© The Authors 2025

1 Introduction

The mid-infrared (MIR) spectra of most objects that are associated with gas and dust, including protoplanetary nebulae, and reflection nebulae, are dominated by broad (about 20–40 cm⁻¹) emission features (3.3, 6.2, 7.7, 8.6, and 11.2 μm), which are referred to as aromatic infrared bands (AIBs, Tielens 2008). These vibrational signature features are generally attributed to the IR fluorescence of large (50–100 C atoms) polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives (Tielens 2013, and references therein). Interstellar IR spectra also show evidence of PAH clusters and very small dust grains (Sellgren 1984; Berné et al. 2007; Pilleri et al. 2012). PAHs are ubiquitous and abundant in the interstellar medium (ISM), where they account for ~10% of the cosmic carbon in the ISM (Sellgren 1984; Puget & Leger 1989; Allamandola et al. 1989). Furthermore, PAHs play an important role in the ionization and energy balance of the ISM (Tielens 2008, and references therein).

Over the past several decades, studies provided detailed information on the evolution processes of PAH species in space (Tielens 2013). In the ISM, PAHs are influenced and constrained by various interstellar environmental factors, such as ultraviolet radiation, electrons, atoms, ions, and other coexisting small molecules (Zhang et al. 2023). Therefore, PAHs may be superhydrogenated (i.e., have excess hydrogen at carbon sites, Cazaux et al. 2016, 2019; Jensen et al. 2019; Hua et al. 2024), partially deuterated (Draine 2006; Yang et al. 2021; Yang & Li 2023a), substituted by other elements (N, O, S, etc. Hudgins et al. 2005; Bauschlicher 1998), or functionalized (such as methyl -CH₃, vinyl -CHCH₂, etc. Hollenbach & Tielens 1999; Yang & Li 2023b). Furthermore, PAH derivative clusters may form through ion-molecule collision reactions in the gas phase (Petrie & Bohme 1993; Böhme 2016; Zhen et al. 2018; Zhen 2019).

As functionalized PAHs, the functional units play a key role in their evolution processes. Studies of the formation and dissociation mechanisms of interstellar nitrogen-containing PAHs were reported. The subtle variations in the peak wavelength of the 6.2 μm emission band are attributed to the nitrogen-containing PAHs (Hudgins et al. 2005). For the nitrogen-containing PAHs, the successive detection of cyano-PAHs recently confirmed the potential and importance of nitrogen-containing PAHs in the ISM in particular. Cyano-PAHs are easier to detect than pure PAHs in the radio band range because their dipole moments are significantly large. The first to be detected was cyanobenzene (C₇H₅N), which has one aromatic ring (McGuire et al. 2018). Subsequent observations have revealed cyanonaphthalene and cyanoindene (C₁₁H₇N and C₁₀H₇N, two aromatic rings, McGuire et al. 2021; Sita et al. 2022), cyanoacenaphthylene (C₁₃H₇N, three aromatic rings, Cernicharo et al. 2024), cyanopyrene (C₁₇H₉N, four aromatic rings, Wenzel et al. 2024, 2025a), and cyanocoronene (C₂₅H₁₁N, seven aromatic rings, Wenzel et al. 2025b).

The evolution of the largest detected interstellar molecular family, 1-, 2-, and 4-cyanopyrene in TMC-1 (with an abundance ratio of about 2:1:2 for these isomers) was studied by Wenzel et al. (2024, 2025a). The authors suggested that the cyano unit is directly added to pyrene to form cyanopyrene isomers, which means that cyano-PAHs are also constrained and influenced by the environmental factors in the interstellar environment.

The PAH clusters are thought to be self-assembled between PAHs and other coexisting molecules in the molecular cloud, and PAH radicals were discussed as the driving force (Zhen 2019; Rapacioli et al. 2006; Rhee et al. 2007; Gavilan Marin et al. 2020). In addition, functional units will also affect the formation and photochemistry process of PAH clusters. Therefore, they have to be considered as well (Bernstein et al. 2002; Walsh 2008; Zhen et al. 2016). Little information is available about the effects of the functional units on the formation and photoevolution of PAH cluster cations, however.

We study the gas-phase formation and photochemical processes of large cyano-containing PAH clusters by combining laboratory experiments with quantum chemical calculations. We consider large PAH (e.g., dicoronylene) cations and small cyanoPAHs (e.g., 9-cyanoanthracene) as the initial precursors for the gas-phase ion-molecule collision reactions. The molecular geometry of DC cations and 9-cyanoanthracene are shown in Fig. 1. As a possible carrier of AIBs, dicoronylene (DC, C₄₈H₂₀, m/z = 596) is selected as an example of large (interstellar) PAHs (Croiset et al. 2016; Zhen et al. 2018). 9-cyanoanthracene (C₁₅H₉N, m/z = 203) is selected as an example of small cyano-containing PAHs. The cyano unit of 9-cyanoanthracene is in the solo-carbon sites. Based on previous studies, the functional units located in solo-carbon sites have a relatively higher chemical reactivity (Yang et al. 2022).

Fig. 1 Optimized structures of [C48H20]+, 9-cyanoanthracene (C15H9N), [C48H19]+ (a), [C48H19]+ (e), and [C48H18]+ (a, b). Atoms are represented as spheres with the following color-coding: Nitrogen is shown in blue, hydrogen is shown in white, and carbon is shown in gray.

2 Experimental results

Briefly, we performed experiments on an apparatus equipped with a quadrupole ion trap and a reflection time-of-flight mass spectrometer (Zhen 2019; Zhen et al. 2019). Further details of the experimental procedures are provided in Appendix A. Dicoronylene (C₄₈H₂₀) was evaporated in an oven, ionized by the electron gun, and then transported into the ion trap in the ion trap chamber. In addition to the main DC cation ([C₄₈H₂₀]⁺, m/z = 596), several residual DC fragments were also observed ([C₄₈H_16/17/18/19]⁺, m/z = 592, 593, 594, and 595) as dehydrogenated DC (DDC) cations. These DDC cations originated from the electron impact ionization of original DC⁺ (Castellanos et al. 2018; Zhen 2019). Another oven was located below the ion trap to vaporize 9-cyanoanthracene (C₁₅H₉N) in the ion trap chamber, which can effuse continuously toward the center of the ion trap. Helium gas was introduced continuously into the trap to thermalize the ion cloud. A third harmonic laser of Nd: YAG (INDI, Spectra-Physics), 355 nm, ~6 ns, operated at 10 Hz, was used to irradiate the trapped cationic species. The resulting mass spectra of the DC/9-cyanoanthracene cluster cations are presented in Figs. 2–4.

A typical mass spectrum of DC/9-cyanoanthracene cluster cations without laser irradiation is shown in Fig. 2. A series of DC/9-cyanoanthracene cluster cations are observed, similar to what was observed for the DC/anthracene cluster cations system (Zhen 2019). The DC/9-cyanoanthracene cluster cations ([(C₁₅H₉N)_nDDC]⁺ with n = [1, 4]) are labeled in detail in a zoomed-in mass spectrum in Fig. 2. The inset spectrum provides more detail on the newly formed DC/9-cyanoanthracene cluster cations: All of them are dehydrogenated, that is, partially H-stripped ion species are the most abundant species, that is, [(C₁₅H₉N)DDC]⁺ (e.g., [(C₁₅H₉N)C₄₈H₁₉]⁺, m/z = 798). In addition, we also observed some additional peaks (m/z = 610, 620, 695, 825, and 865), but were unable to provide assignments. These peaks, for example, m/z = 625, as well as m/z = 825 and 865, might be formed as byproducts from contamination in the ion trap chamber. The peaks of m/z = 610 and 620 might be byproducts of the laboratory synthesis of DC.

Based on the above results and previous studies (Zhen et al. 2018; Zhen 2019), we propose that the DC/9-cyanoanthracene cluster cations are formed through ion-molecule collision reaction between [DDC]⁺ and neutral 9-cyanoanthracene, and that they occur in a step-by-step reaction pathway. The formation pathways of [(C₁₅H₉N)_nDDC]⁺ are shown below, $$\begin{array}{rl}{\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}+& {[{\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)}_{\mathrm{n}-1}(\mathrm{D}\mathrm{D}\mathrm{C})]}^{+}\\ & ⟶{[{\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)}_{\mathrm{n}}(\mathrm{D}\mathrm{D}\mathrm{C})]}^{+},\mathrm{n}=[1,4].\end{array}$$

The photo-fragmentation patterns of DC/9-cyanoanthracene cluster cations upon irradiation at 1.7 and 2.8 mJ (the irradiation time was 0.3 s, from 9.5 to 9.8 s) are shown in Fig. 3 (the blue and red line).

In Fig. 3A, at lower energy (1.7 mJ, blue line), the dissociation of DC/9-cyanoanthracene cluster cations primarily occurs through dehydrogenation (the peaks in the m/z = 770–798 range are products of varying degrees of dehydrogenation) and CN unit loss (the peaks in the m/z = 744–772 range result from CN unit loss accompanied by dehydrogenation), and these two processes occur simultaneously; With increasingly higher laser energy (2.8 mJ, red line), multiple fragmentation steps become more prominent, new fragmentation channels become accessible, and carbon cluster products (e.g., [C₆₀]⁺ and [C₅₈]⁺) clearly formed. For more details, as shown in Fig. 3B, a zoom-in mass spectrum (with higher laser energy, 2.8 mJ) is presented in the range of m/z = 716–812. Based on the previous experimental results (Zhen et al. 2014; Zhen 2019), we conclude that in the range of 776–796, these peaks can be assigned as dehydrogenated molecules ([C₆₃NH_m]⁺); in the range of 750–772 and 725–740, these peaks can be assigned as CN unit lost or C₂ unit lost with more further dehydrogenated ([C₆₂H_m]⁺, [C₆₁NH_m]⁺, and [C₆₀H_m]⁺).

Interestingly, in Fig. 3B, some mass peaks can be attributed to carbon clusters: the nitrogen-containing carbon cluster cations (e.g., [C₆₁N]⁺, m/z = 746 and [C₆₃N]⁺, m/z = 770), and the pure carbon cluster cations (e.g., [C₆₀]⁺, m/z = 720 and [C₆₂]⁺, m/z = 744). In addition, we note that these mass peaks may include all these different types of carbon cluster isomers: N-containing carbon clusters, pure carbon clusters, or carbon clusters with H atoms. For example, [C₆₁N]⁺ and [C₆₂H₂]⁺ have the same mass (m/z = 746), and they may be formed during the process of photolysis at the same time.

[C₆₃N]⁺ has two photodissociation channels: one is formed [C₆₂]⁺ through CN unit lost, and the other is formed [C₆₁N]⁺ through C₂ unit lost. [C₆₂]⁺ and [C₆₁N]⁺ have different photodissociation channels to form [C₆₀]⁺, with C₂ or CN unit lost. This means that the nitrogen atom does not necessarily remain in the large PAHs in the photolysis process. Based on this, the photodissociation channels for [(C₁₅H₉N)DDC]⁺, [C₆₃N]⁺, [C₆₂]⁺, and [C₆₁N]⁺ are shown below, $$\begin{array}{rl}& {\left[\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)\mathrm{D}\mathrm{D}\mathrm{C}\right]}^{+}\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{63}{\mathrm{H}}_{\mathrm{n}}\mathrm{N}\right]}^{+}+2\mathrm{H}/{\mathrm{H}}_2;\\ & {\left[\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)\mathrm{D}\mathrm{D}\mathrm{C}\right]}^{+}\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{62}{\mathrm{H}}_{\mathrm{m}}\right]}^{+}+\mathrm{C}\mathrm{N};\\ & {\left[{\mathrm{C}}_{63}\mathrm{N}\right]}^{+}\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{61}\mathrm{N}\right]}^{+}+{\mathrm{C}}_2,{\left[{\mathrm{C}}_{63}\mathrm{N}\right]}^{+}\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{62}\right]}^{+}+\mathrm{C}\mathrm{N};\\ & {\left[{\mathrm{C}}_{62}\right]}^{+}\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{60}\right]}^{+}+{\mathrm{C}}_2,{\left[{\mathrm{C}}_{61}\mathrm{N}\right]}^{+}\stackrel{\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{60}\right]}^{+}+\mathrm{C}\mathrm{N}.\end{array}$$

Figure 4 shows the resulting mass spectrum of trapped DC/9-cyanoanthracene cluster cations upon irradiation at 3.0 mJ (black line, lower laser energy) and 6.0 mJ (red line, higher laser energy), and then subsequent collision reactions with neutral 9-cyanoanthracene molecules (5.8–9.8 s).

With lower laser-energy irradiation (the black line), the first group is [DDC]⁺ ([C₄₈H_(0–19)]⁺, m/z = 5 76–595), which is produced from the photo-fragmentation processes of original DC⁺ (Lifshitz 2000; Castellanos et al. 2018; Zhen 2019). The second group is [(C₁₅H₉N)DDC]⁺, which formed through the collision reaction between DDC cations with 9-cyanoanthracene molecules. Similarly, the third and fourth groups are [(C₁₅H₉N)₂DDC]⁺ and [(C₁₅H₉N)₃DDC]⁺. With higher laser energy (the red line), the first group is [C_n]⁺, n = [40, 48] and [DDC]⁺ ([C₄₈H_(0–19)]⁺), which were produced from the fragmentation process of the original DC⁺ (Lifshitz 2000; Castellanos et al. 2018). The second group is [(C₁₅H₉N)C_n]⁺, n = [36, 48], which formed through the collision reaction between [C_n]⁺ and 9-cyanoanthracene molecules. Similarly, the third and fourth groups are [(C₁₅H₉N)₂C_n]⁺ and [(C₁₅H₉N)₃C_n]⁺.

Based on this, in the time range of 4.8–5.8 s, the photo-fragmentation pathways upon irradiation are shown below, $$\begin{array}{rl}& {\left[{\mathrm{C}}_{48}{\mathrm{H}}_{20}\right]}^{+}\stackrel{\mathrm{n}\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{48}{\mathrm{H}}_{(0-19)}\right]}^{+}+2\mathrm{H}/{\mathrm{H}}_2\\ & {\left[{\mathrm{C}}_{48}\right]}^{+}\stackrel{\mathrm{n}\mathrm{h}\nu }{\to }{\left[{\mathrm{C}}_{\mathrm{n}}\right]}^{+}+{\mathrm{C}}_2,\mathrm{n}=[36,46].\end{array}$$

In the time range of 5.8–9.88 s, the new cluster cations are formed through a collision formation reaction between [DDC]⁺/[C_n]⁺ and 9-cyanoanthracene molecules step by step. The formation pathways are summarized as follows: $$\begin{array}{rl}{\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}+& {[{\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)}_{\mathrm{m}-1}(\mathrm{D}\mathrm{D}\mathrm{C})]}^{+}\\ & ⟶{[{\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)}_{\mathrm{m}}(\mathrm{D}\mathrm{D}\mathrm{C})]}^{+},\mathrm{m}=[1,3]\\ {\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}+& {\left[{\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)}_{\mathrm{m}-1}\left({\mathrm{C}}_{\mathrm{n}}\right)\right]}^{+}\\ & ⟶{\left[{\left({\mathrm{C}}_{15}{\mathrm{H}}_9\mathrm{N}\right)}_{\mathrm{m}}\left({\mathrm{C}}_{\mathrm{n}}\right)\right]}^{+},\mathrm{m}=[1,3].\end{array}$$

Generally, DC/9-cyanoanthracene cluster cations are effectively formed without or with laser irradiation in the gas phase, which suggests that the chemical reactivity between these [PAH]⁺ and cyano-PAH species is high. Under laser irradiation, the types of reaction products are more diverse. Two types of large cyano-containing PAH cluster cations are observed, [PAH–PAH]⁺ (e.g., [(C₁₅H₉N)₂C₄₈H_(1–19)]⁺) and [carbon clusters–PAH]⁺ (e.g., [(C₁₅H₉N)₂C_n]⁺ n = [36, 48]). Under laser irradiation, the newly formed cluster cations involve a complex photo-fragmentation process; both dehydrogenation and CN/C₂ unit lost channels are identified. Furthermore, we observe pure carbon cluster cations (e.g., [C₆₀]⁺ and [C₆₂]⁺) and nitrogen-containing carbon cluster cations (e.g., [C₆₁N]⁺ and [C₆₃N]⁺), which suggests that the nitrogen atoms or cyano unit may lose or remain in the formed large PAH molecules in the photolysis process.

Fig. 2 Formation pattern of DC/9-cyanoanthracene (C15H9N) cluster cations without laser irradiation. The inset zoomed-in mass spectra reveal more details of the newly formed clusters ([(C15H9N)nDDC]+, n = [1, 4]).

Fig. 3 Panel A: formation pattern of DC/9-cyanoanthracene cluster cations without and with laser irradiation at 1.7 and 2.8 mJ (irradiation times of 0.3 s, from 9.5–9.8 s). Panel B: zoomed-in mass spectrum in the range of m/z = 716–812, revealing the dehydrogenated species and the presence of [C60]+, [C62]+, [C61N]+ and [C63N]+.

Fig. 4 Mass spectrum of DC/9-cyanoanthracene (C15H9N) cluster cations with laser irradiation upon 355 nm irradiation at 3.0 mJ (black line) and 6.0 mJ (red line) (irradiation times of 1.0 s, from 4.8–5.8 s), followed by a collision reaction in the period of 5.8–9.8 s.

3 Theoretical calculation results

To understand the formation mechanisms of cyano-containing PAH clusters and their photo-fragmentation behaviors, we studied the DC/9-cyanoanthracene cluster cations system theoretically. The theoretical calculations were performed with hybrid density functional B3LYP (Becke 1992; Lee et al. 1988) as implemented in the Gaussian 16 program (Frisch et al. 2016). The basis set of the 6-311++G(d, p) was selected and used for these systems. The zeropoint energy and thermal corrections were obtained from the frequency calculation to correct the molecular energy. To account for the intermolecular interactions, the dispersion-correction (D3, Grimme et al. 2011) was considered for the entire system. The geometries of all species were optimized at the local minimum of their potential energy surface.

In addition, we note that due to the higher chemical reactivity of C₁₅H₉N when reacting with PAH (e.g., DDC or carbon clusters) cations, we assumed that there is almost no reaction barrier between PAH cations and C₁₅H₉N. Based on this, we regarded the ion-molecular collision reaction rate between PAH and C₁₅H₉N as higher, which agrees with the obtained experimental results that the cyano-containing PAH clusters formation is efficient.

3.1 Optimized geometric structure of [C₄₈H₂₀]⁺, [C₄₈H₁₉]⁺, [C₄₈H₁₈]⁺, and C₁₅H₉N

The optimized geometric structures of the molecules [C₄₈H₂₀]⁺, [C₄₈H₁₉]⁺, [C₄₈H₁₈]⁺, and C₁₅H₉N are presented in Fig. 1. According to previous experimental and theoretical investigations, adduction reactions tend to occur on the outer carbon sites of the molecules because of their structural symmetry (Elias et al. 2009; Cazaux et al. 2016; Schlathölter et al. 2020). As a result, we only label the available outer carbon sites of PAH molecules in Fig. 1. In addition, we also give the C-C-C angles of PAH cations. These angles differ somewhat from each other, and the smaller C-C-C angle will facilitate the intermolecular hydrogen transfer (Yang et al. 2022).

For [C₄₈H₁₉]⁺ and [C₄₈H₁₈]⁺, we assumed that there is only dehydrogenation without rearrangement of the carbon skeleton during the electron impact ionization and photo-fragmentation process (Zhen et al. 2014; Zhen 2019). [C₄₈H₁₉]⁺ has five-isomer structures as its identical similarity structures, that is, five different carbon sites can be used for the reaction: the first is duo-type, [C₄₈H₁₉]⁺ (a), [C₄₈H₁₉]⁺ (b), [C₄₈H₁₉]⁺ (c), and [C₄₈H₁₉]⁺ (d), C is located in its duo sites; the second is solo type, [C₄₈H₁₉]⁺ (e), C is located in its solo sites. We selected duo-type [C₄₈H₁₉]⁺ (a) and solo-type [C₄₈H₁₉]⁺ (e) presented in Fig. 1. [C₄₈H₁₉]⁺ has an even number of electrons, resulting in a close-shell singlet ground state (the spin multiplicity is 1). [C₄₈H₁₈]⁺ has many isomers. When a PAH loses a first H, the second H is lost from its adjacent side because this allows the formation of a triple bond (Castellanos et al. 2018). Based on this, we selected [C₄₈H₁₈]⁺ (a, b) in this work and show it in Fig. 1. [C₄₈H₁₈]⁺ has an odd number of electrons, resulting in an open-shell doublet ground state (the spin multiplicity is 2).

As presented in Fig. 6, C₁₅H₉N has 11 different outer carbon sites that are available in the adduct reactions, and these carbon sites are labeled 1–11. Carbon site (11) belongs to the CN unit of C₁₅H₉N, while carbon sites 1–4, 5–8, and 10 belong to the >CH unit, and carbon site (9) is the carbon site that connected with the CN unit. The nitrogen site is labeled 12. The 9-cyanoanthracene has an even number of electrons, resulting in a close-shell singlet ground electronic state (the spin multiplicity is 1).

Fig. 5 Optimized structures and formation reactions of [C15H9N....C48H20]+.

Fig. 6 Reaction energies for the formation pathways between [C48H19]+ and 9-cyanoanthracene. The red atoms represent the reaction site..

3.2 Optimized geometric structure and formation pathway of [(C₁₅H₉N)C₄₈H₂₀]⁺

Figure 5 presents the optimized geometric structures and possible formation pathways of [(C₁₅H₉N)C₄₈H₂₀]⁺. [C₁₅H₉N. . ..C₄₈H₂₀]⁺ is a van der Waals cluster, which means that the C₁₅H₉N unit is connected to the [C₄₈H₂₀]⁺ unit by van der Waals forces. To account for the weak interactions (i.e., van der Waals forces) between PAH molecules, we included dispersion correction (D3).

The reaction is exothermic, with 1.10 eV for [C₁₅H₉N. . ..C₄₈H₂₀]⁺. The van der Waals forces can easily link PAHs of this size and shape to form larger clusters. The relatively lower exothermic energies suggest, however, that the van der Waals complexes [C₁₅H₉N. . ..C₄₈H₂₀]⁺ maybe not have readily formed in our laboratory condition. This agrees with the experimental results that the [(C₁₅H₉N)C₄₈H₂₀]⁺ m/z = 799 mass peak is not observed in Fig. 3.

3.3 The optimized geometric structure and formation pathway of [(C₁₅H₉N)C₄₈H₁₉]⁺

[C₄₈H₁₉]⁺ has two types of carbon sites: The solo carbon site (e), and the duo carbon sites (a, b, c, and d). C₁₅H₉N has an active functional unit (-CN unit) and four main types of reaction sites: The solo carbon sites (9 and 10), and the quartet carbon sites (1, 2,3, and 4) on the benzene ring, the carbon site (11) and the nitrogen site (12) in the cyano unit. Based on this, as shown in Fig. 6, with five isomer structure of [C₄₈H₁₉]⁺ and eight typical carbon sites of C₁₅H₉N, 40 possible reaction pathways can be obtained. The exothermic energy for these 40 formation pathways of [(C₁₅H₉N)C₄₈H₁₉]⁺ were obtained, ranges from −1.66 to −3.10 eV. The reaction energy is relatively higher and can stabilize all the molecules. That is, the reaction between [C₄₈H₁₉]⁺ and 9-cyanoanthracene occurs readily.

Figure 7 shows the formation reactions and the optimized structures of some typical isomers of [(C₁₅H₉N)C₄₈H₁₉]⁺. Panel A shows the duo type of [C₄₈H₁₉]⁺ (a) with five positions of C₁₅H₉N: 1, 9, 10, 11, and 12, and panel B shows the solo type of [C₄₈H₁₉]⁺ (e) with five positions of C₁₅H₉N.

As shown in Fig. 7, [(C₁₅H₉N)C₄₈H₁₉]⁺ includes one 9-cyanoanthracene unit and one [C₄₈H₁₉]⁺ unit that connected by one C-C bond ([(C₁₅H₉N)C₄₈H₁₉]⁺ (P_1/9/10/11-a) and [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_1/9/10/11-e)), or by one C-N single bond ([(C₁₅H₉N)C₄₈H₁₉]⁺ (P_12-a) and [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_12-e)). For the C-C single-bonded [(C₁₅H₉N)C₄₈H₁₉]⁺ cluster isomers, C from [C₄₈H₁₉]⁺ is in sp² hybridization, and the other C from 9-cyanoanthracene is in sp³ hybridization with an additional C-H bond. For the C-N single bonded [(C₁₅H₉N)C₄₈H₁₉]⁺ cluster isomer, C from [C₄₈H₁₉]⁺ is in sp² hybridization, and the other N from 9-cyanoanthracene is in sp³ hybridization.

Based on this, we conclude that the formation pathway of the C-N bond isomer has a higher reaction energy than other isomers. The main reason is that the nitrogen atoms in the cyanide unit are more reactive than other carbon atoms and the C-N bonds have a higher bond energy, which stabilizes the formed clusters better. We assumed that the [(C₁₅H₉N)C₄₈H₁₉]⁺ cluster cations produced under laboratory conditions are a mixture that contains all these different isomers. Moreover, isomers of the C-N bonding type may have a more dominant distribution than those of the C-C bonding type.

Fig. 7 Panel A: optimized geometric structures and formation pathways of [(C15H9N)C48H19]+ (P1/9/10/11/12-a). Panel B: same as panel A for [(C15H9N)C48H19]+ (P1/9/10/11/12-e).

3.4 The optimized geometric structure and formation pathway of [(C₁₅H₉N)C₄₈H₁₈]⁺

Figure 8 presents the possible formation pathways between [C₄₈H₁₈]⁺ (a, b) and C₁₅H₉N and the optimized structures of [(C₁₅H₉N)C₄₈H₁₈]⁺. Three possible formation reactions are obtained, [(C₁₅H₉N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}) in panel A, and [(C₁₅H₉N)C₄₈H₁₈]⁺ (P_{1, 2-a, b}) and (P_{9, 10-a, b}) in panel B. To [(C₁₅H₉N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}), a new carbon ring formed and was composed of the C-N unit of C₁₅H₉N and of the C-C unit (carbon site a and b) of [C₄₈H₁₈]⁺ (a, b). To [(C₁₅H₉N)C₄₈H₁₈]⁺ (P_{9, 10-a, b}), a new carbon-bridge ring was formed between the (9, 10) C-C unit of C₁₅H₉N and the C-C unit (carbon site a and b) of [C₄₈H₁₈]⁺ (a, b). To [(C₁₅H₉N)C₄₈H₁₈]⁺ (P_{1, 2-a, b}), a new four carbon ring was formed between the (1, 2) C-C unit of C₁₅H₉N and the C-C unit (carbon site a and b) of [C₄₈H₁₈]⁺ (a, b).

As shown in Fig. 8, all the reactions are exothermic, with exothermic energies of −1.57, −3.08, and −2.42 eV. Interestingly, in some of the obtained cluster isomers, the nitrogen atoms already started to be embedded inside the carbon skeleton. This can be treated as the first formation step of large nitrogen-containing clusters/PAHs.

Fig. 8 Optimized geometric structures and formation pathways of [(C15H9N)C48H18]+.

Fig. 9 Panel A: photo-dehydrogenation pathway of [(C15H9N)C48H19]+ (P1-a/e) and [(C15H9N)C48H19]+ (P9-a). Panel B: photodissociation (cyano unit loss) pathway of [(C15H9N)C48H19]+ (P9-a/e) and [(C15H9N)C48H19]+ (P1-a).

3.5 The photo-fragmentation pathway of [(C₁₅H₉N)C₄₈H₁₉]⁺

In Fig. 9 we show the calculation for the photo-fragmentation pathway of [(C₁₅H₉N)C₄₈H₁₉]⁺. We obtained two types of photo-fragmentation pathways.

We show in Fig. 9A that we obtained the dehydrogenation channels. The dissociation energy is +1.13 and +0.90 eV for H loss from the aliphatic carbon for [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_1-a) and [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_1-e). In contrast, H loss from the aromatic carbon sites takes around +4.31 eV ([(C₁₅H₉N)C₄₈H₁₉]⁺ (P_9-a) to [(C₁₅H₈N)C₄₈H₁₉]⁺ (P_9-a) + H). This suggests that H loss from the aliphatic carbon sites is the dominant dissociation channel. As shown in Fig. 9B, we obtained the CN unit loss channels. The dissociation energy is +2.30 eV and +1.87 eV for CN loss from the aliphatic carbon sites for [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_9-a) and [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_9-e). In contrast, CN unit loss from the aromatic carbon sites takes around +5.63 eV. This suggests that CN loss from the aliphatic carbon sites is the dominant dissociation channel.

3.6 The formation pathway of a nitrogen-containing carbon-ring in large cyano-containing PAH clusters

The study by Zhang et al. (2019) revealed that a cationic fluorene (C₁₃H₁₀) dimer cluster would lead to bowled and curved structure PAHs (C₂₆H₁₂) through a (photo-dehydrogenation) merging process. Based on this, we calculated the continued merging pathway of DC/9-cyanoanthracene cluster cations and show it in Fig. 10.

In Fig. 10A, [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_8-a) have two parallel (equally) possible merging pathways to form [C₆₃H₂₆N]⁺ (P₁) and [C₆₃H₂₆N]⁺ (P₂) through lost 2H or H₂: to [C₆₃H₂₆N]⁺ (P₁), a new seven carbon ring is formed. To [C₆₃H₂₆N]⁺ (P₂), a new eight carbon ring is formed; [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_8-e) have one possible photodissociation pathways to form [C₆₃H₂₆N]⁺ (P₃) through lost 2 H or H₂, and a new nine-carbon ring is formed. In Fig. 10B, first, [(C₁₅H₉N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}) forms [(C₁₅H₈N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}) by losing an H atom. Then [(C₁₅H₈N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}) have two parallel (equally) possible photodissociation pathways to form [C₆₃H₂₂N]⁺: [(C₁₅H₈N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}) → [C₆₃H₂₄N]⁺ (hexagon) → [C₆₃H₂₂N]⁺ and [(C₁₅H₈N)C₄₈H₁₈]⁺ (P_{11, 12-a, b}) → [C₆₃H₂₄N]⁺ (heptagon) → [C₆₃H₂₂N]⁺.

Figure 10 illustrates that the C≡N triple bond in the cyano unit may break during the reaction to form a cyclic structure, ultimately encapsulating the N-atom within the carbon skeleton. Based on this, even after complete dehydrogenation and the initiation of C₂ unit loss, the N-atom remains within the molecule. As shown in Fig. 10, the red carbon stands for the newly formed C rings, and the location of N atoms influences the molecular structure of the newly formed nitrogen-containing PAHs or PAHs species so that they are more homogeneous and diverse. These newly formed nitrogen-containing PAH molecules are all-benzenoid aromatic species with a size of (>60 C-atoms) in the astrophysical relevant range (Croiset et al. 2016), which might serve as a prototypical example for large(r) PAHs.

3.7 The IR spectra of cyano-containing PAH cluster cations

The newly formed cyano-containing PAH cluster cations obtained above may be candidates for the observed IR interstellar bands. They might be formed in the same interstellar area through a coevolutionary network. They might also contribute to the observed interstellar spectrum and motivate spectroscopic studies (Tielens 2013). The IR spectra for these newly formed molecules were theoretically calculated, and the calculations were performed with the 6-311++G(d, p) basis set. The calculated frequencies were scaled with a factor of 0.9670 for all the vibrational modes, and the full width at half-maximum for all spectra bands was set to be 16 cm⁻¹ (Boersma et al. 2014). The computed vibrational normal modes for some typical DC/9-cyanoanthracene clusters are presented in Fig. 11. Panels A1 and A2 show [C₄₈H₁₉]⁺ (a) and [C₄₈H₁₉]⁺ (e), panels B1–F1 show [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_{1/9/10/11/12-a}), panels B2–E2 show [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_{1/9/10/11/12-e}), and panels G1 and G2 show C₁₅H₉N.

As presented in Fig. 11, the IR spectrum is very complex in terms of the intensity and position of the vibrational peaks. In general, the IR spectra of the newly formed cyano-containing PAH clusters are different from the IR spectra of PAH cations and C₁₅H₉N. Some vibrational modes were preserved, others were not, and some new vibrational modes formed. In addition, the IR spectra of each cyano-containing PAH cluster are different, which highly depends on their structure. In the 2100–2300 cm⁻¹ range, a characteristic peak is observed (~4.6 μm), which is attributed to the C≡N (carbon-nitrogen triple bond) stretch vibration. The peak position and intensity of the C≡N stretch vibration change with different binding sites, as illustrated in parts B1–D1, B2–D1, F1, and F2. In E1 and E2, for [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_11-a) and [(C₁₅H₉N)C₄₈H₁₉]⁺ (P_11-e), the C≡N stretch vibration mode is absent, mainly because the C≡N triple bonds in these two molecules are broken.

The spectra we present in Fig. 11 reveal the complexity of the structure of clusters and the complexity of their infrared spectra. Hence, the structures of the cyano-containing PAH cluster cations that formed initially are diverse, and their role in the spectra profile contribution and the spectral features for tentative detections of the cyano-containing PAH cluster cations in the light of the PAH cations and PAHs IR spectra in the ISM is clear.

Fig. 10 Panel A: three large nitrogen-containing PAHs formed from [(C15H9N)C48H19]+ (P8-a/e) (after [(C15H9N)C48H19]+ loss H). Panel B: one large nitrogen-containing PAH formed from [(C15H9N)C48H18]+ (P11, 12-a, b).

4 Astronomical implications

In the interstellar environment, PAH molecules are affected and constrained by different environmental factors over time, such as interstellar UV radiation or other coexisting species (Tielens 2013; Zhang et al. 2023). These environmental factors drive the evolution of PAH molecules during different interstellar evolution periods. This work provided further insight into the evolution processes of large cyano-containing PAH cluster cations.

Recently, Wenzel et al. (2024, 2025a) detected the largest known family of interstellar PAH molecules in TMC-1, including 1-, 2-, and 4-cyanopyrene. Using the Markov chain Monte Carlo (MCMC) method, they measured the column densities of these cyanopyrenes to be ${1.52}_{-0.16}^{+0.18},{0.84}_{-0.09}^{+0.09}$, and ${1.33}_{-0.09}^{+0.10}\times {10}^{12}{\mathrm{c}\mathrm{m}}^{-2}$. The abundance ratio of approximately 2:1:2 reflects the number of equivalent sites on the pyrene (C₁₆H₁₀) molecule, to which the cyano unit can be added. This indicates that the cyanopyrene isomers are formed under kinetic control; that is, the cyano unit is directly added to the pyrene molecule in a hydrogen-rich gas environment at a temperature of about 10 K. Additionally, McGuire et al. (2021) measured the column densities of 1- and 2-cyanonaphthalene to be ${7.35}_{-4.63}^{+3.33}$ and ${7.05}_{-4.50}^{+3.23}\times {10}^{11}{\mathrm{c}\mathrm{m}}^{-2}$, respectively. Cernicharo et al. (2024) reported that the column densities of 1- and 5-acenaphthylene were both ${9.5}_{-0.5}^{+0.5}\times {10}^{11}{\mathrm{c}\mathrm{m}}^{-2}$. These results reflect the number of equivalent sites available for adduction and further support the mechanism of direct CN unit adduction.

The PAHs and cyano-PAHs are influenced by other interstellar molecules through ion-molecule reactions or UV irradiation. Our experimental results show that the gas-phase collision reaction between the large PAH molecules (e.g., (dehydrogenated) DC cations or carbon clusters) and small PAHs (e.g., 9-cyanoanthracene) occurs readily and opens many reaction pathways and a diverse set of large cyano-containing PAH cluster cations. Two types of large cyano-containing PAH cluster cations are observed, [PAH–PAH]⁺ (e.g., [(C₁₅H₉N)₂C₄₈H_(1–19)]⁺) and [carbon clusters–PAH]⁺ (e.g., [(C₁₅H₉N)₂C_n]⁺, n = [36, 48]). The evolution of cyano-PAH cluster cations under laser irradiation was investigated, which mainly involves dehydrogenation and decyanation pathways. Furthermore, the results we obtained also indicate that large cyano-PAHs can undergo photolysis into smaller PAHs through a top-down pathway that forms a more diverse mixture of isomers, including large PAH cluster cations, cyano-PAH cluster cations, graphene cluster cations, and cyano/nitrogen-containing graphene cations. From the calculation, the reaction energies on the different carbon sites are similar. The CN unit plays an important role in the formation and photochemistry processes in which two molecular connection pathways are considered (C-C and C-N bonding type).

A large number of molecules were detected in the ISM (Tielens 2013). Advances in detection techniques have enabled the identification of molecules ranging from small ones with a single aromatic ring (e.g., cyanobenzene) to medium-sized PAHs with up to seven aromatic rings (e.g., cyanocoronene, McGuire et al. 2018, 2021; Cernicharo et al. 2024; Wenzel et al. 2024, 2025a,b). Since cyano-PAHs exist in the ISM, they are expected to produce corresponding emission features in various wavelength bands. To aid searches for large cyano-PAHs in the future, we calculated the spectra of several representative cyanoPAHs and found that their fingerprint features are concentrated at ~4.6 μm. This spectral region agrees well with the observational capabilities of advanced telescopes such as the James Webb Space Telescope (JWST), which is expected to provide support for the study of these more complex molecules.

To illustrate that these characteristic peaks can be identified, we plot again in Fig. 11 the obtained IR spectra of some typical DDC/9-cyanoanthracene cluster cations together with a representative observed spectrum. The observed spectrum is a typical AIB spectrum as seen by the JWST using the Orion Bar atomic PDR template spectrum, as shown in Fig. 12 (top black line). The observed spectrum was adapted from Chown et al. (2024) and Peeters et al. (2024). The spectrum of the cyano/nitrogen-containing PAH cluster cations is somewhat similar to the AIB spectrum. Some spectral patterns agree less well, however. It is difficult to identify the characteristic peaks related to these different types of nitrogen functional units. Near ~4.6 μm, cyano-PAHs exhibit a characteristic C≡N triple bond stretch vibration, which serves as an IR fingerprint feature for the cyano-containing PAHs.

Fig. 11 Computed vibrational normal modes: [C48H19]+ (a), [C48H19]+ (e), C15H9N, [(C15H9N)C48H19]+ (P1/9/10/11/12-a), and [(C15H9N)C48H19]+ (P1/9/10/11/12-e).

Fig. 12 Comparison between the calculated spectrum ([C48H19]+ (a), [(C15H9N)C48H19]+ (P10-a), and [(C15H9N)C48H19]+ (P12-a)) and the observed spectrum. The observed spectrum is a typical AIB spectrum, as observed by JWST using the Orion Bar atomic PDR template spectrum adapted from Chown et al. (2024); Peeters et al. (2024).

5 Conclusions

We investigated the laboratory formation and photochemistry of large cyano-containing PAH cluster cations in the gas phase. With support from theoretical calculations, we determined that cyano-containing PAH cluster species can be formed efficiently. Subsequent photo-processing either converts these cyano-containing PAH cluster species into large nitrogen-containing PAHs (with N atoms incorporated into the PAH structure) or into large PAH cations through the loss of the cyano unit. The cyano unit plays an important role in the formation and photochemistry processes. Additionally, theoretical calculations further revealed the molecular structures of the newly formed clusters, the bonding energies of their reaction pathways, and their infrared spectral features. Furthermore, our research results revealed the complexity of interstellar molecules and the diverse evolution of cyano-containing PAHs in environments that coexist with other PAH species. Our results also offer a plausible clue for the chemical evolution of large cyano/nitrogen-containing molecules or small cyano-bearing dust particles in space.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 12333005 and 12122302), and the Scientific Research Foundation of Education Bureau of Hunan Province (No. 23A0132).

Appendix A Experimental methods

A brief description of the experimental methods is provided. First of all, the dicoronylene (DC, C₄₈H₂₀, Kentax, with purity better than 99.5%, ~580 K) was converted into the gas phase by heating in the oven located in the ion source chamber. Subsequently, these gas DC molecules were ionized by the electron gun (~82 eV) and transported into the ion trap via the ion gate and quadrupole mass filter. Another oven located below the ion trap was used to vaporize 9-cyanoanthracene, (C₁₅H₉N, J&K Scientific, with purity better than 99%, ~328 K) in the ion trap chamber. Then, these molecules would diffuse continuously toward the center of the ion trap. Helium gas was introduced continuously into the trap to thermalize the ion cloud through collisions with a leak valve. Adduct formation presumably occurs under our experimental operating conditions, possibly assisted by collisional stabilization of the chemically bonded intermediate complex by helium atoms.

The third harmonic of an Nd: YAG laser (INDI, SpectraPhysics, 355 nm, the pulse duration of ~6 ns, operated at 10 Hz) was applied to irradiate the ions trapped in the ion trap. The interaction time of the laser and the trapped ions was determined by a beam shutter (Uniblitz, XRS-4), which was externally triggered to ensure that the ion cloud could be irradiated for a specified time. The full-timing sequence was controlled by a high-precision delay generator (SRS DG535).

Our setup operated at a typical frequency of 0.1 Hz; the whole measuring period lasted 10.0 s. We used three-time strategies in this work:

(I). The ion gate was opened at 0.0–9.8 s in order to allow the ion trap to be filled with a certain amount of ions. Meanwhile, ion-molecule collision reactions between these ions and PAH molecules occurred. Then, the resulting mass fragments were measured at 9.88 s;

(II). The ion gate opened (0.0–9.3 s), allowing the ion trap to fill with a certain amount of ions. In this period, the trapped ions reacted with PAH molecules to form new cluster cations. Then, after a short time delay (~0.2 s), the beam shutter opened, and the ion cloud was irradiated (9.5–9.8 s, irradiation times amounting to 0.3 s; that is, typically ~3 pulses, from 9.5–9.8 s). Then, the resulting mass fragments were measured at 9.88 s;

(III). Similarly, the ion trap was filled with some ions at 0.0–4.8 s. The beam shutter was opened at 4.8–5.8 s (typically ~10 pulses), so the trapped ions were exposed to laser irradiation. During the time delay period (5.8–9.8 s, i.e., collision reaction times amounting to 4.0 s), the newly photo-formed ions would collide and react with the PAH molecules, forming a series of large cluster cations. Finally, a negative square pulse (9.88 s) was applied to the end cap of the ion trap. Thus, the cluster cations could be accreted into the time-of-flight (TOF) region, where the resulting mass fragments were measured.

References

All Figures

	Fig. 1 Optimized structures of [C48H20]+, 9-cyanoanthracene (C15H9N), [C48H19]+ (a), [C48H19]+ (e), and [C48H18]+ (a, b). Atoms are represented as spheres with the following color-coding: Nitrogen is shown in blue, hydrogen is shown in white, and carbon is shown in gray.
In the text

	Fig. 2 Formation pattern of DC/9-cyanoanthracene (C15H9N) cluster cations without laser irradiation. The inset zoomed-in mass spectra reveal more details of the newly formed clusters ([(C15H9N)nDDC]+, n = [1, 4]).
In the text

	Fig. 3 Panel A: formation pattern of DC/9-cyanoanthracene cluster cations without and with laser irradiation at 1.7 and 2.8 mJ (irradiation times of 0.3 s, from 9.5–9.8 s). Panel B: zoomed-in mass spectrum in the range of m/z = 716–812, revealing the dehydrogenated species and the presence of [C60]+, [C62]+, [C61N]+ and [C63N]+.
In the text

	Fig. 4 Mass spectrum of DC/9-cyanoanthracene (C15H9N) cluster cations with laser irradiation upon 355 nm irradiation at 3.0 mJ (black line) and 6.0 mJ (red line) (irradiation times of 1.0 s, from 4.8–5.8 s), followed by a collision reaction in the period of 5.8–9.8 s.
In the text

	Fig. 5 Optimized structures and formation reactions of [C15H9N....C48H20]+.
In the text

	Fig. 6 Reaction energies for the formation pathways between [C48H19]+ and 9-cyanoanthracene. The red atoms represent the reaction site..
In the text

	Fig. 7 Panel A: optimized geometric structures and formation pathways of [(C15H9N)C48H19]+ (P1/9/10/11/12-a). Panel B: same as panel A for [(C15H9N)C48H19]+ (P1/9/10/11/12-e).
In the text

	Fig. 8 Optimized geometric structures and formation pathways of [(C15H9N)C48H18]+.
In the text

	Fig. 9 Panel A: photo-dehydrogenation pathway of [(C15H9N)C48H19]+ (P1-a/e) and [(C15H9N)C48H19]+ (P9-a). Panel B: photodissociation (cyano unit loss) pathway of [(C15H9N)C48H19]+ (P9-a/e) and [(C15H9N)C48H19]+ (P1-a).
In the text

	Fig. 10 Panel A: three large nitrogen-containing PAHs formed from [(C15H9N)C48H19]+ (P8-a/e) (after [(C15H9N)C48H19]+ loss H). Panel B: one large nitrogen-containing PAH formed from [(C15H9N)C48H18]+ (P11, 12-a, b).
In the text

	Fig. 11 Computed vibrational normal modes: [C48H19]+ (a), [C48H19]+ (e), C15H9N, [(C15H9N)C48H19]+ (P1/9/10/11/12-a), and [(C15H9N)C48H19]+ (P1/9/10/11/12-e).
In the text

	Fig. 12 Comparison between the calculated spectrum ([C48H19]+ (a), [(C15H9N)C48H19]+ (P10-a), and [(C15H9N)C48H19]+ (P12-a)) and the observed spectrum. The observed spectrum is a typical AIB spectrum, as observed by JWST using the Orion Bar atomic PDR template spectrum adapted from Chown et al. (2024); Peeters et al. (2024).
In the text