76 



III. PORPHYRIN CHEMISTRY 



of four carboxylic groups into the ethyl groups of etioporphyrin has 

 no spectroscopic effect; the separation of the carboxyls by two aU- 

 phatic carbon atoms from the resonance system is able to prevent 



TABLE Vni 



Absorption Spectra of Porphyrins in Hydrochloric Acid" 



"25% HCl unless otherwise stated. 



''Stronger HCl shifts the visible absorption maxima slightly toward the red, not 



decreasing the absorption up to 3 A^ HCl; the Soret band is, however, considerably 



decreased by strong HCl {cf. also 1231, 1515,2097, 26ItO). 



these groups from influencing the spectrum. In uroporphyrin, however, 

 with only one carbon atom between carboxyl and the resonance sj^s- 

 tem of the nucleus, the bands lie more toward the infrared than those 

 of copro-, meso-, or etioporphyrin. 



4.4. Fluorescence 



The red fluorescence of porphyrin solutions in mineral acid or in 

 organic solvents is so strong that it did not escape the notice of the 

 discoverers (2801). Solutions in hydrochloric acid of 0.01 to 1.0 ng. 

 per ml. porphyrin emit a distinctly visible and measurable fluores- 

 cence even under rather weak ultraviolet light. The fluorescence is 

 excited by light absorbed by the Soret band (i.e., violet and ultra- 



