Rotational Characterization of Four-Ring Polycyclic Aromatic Hydrocarbons: Toward the Detection of Fluoranthene and Cyanofluoranthene in Space

The combined use of laboratory rotational spectroscopy and radio astronomical observations remains the most effective approach for identifying molecules in the interstellar medium (ISM). Following the recent detections of several polycyclic aromatic hydrocarbons (PAHs) and their cyano derivatives in the dense Taurus Molecular Cloud (TMC-1), it is reasonable to extend such searches to other PAHs within the same source. In this work, we report a rotational spectroscopy study of commercially available fluoranthene (FA) and its synthesized cyano derivative, 3-cyanofluoranthene (3-CNFA), using chirped-pulse Fourier-transform microwave spectroscopy. The analysis of the rotational spectra, supported by quantum chemical calculations, yielded molecular parameters for the parent species of both FA and 3-CNFA molecules. The experimental data of 3-CNFA were later used for its astronomical search in TMC-1 with the QUIJOTE line survey but proved unsuccessful. Despite the nondetection of 3-CNFA in this source, the upper limit to its abundance was established. The experimental data will support future astronomical searches in the ISM.