Mechanisms of Excited State Formation

FIGURE 4.3 Various processes leading to excited state formation and their inverses. See text for explanation. From Brocklehurst (1970). 

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V.5 Mechanism of Excited State Formation from Dioxetans... 

Quendiii of Exdted States.—Much research has been adapted from early observations that micelles could drastically alter the fluorescence lifetime of pyrene and the efficiency of excimer formation. The various mechanisms of excited-state transformation have been reviewed. " The polarity of the micellar surface may be estimated from the fluorescence spectrum of pyrene 2-carboxaldehyde " giving rise to the conclusion that sodium laurylsulphate (e=45) has a more polar surface than cetyltrimethylammonium bromide (e = 18). The fluidity of the site of solubilization may be estimated from fluorescence polarization " in surfactants of general structure (104). The Si... 

Figure 3.19 Resolution of the concerted versus stepwise mechanism of excited-state double proton transfer in doubly hydrogen-bonded dimer of 7-azaindole [90, 91]. Schematic presentations of (a) the concerted, one-step, synchronous double proton transfer in 7-azaindole dimer and (b) the stepwise proton transfer following formation...

Ashokkumar M, Vu T, Grieser F (2004) A quest to find the mechanism for the formation of excited state metal atoms during acoustic cavitation. Proc 18th Int Congr Acoust 4 2935-2936... 

On the basis of mechanistic studies, mainly on these isolable cychc four-membered peroxides (1 and 2), two main efficient chemiexcitation mechanisms can be defined in organic peroxide decomposition (i) the unimolecular decomposition or rearrangement of high-energy compounds leading to the formation of excited-state products, exemplified here in the case of the thermal decomposition of 1,2-dioxetane (equation i)". 5,i9. 

The CIEEL mechanism has been utilized to explain the catalyzed decomposition of several cyclic and linear peroxides, including diphenoyl peroxide (4), peroxyesters and 1,2-dioxetanones. Special interest has focused on this mechanism when it was utilized to explain the efficient excited state formation in the chemiexcitation step of the firefly s luciferin/luciferase bio luminescence. However, doubts have been voiced more recently about the validity of this mechanistic scheme, due to divergences about the... 

This biradical-like concerted mechanism, in which the kinetic features reflect the biradical character and the formation of excited-state products can best be rationalized by the concerted namre of the complex reaction coordinate, was proposed to optimally reconcile the experimentally determined activation and excitation parameters of most 1,2-dioxetanes studied and has been called the merged mechanism . Specifically, bofh fhermal sfabil-ity and singlel and friplef quanfum yields in fhe series of mefhyl-subsliluled 1,2-dioxelanes, including fhe parenf 1,2-dioxefane" , could be readily rationalized on the basis of the merged mechanism and qualitative quanmm mechanics considerations . 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

Further examination of "reductive oxidation" ECL using polyaromatic compounds in non-aqueous media has revealed three signific ant features of the luminescence mechanism (10). First, the cyclic voltammograms fojj the reduction of the polyaromatic compounds in the presence of S2O8 were of a highly catalytic type. Second, the efficiency of ECL was qualitatively dependent on the stability of the aromatic radical cation rather than of the aromatic radical anion. Third, the importance of the aromatic radical cation ion in the mechanism for the formation of excited states was illustrated using a tertiary reactant system. The results of these studies are summarized below. 

Most compounds which undergo a photochemical reaction do not simultaneously show photoluminescence. It is then more difficult to prove that a reaction induced by ac electrolysis proceeds via the intermediate formation of excited states. A different mechanism may be in operation. In this case the chemical transformation occurs in the reduced and/or oxidized form. The back electron transfer merely regenerates the charges of the starting compound ... 

As discussed above, a chemical transformation which occurs during the ac electrolysis does not require the intermediate formation of excited states. The chemical reaction may take place in the reduced and/ or oxidized form of a compound. Nevertheless, in this case the electrolysis may still lead to the same products as those of the photolysis due to the obvious relationship between electronic excitation and redox processes. It will be then quite difficult to elucidate the mechanism of electrolysis. This reaction type may apply to the electrochemical substitution of Cr(CO) (59). 

The ES-mechanism of Frenkel-pair formation as a result of excitation of Rydberg atomic states was confirmed by recent molecular dynamics calculations [28,29]. After the bubble formation the surrounding ground state atoms appear to have moved to the second shell. It was found that the second-nearest neighboring vacancy-interstitial pairs could create the permanent defects, which remain in the lattice after exciton annihilation (Fig.Sb) [29],... 

The most significant conclusion reached from investigation of the chemistry of the secondary peroxyesters is that the energy released on thermal conversion to the ketone and the carboxylic acid can be directed to the formation of excited state products. However, the specific structure of the secondary peroxyester controls the specific mechanism of chemiexcitation and the yield of excited state product obtained. These findings point the way to further exploration of the chemistry of these compounds. 

As a second example of intersystem crossing mechanism in biochromophores we include here the case of the DNA pyrimidine nucleobases, starting by the uracil molecule [91]. In previous sections we presented a model for the rapid internal conversion of the singlet excited rationalizes the ultrafast decay component observed in these systems, both in the gas phase and in solution. Despite the short lifetimes associated to this state, which is the main contributor to the photophysics of the system, formation of photodimers PyroPyr has been observed for the monomers in solution, as well as in solid state, for oligonucleotides, and DNA [92], Since the sixties, the determination of the mechanism of the photoinduced formation of cyclobutane dimers has been the subject of numerous studies [92, 93-97], One of the most classic models that has been proposed for the photodimerization of Pyr nucleobases in solution invokes photoexcitation of a molecule to a singlet state followed by population of a triplet state by an intersystem crossing mechanism... 

Quenching The deactivation of an excited molecular entity intermolecularly by an external environmental influence (such as a quencher) or intramolecularly by a substituent through a nonradiative process. When the external environmental influence (quencher) interferes with the behavior of the excited state after its formation, the process is referred to as dynamic quenching. Common mechanisms include energy transfer, charge transfer, etc. When the environmental influence inhibits the excited state formation the process is referred to as static quenching. 

Both particles, electron and hole—coming from the different electrodes—move from opposite directions towards the recombination layer. There they can combine and form excitons. This may happen near to the layer interface, on matrix molecules within the layer, and/or at doped emitter molecules. In suitable cases, as required for OLEDs, this leads to a population of excited states of the emitter material which subsequently emits light. Obviously, this process should occur with high efficiency. Details of the mechanism of exciton formation and population processes of excited emitter states are discussed in the next section. 

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