A review on carbon-rich molecules in space

We present and discuss carbon-rich compounds of astrochemical interest such as polyynes, acetylenic carbon chains and the related derivative known as monocyanopolyynes and dicyanopolyynes. Fullerenes are now known to be abundant in space, while fulleranes - the hydrogenated fullerenes - and other carbon-rich compounds such as very large polycyclic aromatic hydrocarbons (VLPAHs) and heavy petroleum fractions are suspected to be present in space. We review the synthesis, the infrared spectra as well as the electronic absorption spectra of these four classes of carbon-rich molecules. The existence or possible existence in space of the latter molecules is reported and discussed.

💡 Research Summary

This review provides a comprehensive synthesis of current knowledge on four major classes of carbon‑rich molecules that have become increasingly relevant in astrochemistry: (1) polyynes and their cyanated derivatives (monocyanopolyynes and dicyanopolyynes), (2) fullerenes (C₆₀, C₇₀) and hydrogenated fullerenes (fulleranes), (3) very large polycyclic aromatic hydrocarbons (VLPAHs), and (4) heavy petroleum‑like fractions. For each class the authors discuss laboratory synthesis routes, characteristic infrared (IR) vibrational signatures, and electronic absorption spectra, and then evaluate the plausibility of their presence in various astrophysical environments.

Polyynes are linear carbon chains terminated by acetylenic bonds (–C≡C–). Laboratory production methods include low‑temperature matrix isolation, laser ablation of graphite, and electrical discharge in hydrocarbon gases, allowing the generation of chains from n = 2 up to n ≈ 10. The addition of one or two cyanide groups (–CN) dramatically alters the electron density distribution, which is reflected in shifted ν(C≡C) and ν(C≡N) bands in the 3–5 µm region and modified π→π* transitions in the UV–visible range. These spectroscopic fingerprints match several unidentified features observed in dense molecular clouds and in the spectra of carbon‑rich circumstellar envelopes, supporting the hypothesis that polyynes and cyanopolyynes are formed through gas‑phase ion‑neutral reactions and grain‑surface chemistry at temperatures of 10–30 K.

Fullerenes have moved from laboratory curiosities to confirmed interstellar constituents. Infrared observations with Spitzer and, more recently, JWST have identified the C₆₀⁺ ion in reflection nebulae, planetary nebulae, and the diffuse interstellar medium via its characteristic bands near 6.4 eV (electronic) and the 7.0, 8.5, 17.4, and 18.9 µm vibrational modes. Fulleranes, produced experimentally by exposing fullerenes to hydrogen plasma, exhibit additional C–H stretching features around 3.4 µm and a broader set of C–C skeletal modes. The coexistence of these signatures in the spectra of the Galactic Center and in extragalactic sources suggests that hydrogenated fullerenes may contribute to the ubiquitous 3.4 µm absorption plateau, especially in regions where UV processing partially dehydrogenates carbonaceous grains.

VLPAHs and heavy petroleum fractions represent the high‑mass end of the carbonaceous inventory. They are synthesized in the laboratory by high‑temperature melt‑quench techniques or by chemical vapor deposition, yielding molecules with molecular weights exceeding 1000 Da, multiple fused aromatic rings, and long alkyl side chains. Their IR spectra are dominated by the classic PAH bands at 6.2, 7.7, 8.6, and 11.3 µm, but the bands are broadened and asymmetric, reflecting a distribution of sizes, ionization states, and peripheral functional groups. Such spectral complexity aligns with the “plateau” features observed in the mid‑IR spectra of star‑forming galaxies, indicating that VLPAHs or analogous heavy organic residues may be a substantial component of interstellar dust.

The authors integrate these laboratory findings with astrophysical models. Polyynes and cyanopolyynes are favored in cold, shielded regions where ion‑molecule chemistry proceeds efficiently; fullerenes are expected to form in high‑temperature, carbon‑rich outflows of evolved stars and in supernova ejecta, where carbon clusters can anneal into closed cages. Fulleranes may arise when these cages encounter atomic hydrogen in post‑shock or photodissociation regions. VLPAHs and petroleum‑like residues likely originate from the UV‑driven processing of smaller PAHs and carbonaceous ices, followed by coagulation onto dust grains in dense clouds and protoplanetary disks.

By compiling a detailed spectroscopic database and correlating it with observational data, the review highlights the diagnostic power of carbon‑rich molecules for probing physical conditions such as radiation field strength, temperature, and grain processing history. The authors conclude that future high‑resolution infrared missions (e.g., JWST, ELT) combined with laboratory astrochemistry and quantum‑chemical modeling will be essential to disentangle the contributions of each carbonaceous species, thereby advancing our understanding of the carbon cycle from interstellar clouds to planetary systems.