Excited states generated

In the two most important types of photosensitization, both of which are in accord with the Wigner rule, a triplet excited state generates another triplet and a singlet generates a singlet ... 

Rat oncomodulin, a parvalbumin-1 ike tumor protein that has two tyrosine residues but no tryptophan, exhibits fluorescence emission at 301 and 345 nm.(135) Upon binding two moles of Ca2+ per mole of oncomodulin, the 301-nm intensity increases while the 345-nm band decreases. These results were explained in terms of acidic side chains involved in either binding Ca2+ or accepting a proton on excited-state generation of tyrosinate. The cloned... 

Shoaf, A. R., R. C. Allen, and R. H. Steele. Electronic excitation state generation in mammalian systems Mechanism-role-pathology. Second Annual Meeting American Society for Photobiology, July 22-26, 1974. Unhrerdty of British Colnm-bia. Vancouver. B. C. 

We have previously investigated ligand release in the 6-coordinate piperidine complexes of Ni octaethylporphyrin (10). For Ni(OEP), formation of the 6-coordinate complex is not complete, and so, the picture is complicated by the presence of both 4- and 6-coordinate species in the initial sample. However, upon excitation with the pulsed laser the relative proportions of the two Ni(OEP) species change as determined by changes in the relative intensities of the 4- and 6-coordinate sets of Raman marker lines. Thus, axial ligand release is observed in the excited state generated during the... 

The contemporary trends of dioxetane chemistry include a number of fundamental and applied aspects. The fundamental aspects encompass the stereoselective synthesis and the transformations of novel chiral dioxetanes, as well as the mechanistic studies on the thermal, electron-transfer-induced and catalytic dioxetane decomposition. The emphasis lies on the elucidation of the excited-state generation in these chemiluminescent processes. 

A wealth of experimental data on the thermal dioxetane decomposition and the excited-state generation in the thermolysis process has been comprehensively surveyed in previous reviews". During the last decade, computational elucidation of the thermal cleavage received major attention and in the present subsection we consider the relevant smd-ies. Computations on the dioxetane thermolysis were conducted by both ab initio and semiempiricaP methods at different levels of sophistication. 

Clearly, a detailed knowledge of the electron-transfer process is required if more effective CIEEL-triggerable chemiluminescent systems are to be rationally designed, rather than engage in an empirical trial-and-error hunt. To understand the excited-state generation process, the nature of the CIEEL emitter and the chemiexcitation mechanism should be established. 

Rauhut and coworkers proposed the occurrence of a charge transfer complex between the HEI and the ACT in order to explain the electronically excited-state generation in the peroxyoxalate system. Chemiluminescence quantum yield (4>cl) measurements with different activators have shown that the lower the ACT half-wave oxidation potential (Ei/2° ) or singlet energy (Es), the higher the electronically excited-state formation rate and 4>cl- According to the mechanistic proposal of Schuster and coworkers for the CIEEL... 

Like other peroxides, also dioxetanes are sensitive to the presence of metal ions and their complexes, which catalyze the decomposition of the dioxetane molecule. In most cases, this decomposition is dark, i.e. no chemiluminesce is generated in such a catalytic cleavage42. An informative exception, for instance, constitutes the chemiluminescent decomposition of the dioxetane 19 in Scheme 13, initiated by the ruthenium complex Ru(bipy)3Cl243. It has been shown that this chemiexcitation derives from the valence change of the ruthenium ion in the process Ru3+ I e — Ru2+, for which the efficiency of the excited-state generation may be as much as 40%44. Hence, when the radical anion of the carbonyl cleavage fragment from the dioxetane and the Ru3+ ion are formed in... 

Figure 8.13 Chemiluminescence from excited state generated by radical-ion recombination.

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... 

Furthermore, excited states generated electrochemically may be not only emissive but also reactive. The possibility of such an "electrophotochemistry" (epc) has been considered before (34). But real examples were discovered only quite recently and will be discussed later (35,36). However, chemical transformations induced by ac electrolysis may not only proceed via excited states. Other mechanisms can be also consistent with these observations. While this extends the range of reaction types of ac electrolysis, it complicates the elucidation of the real mechanism. Examples of the various reaction types are presented in the following sections. 

Galardy et al. (1973) introduced acetophenones and benzophenones for photoaffinity labeling (see also Martyr and Benisek, 1973 Katzenellenbo-gen et al., 1974). The properties of excited states generated from these and other a,P-unsaturated ketones are well understood (Turro, 1979). In most cases a triplet excited state is formed that abstracts a hydrogen atom from a donor yielding two radicals which subsequently couple. Unreacted excited species relax to the ground state and may be excited repeatedly until they react (Fig. 2.5). 

For photochemical purposes only absorption and luminescence are of importance absorption is the main method of excited state generation, whereas luminescence belongs to photophysical processes, which compete with the photoreactions in the excited state deactivation. 

Absorbed light intensity (hi) controls the rate of the excited state generation... 

Protonated A-nitroso amines undergo photolytic decomposition from the excited state generating transient aminium radicals and nitric oxide. Aminium radicals, which are electrophilic radicals, initiate the addition to alkenes to give //-amino nitroso compounds or oc-amino oximes (Section 7.2.5.2), however, under an oxygen atmosphere, //-nitryloxy amines (/i-amino nitrates) are produced through peroxynitrites intermediates97-99. 

Figure 1. Three different types of electron transfer processes. Bottom box Those chromophores bearing an asterisk (e.g., C ) indicate that they are in an electronically excited state, generated by absorption of light.

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