EXCITATION OF POLYENES AND PORPHYRINS 115 



of "activators" bring the fluorescence up to full strength in both com- 

 pounds. These activators are polar substances that can form hydrogen 

 bonds, e.g., water, alcohols, acids, or amines. They showed that the 

 activators form a 1-to-l complex with the chlorophyll and that the com- 

 plex is the fluorescing agent. 



The discussion in this chapter suggests a hypothesis that might account 

 for this fluorescence behavior. It is that there is another electronic 

 singlet state in dry chlorophyll a or b, below but close to the ^Qx state, 

 possibly a ^l\ or ^IF^ state of the carbonyl group in the isocyclic ring of 

 chlorophyll (phorbide type). Figure 2-12 shows that in a molecule the 

 size of chlorophyll, with the x-transitions moved down to 16,000 cm~^ 

 they might just be catching up with the carbonyl n-transitions. A low 

 ^L^ or a low W state, with transitions to ground between -iO and 1000 

 times weaker than for ^Q^, would be relatively nonfluorescent. (This 

 would be a favorable situation for finding phosphorescence.) It would 

 not be detected in absorption on the side of the strong ^Qx band until it 

 was several hundred wave numbers lower (see the left-hand side of Fig. 

 2-19); and it would move to the blue in the presence of polar mole- 

 cules or protons, as described earlier in this chapter, perhaps passing 

 above the ^Q^ and permitting the fluorescence to take place. Precisely 

 such crossings between low /i-transitions and x-transitions in polar sol- 

 vents have now been identified for simpler molecules (McConnell, 1952). 



The crossings, if they exist, could be of significance in biological por- 

 phyrins if they served to control the fluorescent and phosphorescent life- 

 times and relative populations during irradiation. 



The possibility that the phosphorescent triplet is ^U or ^W seems to be 

 ruled out by the short reported lifetimes and by the small n-g singlet- 

 triplet separation calculated by Reid (1953). 



If this explanation is correct, the peculiar fluorescence behavior is not 

 directly related to the considerable changes in the absorption spectra 

 which Livingston and coworkers (1949) also found between the dry and 

 wet chlorophyll. (The shift in the n-transition would be almost undetect- 

 able.) Instead, these spectral changes are simply evidence of the change 

 in the spectroscopic moment of the ring substituents when the activator 

 complex is formed. 



For porphyrins capable of forming such complexes, probably most of 

 the spectra now in the literature should be ascribed to the complexed 

 molecule, and spectroscopic moments determined from them should be 

 attributed to the complex. 



ELECTRON DENSITY AND EXCITATION 



The TT-electron densities in the ground and excited states of porphin 

 and tetrahydroporphin are given in Table 2-4, as computed from the 

 LCAO wave functions of Longuet-Higgins and coworkers (1950). These 



