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 (kET) and charge recombination (kCR) rate constants for RDA ≈ 1.8 nm are estimated to be kET > 2 × 1012 s–1 and kCR ≈ 1011 s–1 order, respectively. Thus, SPP enables long-range, fast PFT via a new π-way mechanism.