Nonlinear optical organic co-crystals of merocyanine dyes and phenolic derivatives with short hydrogen bonds

We investigated the co-crystallization of merocyanine dyes M (M: R-N⁺C₅H₄-CH=CH-C₆H₄O ^-, M1: R=CH₃, M2: R=HO-CH₂-CH₂-) with phenolic and aniline derivatives and show that the short or very short hydrogen bond between the two phenolic oxygen atoms which lead to a self-assembly of the M dyes and phenol derivatives is the key steering force for the co-crystallization process. The co-crystal formation was studied by melting point determination, by second-harmonic generation using the Kurtz and Perry powder test, and X-ray structural analysis. We present detailed results on the growth, polymorphism, and nonlinear optical properties of co-crystals of derivatives of M1 (M2) and m-nitrophenol (mNP) and co-crystals of M2 and methyl 2,4-dihydroxybenzoate (MDB). Three phases of the co-crystal M2·mNP and three phases of the co-crystal M2·MDB were found. Both M2·MDB(I) and (II) have the same crystal structure (space group symmetry Cc determined by X-ray diffraction), but show different linear and nonlinear optical properties. This unusual property in the co-crystals of M2·MDB(I) and (II) may be caused by a different proton location of the short hydrogen bond (O-H-O) in the aggregation between M2 and MDB which could not be resolved by X-ray diffraction.