Muoniated radical states in the organic semiconductor phthalocyanine J. Piroto Duarte,* R. C. Vilão, H. V. Alberto, J. M. Gil, Francisco P. S. C. Gil, A. Weidinger, and N. Ayres de Campos Physics Department, University of Coimbra, P-3004-516 Coimbra, Portugal K. Fostiropoulos Hahn-Meitner Institute Berlin, Glienicker Strasse, 100, D-14109 Berlin, Germany Received 13 October 2005; revised manuscript received 6 January 2006; published 21 February 2006 Phthalocyanine samples of ZnPc, H 2 Pc, and CuPc were investigated with the muon spin rotation technique. In ZnPc and H 2 Pc, three muoniated radical states of paramagnetic origin were identified, two of which have hyperfine interactions in the range 110– 150 MHz and correspond to muonium addition at the outer benzene rings. The third state has a smaller hyperfine interaction about 25 MHz and is tentatively assigned to addition at bridging nitrogen atoms. CuPc has an unpaired electron from the Cu atom, which originates a diamagneti- clike signal upon muonium addition. The signal has two components with very different relaxation rates, corresponding to two different spatial couplings of the Cu electron with the muonium’s electron. DOI: 10.1103/PhysRevB.73.075209 PACS numbers: 71.55.Ht, 61.72.Ww, 76.75.+i I. INTRODUCTION Organic semiconductors have been envisaged in recent years as top candidates for the development of low-cost elec- tronic and optoelectronic devices. 1–3 Research on these com- pounds has consequently experienced a steep increase, and many aspects of their structural and transport properties, par- ticularly of conjugated polymers, have been revealed lately. Among the nonpolymeric organic semiconductors, phtha- locyanines Pc are of great interest because of their applica- tion in solar cells 4 and organic displays. 5 Phthalocyanines derive from porphyrins by the addition of benzene rings to the four pyrrole units along with substitution of the methine bridges by azamethine ones see Fig. 1. Like what happens with conjugated polymers, their semiconducting behavior stems from the intermolecular overlap of the highly delocal- ized -orbitals formed as a result of the alternate saturated- unsaturated bond structure of each molecule. The performance of Pc-based devices is known to be en- hanced by doping with small organic molecules, 6 although the impact on charge-carrier transport mechanisms is far from being fully understood. Most of the information avail- able has been obtained with macroscopic techniques like re- sistivity or Hall effect measurements, which focus on the slow components of charge-carrier diffusion. 7 Knowledge obtained with microscopic techniques is fundamental to un- derstand charge-carrier transport and doping effects in phtha- locyanines, allowing for a more efficient optimization of de- vices. In the present paper we give an account of experimental results obtained for undoped phthalocyanines with the muon spin rotation SR technique. Akin to NMR, the SR tech- nique uses the spin of positive muons implanted in the ma- terial to probe the local environment, 8,9 but has a much greater sensitivity due to the high degree of initial spin po- larization of the implanted muons. SR investigations on conjugated polymers have been re- ported in the past. 10,11 It is generally assumed that the posi- tive muon captures one electron upon stopping, forming a neutral system known as muonium Mu. Muonium adds to unsaturated molecules by breaking double bonds, forming muoniated radicals. If the radical’s unpaired electron remains localized near the positive muon, a contact hyperfine field at the muon exists, and site assignments can be made on the basis of hyperfine interaction values. Here, we focus our at- tention on muon site assignments and the local electronic properties of phthalocyanines in the vicinity of the muon. These results lay ground for a planned study of charge carrier transport in doped phthalocyanines. II. EXPERIMENTAL DETAILS Three -form phthalocyanine compounds differing on the central inclusion were studied as representatives of the Pc family: ZnPc nonmagnetic central atom, Fig. 1, CuPc magnetic central atom, and H 2 Pc metal-free phthalocya- FIG. 1. Molecular structure of zinc phthalocyanine ZnPc. The possible muon addition sites investigated in this work are indicated with the Roman numerals I, II and letters a – d. Electronic structure calculations yield stable positions for sites I and II only see Sec. IV. PHYSICAL REVIEW B 73, 075209 2006 1098-0121/2006/737/0752096/$23.00 ©2006 The American Physical Society 075209-1