FLUORESCENCE OF CHLOROPHYLL DERIVATIVES 749 



365, 404.7, 435.8 or 546 m/x, or by white light filtered through a red, orange 

 yellow, violet, blue or green filter. 



Whether chlorophyll is capable of emittmg a weak, but long-lasting 

 infrared fluorescence (originating in a metastable state, cj. page 753) is 

 uncertain. Calvin and Dorough (1947) have described such a "phosphores- 

 cence," but Livingston and co-workers (1948) could not confirm their ob- 

 servations (c/. page 795). 



Figure 23. IB shows, beside the fluorescence spectra of the two chlorophylls, also 

 those of the two pheophorbides, and of two porphyrins. The pheophorbides {i.e., 

 chlorophyllides in which hydrogen has been substituted for magnesium) fluoresce not 

 less strongly than the chlorophylls or chlorophyllides themselves; but certain other sub- 

 stitutions in the same position in the molecule {e. g., copper instead of magnesium) cause 

 complete disappearance of fluorescence. 



Stern and Molvig (1935, 1936^), Stern and Dezelic (1936) and Stern 

 (1938) have investigated the fluorescence of numerous porphyrins and 

 chlorins. They found that, similariy to chlorophyll, all of them fluoresce 

 with red light, even when excited by violet or ultraviolet radiation. The 

 main fluorescence band always lies close to the first absorption band in the 

 red — whether this band is the weakest of the whole absorption spectrum 

 (as in some porphyrins) or the strongest one (as in chlorins and phorbins). 

 Stern (1938) found that tetrapyrrole compounds without the closed por- 

 phin ring system (e. g., the bile pigments), as well as compounds in which 

 the conjugation in the porphin ring is interrupted, do not obey this rule, 

 and do not show sharp fluorescence bands at all. He therefore considered 

 a sharp red fluorescence band as an important characteristic of the all- 

 round conjugated porphin ring system. 



According to the term systems given in figures 21.9 and 21.25, the ap- 

 pearance of the red band in fluorescence, to the exclusion of all the other 

 bands, means that all excitation energy in excess of that corresponding to 

 the lowest, nonvibrating, excited electronic state is dissipated before 

 fluorescence can occur — probably first by internal distribution of this 

 energy among vibrations within the pigment molecule, a process known 

 as "internal conversion," and then by gradual transfer of vibrational quanta 

 to the medium. The dissipation is interrupted at the lowest excited level, 

 whether this is level .4 (in porphyrins), Y (in chlorins and phorbins) or 

 Z (in bacteriochlorophyll), long enough to allow a significant proportion 

 of the excitation energy to escape as fluorescence. 



It was suggested by Franck and Herzfeld (1937) that the capacity of 

 chlorophyll to convert rapidly quanta of larger size into smaller red (quanta 

 may be important for the function of this pigment in photosynthesis, be- 

 cause it prevents the occurrence of undesirable photochemical reactions 

 that could be sensitized by the larger quanta. This surmise may or may 

 not be correct, but since the same property is shared by all porphyrins and 

 chlorms, it cannot explain the special suitability of chlorophyll as photo- 

 catalyst in photosynthesis. 



