Structural studies into πˑˑˑπ interactions and their cooperativity effect on the spin crossover behaviour of a novel series of naphthalimide compounds

Abstract

The crystal engineering of metal-organic materials is an active area of research in the field of materials science with a wide variety of diverse functions, properties, and promising applications. Prominent work has been pioneered over the last few decades in crystal engineering where structure directing components have been systematically built into molecular building blocks. However, this has not been fully exploited, particularly in the field of magnetically switchable materials. The numerous interesting properties of spin crossover (SCO) active systems, combined with the current trend to develop molecular electronics and machines has resulted in a dramatic increase in the exploration compounds exhibiting this phenomenon. Modifying the solid-state interactions between metal complexes is essential for controlling the nature of the SCO event. One approach to achieve this is to use supramolecular chemistry to assemble complexes into high ordered arrays through non-covalent supramolecular interactions. Hydrogen bonding is typically the main tool used to control the formation of networks in crystal engineering due to its reproducible, well defined, and directional properties. Previous work has investigated the effect that hydrogen bonding and halogen bonding has on the cooperative nature of the SCO event, which proposed that other supramolecular interactions can also alter the nature of this cooperativity. The focus of this work is on utilising π⋯π interactions to systematically modify the SCO transition. The use of π⋯π stacking interactions has been become prevalent since the discovery that the electron density within the π systems defines the strength of the interaction. The established order of stability in the interaction of two π systems is π-deficient⋯π-deficient > π-deficient⋯πrich > π-rich⋯π-rich. A key aim of this project is to exploit π⋯π stacking interactions for the engineering of magnetically switchable metal-organic supramolecular networks. Naphthalimide-based functional groups were identified as the target for this project because of: a) their inherent ability to induce SCO in Fe(II); b) the long range ordering achieved through πstacking and c) the interesting photophysical properties of the 1,8-naphthalimide moiety. The electron deficient 1,8-naphthalimide systems have not only been utilised as ligand scaffolds for metal complexes, but also investigated as non-coordinating anions to incorporate this structure directing group into the lattice. While systematically varying the nature of substituents on naphthalimide backbone, we will use quantum crystallography methods to develop an understanding of how subtle changes in electron withdrawing/donating substituents influence the nature of interactions, and accordingly how π⋯π interactions influence magnetic properties. The calculation of the intermolecular interaction energies has resulted in an array of information which provides insights, that we wish to develop into detailed structure function relationship, thereby increasing control over the behaviour of magnetic materials.

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Identifiers

ORCID for Jonathan A Kitchen: orcid.org/0000-0002-7139-5666

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