Multiscale study of mononuclear Co^II SMMs based on curcuminoid ligands

Abstract

This work introduces a novel family of Co^II species having a curcuminoid (CCMoid) ligand, 9Accm, attached, namely [Co(9Accm)₂(py)₂] (<b>1</b>) and [Co(9Accm)₂(2,2'-bpy)] (<b>2</b>), achieved in high yields by the use of a microwave reactor, and exhibiting two different arrangements for the 9Accm ligands, described as "<i>cis</i>"(<b>2</b>) and "<i>trans</i>"(<b>1</b>). The study of the similarities/differences of the magnetic, luminescent and surface behaviors of the two new species, <b>1</b> and <b>2</b>, is the main objective of the present work. The determined single-crystal structures of both compounds are the only Co^II-CCMoid structures described in the literature so far. Both compounds exhibit large positive <i>D</i> values, that of <b>1</b> (<i>D</i> = +74 cm^-1) being three times larger than that of <b>2</b> (<i>D</i> = +24 cm^-1), and behave as mononuclear Single-Molecule Magnets (SMMs) in the presence of an external magnetic field. Their similar structures but different anisotropy and SMM characteristics provide, for the first time, deep insight on the spin-orbital effects thanks to the use of CASSCF/NEVPT2 calculations implementing such contributions. Further magnetic studies were performed in solution by means of paramagnetic ¹H NMR, where both compounds (<b>1</b> and <b>2</b>) are stable in CDCl₃ and display high symmetry. Paramagnetic NMR appears to be a useful diagnostic tool for the identification of such molecules in solution, where the resonance values found for the methine group (-CH-) of 9Accm vary significantly depending on the <i>cis</i> or <i>trans</i> disposition of the ligands. Fluorescence studies show that both systems display chelation enhancement of quenching (CHEQ) with regard to the free ligand, while <b>1</b> and <b>2</b> display similar quantum yields. Deposition of <b>1-2</b> on HOPG and Si(100) surfaces using spin-coating was studied using AFM; UV photoemission experiments under the same conditions display <b>2</b> as the most robust system. The measured occupied density of states of <b>2</b> with UV photoemission is in excellent agreement with theoretical DFT calculations.