Engineering Excited-State Dynamics: [N···I···N]⁺ Halogen Bonds Unlocks Efficiently Emissive Quinoline

Abstract

Fluorescent materials, owing to their excellent photophysical properties, are highly influential in both fundamental research and industrial applications. This study introduced the novel concept of “halogen-bonded induced emission (HIE)” for the first time, based on the unique [N···I···N]⁺ halogen bonds. Contrasting sharply with the weak fluorescence of monomers, quinoline and its derivatives demonstrate markedly enhanced photoluminescence intensity accompanied by higher quantum yields upon formation of [N···I···N]⁺ complexes. Single-crystal analysis confirmed that QL-I adopts the expected linear three-center-four-electron coordination geometry (N-I-N bond angle ~178.4°), with the two quinoline rings nearly coplanar and stacked in a slipped arrangement. This configuration effectively extends the π-conjugation while minimizing detrimental intermolecular π-π stacking. The experiments further revealed the underlying emission mechanism: the halogen bond stabilizes the excited-state energy levels, suppresses nonradiative decay pathways, thereby enhancing radiative transitions as well as overall fluorescence efficiency. Furthermore, doping the iodine complexes into poly(methyl methacrylate) (PMMA) successfully yielded highly luminescent fluorescent films. This study not only offers deep insights into the impact of the [N···I···N]⁺ halogen bond as a weak interaction in optical applications, but also provides a more convenient and efficient pathway for developing supramolecular photofunctional materials with high luminescence efficiency.

Keywords

[N···I···N]⁺ halogen bonds / Fluorescence enhancement / Supramolecular self-assembly / Highly efficient luminescence / Supramolecular chemistry / Information encryption / Fluorescent film / Anti-counterfeiting

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