Luminescent core-isolated solvent-free liquids as a soft material platform for optical gas sensing

Abstract

Solvent-free functional molecular liquids have attracted great interest as a new class of stimuli-responsive soft materials, yet their potential as optical gas sensors remains unexplored. Conventionally, luminescent organic molecules are employed in combination with a solid support or matrix. However, their performance in chemical sensing and optoelectronic devices is often hindered by adverse phenomena such as aggregation, concentration quenching, and photodegradation. In this study, we employ a strategy to isolate and wrap a phosphorescent Pt(II)-porphyrin core with bulky yet flexible branched alkyl chains, resulting in a solvent-free liquid at room temperature that demonstrates excellent properties for sensing oxygen (O₂) gas. Compared to reference material composed of Pt(II)-tetraphenylporphyrin and a highly gas-permeable polymer matrix, our Pt(II)-porphyrin liquid shows comparable sensitivity (I₀/I₁₀₀ = 75~90), better linearity, and greater photostability in its O₂-responsive phosphorescence. This is attributed to the high homogeneity and gas solubility of the liquids, as well as to the shielding of luminescent-core units by bulky alkyl chains. The liquid nature of the materials allows for ratiometric sensing, where the compatibility of a phosphorescent Pt(II)-porphyrin liquid (O₂-sensitive) and a fluorescent alkyl-pyrene liquid (O₂-insensitive) enables reproducible monitoring of O₂ concentration without specific calibration. Indeed, these results highlight the significant benefits of core-isolated luminescent liquids in diverse sensing applications.