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

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 primary and stimulated echoes. The temperature variations from room temperature and lower to liquid helium indicated interesting characteristics, 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 pi-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 quantum mechanical tunneling effect of the methyl substituents.