Electron spin relaxation studies of conducting polypyrroles: The difference of the relaxation processes between highly conducting and semiconducting polypyrroles

The properties of electron spins in highly conducting (sigma = 50 S cm(-1)) as well as in semiconducting (sigma = 10(-2) S cm(-1)) polypyrroles are investigated on the basis of the electron spin relaxation times (T-1 and T-2) measured with pulsed EPR. The temperature dependence of T-1 and T-2 in the good conductors is explicitly different from those in semiconductors. Redfield theory of spin relaxation, applied in order to evaluate the difference, makes it possible to estimate the correlation times and the fluctuating local fields for the two types of polypyrroles. The spin correlation time of good conductors is much shorter than that of semiconductors, a fact that apparently demonstrates that the electron spins in PPy move more rapidly with increasing conductivity. Also the local field in semiconducting PPy is found to be constant over the whole temperature range, while the good conductors exhibited an increasing local field with increasing temperature. (C) 1998 Elsevier Science Ltd. All rights reserved.