NONLINEARITY ENHANCEMENT BY SOLID-STATE PROTON-TRANSFER - A NEW STRATEGY FOR THE DESIGN OF NONLINEAR-OPTICAL MATERIALS

Theoretical considerations of the molecular hyperpolarizabilities (bet a) of donor-acceptor substituted aromatic molecules show that the tran sfer of a proton from the electron-donor group of an acid to the elect ron acceptor group of a base could increase beta for both resulting io nic species relative to the corresponding neutral molecules. To take a dvantage 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 sec ond harmonic signal by the Kurtz and Perry powder test. The material c rystallizes in the orthorhombic space group P2(1)2(1)2(1) and is compo sed of 4-(dimethylamino)pyridinium 4-nitrophenolate 4-nitrophenol (DMA PNP2). Theoretical calculations ofthe hyperpolarizabilities predict th at 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 equimola r melt of 4-hydroxypyridine and 4-nitrophenol. In this cocrystal, whic h forms in the noncentrosymmetric orthorhombic space group Pna2(1), th e pyridine derivative is present as the 4-pyridone tautomer. Though pr oton transfer occurs for only one of the two crystals presented here, the work with DMAPNP2 illustrates that new, highly efficient NLO mater ials can be designed by using acid/base chemistry.