Crystal Growth and Morphology Control of OH1 Organic Electrooptic Crystals

We report on the growth of large-size phenolic configurationally locked polyene OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) organic bulk crystals. OH1 has been recently identified as very promising for electro-optic and THz-wave applications, with several advantages compared to the benchmark ionic salts such as DAST (4′-dimethylamino-N-methyl-4-stilbazolium tosylate) and DSTMS (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6-trimethylbenzene sulfonate). We investigate the thermodynamic equilibrium conditions, the solubility, and the metastable zone of OH1/methanol system. The intermolecular hydrogen-bond interactions between the OH1 and methanol molecules impede the main supramolecular interactions between the OH1 molecules, which results in a strong delay in nucleation and provides a very large metastable-zone width. We furthermore studied the crystal morphology and the solvent effect by analyzing the relationship between the morphology and the molecular orientation at the crystal surface as well as the growth direction. The growth kinetics of specific OH1 crystal faces for different growth methods (slow evaporation or supercooling) has been studied. In the OH1/methanol system with a large metastable-zone width, we can grow large OH1 crystals and control their morphology. Thinner or thicker crystals of about 15 × 12 × 1 mm3 and 18 × 17 × 4 mm3, respectively, with good optical quality can be grown in roughly 2 weeks time.