4. Absorption, Fluorescence, and Phosphorescence^ 



Molecules send us messages through photons, be it through 

 photons they absorb, be it through photons they emit. This chapter 

 will be devoted to the evaluation and decoding of some of these 

 messages in relation to the problem of biological energy transmis- 

 sion. 



If a photon hits a molecule and finds no electron which can take 

 up its energy, then it passes this molecule without leaving a dis- 

 turbance behind. If it does find such an electron, then this electron 

 absorbs the photon and becomes excited to a higher energy level. 

 In order to transmit £* a substance must be able to accept it. The 

 light absorption tells us what energies a molecule is capable of 

 accepting. 



The fate that the E* thus communicated to a molecule in vitro 

 may meet varies according to the qualities of the excited electron 

 and the molecule which contains it. Electrons of the outer shells 

 of the molecules belong mostly to an individual atom and are 

 bonded to its nucleus. If such a bonded electron is excited, it is 

 likely to cause the nucleus to vibrate around its equilibrium posi- 

 tion and spend its E* which, eventually, becomes dissipated as 

 heat. Evidently, such an electron would be unfit to act as an 

 energy transmitter. 



Molecules containing conjugated double bonds have nonbonded 

 electrons which do not belong to single atoms but to the whole 

 conjugated system. These v electrons, as a rule, are more easily 



*The student interested in fluorescence and related phenomena will find 

 two excellent monographs written on this line by Forster and Pringsheim. 

 The present knowledge about the triplet state has been reviewed lately by 

 Kasha and McGlynn. 



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