Some organic and organometallic molecules with remarkably large second hyperpolarizabilities

The structure-polarization relationship has been investigated in a series of polyenes. Various conjugation patterns have been studied and their effect on the␣polarizabilities and hyperpolarizabilities has been commented upon. Two classes of conjugated oligomers have been shown to have very large second hyperpolarizabilities. It is shown that the known potential of the conjugated chains to lead to large nonlinearities is enhanced by substituting one or more hydrogen atoms with lithium. This interesting and useful point is documented by calculating the hyperpolarizabilities of several selected organolithium derivatives presenting a variety of molecular structures. The largest computed nonlinearity is γ = 4 × 107 a.u., which is an exceptionally large value. The present computations have been performed using the semi-empirical approaches MNDO, PM3 and MNDO/d. This choice of well-tested semi-empirical models, in connection with the available literature values (theoretical and experimental), supports the validity of the reported findings.