24 PROBLEMS IN PHOTOSYNTHESIS 



no chemical reaction, will occur. In the pyridine molecule, however, the 

 aromatic C — N bonds arc much weaker than the C — C bonds of the benzene 

 molecule so that photochemical dissociation, but no fluorescence, has been 

 observed (21). 



Generally speaking", we have fluorescence when a molecule absorbs radia- 

 tion energy without using it. As Szent-Gyorgyi (52) pointed out, fluorescence 

 may not have any biological significance. The biological role of molecules 

 is not to emit but to transinit radiation energy. As a matter of fact, biologi- 

 cally active molecules should exhibit very little or no fluorescence as some of 

 the energy absorbed may be lost. Chlorophyll, which we must consider to 

 be the most important energy transmitter, shows very little fluorescence in 

 vivo while its fluorescence in organic solvents is particularly impressive 

 (see § 2). 



•T, 



■T, 



Fig. 7. Schematic representation of photo- 

 chemical excitation. G: ground state. Si and 

 Si: singlet states. 7\ and 7%.' triplet states. 

 A: absorbed radiation. E: energy loss. F: 

 y/f fluorescence. The short arrows indicate conver- 



sions with change in spin. 



Upon irradiation with ultraviolet, the red dye Rhodamin B shows orange 

 fluorescence. Figure 7 depicts schematically what happens. The normal 

 ground state of the molecule is indicated by the line G. The arrow GS^i 

 represents the excitation the molecule undergoes in absorbing the radiation 

 energy. By a change of energy — without radiation — the molecule is 

 brought from the state ■5*2 into the state ^'i. Finally the arrow S^G shows the 

 emission of radiation, which is called fluorescence. This type of excitation 

 whereby an electron arrives at a higher energy level and afterwards returns 

 to the ground state is termed singlet excitation. As has been pointed out in 

 § 8, only two electrons with antiparallel spin can be present on the same 

 energy level. In the case of singlet excitation one of the two electrons jumps 

 on to a higher level and afterwards returns to its companion. This process 

 is quite correct from a quantum mechanical point of view (Fig. 8). The 

 possibility exists, however, that the excited electron, on its way to the higher 

 level, obtains opposite spin so that its magnetic momentum gets the same 

 sign as that of the electron left behind. In this case, the excited electron 

 can no longer return to the ground state, as two electrons with parallel 

 spins cannot coexist on the same level. The excited electron must therefore 

 remain on the higher level indicated in Figure 7 by the lines Ti or T^. The 

 electron is then trapped in the high energy level. This is called the triplet 

 state which, due to some loss of energy, lies somewhat lower than the cor- 

 responding singlet state. The trapped electron emitting radiation can 



