Suppression of pyrene excimers through sulfur-bridge oxidation and steric shielding

Abstract

Excimer formation is an intrinsic limitation of pyrene-based luminophores, often leading to broadened emission and reduced color purity in the condensed phase. Herein, we report a molecular design strategy to suppress excimer emission by combining steric shielding of the pyrene core with oxidation-state control of a sulfur bridge. Introduction of bulky tert-butyl substituents restricts intermolecular π–π interactions, while coupling with p-toluenethiol followed by stepwise oxidation affords three structurally analogous derivatives: sulfide (PS), sulfoxide (PSO), and sulfone (PSO₂). Single-crystal X-ray diffraction analysis reveals that increasing sulfur oxidation enhances molecular rigidity and progressively disrupts cofacial stacking between pyrene units. Consequently, all compounds exhibit higher photoluminescence quantum yields and longer excited-state lifetimes in thin films than in solutions, indicating efficient suppression of aggregation-induced quenching. Photophysical investigations demonstrate a distinct evolution of excited-state character: PS shows hybridized local-excited/charge-transfer (HLCT) emission, whereas PSO and PSO₂ predominantly display localized excited states. Remarkably, PSO exhibits mixed LE–CT emission with an exceptionally narrow full width at half maximum of 28 nm in solution, reflecting a rigid and well-confined excited state. Solvatochromic measurements and quantum chemical calculations consistently support the oxidation-dependent modulation of the emissive state. Additionally, higher sulfur oxidation states impart enhanced thermal stability to the luminophores. This study establishes sulfur oxidation-state engineering as an effective molecular strategy to control packing, excited-state nature, and excimer suppression in pyrene-based emitters, providing fundamental guidelines for designing high-purity solid-state organic luminophores.