Exploring bonding and the impact of non-covalent interactions on solid-state photoswitching by non-spherical structure refinements

Lauren E. Hatcher

School of Chemistry, Cardiff University, Park Place, Cardiff, CF103AT, UK
e-mail: HatcherL1@cardiff.ac.uk

Solid-state photo-switches have a variety of applications including data-storage, solar energy and photocatalysis.1-3 Even in systems that can accommodate movement of whole atoms or molecules, non-covalent interactions can control various aspects of switching, including likely reaction pathways, the photo-products obtained or the excited-state population achieved. This is particularly true of intermolecular interactions, e.g. hydrogen bonds, that must be broken throughout the solid to facilitate photo-switching in the bulk.4 Thus, non-covalent interactions are often key to explaining important structure-property correlations.

While single-crystal X-ray diffraction (SCXRD) refinements using the traditional Independent Atom Model provide atomic-scale information before, after and even during photo-switching, often information on non-covalent interactions is unavailable or, at best, can only be inferred from refined parameters (e.g. bond lengths/angles). Experimental charge density refinements can directly refine the electron density based on more accurate, non-spherical models e.g. multipolar refinements, providing unique understanding of the fine electron density.5-7 However, such experiments typically require very high resolution data (<0.5 Å). As many issues typical for photocrystallographic studies can significantly limit the diffraction data quality e.g. light damage, phase transitions or disorder due to partial excitation, these studies present a significant challenge for traditional charge density analysis.

More recently, a semi-empirical approaches have emerged that sit between experimental charge density refinements and ab-initio charge density calculations.8 NoSpherA2 (Non-Spherical Atoms in Olex2) is one such approach that has already been applied to several interesting crystallographic problems.9-11 The software utilises Hirshfeld atom refinement (HAR) to calculate non-spherical atomic form factors, then refines these non-spherical atom shapes against the experimental electron density obtained by SCXRD.8 Through this combination of quantum chemical calculations and experimental electron density refinement a greatly improved crystallographic model is obtained, providing new insight into intra- and intermolecular bonding. NoSpherA2 has advantages over experimental charge density analysis as it can be applied to materials that do not diffract to such high resolution as needed for multipolar refinements, and materials containing disorder.

We here-in present an application of NoSpherA2 to photo-switchable linkage isomer crystals. Using non-spherical structure refinements, we uncover the non-covalent interactions present in ground-state and excited-state isomers and explore the nature of the bonding between the isomerisable ligand and the metal. This insight can be used to rationalise key properties, e.g. stability/lifetime of the photoexcited-state, knowledge that can iteratively be applied to design new materials tailored for particular applications. The results show the applicability of NoSpherA2 for photocrystallographic refinements and recommend its application to other photoswitchable and photocatalytic materials.

References:

[1] Zeng et al., ACS Catalysis, 2016, 6, 7935-7947.

[2] Tsai et al., Science, 2018, 360, 67.

[3] Sato, Nat. Chem., 2016, 8, 644-656.

[4] Hatcher and Raithby, CrystEngComm, 2017, 19, 6297-6304.

[5] Stevens and Coppens, Acta Cryst. A, 1979, 35, 536-539.

[6] Coppens et al., Acta Cryst. A, 1979, 35, 63-72.

[7] Hansen and Coppens, Acta Cryst. A, 1978, 34, 909-921.

[8] Kleemiss et al., Chem. Sci., 2021, 12, 1675-1692.

[9] Saunders et al., CrystEngComm, 2021, 23, 6180-6190.

[10] Chocolatl Torres et al., Acta Cryst. E, 2021, 77, 681-685.

[11] Novelli et al., Acta Cryst. B, 2021, 77, 785-800.