ENERGY TRANSFER BETWEEN DIFFERENT PIGMENT MOLECULES 1309 



and another which was not re-absorbed. By plotting these ratios as func- 

 tion of the thickness of the layer, curves were obtained which agreed with 

 the theoretical equations (derived from Beer's law) as long as the layer was 

 not thinner than the wave length of light. In thinner layers, systematic 

 de\nations from the simple absorption theory were found which were in- 

 terpreted as evidence that, in such thin layers, the loss of fluorescence 

 cjuanta caused by resonance transfer became significant. 



In following the picture of energy transfer still further into the field of 

 nonresonating systems, one notes that, since the period of a single molecular 

 vibration ma,y be sufficient to achieve transfer of a quantum immedi- 

 ately after its absorption, the probability of transfer may be significant 

 also if electronic resonance can be established for the duration of one (or 

 a few) molecular vibrations during the process of internal energy dissipa- 

 tion in the primary absorber. For example, a molecule that had absorbed 

 an ultraviolet quantum, and is engaged in internal conversion of the latter 

 into vibrational quanta, must, during this process, assume for a short time 

 a configuration in which it is in electronic resonance with a molecule that 

 absorbs only visible quanta. It passes through this configuration so 

 rapidly that no emission of visible fluorescence can be noticed (except per- 

 haps by an ultrasensitive, photon counting method) ; and yet the time of 

 passage may be long enough for the excitation energy to migrate by reson- 

 ance. In this way, one could perhaps explain (as first suggested by Franck 

 and Li\angston 1949) such phenomena as the dissociation of the myoglobin- 

 carbon monoxide compound by ultraviolet light absorbed in the protein 

 moiety of the porphyrin-protein complex (Bucher and Kaspers 1947). 

 Two possible alternatives are, however: (1) resonance transfer of the 

 ultraviolet quantum as such from the protein to the chromophore (which 

 has absorption bands also in the ultraviolet !) ; and (2) conversion of elec- 

 tronic excitation into vibrational energy in the protein, followed by its 

 spreading into the porphyrin ("intermolecular heat conduction"). The 

 second alternative is not possible in the case of Bannister's (1953) study of 

 the intensity of fluorescence of phycocyanin excited by various wa^■ e lengths 

 between 230 and 360 mn, in which he found the same quantum yield every- 

 where, including the absorption band at 277 m/x, where about 50% of the 

 total absorption can be attributed to the protein moiety of the chromopro- 

 teid (characteristic band of the aromatic amino acids tyrosin and trypto- 

 phan !) . One of the two above-mentioned resonance transfer mechanisms 

 must be operative in this case. 



The energy transfer (sensitized fluorescence) between organic molecules in the vapor 

 phase (aniline -^ indigo; benzene — *• aniline) was described by Prileshajeva and co- 

 workers (]934, 1937) and Terenin (1943). Energy' transfer between molecules of this 

 type in crystals was studied quite extensively, the best known case being that of naph- 



