Photoinduced Radical Cations Enable Anti-Kasha Emission in a Pyrene-Based Azacationic Ladder Polymer

Conjugated ladder polymers are a unique class of macromolecules, characterized by their rigid and thermally stable structures. This work presents the synthesis and characterization of a pyrene-based azacationic ladder polymer (polymer A). Spectroscopic analysis points to the generation of radical cationic units during photoreduction, while cationic species are retained in the polymer backbone, thereby enabling in situ n-type doping. Unconventional anti-Kasha emission, with a maximum at 490 nm, appears to originate from these radical species in solution. In toluene, the lower dipole moment of the solvent leads to dual emission: anti-Kasha emission from radical cations and an S₁ → S₀ transition from polycationic units at 780 nm. This interpretation is supported by density functional theory/time-dependent density-functional theory calculations, which indicates that the large energy gap between the D₃ and D₂ states of the radical cationic units may inhibit internal conversion, allowing anti-Kasha behavior. Despite their potential reactivity, the cationic and radical cationic species remain stable in solution in the dark for over 110 h. To the authors' best knowledge, polymer A is the only ladder-type conjugated polymer reported to exhibit anti-Kasha emission together with light-induced n-type doping behavior.