Anion-cation annihilation

C. Anion-Cation Annihilation and Charge-Transfer Luminescence 433... 

If —AH° is nearly marginal to E,., the T-route can contribute to the formation of A in addition to the S-route, and the reactions are said to follow the ST-route. A typical system with such a route is the rubrene anion-cation annihilation (115-117). 

This association has its counterpart that was also variously described as an encounter complex, a nonbonded electron donor-acceptor (EDA) complex, a precursor complex, and a contact charge-transfer complex.10 For electrically charged species such as anion/cation pairs (which are relevant to ion-pair annihilation), the pre-equilibrium association results in contact ion pairs (CIP)7 (equation 3)... 

Feldberg68,69 has made a valuable analysis of the relationship of the light produced in a double potential step electrochemiluminescence experiment to the current, time, and kinetic parameters involved. The analysis presumes that the reaction which produces excited states is cation-anion radical annihilation which occurs when the radical ions, separately produced, diffuse together in the solution near the electrode. The processes that Feldberg initially considered were eqs. (7)—(13). The assumptions involved are that decay of the excited state... 

The Type 3 SN2 reaction between Cl- + CH3SHf is interesting because it represents a formal anion-cation recombination through substitution. Because charges are annihilated in forming the transition state, polar solvents will significantly destabilize product formation. Fortunately, the loss in solvation of the two ions is compensated for by electrostatic attractions in bringing the two reactant species into contact. Therefore, the outcome of an SN2 reaction of Type 3 depends on the balance of Coulomb stabilization and solvent destabilization. The reactant and product diabatic states are defined as follows in MOVB theory ... 

The formation of the radical anion/cation in the diffusion layer is followed by an annihilation reaction. If there is sufficient energy in this electron transfer to populate the excited state of the hydrocarbon, luminescence results. The energy available is determined by the redox potential of the ions. 

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

Therefore, the sequence of reactions illustrated in Fig. 1 catalytically (the anthraquinone is regenerated) injects a radical cation into a DNA oligonucleotide that does not simultaneously contain a radical anion. As a result, the lifetime of this radical cation is determined by its relatively slow bimolecular reaction with H20 (or some other diffusible reagent such as 02- ) and not by a rapid intramolecular charge annihilation reaction. This provides sufficient time for the long distance migration of the radical cation in DNA to occur. 

Ionic polymerizations are generally much faster than radical polymerizations. Both cationic and anionic polymerizations typically proceed with much higher concentrations of propagating centers (10r -10 2 molar) than in radical polymerizations (lpropagating centers do not annihilate each other as do radicals. 

According to Hercules 5> a measure of the relationship between direct excitation of the first excited singlet state by radical-ion recombination and triplet-triplet annihilation is the entropy factor FAS, estimated to be on average 0.2 eV. The enthalpy of the radical cation-radical anion recombination can be measured as the difference between the redox potentials 1/2 Ar—Ar (oxidation) and 1/2 Ar—Ar<7> (reduction). This difference has to be corrected by the entropy term. If this corrected radical-ion recombination enthalpy is equal to or larger... 

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

Thermal annihilation of carbonylmetallate anions by carbonylmetal cations... 

In one of the first experimental studies where ion radical annihilation in solution was considered as an emissive possibility, Yamamato, Nakato, and Tsubomura61 found that Y,Y,Y, Y -tetramethyl-p-phenyl-enediamine (TMPD) and pyrene when irradiated in the ultraviolet in a glass at low temperature formed Wurster s blue cation radical, pyrene anion radical, and solvated electrons. When the glass was warmed, thermoluminescence was observed. A similar emission was observed when a previously irradiated mixture of TMPD and 2-methylnaph-thalene was warmed. The emission in both instances was ascribed to charge-transfer fluorescence resulting from combination of a cation radical with an anion radical. 

In a suggested alternative mechanism, lower energy triplets produced by the cation-anion annihilation produce fluorescent singlets by triplet-triplet annihilation.73 This possible pathway cannot be overlooked on two accounts. First, it is known from phosphorescence emission that triplets are produced in these systems13,15,60 and second, the double potential step data analyzed by the Feldberg method seems to be in agreement with such a double annihilation mechanism.65 If triplets are initially produced in high local concentrations by the redox reaction in... 

At this point it is necessary to consider the mechanism of electron-transfer luminescence in solutions which cannot involve ion-radical annihilation because both cation and anion of the fluorescer are not formed. Such emission can be achieved by treating anion radicals with chemical oxidants or electrochemically under conditions where the corresponding cation cannot be produced, and it may also be achieved by electrochemical reduction of cations without producing the corresponding anion. In addition to triplets, three types of processes and pathways have been proposed to help explain why such emission occurs. These may be described as (7) impurities, (2) ion-radical aggregates, and (5) heterogeneous electron transfer. It is evident63 that impurities,... 

It is emphasized that the terms excimer2 and exciplex3,4 are reserved here for homomolecular and heteromolecular excited double molecules formed after the act of light absorption by one component in a process of photoassociation, in the absence of spectroscopic or cryoscopic evidence for molecular association in the ground state. Recent findings indicate that excimer (or exciplex) formation may also result from triplet-triplet annihilation,5,8 cation-anion combination7 (doublet-doublet-annihilation), and electron capture by the (relatively stable) dimer (or complex) cation8 these processes are discussed in Section VII. 

ECL emission has been also observed in the mixed ECL systems involving PAHs with reaction partners like aromatic amines or ketones forming radical cations D + or radical anions A-, respectively.114127 Such approach solves the problems caused by the instability of ECL reactants but lowers distinctly the free energy available for the formation of an excited state. Usually, the energy released in electron transfer between A- + D + ions is insufficient to populate emissive 11A or D states directly and the annihilation of the radical ions usually generates only nonemissive3 A or 3 D triplets that produce light via triplet-triplet annihilation. Consequently ECL efficiencies in the mixed ECL systems are usually very low. Only in some cases, when radiative electron transfer between A + D+ species is operative, relatively intense [A D + ] exciplex emission can be observed. 

As can be seen from Table X, bright eel emission is observed only when both radical-anion and radical-cation are of moderate stability. The mechanism of the eel emission has been studied in some detail. A cation-anion annihilation delivers insufficient energy to reach the excited singlet state directly. Probably, ion-radical aggregates are involved and multiple electron transfer results in sufficient energy accumulation. 

This chemoluminescence results from interaction of 156 (generated from 155 under thermal conditions) and 1,3-DIBF (formed in a minor amount from 156). The first step is the formation of an encounter complex. Electron transfer generates a peroxide radical anion of 156 and a radical cation of 1,3-DIBF. Cleavage of the 0-0 bond in the radical anion of 156 forms an o-dibenzoylbenzene radical anion. Annihilation of the oppositely charged ions gives an excited singlet of 1,3-DIBF (with subsequent fluorescence) (82JA1041). 

In organic ECL reactions, the luminescent species are generally derivatives of polyaromatic hydrocarbons where A and B in Eqs. (1) through (4) can be either the same species (leading to self-annihilation) or two different PAHs with either being the analyte (mixed system). Some examples of both self-annihilation and mixed system ECL reactions of organic molecules are listed in Tables 1 and 2. One well-studied example is the self-annihilation reaction between the anion and cation radicals of 9,10-diphenylanthracene (DPA) via an S-route in acetonitrile resulting in blue fluorescence characteristic of DPA [17] ... 

Figure 1 Proposed mechanism for the generation of the radical anion and radical cation of perylene diimide and self-annihilation of the two to yield the triplet excited state.

Pt2(P205H2)4, also known as Pt2(POP)4, in the presence of tetrabutylam-monium (TBA) salts as electrolyte [19,20], ECL has also been observed from palladium and platinum a, 3,y,5-tetraphenylporphyrin complexes, and also via self-annihilation of the electrogenerated anion and cation radicals [21],... 

By annihilation of a radical cation and a radical anion neutral species are formed and light is emitted. Reduction and oxidation of the neutral compounds regenerates the radical ions for a new cycle. By fast repetition of this cycle, e.g., via electrolysis on an AC-line, continous conversion of current into light should be possible. 

Bard etaL 5S6>5571 and Visco etaL 558) have quantitatively analyzed the intensity of pulsed ECL of 9,10-diphenylanthracene, tetraphenylpyrene and rubrene. By computer simulation of the electrode process and the subsequent chemical reactions the rates for chemical decay of the radical ions could be determined. Weaker ECL with fluorescence emission 559 or electrophosphorescence S60) occurs if the radical anion R - reacts with a dissimilar radical cation R,+ of insufficient high oxidation potential to gain enough energy for fluorescence emission, that is, if ht fluorescence) >23.06 (Ej >+. -Ej -.), e.g., in the annihilation of the anthracene radical anion with Wurster s blue. For these process the following schemes are assumed (Eq. (242) ) ... 

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