Halogen-Terminated Carbon Atomic Wires by Laser Ablation in Halogenated Organic Solvents: Synthesis and Characterization
We report the synthesis of halogenated carbon atomic wires (halopolyynes) via pulsed laser ablation in liquid and their comprehensive Raman characterization. Using dichloromethane and dibromomethane-containing solutions, we produced polydisperse mixtures of monohalogenated (HC$_{2n}$X) and dihalogenated (XC$_{2n}$X) polyynes (X=Cl, Br; n=3-10). High-performance liquid chromatography enabled separation and analysis of these compounds, while chemical derivatization and mass spectrometry confirmed their molecular structures. A possible formation mechanism is proposed, involving carbon chain polymerization and termination by hydrogen and halogen atoms from atomized solvent molecules during the plasma phase. UV-Vis absorption and synchrotron-based UV Resonance Raman spectroscopy reveal that halogen terminations act as auxochromes through p-$π$ conjugation between their lone-pair electrons and the sp-carbon backbone, extending effective conjugation length. This interaction reduces bond length alternation and moderately redshifts vibronic absorption and vibrational modes. Resonance Raman spectra show selective overtone enhancement and vibrational anharmonicity consistent with carbyne-like materials. These findings expand the synthetic approaches for halogenated carbon atomic wires and establish halopolyynes as a platform for tailoring electronic and optical properties of sp-carbon wires.