Photochemical reactions phosphorescence

The physical basis of spectroscopy is the interaction of light with matter. The main types of interaction of electromagnetic radiation with matter are absorption, reflection, excitation-emission (fluorescence, phosphorescence, luminescence), scattering, diffraction, and photochemical reaction (absorbance and bond breaking). Radiation damage may occur. Traditionally, spectroscopy is the measurement of light intensity... 

Direct Photolysis. Direct photochemical reactions are due to absorption of electromagnetic energy by a pollutant. In this "primary" photochemical process, absorption of a photon promotes a molecule from its ground state to an electronically excited state. The excited molecule then either reacts to yield a photoproduct or decays (via fluorescence, phosphorescence, etc.) to its ground state. The efficiency of each of these energy conversion processes is called its "quantum yield" the law of conservation of energy requires that the primary quantum efficiencies sum to 1.0. Photochemical reactivity is thus composed of two factors the absorption spectrum, and the quantum efficiency for photochemical transformations. 

Among the many excited singlet and triplet levels, 5i and Ti have distinct properties. They are in general the only levels from which luminescence is observed (Kasha rule) also most photochemical reactions occur from Sr or Ti. Here we discuss the characterization of the lowest triplet state by electronic spectroscopy. First we treat the theoretical background that allows the absorption spectra of conjugated systems to be described, and then we discuss the routes that lead to phosphorescence emission and Ti- - Sq absorption intensity. Details of the experimental methods used to determine triplet-triplet and singlet-triplet absorption spectra, as well as phosphorescence emission spectra are given in Chapters III, IV, and V. Representative examples are discussed. 

It is a little difficult to relate these observations of phosphorescence in low-temperature matrices (which in both cases are composed of molecules which at room temperature photoreact with the pyrimidines) to the observed photochemical reactions. The fluorescence of the pyrimidines in frozen aqueous matrices may be weak because the excited molecules are quenching each other reactively—an argument strengthened by the observation110 that addition of ethanol to the solution strengthens the phosphorescence but prevents dimer formation.29 No clear-cut conclusions can yet be provided by these studies. 

Some solvents containing heavy atoms can induce enhancement of phosphorescence at the expense of fluorescence, e.g. ethyl iodide, nitro-methane, CS2 (external heavy atom effect). Irreversible conversion to ionic or radical products is often observed. Hence the system changes with time and the process should be classed a photochemical reaction distinct from the reversible quenching reactions discussed above. For example for anthracene and carbon tetrachloride ... 

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