The Quenching of Anthracene Fluorescence by Weak Quenchers

Abstract

The transition from strong to weak quenching The quenching of the fluorescence of anthracene by aniline, ethyl iodide, carbon tetrachloride and bromobenzene was measured as a function of fluidity. The fluidity was varied by mixing purified heavy paraffin with petrol ether. If quenching is considered to occur only when quencher and fluorescer are adjacent to one another (an encounter), it is possible to derive an equation relating the quenching constant with the rate constant for quenching in encounters, and the rate constants for diffusion into and out of encounters. The equation is Lim[Q]→O f0/f-1/[Q] = k0 = k'3(K+k'1)/1+k'2+k'3 (i) Where k3 = rate constant for quenching in encounters K1 = rate constant for diffusion into encounters K2 = rate constant for diffusion out of encounters K = k1/k2 f0 and f are the fluorescence intensities before and after the addition of quencher. The prime denotes multiplication by ζ , the lifetime of fluorescence in absence of quencher. The agreement of this equation with experimental quenching constants determined at different fluidities is satisfactory. k1 is calculated at each fluidity, from SO2 quenching data (where k0 for SO2 = k'1+K). K as might have been expected, is of the order unity, while k3 decreases as the quencher becomes weaker, as is shown in the following table:- Aniline k'3 = 100 K = 3.5 Carbon tetrabromide 40 8 Ethyl iodide 8 to 12 2 to 3 Carbon tetrachloride 0.5 to 1 2 to 3 Bromobenzene 0.6 to 1 2 to 3 The difficulty of estimating K and k'3 separately, becomes greater as the quenching constant decreases. Other methods of determining these constants are therefore investigated. K may be found from the effect of the quencher on the quenching constant of a strong (diffusion controlled) quencher. The quenching of anthracene fluorescence by oxygen in the presence of ethyl iodide was measured in petrol ether. K was found to be 3.5=-0.5, in fair agreement with the figure given in the table. K may also be calculated from measurements of the solubility of the fluorescer in the solvent and quencher respectively, but the calculation holds only for s-regular solution. This assumption may explain why K calculated for ethyl iodide in petrol ether (K = 2.9) does not agree so well with the above figures. Lifetime of fluorescence The quenching constant deduced from the Stern-Volmer law, K = (f0/f – 1)/[Q] is equal to the ratio of the specific rate constants for quenching and for fluorescence emission (that is the reciprocal of the lifetime). It is of some importance to be able to estimate the lifetime of fluorescence of anthracene in different solvents in order to assess the effect of solvent on the rate constant for quenching. An attempt is made to calculate the lifetime from ζ= ζ eQ where ζ = observed lifetime Q = absolute efficiency of fluorescence ζe = ideal lifetime of fluorescence, in principle calculable from the absorption spectrum. The theory of the measurement of absolute quantum efficiencies of fluorescence is reinvestigated. A large correction must be made allowing for the refractive index of the solvent. This arises from the fact that light passing from a medium of high refractive index to one of low, is spread out, resulting in a decrease in the light passing a fixed aperture. Fluorescence efficiencies determined range from 0.23 to 1.03 (=-0.02). The latter figure (that for 9,10 dimethyl anthracene in benzene) should presumably be unity. The lifetime of anthracene fluorescence in bezene is calculated to be 3.7+- 0.2x10-9 secs; in other solvents the lifetime is higher (approximately 5.2x10 -9 secs). The effect of solvent, temperature and the structure of the quencher, on the quenching constant. Variations in the quenching constant with solvent and temperature, after corrections for changes in fluidity and lifetime have been applied, must be ascribed to both the K and k3 terms, since K = e∆Se/R . e-∆He/RT and k3 = P Z T1/2 e-E'/RT where ∆Se and ∆He = entropy and heat of formation of encounters PZ = a probability factor x the collision frequency in encounters E' = activation energy if any, to the quenching reaction. It is found that (E'+∆He) varies from -1 to +3 k cals/mol., and the non temperature dependent part (PZ) from 10 6 to nearly 10 11 collisions /mol/sec. The PZ factor appears to depend on the existence of electron attracting or repelling groups in the quencher molecule, suggesting that quenching is governed by the ease of electron transfer.