Fluorescence and phosphorescence quenching

Ruthenium(II) bipyridyl and Cr(III) aquo complexes luminesce strongly when photostimulated. The emission of light can be quenched effectively by such species as oxygen, paraquat, Fe(II) aquo complexes, Ru(II) complexes and Cr(NCS)i (Sutin [15]). Pfeil [16] found that the quenching rate coefficients are typically a third to a half of the value which might be predicted from the Smoluchowski theory [3]. 

Upon ejection from an ion or molecule by photoionization or high energy radiolysis, the electron can be captured in the solvent to form an anionic species. This species is called the solvated electron and has properties reminiscent of molecular anions redox potential of —2.75eV and diffusion coefficient of 4.5 x 10-9 m2 s-1 (Hart and Anbar [17]) in water. Reactions between this very strong reductant and an oxidising agent are usually very fast. The agreement between experimental results and the Smoluchowski theoretical rate coefficients [3] is often close and within experimental error. For instance, the rate coefficient for reaction of the solvated (hydrated) electron in water with nitrobenzene has a value 3.3 x 10+1° dm3 mol-1 s-1. 

Another mechanism for the quenching of fluorescence or phosphorescence depends upon the possibility that the energy of the excited state (exciton) can be transferred to a molecule (acceptor) by a non-radiative mechanism. The two most probable such mechanisms are those of the dipole—dipole interaction and the exchange effect (Forster [12] and Dexter [13]) which may extend 2—5 and lnm, respectively. Except in very mobile solvents, the dipole—dipole interaction completely dominates diffusion and makes it difficult to observe the quenching behaviour characteristic of a diffusive process. The exchange effect is much less strong and is more comparable with diffusive quenching. Marshall et al. 

3 QUENCHING OF LUMINESCENCE FROM METAL COMPLEXES BY ELECTRON TRANSFER 

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In polymer blends the features one would like to characterize are the amount of interphase in the system and the nature of its properties. An interesting approach to this probln takes advantage of the different sorption properties of oxygen in the two polymers making up the blend. Sorption is the product of solubility times diffusion. Fluorescence and phosphorescence quenching provide a measure of sorption For the process... 

Physical methods Physical methods include photometric absorption and fluorescence and phosphorescence inhibition, which is wrongly referred to as fluorescence quenching [1], and the detection of radioactively labelled substances by means of autoradiographic techniques, scintillation procedures or other radiometric methods. These methods are nondestructive (Chapt. 2). 

In order to clear up the mechanism of inactivation of excited states, we examined the processes of quenching of fluorescence and phosphorescence in PCSs by the additives of the donor and acceptor type253,2S5,2S6 Within the concentration range of 1 x 1CT4 — 1 x 10"3 mol/1, a linear relationship between the efficiency of fluorescence quenching [(/0//) — 1] and the quencher concentration was found. For the determination of quenching constants, the Stem-Volmer equation was used, viz. 

The selection rules for quenching are different for fluorescence and phosphorescence. Hence, in Eqs. (3.1) and (3.3), quenchers of phosphorescence, [Qp], and fluorescence, [Q/], are distinguished because molecules that quench one may not quench the other with the same efficiency. 

The intersystem crossing process has opposite effects on the yields of fluorescence and phosphorescence since it depletes the singlet state and populates the triplet state. It is commonly known that heavy ions, such as iodide and bromide, increase intersystem crossing by spin-orbit coupling.(1617) For proteins, fluorescence can be quenched as phosphorescence yield is enhanced. 8,19) However, although the phosphorescence yield is increased, the lifetime is decreased. This effect arises because spin-orbit coupling, which increases the intersystem crossing rate from 5, to Tt, also increases the conversion rate from T, to S0. 

B. Somogyi, J. A. Norman, and A. Rosenberg, Gated quenching of intrinsic fluorescence and phosphorescence of globular proteins, Biophys. J. 50, 55-61 (1986). 

A third method of looking for triplet-state intermediates in a reaction is to correlate the extent of quenching of phosphorescence of an excited species with the amount of chemical reaction under the specific reaction conditions. The possibility of this has led to numerous measurements of the fluorescence and phosphorescence of the pyrimidines and purines. 

Sometimes the fluorescence and phosphorescence spectra of a compound in solution overlap. They may be separated as follows. If a suitable triplet energy acceptor is added, this will quench the phosphorescence, leaving the fluorescence unaffected, while a suitable triplet donor will sensitize the phosphorescence in the absence of any fluorescence. Back-strom and Sandros have analyzed the total luminescence spectra of biacetyl, benzil, and anisil in this way, using pyrene as the triplet acceptor and benzophenone as triplet donor.3... 

Since the heterocyclic-substituted platinum 1,2-enedithiolates are dual emitters with only one emission that is oxygen quenched, ratiometric oxygen analysis is possible (see Fig. 3) (21, 29). While fluorescence and phosphorescence intensities vary with changes in optical clarity, fluctuations in the source and detector, and photobleaching of the emitter, the intensity ratio of a dual emitter (phosphorescence-fluorescence) does not. As such, the 3I/1I intensity ratio can be used in place of I, in Eq. 3 to generate Eq. 7 and 8 (29-31, 69). 

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-... 

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. 

Measurements of the quantum yields of fluorescence and phosphorescence of [3] (Davidson et ai, 1980a) showed that for many of the compounds fluorescence quenching in rigid matrices is elTicient. However, in all cases 0p + 0F and therefore on cooling many of the molecules must adopt conformations in which fluorescence quenching can take place but the interaction does not lead to triplet production. Thus there is apparently a conformational requirement for triplet production via the external heavy atom effect. 

When investigating the quenching of the triplet state by oxygen as well as by butene-2, a value of 3x 10 l.mole sec was estimated" from similar studies, ki 10 sec was determined. The rate coefficients reported for the fluorescence" and phosphorescence steps were 4x10 secand3 x 10 sec, respectively. 

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