Return Electron Transfer Research Articles

Radical cations and triplet states of 1,2-disubstituted cyclopropanes: comparison of potential surfaces

Radical ion pairs generated by photo-induced electron transfer from 1,2-disubstituted cyclopropanes to various acceptors undergo return electron transfer in pairs of singlet and triplet multiplicity. The pair energies relative to the reactant ground states and to accessible triplet states, respectively, determine the competition between the recombination pathways. The potential surfaces of the radical cations and triplet states of 1,2-diphenyl-, 1, and 1,2-dimethylcyclopropane, 2, have been examined by density functional theory calculations. The radical cation surfaces have minima at geometries that retain significant bonding between C-1 and C-2, preventing geometric isomerization of the radical cations. The triplet potential surfaces are dissociative with minimal rotational differentiation at long distances between C-1 and C-2.

Photoinduced charge shift and charge recombination through an alkynyl spacer for an expanded acridinium-based dyad

The photophysical and electrochemical properties of a dyad comprising an expanded acridinium light-harvester, coupled via a triple bond to a trialkyloxybenzene donor, are described. Laser excitation of the dyad in 1,2-dichloroethane (DCE) results in rapid charge shift (5 ps) followed by slower charge recombination (81 ps) to completely restore the ground state. Discrimination between forward and return electron transfer (k(ret)/k(for) ~ 16) is rather poor. There is no indication of triplet formation on the expanded acridinium-based group following charge recombination.

Activation of aliphatic C–H bonds by tetracyanobenzene photosensitization. A time-resolved and steady-state investigation

The photochemistry of 1,2,4,5-tetracyanobenzene (TCB) in acetonitrile in the presence of representative aliphatic donors (cyclohexane, 1,4-dioxane and triethylamine) has been investigated by femtosecond (fs) and nanoseconds (ns) flash photolysis as well as steady state irradiation in order to define the kinetic frame for the activation of aliphatic C–H bonds. Unlike in the case of aromatics, no ground state complex (except possibly in the case of triethylamine) or exciplex is formed. The lowest excited singlet is quenched and forms the free radical ions. The efficiency of the process, measured by the yield of TCB˙− (0.84 with cyclohexane, 0.15 with 1,4-dioxane, 0.37 with triethylamine) depends on the rate of return electron transfer, low for a high exothermicity. The chemical reaction following the electron transfer step depends on the properties of the radical cation. Thus, with cyclohexane deprotonation is slow and does not occur measurably on a microsecond scale, while on a longer time scale, the formation of alkyl radicals competes with back electron transfer between the free ions. The deprotonation of amine radical cations is faster and the resulting α-aminoalkyl radicals reduce a further molecule of TCB, causing the accumulation of the radical anion which is indefinitely stable in the absence of oxygen.

ChemInform Abstract: Photochemical Generation, Isomerization, and Oxygenation of Stilbene Cation Radicals.

The cation radicals of cis- and trans-stilbene and several of their ring-substituted derivatives have been generated in solution directly by means of pulsed-laser-induced electron transfer to singlet cyanoanthracenes or indirectly via electron transfer from biphenyl to the singlet cyanoanthracene followed by secondary electron transfer from the stilbenes to the biphenyl cation radical. Transient absorption spectra of the cis- and trans-stilbene cation radicals generated by secondary electron transfer are similar to those previously obtained in 77 K matrices. Quantum yields for radical ion-pair cage escape have been measured for direct electron transfer from the stilbenes to three neutral and one charged singlet acceptor. These values increase as the ion-pair energy increases due to decreased rate constants for radical ion-pair return electron transfer, in accord with the predictions of Marcus theory for highly exergonic electron transfer. Cage-escape efficiencies are larger for trans- vs cis-stilbene cation radicals, possibly due to the greater extent of charge delocalization in the planar trans vs nonpolar cis cation radicals. Cage-escape stilbene cation radicals can initiate a concentration-dependent one way cis- {yields} trans-stilbene isomerization reaction.

Off the Back or on the Side: Comparison of meso and 2‐Substituted Donor‐Acceptor Difluoroborondipyrromethene (Bodipy) Dyads

AbstractThe preparation of several difluoroborondipyrromethene (Bodipy) dyads is described incorporating covalently attached hydroquinone/quinone groups at the 2‐position (BD‐SHQ, BD‐SQ, BD‐SPHQ, BD‐SPQ). The compounds, currently under investigation as chemical sensors for reactive oxygen species, show various levels of fluorescence depending on the oxidation state of the appended group. The 19F NMR spectrum for BD‐SHQ in CDCl3 at room temperature reveals the two fluorines are inequivalent on the NMR timescale. In contrast, the 19F NMR spectrum for the counterpart quinone compound, BD‐SQ, is consistent with two equivalent fluorine atoms. The two results are interpreted as the quinone is free to rotate around the connector bond, whereas this motion is restricted for the hydroquinone group and makes the fluorines chemically inequivalent. Cyclic voltammograms recorded for all derivatives in CH2Cl2 electrolyte solution are consistent with typical Bodipy‐based redox chemistry; the potentials of which depend on factors such as presence of the phenylene spacer and oxidation state of the appended group. A comparison of the electrochemical behaviour with the counterpart meso derivatives reveals some interesting trends which are associated with the location of the HOMO/LUMOs. The absorption profiles for the compounds in CH3CN are again consistent with Bodipy‐based derivatives, though there are some subtle differences in the band‐shapes of the closely‐coupled systems. In particular, the absorption spectra for the dyad, BD‐SQ, in a wide range of solvents are appreciably broader than for BD‐SHQ. Femtosecond transient absorption spectroscopy performed on the hydroquinone derivatives, BD‐SHQ and its meso analogue is interpreted as electron transfer occurs from the hydroquinone unit to the first‐excited singlet (S1) state of the Bodipy center, followed by ultrafast charge recombination to reinstate the ground state. The coupling of OH vibrations to the return electron transfer process is invoked to explain the lack of clear identification of the charge‐separated state in the transient records.

Pyrylogens: Synthesis, Structural, Electrochemical, and Photophysical Characterization of a New Class of Electron Transfer Sensitizers

The synthesis and photophysical properties of a new series of dicationic electron transfer sensitizers have been reported. These new materials, pyrylogens, are hybrids of pyrylium cations and Viologen dications. Electron transfer reactions of neutral organic substrates using these new sensitizers generate radical-cation/radical-cation pairs whose repulsive (repellent) interaction is designed to compete with energy wasting return electron transfer (RET) by enhancing diffusive separation and formation of solvent separated ions.

Biradicals by triplet recombination of radical ion pairs

Radical ion pairs generated by photoinduced electron transfer may undergo return electron transfer (RET) in pairs of singlet or triplet multiplicity. RET efficiencies are determined by the free energy of RET and the topologies of the potential surfaces of parent molecule, radical ion and triplet state. If radical ion geometries are different from the corresponding triplet states, RET occurs either with cleavage ("dissociative" RET; 1,2-diphenylcyclopropane radical cations) or formation of C-C bonds ("associative" RET; norbornadiene radical cation). Radical ions of some strained ring compounds spontaneously undergo ring-opening; RET to such species form ring-opened triplets without major geometry changes. CIDNP spectroscopy offers unique insights into triplet RET.

Return Electron Transfer in Radical Ion Pairs of Triplet Multiplicity

AbstractFor Abstract see ChemInform Abstract in Full Text.

Bond-Coupled Electron Transfer Processes: Cleavage of Si−Si Bonds in Disilanes

Photooxidation of 1,1,2,2-tetra-tert-butyl-1,2-diphenyldisilane, 1, by triplet sensitizers in CHCl3/CCl4 solutions yields chlorosilane, 2, in high chemical yield. The quantum yield for formation of 2 depends on the energy of the ion radical pair formed following initial electron transfer. Dissociative return electron transfer (DRET) is proposed as the mechanism for the highly efficient Si-Si bond cleavage in 1. DRET may be a useful strategy for the fragmentation of other such bonds in di-, oligo-, and polysilanes as well as other group 4A compounds using a variety of sensitizers with different spectral properties.

SOLVENT REORGANIZATION ENERGY WITH DIELECTRIC GREEN FUNCTIONAL AND ITS APPLICATION TO RETURN ELECTRON TRANSFER IN TETRACYANOETHYLENE-HEXAMETHYLBENZENE SYSTEM

Based on classical electrodynamics, the proper integral formula for general cases is adopted to estimate the electrostatic free energy difference between two given states, e.g. equilibrium and non-equilibrium states in the charging process of solute molecule in solution. With adequate consideration of the change of dielectric property in establishing the non-equilibrium state, the electrostatic free energy for this state can be obtained in a form differing from the traditional theories. The general formula of the solvent reorganization energy is derived according to the definition of the free energy difference between the non-equilibrium and equilibrium states due to the same electronic state of the solute molecule. Applying it to the two-sphere model and single-sphere model, we obtain the new expressions of the solvent reorganization energy, which are convenient to be used in practice. In the investigation of the return electron transfer of tetracyanoethylene-hexmethylbenzene system, good results can be obtained by using our expression when they are compared with those from the experiment.

Doublet−Quartet Intersystem Crossing and Electron Transfer of Cobalt(II) Moieties in [LRuIII(L−L)CoIIL]5+ (L = 2,2‘:6‘,2‘ ‘-Terpyridine and L−L = A Bridging Ligand)

Return electron transfer (RET) and intersystem crossing (ISC) of [(tpy)RuIII(tpy-ph-tpy)2,4CoII(tpy)]5+ (tpy = 2,2‘:6‘,2‘ ‘-terpyridine and tpy-ph-tpy = 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene) and [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy-tpy = 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarter-pyridyne) produced on the subpicosecond laser excitation have been investigated in a wide temperature range. A biexponential recovery process of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ and [CoIII(t)] = [2CoII(0)](1 − [fα exp(−kαt) + fβ exp(−kβt)]) occurs immediately after the production of [2RuIII(tpy)(tpy-pph-tpy)2CoII(tpy)]5+ in the excited-state electron-transfer quenching of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+. Four parameters of biexponential recovery, kα, fα, kβ, and fβ in a range of 180−300 K, are interpreted in terms of RET and ISC reactions among three states: [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+, [(tpy)2RuIII(tpy-tpy)4CoII(tpy)]5+, and [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+. RET of [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+ regenerating [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ with kRET(D) of ∼4 × 1010 s-1 competes with doublet−quartet ISC with kQD of 0.20 × 1010 s-1 at of 0.044 eV and 2.8 × 1010 s-1 at of −0.01 eV. Meanwhile, the rate constant of the quartet−doublet ISC, kDQ (∼1.6 × 1010 s-1at of −0.01 eV), is much larger than RET of [(tpy)2RuIII(tpy-ph-tpy)4CoII(tpy)]5+. Dependences of kQD on give rise to a rough estimation of reorganizational free energy (0.03 eV). As for [(tpy)RuIItpy-tpy)CoIII(tpy)]5+, kRET(D), kDQ, and kQD are evaluated to be 2.3 × 1011 s-1, 1.5 × 1011 s-1 and 1.5 × 1011 s-1 at of −0.01 eV.

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.

Associative return electron transfer. A bond-coupled electron transfer in the photoreactions of cyclopropylamines

The dynamics of the geminate radical-ion pairs formed by electron transfer to the excited states of cyanoanthracenes from 2-phenylcyclopropylamines are dominated by exothermic bond cleavage of the amine radical cations. Quantitative studies of product formation as a function of the energetics of the photochemical and corresponding thermal reactions provide support for a novel mechanism in which return electron transfer in the geminate pair occurs in concert with bond formation from the ring-opened radical cations. This bond-coupled electron transfer process is referred to as an associative return electron transfer reaction. The important features of the associative electron transfer process that explain the experimental observations are described in terms of potential energy surfaces and competition between adiabatic and non-adiabatic deactivation paths.

Kinetics of Isomerization via Photoinduced Electron Transfer. I. Spectral Analysis and Structural Reorganization of Hexamethyl Dewar Benzene Exciplexes

It is well recognized that emission spectra from the reactions of excited-state electron acceptors (A*) with hexamethyl Dewar benzene (D) are typically dominated by fluorescence from exciplexes of its valence isomerization product hexamethylbenzene (B) (i.e., A•-B•+). We were able to obtain well-defined emission spectra of the A•-D•+ exciplexes with several cyanonaphthalenes as the excited-state electron acceptors by subtraction of the dominant A•-B•+ fluorescence from the total emission. Interestingly, a comparison of band shapes and maxima between the exciplexes of D and B reveals that the reorganization energy for return electron transfer in A•-D•+ is much larger than in A•-B•+ (∼1.2 vs 0.57 eV). Furthermore, a comparison between exciplexes of D and of 1,2-dimethylcyclobutene as a model compound showed that a greater reorganization energy is associated with return electron transfer from the A•-D•+ exciplexes. DFT calculations identified much of the “excess” reorganization energy in A•-D•+ with a change in the dihedral angle (flap angle) between the two cyclobutene moieties, which is ∼12° smaller in D•+ than in D. Approximately one-fourth of the total reorganization energy of ∼1.2 eV for the D exciplexes is due to this angle deformation. Spectra of the exciplexes of B and of model olefins were analyzed by using a familiar two-mode model whereas those of D required a three-mode model to account for the intermediate frequency (491 cm-1) mode associated with the flap angle deformation. Remarkably, although the driving forces for return electron transfer are nearly identical for the exciplexes of D and B with the same acceptor, the rate constants for nonradiative return electron transfer are predicted to be 4 to 5 orders of magnitude greater for the A•-D•+ exciplexes because of their larger reorganization energies.

Intersystem Crossing in Charge-Transfer Excited States

Absolute rate constants for intersystem crossing (ISC) in exciplexes formed between cyanoanthracenes as acceptors and alkylbenzenes as donors in solvents with low to moderate polarity have been measured. The extent of charge transfer in these particular exciplexes has been determined previously, and thus the dependence of the ISC rate constant on this parameter is obtained. A smooth transition of exciplexes with high locally excited state character to those with essentially pure charge-transfer character argues in favor of a mechanism of intersystem crossing involving a spin-forbidden return electron transfer process to form the locally excited triplet directly. The proposed mechanism is supported by the observed energy gap dependence of the rate constant, and by deuterium isotope effects. External heavy-atom effects in chlorinated solvents are observed. Analysis of the rate constant for intersystem crossing for the pure charge-transfer exciplexes using electron-transfer theory allows determination of the matrix element for the spin-forbidden electron-transfer reaction. It is found to be roughly 3000 times smaller than the matrix element for corresponding spin-allowed electron transfer in similar systems. General principles for the factors that control the mechanisms of intersystem crossing in excited charge-transfer states are discussed.

Temperature-Independent Rate of Electron-Transfer between a Cobalt(II) and a Ruthenium(III) of Doublet Electronic Configuration

A study of laser kinetic spectroscopy on [RuII(tpy)(BL)CoIII(tpy)]5+ (BL: 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene (btbz) and 6‘,6‘ ‘-bis(2-pyridyl)- 2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarter-pyridine (bpqp)) revealed rise-and-decay of a triplet Ru-to-ligand charge-transfer state, 3CT(Ru), and an electron-transfer product of [(tpy)RuIII(BL)CoII(tpy)]5+. 3CT(Ru) underwent electron-transfer with a rate constant of 0.3 × 1012 and 1 × 1012 s-1 to produce [(tpy)RuIII(BL)CoII(tpy)]5+ with a doublet electronic configuration dπ6dσ* of cobalt(II). The return electron transfer (RET) of [RuIII(tpy)(BL)CoII(tpy)]5+ (BL:btbz and bpqp) in acetonitrile occurred with a rate constant of 1 × 1010 and 6 × 1010 s-1 at 298 K, respectively, which are much faster than intramolecular RET of [(bpy)2Ru(BL)Co(bpy)2]5+ from cobalt(II) with a quartet electronic configuration of dπ5dσ*2. The magnitude of reorganization energy (0.95 ± 0.15 eV) and electronic coupling matrix element (0.8 and 2 meV) are evaluated from the independence of RE...

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).

Two-Electron Sensitization: A New Concept for Silver Halide Photography

The primary process in conventional photography involves electron transfer from an excited dye molecule into the conduction band of a silver halide microcrystal. Repeated events of this type ultimately lead to formation of a small, stable cluster of silver atoms in the silver halide that acts as the latent image, along with the one-electron oxidized forms of the dye molecules. Here we describe a new concept for increasing the efficiency of photographic systems, two-electron sensitization, which makes use of the chemical potential stored in the oxidized dyes. In conventional photography, subsequent reactions of the oxidized dyes are not controlled and may in fact include counterproductive return electron transfer reactions (recombination). In the two-electron sensitization scheme, an appropriately designed electron donor molecule, X−Y, that is added to the photographic dispersion transfers an electron to the oxidized dye to give a radical cation, X−Y•+. The X−Y•+ then undergoes a fragmentation reaction to give a radical, X•, and a stable cation, Y+. The radical X• is chosen to be sufficiently reducing so that it can inject an electron into the silver halide conduction band. In this way, the oxidized dye, which is a strong oxidant, is replaced by the radical, X•, which is a strong reductant. The two-electron transfer scheme has the potential of doubling the photographic speed because two electrons are injected per absorbed photon. Here we describe the mechanistic details of the two-electron sensitization scheme and the structural and energetic criteria for the X−Y molecules. Several electron-rich carboxylate molecules that meet these criteria have been identified. Solution-phase experiments to determine the fragmentation (decarboxylation) kinetics and the reducing power of the resultant radicals are described. Photographic data demonstrate that increases in sensitivity by factors approaching 2 can be obtained, confirming the viability of the two-electron sensitization concept.

Ultrafast dynamics of the excited tetracyanoethylene–toluene electron donor–acceptor complex

The ultrafast spectroscopy of the electron donor–acceptor complex of tetracyanoethylene with toluene has been investigated in several polar and non-polar solvents. After ultrafast excitation of the charge transfer band, formation and then decay of the contact ion pair is observed. The decay time of the contact ion pair due to charge recombination is, in most cases, longer than the solvation time and is observed on a time scale from 1 ps to 1 ns, depending on the solvent. Using a conventional non-adiabatic theory of electron transfer, absolute rate constants for the non-radiative return electron transfer, varying over three orders of magnitude, can be predicted from limited information obtained from the analysis of the radiative processes in non-polar solvents only. The model fails in cases where solvent relaxation is slower or comparable to the rate of the return electron transfer.

Photochemistry of phthaloylcysteine, its methyl ester and C-unprotected S-alkyl derivatives

N-Phthaloyl cysteine derivatives 1a-d were photochemically transformed by elimination, decarboxylation, and via electron transfer cyclization to the products 2,3,4 and 6–8. The spin selectivities of singlet and triplet pathways were investigated in acetonitrile and acetone. The excited singlets were prone to elimination and γ-H abstractions (e.g. formation of 5) whereas the triplets cyclized to thiazinoisoindoles. This behaviour can be correlated with the efficiencies of forward and return electron transfer steps versus homolytic hydrogen abstractions as examplified for the cysteine substrate 9.