Excited state annihilation reaction

Oxidized and reduced species can be produced when the excimer or exciplex is formed in a polar medium. The excited state-excited state annihilation reaction is another bimolecular process transforming the excited-state energy. An excited state of a higher energy, A in Equation 6.81, or charge separation (Equation 6.82) can be produced in the annihilation process. 

FIGURE 12.6 Schematic of how lateral intermolecular energy transfer across the semiconductor surface can lead to a second-order excited-state annihilation reaction. First-order excited-state relaxation was observed for sensitizers with short excited-state lifetimes, t < 50 ns, and was predominant at low irradiances and surface coverages for all sensitizers. 

The decomposition of dioxetanone may involve the chemically initiated electron-exchange luminescence (CIEEL) mechanism (McCapra, 1977 Koo et al., 1978). In the CIEEL mechanism, the singlet excited state amide anion is formed upon charge annihilation of the two radical species that are produced by the decomposition of dioxetanone. According to McCapra (1997), however, the mechanism has various shortfalls if it is applied to bioluminescence reactions. It should also be pointed out that the amide anion of coelenteramide can take various resonance structures involving the N-C-N-C-O linkage, even if it is not specifically mentioned. 

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)...

An external magnetic field was observed to have practically no effect on the intensity of the thianthrene (430 nm) emission, indicating that no triplet states are involved. The shorter-wavelength emission of the oxadiazole 102, however, is probably due to a triplet-triplet annihilation reaction of diphenyloxadiazole triplets. These are produced in the radical-ion reaction between 101 and 102, yielding thianthrene excited-singlet molecules and diphenyl-oxadiazole excited-triplet molecules ... 

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. 

Two main CL pathways are then possible If sufficient energy is available, an electron transfer reaction where the singlet excited state of A is accessible (3) otherwise an energy-deficient route whereby the energy from two triplet excited-state species are pooled, to provide sufficient energy to form the singlet excited state, in what is termed a triplet-triplet annihilation reaction, (4) and (5). 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

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... 

Some years later, at the beginning of the 1970s, first ECL system based on the luminescent transition metal complex tris(2,2 -bipyridine)ruthenium(II)-Ru (bipy)32 + -has been reported.11 It was shown that the excited state 3 Ru(bipy)32 + can be generated in aprotic media by annihilation of the reduced Ru(bipy)31 + and oxidized Ru(bipy)33 + ions. Due to many reasons (such as strong luminescence and ability to undergo reversible one-electron transfer reactions), Ru (bipy)32+ later has become the most thoroughly studied ECL active molecule. 

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. 

Consequently, the excited 3 Ru(bipy)32+ state can be produced via three different routes (i) Ru(bipy)3+ oxidation by TPrA"+ cation radical, (ii) Ru(bipy)33+ reduction by TPrA" free radical, and (iii) the Ru(bipy)33 + and Ru(bipy)3 + annihilation reaction. The ECL intensity for the first and second waves was found to be proportional to the concentration of both Ru(bipy)32+ and TPrA species in a very large dynamic range with reported detection limits as low as 0.5 pM155 for Ru(bipy)32+ and 10 nM156 for TPrA. In addition to Ru(bipy)32+, many other metal chelates and aromatic compounds or their derivatives can produce ECL in the presence of TPrA as a coreactant upon electrochemical oxidation (cf. Chapter 4 in the Bard s ECL monograph.32). 

This complex has been widely used in sensing applications since both radical ions of the complex are relatively stable to decomposition reactions. Many systems using this chromophore exist in which ECL is produced at a single electrode via coreactant oxidation or reduction schemes as discussed in the first segment of this section [Eqs. (5) through (9)]. For example, the reduction product of the peroxydisulfate dianion, S2Og, can function as an oxidant in the ECL reaction by annihilation with the electrochemically generated Ru1+ to yield the MLCT excited state of the Ru(II) complex by the mechanism [24] ... 

A question that arises in consideration of the annihilation pathways is why the reactions between radical ions lead preferentially to the formation of excited state species rather than directly forming products in the ground state. The phenomenon can be explained in the context of electron transfer theory [34-38], Since electron transfer occurs on the Franck-Condon time scale, the reactants have to achieve a structural configuration that is along the path to product formation. The transition state of the electron transfer corresponds to the area of intersection of the reactant and product potential energy surfaces in a multidimensional configuration space. Electron transfer rates are then proportional to the nuclear frequency and probability that a pair of reactants reaches the energy in which they have a common conformation with the products and electron transfer can occur. The electron transfer rate constant can then be expressed as... 

It is also possible for species that are created in ECL reactions to interact with each other in ways that interfere with the generation of ECL and partially, if not completely, quench the emission. For example, one difficulty in direct sensing of coreactants is that the coreactant may also quench the luminescence of the excited state generated in the annihilation process. This difficulty was recognized several years ago by Bard and coworkers in the examination of the [(bpy)3Ru]2+/S20g system [24], Luminescence arises upon reduction of the Ru(II) complex and reduction of S20g mediated by the Ru(I) complex formed. The intermediate SOJ ion formed is a powerful oxidant and annihilation with the Ru(I) complex will yield the excited state of Ru (II) complex [Eq. (13d)]. However, the persulfate ion is an effective quencher of the MLCT excited state of the Ru(II) complex. Figure 9 shows the observed ECL intensity for this system... 

It should be mentioned here that the processes which are involved in the appearance of an eel of Pt2(pop) - are associated with changes in the metal-metal bonding of this binuclear complex (38-40, 42,44,46,47). The Pt-Pt bond order which is zero in the ground state is increased to 0.5 by oxidation as well as by reduction. The annihilation reaction leads to the formation of Pt2(pop) as the ground (bond order = 0) and excited state (bond order =1). A related case which was reported quite recently is the eel of Mo Cl The metal-... 

In the presence of an excess of TMPD the complex is still reduced, but TMPD is oxidized during the electrolysis. Since the oxidation potential of TMPD is much lower than that of the complex, the annihilation reaction of the complex anion and TMPD+ does not provide enough energy to generate the complex in the excited state. Quite an analo-... 

This is a stepwise function that approaches unity with increasing AG , because the excited state becomes the unique reaction product when the recombination to the ground state is switched off. This function, shown in Figure 3.54, resembles the experimental results obtained in Refs. 191-193. A typical example of such data is shown in Figure 3.55. However, the plateau approached by most of the curves obtained experimentally is lower than 1. This can be an indication of some unknown channel of charge recombination or additional quenching of excitations by either the survived ions or through biexcitonic annihilation of triplets [194] (see Section XIII.C). 

The free energy of the annihilation reaction can be evaluated from the one-electron potentials for oxidation and reduction of the species involved and the zero-zero emission energy, Eqq, of the luminescent product. The excited state formed can be either singlet or triplet spin multiplicity (or both). If strict spin conservation is observed in the process, the products will contain 75 % triplet and 25 % singlet however, hyperfine interactions provide alternate avenues for angular momentum conservation [13, 14]. 

The constants of the equation were obtained from simulations and the process applies only to generation of singlet excited states at diffusion-limited annihilation rates. Nonetheless, the expression provided an experimental approach for determining efficiencies of the production of emitting excited states in annihilation reactions. Simulations for systems that react via triplet formation and subsequent triplet-triplet annihilation were also developed [40b] and they illustrated that the two mechanisms can be distinguished by analysis of intensity-time profiles. 

If the exothermicity of the annihilation of the given ions is still smaller than the energy of the excited triplet states, the reaction is generally not of interest, although it has been shown that such systems may still produce light. This last case, however, corresponds formally to radiative electron transfer from R to R+ (the E-route). It should be described in terms of the competition between radiative and radiationless transition in the inverted Marcus region. 

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