Halopolyynes: Synthesis, Structure, and Spectroscopy
- Halopolyynes are linear sp-carbon chains capped with halogen atoms that exhibit a polyynic structure with alternating triple and single bonds.
- They are produced via pulsed laser ablation in liquid using halogenated solvents, enabling simultaneous chain formation and halogen termination.
- Halogen termination modulates bond-length alternation, induces optical redshifts in UV–Vis spectra, and alters Raman signatures, providing chemical handles for further functionalization.
Halopolyynes are linear -carbon chains whose ends are capped with halogen atoms rather than exclusively by hydrogen or organic substituents. In the reported system, they comprise monohalogenated species of the form and dihalogenated species of the form , with or and denoting the number of -carbon atoms in the backbone. For chains written as alternating units, and the general structural representations are and 0. Halopolyynes were obtained by pulsed laser ablation in liquid (PLAL) using halogenated organic solvents, constituting the first synthesis of halogenated organic molecules via PLAL and the first Raman characterization of long halogenated carbon atomic wires (Marabotti et al., 8 Jul 2025).
1. Definition, structural classes, and relation to other carbon atomic wires
The reported halopolyyne families are monohalogenated 1 with 2 or 3 and 4–5, and dihalogenated 6 with 7–8 for chlorine and 9–0 for bromine (Marabotti et al., 8 Jul 2025). The chains are polyynic rather than cumulenic: they are described as alternating triple and single bonds, and the backbone is naturally expressed as a sequence of 1 units.
Relative to hydrogen-terminated 2, methyl-terminated 3, and cyano-terminated 4 or 5 polyynes, halogen terminations act as weak electron donors. In the reported comparison, this weak-donor behavior induces moderate vibronic redshifts in UV–Vis spectra, with effects larger than 6 but smaller than strong 7-conjugating groups such as 8; reduces bond length alternation (BLA) slightly and downshifts the collective 9 Raman mode; increases hydrophobicity and thus retention times in reversed-phase HPLC; provides distinctive isotopic fingerprints, especially for bromine, upon derivatization and mass spectrometry; and offers synthetic handles for further functionalization, including oxidative addition to Pd (Marabotti et al., 8 Jul 2025).
The BLA descriptor used for these chains is
0
Within the reported framework, halogen termination decreases BLA relative to hydrogen-capped analogues, but the wires remain polyyne-like rather than cumulene-like. This distinction is central to their classification as carbon atomic wires with preserved polyyne spectroscopic signatures.
2. PLAL synthesis and the proposed formation mechanism
Halopolyynes were synthesized by PLAL from a graphite target of 1 purity using an Nd:YAG laser at the fundamental wavelength 2, pulse duration 3, repetition rate 4, and pulse energy 5, focused with a 6 focal length lens. The spot radius was reported as 7, corresponding to a fluence 8. The ablation media were 9 dichloromethane (DCM, stabilized with amylene), or a 0 v/v mixture of cyclohexane and dibromomethane (DBM), with cyclohexane required because DBM density exceeds that of graphite. Isopropanol was used for reference ablations. Each run lasted 1; for concentration, 2 ablations were pooled and processed, and the products were transferred to acetonitrile for HPLC compatibility (Marabotti et al., 8 Jul 2025).
The formation mechanism is described in terms of a confined, high-temperature plasma plume generated at the target–liquid interface. Carbon species from the target—3, 4, radicals, and ions—coexist with fragments arising from solvent atomization. Atomized halogenated solvents supply H and X atoms, enabling polymerization of carbon species into growing 5 chains followed by termination with H, X, or X/X pairs. The qualitative outcomes are 6, 7, and 8 (Marabotti et al., 8 Jul 2025).
The product distributions were polydisperse. For the chlorinated series, species extended from 9 to 0 for monohalogenated chains and up to 1 for dihalogenated chains; for the brominated series, monohalogenated chains extended from 2 to 3 and dihalogenated chains up to 4. Relative yields were reported as chromatographic area ratios. For chlorinated mono-halopolyynes, the relative abundance was highest at short length: 5 of 6, 7, 8, 9, and 0. Brominated mono-halopolyynes were lower overall in the cyclohexane/DBM medium, with 1 of 2, 3, 4, and 5. Dihalogenated species were much less abundant: 6 was 7–8 lower than 9, and 0 was 1–2 lower than 3 (Marabotti et al., 8 Jul 2025).
The length dependence of the yields indicates that halogen termination is more probable for short chains and becomes less probable as 4 increases. The reported explanation attributes this trend to the short lifetimes and high reactivity of halogen radicals, together with faster degradation of longer halogen-capped chains under PLAL.
3. Separation, chromatographic behavior, and structural identification
The mixtures were separated by RP-HPLC on a Shimadzu Prominence UFLC with diode-array UV–Vis detection and fraction collection, using a Phenomenex Luna 5 C18(2), 6, 7 column and an aqueous gradient compatible with acetonitrile-transferred samples (Marabotti et al., 8 Jul 2025). The chromatographic behavior is consistent with chain-length and termination dependence. Retention times increase both with increasing chain length and with halogen substitution, reflecting the reported increase in hydrophobicity induced by halogen end groups.
For the chlorine series obtained from DCM ablations, representative values were 8 at 9 with 0, 1 at 2 and 3, 4 at 5 and 6, 7 at 8 and 9, 0 at 1 and 2, 3 at 4 and 5, 6 at 7 and 8, 9 at 00 and 01, 02 at 03 and 04, 05 at 06 and 07, 08 at 09 and 10, and 11 at 12 and 13. For the bromine series from cyclohexane/DBM, representative values were 14 at 15 and 16, 17 at 18 and 19, 20 at 21 and 22, 23 at 24 and 25, 26 at 27 and 28, 29 at 30 and 31, 32 at 33 and 34, 35 at 36 and 37, and 38 at 39 and 40 (Marabotti et al., 8 Jul 2025).
Structural confirmation relied on derivatization that exploits halogen reactivity. Oxidative addition with 41 was used to form cationic complexes of the type 42 upon halide abstraction under ESI conditions. The reaction scheme was described as 43, followed by formation of 44 in the ion source, where 45 is the abstracted halide anion (Marabotti et al., 8 Jul 2025).
ESI-HRMS on a Bruker qTOF compact detected cations corresponding to mono-halopolyynes. The reported derivative cations were 46 at 47, 48 at 49, 50 at 51, and 52 at 53, each showing characteristic isotopic patterns diagnostic of Pd/P and of Cl or Br. Dihalogenated 54 species were not observed in mass spectrometry, with the reported explanation being lower concentration and lower stability in the ion source.
4. Electronic structure, UV–Vis response, and bond-length alternation
All reported halopolyynes display sharp vibronic progressions in UV–Vis absorption that are characteristic of polyynes, and the series redshift monotonically with increasing 55 (Marabotti et al., 8 Jul 2025). Halogen termination adds an additional redshift relative to hydrogen-capped analogues. The effect is termination-sensitive: bromine produces larger redshifts than chlorine, and dihalogenated chains redshift more than monohalogenated chains.
Quantitatively, for 56 the reported maxima are 57 for 58, 59 for 60, 61 for 62, 63 for 64, and 65 for 66. For 67, the corresponding values are 68, 69, 70, 71, and 72. For 73, they are 74, 75, 76, 77, and 78. For 79, the reported values are 80 for hydrogen, 81 for chlorine, and 82 for bromine termination. The representative redshifts are therefore 83, 84, 85, and 86 at 87; 88, 89, 90, and 91 at 92; 93, 94, 95, and 96 at 97; and 98 and 99 for the mono-halogenated 00 chains (Marabotti et al., 8 Jul 2025).
The reported interpretation is that halogen termination behaves as a weak electron donor. Partial charge injection enhances 01-conjugation and reduces the bandgap, consistent with the observed optical redshift. Density-functional-theory trends further show that BLA decreases with halogen termination, with the ordering described as 02 and 03 for the magnitude of the halogen-induced effect, and with dihalogenated chains more affected than monohalogenated ones. The effect is most pronounced for shorter chains and diminishes as 04 increases (Marabotti et al., 8 Jul 2025).
The vibronic envelopes were analyzed through Huang–Rhys factors. These factors increase with 05, as typical for carbon atomic wires, but halogen termination does not significantly change them relative to hydrogen-capped analogues. The reported conclusion is that halogen end groups induce limited hyperconjugation compared with more strongly 06-like end groups such as 07.
5. UV Resonance Raman signatures, overtone behavior, and anharmonicity
The vibrational characterization was carried out by synchrotron-based UV Resonance Raman spectroscopy at Elettra BL10.2-IUVS with tunable deep-UV excitation from 08 to 09, using excitation energies resonant with the 10 vibronic transitions of the selected halopolyynes (Marabotti et al., 8 Jul 2025). The dominant feature is the fundamental ECC (11) mode, assigned to the collective stretching of the triple bonds. Its frequency decreases with increasing chain length and with increasingly donating terminations; dihalogenated species downshift more than monohalogenated species, and bromine produces a larger downshift than chlorine.
Representative experimental ECC frequencies are 12 at 13 with DFT value 14, 15 at 16 with DFT value 17, and 18 at 19 with DFT value 20. More generally, the ECC mode falls from approximately 21–22 for shorter chains toward approximately 23–24 for longer chains, and halogenation further downshifts the mode relative to 25 (Marabotti et al., 8 Jul 2025).
A notable Raman feature is selective overtone enhancement. When excitation is resonant with the 26 transition rather than the 27 transition, the second-order overtone 28 is selectively enhanced and can exceed the intensity of the fundamental 29. This behavior was demonstrated for 30 at 31 and 32, for 33 at 34 and 35, and for 36 at 37. The reported interpretation is that overtone-selective resonance is an emerging fingerprint of carbyne-like systems (Marabotti et al., 8 Jul 2025).
The vibrational anharmonicity parameter 38 was extracted from overtone positions according to
39
where 40 is the fundamental ECC wavenumber and 41 is the second overtone wavenumber, both in 42. The measured values are 43 for 44 and 45 for 46. The corresponding predictions from the previously proposed universal anharmonicity law for carbyne-like materials are 47 and 48. The reported agreement constitutes the first independent confirmation of that universal law in a new class of carbon atomic wires (Marabotti et al., 8 Jul 2025).
6. Classification, stability, limitations, and chemical utility
Halopolyynes are classified within carbyne-like materials because they share the core spectroscopic hallmarks of that family: vibronically structured UV–Vis series that redshift with chain length and with more donating terminations; a strong, length-dependent ECC Raman mode near 49–50; selective enhancement of the 51 overtone under specific resonance conditions; and strong vibrational anharmonicity with 52 following the universal law across chain lengths (Marabotti et al., 8 Jul 2025). Relative to 53 they display moderate additional redshifts and ECC downshifts; relative to cumulenes, they retain polyyne-like BLA, although reduced by halogens, and they preserve overtone behavior typical of polyynes.
The work also emphasizes stability constraints. Long halopolyynes are described as unstable, and halogen-capped chains can be more reactive than 54. A specific precedent is noted for iodinated chains: 55 beyond approximately 56 carbons are explosive near approximately 57–58 in wet synthesis. During PLAL, longer halogenated chains are reported to degrade faster than hydrogen-capped analogues, with dihalogenated chains still less stable. Handling recommendations include working in acetonitrile, avoiding prolonged exposure to high-intensity light and heat, processing HPLC fractions quickly, and performing UVRR at low concentrations of 59–60 (Marabotti et al., 8 Jul 2025).
Several limitations are explicitly identified. PLAL intrinsically produces broad 61-distributions, so polydispersity is inherent. Although PLAL offers simplicity and flexibility, it provides limited fine control over chain length and termination compared with state-of-the-art wet syntheses. Brominated and dihalogenated products are obtained at lower yields, and the incompatibility of DCM or DBM media with aqueous mobile phases requires transfers or extractions before HPLC (Marabotti et al., 8 Jul 2025).
At the same time, halogen termination offers chemically useful tunability. Changing chain length and halogen identity modulates the bandgap as reflected in UV–Vis positions, alters conjugation through BLA changes, and shifts the ECC mode and overtone behavior. The reported potential applications include nanoelectronics, where the compounds may serve as molecular wires and interconnects; optoelectronics and photonics, as UV absorbers with tailored vibronic structure; sensing through UVRR signatures; and labels or metrology through distinctive isotopic MS fingerprints after derivatization (Marabotti et al., 8 Jul 2025). The end groups also provide downstream synthetic handles: oxidative addition to Pd was demonstrated directly, and the reported chemistry points to cross-coupling for attachment of functional end groups or structural extension, as well as the synthesis of push–pull chains, organometallic caps, and polymeric architectures.
In this formulation, halopolyynes occupy a distinct position among carbon atomic wires. They preserve the defining carbyne-like spectroscopic behavior of polyynes while introducing halogen-dependent control over optical, vibrational, and chemical properties, and they do so through a one-step physical synthesis route that expands the available synthetic toolbox for termination-engineered carbon-rich architectures (Marabotti et al., 8 Jul 2025).