Vibrational versus electronic first hyperpolarizabilities of π-conjugated organic molecules:: an ab initio Hartree-Fock investigation upon the effects of the nature of the linker

Ab initio RHF calculations have been used to study the relative variation of beta(e) and beta(v) in D/A (NH(2)/NO(2)) substituted pi-conjugated compounds as a function of the specific nature, length, and charge of the linker. For the vibrational property we have separated off the contribution due low-frequency to (omega less than or equal to 100 cm(-1)) modes, which have long response times and are associated primarily with torsional motions. For all cases considered the beta(v) (>100)(0)/beta(e)(0) ratio lies in the range 1.35-6.58. In our systematic comparisons we have found that increases in beta(e) are most often accompanied by increases in beta(v). Similar behavior has also been obtained previously for a series of mono- and di-substituted benzenes. However, there are counter-examples as well: when the conjugation is broken by inserting an sp(3) defect, when changing the nature of the aromatic rings in stilbene-like linkers and, more importantly, upon doping alpha,omega-nitro,amino all-trans polyene chains. (C) 1999 Elsevier Science B.V. All rights reserved.