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

Poly-p-phenylene wires are critically important as charge-transfer materials in photovoltaics. A comparative analysis of a series of poly-p-phenylene (^RPP_n) wires, capped with isoalkyl (^iAPP_n), alkoxy (^ROPP_n), and dialkylamino (^R2NPP_n) groups, shows unexpected evolution of oxidation potentials, i.e., decrease (−260 mV) for ^iAPP_n, while increase for ^ROPP_n (+100 mV) and ^R2NPP_n (+350 mV) with increasing number of p-phenylenes. Moreover, redox/optical properties and DFT calculations of ^R2NPP_n/^R2NPP_n^+• further show that the symmetric bell-shaped hole distribution distorts and shifts toward one end of the molecule with only 4 p-phenylenes in ^R2NPP_n^+•, while shifting of the hole occurs with 6 and 8 p-phenylenes in ^ROPP_n^+• and ^iAPP_n^+•, respectively. Availability of accurate experimental data on highly electron-rich dialkylamino-capped ^R2NPP_n together with ^ROPP_n and ^iAPP_n allowed us to demonstrate, using our recently developed Marcus-based multistate model (MSM), that an increase of oxidation potentials in ^R2NPP_n arises due to an interplay between the electronic coupling (H_ab) and energy difference between the end-capped groups and bridging phenylenes (Δε). A comparison of the three series of ^RPP_n with varied Δε further demonstrates that decrease/increase/no change in oxidation energies of ^RPP_n can be predicted based on the energy gap Δε and coupling H_ab, i.e., decrease if Δε < H_ab (i.e., ^iAPP_n), increase if Δε > H_ab (i.e., ^R2NPP_n), and minimal change if Δε ≈ H_ab (i.e., ^ROPP_n). MSM also reproduces the switching of the nature of electronic transition in higher homologues of ^R2NPP_n^+• (n ≥ 4). These findings will aid in the development of improved models for charge-transfer dynamics in donor–bridge–acceptor systems.