Electronically excited molecules physical properties

The redistribution of electrons associated with electronic excitation of a molecule changes all its physical properties, in particular its acidity (Section 5.3) and its redox properties. Electronically excited molecules are both strong oxidants and reductants (Section 4.1, Figure 4.1). Diffusional encounter of an excited molecule M with an electron donor D or acceptor A will therefore often result in the formation of a contact ion pair or solvent-separated ion pair, depending on whether electron transfer occurs at a certain distance with at least one solvent molecule between M and D or A or only upon direct contact. For example, irradiation of naphthalene in an apolar solution containing iV.jV-dimethylaniline leads to the diffusion-controlled formation of a radical ion contact pair (Scheme 5.1). 

The pH of the solution is also an important parameter that will influence the luminescence characteristics of organic species that exhibit acid-base properties. In many instances, the chemical and physical properties of electronically excited molecules differ markedly from those of the ground-state molecules, because of the different electronic distribution. Therefore, most of the excited molecules show protonation constants (log Kg) which differ greatly from those measured in their ground states. Differences in log Kg of more than 6 units have been observed in a variety of compounds. As for the ground state, acid-base equilibria in the excited state are drastically altered by the surfactant aggregates, which can result in a further increase in sensitivity. 

The majority of heterogeneous chemical and physical-chemical processes lead to formation of the intermediate particles - free atoms and radicals as well as electron- and oscillation-excited molecules. These particles are formed on the surface of solids. Their lifetime in the adsorbed state Ta is determined by the properties of the environment, adsorbed layer, and temperature. In many cases Ta of different particles essentially affects the rate and selectivity of heterogeneous and heterogeneous-homogeneous physical and chemical processes. Therefore, it is highly informative to detect active particles deposited on surface, determine their properties and their concentration on the surface of different catalysts and adsorbents. 

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