Electron Spin Resonance Study of Molecular Orientation and Dynamics of Phenyl Imino and Nitronyl Nitroxide Radicals in Organic 1D Nanochannels of Tris(o-phenylenedioxy)cyclotriphosphazene

Abstract

Imino and nitronyl nitroxide (IN and NN, respectively) radicals such as phenyliminonitroxide (PhIN) and phenylnitronylnitroxide (PhNN), respectively, were dispersed in the organic 1D nanochannels of tris( o-phenylenedioxy)cyclotriphosphazene (TPP). Electron spin resonance (ESR) measurements were conducted on these inclusion compounds (ICs) in the temperature range 4.2-300 K. The modulated-septet ESR spectra of TPP ICs using PhIN observed in the range 165-258 K were reproduced with the EasySpin program package using a model in which some of the PhIN molecules underwent uniaxial rotational diffusion in the TPP nanochannels around the molecular long axis corresponding to the principal y-axis of the g tensor. However, for the TPP IC using PhNN, complicated ESR spectra were observed, which were not consistent with the modulated quintet observed in solution or in the fast-motion limit in solids. These spectra were reproduced by the superposition of a quintet originating from rotational diffusion of PhNN molecules and a septet based on rotational diffusion of PhIN molecules generated in the synthetic process. The rotational diffusion activation energies of PhIN and PhNN in the TPP nanochannels were estimated to be 19 and 45 kJ mol using an Arrhenius plot, respectively. These were consistent with that for NN radicals in the 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) (37-54 kJ mol) with a larger pore diameter than TPP reported in our previous study, or with that for 4-substituted-2,2,6,6-tetramethyl-1-piperidinyloxyl (4-X-TEMPO) in TPP nanochannels (5-26 kJ mol) with regard to molecular size or host-guest or guest-guest interactions. These results indicate that not only the NN group but also IN may be used for the clarification of chemical or biological structures of nanomaterials such as nanosized cavities.

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

“…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.…”

“…The isotropic values of the g and A tensors of these radicals have been reported. , The g and A tensor components of the BzNN radicals should be examined in more detail in the dispersed state to gain insight into the nature of the 4- Y -PhBzNN radicals that can be potentially used as spin probes in the ESR spin probe technique. Details of the ESR spectra of phenyliminonitroxide (PhIN ( 8 ); Z = H; (Figure d)) present in organic matrices and 1D nanochannels have also been reported. , The use of these radicals can help identify suitable probe radicals for further studies. The structures and shapes of the matrices, and the interactions between the matrices and guest radicals should be taken into consideration while selecting the suitable radicals for the studies.…”

“…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).…”