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

## TL;DR

A new pyrene-based polymer shows unusual light emission when exposed to light, due to the formation of radical cations.

## Contribution

The study reports a pyrene-based ladder polymer that exhibits anti-Kasha emission and light-induced n-type doping.

## Key findings

- The polymer shows anti-Kasha emission at 490 nm from radical cations.
- Dual emission is observed in toluene with transitions at 780 nm and 490 nm.
- Radical cationic species remain stable in solution for over 110 hours in the dark.

## Abstract

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.

Dual emission from a novel conjugated azacationic ladder polymer containing pyrene is observed in solution: conventional emission at 780 nm from S1 → S0 transitions of cationic units and anti‐Kasha emission (D3 → D0) at 490 nm from photogenerated radical cationic sites. This unique behavior arises from the coexistence of distinct charged chromophores within the rigid polymer backbone.© 2026 WILEY‐VCH GmbH