ESR Study of Molecular Orientation and Dynamics of Nitronyl Nitroxide Radicals in CLPOT 1D Nanochannels

Abstract

New inclusion compounds (ICs) were prepared using the organic 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) as a nanosized template and nitronyl nitroxide (NN) radicals such as phenylnitronylnitroxide (PhNN) and p-nitrophenylnitronylnitroxide (p-NPNN). ESR measurements below 255 K for the CLPOT ICs diluted with spacer molecules gave rigid limit spectra similar to that for PhNN molecules in a glassy ethanol matrix at low temperature, which suggests isolation of the radical molecules. ESR measurements for them in the range of 290-400 K gave a modulated quintet ESR signal, which suggested uniaxial rotational diffusion of NN radicals in the nanochannels approximately around the principal y-axis of the g-tensors. In the ESR measurements to 430 K for the [(CLPOT)2-(p-NPNN)0.07] IC without spacers, the broader line width than the case in dilution was observed by inter-radical dipolar interaction. In every case, the rotational diffusion activation energies of NN radicals in the CLPOT nanochannels were several times larger than those of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical derivatives (4-X-TEMPO) in CLPOT nanochannels. This is expected due to the larger molecular size of NN radicals than 4-X-TEMPO or stronger interaction between NN radicals and the surrounding host or guest molecules.

Investigation of Various Organic Radicals Dispersed in Polymethylmethacrylate Matrices Using the Electron Spin Resonance Spectroscopy Technique

“…The lowest component of the g tensor should be observed perpendicular to the molecular plane of the BzNN group, that is , along the principal z axis . It should be approximately 2.0023. ,, As g xx > g yy > g zz for the 4- Y -PhBzNN radical, the intensity of the g xx component is observed at the region of the lowest magnetic field.…”

“…The Hamiltonian functions reported in the literature were used to describe the free radicals dispersed in the organic matrices and 1D nanochannels. ,,− Previously reported calculation methods were followed to describe the system where PMMA was used as the organic matrix. NO, 2- X -NN, 4- Y -PhBzNN, and 2- Z -IN radicals in the matrices were characterized as follows using the spin Hamiltonian: where β e , B , g , A (N i ), A (H j ), β n , g n (N i ), g n (H j ), Ŝ , and Î denote the Bohr magneton; laboratory magnetic flux density vector; electron spin g tensor; hyperfine tensor for the i th 14 N nucleus of the NO, 2- X -NN, 4- Y -PhBzNN, and 2- Z -IN radicals ( i = 1 or 2); superhyperfine tensor of j th 1 H nucleus of 4- Y -PhBzNN ( j = α, β, o , m , or p ; the protons at α and α′, β and β′, o and o ′, or m and m ′ are equivalent due to molecular symmetry; Figure c); nuclear magneton; nuclear spin g tensor of the i th 14 N nucleus of the NO, 2- X -NN, 4- Y -PhBzNN, and 2- Z -IN radicals; nuclear spin g tensor of the j th 1 H nucleus of 4- Y -PhBzNN; electron spin operator; and nuclear spin operator, respectively.…”

“…The principal axes of the g tensor of the NO, 2- X -NN, and 2- Z -IN radicals were defined following previous reports. ,,,, The g tensor corresponding to the 4- Y -PhBzNN radical was then determined. The unpaired electron present in the BzNN group was present in the π orbital.…”

“…In the past decade, the ESR spin probe technique has been used to study the organic 1D nanochannels of 2,4,6-tris (4-chlorophenoxy)-1,3,5-triazine and ( o -phenylenedioxy)­cyclotriphophazene crystals using 2-substituted nitronylnitroxide (2- X -NN, 3 – 5 , Figure b). , Nitronylnitroxide (NN) radicals are common organic radicals. Several radicals belonging to this group of radicals behave as organic magnets at low temperatures. , Various NN radicals are present in the ligands in magnetic metal complexes − and metal–organic frameworks (MOFs) .…”

Nitroxide radicals appended to phthalonitriles: synthesis, structural characterization and photophysical properties

“…Among nitroxides, nitronyl (NN) and imino nitroxide (IN) radicals are proven to be a versatile family of spin labels as NNs are transformed into INs by reactive radicals generated in biological processes or decay completely to diamagnetic species; both cases can be easily monitored thanks to distinctive pattern in their EPR spectra (Joseph et al, 1993;Rosen et al, 2003;Medvedeva et al, 2004;Samuni et al, 2010;Wang et al, 2011;Sousa et al, 2016). NN and IN based spin probes are ideally suited for investigating the dynamic aspects of molecular interactions because of their covalent incorporation into biologically critical architectures (Okamoto et al, 2005;Ise et al, 2006;Tanaka et al, 2007;Maekawa et al, 2007Maekawa et al, , 2010Atsumi et al, 2012;Wright et al, 2014;De Zotti et al, 2017) and noncovalent interactions (Nakabayashi et al, 2007;Peresypkina et al, 2010;Vostrikova et al, 2011;Kobayashi et al, 2014Kobayashi et al, , 2018Strizhakov et al, 2014). On the other hand, nitrile-containing compounds are known to form protein-ligand interactions by hydrogen bonding, hydrophobic interactions and covalent bonding with numerous protein residues (Wang et al, 2018).…”