Excited state annihilation

The previous examples of eel were interpreted on the basis of a relatively simple mechanism. In these cases the back electron transfer generates directly the emitting excited state (annihilation). However, in more complicated systems back electron transfer and formation of an emitting state may be separate processes... 

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. 

The excited-state decay kinetics of Ru(bpy)2(dcb) + -Ti02 immersed in neat acetonitrile, probed by transient absorption spectroscopy, exhibited nonexponential kinetics. By minimizing the excitation irradiance, near exponential kinetics were observed for excited-state decay. However, at high excitation irradiance, second-order kinetics were found to fit the experimental data well. These observations are consistent with competitive first- and second-order processes attributed to radiative and nonradiative excited-state deactivation, Eq. 21, proceeding in parallel with excited-state annihilation, Eq. 22 ... 

Early work showed that low light intensity excitation sources had to be used to avoid excited-state annihilation processes [193, 194]. Using a low-intensity mode-locked laser source and single photon counting, Muenter measured the rates of fluorescence decay, k, for some typical monomeric cyanine dyes on silver chloride and silver bromide cubic crystals [195] k was obtained from Eq. (108) for k, and assuming that kf + A nr) could be determined from measurements of dye lifetimes in... 

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. 

Many transition metal systems are open-shell systems. Due to the presence of low-energy excited states, it is very common to experience problems with spin contamination of unrestricted wave functions. Quite often, spin projection and annihilation techniques are not sufficient to correct the large amount of spin contamination. Because of this, restricted open-shell calculations are more reliable than unrestricted calculations for metal system. Spin contamination is discussed in Chapter 27. 

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. 

Cramer, C. J., Dulles, F. J., Giesen, D. J., Almlof, J., 1995, Density Functional Theory Excited States and Spin Annihilation , Chem. Phys. Lett., 245, 165. 

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)...

Much less is known about excited-state dynamics of carotenoid J-aggregates, as only zeaxanthin J-aggregates have been studied to date. Only two decay components of -5 and 30ps were needed to fit the kinetics recorded at the maximum of the Sj-S band, Figure 8.8. Since no annihilation studies were carried out, the origin of these components is not known. It is likely that the 5ps lifetime is due to annihilation whereas the 30 ps component corresponds to the. S, lifetime, which is even longer than that of the H-aggregates. 

In 1978 Wrighton and his group showed that the complex Re(o-phen)-(CO)-Cl undergoes eel from its lowest excited state which lies about +2.3 eV above the ground state (34). The annihilation is energy sufficient. The oxidation of the neutral complex occurs at E. - 1.3 V vs. SCE while the reduction takes place at -1.3 V. 

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... 

The fluorescence spectra measured just upon ablation are given in Figure 2A as a function of laser fluence. The contribution below 370 nm was suppressed, as a Hoya L37 filter was used in order to cut off the laser pulse. Fluorescence spectra of this polymer film consist of sandwich (max. 420 nm, lifetime 35 ns) and partial overlap (max. 370 nm, lifetime 16 ns) excimers (20). The latter excimer is produced from the initially excited monomer state, while the sandwich excimer from the partial overlap excimer and the monomer excited states. Since these processes compete with efficient interactions between identical and different excimers (Si - Si annihilation) (12), the sandwich excimer is quenched to a greater extent compared to the partial overlap one under a high excitation. Actually the fluence-dependent spectral change around the threshold can be interpreted in terms of Si - Si annihilation. 

This mechanism has been formulated in analogy to the known electrochemiluminescence, in which radical-ion annihilation generated at opposite electrodes leads to the formation of the electronically excited state (Scheme 2) . The difference between the CIEEL mechanism and electrochemiluminescence is that, in the former, the radical ions—whose annihilation is responsible for the formation of the excited state—are formed chemically by electron transfer to high-energy peroxides and subsequent bond cleavage or rearrangements. 

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... 

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