Spin-lattice and spin-spin relaxation times of 2,4,6-tri-tert-butyl phenoxyl and 2,6-di-tert-butyl-4-methyl phenoxyl in diamagnetic crystals: Relaxation mechanism and influence of molecular motions

Abstract

The spin-lattice and phase-memory times were determined for 2,4,6-tri-tert-butyl phenoxyl and 2,6-di-tert-butyl-4-methyl phenoxyl dispersed in a diamagnetic matrix by use of p¡ and stimulated echoes. The temperature variations from room temperature and lower to liquid helium indicated interesting characte¡ albeit there were no echo signals between 150 and 100 K. It may be concluded that the relaxation times vary depending on the motional fluctuation of substituted molecular groups on the n-conjugated framework. In particular, the methyl rotational motion of the tert-butyl groups at the ortho-positions or the para-position is a dominant contribution, and tert-butyl rotation itself and additional methyl rotation at the para-position are also responsible for the shortening, especially in the phase-memory times. The shortening of the relaxation times when approaching liquid helium temperature is probably due to the quanturn mechanical tunneling effect of the methyl substituents.

Substituent Effect on g-Tensor: Multifrequency ESR Study and DFT Calculation of Polycrystalline Phenoxyl Radicals in Diamagnetic Crystals

“…It was confirmed that both methods could produce the corresponding phenoxyl radicals diluted in the diamagnetic crystals, i.e., the primary phenol, 11,15 giving the same ESR spectra. (I′) can be obtained only by UV irradiation, since, in solution, the generated (I′) can instantly dimerize (as shown in Scheme 2), becoming diamagnetic and ESR-silent.…”

“…Precursor phenols purchased commercially (I−V) or synthesized following the procedure described in the literature (VI 11 ) were recrystallized from a toluene solution at room-temperature several times and UV-irradiated in vacuo for 4 or 5 h for the radical generation with a high-pressure mercury lamp manufactured by Ushio Electric Inc (shown in Scheme ) − . It was found that these generated radicals are dispersed homogeneously in the diamagnetic matrixes of the corresponding phenols, , just like the system of γ-irradiation .…”

“…Precursor phenols purchased commercially (I−V) or synthesized following the procedure described in the literature (VI 11 ) were recrystallized from a toluene solution at room-temperature several times and UV-irradiated in vacuo for 4 or 5 h for the radical generation with a high-pressure mercury lamp manufactured by Ushio Electric Inc (shown in Scheme ) − . It was found that these generated radicals are dispersed homogeneously in the diamagnetic matrixes of the corresponding phenols, , just like the system of γ-irradiation . On the other hand, it was found that the oxidation reactions with chemical reagents such as lead dioxide or potassium ferricyanide yield almost quantitatively the corresponding radicals through abstraction of a hydrogen atom from a phenoric hydroxyl group.…”

Synthesis and Characterization of Face-to-Face Arylsulfanyl and Phenoxyl Radicals Embedded in Mesoporous Silicas

“…Thanks to the silica nanostructure, the relaxation times are longer than those measured in solution for the same radicals; they are even longer than those of the nitroxides typically used in DNP NMR as polarizing agent . In addition, two other points drew our attention: (i) SBA n -SO exhibits relaxation times that are on the order of those of SBA n -OO and (ii) the two sulfur- and oxygen-centered radicals have different g -values (see CW, X-, Q-, and W-band results).…”