Bimolecular phosphorescence

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

To minimize quenching of triplets due to bimolecular collisions, phosphorescence studies are generally carried out in rigid glasses at 77°K. Some materials which may be used to produce these glasses are listed in Table... 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

The molecules which have reached Ti will now react with a rate constant kr (unimolecular reaction) or [N] (bimolecular reaction with a ground state partner N) in competition with radiative (phosphorescence of rate constant P), non-radiative (A ) deactivations as well as quenching processes ( q[Q]) so that the final reaction quantum yield of the primary process is... 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

Bimolecular deactivation (pathway vii, Fig. 1) of electronically excited species can compete with the other pathways available for decay of the energy, including emission of luminescent radiation. Quenching of this kind thus reduces the intensity of fluorescence or phosphorescence. Considerable information about the efficiencies of radiative and radiationless processes can be obtained from a study of the kinetic dependence of emission intensity on concentrations of emitting and quenching species. The intensity of emission corresponds closely to the quantum yield, a concept explored in Sect. 7. In the present section we shall concentrate on the kinetic aspects, and first consider the application of stationary-state methods to fluorescence (or phosphorescence) quenching, and then discuss the lifetimes of luminescent emission under nonstationary conditions. 

Since the transition moment of the spin-forbidden T,- Sq transition is very small, the natural lifetime rj of the triplet state is long. Consequently, radiationless processes can compete with phosphorescence in deactivating the T state. Of particular importance are collision-induced bimolecular processes (cf. Section 5.4), and phosphorescence of gases and liquid solutions is relatively difficult to observe (Sandros and Backstrom, 1962). An exception is biacetyl with a very short T, lifetime rj. (Cf. Figure 5.3.) Phosphorescence spectra are commonly measured using samples in solvents or mixed solvents that form rigid glasses at 77 K (such as EPA = ether-pentane-alcohol mixture). (Cf., however. Example 5.6). 

The spectrum of Nph form on aerosil is not resolved. The wide-band fluorescence contribution relative to the molecular emission is large, afterglow is not observed. The wideband excitation spectrum at 400 nm is shifted relatively to that of a molecular form by 10 nm. For zeolites this is mainly CTC and for aerosil a bimolecular associate of Nph. When adsorbing from a vapor phase, the emission spectrum of Nph in a zeolite consists of a continuous structureless band which is a superposition of CTC and dimers adsorbed at the outer surface (Fig. 3a).In the case of co-adsorption of water vapor or hexane the spectrum transforms with the appearance of structured fluorescence and phosphorescence components (Fig.3b). The coadsorbate seems to promote breaking up of dimers and diffusion of molecules in zeolite cages. 

Gx being the generation rate of triplets, /J0 is the sum of the radiative kr and nonradiative fenr decay, hence the inverse phosphorescence lifetime (/J0 = knr + kr = l/rphos)- 7tta is the bimolecular triplet-triplet-annihilation (TTA) constant. The last term in Eq. (16) includes all losses due to bimolecular interaction of triplets with impurities such as paramagnetic quenchers and charge carriers, y is the rate constant for this process and n, the concentration of the impurity. For the present... 

ES2 has its own decay processes including radiative (k, termed phosphorescence) and non-radiative deactivation (k ), unimolecular reaction to product(s) (kp) and bimolecular quenching by energy or electron transfer (kq) to another species. For this model the ES2 lifetime is defined by... 

The modulation technique mentioned above has been used to identify triplet excimers in 1,2-benzanthracene and 1,2 3,4-dibenzanthracene at high solute concentrations167 and the differences between luminescence from naphthalene in fluid solution in the temperature range 353—173 and naphthalene in a rigid solution at 77 have been ascribed to phosphorescence from a triplet excimer.168 Excimer formation in solid poly-(2-vinylnaphthalene) and polystyrene is found to be dependent on the temperature at which the film is cast, and a statistical model based on the rotational isomeric state approximation has been used to formulate an expression for the fraction of excimer sites in the solid systems.168 Kinetic equations for dimer formation and decay, based on the statistical mechanics of ideal gases, have been obtained. These equations, derived from the N-atom von Neumann equation, take into account both bimolecular and termolecular equations.157 158 160... 

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