52 LIGHT AND LIFE 



E. Low Absorption Intensity 



It is observed empirically that allowed tt ^- tt* bands in polyatomic 

 molecules may have oscillator strengths from 0.1 to approx. 1, with 

 molar absorption coefficients frequently in the range 10,000 to 100,000 

 or more. 



On the other hand, n -^ tt* transitions, even if symmetry allowed, 

 have oscillator strengths of the order of magnitude of 10--, with 

 molar absorption coefficients in the range 300 to 2000. A study of 

 the spatial distributions for TT-orbitals and n-orbitals (Fig. 3, 4) 

 will indicate one cause of this low intensity: the relatively good or- 

 bital overlap for tt ^^ tt* promotion and the relatively poor overlap 

 for n -^ TT* promotion. Orgel (30) has examined other sources of low 

 intensity in w -» tt* transitions in addition to spatial overlap, includ- 

 ing the effect of charge redistribution upon 7i -^ tt* promotion, and 

 the diminution of intensity arising from hybrid lone-pair orbitals, 

 in which only one of the hybrid components (s-orbitals in the case 

 of sp~ hybrid orbitals on the N of N-heterocyclics) contributes to the 

 intensity of an n -^ tt* transition. 



F. Unique Polarization 



In section IV-A the polarization of n -^ tt* singlet-singlet transitions 

 was shown to be out-of-plane for the N-heterocyclic molecules, if al- 

 lowed (cf. Fig. 10) , and the tt -^ tt* transitions in-plane, by the use 

 of group theory. For singlet-triplet transitions, these polarizations 

 are reversed, as shown by Clemen ti and Kasha (5) . 



Physically it is easy to see that the singlet-singlet n -^ tt* transi- 

 tions should be out-of-plane polarized. The 77-orbitals of the mole- 

 cules of Figs. 3, 4, and 5 are symmetric to the molecular plane. The 

 TT-orbitals are antisymmetric to the molecular plane. An overlap in- 

 tegral between the ?i-orbitals and the 7r-orbitals over the whole space 

 of the molecule will obviously be equal to zero, since the negative 

 part of the overlap will cancel the positive part. However, an electric 

 dipole may be antisymmetric to the molecular plane: when a dipole 

 ojjerator acts on (nudtiples) an orbital distril)ution Avhich is antisym- 

 metric to the same plane, a synnnctric distribution function is ob- 

 tained. Thus, an integral such as the transition moment integral (up- 

 per part of Fig. 11) will be non-vanishing if the electric vector of the 

 light is polarized out-of-plane. The recent results Innes, et al. (Ha), 

 on rotational analysis of the first ultraviolet absorption band of the 

 diazines, confirm the polarization deductions. 



