Molecular Design and Development of Materials with Second-Harmonic Generation (SHG) Through Self-Assembly of Supramolecular Systems

Materials having a non-centrosymmetric (NC) structure show an assortment of interesting properties such as second-harmonic generation (SHG), piezoelectric or ferroelectric properties. To achieve this most outstanding objective, breaking of the inversion symmetry is a prerequisite of the material for having a static electric field. In our work, we are designing and preparing function-inspired custom-made building blocks for the engineering of functional materials with SHG properties.<br/>We have recently demonstrated to prepare a metal-organic framework (MOF) based crystalline, NC thin film by liquid-phase quasi-epitaxial growth (referred as SURMOF) by self-assembly of the asymmetric-bipyridines aided by computational theory and advanced simulations. The thin film with integrated electric field showed an electrostatic potential of up to 0.9 eV. Additionally, the NC properties were proven independently by the detection of a SHG signal. The directed self-assembly of 2,6-dimethyl-4,4′-bipyridine resulted from a difference in the binding energy of the opposite coordinating pyridyl nitrogen atoms, caused by the electronic effect of the adjacent methyl groups on one pyridyl moiety having an electron-donating effect on the coordinating nitrogen atom. By further optimizing the SURMOF synthesis and the design of the dipole, we want to improve the quality of the SURMOF and enhance the non-linear optical (NLO) properties.^[1]<br/>Materials modulation with controlled geometries at molecular scale and to avoid the center symmetry trap, which is the quenching of a macroscopic electrostatic field by favoring an antiparallel formation of dipoles, the linker design was altered and a library of dipolar pillars based on the 4,4’-bipyridine structure for functional materials with NLO properties was prepared. Furthermore, a protection/deprotection strategy for the preparation of SURMOFs was developed and investigated to better control the orientation of the dipoles in the material. Of course, molecular systems do not have to be limited to the above-mentioned specific building blocks and exploring non-conventional more ambitious structures will surely lead to new features advancing SHG materials.<br/>[1] A. Nefedov, R. Haldar, Z. Xu, H. Kühner, D. Hofmann, D. Goll, B. Sapotta, S. Hecht, M. Krstić, C. Rockstuhl, W. Wenzel, S. Bräse, P. Tegeder, E. Zojer, C. Wöll, <i>Adv. Mater.</i> <b>2021</b>, <i>33</i>, 2103287.