S1 Supporting Material for 2-(9,10-Anthraquinon-2-yl)-4,4,5,5-tetramethyl-4,5- dihydro-1H-imidazole-3-oxide-1-oxyl: polymorphism in a conjugated anthraquinone-substituted nitronylnitroxide Handan Akpinar, a John A. Schlueter b and Paul M. Lahti a, * a Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States; E-mail: lahti@chem.umass.edu b Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States; E-mail: jaschlueter@anl.gov Synthetic details for AntQNN. Page S2 Figure S1. ATR FT-IR spectra for neat α-phase (above) and β-phase (below) S3 of AntQNN. Figure S2. Plane-to-plane distance in angstroms for anthraquinone π-stacked dyads S4 in α-AntQNN. Figure S3. Plane-to-plane distance in angstroms for end-ring-only anthraquinone π- S4 stacking in β-AntQNN. Figure S4. β-AntQNN crystallographic lattice parameter dependence on S5 temperature. Figure S5. Inter-ring plane-plane torsion in α-AntQNN and β-AntQNN. S6 Figure S6. Solution UV-vis absorbance spectra for AntQNN. S7 Figure S7. Visual comparison of solid α -AntQNN and β-phase AntQNN. S6 Figure S8. ORTEP diagram for α-AntQNN at 293 K. S8 Figure S9. ORTEP diagram for β-AntQNN at 300 K. S8 Figure S10. EPR spectrum of AntQNN, room temperature. S9 Beta-Phase Single Crystal X-ray Diffraction procedures; crystallographic S10 figure generation. Table S1. Crystallographic data and structure refinement for AntQNN polymorphs. S11 Equations used for magnetic analyses. S12 Figure S11. Spin pairing fit for α-AntQNN magnetic susceptibility temperature S13 dependence (external field 1000 Oe). Figure S12. 1-D AFM Bonner-Fisher fit for α-AntQNN magnetic susceptibility S13 temperature dependence (external field 1000 Oe). Figure S13. 1-D AFM Bonner-Fisher fit for β-AntQNN magnetic susceptibility S14 temperature dependence (external field 1000 Oe). Figure S14. Molar magnetization plots at 1.8 K for α-AntQNN and β-AntQNN. S14 Computational Modeling for Exchange Involving Close NO Contacts. S15 Electronic Supplementary Material (ESI) for Chemical Communications This journal is © The Royal Society of Chemistry 2013