ENERGY TRANSFER IN PHOTOCHEMICAL SYSTEMS 51 



complicated by the possible occurrence of other effects such as the 

 equilibrium formation of relatively stable, nonfluorescent dimers 

 (Rabinowitch and Epstein, 1941). In many cases, the nature of the 

 concentration dependence and also the effects of temperature and 

 viscosity enable one to decide between the mechanisms. If resonance 

 transfer is to lead to self-quenching, some of the molecules must be in 

 a nonfluorescent state and, furthermore, the lifetime of such a state 

 must be comparable to or longer than the average time which the 

 excitation energy spends in any one molecule. Forster ( 1951 ) suggests 

 that such a nonfluorescent energy sink is a statistical dimer. 



4. Quenching of fluorescence by solutes. This phenomenon is essen- 

 tially similar to self-quenching in that the final energy acceptor must 

 be nonfluorescent, the electronic excitation energy eventually being 

 degraded into the thermal energy of the solvent. Interpretations in 

 terms of resonance transfer may be complicated by the occurrence of 

 intermolecular spin-orbital perturbations (Kasha and McGlynn, 1956) 

 leading to an increased probability of an intersystem crossing into the 

 triplet state. 



It would be impossible here to review all of the many systems in 

 which the above phenomena have been observed. However, we will 

 mention a few of the more significant examples. Forster (1949), in a 

 study of the concentration dependence of the quenching of the fluores- 

 cence of solutions of tryptaflavine by rhodamine B, demonstrated that 

 nonradiative energy transfer occurs efficiently at distances as great as 

 70 A, in agreement with the predictions of his theory. Similarly, 

 Lavorel (1957), in a study of alkaline fluorescein solutions, was able 

 to calculate that a resonance transfer of energy occurred over an 

 average of about 300 molecules. Of particular biological significance 

 is the demonstration by Watson and Livingston (1950) and by Duy- 

 sens (1951) of the sensitization of chlorophyll a fluorescence in 

 methanol solution by chlorophyll b. In addition, there have been a 

 number of studies of energy transfer in protein-dye conjugates (Shore 

 and Pardee, 1956) whereby energy absorbed in the protein (e.g., 

 lysozyme, bovine, plasma albumin, chymotrypsinogen, and ribonucle- 

 ase) portion of the conjugate excited the fluorescence of the dye (e.g., 

 l-dimethylaminonaphthalene-5-sulfonyl chloride) . 



A series of very interesting experiments by Terenin and Ermolaev 



