Introduction:

1. Magnetic Circular Dichroism of Discotic Organic Semiconductor Crystalline Thin Films C. Lamarche1, Z. Pan1, L. Manning1, N. Rawat1, T. Tokumoto2, J. Cherian2, S. McGill2,andM. Furis1. 1Physics Department and the Material Science Program, University of Vermont, Burlington VT 05405. 2National High Magnetic Field Laboratory, Optics Program, Florida State University, Tallahassee, FL 32306. • Introduction: • This summer, the goal of my research has been to study thin films of discotic organic molecules which form crystals when deposited onto a substrate. This is a typical example of a complex material system, where the electronic and magnetic properties are highly anisotropic, and distinctly different from those of single molecules. I investigated one particular group of these crystal-forming molecules, the phthalocyanines, which allow for the encapsulation of a metal ion with oxidation state 2+. [1] Using magnetic circular dichroism as a method to probe the magnetism of two different metal-phthalocyanines, Cu and Zn, I observed the effect that the metal ion has on the magnetic properties of the crystal as a whole. Given that many of these properties are only apparent at high magnetic field, and low temperature, the Magnet Lab was the ideal location for these experiments. • Analysis: • Three different features are associated with magnetic circular dichroism. The A-term, see (2) above, is due to the Zeeman splitting of the orbitally degenerate excited state, and results in a derivative-like shape. The B-term, see (3) above, is seen when two excited states are close in energy, but both are relatively far from the ground state. These excited states are mixed by the magnetic field. Finally, the C-term, see (4) above, results from the splitting of a degenerate ground state, making it temperature dependent. [3] • Further, the change in amplitude of the magnetic circular dichroism signal with magnetic field can shed light on the magnetic ordering of the system (e.g. paramagnetic, diamagnetic, etc…). Data (CuPc): 10T 1.5K 400mK • Materials and Methods: • Cu-octabutoxy phthalocyanine and Zn-octabutoxy phthalocyanine thin films were prepared using the solution-processed hollow pen writing technique, developed at the University of Vermont. [2] This method produces films with grain sizes as large as a millimeter. • These metal-phthalocyanines are square-planar, and have D4h symmetry. The eight butoxy aliphatic chains make the molecules soluble in organic solvents, which is critical for this type of deposition. 0T 647nm Feature • Conclusions: • Both the Cu and Zn phthalocyanine films seem to have exhibited all three magnetic circular dichroism features, with the C-terms being the most uncommon. • The magnetic ordering of the Zn phthalocyanine was observed to be diamagnetic, down to a temperature of 1.5K. The Cu phthalocyanine, however, shows paramagnetic behavior at low temperature (~10K), which changes to diamagnetic behavior at higher temperature. • Magnetic circular dichroism, which measures the differential absorption of left and right circularly polarized light, was used to study the magnetic properties of these films. • Magnetic fields as large as 10T, and temperatures as low as 400mK, were used to study these crystals. MCD Data (ZnPc): B Field(T) 665nm Feature • Current/Future Work: • Magnetic circular dichroism is now being performed in the new Florida-Split Helix Magnet, at room temperature, with a magnetic field strength of up to 22T. • Future studies will be conducted at temperatures as low as 4.2K with magnetic fields as large as 25T. 1.5K 1.5K Wavelength(nm) Literature Cited: [1] Heutz S., Mitra C., Wu W., Fisher A., Kerridge A., Stoneham M., Harker T., Gardener J., Tseng H., Jones T., Renner C., Aeppli G. Molecular Thin Films: A New Type Of Magnetic Switch. Advanced Materials 19, (2007). [2] Headrick R., Wo S., Sansoz F., Anthony J. Anisotropic Mobility In Large Grain Size Solution Processed Organic Semiconductor Thin Films. Applied Physics Letters 92, (2008). [3] Kobayashi N., Nakai K. Applications Of Magnetic Circular Dichroism Spectroscopy To Porphyrins And Phthalocyanines. Chemical Communications 40, (2007). Acknowledgements: I would like to thank Dr. Stephen McGill, Dr. TakahisaTokumoto, and Judy Cherian for hosting me at the Magnet Lab, and providing useful insight into the physics behind the experiments. I would also like to thank the dedicated people at the Center for Integrating Research and Learning, without whom my involvement in this research would not have been possible. Finally, I would like to thank Dr. MadalinaFuris and Zhenwen Pan for their continued support and encouragement.