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

This work introduces a novel family of Co-II species having a curcuminoid (CCMoid) ligand, 9Accm, attached, namely [Co(9Accm)(2)(py)(2)] (1) and [Co(9Accm)(2)(2,2'-bpy)] (2), achieved in high yields by the use of a microwave reactor, and exhibiting two different arrangements for the 9Accm ligands, described as 'cis'(2) and 'trans'(1). The study of the similarities/differences of the magnetic, luminescent and surface behaviors of the two new species, 1 and 2, 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 D values, that of 1 (D = +74 cm(-1)) being three times larger than that of 2 (D = +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 H-1 NMR, where both compounds (1 and 2) 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 cis or trans disposition of the ligands. Fluorescence studies show that both systems display chelation enhancement of quenching (CHEQ) with regard to the free ligand, while 1 and 2 display similar quantum yields. Deposition of 1-2 on HOPG and Si(100) surfaces using spin-coating was studied using AFM; UV photoemission experiments under the same conditions display 2 as the most robust system. The measured occupied density of states of 2 with UV photoemission is in excellent agreement with theoretical DFT calculations

Multiscale study of mononuclear Co" SMMs based on curcuminoid ligands

This work introduces a novel family of Co" species having a curcuminoid (CCMoid) ligand, 9Accm, attached, namely [Co(9Accm)₂(py)₂] (1) and [Co(9Accm)₂(2,20-bpy)] (2), achieved in high yields by the use of a microwave reactor, and exhibiting two different arrangements for the 9Accm ligands, described as "cis"(2) and "trans"(1). The study of the similarities/differences of the magnetic, luminescent and surface behaviors of the two new species, 1 and 2, is the main objective of the present work. The determined single-crystal structures of both compounds are the only Co"-CCMoid structures described in the literature so far. Both compounds exhibit large positive D values, that of 1 (D = +74 cm⁻¹) being three times larger than that of 2 (D = +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 (1 and 2) 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 cis or trans disposition of the ligands. Fluorescence studies show that both systems display chelation enhancement of quenching (CHEQ) with regard to the free ligand, while 1 and 2 display similar quantum yields. Deposition of 1-2 on HOPG and Si(100) surfaces using spin-coating was studied using AFM; UV photoemission experiments under the same conditions display 2 as the most robust system. The measured occupied density of states of 2 with UV photoemission is in excellent agreement with theoretical DFT calculations

Multiscale study of mononuclear Co" SMMs based on curcuminoid ligands

This work introduces a novel family of Co" species having a curcuminoid (CCMoid) ligand, 9Accm, attached, namely [Co(9Accm)₂(py)₂] (1) and [Co(9Accm)₂(2,20-bpy)] (2), achieved in high yields by the use of a microwave reactor, and exhibiting two different arrangements for the 9Accm ligands, described as "cis"(2) and "trans"(1). The study of the similarities/differences of the magnetic, luminescent and surface behaviors of the two new species, 1 and 2, is the main objective of the present work. The determined single-crystal structures of both compounds are the only Co"-CCMoid structures described in the literature so far. Both compounds exhibit large positive D values, that of 1 (D = +74 cm⁻¹) being three times larger than that of 2 (D = +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 (1 and 2) 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 cis or trans disposition of the ligands. Fluorescence studies show that both systems display chelation enhancement of quenching (CHEQ) with regard to the free ligand, while 1 and 2 display similar quantum yields. Deposition of 1-2 on HOPG and Si(100) surfaces using spin-coating was studied using AFM; UV photoemission experiments under the same conditions display 2 as the most robust system. The measured occupied density of states of 2 with UV photoemission is in excellent agreement with theoretical DFT calculations

Direct Evidence of an Excited-State Triplet Species upon Photoactivation of the Chlorophyll Precursor Protochlorophyllide

The chlorophyll precursor protochlorophyllide (Pchlide), which is the substrate for the light-driven enzyme protochlorophyllide oxidoreductase, has unique excited-state properties that facilitate photocatalysis. Previous time-resolved spectroscopy measurements have implied that a long-lived triplet state is formed during the excited-state relaxation of Pchlide, although direct evidence of its existence is still lacking. Here we use time-resolved electron paramagnetic resonance (EPR) in combination with time-resolved absorption measurements at a range of temperatures (10–290 K), solvents, and oxygen concentrations to provide a detailed characterization of the triplet state of Pchlide. The triplet decays in a biphasic, oxygen-dependent manner, while the first reported EPR signature of a Pchlide triplet displays both emissive and absorptive features and an antisymmetric spectrum similar to other porphyrin triplet states. This work demonstrates that the Pchlide triplet is accessible to various cryogenic spectroscopic probes over a range of time scales and paves the way for understanding its potential role in catalysis