Engineering of polar molecular crystals with optimized chromophoric orientation for nonlinear optics

We review here two new directions in the area of molecular crystal engineering for nonlinear optics where the alignment, tuning and optimization of the acentric orientation of highly extended, dipolar chromophores in the crystalline state will be described. The use of the non-rod-shaped framework-hydrazone skeleton - as a π-conjugation core, due to a high preference for a lambda-shaped packing motif is a very useful tool to acentricatly align and optimize chromophoric orientation in the solid state for nonlinear optical effects such as second harmonic generation. The supramolecular synthetic approach based on the short hydrogen bond induced self-assembling of molecular aggregates shows a high tendency to induce an acentric packing and a higher degree of design feasibility in tuning and optimizing the chromophoric orientation within the crystalline lattice. In addition, co-crystals obtained by this approach often show greatly improved crystal and physical properties relative to its starting components.