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 (Δ E ) 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. Copyright © 2004 John Wiley & Sons, Ltd.