Anthracene, triplet state from flash photolysis

Absorption spectra of electronically excited states may be observed in flash photolysis studies. Porter has established the existence of the triplet state in a wide range of organic compounds in the liquid and gaseous phases. For example, the first triplet state of anthracene is populated by radiationless conversion from a photochemically excited singlet molecule, and may be observed by the absorption to the second triplet level. Absolute measurements of the triplet concentration may be made by determinations, from the absorption spectra, of the depletion of the singlet state. Similar results have been obtained with a variety of hydrocarbons, ketones, quinones and dyestuffs. 

Limited numbers of studies have been reported on the properties of molecules in the T states. Two-color two-laser flash photolysis can be applied to study photoinduced reactions from the T states. It has been reported that the main reaction path from the T states is the triplet ENT to the triplet quenchers. To the best of our knowledge, there has been only one report on the ELT from the T state. Wang et al. [48] reported the ELT from anthracene(T2) to ethyl bromoacetate. However, no detailed mechanism of the ELT from the T state has been reported. In this section, we summarize our recent systematic study of the intermolecular ELT from a series of substituted naphthalenes (NpD) in the T state to electron acceptors [85, 86]. 

For 12 and 13 type dyes with various counterions in nonpolar media, electron transfers via singlet and triplet states were distinguished by flash photolysis. The rate constant for electron transfer from the first excited singlet state of 02 in methanol to benzoquinone and other suitable acceptors is = (1-3) X 10 ° M secr. Electron transfer, involving the triplet state of T2-T4, was performed with benzoquinone and ascorbic acid in alcohols. ESR results with various cyanine dyes were interpreted by electron transfer involving the superoxide radical anion. Electron transfer competes with delayed fluorescence and trans—Kis isomerization when induced by energy transfer from triplet anthracene to 13 dyes. Charge-transfer states were reported for cyanine-oxonol ion pairs. 

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