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