Layer-By-Layer Assembly of Asymmetric Linkers into Non-Centrosymmetric Metal Organic Frameworks: A Thorough Theoretical Treatment

Layer-by-layer synthesis of surface-coordinated metal-organic frameworks (SURMOF) enables the assembly of asymmetric, dipolar linkers into non-centrosymmetric pillar-layered structures. Using appropriate substrate terminations can yield oriented growth with the dipoles aligned perpendicular to the surface. The aligned pillar linkers give rise to a built-in electrostatic field. In addition, the non-centrosymmetric structure of the SURMOF gives rise to intriguing nonlinear optical features, such as second harmonic generation. Previous research with methyl-functionalized bipyridine pillar linkers have demonstrated that this approach works in principle, but so far the total degree of alignment is only very small. Herein, a multiscale modelling approach is presented for in-silico SURMOF assembly to identify and overcome limitations in the growth of pillar-layered SURMOFs and to develop a strategy to maximize linker alignment. Using master equation models and kinetic Monte Carlo simulations, it is found that the formation of a highly ordered state corresponding to the thermodynamic equilibrium is often prevented by long-lasting transient effects. Based on ab initio binding energies for a wide selection of hypothetical pillar linkers, a fast-binding, slow-relaxation scheme is able to be identified during the SURMOF growth for a range of different pillar linkers. These observations allow them to derive a rational strategy for the design of novel linkers to yield SURMOF-based non-centrosymmetric materials with substantially improved properties.