From p-Type to n-Type: Chlorination-Induced Electronic and Structural Modulation of Axially Substituted Siloxane...

The development of air-stable n-type organic semiconductors remains a critical challenge for organic thin-film transistor (OTFT) technologies. Silicon phthalocyanines (SiPcs) bearing axial siloxane substituents have recently achieved record charge carrier mobilities but exhibit p-type behavior due to their relatively high-lying frontier orbital energies. Herein, we report the synthesis, characterization, and OTFT integration of two new chlorinated siloxane silicon phthalocyanines, (Si4O)2SiPcCl8 and (Si4O)2SiPcCl16, designed to lower the lowest unoccupied molecular orbital (LUMO) energy and enable n-type transport. Peripheral chlorination effectively reduces the LUMO levels to -3.8 and -4.0 eV, respectively, confirming the desired electronic modulation. Single-crystal X-ray diffraction reveals that chlorination disrupts the dense π-stacking observed in the parent unchlorinated derivative through two distinct mechanisms: increased intermolecular separation in (Si4O)2SiPcCl8 and severe core distortion in (Si4O)2SiPcCl16. These structural perturbations directly impact device performance: (Si4O)2SiPcCl8 OTFTs achieve average electron mobilities of 1.6 × 10-2 cm2 V-1 s-1, comparable to one of the best-performing alkyl-substituted SiPcs, while the highly distorted (Si4O)2SiPcCl16 derivative exhibits an order-of-magnitude lower mobility. Thin-film characterization by powder X-ray diffraction and atomic force microscopy, corroborated by density functional theory calculations, demonstrates that the (Si4O)2SiPcCl8 films adopt a favorable texture aligning the high-mobility crystallographic axis parallel to the substrate. This work establishes clear structure-property relationships for chlorinated SiPcs and provides a rational framework for the design of high-performance n-type phthalocyanine semiconductors.