R. S. BECKER AND M. KASHA 31 



an excited ( antibonding; cf. Coulson, 1947) 7r-orbital; (b) the excita- 

 tion of a nonbonding electron of the N-atom in pyridine to an excited 

 (antibonding) 7r-orbital. 



For our purposes the characteristics of n -^ tt- transitions which are 

 of interest are: 



(1) n ^ TT-absorptions (even if allowed by spectroscopic selection 

 rules ) are weaker tlian corresponding tt -> 7r-absorptions ( Kasha, 

 1950). This means, in view of the relation discussed in Section III 

 ( 1 ) , that n,7r-excited states will be much longer Hved than analogous 

 7r,7r-excited states, with the consequences discussed previously. 



( 2 ) n -^ TT-absorptions show a blue shift upon change of solvent 

 from hydrocarbon type to hydroxylic type (Kasha, 1950; McConnell, 

 1952). Blue shifts of as much as a few hundred to a few thousand 

 wave numbers (cm"^) have been observed. Recently it has been 

 estabhshed (Brealey and Kasha, 1954) that these blue shifts may be 

 ascribed largely to hydrogen bonding of the solvent to the n-electrons 

 of the solute molecule. 



(3) If n -^ TT-absorptions fall at lower energy or longer wavelength 

 than any tt -^ 7r-absorption, then the molecule will be nonfluorescent 

 (Kasha, 1950). Examples are many-fold, in which the nonfluorescence 

 may be attributed to this juxtaposition of energy levels. Thus, all 

 ahphatic ketones and many aromatic ketones and aldehydes are non- 

 fluorescent and have n,7r levels lower than 7r,7r. In these, $f° = 0, and 

 <l>p° ^ 1, so that (only) very strong phosphorescences are generally 

 observed in rigid glass solutions. Other examples are most nitro 

 compounds, quinones, azo compounds, and simple N-heterocychcs. 



The application of these interpretations to the chlorophylls may be 

 made as follows. Both chlorophyll a and chlorophyll b contain carbonyl 

 groups, C = O. Isolated carbonyl groups will have ultraviolet n -^ -n-ab- 

 sorptions, similar to that in acetone; these will not interest us. How- 

 ever, C = O groups conjugated with other parts of the molecule may 

 have n -^ 7r-absorption at quite long wavelengths. Moreover, the more 

 highly conjugated the carbonyl group, the stronger will the n -^ 7r-ab- 

 sorption be, although in all cases somewhat less strong than pure 

 TT-electron absorptions. Moreover, such n -> 7r-absorptions should 

 blue shift in hydroxylic solvents, or any solvents capable of forming 

 molecular complexes specifically involving the n-electrons of the chlo- 

 rophyll carbonyl groups. 



