26 PROBLEMS IN PHOTOSYNTHESIS 



Excited molecules have a longer life in the triplet state than in the singlet 

 state, as the electron is quasi-trapped. The probability of the singlet- 

 triplet change can be considerably increased by the addition of oxygen or 

 iodides. Kasha (31) found that the singlet spectrum of dichloronaphthalene 

 can be changed into the triplet spectrum by the addition of ethyl iodide. 

 Decrease of temperature to far below ° C destroys the fluorescence of dyes 

 and of some biological substances. Phosphorescence occurs instead of 

 fluorescence because the molecules are brought into the triplet state. 



Chi 



^ — >Chl* 



fluorescence 



transition into 



a long lived ^sensitization 



triplet state 



Fig. 11. The photochemical behavior of excited chlorophyll. 



§ 12 The Excited State of the Chlorophyll Molecule 



There are two different ways in which chlorophyll can use light energy: 

 fluorescence and the photochemical process of photosynthesis. The excited 

 chlorophyll molecule may, depending on the conditions prevailing, either 

 show fluorescence or change into long-lived active forms (triplet state), in 

 which absorbed light energy can be stored for photochemical action. The 

 scheme in Figure 1 1 shows both possibilities; the excited chlorophyll molecule 

 is represented by Chi*. The greater the fluorescence, the smaller the photo- 

 synthetic activity (32). Wassink (62) tried to establish a correlation between 

 fluorescence and photosynthesis. Franck (22, 23, 24, 46) considered fluo- 

 rescence can be ignored so that — as has also been pointed out by Uri (54) — 

 there is no evidence to support Wassink's views. 



Quite a number of hypotheses on the change of Chi* into long-lived 

 forms have been established. The older assumption that Chi* may give 

 off H atoms 



Chi* -^ H + Chi,, 



Chl^^ representing an oxidized or a dehydrogenated chlorophyll molecule, 

 is not accepted today (8). It is indeed difficult to imagine how a C — H 

 bond having a bond energy of 87000 cal/mole can be split in the excited 

 molecule by the uptake of only 40000 cal/mole radiation energy (red part of 

 the spectrum). This might be possible in the blue or the violet (photo- 

 chemical equivalent: 70000 cal/mole) together with thermal energy, but 



