Partial Fluorination as a Strategy for Crystal Engineering of Rubrene Derivatives

Ogden, W. A.; Ghosh, S.; Bruzek, M. J.; McGarry, K. A.; Balhorn, L.; Young, V., Jr.; Purvis, L. J.; Wegwerth, S. E.; Zhang, Z.; Serratore, N. A.; Cramer, C. J.; Gagliardi, L.; Douglas, C. J.
Cryst. Growth Des. 2017, 17, 643 (doi:10.1021/acs.cgd.6b01497).

Through a close examination of the intermolecular interactions of rubrene (1a) and select derivatives (1b–1p), a clearer understanding of why certain fluorinated rubrene derivatives pack with planar tetracene backbones has been achieved. In this study we synthesized, crystallized, and determined the packing structure of new rubrene derivatives (1h–p). Previously, we proposed that introducing electron-withdrawing CF₃ substituents induced planarity by reducing intramolecular repulsion between the peripheral aryl groups (1e–g). However, we found that in most cases, further increasing the fluorine content of rubrene led to twisted tetracene backbones in the solid state. To understand how rubrene (1a) and its derivatives (1b–p) pack in the solid state, we (re)examined the crystal structures through a systematic study of the close contacts. We found that planar tetracene cores occur when close contacts organize to produce an S symmetry element about a given rubrene molecule. We report the first instance of rubrene derivatives (1l and 1n) that pack in a 2D-brick motif. The prospects for new rubrene derivatives in semiconductors were estimated by calculating the reorganization energies of the monomers and transfer integrals of the dimers we observed. Our work allows for the rational design and improved crystal engineering of new rubrene derivatives.