Relation Between Bond-Length Alternation and Second Electronic Hyperpolarizability of Conjugated Organic Molecules

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Science  09 Jul 1993:
Vol. 261, Issue 5118, pp. 186-189
DOI: 10.1126/science.261.5118.186


The solvent dependence of the second hyperpolarizability, γ, of a variety of unsaturated organic compounds has been measured by third harmonic generation at 1907 nanometers. It is seen that the measured γ is a function of solvent polarity. These solvent-dependent hyperpolarizabilities are associated with changes in molecular geometry from a highly bond-length alternated, polyene-like structure for a formyl-substituted compound in non-polar solvents, to a cyanine-like structure, with little bond-length alternation, for a dicyanovinyl-substituted compound in polar solvents. By tuning bond-length alternation, γ can be optimized in either a positive or negative sense for polymethine dyes of a given conjugation length.