Peculiarities in the infrared emission of PAH-C$_{60}$ adducts

Abstract

The coexistence of PAHs and the C$_{60}$ fullerene in different astrophysical environments can give rise to the formation of new complex species denoted as PAH-C$_{60}$ adducts, which may contribute to the infrared (IR) emission observed. These PAH-C$_{60}$ adducts have been previously reported experimentally due to the high reactivity between PAHs and C$_{60}$. From the astrophysical point of view, however, they have not been considered in detail yet. Here we have performed a combined experimental and theoretical study in order to characterize the IR spectra of PAH-C$_{60}$ adducts, including multiple adducts. By using new advanced experimental techniques, we have been able to synthesize some specific PAH-C$_{60}$ adduct isomers, and measured their IR spectra. These experimental data are used to correct their harmonic scaled spectra, as obtained from quantum-chemistry calculations performed at the DFT level under the B3LYP-GD3/6-31+G(d) approach. This way, we simulate the IR ($\sim$3$-$25 $μ$m) spectra of multiple PAH-C$_{60}$ adducts, composed by a different number of PAH units: mostly one or two units. In addition, the chemical kinetics data available in the literature are used to tentatively estimate the possible order of magnitude of the abundances of these PAH-C$_{60}$ adducts using the available observational data. Essentially, our results reveal a possible strong modification of the IR spectra when astronomically estimated abundances are considered. Several spectral peculiarities are observed, such as a broad $\sim$3.4-3.6 $μ$m feature, and important modifications in the 6-10 and 12-16 $μ$m spectral regions together with contributions to the C$_{60}$ features at 7.0 and 18.9 $μ$m. Interestingly, these PAH-C$_{60}$ adducts lack aliphatic CH bonds, but they display IR features around 3.4 $μ$m, challenging previous interpretations of this astronomical feature.

Figures (13)

• Figure 1: Structure of the several PAHs considered to build the PAH-C$_{60}$ adducts. The chemical formula, name and notations are also displayed.
• Figure 2: The three archetypal PAH-C$_{60}$ adducts synthesized by supramolecular mask strategy and used for the scaling factors validation: 1An$C_{60}$ (mono-anthracene), 2An$C_{60}$ (bis-anthracene) and 1Pn$C_{60}$ (mono-pentacene) PUJALS2022. The notation follows the legend in Figure \ref{['fig1']}, adding the number of PAH units and the C$_{60}$ identification.
• Figure 3: Experimental (red) versus scaled harmonic B3LYP-D3/6-31+G* (black) spectra for the three archetypal PAH-C$_{60}$ adducts used for the scaling factors validation: 1An$C_{60}$, 2An$C_{60}$ and 1Pn$C_{60}$ (Fig.\ref{['fig2']}). The experimental spectra of the pristine PAHs (light blue) are included to ease the discussion. (A): Spectra built from scaling factors in Table \ref{['tb1']}. (B): Spectra built from empirical-corrected scaling factors for wavelengths < 5 $\mu$m ( > 2000 cm$^{-1}$) (see the text for more details). The similarity score value for each case is also indicated. The theoretical spectra have been constructed in the wavenumber scale by the convolution of a Lorentzian function with FWHM = 4 cm$^{-1}$, in order to ease the comparison with the experimental spectra.
• Figure 4: Scaled theoretical IR spectra of the mono PAH-C$_{60}$ adducts (one PAH unit: $\rm C_{60}$-$\rm C_x H_y$) modeled at the B3LYP-D3/6-31+G* level. All spectra were convolved with a Lorentzian function of FWHM = 0.02 $\mu$m. In the case of 2H-indene (In) and indenyl (Iyl), the blue and red arrows highlight the binding of the PAH with a pentagon and hexagon ring to the C$_{60}$ cage, respectively. For indenyl, with a single bond connecting its pentagon to C$_{60}$, a green arrow has been used instead. In all panels, the red dashed spectra correspond to the isolated pristine PAH, while the blue dashed lines mark the four strongest C$_{60}$ features ($\sim$7.0, 8.5, 17.4 and 18.9 $\mu$m). The black arrows on the spectra indicate those characteristic features (specific of each adduct), which are free of contribution from C$_{60}$, the pristine PAH and even from the other adducts.
• Figure 5: Theoretical scaled IR spectra of PAH-C$_{60}$ bis-adducts from 2H-indene (2InC$_{60}$, a-c) and indenyl (2IylC$_{60}$, d-e). The red dashed spectra correspond to the mono-adduct analogues, while the blue dashed lines mark the four strongest C$_{60}$ features ($\sim$7.0, 8.5, 17.4 and 18.9 $\mu$m). Note that the convolution parameters are the same as in Figure \ref{['fig4']}. For a detailed discussion on the models and spectral features we refer to Appendix \ref{['bis']}.