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 Original Research Article

Ab initio RHF calculations have been used to study the relative variation of βe and βv in D/A (NH2/NO2) substituted π-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 (ω≤100 cm−1) modes, which have long response times and are associated primarily with torsional motions. For all cases considered the βv; >100(0)/βe(0) ratio lies in the range 1.35–6.58. In our systematic comparisons we have found that increases in βe are most often accompanied by increases in βv. 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 sp3 defect, when changing the nature of the aromatic rings in stilbene-like linkers and, more importantly, upon doping α,ω-nitro,amino all-trans polyene chains.