Spin soliton in aπ-conjugated ladder polydiacetylene

Abstract

The electronic structure and the spin dynamics of a spin soliton observed in a polydiacetylene have been investigated using multifrequency pulsed and continuous-wave (cw) electron paramagnetic resonance (EPR), electron spin echo (ESE) spectroscopy, electron-nuclear double resonance (ENDOR), and electron-nuclear-nuclear triple resonance (TRIPLE). The undoped ladder polymer in which two polydiacetylene wires are linked by bridges consisting of two triple bonds exhibits a strong EPR signal. In contrast, no EPR signal is observed in the ladder-structured polydiacetylene, where the bridges consist of single bonds. The observed bridge effect on the probability of formation of paramagnetic centers is interpreted in terms of a quasidegenerate ground state due to the π-conjugated ladder structure that creates a π-bond kink corresponding to a spin soliton. High-frequency and high-field EPR reveals a small anisotropy of the g tensor (principal values g 1 =2.0037, g 2 =2.0028, and g 3 =2.0023) for the spin soliton. By nutation spectroscopy, the spin multiplicity was confirmed to be S=½. The hyperfine pattern attributed to the β protons was detected by ENDOR and TRIPLE methods, providing evidence for both the positive and negative spin densities of the soliton wave function. These experimental results led to the conclusion that the unpaired electron of the spin soliton was substantially delocalized over the two polydiacetylenes through the π-conjugation. At temperatures (T) below 50 K, the width of the EPR spectrum narrows by about 45% with increasing temperature and the width remains nearly constant for T>50 K. The pronounced change in the line width for T<50 K suggests the presence of a mobile spin soliton in the ladder-structured polydiacetylene. A temperature dependence of the electron spin system's phase memory time T M for T<50 K was also observed by ESE spectroscopy. The decay of the ESE exhibits a nonexponential behavior, with T M increasing monotonically with rising temperature These findings represent clear experimental evidence for motional narrowing phenomena. Based on a one-dimensional random-walk model for the unpaired electron spin along the ladder polymer, the soliton's diffusion rate constant was estimated from an analysis of the ESE dephasing process. An Arrhenius plot of the diffusion rate was asymptotic to 8×10 7 s - 1 at very low temperature, which is two orders of magnitude slower than that of the soliton in trans-polyacetylene.