INFLUENCE OF MEDIUM 635 



10~^ sec, energy dissipation within 5 X 10~^^ sec. would reduce the yield 

 of fluorescence to <0.01% and thus make it practically unobservable.) 



Similar considerations apply to state A. The ease with which states 

 .4 and B are transformed into state Y (as shown by the excitation of red 

 fluorescence with yellow or blue light, cf. page 748) indicates that the po- 

 tential energies of states A, Y and B (plotted against some appropriate 

 "configiH'ation co-ordinate") give curves of the type shown in figure 21.22. 

 At point M, the electronic excitation energy of state B is easily transformed 

 into the (smaller) electronic excitation energy of state Y, plus a large 

 amount of vibrational energy. 



The yield of red fluorescence of chlorophj'll in solution is of the order of 

 10% (cf. chapter 23). This shows that the actual life-time of state Y in 

 solution is of the order of one tenth of the above calculated "natural" life- 

 time, i. c, about 5 X 10"^ sec. The various "quenching" processes that 

 may contribute to this shortening of the life-time of excited molecules will 

 be discussed in chapter 23 (page 755). 



B. Influence of Medium on Absorption Spectrum of 

 Chlorophyll and Bacteriochlorophyll* 



We have spoken so far of the absorption spectra of chlorophyll antl its 

 derivatives in solution as though they were determined only by the chemi- 

 cal structure of these compounds. However, the absorption spectra also 

 are affected by changes in the nature of the solvent, and even more strongly 

 by adsorption on solids, or by the formation of colloidal aggregates. These 

 spectroscopic changes are caused by interaction between the light-absorbing 

 molecules and their neighbors. Kundt had noticed as early as 1878 that 

 the absorption bands of many dyestuffs are shifted toward longer waves 

 with increasing refractivity of the solvent. This relation appears plausible 

 in the light of London's theor}^, which establishes a connection between 

 the capacity of molecules to refract light and the intensity of intermolecular 

 forces — both properties being determined by polanzability of the molecules. 



Parallelism between molecular attraction and polarizability presumes 

 the absence of other, chemical or physical forces between the interacting 

 molecules. Such forces may arise if the molecules bear electric charges, 

 dipole moments, or possess incompletely saturated valencies; therefore, 

 we can expect Kundt 's rule to apply primarily only to neutral, nonpolar, 

 saturated molecules in nonpolar, saturated solvents. The rule may also 

 apply to series of polar or nonsaturated molecules in which the additional 

 interactions due to dipoles or residual valencies are approximately constant 

 (e. g., to a homologous series of alcohols). 

 * Bibliography, page 669. 



