Tuning mesomorphic, spectral and nonlinear optical behavior in chalcogenophene triads: the role of oxygen, sulfur, and selenium

Abstract

Understanding how heteroatom identity determines competing functional properties is essential for the rational design of multifunctional organic materials. Here, we report the synthesis of nine new chalcogenophene-based compounds in which oxygen, sulfur, or selenium is systematically incorporated into both the rigid core and terminal substituents. This homologous and isosteric molecular platform enables direct correlation of mesomorphic organization, photophysical response, and third-order nonlinear optical behavior within a single structural framework. We demonstrate that chalcogen substitution induces distinct and competing regimes of molecular self-assembly and excited-state electronic response. Thiophene-containing cores promote robust nematic mesophase formation (e.g., a nematic range of ~80 °C), whereas oxygen-containing derivatives favor ecicient radiative decay and enhanced third-order nonlinear optical response, yielding rare deep-blue emission with high fluorescence quantum yields up to 0.60 and color coordinates approaching the Rec.709 standard. Third-harmonic generation (THG) measurements in PMMA thin films at 1064 nm reveal χ⁽³⁾ values up to 12.34 × 10^-22 m² V^-2 , highlighting a regime in which high emission eciciency can coexist with strong third-order response despite limited mesomorphism. In contrast, increased chalcogen polarizability does not translate into simultaneous optimization of mesomorphic, emissive, and nonlinear properties.Terminal thioalkyl substitution consistently enhances fluorescence eciciency across the series. Quantum-chemical analysis reveals that these trends arise from a balance between electronic delocalization, backbone planarity, and excited-state reorganization, rather than polarizability alone, establishing transferable structure-property design principles for multifunctional organic materials.