Insight into Stable, Concentrated Radicals: from Sulfur-functionalized Alkyne-rich Crystalline Frameworks to Simple Alkyne Motifs

Abstract

Organic radicals feature versatile unpaired electrons key for photoelectronic and biomedical applications, but remain difficult to access in stable concentrated forms. We disclose easy generation of stable, concentrated radicals from various alkynyl phenyl motifs, including 1) sulfur-functionalized alkyne-rich organic linkers in crystalline frameworks; 2) the powders of these molecules alone; 3) simple diethynylbenzenes. For Zr-based framework, generation of radical-rich crystalline framework was achieved by thermal annealing in the range of 300‒450 °C. For terminal alkynes, electron paramagnetic resonance signals (EPR; indicative of free radicals) arise after air exposure or mild heating (e.g., 70 °C). Further heating (e.g., 150 °C for 3 hours) raises the radical concentrations up to 3.30 mol kg‒1. For the more stable internal alkynes, transformations into porous radical solids can also be triggered, albeit at higher temperatures (e.g., 250 to 500 °C). The resulted radical-containing solids are porous, stable to air as well as heat (up to 300‒500 °C) and exhibit photothermal conversion capacity. The formation of radicals can be ascribed to extensive alkyne cyclizations, forming defects, dangling bonds and the associated radicals stabilized by the polycyclic π-systems.