Halogen-Terminated Carbon Atomic Wires by Laser Ablation in Halogenated Organic Solvents: Synthesis and Characterization

Abstract: We report the first synthesis of halogenated organic molecules via pulsed laser ablation in liquid, specifically halopolyynes, i.e., carbon atomic wires terminated by halogen atoms, and the first Raman characterization of long halogenated wires. Using dichloromethane and a dibromomethane-containing solution, we produced a polydisperse mixtures of monohalogeanted (HC$_n$X) and dihalogenated (XC$_n$X) polyynes (X=Cl, Br; n=6-20). High-performance liquid chromatography enabled us to separate and analyze these compounds, while chemical derivatization and mass spectroscopy confirmed their molecular structures. Their formation mechanism involves carbon chain polymerization and termination by halogen atoms from atomized solvent molecules during the plasma phase. UV-Vis absorption and synchrotron-based UV Resonance Raman spectroscopy revealed that halogen terminations act as weak electron donors, slightly affecting conjugation, bond length alternation, and moderately redshifting vibronic absorption and vibrational modes. Resonance Raman spectra show selective overtone enhancement, an emerging signature behavior of carbyne-like systems. Vibrational anharmonicity measurements confirm that halopolyynes follow the universal anharmonicity law previously proposed for carbyne-like materials, solidifying their classification within this family. These findings not only expand the synthetic toolbox for carbon atomic wires but also establish halopolyynes as a versatile platform for tailoring wire terminations and developing novel materials with tunable electronic and optical properties.