optical properties of nucleic acids 533 



1. General Theory 



The nature of an electronic transition in a molecule caused by the ab- 

 sorption of radiation of appropriate frequency requires that for maximum 

 probability the electric vector of the radiation should have a definite di- 

 rection with respect to the chromophoric group of the molecule. For a 

 linear conjugated system, such as a polyene, the electric vector must be 

 parallel to the direction of the conjugated multiple bonds, while for a planar 

 conjugated system, of which benzene is the simplest example, the electric 

 vector must be in the plane of the ring. The absorption is greatest when 

 these conditions are satisfied. If the electric vector is perpendicular to the 

 plane of the ring the transition probability, and therefore the intensity of 

 absorption, is reduced, though not necessarily to zero, because of inter- 

 action between the two modes of excitation. These polarization properties 

 apply both to allowed transitions and also to the much weaker transitions, 

 e.g., that giving rise to the 260-m)u benzene band, which acquire allowed 

 character because of the disturbing effect of simultaneous changes in the 

 vibrational states. If the ring chromophore is unsymmetrical, the strength 

 of the transition may also depend on the direction of the electric vector 

 in the plane of the ring. 



The intensity of absorption of polarized radiation by a crystal or oriented 

 specimen will therefore depend on the orientation of the electric vector of 

 the radiation with respect to the absorbing groups of the constituent mol- 

 ecules, and hence in the case of a crystal, with respect to the crystallographic 

 axes. For an oriented fiber or sheet, the axis of reference will be the fiber 

 axis or shear direction, as noted above. The crystal or specimen will there- 

 fore require more than one absorptivity to express its absorption properties, 

 and may be described as dichroic (or trichroic if three such coefficients are 

 required). These are known as the principal absorptivities and can be re- 

 ferred to specific directions with the crystal or system. Since the anisotropy 

 of refractive index (birefringence) for unabsorbed light in a region of trans- 

 parency is also a conseciuence of the molecular orientation of the specimen, 

 the directions of the principal refractive indices, and hence of the extinction 

 directions, will be related to, and for uniaxial orientation, will coincide 

 with, the directions of the principal absorptivities. The birefringence and 

 dichroism are not necessarily parallel indications of the orientation of the 

 same groups in a molecule since in a region of transparency the birefringence 

 is the sum of contributions by all the bonds in the molecule whereas in an 

 absorbing region the dichroism is a characteristic of the absorbing groups 

 only. 



Absorption measurements in solution refer to randomly oriented mol- 

 ecules (except for the special case of solutions of long-chain macromolecules 



