Controlling the Conductivity of Cyclo[8]pyrrole Polyiodide Cocrystals via Minor Substituent Group Changes

Substituent group modifications can influence the conductivity of organic materials. However, achieving several orders of magnitude increases in conductivity through minor substituent changes remains a challenge. Here, we report the observation of such large changes in cocrystals of cyclo[8]pyrroles and polyiodides. Two cocrystals were prepared, one from all-ethyl-substituted cyclo[8]pyrrole (<b>1</b>^<b>•+</b>), which forms a 2D stacked structure [(<b>1</b>^<b>•+</b>)₂⊃(I₇)^-<b>•</b>(I₂₄)^-], and the other from a methyl-ethyl-substituted analogue (<b>2</b>^<b>•+</b>), which yields a 3D layered structure [<b>2</b>^<b>•+</b><b>•</b>(I₁₆)^-]. The methyl-ethyl-substituted cocrystal exhibited an approximately 1000-fold higher conductivity (6.1 × 10^-1 S/cm) than its all-ethyl counterpart (4.2 × 10^-4 S/cm). Cocrystal [<b>2</b>^<b>•+</b><b>•</b>(I₁₆)^-] demonstrated good stability, retaining the bulk of its conductivity after being exposed to air for four months or upon heating to 100 °C. The present findings highlight how substituent effects, a molecular feature readily amenable to modification, can have a profound effect on cocrystal conductivity. This work thus sets the stage for further optimization of high-performance organic conductors.