Nodal Arrangement of HOMO Controls the Turning On/Off the Electronic Coupling in Isomeric Polypyrene Wires

The charge transfer along π-conjugated wires is largely governed by the interchromophoric electronic coupling that depends on the geometry (e.g., interchromophoric dihedral angle) and electronic structure of the chromophores. Herein, we demonstrate that stabilization of the cationic charge (hole) in polypyrene cation radicals and the extent of hole delocalization can be easily controlled by modulating the nodal arrangement of the HOMO. For example, 2,2′-linked para-polypyrenes show nonexistent electronic coupling owing to a nodal arrangement of HOMO that is unfavorable for orbital overlap, despite a favorable interchromophoric dihedral angle. A repositioning of the linkage between two pyrenes from para to meta positions produces a far less favorable interchromophoric dihedral angle, yet the electronic coupling turns on due to a favorable nodal arrangement of HOMO, which allows interchromophoric orbital overlap. This surprising finding has been demonstrated through the synthesis and systematic examination of the redox and optical properties of meta-polypyrenes (m-Py_n), which reveals a sizable delocalization of the hole in m-Py_n^+• that extends to three pyrene units, only two benzenoid units less than in typical poly-p-phenylene wires. These findings of widespread interest, supported by density functional theory (DFT) and the Marcus-based multistate model, will impact the rational design of new charge-transfer materials for photovoltaic and molecular electronics applications.