824 LIGHT AND LIFE 



The Nature aiid Significance of n — > tt* Transitions 



Michael Kasha has dealt in detail with the n -^ tt* transitions in 

 polyatomic molecules, the understanding of which he has very largely 

 elucidated himself. The application to deoxyribose nucleic acid and 

 to chlorophyll is of obvious importance in understanding the relation 

 of the optical phenomena observed in molecular spectroscopy to the 

 biological functions of these molecules. The first broad distinction in 

 this area of work was made by Burawoy, who noted that certain ab- 

 sorption bands in molecules shift toward the red in solvents of higher 

 dielectric constant, whereas others shift toward the blue under the 

 same conditions. The former are now recognized to be tt — » tt* 

 transitions; the latter, in most cases, n — > tt* transitions. 



Kasha points out that the lone-pair electrons which characteris- 

 tically give rise to the lowest excited singlet states may originate either 

 from n (non-binding) orbitals which cannot conjugate with vr-orbitals, 

 or from /-orbitals which can do so. The carbonyl group of formalde- 

 hyde or acetone and the aza-nitrogen of pyridine supply examples of 

 n-orbitals. Examples of /-orbitals are to be found on the nitrogen 

 of the pyrrole ring, the amino group of the aniline molecule, or the 

 central carbon atom of triarylmethyl radicals. Because of the con- 

 jugation of these orbitals with the 77-orbitals of the molecule, only 

 TT -^ TT* transitions are to be expected. But if, in the aniline or 

 triarylmethyl structure, there is a twist of 90° at the C-N or corre- 

 sponding bond, there would be no conjugation of the /-orbital with 

 the TT-orbitals; and at intermediate degrees of twisting a transition 

 from the /-orbital to an empty antibonding vr-orbital could occur. 

 This is labeled by Kasha an / — > fl^ transition to distingiush it from 

 an n -^ tt* transition. The spectjal changes of the two types do, 

 however, resemble one another, though their polarizations and in- 

 tensities contrast sharply. 



Formaldehyde has three weak absorption bands. One at 1650 A 

 has been identified as a tt ^ 77* transition; one at 1900 A as perhaps 

 an 71 ^ TT* transition; a very weak 2900 A absorption band as almost 

 certainly an ;/ -^ tt* transition. In the N-heterocyclic molecules, so 

 important biologically, the longest wavelength singlet-singlet absorp- 

 tion bands are // -^ tt* transitions. 



7 he molecular orl^itals in which the electrons move can be classi- 

 fied according to their symmetry properties. According to quantum 

 mechanics, an orbital is either synnnctrical (its sign remaining un- 

 changed upon exchange of coorcHnatcs) or antisynnnetrical (its 



