SOME PHOTOCHEMICAL CONSIDERATIONS 23 



nance in chloroplasts is also observed in the dark, though to a lesser degree. 

 Commoner's studies on isolated chloroplasts and also on living Chlorella 

 support the view that in photosynthesis there must be a univalent electron 

 transport and that the chloroplast acts as a semiconductor (5) (see § 55). The 

 question arises whether the electron spin resonance observed in chloroplasts 

 is due to enzymatic processes. However, Calvin and Sogo found that 

 cooling" to —140° C did not influence the phenomenon. 



§ 11 Excited States of Molecules* 



When a system absorbs light, the equivalent of the energy absorbed is not 

 lost but has to reappear either in another form or as radiation energy. It is 

 changed into thermal energy when the temperature of the system is increased. 

 It is changed into chemical energy when an endothermic chemical reaction is 

 involved, as is the case in photosynthesis. The radiation energy absorbed, 

 however, may also be emitted again as radiation energy; this is called photo- 

 luminescence. Each photoluminescence exhibits some kind of inertia, as 

 some time passes between the removal of the incident light stimulus and the 

 end of the luminescent phenomenon. If this time span is shorter than 10"^ 

 sec, we have fluorescence. If it is longer than 10~^ sec, we speak of phos- 

 phorescence.** 



Absorption of radiation energy means absorption of a quantum by an 

 electron bringing the latter on to a higher energy level, i.e., bringing it into 

 an excited state. When this process deals with electrons of the outer shells 

 of the atoms, vibration of the nuclei occurs; this means nothing but a change 

 of radiation energy into thermal energy. In molecules with conjugated 

 double bonds*** we have so-called tt electrons which, not being bonded to 

 any definite nuclei, belong to the whole molecule. When radiation energy 

 is absorbed by tt electrons, emission of radiation energy of lower frequency 

 takes place. If the molecule contains weak bonds with a dissociation energy 

 lower than the radiation energy absorbed (excitation energy), the weak bonds 

 may be changed, the absorbed energy being used up. This is a chemical 

 change and no radiation will be emitted. We may illustrate this by the 

 benzene molecule, the bonds of which require a dissociation energy of at 

 least 126000 cal/mole. According to equation 2, the excitation energy of 

 light of the wave-length 2700 A amounts to 106000 cal/mole. The dissocia- 

 tion energy is thus higher than the excitation energy, and fluorescence, but 



* For literature on the excited states of molecules, see Laidler (34), Pringsheim (39), Reid (40). 

 ** This differentiation according to Forster (21 ), though arbitrary, is less complicated than the 

 definitions given by Perrin (38) and by Lewis and Kasha (30, 35). See also page 25. 

 *** Conjugated double bonds are written as 



— CH=CH— CH=CH— CH=etc 



Organic molecules showing photoluminescence may belong to different categories but they have 

 such conjugated double bonds in common. 



