2-(p-Diethylaminostyryl)pyrrole (I) and 2-[5-diethylaminothienyl]vinyl]pyrrole (II) derivatives with systematic variation of the acceptors have been synthesized and their first hyperpolarizabilities were measured.
The b values increased systematically as the aromatic resonance energy decreased. Moreover, the value of b for the former increased gradually as the acceptor strength increased.
The opposite trend observed in the latter series of compounds has been attributed to the distorted structure caused by the steric hindrance between the N-methyl group and the acceptor moiety.