radical trapping agent via H-atom transfer and an alkene radical cation trapping agent via electron transfer. [reaction: see text].     

The photophysics of alkali naphtholates: lifetimes of contact and solvent-separated ion pairs present simultaneously in weakly polar solvents

The solvent effect on the emission and absorption properties of the β-naphtholate anion was studied and the lifetimes measured. In all solvents only a single peak is observed in the spectra. In spite of the lack of resolved spectral bands, the combination of shifts and lifetimes made it possible to demonstrate the simultaneous presence of contact and solvent-separated ion pairs in weakly polar solvents like tetrahydrofuran.     

Formation and decay of localized contact radical ion pairs in DNA hairpins

The dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing diphenylacetylene-4,4'-dicarboxamide linkers have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. The lowest excited singlet state of the linker is capable of oxidizing nearest neighbor adenine as well as guanine. A large wavelength shift in the transient absorption spectrum accompanies the conversion of the singlet linker to its anion radical, facilitating the investigation of electron-transfer dynamics. The rate constants for charge separation are dependent upon the oxidation potentials of the neighboring nucleobase donors but not upon the identity of nonnearest neighbors. Thus, the charge separation processes yield a contact radical ion pair in which the positive charge is localized on the neighboring nucleobase. Rate constants for charge recombination are dependent upon the identity of the first and second nearest-neighbor nucleobases but not more remote bases. This dependence is attributed to stabilization of the contact radical ion pair by interaction with its nearest neighbor. The absence of charge migration to form a base-pair separated radical ion pair is a consequence of Coulombic attraction in the contact radical ion pair and the low effective dielectric constant (epsilon < 7) experienced by the contact radical ion pair. Photoinduced charge injection to form a base-pair separated radical ion pair is necessary in order to observe charge migration.     

Dynamic nature of the transition state for the SN1 reaction mechanism of diphenylmethyl acetates

Picosecond absorption spectroscopy is employed in the study of the reaction dynamics for the contact ion pairs produced upon the photolysis of a series of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol. From the temperature dependence of the rate constants, the activation parameters associated with covalent bond formation and diffusional separation to the solvent-separated ion pair are obtained. The activation parameters for bond formation are examined within the context of the Hynes theory for solvent dynamical effects on the passage through the transition state; deviations from the transition-state theory are found to be large. Factors that control nucleophilicity are discussed. Finally the validity of applying the Marcus equation to the SN1 reaction mechanism is addressed.     

Investigation of geminate recombination of radical ion pairs generated by dissociation of exciplexes in moderately polar solvents using the photoconductivity technique

A new approach to determination of the recombination rate of radical ion pairs in moderately polar solvents is presented. It is based on an investigation of transient photocurrents caused by dissociation of exciplexes generated in photoinduced electron transfer reactions. It has been shown that the recombination rate of geminate ion pairs can be found from the photocurrent rise time. We have applied such an approach to transient photocurrents observed by Hirata et al. [Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 87 (1983) 1659] for the pyrene/dicyanobenzene system in solvents of moderate polarity. The increase of the obtained recombination rate of photogenerated ions with increasing polarity of solvent testifies that ions recombine mainly by the backward electron transfer from the dicyanobenzene anions to solvent-separated cations of pyrene.     

Bonded exciplexes. A new concept in photochemical reactions

Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5-0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 x 10(9) M(-1)s(-1), even though formation of a conventional radical ion pair, A*-D*+, is endothermic by approximately 0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a "bonded exciplex", A- -D+. The bonded exciplex can be formally thought of as resulting from bond formation between the unpaired electrons of the radical ions A*- and D*+. The covalent bonding interaction significantly lowers the energy of the charge-transfer state. As a result of this interaction, the energy decreases with decreasing separation distance, and near van der Waals contact, the A- -D+ bonded state mixes with the repulsive excited state of the acceptor, allowing efficient reaction to form A- -D+ even when formation of a radical ion pair A*-D*+ is thermodynamically forbidden. Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciplex state.     

Effect of nucleophilic solvent on the kinetic parameters of the reactions of unimolecular heterolysis. Mechanism of the covalent bond heterolysis

Reactions of unimolecular heterolysis occur through consecutive formation of four ion pairs: contact, spatially separated, separated by one solvent molecule, and solvent-separated. In the limiting stage, the contact ion pair interacts with the solvent cavity. Nucleophilic solvation hinders the separation of ions in the transition state. At the heterolysis of secondary substrates this is compensated by the nucleophilic solvation of the incipient carbocations from the rear and the reaction rate does not depend on the solvent nucleophilicity. In the case of heterolysis of tertiary substrates, only partial compensation occurs, and nucleophilic solvent reduces the reaction rate through reducing the activation entropy.     

Types,Symbolism of Representation of Steric Structure,and Conformations of Ion Pairs. Stereochemical Version of the Generalized Rule of Elimination

Analysis of published data from the standpoint of the generalized rule of elimination demonstrated that in addition to contact and solvent-separated ion pairs, in elimination a species of a third type is generated, called spatially separated ion pair. This is an intermediate formed on the pathway of transformation of a contact ion pair to a solvent-separated one. Each of these ion pairs preserves its initial configuration of the bond C-nucleofuge starting from its origination to transformation into an elimination product, demonstrating discrete and inherent regio- and stereoselectivity: a contact ion pair shows nucleophilically controlled syn reactivity, and spatially- and solvent-separated ion pairs, electrophilically controlled syn and anti reactivity, respectively. The generalized rule of elimination allows almost faultless prediction of regio- and stereo- selectivity, being applicable to interpretation of even those published data which appear surprising or abnormal from the standpoint of the modern theoretical views.     

Electronic Structure and Structure of Ion Pairs of Anionic σ Adducts of Polynitroarenes with Acetonate Ion

The anionic Yanovskii adducts of 1,3,5-trinitro- and 1,3-dinitrobenzenes, 1,3-dinitronaphthalene, and 3,5-dinitrobenzoic acid with sodium, potassium, and tetrabutylammonium acetonates in low-polarity solvents exist mostly as contact ion pairs, while in polar solvents, as solvent-separated ion pairs and free ions. Lowering the temperature increases the fraction of solvent-separated ion pairs in low-polarity solvents and of free ions in polar solvents, by shifting the equilibria to stronger solvated ionic species. By quantum-chemical calculations of the geometry and electronic structure of the anions and of the electronic absorption spectra of the free ions and ion pairs, as well as by a spectrophotometric study of adducts with solvents of various polarity it was established that the cation on ion-pair formation coordinates with the 4-nitro group with respect to the pyramidal node.     

Self-assembled single-walled carbon nanotube:zinc-porphyrin hybrids through ammonium ion-crown ether interaction: construction and electron transfer

An ammonium ion-crown ether interaction has been successfully used to construct porphyrin-single-walled carbon nanotube (SWNT) donor-acceptor hybrids. The [18]crown-6 to alkyl ammonium ion binding strategy resulted in porphyrin-SWNT nanohybrids that are stable and soluble in DMF. The porphyrin-SWNT hybrids were characterized by spectroscopic, TEM, and electrochemical techniques. Both steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited state of the porphyrins and free-energy calculations suggested that electron-transfer quenching occurred. Nanosecond transient absorption spectral results supported the charge-separation quenching process. Charge-stabilization was also observed for the nanohybrids in which the lifetime of the radical ion pairs was around 100 ns. The present nanohybrids were also used to reduce the hexyl viologen dication (HV2+) and to oxidize 1-benzyl-1,4-dihydronicotinamide in solution in an electron-pooling experiment. Accumulation of the radical cation (HV.+) was observed in high yields, which provided additional proof for the occurrence of photoinduced charge separation. The present study demonstrates that a hydrogen-bonding motif is a successful self-assembly method to build SWNTs bearing donor-acceptor nanohybrids, which are useful for light-energy harvesting and photovoltaic applications.     

State in solution, structure, and regioselectivity of reactions of the lithium 1-(2-methoxyphenyl)-3,3-diphenylpropyne derivative

According to the spectrophotometric data, the lithium 1-(2-methoxyphenyl)-3,3-diphenylpropyne derivative in diethyl ether exists as contact ion pairs, while in THF, according to the spectrophotometric and¹³C NMR data, solvent-separated ion pairs are predominantly formed. According to the¹³C NMR data, the carbanion in the solventseparated ion pairs has a structure close to the propargylic type. The regioselectivity of reactions of the lithium derivative with ethyl halides in diethyl ether, THF, and hexamethyphosphoramide, with benzyl chloride in the first two solvents, and with methanol in THF were studied. The protonation with methanol proceeds exclusively at the allenylic center (C-1) while the ethylation and especially benzylation proceed predominantly at the propargylic center (C-3). The selectivity of ethylation of the propargylic center of both solvent-separated ion pairs in THF and contact ion pairs in diethyl ether increases as the hardness of the ethylating agent increases, and in the case of the same ethyl halide, the selectivity increases from the solvent-separated ion pairs to the contact ion pairs. The spectral data obtained and the data on changes in the regioselectivity do not allow one to believe that the contact ion pairs of the lithium derivative in ether exhibit the intramolecular coordination of the lithium cation to the methoxy group, which might lead to the allenylic structure of contact ion pairs of this derivative. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2043–2051, November, 1997.     

Corrole-fullerene dyads: formation of long-lived charge-separated states in nonpolar solvents

The first example of covalently linked free-base corrole-fullerene dyads is reported. In the newly synthesized dyads, the free-energy calculations performed by employing the redox and singlet excited-state energy in both polar and nonpolar solvents suggested the possibility of electron transfer from the excited singlet state of corrole to the fullerene entity. Accordingly, steady-state and time-resolved emission studies revealed efficient fluorescence quenching of the corrole entity in the dyads. Further studies involving femtosecond laser flash photolysis and nanosecond transient absorption studies confirmed electron transfer to be the quenching mechanism, in which the electron-transfer product, the fullerene anion radical, was able to be spectrally characterized. The rate of charge separation, kCS, was found to be on the order of 10(10)-10(11) s(-1), suggesting an efficient photoinduced electron-transfer process. Interestingly, the rate of charge recombination, kCR, was slower by 5 orders of magnitude in nonpolar solvents, cyclohexane and toluene, resulting in a radical ion-pair lasting for several microseconds. Careful analysis of the kinetic and thermodynamic data using the Marcus approach revealed that this novel feature is due to appropriately positioning the energy level of the charge-separated state below the triplet states of either of the donor and acceptor entities in both polar and nonpolar solvents, a feature that was not evident in donor-acceptor dyads constructed using symmetric tetrapyrroles as electron donors.     

Ion pairing and the rotational mobility of organic ions in solution

Carbon-13 dipolar spin-lattice relaxation times can be used to study microscopic ion mobility in solvent-separated and contact ion-pair systems. Two chemically stable ion pairs were studied. Cyclohexylammonium formate observed in a number of solvents allows correlation of relaxation times—and therefore ion rotational mobility—with empirical solvent polarity indices. Estimation of the effective anion radius shows a change of a factor of three in size arising from solvation/ion-pairing effects. Trimesate trianion (1,3,5-tricarboxybenzene) with differing cations present in solution is a good probe of changes in the solvation sphere and degree of ion aggregation. Variable temperature studies give an activation energy for overall ionic reorientation of c. 5 kcal/mol.     

Manipulating the production and recombination of electrons during electron transfer: Femtosecond control of the charge-transfer-to-solvent (CTTS) dynamics of the sodium anion

The scavenging of a solvated electron represents the simplest possible electron-transfer (ET) reaction. In this work, we show how a sequence of femtosecond laser pulses can be used to manipulate an ET reaction that has only electronic degrees of freedom: the scavenging of a solvated electron by a single atom in solution. Solvated electrons in tetrahydrofuran are created via photodetachment using the charge-transfer-to-solvent (CTTS) transition of sodide (Na(-)). The CTTS process ejects electrons to well-defined distances, leading to three possible initial geometries for the back ET reaction between the solvated electrons and their geminate sodium atom partners (Na(0)). Electrons that are ejected within the same solvent cavity as the sodium atom (immediate contact pairs) undergo back ET in approximately 1 ps. Electrons ejected one solvent shell away from the Na(0) (solvent-separated contact pairs) take hundreds of picoseconds to undergo back ET. Electrons ejected more than one solvent shell from the sodium atom (free solvated electrons) do not recombine on subnanosecond time scales. We manipulate the back ET reaction for each of these geometries by applying a "re-excitation" pulse to promote the localized solvated electron ground state into a highly delocalized excited-state wave function in the fluid's conduction band. We find that re-excitation of electrons in immediate contact pairs suppresses the back ET reaction. The kinetics at different probe wavelengths and in different solvents suggest that the recombination is suppressed because the excited electrons can relocalize into different solvent cavities upon relaxation to the ground state. Roughly one-third of the re-excited electrons do not collapse back into their original solvent cavities, and of these, the majority relocalize into a cavity one solvent shell away. In contrast to the behavior of the immediate pair electrons, re-excitation of electrons in solvent-separated contact pairs leads to an early time enhancement of the back ET reaction, followed by a longer-time recombination suppression. The recombination enhancement results from the improved overlap between the electron and the Na(0) one solvent shell away due to the delocalization of the wave function upon re-excitation. Once the excited state decays, however, the enhanced back ET is shut off, and some of the re-excited electrons relocalize even farther from their geminate partners, leading to a long-time suppression of the recombination; the rates for recombination enhancement and relocalization are comparable. Enhanced recombination is still observed even when the re-excitation pulse is applied hundreds of picoseconds after the initial CTTS photodetachment, verifying that solvent-separated contact pairs are long-lived, metastable entities. Taken together, all these results, combined with the simplicity and convenient spectroscopy of the sodide CTTS system, allow for an unprecedented degree of control that is a significant step toward building a full molecular-level picture of condensed-phase ET reactions.     

SOLVENT INFLUENCES ON THE SINGLET QUENCHING OF CHLOROPHYLL a BY 2, 5-DIMETHYL-p-BENZOQUINONE

Abstract— The quenching of chlorophyll a excited singlet states by 2.5-dimethyl- p -benzoquinone has been investigated in solvents of varying viscosity and polarity. The observed singlet lifetimes showed little variation in several hydrocarbon solvents. Stern-Volmer constant K depends on the viscosity of the solvent, although cyclic and straight-chain hydrocarbons behave somewhat differently. The decrease of the K values with increase of viscosity suggests that the quenching mechanism is at least partly dynamic, although there is evidence for static quenching as well. The influence of solvent polarity on the K values was found to be insignificant, which is consistent with a very short-lived ion pair intermediate formed by electron-transfer quenching.     

The stereoregularity of polystyrenes obtained by different ion pairs of polystyryl alkali salts

The stereoregularity of polystyrenes obtained with sodium, potassium, rubidium, and cesium naphthalenes in various solvents was determined by ¹³C-NMR spectroscopy. Polystyrenes produced by contact ion pairs of polystyryl cesium in dioxane and tetrahydrofuran (THF) had the proportions of a 57–58% racemic dyad (P_r), whereas the P_r values increased to 65 and 69% by solvating Cs⁺ counterions in dimethoxyethane and by agent-separating them with crown ether, respectively. Polystyrene obtained by contact ion pairs of polystyryl sodium in dioxane showed a P_r of 66%; polymers produced by solvent-separated ion pairs of polystyryl sodium in THF at ?78°C had a P_r of 71%. A polymerization system which contained alkali counterions with large ionic radii and solvents with low dielectric constants in which only contact ion pairs existed produced polystyrenes with isotactic-rich configurations. The stereoregularity of polystyrene produced by contact ion pairs of polystyryl potassium and rubidium in tetrahydropyran (THP) occurred intermediately between that of polymers obtained in diethyl ether and THF. It was concluded that the stereoregulation of contact ion pairs may be closely related to the interionic distance of the ion pair.