Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

156 EXPERIMENTAL METHODS FOR FAST REACTIONS(iu) Much work has been done on acid-base reactions of the typeandA*H+B = A*-+HB+A*+HB = A*H+ +B-.An example of group (iu) which also illustrates rather well the fact that A andA* frequently behave very differently has been reported by Urban and Weller".The fluorescence of certain substituted amino-pyrenes of the typewas studied in various alcohols. In the ground state these amino-pyrenes (ArNRJare weak acids but in the excited state they are very strong acids, having dissociationconstants between about 10' and lo6 mole.1-'. Thus in strongly acidic media theunexcited species is mainly in the protonated form ArNR2H+. From restrictionsof the Franck-Condon type it is to be expected that the excited form will beAr*NR2H+ and that its molecular environment will be that appropriate to theground state, even though in the excited state the deprotonated form is thermodynamicallythe more stable. However, at room temperature the fluorescence spectrumwas found to be due to Ar*NR2, and this was explained in the following way.The deprotonation reactionAr*NR2H+ 2 Ar*NR2 + H+ (6)is so fast that the lifetime of Ar*NR2H+ reacting by (6) is much less than itsfluorescence lifetime. As the temperature was lowered, another emission spectrumappeared and the &st decreased in intensity. The deprotonation reaction (6) wasevidently being slowed down, so that k,, eventually became much less than thereciprocal fluorescence lifetime of Ar*NR2 H+. Thus during the lifetime ofAr*NR,H+ no more proton loss occurred and only fluorescence due to this specieswas observed. The lifetimes T~ were not actually measured, but there was very goodreason to suppose that they were about the same as for unsubstituted pyrene, ofthe order of lo-' sec. One very interesting result of this study was that, althoughthe dissociation constants varied over four orders of magnitude, klz was found tobe independent of the substituents on the ammonium-pyrene, but depended significantlyon the alcohol used as solvent. It was concluded that the rate-determiningstep was independent of factors involving the other groups on the ammoniumpyrene,e.g., changes in solvation around the sulphonate groups, and that the impor-