Spin Exchange Interaction in Substituted Copper Phthalocyanine Crystalline Thin Films.

Metal phthalocyanines (MPcs) are often presented as the archetype of small molecule organic semiconductors because they are employed in the vast majority of organic optoelectronics applications1,2,3. These semiconductors form an interesting class of materials whose electronic and optical properties are the result of an interplay between localization and delocalization of spin and charge carriers. 3-d transition MPcs are fully compatible with all organic device processing techniques and, most importantly, the presence of the metal ion results in a potential for customization of electronic properties4,5,6. Despite the significant body of research work that explores the electronic properties of these molecules, magnetism studies of MPcs in the crystalline phase are especially rare and raise numerous challenges with regards to unambiguously identifying the exchange mechanisms and electronic states involved in the observed magnetic ordering7,8,9,10. Theoretical models predict that in crystalline 3d-transition MPc thin films, a spin-dependent exchange interaction exists between the delocalized π-electrons of the phthalocyanine C-N ring and the localized unpaired d-shell electron spin of the central metal ion8,9,11. These exchange mechanisms are predicted to be very different depending on the molecular ordering in the crystalline phase, the number of unpaired spins on the d-orbitals of the central ion, and the energy and symmetry of these orbitals relative to those of the ligand π-electrons. Elucidating these exchange mechanisms is very important for magneto-optics and spintronics applications12,13, especially in the context of the great progress made in recent years on developing crystalline small molecule thin films with very large electron mobilities14,15,16. A comprehensive study of spin exchange interactions in these materials must necessarily start with Copper Phthalocyanine (CuPc), the most studied and best understood member of this family in terms of its electronic and magnetic properties7,8,10. The d9 configuration of the Cu2+ ion results in a single unpaired electron spin located in a or b1g orbital. Magnetization measurements reveal the presence of an antiferromagnetic coupling between Cu ions, characterized by a Curie -Weiss temperature of −2.6 K10,17, but they cannot identify the nature of this antiferromagnetic exchange. These experimental observations are supported by theoretical studies that predict the S = 1/2 Cu spins engage in a Coulomb-like indirect exchange interaction with the delocalized π electrons18. In this research article, we report the results of Magnetic Circular Dichroism (MCD) spectroscopy experiments that identify the electronic state within the HOMO-LUMO bandgap manifold which mediates the spin-dependent indirect exchange between neighboring Cu ions using crystalline thin films of copper(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (Cu-OBPc) with macroscopic grain sizes. In analogy to Diluted Magnetic Semiconductors (DMS)19,20,21,22,23, a quantitative estimation for the strength of this exchange is obtained in the form of an enhanced effective g-factor for ligand electrons.