Kasha phosphorescence

Spectroscopists observed that molecules dissolved in rigid matrices gave both short-lived and long-lived emissions which were called fluorescence and phosphorescence, respectively. In 1944, Lewis and Kasha [25] proposed that molecular phosphorescence came from a triplet state and was long-lived because of the well known spin selection rule AS = 0, i.e. interactions with a light wave or with the surroundings do not readily change the spin of the electrons. 

In 1944, Lewis and Kasha (52) identified phosphorescence as a forbidden" transition from an excited triplet state to the ground singlet state and suggested the use of phosphorescence spectra to identify molecules. Since then, phosphorimetry has developed into a popular method of analysis that, when compared with fluorometry, is more sensitive for some organic molecules and often provides complimentary information about structure, reactivity, and environmental conditions (53). 

Spectroscopists interested in elucidation of the molecular energy schemes studied the phosphorescence emission of over 200 compounds, of which 90 were tabulated by Lewis and Kasha in 1944. They classified phosphorescing substances in two classes, based on the mechanism of phosphorescence production. The first group comprises minerals or crystals named phosphors, where the individual molecule is not phosphorescent as such, but emits a shining associated with the presence of some impurity localized in the crystal. This type of phosphorescence cannot be attributed to a concrete substance. The second type of phosphorescence emission is attributed to a specific molecular species, being a pure substance in crystalline form, adsorbed on a suitable surface or dissolved in a specific rigid medium [22],... 

When the excited triplet state is populated, rapid vibrational relaxation and possibly internal conversion may occur (if intersystem crossing takes place to an excited triplet of greater energy than Ti). Thus the excited molecule will relax to the lowest vibrational level of the Ti state, from where phosphorescence emission can occur in compliance with Kasha s rule. 

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. 

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... 

Figure 3.29 (a) Outline of the absorption, A fluorescence, F and phosphorescence, P spectra of a rigid polyatomic molecule. X = wavelength, vertical axis = absorbance (A) or emission intensity (F, P). (b) The Stokes shift of the absorption and fluorescence spectra is defined as the difference between their maxima. When this shift is small, there is a substantial spectral overlap between absorption and emission, (c) Jablonski diagram and outline of the absorption and fluorescence spectra of azulene, an exception to Kasha s rule. The energy gap between S0 and Sj is very small, that between Sj and S2 is very large... 

The emission behavior is embodied in two rules Kasha s rule and Vavilov s law [91]. Kasha s rule states that if a molecule emits a photon, it will always originate from the lowest excited state of a given spin multiplicity. If the emission occurs from the lowest singlet excited state it is called fluorescence and when the lowest excited state is a triplet it is known as phosphorescence. In addition, Vavilov s law implies that the fluorescence quantum yield (F) is essentially independent on the excitation wavelength. 

Where chemical processes such as protonation are concerned, it is the general rule that only the first excited singlet state (Sj) and the first excited triplet state (T are involved. This is closely related to Kasha s rule for radiation emission (Kasha, 1950) fluorescence always occurs from the lowest excited singlet state and phosphorescence from the lowest triplet. Since experimental conditions are often arranged so that protonation is in competition with emission, i.e. so that their rates are similar, these rules are easily understood in terms of the much shorter time ( 10-1 2 s) required for the Sj state to be reached from the higher states produced immediately on absorption... 

McGlynn Mid Boggus describe the phenomenon thus absorption in the charge transfer bMid is followed either by the converse emission or by intersystem crossing (according to Kasha [124]) to a dissociative level of the complex which yields the aromatic in its first excited triplet state. The aromatic hydrocarbon then phosphoresces. 

Jablonski tvas the first to suggest a simple set of three electronic levels to explain many of the phenomena of fluorescence and phosphorescence. Lewis and Kasha later identified one of these states as a triplet state. We do noli reproduce a Jablonski diagram because it has appeared so often. 

There had also been experimental advances calling for theoretical explanations. For example in 1944 Michael Kasha with GN Lewis [200] discovered that the emitting state in the long-lived phosphorescence of simple aromatics was an excited triplet state. There was at that time no theory of the energies and other properties of these states. It was a good time to be starting in these fields of enquiry. 

Solutions of unsubstituted monoheterocyclic compounds, pyridine, pyrrol, furan and thiopene are non-fluorescent. According to Kasha and Reid17, the near ultra-violet absorption spectra of N-heterocyclic compounds like pyridine, pvrazine and phenazine include n tt transitions and the excited state undergoes, with high probability, a radiationless transition to a lower triplet metastable state and hence few molecules remain in the excited state long enough to fluoresce. The presence of molecules in the triplet state has been shown by their phosphorescence. 

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