Phosphorescence

Phosphorescence is a spin-forbidden radiative transition between states of different multiplicity, usually from the lowest vibrational level of the lowest excited triplet state, Ti(v = 0). 

Phosphorescence, together with fluorescence, is called luminescence is emission from long-lived triplet excited states. The intensity depends on whether faster processes occur in the excited state. The triplet states are, in most cases, the lowest energy excited states. 

The triplet state is considerably more long-lived than the singlet state roughly speaking, the lifetime is on the microsecond rather than on the nanosecond scale. In any case, phosphorescence is an important part of the emission. 

The natural lifetime of the triplet state = Mkf may be estimated from the observed lifetime and the quantum yields of fluorescence and phosphorescence. According to Equation (5.11) 

If internal conversion and all energy-transfer processes and photochemical reactions are negligible, )st — I d one obtains 

The natural lifetime to varies between 10 s and many seconds. 

As a result of the heavy-atom effect or the effect of paramagnetic molecules such as Oi, which both enhance S — T absorption (cf. Section 1.3.2), phosphorescence T - S as well as the rate constants kn f and kyi of intersystem crossing will be favored. The consequence according to Equation (5.11) is an increase in whereas r/p will increase or decrease depending on which of the two processes, radiationless deactivation of the triplet state or phosphorescence, is more strongly favored. Frequently, an increase in the 

When 4 p is independent of the excitation wavelength the extreme sensitivity associated with emission spectroscopy can be utilized to obtain Sg-T absorption spectra by measuring phosphorescence excitation spectra (Mar-chetti and Kearns, 1967). The principle of the method is the same as for 

Phosphorescence would predominate in the former case, fluorescence in the latter. 

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- 

Optical absorption to a higher triplet has afforded further evidence that the emitting state in phosphorescence is a triplet. Intense irradiation of a boric acid glass containing fluorescein leads to the appearance of a new absorption band due to triplet-triplet absorption. Flash photolysis, in which a sample is exposed to a brief, intense flash of light, can be used to produce high transient concentrations of triplet species kinetic absorption spectroscopy of the system enables the build-up and decay of several singlet and triplet levels to be followed as a function of time. 

Phosphorescence most commonly follows population of Ti via ISC from Si, itself excited by absorption of light. The Ti state is usually of lower energy than Si, and the long-lived (phosphorescent) emission is almost always of longer wavelength than the short-lived (fluorescent) emission. The relative importance of fluorescence and phosphorescence depends on the rates of radiation and ISC from Si the absolute efficiency depends also on intermolecu-lar and intramolecular energy-loss processes, and phosphorescent emission competes not only with collisional quenching of Ti but also with ISC to So- 

A major advantage of four-component methods, in which not only the ground state but also excited states are accessible (Cl, MCSCF or Fock-space CC methods), is that electronic transitions, which are spin forbidden in nonrelativistic theory, can be studied due to the implicit inclusion of spin-orbit coupling. Four-component methods are thus able to describe phosphorescence phenomena adequately. However, only a little work has been done for this type of electronic transitions and almost all studies utilize approximate descriptions of spin-orbit coupling (see, for instance, Christiansen et al 2000). 

Chemiluminescence, bioluminescence, and electrochemiluminescence are types of luminescence in which the excitation event is caused by a chemical, biochemical, or electrochemical reaction and not by photoillumination. 

Although there are no reported experimental data upon phosphorescent emission from the carbonyl halides under scrutiny in this book, there has been some effort expended in calculating the transition energies, polarizations and lifetimes for possible excited triplet states (see Table 17.14) [337,1344]. These predictions should expedite the interpretation of any data 

An early prediction by El-Sayed [588b] that the halogen atoms should have a negligible influence upon the t — n phosphorescent lifetimes of these compoimds (relative to COHj) has, in more recent work [337], been refuted. A marked dependence in the predicted lifetimes is revealed in the data in Table 17.14, and this has been attributed [337] to mixing between the halogen and the oxygen atomic orbitals. 

CALCULATED TRANSITION ENERGIES AND LIFETIMES FOR SOME EXCITED TRIPLET STATES OF THE CARBONYL HALIDES 

Molecule Excited State Transition Transit ion Energy/eV Li fet ime/ms Ref. 

In the Sections above, various aspects of the electronic structure of the carbonyl halides have been discussed in some detail, and it is now appropriate to consider what insight this knowledge yields concerning their chemical reactivity. In particular, their reactivity towards nucleophilic and electrophilic substitutions will be examined. 

The ground state of a typical phosphorescent molecule is a singlet because its electrons are aU paired the excited state to which the absorption excites the molecule is also a singlet. The peculiar feature of a phosphorescent molecule. 

46 The sequence of steps leading to phosphorescence. The important step is the intersystem crossing from an excited singlet to an excited triplet state. The triplet state acts as a slowly radiating reservoir because the return to the ground state is very slow. 

Apart from a small number of cofactors, such as the chlorophylls and flavins, the majority of the building blocks of proteins and nucleic acids do not fluoresce strongly. Four notable exceptions are the amino acids tryptophan (/labs 280 nm and /lfi oj = 348 nm in water), tyrosine (labs 274 nm and Afiuo, -303 nm in water), and phenylalanine (Aabs == 257 nm and = 282 nm in 

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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.

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. 

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. 

Zeng Y, Biczok L and Linschitz H 1992 External heavy atom induced phosphorescence emission of fullerenes the energy of triplet Cgg J. Phys. Chem. 96 5237-9... 

White phosphorus is very reactive. It has an appreciable vapour pressure at room temperature and inflames in dry air at about 320 K or at even lower temperatures if finely divided. In air at room temperature it emits a faint green light called phosphorescence the reaction occurring is a complex oxidation process, but this happens only at certain partial pressures of oxygen. It is necessary, therefore, to store white phosphorus under water, unlike the less reactive red and black allotropes which do not react with air at room temperature. Both red and black phosphorus burn to form oxides when heated in air, the red form igniting at temperatures exceeding 600 K,... 

When exposed to sunlight or when heated in its own vapor to 250oC, it is converted to the red variety, which does not phosphoresce in air as does the white variety. This form does not ignite... 

On the average, one part of radon is present ot 1 x IO21 part of air. At ordinary temperatures radon is a colorless gas when cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. It has been reported that fluorine reacts with radon, forming a fluoride. Radon clathrates have also been reported. 

The total fluorescence (or phosphorescence) intensity is proportional to the quanta of light absorbed, To — T, and to the efficiency (f>, which is the ratio of quanta absorbed to quanta emitted ... 

The fraction of absorbed photons that produce a desired event, such as fluorescence or phosphorescence (4>). 

Phosphorescence is most favorable for molecules that have n transitions,... 

In an emission spectrum a fixed wavelength is used to excite the molecules, and the intensity of emitted radiation is monitored as a function of wavelength. Although a molecule has only a single excitation spectrum, it has two emission spectra, one for fluorescence and one for phosphorescence. The corresponding emission spectra for the hypothetical system in Figure 10.43 are shown in Figure 10.44. 

The basic design of instrumentation for monitoring molecular fluorescence and molecular phosphorescence is similar to that found for other spectroscopies. The most significant differences are discussed in the following sections. 

A fluorescence or phosphorescence spectrum in which the emission intensity at a fixed wavelength is measured as a function of the wavelength used for excitation. 

Molecular Phosphorescence Instrumentation for molecular phosphorescence must discriminate between phosphorescence and fluorescence. Since the lifetime for fluorescence is much shorter than that for phosphorescence, discrimination is easily achieved by incorporating a delay between exciting and measuring phosphorescent emission. A typical instrumental design is shown in Figure 10.46. As shown... 

Block diagram for molecular phosphorescence spectrometer with inset showing how choppers are used to isolate excitation and emission. 

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