Multi-Edge Resonant Tender X-ray Diffraction for Probing the Crystalline Packing of Conjugated Polymers

Resonant X-ray diffraction exploits energy-dependent changes in X-ray scattering across an elemental absorption edge to provide additional information about the organization of atoms within the unit cell. Here, we demonstrate the utility of performing resonant X-ray diffraction at multiple absorption edges to provide complementary information about atomic positions and bond orientation. These experiments are achieved through the chlorination of a well-studied naphthalene diimide bithiophene copolymer, P(NDI2OD-T2), to produce P(NDI2OD-T2Cl2), enabling measurements at sulfur and chlorine K-edges. Strong modulation of the diffraction intensity is observed at both edges but with significant differences in the observed diffraction anisotropy. These differences in diffraction anisotropy arise due to differences in the orientation of sulfur and chlorine bonds within the unit cell. Simulations of the supramolecular organization of P(NDI2OD-T2Cl2) have also been performed, with P(NDI2OD-T2Cl2) showing a similar lowest energy packing geometry to unchlorinated P(NDI2OD-T2). Comparing the simulated unit cells with the experimental results, we find that the experimental results are best explained by a mixture of anti and syn conformers in thin film samples of P(NDI2OD-T2Cl2). This observation is in line with the smaller energy barrier computed at the quantum chemical level between the two conformers providing a higher flexibility of the conjugated backbones.