Phenolic Polyene Crystals with Tailored Physical Properties and Very Large Nonlinear Optical Response

We report on the design of the nonlinear optical organic phenolic polyene crystals for tailoring their physical properties to achieve optimized crystals for applications. New phenolic OH2 (2-(3-(4-hydroxystyryl)-5-methylcyclohex-2-enylidene)malononitrile) molecules keep the main supramolecular interaction sites and the high hyperpolarizability of the previously studied state-of-the-art OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) and OH3 (2-(3-(4-hydroxystyryl)cyclohex-2-enylidene)malononitrile) molecules but contain different substituents in the non-π-conjugated part of the molecule, which affects the crystal packing and the physical properties. The acentric OH2 crystals exhibit an improved molecular ordering compared to OH1 crystals, leading to a high order parameter cos 3 θ p = 0.92, which is close to optimal for electro-optics and terahertz generation applications. They also exhibit a considerably improved solution growth characteristics compared to the isomorphous OH3 crystals, based on the increase of the solubility by a factor of 2. The acentric OH2 crystals exhibit a large macroscopic nonlinearity with up to twice the second harmonic generation efficiency of OH1 and OH3 crystals as well as the possibility to grow bulk crystals suitable for optical investigations. Furthermore, we evaluate theoretically that the head-to-tail intermolecular hydrogen bonds occurring in phenolic polyene crystals may enhance the hyperpolarizability of the molecules by 50% or more.