Phosphorescent Organic Molecules

FIGURE 11. Optical detection of ESR of the triplet state of benzophenone showing the fine and superfine structure of aNL = 1 transitions. 

The zero-field splitting parameters of some aromatic 

FIGURE 12. ESR spectrum of the triplet state of naphthalene showing the fine structure and the hyperfine splittings. From Varian Associates EPR at Work Series, No. 26. 

FIGURE 13. ESR spectrum of randomly oriented photoexcited naphthalene triplets showing the aM = 1 transitions. Taken from reference (283). 

However, for n,ir triplet aromatic ketones and aldehydes, the increased spin delocalization onto the rings should reduce the zero-field splitting, and the failure of their ESR detection may be attributed mainly to their short lifetimes. Hayashi and coworkers (290) have examined the ESR fine-structure parameters of a number of molecular complexes in their phosphorescent state and have derived the dependence of the fine-structure values upon the degree of charge-transfer character in the lowest triplet state of the molecular complexes. Some of their results are collected in Table 15. 

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Confirmation that the emitting species in phosphorescent organic molecules is a triplet has come from several sources. In the 1940s it was discovered that a solution of fluorescein in boric acid glass became paramagnetic under intense irradiation more recently it has been shown that the paramagnetism and the phosphorescence decay at identical rates when irradiation ceases. The electron paramagnetic resonance (EPR) technique is capable of detect-... 

Figure Bl.1.3. State energy diagram for a typical organic molecule. Solid arrows show radiative transitions A absorption, F fluorescence, P phosphorescence. Dotted arrows non-radiative transitions.

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

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). 

Under certain conditions phosphorescence can be observed at room temperature from organic molecules adsorbed on solid supports such as filter paper, silica and other chromatographic supports. 

For organic molecules, the lifetime of the singlet state ranges from tens of picoseconds to hundreds of nanoseconds, whereas the triplet lifetime is much longer (microseconds to seconds). However, such a difference cannot be used to make a distinction between fluorescence and phosphorescence because some inorganic compounds (for instance, uranyl ion) or organometallic compounds may have a long lifetime. 

Triplet state acidities, pKj, have been obtained by Porter and his school from phosphorescence studies, using flash photolysis techniques. The singlet, triplet and ground state acidity constants of some organic molecules are given in Table 4.2. 

The fluorescence of organic molecules and ions in solution is a photoluminescent process that decreases extremely rapidly when the excitation ceases, in contrast to phosphorescence, which latter has a much slower decrease and is seldom used for analysis. Finally, chemiluminescence, the emission of light during a chemical reaction, is involved in some analytical applications. 

It may be noted that this simple collision process may be complicated by the addition196,200 of a 2-butene to the excited benzene to form a benz-valene-butene adduct.176 The quenching of phosphorescence of organic molecules by oxygen may be explained53,86,140 by the triplet-triplet annihilation mechanism86,129,167... 

Figure 23-14 Potential energy diagram for the ground state S0 and the first excited singlet S, and triplet Tj states of a representative organic molecule in solution. G is a point of intersystem crossing Sj —> T,. For convenience in representation, the distances r were chosen rS() < rSj < rT thus, the spectra are spread out. Actually, in complex, fairly symmetric molecules, rS(. rs < rT and the 0-0 absorption and fluorescence bands almost coincide, but phosphorescence bands are significantly displaced to the lower wavelengths. From Calvert and Pitts,2 p. 274.

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

On the other hand, most chemists and many physicists leading with polyatomic organic molecules currently employ the mechanistic definitions advanced by G. N. Lewis and shown in Figure 1. Thus, fluorescence is defined as a radiative transition between states of like multiplicity, e.g., 5 x - So + hv. Phosphorescence is a radiative transition between states of different multiplicity. In organic molecules the process is usually associated with spin-forbidden transitions such as Ti - S0 + hv". 

It would be desirable to insert a probe into the polymer to ascertain the local environmental conditions. In addition to having microscopic dimensions, the probe must act as a timing device which specifies the time-scale of the observation. Such a probe is a fluorescent molecule. Its dimensions are about the size of a monomer residue, namely of the order of 10 A, and the lifetime of fluorescence, r, varies between about 10-9— 10"7 sec., depending on the fluorescent compound and the medium (9). Still longer time-scales, namely, 10"4—10 sec., are achieved with organic molecules in the phosphorescent state (21). 

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