Excited states, chemical generation

Modem photochemistry (IR, UV or VIS) is induced by coherent or incoherent radiative excitation processes [4, 5, 6 and 7]. The first step within a photochemical process is of course a preparation step within our conceptual framework, in which time-dependent states are generated that possibly show IVR. In an ideal scenario, energy from a laser would be deposited in a spatially localized, large amplitude vibrational motion of the reacting molecular system, which would then possibly lead to the cleavage of selected chemical bonds. This is basically the central idea behind the concepts for a mode selective chemistry , introduced in the late 1970s [127], and has continuously received much attention [10, 117. 122. 128. 129. 130. 131. 132. 133. 134... 

Shimomura, O. (1982). Mechanism of bioluminescence. In Adam, W., and Cilento, G. (eds.), Chemical and Biological Generation of Excited States, pp. 249-276. Academic Press, New York. 

For reviews, see Turro, N.J. Ramamurthy, V. in de Mayo Rearrangements in Ground and Excited States, vol. 3 Academic Press NY, 1980, p. 1 Murray, R.W. in Wasserman Murray, Ref. 239, p. 59. For a general monograph, see Adam, W. Cilento, G. Chemical and Biological Generation of Excited States Academic Press NY, 1982. 

Excited states can also decay by means of chemical reaction via heterolytic bond cleavage, leading to ions, or by homolytic bond cleavage generating free radicals. 

The determination of the laser-generated populations rij t) is infinitely more delicate. Computer simulations can certainly be applied to study population relaxation times of different electronic states. However, such simulations are no longer completely classical. Semiclassical simulations have been invented for that purpose, and the methods such as surface hopping were proposed. Unfortunately, they have not yet been employed in the present context. Laser spectroscopic data are used instead the decay of the excited state populations is written n (t) = exp(—t/r ), where Xj is the experimentally determined population relaxation time. The laws of chemical kinetics may also be used when necessary. Proceeding in this way, the rapidly varying component of AS q, t) can be determined. 

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

Figure 12.1.4 A chemielectronic step, i.e., a step in which the chemical energy of an intermediate is converted into electronic energy in a fluorescent dye. Here the C2O4 intermediate releases energy as it dissociates into two carbon dioxide molecules. The energy is transferred to the accompanying fluorescent dye to generate an excited state of the dye.

Faulkner, L. R. Glass, R. S. In "Chemical and Biological Generation of Excited States" Adam, W. Cilento, G., Eds. Academic Press New York, 1982 chapter 6. 

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

A.J. Bard, University of Texas The fact that one can generate chemiluminescence in polymer films containing Ru-(bpy)3 2 implies that the excited state may not be quenched completely by electron transfer reactions. Are the photoreactions you describe thermodynamically uphill (i.e., with chemical storage or radiant energy) or are they photocatalytic ... 

Electrogenerated chemiluminescence (ECL) is the process whereby a chemiluminescence emission is produced directly, or indirectly, as a result of electrochemical reactions. It is also commonly known as electrochemiluminescence and electroluminescence. In general, electrically generated reactants diffuse from one or more electrodes, and undergo high-energy electron transfer reactions either with one another or with chemicals in the bulk solution. This process yields excited-state molecules, which produce a chemiluminescent emission in the vicinity of the electrode surface. 

For chemical systems of interest, photolysis produces intermediates, such as radicals or biradicals, whose energetics relative to the reactants are unknown. The energetics of the intermediate can be established by comparison of the acoustic wave generated by the non-radiative decay to create the intermediate, producing thermal energy , with that of a reference or calibration compound whose excited-state decay converts the entire photon energy into heat, / (ref). The ratio of acoustic wave amplitudes, a, represents the fraction of the photon energy that is converted into heat. 

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