Photochemistry phosphorescence

Lewis made additional valuable contributions to the theory of colored substances, radiation, relativity, the separation of isotopes, heavy water, photochemistry, phosphorescence, and fluorescence. As a major in the U.S. Army Chemical Warfare Service during World War I, he worked on defense systems against poison gases. From 1922 to 1935 he was nominated numerous times for the Nobel Prize in chemistry. Lewis s death, while measuring the dielectric constant of hydrogen cyanide on March 23, 1946, precluded his receiving the prize, which is not awarded posthumously, see also Acid-Base Chemistry Lewis Structures. 

The triplet state has received rather less attention than the singlet in micellar photochemistry. Phosphorescent decay of solubilized polynuclear aromatic hydrocarbons from their Tj state may be observed in heavy-metal ion lauryl sulphate micelles. This involves a conventional intersystem crossing from Si- Ti promoted by spin-orbit coupling with the heavy atom. 1-Bromonaphthalene readily forms a triplet excited-state in micelles, which may be quenched by added sodium nitrite in water, the lifetime then being reduced from 2.8 x 10 s to 5 x 10 s. There is efficient triplet energy transfer from N-methylphenothiazine (Ti) to trans-stilbene (So), which is irreversible but reversible energy-transfer to naphthalene (So) occurs. ... 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

Low phosphorescence efficiency, however, leaves fast radiationless decay as the prime course of inefficient photochemistry. 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Phosphorescence and photochemistry of aromatic amino acids have been reported.481-483 Triplet states of nucleic acids have also been detected. For example, the phosphorescence of DNA equals the sum of the slow emissions from deoxyadenosine and deoxyguanosine monophosphates present, indicating that only the purine bases phosphoresce.484... 

The excited states are produced through the absorption of light. This is a major process of luminescence in photochemistry. As mentioned in section 3.4, a distinction is made between fluorescence and phosphorescence . The conditions required for their observation are discussed below. 

As we shall see, n —> tt singlet and triplet states of carbonyl compounds play an important role in photochemistry. Aldehydes and ketones display all the characteristics of absorption, fluorescence, phosphorescence, and intersystem crossing (5, —> T,) illustrated in Figure 28-1. Generally, they are more efficient at intersystem crossing than are unsaturated hydrocarbons, perhaps because the energies of the S and T states involved are not widely different. 

The photochemistry of 6-arylpropiophenones has been examined in Silicalite and other zeolites The channel structure of Silicalite prevents intramolecular quenching by the 6-phenyl group in 6-phenylpropiophenone and leads to a dramatic enhancement of the phosphorescence. 

Abstract Photochemistry is concerned with the interaction between light and matter. The present chapter outlines the basic concepts of photochemistry in order to provide a foundation for the various aspects of environmental photochemistry explored later in the book. Electronically excited states are produced by the absorption of radiation in the visible and ultraviolet regions of the spectrum. The excited states that can be produced depend on the electronic structure of the absorbing species. Excited molecules can suffer a variety of fates together, these fates make up the various aspects of photochemistry. They include dissociation, ionization and isomerization emission of luminescent radiation as fluorescence or phosphorescence and transfer of energy by intramolecular processes to generate electronic states different from those first excited, or by intermo-lecular processes to produce electronically excited states of molecules chemically different from those in which the absorption first occurred. Each of these processes is described in the chapter, and the ideas of quantum yields and photonic efficiencies are introduced to provide a quantitative expression of their relative contributions. 

In addition to absorption and stimulated emission, a third process, spontaneous emission, is required in the theory of radiation. In this process, an excited species may lose energy in the absence of a radiation field to reach a lower energy state. Spontaneous emission is a random process, and the rate of loss of excited species by spontaneous emission (from a statistically large number of excited species) is kinetically first-order. A first-order rate constant may therefore be used to describe the intensity of spontaneous emission this constant is the Einstein A factor, Ami, which corresponds for the spontaneous process to the second-order B constant of the induced processes. The rate of spontaneous emission is equal to Aminm, and intensities of spontaneous emission can be used to calculate nm if Am is known. Most of the emission phenomena with which we are concerned in photochemistry—fluorescence, phosphorescence, and chemiluminescence—are spontaneous, and the descriptive adjective will be dropped henceforth. Where emission is stimulated, the fact will be stated. 

Fig. 7 Jablonski diagram showing absorption, fluorescence, and phosphorescence. (Reproduced from R. P. Wayne, Principles and applications of photochemistry, Oxford University Press, Oxford, 1998. By permission of Oxford University Press)...

---