First hyperpolarizabilities of triazine derivatives. Ab initio studies and Hammett correlation

First hyperpolarizabilities (β) of triazine derivatives were studied by the ab initio method (HF/6-31G). The β values of these molecules increase with a stronger donor and as the conjugation length increases, probably because the electronic charge becomes more delocalized and the HOMO-LUMO energy gap (ΔE) and the bond length alternation (BLA) decrease with variation of the chromophore structure. Also, the susceptibility of β to the donor strength is found to be larger for a more elongated substrate. Noteworthy is the excellent linear relationship between β and BLA and also the gas-phase substituent constants (σ_gas⁺). This result may serve as a useful guideline for the design of two-dimensional octupoles with large first hyperpolarizabilities.