Radical Formation Initiated by the Biphotonic Excitation of Acridone
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概要
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The photochemical formation of alcohol radical has been found to occur in the rigid solvent of ethanol (77 K) and in the polyvinyl alcohol (PVA) film (77 K and at room temperature), when acridone is dissolved as a sensitizer. It has been established that by absorbing two photons stepwise acridone gives rise to radicals which abstract hydrogen atoms from alcohol molecules, this being called the radical reaction mechanism and abbreviated as R-R mechanism. The energies necessary for the radical formation were estimated by determining energy levels of acridone and examining the wavelength dependence of the reaction. They differed for ethanol (77 K), PVA (77 K) and PVA (room temp.), perhaps due to the cage effect. The feature of the R-R mechanism has been compared with that of the energy transfer type (E-T mechanism) in which the energy is transferred from the higher triplet of solute to solvent molecules. When methyl iodide with a lower dissociation energy is added to an ethanol solution of acridone (77 K) and is then irradiated by an exciting light ineffective for R-R mechanism, methyl radical is formed by E-T mechanism. When the two types of reaction were energetically possible, E-T mechanism occurred more easily than R-R mechanism.
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