Nonlinearity Enhancement by Solid-State Proton Transfer: A New Strategy for the Design of Nonlinear Optical Materials

Abstract

Theoretical considerations of the molecular hyperpolarizabilities (β) of donor−acceptor substituted aromatic molecules show that the transfer of a proton from the electron-donor group of an acid to the electron acceptor group of a base could increase β for both resulting ionic species relative to the corresponding neutral molecules. To take advantage of this, the cocrystallization of 4-nitrophenol with various organic bases was studied. Cocrystallization of 4-nitrophenol with 4-(dimethylamino)pyridine gives yellow crystals that display a strong second harmonic signal by the Kurtz and Perry powder test. The material crystallizes in the orthorhombic space group P212121 and is composed of 4-(dimethylamino)pyridinium 4-nitrophenolate 4-nitrophenol (DMAPNP2). Theoretical calculations of the hyperpolarizabilities predict that the values of β are greater for the 4-(dimethylamino)pyridinium and 4-nitrophenolate ions than for the corresponding neutral molecules. A second noncentrosymmetric material has been grown ...