In Situ Tailor-Made Additives for Molecular Crystals: A Simple Route to Morphological Crystal Engineering

We report on a new morphological crystal engineering technique introducing in situ tailor-made additives for organic molecular crystals. In the crystal growing process, phenolic substrate molecules are transformed to in situ tailor-made phenolate additives by adding a small amount of base, without any additional synthetic and purification processes. To demonstrate the in situ tailor-made additive technique, phenolic OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) crystal, exhibiting highly efficient nonlinear optical and terahertz generation responses, is chosen as substrate crystal. The in situ tailor-made additive molecules containing a phenolate group exhibit considerably different stereoselective interactions with specific crystal surfaces of phenolic substrate OH1 crystals compared to analogous conventional tailor-made additives. The stereospecific interactions result in a morphological change of the grown OH1 crystal more optimal for photonic applications. To show the usefulness of the in situ tailor-made additive technique for applications as well as for fundamental research, newly created parallel surfaces in rectangular rod-shaped OH1 crystals grown in the presence of tailor-made additive are used to demonstrate the THz wave generation by optical rectification, in which one single OH1 crystal can be used in two different optical configurations. Therefore, the in situ tailor-made additive technique is very useful for morphological crystal engineering in fundamental research and various practical applications.