Molecular Orientation and Dynamics of Different Sized Organic Radicals Included in Organic 1D Nanochannels

Abstract

The molecular orientation and dynamics of di-t-butylnitroxide (DTBN) and 4-oxo-2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPONE) radicals in the organic 1D nanochannels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were investigated by examining inclusion compounds (ICs) diluted by the co-inclusion of nonradicals using electron spin resonance (ESR). Spectral simulation showed that the axial rotation of DTBN or TEMPONE molecules is excited in TPP nanochannels with activation energies of 3 and 10 kJ mol(-1), respectively. The rotation axes for both DTBN and TEMPONE were determined to be almost parallel to the principal y-axis of the g tensor, which agrees with the result reported for 2,2,6,6-tetramethyl-1-piperidinyl-1-oxyl (TEMPO) in TPP nanochannels, which has the activation energy of 5 kJ mol(-1). These results indicate that the molecular orientations of guest nitroxide radicals are almost independent of the molecular sizes of the guest radicals in TPP nanochannels, although the molecular dynamics are dependent on the molecular sizes.

ESR study of molecular orientation and dynamics of TEMPO derivatives in CLPOT 1D nanochannels

“…. These were simulated using the g and A tensor components shown in Table . The Gaussian and Lorentzian line‐width components were 0.56 and 0.24 mT for 1c/2b and 0.50 and 0.34 mT for 1c/2c .…”

“…The principal axis systems xyz of the g and A tensors can be assumed as previously reported (Fig. S2), that is, z perpendicular to the CNC plane, x parallel to the N–O bond, and y perpendicular to the zx plane.…”

“…The procedure employed to obtain the rotation axes of the radicals in the nanochannels has been previously reported . Nitroxide free radicals orientated in 1D nanochannels are characterized by the spin Hamiltonian: where β e , B , g , A , β n , g n , Ŝ , and Î are the Bohr magneton, the laboratory magnetic flux density vector, the electron spin g ‐tensor, the hyperfine tensor of the 14 N nucleus in the nitroxide radical, the nuclear magneton, the nuclear spin g‐ factor, the electron spin operator, and the 14 N nuclear spin operator in the nitroxide radical, respectively.…”

“…The rotation axis of 4‐X‐TEMPO radical was defined by polar and azimuthal angles, θ and ϕ , respectively, relative to the principal axis system of the g ‐tensor of 4‐X‐TEMPO. Although four possible rotation axes were derived within the domains defined by 0 ≤ θ ≤ π and 0 ≤ ϕ ≤ 2 π , only the ( θ 0 , ϕ 0 ) values are given in the following discussion …”

“…The molecular orientations and dynamics of differently sized TEMPO derivatives substituted at the 4‐position (4‐X‐TEMPO; 1 in Scheme ) in TPP nanochannels have been previously examined . The activation energy, E a , associated with the uniaxial rotational diffusion of each radical in the TPP nanochannels, was found to increase with increasing molecular size.…”