Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

5 FLUORESCENCE QUENCHING 153is then given by T~ = tan A+/2zv. The modulation may be brought about by passingthe light through a liquid cell across which a suitable periodic disturbance hasbeen applied, for example, ultrasonic waves can cause a suitable liquid column tobehave like a diffraction grating (the Debye-Sears effect, cf. Section 3). Other rapidperiodic-switching devices such as a Kerr cell between two crossed polarizers may beused (although the opaqueness of nitrobenzene to uv light and its tendency tophotodecomposition make it a somewhat unsuitable Kerr cell filler).How may these measured functions be related to the rate coefficients of reactionsof the excited species? It is useful to consider how the excitation arises and the variousways in which the excited species may dissipate its excess energy. Suppose Ais excited to A* which reacts with B to give products. There are two distinct mechanismswhich may be considered for such a fluorescence quenching process. Beforethe transfer of energy from A* to B is possible the species must form an encountercomplex in which the solvent cages of A* and B have been sufficiently modified toallow significant chemical interaction between the two reactants. By the dynamic,or diffusional, pathway A* is formed in comparative isolation from B, and the encountercomplex is produced as a second stepA( + B) + A*( + B) * A*.B 2 productsThe static path involves the excitation of a molecule of A which is already inclose proximity to BA + B * A*.B 3 productsConsider, now, how the energy is dissipated. In the absence of B, A* may lose itsenergy either as fluorescence emission or in some non-radiative process such as interactionwith the solvent. On the convention used by For~ter'~ and Weller" the ratecoefficients or probabilities for these two processes are denoted by nf and nd sec-',respectively. The lifetime T~ of the excited species is then (nf+nd)-' sec and thequantum yield $o of the fluorescence process is nf/(nf+nd) = nf-xo. When B ispresent, a third method of energy dissipation from A* is by reaction with B; forthis a pseudo-first order rate coefficient k2cB sec-' may be assigned. The lifetimeof A*(T) is now given byand the quantum yield 4J byThus the ratio of the quantum yields (which is equal to the ratio of the measuredfluorescence intensities with and without B present, Z/Zo) is given byReferences pp. 176-1 79