Nonlinearity enhancement by solid-state proton transfer: A new strategy for the design of nonlinear optical materials

Theoretical considerations of the molecular hyperpolarizabilities (beta) 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 beta 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 P2(1)2(1)2(1) and is composed of 4-(dimethylamino)pyridinium 4-nitrophenolate 4-nitrophenol (DMAPNP2). Theoretical calculations ofthe hyperpolarizabilities predict that the values of beta are greater for the 4-(dimethylamino)pyridinium and 4-nitrophenolate ions than for the corresponding neutral molecules. A second noncentrosymmetric material has been grown from an equimolar melt of 4-hydroxypyridine and 4-nitrophenol. In this cocrystal, which forms in the noncentrosymmetric orthorhombic space group Pna2(1), the pyridine derivative is present as the 4-pyridone tautomer. Though proton transfer occurs for only one of the two crystals presented here, the work with DMAPNP2 illustrates that new, highly efficient NLO materials can be designed by using acid/base chemistry.