Intramolecular Long-Range Photoinduced Electron Transfer Using π‑Way of a Step-π-Conjugated Polymer

Photoinduced electron transfer (PET) along a π-scaffold with breaks remains poorly understood. To address this, we showed that a step-π-conjugated polymer (SPP)made of alternating pyrrole rings and a single sp³ carbonenables fast PET through its backbone. In this system, fluorescence quenching occurred when a perylene (Pe) donor was photoexcited in the presence of an anthraquinone (AQ) acceptor. To clarify the quenching mechanism, we considered four possibilities: (i) long-range (pseudo) through-bond PET, (ii) intermolecular through-space PET, (iii) short-range PET (through-space and through-bond), and (iv) intramolecular through-space PET. To systematically test which process dominated, we synthesized three polymers: water-soluble SPP, amphiphilic SPP, and poly-(methacrylic acid) sodium salt (PMANa), which lacks π-conjugation. The fluorescence data of these polymers indicate that long-range (pseudo) through-bond PET is the main mechanism in the SPP system. Moreover, femtosecond transient absorption spectroscopy reveals a Pe radical cation (Pe^•+) and AQ radical anion (AQ^•-), confirming charge separation through the step-π-conjugated backbone. The PET (k_ET) and charge recombination (k_CR) rate constants for R_DA ≈ 1.8 nm are estimated to be k_ET > 2 × 10¹² s^-1 and k_CR ≈ 10¹¹ s^-1 order, respectively. Thus, SPP enables long-range, fast PFT via a new π-way mechanism.