Synthesis, structure, thermal and nonlinear optical properties of a series of novel D–π–A chromophores with varying alkoxy substituents

Six new derivatives of the previously reported chromophore 2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile (OH1) with potential application in electrooptics were synthesized and their crystal structures characterized using a single crystal X-ray technique. The presented chromophores have a D–π–A structure with a phenol donor, ring locked polyene conjugated bridge and dicyanomethylydene acceptor. In order to potentially alleviate problems with the quality of melt grown films of OH1, the melting point was tuned through the introduction of alkoxy groups of various lengths and onto various positions on the chromophore core as well as through the modification of the bridge structure. We show that the melting point of these materials can be significantly decreased with the increased number of carbon atoms in alkoxy group. However, the molecular arrangement was found to be highly sensitive to the addition of substituents, and regardless of the size and position of the alkoxy groups, all presented chromophores formed centrosymmetric structures. The distinctions in crystal packing are explained by the mutual arrangement of donor (OH) and H-acceptor (OAlk, C(CN)2) groups in the molecules. Static first order hyperpolarizabilities along with molecular geometry in isolated state were calculated using DFT.