The relation between colour and molecular structure in organic compounds

Open Access. Scanned from Vol.1 of Scientific Papers of C.V. Raman. Ed. by S. Ramaseshan. Published in 1988 by the Indian Academy of Sciences, Bangalore.(a) After a brief review of the existing theories, the paper considers the relationship between colour and constitution in organic compounds from a new and fundamental standpoint suggested by a comparison of the structure and the physical properties of diamond and graphite. (b) A remarkable and very general perallelism between the colour or light absorption, and the degree of optical, electrical, and magnetic anisotropy of the molecules is revealed on an examination of the data of(a) light absorption, (b) light scattering, (c) electric birefringence, (d) magnetic birefringence, (e) magnecrystalline behaviour, and (f) crystalline birefringence of carbon compounds. Those structures and groups that tend to favour the development of colour also tend in general to enhance the degree of optical, electrical and magnetic anisotropy of the molecule, The facts covered by the "theory of chromophores and auxochromes", the "quinonoid hypothesis" and "the strain theory of colour" are simultaneously included within this single generalisation. , (c) A theoretical basis for the generalisation is furnished by the electron theory of dispersion, taken together with certain hypotheses (due to J J Thomson and L Silberstein) regarding the origin of the optical anisotropy of molecules, which are supported by observational data regarding refractivity, dispersion and pleochroism of organic compounds. (d) 7he mechanism of light-absorption is briefly discussed, with reference to electrical conduotivity, and the special type of photoconductivity observed in illuminated crystals of high refractive index