Energy Gap between the Poly‑<i>p</i>‑phenylene Bridge and Donor Groups Controls the Hole Delocalization in Donor–Bridge–Donor Wires

Poly-<i>p</i>-phenylene wires are critically important as charge-transfer materials in photovoltaics. A comparative analysis of a series of poly-<i>p</i>-phenylene (<sup><i><b>R</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>) wires, capped with isoalkyl (<sup><i><b>iA</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>), alkoxy (<sup><i><b>RO</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>), and dialkylamino (<sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>) groups, shows unexpected evolution of oxidation potentials, i.e., decrease (−260 mV) for <sup><i><b>iA</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>, while increase for <sup><i><b>RO</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> (+100 mV) and <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> (+350 mV) with increasing number of <i>p</i>-phenylenes. Moreover, redox/optical properties and DFT calculations of <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>/<sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub><sup>+•</sup> further show that the symmetric bell-shaped hole distribution distorts and shifts toward one end of the molecule with only 4 <i>p</i>-phenylenes in <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub><sup>+•</sup>, while shifting of the hole occurs with 6 and 8 <i>p</i>-phenylenes in <sup><i><b>RO</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub><sup>+•</sup> and <sup><i><b>iA</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub><sup>+•</sup>, respectively. Availability of accurate experimental data on highly electron-rich dialkylamino-capped <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> together with <sup><i><b>RO</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> and <sup><i><b>iA</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> allowed us to demonstrate, using our recently developed Marcus-based multistate model (MSM), that an increase of oxidation potentials in <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> arises due to an interplay between the electronic coupling (<i>H</i><sub>ab</sub>) and energy difference between the end-capped groups and bridging phenylenes (Δε). A comparison of the three series of <sup><i><b>R</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> with varied Δε further demonstrates that decrease/increase/no change in oxidation energies of <sup><i><b>R</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub> can be predicted based on the energy gap Δε and coupling <i>H</i><sub>ab</sub>, i.e., decrease if Δε < <i>H</i><sub>ab</sub> (i.e., <sup><i><b>iA</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>), increase if Δε > <i>H</i><sub>ab</sub> (i.e., <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>), and minimal change if Δε ≈ <i>H</i><sub>ab</sub> (i.e., <sup><i><b>RO</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub>). MSM also reproduces the switching of the nature of electronic transition in higher homologues of <sup><i><b>R2N</b></i></sup><b>PP</b><sub><i><b>n</b></i></sub><sup>+•</sup> (<i>n</i> ≥ 4). These findings will aid in the development of improved models for charge-transfer dynamics in donor–bridge–acceptor systems.