Solvent-separated Radical Ion Pair Research Articles

Water modulates the chemiexcitation efficiency of the peroxyoxalate reaction: Enhancement at low and decrease at higher concentrations

Chemiluminescence transformations are widely utilized in analytical and bioanalytical applications. Specifically, the peroxyoxalate system can be employed for sensitive quantification of a wide variety of analytes. Under anhydrous conditions, the peroxyoxalate reaction shows extremely high emission quantum yields, comparable to highly efficient bioluminescence systems. However, in aqueous media, important for bioanalytical applications, the emission efficiency suffers a significant decrease of several orders of magnitudes, although the reason for this loss in efficiency is unknown up to now.In the present work, we show the results of a detailed photophysical study on the effect of water on the chemiexcitation efficiency of the peroxyoxalate reaction using bis(2,4-dinitrophenyl) oxalate as reagent and imidazole as base catalyst in the presence of different chemiluminescence activators. It is shown that in 1,2-dimethoxyethane/water mixtures (DME:H2O), singlet quantum yields increase with increasing the water concentrations up to 0.7 mol/L, followed by a steady decrease at higher water concentrations. The decrease in quantum yields is not predominantly due to concurrent ester hydrolysis, as quantum yields could be corrected for this side reaction. This indicates that the quantum yield decrease is due to the diminished chemiexcitation efficiency in the presence of water. Using different activators, it is shown that chemiexcitation occurs via electron transfer and electron back-transfer steps between a high-energy intermediate and the activator. Additionally, its efficiency is governed by the energetics of the electron back-transfer step, in agreement with the Chemically Initiated Electron Exchange Luminescence (CIEEL) mechanism, also in partially aqueous environment. Lower excitation efficiency in aqueous media may be due to formation of solvent-separated radical ion pairs, inhibiting chemiexcitation.

Photoreduction of Azaoxoisoaporphines by Amines: Laser Flash and Steady‐State Photolysis and Pulse Radiolysis Studies

Photoreduction of 7H-benzo[e]perimidin-7-one (3-AOIA, A1) and its 2-methyl derivative (2-Me-3-AOIA, A2) by non-H-donating amines (1,4-diazabicyclo[2.2.2]octane [DABCO]; 2,2,6,6-tetramethylpiperidine [TMP]), and a hydrogen-donating amine (triethylamine [TEA]), has been studied in deaerated neat acetonitrile solutions using laser flash photolysis (LFP) and steady-state photolysis. The triplet excited states of A1 and A2 were characterized by a strong absorption band with λmax = 440 nm and lifetimes of 20 and 27 μs respectively. In the presence of tertiary amines, both triplet excited states were quenched with rate constants close to the diffusional limit (kq ranged between 10(9) and 10(10) M(-1) s(-1)). The transient absorption spectra observed after quenching with DABCO and TMP were characterized by maxima located at 460 nm and broad shoulders in the range of 500-600 nm. These transient species are attributed to solvent-separated radical ion pairs and/or to isolated radical anions. In the presence of TEA, these transients undergo proton transfer, leading to the neutral hydrogenated radicals, protonated over the N1- and O-atoms. Transient absorption spectra of these transients were characterized by maxima located at 400 and 520 nm and 430 nm respectively. Additional support for these spectral assignments was provided by pulse radiolysis (PR) experiments in acetonitrile and 2-propanol solutions.

Photoreduction of Oxoisoaporphines by Amines: Laser Flash and Steady-State Photolysis, Pulse Radiolysis, and TD-DFT Studies

Photoreduction of oxoisoaporphine (OIA) (1-aza-benzo-[de]anthracen-7-one) and its 5-methoxy (5-MeO-OIA) derivative by selected amines (two non-alpha-hydrogen-donating amines (1,4-diaza[2.2.2]-bicyclooctane (DABCO) and 2,2,6,6-tetramethylpiperidine (TMP)) and three alpha-hydrogen-donating amines (triethylamine (TEA), diethylmethylamine (DEMA), and dimethylethylamine (DMEA))) has been studied in deaerated neat acetonitrile solutions using laser flash and steady-state photolysis. The triplet excited states of OIA and 5-MeO-OIA are characterized by intense absorption maxima located at lambda(max) = 450 nm and lifetimes of 34.7 +/- 0.5 and 44.6 +/- 0.4 micros, respectively. In the presence of tertiary amines, both triplets are quenched with a rate constant that varies from the near diffusion limit (>10(9) M(-1) s(-1)) to a rather low value (approximately 10(7) M(-1) s(-1)) and shows the expected dependence on the reduction potential for one-electron-transfer reactions. The transient absorption spectra observed after quenching of the respective triplet states are characterized by distinct absorption maxima located at lambda(max) = 480 and 490 nm (for OIA and 5-MeO-OIA, respectively) and accompanied by broad shoulders in the range of 510-560 nm. They were assigned to either solvent-separated radical ion pairs and/or isolated radical anions. In the presence of alpha-hydrogen-donating amines these species undergo protonation that leads to the formation of neutral hydrogenated radicals A1H(*)/A2H(*) with two possible sites of protonation, N and O atoms. Pulse radiolysis and molecular modeling together with TD-DFT calculations were used to support the conclusions about the origin of transients.

Charge Separation Distance for Flexible Donor−Bridge−Acceptor Systems after Electron-Transfer Quenching

Photoinduced transient dipole experiments are used to measure the effective charge separation distance, which is equivalent to the photoinduced change in dipole moment divided by the electron charge of flexible electron-donor/acceptor systems, D-(CH2)n-A, where D is 4- N,N-dimethylaniline, A is 9-anthryl, and n = 3, 4. We find that the dipole moments increase strongly with solvent polarity. For the compound with n = 4 (DBA4), analysis of dipole signals indicates that the effective charge separation distances in toluene, 1,4-dioxane, ethyl acetate, tetrahydrofuran, dichloromethane, 1,2-dichloroethane, 2-methylpentanone-3, 3-pentanone, and benzonitrile are 2.2, 2.5, 4.5, 4.7, 5.5, 5.5, 4.8, and 6.3 A, respectively. These values can be understood as the root-mean-square charge separation distance in the solutions of different solvents. We assume that the folded contact configuration has a separation distance of 3.5 A, the extended, solvent-separated configuration has a separation distance of 8.0 A, and that they are the only two stable species after electron-transfer quenching. The formation efficiencies of contact radical ion pairs (CRIPs) and solvent-separated radical ion pairs (SSRIPs) are estimated in different solvents. The results indicate that a significant fraction of the ion pairs exist as solvent-separated ion pairs when the dielectric constant of the solvent is larger than 10. These results indicate that electron-transfer quenching can indeed happen at large separations in polar solvents. They also reveal that there is a barrier for ion pairs formed at large separations, hindering collapse to a contact separation of around 3.5 A.

The Behavior of Exciplex Decay Processes and Interplay of Radiationless Transition and Preliminary Reorganization Mechanisms of Electron Transfer in Loose and Tight Pairs of Reactants

Exciplex emission spectra and rate constants of their decay via internal conversion and intersystem crossing are studied and discussed in terms of conventional radiationless transition approach. Exciplexes of 9-cyanophenanthrene with 1,2,3-trimethoxybenzene and 1,3,5-trimethoxybenzene were studied in heptane, toluene, butyl acetate, dichloromethane, butyronitrile, and acetonitrile. A better description of spectra and rate constants is obtained using 0-0 transition energy and Gauss broadening of vibrational bands rather than the free energy of electron transfer and reorganization energy. The coincidence of parameters describing exciplex emission spectra and dependence of exciplex decay rate constants on energy gap gives the evidence of radiationless quantum transition mechanism rather than thermally activated medium reorganization mechanism of charge recombination in exciplexes and excited charge transfer complexes (contact radical ion pairs) as well as in solvent separated radical ion pairs. Radiationless quantum transition mechanism is shown to provide an appropriate description also for the main features of exergonic excited-state charge separation reactions if fast mutual transformations of loose and tight pairs of reactants are considered. In particular, very fast electron transfer (ET) in tight pairs of reactants with strong electronic coupling of locally excited and charge transfer states can prevent the observation of an inverted region in bimolecular excited-state charge separation even for highly exergonic reactions.

Electron Transfer in a Radical Ion Pair: Quantum Calculations of the Solvent Reorganization Energy

Results are presented for an investigation of intermolecular electron transfer (ET) in solution by means of quantum calculations. The two molecules that are involved in the ET reaction form a solvent-separated radical ion pair. The solvent plays an important role in the ET between the two molecules. In particular, it can give rise to specific solute-solvent interactions with the solutes. An example of specific interactions is the formation of a hydrogen bond between a protic solvent and one of the molecules involved in the ET. We address the study of this system by means of quantum calculations on the solutes immersed in a continuum solvent. However, when the solvent can give rise to hydrogen bond formation with the negatively charged ion after ET, we explicitly consider solvent molecules in the solute cavity, determining the hydrogen bond energetic contribution to the overall interaction energy. Solute-solvent pair distribution functions, showing the different arrangement of solvent molecules before and after ET in the first solvation shell, are reported. We provide results of the solvent reorganization energy from quantum calculations for both the two isolated fragments and the ion pair in solution. Results are in agreement with available experimental data.

Solvent-Separated Radical Ion Pairs and Free Ion Yields. 1. Effect of Temperature on Free Ion Formation in Solution

Free ion yields from geminate ion pairs formed after photoinduced electron transfer are measured by the transient photocurrent method in three moderately polar solvents. Photoexcited 9,10-dicyanoanthracene (DCA) is used as the electron acceptor and alkyl-substituted benzenes as donors. It was found that, generally, there is no significant change in free ion yield as the temperature is increased. On the basis of a theoretical model developed under Collins−Kimball boundary conditions, several factors are analyzed, including dielectric constants and viscosities of solvents, initial separation distance distribution of geminate ion pairs, and temperature-induced changes in recombination rates. By comparing escape probabilities of geminate ion pairs calculated at different initial formation and recombination/separation distances with measured free ion yields, we show that free ions are mainly produced from the solvent-separated ion pairs that are initially formed after electron transfer quenching of photoexcite...

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

Nanosecond pulsed electron paramagnetic resonance spectroscopy is applied to characterize exponential decay constants (β) of the squared electronic coupling matrix element (VDA2) in transient, solv...

Viscosity effects on 9,10-dicyanoanthracene-sensitized photooxidation of N,N-dibenzylhydroxylamine and its derivatives in protic polar solvents

An analysis of viscosity effects on the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by the title hydroxylamines (1) and oxygen in protic polar solvents revealed the occurrence of a diffusion-limited emission quenching for both the quenchers. It was found that the viscosity dependence of the former quenching rate constant is a major factor that controls the magnitude of dependence of the sensitized oxidation efficiency at a given hydroxylamine concentration on the solvent viscosity. The interesting finding that an increase in solvent viscosity exerts only a minor effect on the limiting quantum yield for the reaction led us to conclude that the increased viscosity lowers the rates for both processes of the return electron transfer and the diffusive separation of radical ions to almost the same extent. This finding substantiates the intervention of a solvent-separated radical ion pair as a key intermediate. Based on molecular size effects on the limiting quantum yield in methanol, it was suggested that an unpaired electron generated on the hydroxylamino nitrogen is not appreciably delocalized into the aromatic rings, thus permitting strong electrostatic and hydrogen bonding interactions between the caged 1 cation radical–DCA anion radical pair and protic polar solvent molecules.

Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (r eff) effectively contribute to the CIDEP generated from diffusive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into account the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperfine-dependent RPM polarization (P RPM) phase is varied with the driving force (−ΔG CR) for the charge-recombination (CR) process and that the boundary −ΔG CR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distance,r eff=1.2 nm, between the ion radicals. For the first time, ther eff is well described by the exponent parameter (β) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate ther eff and theP RPM in homogeneous liquid polar solvents.

Variations in efficiencies of triplet state and exciplex formation following fluorescence quenching of 9,10-dicyanoanthracene due to charge transfer interactions.

Data is presented on the quenching of 9,10-dicyanoanthracene by benzene derivatives in acetonitrile. The quenching occurs via a charge transfer mechanism with the quenching rate constants exhibiting a Rehm-Weller dependence on the free energy change of the electron transfer reaction. The quenching of the prompt fluorescence brings about an increase in the delayed fluorescence of DCA as a result of intersystem crossing in the exciplex, and a modified Wilkinson's plot has been used to determine the efficiency of triplet formation during the quenching of DCA fluorescence by benzene derivatives. We suggest that intersystem crossing yields in the exciplex are unity, and variations in triplet state yields as a result of singlet state quenching reflect partitioning between exciplex formation and solvent-separated radical ion pair (SSRIP) formation. The data clearly show competition between exciplex formation and SSRIP formation, with the latter becoming dominant when the free energy for electron transfer exceeds the solvent reorganisation energy.

The solution dynamics of photoinduced geminate radical ion pairs: free ion formation

For geminate radical ion pairs (RIPs) formed by electron transfer quenching of excited 9,10-dicyanoanthracene (DCA) using benzene donors substituted with different bulky groups in several moderately polar solvents, the recombination rates at a variety of separation distances are measured. We used photocurrent techniques together with single photon timing measurements. For the first time, increased recombination rate constants with increasing RIP separation distance are observed. It is concluded that the dominant mechanism for the recombination of solvent separated radical ion pairs (SSRIP) involves a direct return electron transfer from SSRIPs by tunneling, thus by-passing the contact radical ion pairs (CRIP).

Photoinduced electron transfer between acenaphthylene and 1,4-benzoquinones. Formation of dimers of acenaphthylene and 1 ∶ 1-adducts and effect of excitation mode on reactivity of the charge-transfer complexes

Photochemical reactions of acenaphthylene (ACN) with 1,4-benzoquinones (BQs) of varying reduction potentials in solution have been investigated in order to determine final products and quantum yields of the reactions and to get an insight into the factors which govern their reactivities. The products were the two isomeric dimers of ACN and three types of 1 ∶ 1-adducts (cyclobutanes, furans, and oxetanes) between ACN and BQs. However, the product distribution varied widely with the substitution on the BQ skeleton. Two modes of excitation, that is, selective excitation of the charge transfer (CT) complex (the CT mode; typical wavelength: 546.1 nm) and direct excitation of ACN or BQs (the direct mode; typical wavelength: 435.8 nm), essentially gave similar product distributions when the reaction took place through both modes. However, the direct mode showed higher quantum yields for the reactions, Φ435.8, than the CT mode, Φ546.1. Moreover, Φ546.1 tended to increase with increase of free energy gap, −ΔGBET, between the ground state CT complexes and the resulting radical ion pair (RIP). These observations can be rationalized by a mechanism involving a distinctive RIP as an intermediate generated by photoinduced electron transfer in each mode. Thus, the solvent-separated radical ion pair (SSIP) produced in the direct mode excitation will undergo dissociation to the free radical ions (FRIs), ACN+˙ and BQ−˙, affording final products competing with backward electron transfer (BET). In contrast, the contact radical ion pair (CIP) produced in the CT mode excitation much more rapidly deactivates to the ground state than dissociates to FRIs via SSIP due to faster BET, whose rate depends on −ΔGBET. The 1 ∶ 1-adducts can be formed from either a cage reaction inside the RIP (CIP or SSIP) or reaction between FRIs, whereas the dimers of ACN should be formed from reaction of ACN+˙ with the ground state ACN.

Reaction Pathways of the Electron Transfer from Photoexcited 10-Methylphenothiazines to Electron Acceptors in Polar Solvents. Effects of Magnetic Fields and Heavy Atoms on Efficiency of Free Ion Formation

The lowest triplet excited states of (2-substituted) 10-methylphenothiazine were found to be quenched by various electron acceptors in polar solvents such as 2-propanol and acetonitrile through electron transfer (ET). The transient absorption and time-resolved EPR spectra indicated that the radical cation of the phenothiazine and radical anions of the acceptors were formed as the ET products in moderate to high yields. These free radical ions were formed via two types of intermediates, (i) a triplet contact radical ion pair (3CRIP) or a triplet exciplex (3Ex*) and (ii) a triplet solvent-separated radical ion pair (3SSRIP). In the quenching by the Br-substituted acceptors, a large fraction of 3CRIP (or 3Ex*) was deactivated to the singlet ground states due to the breakdown of the spin-forbiddance by strong spin−orbit coupling. On the other hand, 3CRIP (or 3Ex*) containing no heavy atom was mainly transformed into 3SSRIP by solvation. 3SSRIP decayed through either the separation to free ions or the triplet−...

Magnetic field effects on the triplet exciplex dynamics in the duroquinone-N,N-dimethylaniline derivative systems

Magnetic field effects (MFEs) on the radical yield in the photoinduced electron transfer reaction from the p-halogen derivatives (4XDMA) of N,N-dimethylaniline to the excited triplet state of duroquinone (DQ) have been investigated in alcoholic solutions at room temperature. In 1-propanol and 1-butanol solutions, the radical yields decreased as the magnetic field increased and became nearly constant at 1-1.8 T in the DQ-4BrDMA and DQ-4IDMA systems, suggesting that the spin-orbit coupling interaction due to the heavy atoms governs the radical yield. On the other hand, in the methanol solution MFE due to a radical pair mechanism was observed. We concluded that the key intermediate to determine the radical yield is the triplet exciplex or contact radical ion pair in the 1-propanol and 1-butanol solutions, while it is the solvent-separated radical ion pair in the methanol solution.

Exciplex Formation in Complexes between Cyclophane Hosts and Aromatic Guests: Evidence That the Ground-State Complexes Exist in More Than One Distinct Geometry

Several naphthalene derivatives and fluorene form exciplexes with cyclophane 1 when the host−guest complexes are photoexcited. The electron-rich aromatic units of host 1 act as electron donors (D), while the aromatic guests serve as electron acceptors (A). A linear dependence between exciplex fluorescence energy (hνemax) and the difference between the oxidation potential of the host and the reduction potential of the guest (EDox − EAred) has been established for the exciplexes observed with fluorene, naphthalene, and 1-fluoronaphthalene, in accord with data from previous studies in which triethylamine is the donor with these and other aromatic acceptors. Points for N-acetyl-2-naphthylalanine, 1-methylnaphthalene, 1-chloronaphthalene, and 2-chloronaphthalene deviate from the line, indicating reduced exciplex stability resulting from unfavorable steric interactions between host and guest. The guests with the highest reduction potentials, 1- and 2-cyanonaphthalene, exhibit extensive fluorescence quenching with little or no observable exciplex fluorescence, suggesting efficient formation of solvent-separated radical-ion pairs. Time-resolved fluorescence experiments reveal that the exciplexes arise via two independent pathways, one of which occurs very rapidly. The data indicate that the ground-state host−guest complexes exist at equilibrium in at least two distinct geometries.

Photoinduced Electron Transfer between Acenaphthylene and Tetracyanoethylene: Effect of Irradiation Mode on Reactivity of the Charge-Transfer Complex and the Resulted Radical Ion Pair in Solution and Crystalline State

The mechanism of photodimerization of acenaphthylene (ACN) and of reactions with tetracyanoethylene (TCNE) by electron transfer (ET) has been investigated in solution and solid state to elucidate the role of the radical cation of ACN (ACN+•) in formation of the cisoid-dimer (cisoid-1) and the transoid-dimer (transoid-1) of ACN and addition products to TCNE. Selective excitation of the 1:1 charge-transfer (CT) complex between ACN and TCNE with light of >500 nm did not result in any reaction in acetonitrile (AN) or 1,2-dichloroethane (DCE). On the other hand, direct irradiation of ACN with light of >400 nm in solution in the presence of TCNE gave cisoid-1 and transoid-1 as the major products together with a [2 + 2]-adduct (2) and two isomeric [2 + 2 + 2]-adducts (3 and 4) of ACN and TCNE as minor products. Distinction of photochemical reactivity between selective CT excitation and direct excitation of ACN can be attributed to faster backward electron transfer (BET) from the contact radical ion pair (CIP) on CT excitation than from the solvent-separated radical ion pair (SSIP) on direct excitation of ACN due to very low energy for BET, as low as 1.34 V. Effect of [TCNE] on quantum yield for the dimerization of ACN and on the cisoid/transoid ratio of the resulted 1 rationalizes the mechanism involving the singlet and triplet SSIP; the former tends to undergo BET, but the latter undergoes dissociation to ACN+•, followed by formation of dimeric radical cation of ACN, ACN2+•, finally leading to 1. A possible mechanism for formation of 3 and 4 is discussed on the basis of concentration dependence of ACN. Contrary to photochemical inertness of the CT complex in solutions, CT excitation of the 1:1 crystal of ACN and TCNE (ACN·TCNE) gave 2 as the sole product. The selective formation of 2 indicates that fixation of the two alkenic CC double bonds in ACN·TCNE separated by 3−4 Å in both the excited CT state and the resulted CIP retards the deactivation and BET but enables them to undergo cycloaddition.

Enantiodifferentiating photoisomerization of (Z )-cyclooctene and (Z,Z )-cycloocta-1,3-diene sensitized by chiral aromatic amides

Optically active benzene(poly)carboxamides with C2-symmetric pyrrolidine moieties have been employed as chiral sensitizers for the geometrical photoisomerizations of (Z)-cyclooctene (1Z) and (Z,Z)-cycloocta-1,3-diene (1ZZ) in some solvents at varying temperatures. ortho-Substituted di- and tetra-amides used as chiral sensitizers give enantiomeric excesses (ee) up to 14% for both 1Z and 1ZZ in pentane at low temperatures. The use of polar solvents dramatically diminishes the product’s ee, due to the intervention of a free or solvent-separated radical ion pair between 1 and the chiral sensitizer.

Dynamics of the Singlet Excited States of Diarylethenes in the Presence of Charge-Transfer Interactions. A Picosecond Laser Flash Photolysis Study

Charge-transfer interactions of singlet excited trans isomers of n-styrylnaphthalene (n-StN, with n = 1 and 2), 9-styrylanthracene (9-StAn), and n-styrylphenanthrene (n-StPh, with n = 1, 2, and 9) with electron donors (N,N-diethylaniline, DEA, and 4-bromo-N,N-dimethylaniline, BrDMA) and acceptors (p-dicyanobenzene, DCNB) were studied in polar and nonpolar solvent at room temperature by absorption picosecond laser flash photolysis. The transient absorption formed within the laser pulse, already attributed to S1 → Sn and S2 → Sn transitions, were replaced, in the presence of amine quenchers, by the absorption of the exciplex (nonpolar solvent) or of contact radical ion pairs (CRIP, polar solvent). The time evolution of these transients showed the formation of the solvent-separated radical ion pair (SSRIP) and then of the longer-lived absorption of triplet state and/or radical anions. In the presence of DCNB in polar solvent the evolution of the first excited singlet showed the formation of CRIP, SSRIP, and ...

Metal-Ion Detection by the Magnetic-Field-Sensitive Fluorescence of Intramolecular Exciplexes Containing Aza-Crown-Ether Moieties as Electron Donor

Magnetic field effects (MFE) detected by intramolecular exciplex fluorescence have been studied in acetonitrile solutions for new donor−acceptor compounds in which pyrene is linked to aza-15-crown ether by a polymethylene chain with 3, 9, and 16 CH2 units. It was found that addition of alkali and alkaline-earth ions to the solutions decreases the observable MFE. The longer the chain and the greater the stability constants of the aza-crown complexes with metal ions, the more pronounced is the influence of the added metal ions. The data obtained are rationalized on the basis of a kinetic scheme that specifically includes photoejection of metal ions from the crown-ether complexes, accompanied by Coulombic interaction of the ions released with the solvent-separated radical ion pairs, the so-called special salt effect.