PHOTOSYNTHETIC PIGMENTS 2$ 



concentration of phenyl hydrazine actually stimulates fluor- 

 escence of chlorophyll in benzene). The fluorescence is 

 also quenched by oxygen to an extent of about 20% in air. 

 Removal of the magnesium to form the phaeophytin has 

 little effect on the fluorescence but replacement of the mag- 

 nesium by copper abolishes the fluorescence. The porphyrin 

 series is similar, both free porphyrins and the magnesium 

 derivatives being fluorescent whilst the copper derivatives 

 are not. 



3. Chemiluminescence. The luminescence of tetra- 

 pyrrolic compounds when heated in organic solutions was 

 first discovered with the compound magnesium phthalo- 

 cyanine. Free porphyrins or metallic derivatives other than 

 the magnesium compounds, with the exception of zinc 

 which shows a weaker effect, do not luminesce. Thus the 

 ability of a pigment to luminesce does not simply depend on 

 the capacity for fluorescence but in the case of these tetra- 

 pyrrolic compounds rather on the presence of magnesium. 

 A solution of chlorophyll in tetralin (1:2:3: 4-tetrahydro- 

 naphthalene) shows a strong orange-red glow when the 

 temperature is raised to about 200° C, the spectrum of 

 the emitted light resembling the fluorescence spectrum of 

 chlorophyll. The maximum is, however, shifted nearer to 

 the violet due in the first place to the thermal degradation of 

 the side groups of the chlorophyll molecule. Other organic 

 liquids can be used in place of tetralin, e.g. higher paraffins, 

 natural oils or fats. 



4. Photochemical sensitizations. As early as 1874 Becquerel 

 showed that chlorophyll could be used as a s^ensitizer for the 

 photographic plate and since that time a variety of reactions 

 photocatalysed by chlorophyll have been described. The 

 reactions fall into two classes: {a) oxidations involving mole- 

 cular oxygen and {h) oxido-reductions involving hydrogen 

 transfer between two reactants. 



In the first class Gaffron (1927) showed that chlorophyll 

 photocatalysed the oxidation of allyl thiourea in acetone or 

 of thiourea in pyridine. The efficiency decreases as the con- 

 centration of thiourea is decreased or the concentration of 



