A novel class of phenol–pyridine co-crystals for second harmonic generation

Abstract

To test the approach of combining both ionic and hydrogen-bonding interactions for the design of non-linear optical (NLO) materials, a number of phenol–pyridine co-crystals have been synthesized and their NLO properties investigated. The co-crystals are characterized by second harmonic generation measurements as well as the more conventional methods of melting point measurements, infrared and nuclear magnetic resonance spectroscopy. To investigate whether the phenol co-crystals are organic salts, the 2-methoxy-4-nitrophenol–4-(dimethylamino)pyridine (2:1) co-crystal 6 and the 2-methoxy-4-nitrophenol–4-pyrrolidinylpyridine–water (1:1:1) co-crystal 8 are further characterized by X-ray single-crystal diffraction. Crystal structure analyses reveal that both 6 and 8 are ionic co-crystals (or organic salts) composed of a phenoxide anion, a pyridinium cation and a neutral molecule. In the two co-crystals, the phenoxide, pyridinium and neutral molecules are held together by ionic attractions as well as hydrogen-bonding interactions. Both 6 and 8 crystallize in non-centrosymmetric structures [Pna2 ₁ (orthorhombic), a=6.880(4), b=38.40(1), c=8.454(3) Å, Z=4, D _c =1.369 g cm ^-3 and R=0.051 for 6 and Cc (monoclinic), a=7.302(3), b=23.518(2), c=9.940(1) Å, β=107.12(2)°, Z=4, D _c =1.365 g cm ^-3 and R=0.036 for 8]. In addition to X-ray structure determination, it is possible to predict whether phenol–pyridine co-crystals are organic salts based on the ΔpK _a [pK _a (pyridine)-pK _a (phenol)] and stoichiometric ratio of the co-crystals. Preliminary results suggest that this type of co-crystals, particularly for the ionic co-crystals, may have a higher chance of forming non-centrosymmetric structures than the normal achiral organic compounds.