Muoniated radicals in the organic semiconductor zinc-phthalocyanine

muSR experiments were conducted on the organic semiconductor zinc-phthalocyanine (Zn-Pc) in order to identify and characterize the muon states in this material. Transverse-field experiments at 0.5 mT reveal the existence of paramagnetic muon states in Zn-Pc, with the formation probability of nearly 100%. Two different muoniated radicals were detected, with hyperfine parameters of 127 and 142 MHz; muonium is thus bound to the Zn-Pc molecule at two different sites. Preliminary longitudinal field measurements indicate the onset of spin dynamics above 200K. The observed relaxation is interpreted as due to spin exchange scattering with charge carriers. (C) 2002 Elsevier Science B.V. All rights reserved.