Gas-phase preparation of azulene (C₁₀H₈) and naphthalene (C₁₀H₈) via the reaction of the resonantly stabilized fulvenallenyl (C₇H₅˙) and propargyl (C₃H₃˙) radicals

Abstract

Synthetic routes to the 10π Hückel aromatic azulene (C₁₀H₈) molecule, the simplest polycyclic aromatic hydrocarbon carrying an adjacent five- and seven-membered ring, have been of fundamental importance due to the role of azulene – a structural isomer of naphthalene – as an essential molecular building block of saddle-shaped carbonaceous nanostructures such as curved nanographenes and nanoribbons. Here, we report on the very first gas phase preparation of azulene by probing the gas-phase reaction between two resonantly stabilized radicals, fulvenallenyl and propargyl , in a molecular beam through isomer-resolved vacuum ultraviolet photoionization mass spectrometry. Augmented by electronic structure calculations, the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism affords a versatile concept for introducing the azulene moiety into polycyclic aromatic systems thus facilitating an understanding of barrierless molecular mass growth processes of saddle-shaped aromatics and eventually carbonaceous nanoparticles (soot, interstellar grains) in our universe.

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Introduction

Since the very first synthesis of the non-alternant 10π-Hückel aromatic azulene molecule (C₁₀H₈) by Pfau and Plattner in 1939 from octahydronaphthalene (Scheme 1),¹ five-, six-, and seven-membered organic ring moieties have emerged as fundamental molecular building blocks in 2-dimensional (graphenes, nanoflakes),^2–5 3-dimensional (nano bowls, nanotubes, fullerenes),^3,6–11 and saddle-shaped¹² polycyclic aromatic hydrocarbons (PAHs) (Scheme 2). These carbon-based nanostructures are of fundamental significance in the fields of material science, physical (organic) chemistry and astrophysics. This is due to their exceptional photophysical properties from the narrow highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) energy gaps¹³ and resonance delocalization,¹⁴ exploitation as organic field-effect transistors (OFETs),^15,16 organic conductors,¹⁷ and non-linear optical materials,¹⁸ as well as potential carriers of selected diffuse interstellar bands (DIBs)^19,20 – discrete absorption features overlaid on the interstellar extinction curve covering the blue visible (400 nm) to the near-infrared (1.2 mm) region. Aromatic structures also serve as precursors to carbonaceous nanoparticles in combustion systems (‘soot particles’)²¹ and in circumstellar envelopes of carbon-rich Asymptotic Giant Branch (AGB) stars^22–24 along with planetary nebulae as their descendants (‘circumstellar grains’).^22,25–27 Whereas a mechanistical framework of the formation of PAHs carrying five- and six-membered ring moieties is beginning to emerge,²⁸ the gas-phase synthetic routes to polycyclic aromatic hydrocarbons incorporating seven membered rings, in particular those carrying azulene building blocks in which a five membered ring is annulated to a seven membered ring, are still elusive.^15,29

Scheme 1 Molecular structures of the azulene (1a and 1b) and naphthalene (2) isomers (C₁₀H₈); atoms are color coded in gray (carbon) and white (hydrogen).

Scheme 2 Representative polycyclic aromatic hydrocarbons (PAHs) carrying five- (left), six (center) and seven-membered rings (right) as in the C₄₀ nanobowl (C₄₀H₁₀, 3), circumcoronene (C₅₄H₁₈, 4) and [7]-circulene (C₂₈H₁₄, 5). The incorporation of five-membered rings results in a positive curvature, whereas seven-membered ring inflict a saddle-shaped structures as the result of a positive and negative curvature.

Classical preparative organic synthesis to azulene derivatives exploit, e.g., active methylene reagents via Nozoe's method,^30,31 Scholl-type oxidative cyclization³² and Aldol condensations coupled with Vilsmeier–Hack reactions and Suzuki coupling³³ effectively incorporating azulenes into helical nanographenes with twisting angles exceeding 16° (ref. 32 and 34) and non-planar acenes moieties isoelectronic to pentacene (C₂₂H₁₄), hexacene (C₂₆H₁₆) and heptacene (C₃₀H₁₈).³² Seven-membered rings were also incorporated into polycyclic molecules via palladium-catalyzed intramolecular arylation efficiently yielding C₃-symmetric propeller shaped PAHs bearing three seven-membered rings and three [4]helicene units.³⁵ Würthner et al. established an elegant synthetic protocol for preparing saddle-shaped corannulene derivatives bearing both a positive and negative curvature at the corannulene and annulated seven-membered ring moieties.³⁶ Nevertheless, regardless of these astonishing advances on the preparative synthesis and characterization of azulene derivatives, gas-phase molecular mass growth processes to azulene – the simplest representative of a bicyclic, 10π aromatic molecule formally accessed through fusion of cyclopentadiene and cycloheptatriene rings – along with its derivatives in high-temperature combustion systems and extraterrestrial settings have not been reported yet, but are fundamental to unravel the gas phase preparation of carbonaceous nanostructures such as curved nanographenes^37,38 and nanoribbons.³⁹

Here, we provide persuasive testimony on the first gas phase preparation of the azulene molecule (C₁₀H₈) along with its naphthalene isomer (C₁₀H₈) initiated through the reaction of two resonantly stabilized free radicals – fulvenallenyl and propargyl – via de-factor ring annulation within a chemical microreactor^40–42via the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism. The reaction products were sampled in a molecular beam via electron ionization^43–45 and isomer-specifically via fragment-free photoionization of the neutral products exploiting tunable vacuum ultraviolet (VUV) light followed by detection of the ionized molecules in a reflection time-of-flight mass spectrometer^41,46 (Re-TOF-MS; Fig. S1 and S2, ESI†). Augmented by electronic structure calculations, the unexpected gas phase preparation of azulene from the bottom-up serves as an unconventional entry point to curved aromatics in combustion systems^21,47 and in carbon-rich, circumstellar environments.^48–50 The fulvenallenyl radical can be generated from fulvenallene (C₇H₆)^51,52 through hydrogen abstraction at elevated temperatures above 1000 K as in combustion flames and in circumstellar envelopes close to the central star or via photolysis of the C–H bond by the internal ultraviolet field existing even deep inside molecular clouds such as the Taurus Molecular Cloud-1 (TMC-1), where fulvenallene (C₇H₆)⁵³ and the propargyl radical ⁵⁴ have been detected at high fractional abundances of (2.7 ± 0.3) × 10⁻¹⁰ and (8.7 ± 0.7) × 10⁻⁹, respectively. This proof-of-concept study exposes a benchmark of an unconventional, barrierless reaction involving two resonantly stabilized free radicals (RSFRs) leading to the very first gas-phase preparation of azulene along with its naphthalene isomer (Scheme 1). This route signifies a facile, illustrative mechanism to methodically build-up even more complex, previously elusive functionalized and/or side-chain substituted azulenes in the gas phase through reactions of (alkyl) substituted fulvenallenyl and/or propargyl radicals thus accessing the previously elusive gas phase chemistry of a key class of 10π-Hückel aromatics: azulenes.

Results & discussion

Electron ionization detection

Propargyl (; 39 amu) and fulvenallenyl (; 89 amu) radicals were prepared in situ in a resistively heated silicon carbide microreactor via pyrolysis of helium-seeded phthalide (C₈H₆O₂)^55,56 and propargyl bromide (C₃H₃Br),⁴⁰ (Fig. S1 and S3†) respectively. The temperature-dependent ion profiles of the parent ions of the radicals along with the C₁₀H₈ isomers are presented in Fig. 1; the neutral reactants and products were ionized by electron ionization at 13 ± 2 eV and sampled by an orthogonal extraction mass spectrometer (Fig. S1†).^57–59 Ion counts at m/z = 89 only emerge at temperatures exceeding 1400 ± 50 K. This signal arises predominantly from ionization of the fulvenallenyl radical as demonstrated in prior isomer-resolved experiments.⁵⁵ Ion signal at both m/z = 89 (C₇H₅⁺) and m/z = 128 (C₁₀H₈⁺) rises as the temperature increases from 1400 to 1700 K, suggesting that the fulvenallenyl radical (; 89 amu) represents a key reactant to form C₁₀H₈ (128 amu) isomers. The detection of C₁₀H₈ supports earlier modeling work that proposed the reaction of propargyl with fulvenallenyl as a viable source of naphthalene (C₁₀H₈) in combustion environments.⁶⁰ However, to support this claim, an isomer-selective analytical technique, i.e. soft photoionization, is required to identify the nature of the isomer(s) formed along with their branching ratios.

Fig. 1 Temperature-dependent ion count profiles. The red, black and blue lines exhibit respectively ion counts of propargyl (; 39 amu), fulvenallenyl (; 89 amu) radicals and C₁₀H₈ isomers (128 amu) in various temperatures in experiments. Error bars include uncertainties in the temperature and shot-to-shot reproducibility.

Photoionization mass spectra

A representative photoionization mass spectrum collected at a photon ionization energy of 9.00 eV is displayed in Fig. 2 for the reaction of the fulvenallenyl (; 89 amu) radical with the propargyl (; 39 amu) radical. These radicals were generated in situ in the microreactor at 998 ± 10 K via pyrolysis of helium-seeded trichloromethyl-benzene (C₇H₅Cl₃) – accounting for the higher yields of fulvenallenyl from trichloromethyl-benzene compared to phthalide pyrolysis⁵⁶ – and propargyl bromide (C₃H₃Br),⁴⁰ respectively (Fig. S2, S3† and 2c). Control experiments were also carried out by helium-seeding only the trichloromethyl-benzene precursor at 298 K (Fig. 2a). Only ion counts at m/z = 194 (C₇H₅³⁵Cl₃⁺), 195 (¹³CC₆H₅³⁵Cl₃⁺), 196 (C₇H₅³⁵Cl₂³⁷Cl⁺), 197 (¹³CC₆H₅³⁵Cl₂³⁷Cl⁺), 198 (C₇H₅³⁵Cl³⁷Cl₂⁺), 199 (¹³CC₆H₅³⁵Cl³⁷Cl₂⁺) and 200 (C₇H₅³⁷Cl₃⁺) were detected. When the temperature rises to 998 ± 10 K, the aforementioned ion counts of the trichloromethyl-benzene precursor disappear (Fig. 2b), indicating its quantitative decomposition. The stepwise elimination of chlorine (Cl) atoms from the trichloromethyl-benzene precursor with an increasing temperature of the microreactor is evident from the mass spectra (marked in green in Fig. 2b, c and S4–S7†); as products of the self-recombination of (89 amu), C₇H₄ (88 amu) and/or C₇H₃ (87 amu), prominent ion peaks emerge at m/z = 174 (C₁₄H₆⁺), 176 (C₁₄H₈⁺), and 178 (C₁₄H₁₀⁺) (Fig. S8†). Once propargyl bromide (C₃H₃Br) is introduced to the reactor, ion counts at m/z = 39, 78, 128 and 129 (Fig. 2c and d) emerge; these can be associated with (39 amu), C₆H₆ (78 amu), C₁₀H₈ (128 amu) and ¹³CC₉H₈ (129 amu), which are clearly absent in the control experiments (Fig. 2a and b). It shall be highlighted that a previous investigation of the propargyl radical self-reaction exploiting a chemical micro reactor did not result in any molecular mass growth processed beyond benzene and its acyclic C₆H₆ (78 amu) isomers.⁶¹ Consequently, accounting then for the molecular weight of reactants and the product, signal at m/z = 128 (C₁₀H₈⁺) originates from the reaction of the fulvenallenyl radical (, m/z = 89) with the propargyl radical (, m/z = 39) (Fig. 2d).

Fig. 2 Mass spectra recorded at the photoionization energy of 9.00 eV. (a) Trichloromethyl-benzene (C₇H₅Cl₃)/helium (He) system at 373 ± 10 K; (b) trichloromethyl-benzene (C₇H₅Cl₃)/helium (He) system at 998 ± 10 K; (c) trichloromethyl-benzene (C₇H₅Cl₃)/propargyl bromide (C₃H₃Br) system at 998 ± 10 K; and (d) magnified comparison of m/z = 128 between the reaction and control experiments. The signal at m/z = 128 is highlighted in red.

Photoionization efficiency (PIE) curves

With the detection of a hydrocarbon molecule of the molecular formula C₁₀H₈ (128 amu) formed via the fulvenallenyl – propargyl radical–radical reaction, we are focusing now our attention to the elucidation of the structural isomer(s) synthesized in this reaction. This requires a detailed analysis of the photoionization efficiency (PIE) curve, which reports the intensity of the ion at m/z of 128 (C₁₀H₈⁺) as a function of the photon energy from 7.20 to 9.00 eV.

First, the experimental PIE curve (black) of m/z = 89 (C₇H₅⁺) in trichloromethyl-benzene (C₇H₅Cl₃)/propargyl bromide (C₃H₃Br) system is shown in Fig. 3a, along with experimental PIE curve (orange) of fulvenallenyl (, m/z = 89) generated from phthalide pyrolysis (Fig. S9–S10†).^56,62,63 The onset of the ion counts at 8.20 ± 0.05 eV is consistent with the adiabatic ionization energy of fulvenallenyl of 8.19 ± 0.02 eV;^56,63 further, both PIE curves match exceptionally well revealing that fulvenallenyl is generated from both the trichloromethyl-benzene and phthalide precursors (Fig. 3a and S11†). Second, the experimentally collected PIE curve at m/z = 128 is displayed in Fig. 3b (black) and overlayed with the reference curves of the C₁₀H₈ isomers azulene (blue) and naphthalene (green), as well as the ¹³C-isotopic signal ¹³CC₉H₇ from m/z = 127. The experimental PIE curve can be accurately replicated by a linear combination of the PIE curves of azulene (C₁₀H₈), naphthalene (C₁₀H₈) and the natural ¹³C contribution of the radical(s) (¹³CC₉H₇) with the PIE curve of m/z = 127 displayed in Fig. S5.† Overall, the onset of ion counts of the experimental PIE curve at 7.40 ± 0.05 eV agrees exceptionally well with the adiabatic ionization energy (IE) of azulene recorded in the present work (IE = 7.41 ± 0.02 eV, Fig. S12b†),⁶⁴ while the contribution of naphthalene (IE = 8.14 ± 0.01 eV)^64,65 becomes evident at photon energies of 8.15 ± 0.05 eV and higher. Therefore, our experimental data provide compelling evidence on the formation of both azulene and naphthalene under our experimental conditions. Accounting for the absolute photoionization cross sections,⁶⁵ branching ratios of azulene versus naphthalene are derived to be 5.7 ± 0.7% versus 94.3 ± 11.3%. We would like to stress that the inclusion of monocyclic C₁₀H₈ isomers carrying, e.g., a benzene ring with a C₄H₃ side chain such as 4-phenylvinylacetylene (IE = 8.25 ± 0.05 eV) and trans-1-phenylvinylacetylene (IE = 8.20 ± 0.05 eV),⁶⁶ cannot replicate the experimentally derived PIE at m/z = 128; only upper limits of 4-phenylvinylacetylene and trans-1-phenylvinylacetylene of up to 2% of the ion counts can result in acceptable fits (Fig. S13†). These conclusions are also supported by the analysis of the PIE curve at m/z = 129 which results from ¹³C isotopes (10 × 1.109% natural abundance) of C₁₀H₈ isomers, azulene and naphthalene (Fig. 3c and S14†).

Fig. 3 Experimental and reference photoionization efficiency (PIE) curves for species at m/z = 89 (a), 128 (b) and 129 (c) in the trichloromethyl-benzene (C₇H₅Cl₃)/propargyl bromide (C₃H₃Br) system at 998 ± 10 K. The overall error bars consist of three parts: ±10% based on the accuracy of the photodiode, ±5% considering the injection stability and a 1σ error of the PIE curve averaged over the individual scans. The color-coded lines refer to the reference PIE curves of azulene (blue) and naphthalene (green) isomers. The orange line means experimental PIE curve of fulvenallenyl generated from the traditional phthalide pyrolysis revealing an excellent match with fulvenallenyl prepared from trichloromethyl-benzene. The red lines show the overall fit via the linear combination of the reference curves.

Potential energy surfaces

With the explicit experimental detection of azulene and naphthalene isomers (C₁₀H₈) formed via the gas phase reaction of the fulvenallenyl radical with the propargyl radical in the gas phase, it is our objective to unravel the underlying reaction mechanisms. In case of complex systems, it is of advantage to combine the experimental results with electronic structure calculations (Section 6†) to explore synthetic routes to naphthalene (p1) and azulene (p2) with relative energies of the products, intermediates and transition states calculated with an accuracy of ± 4 kJ mol⁻¹. Fig. 4 illustrates resonance structures of the fulvenallenyl and propargyl radicals depicting their singly occupied molecular orbitals (SOMO) and distribution of the electron spin density. Fulvenallenyl can be considered as an analog of propargyl with the five-membered ring moiety of fulvenallenyl substituted by the CH₂ group of propargyl. While the propargyl radical possesses only two resonance structures with the unpaired electron delocalized between the CH (tail) and CH₂ (head) ends, fulvenallenyl features six resonance structures, four of which are symmetrically unique with the radical distributed between the exocyclic CH end and each of the five carbon atoms in the five-membered ring. This is supported by the structure of the singly occupied molecular orbital (SOMO) and the computed spin distribution which shows the highest spin density in the ipso position in the ring followed by the terminal, exocyclic methylidyne moiety (CH) carbon and the carbon atoms in the meta positions in the ring. The ortho carbon atoms in the ring carry the lowest – by the absolute value – spin density. Therefore, there are four chemically distinct sites for the propargyl radical addition to fulvenallenyl, although the addition to the ortho positions is anticipated to be least likely. Altogether, considering the head–tail delocalization of spin density in propargyl, the fulvenallenyl – propargyl reaction can be initiated via eight distinct entrance channels.

Fig. 4 Schematic of electron distribution including computed partial charges on individual carbon atoms, singly occupied molecular orbitals (SOMO) and spin density distribution including computed spin densities on C atoms in fulvenallenyl (, top row) and propargyl (, bottom row) radicals.

Naphthalene formation

The calculations of the relevant C₁₀H₈ potential energy surface (PES) portray that these eight entrance channels can be subdivided into two groups, where the rearrangements of the initial complexes formed in each of the group appear to have relatively low barriers and thus are faster than the competing isomerization reactions (Fig. 5a and b). The first group (Fig. 5a) includes propargyl tail addition to the ipso, ortho and meta positions in the five-membered ring of fulvenallenyl together with propargyl head addition to the CH end (tail) of fulvenallenyl ; these additions lead to intermediates i1, i2, i13 and i14, respectively. Here, the ring additions are interconnected by facile migrations of the propargyl moiety between neighboring carbon atoms in the ring, whereas the initial complex of the head addition easily interconverts to the complex formed by the tail addition to the ipso position by the rebound occurring via a six-membered ring transition state (TS). Seemingly, the second group (Fig. 5b) combines propargyl head additions to the carbon atoms in the ipso, ortho and meta positions with the tail addition to the tail of fulvenallenyl accessing intermediates i15, i16, i17 and i18, respectively; from these, the propargyl can easily rebound from the initial complex for the ipso-ring addition to the initial complex for the tail-to-tail addition. Fig. 5 compiles only the most favorable pathways leading to naphthalene (p1), azulene (p2), fulvalene (p3) and 5-methylene-5H-indene (p4). The preferential pathways to naphthalene (Fig. 5a, pink) initiate from the initial complexes for tail addition to the ipso and ortho carbons in the ring, i.e., intermediates i1 and i2, residing 216 and 245 kJ mol⁻¹ lower in energy than the reactants, respectively. Although i2 is thermodynamically more stable than i1, it is likely that the formation of i2 is kinetically hindered because of the low spin density on the ortho carbon atom. Nevertheless, i1 rearranges to i2via a relatively low barrier of 90 kJ mol⁻¹. The pathway from i2 to naphthalene involves cyclization to a ten-membered ring with the initial five-membered ring being open (i2 → i3), contraction of the ten-membered ring to a bicyclic structure featuring fused six- and five-membered rings with an out-of-ring methylene (CH₂) group (i3 → i4), two consecutive 1,2-hydrogen shifts around the six-membered ring (i4 → i5 → i6), leading to 2-methyleneindene (i6), insertion of the CH₂ group in the five-membered ring leading to its expansion to a six-membered ring (i6 → i7 → i8) and finally, 1,2-hydrogen migration i8 → p1 to naphthalene. The highest energy transition state along the pathway from i2 to naphthalene resides 112 kJ mol⁻¹ lower in energy than the separated reactants and corresponds to the hydrogen migration step i4 → i5.

Fig. 5 Most favorable reaction pathways of the reaction of fulvenallenyl with propargyl (a) to naphthalene and azulene and (b) to fulvalene. See Fig. S15 in ESI† for structures of representative transition states.

Azulene formation

Alternatively, i2 can give rise to azulene (Fig. 5a, blue) via a multistep pathway featuring 1,2-hydrogen migration (i2 → i9), a six-membered ring closure (i9 → i10), another hydrogen shift (i10 → p4) resulting in a rather stable 5-methylene-5H-indene species p4. 5-Methylene-5H-indene can further be subjected to insertion of the out-of-ring methylene group to the six-membered ring expanding it to a seven-membered ring (p4 → i11 → i12), with a hydrogen shift completing the formation of azulene (i12 → p2), which is 534 kJ mol⁻¹ exoergic with respect to the reactants. The highest in energy transition state along this pathway corresponds to the hydrogen shift i10 → p4. Interestingly, in both routes from i2 to naphthalene and azulene, the critical ring closure steps leave an out-of-ring methylene group, which later inserts either into five- or six-membered ring. This mechanism is reminiscent of the so-called methylene walk mechanism in a hydrogen atom assisted isomerization of azulene to naphthalene occurring on the PES.^67,68 The computationally predicted difference in enthalpies of formation of naphthalene and azulene of 147 ± 4 kJ mol⁻¹ agrees closely with the NIST derived value of 158 ± 10 kJ mol⁻¹.

Fulvalene formation

The channels proceeding through the second group of entrance channels represented by the tail-to-tail addition forming i18 provide a facile route to fulvalene (IE = 8.70 eV,⁶⁹Fig. 5b, green). In particular, the side chain in i18 undergoes a five-membered ring closure to i19, where a 1,2-shift from the CH₂ moiety to the bare carbon atom in the newly formed ring produce fulvalene (p3). Here, the highest energy along the i18 → i19 → p3 pathway corresponds to the ring closure step and is located 139 kJ mol⁻¹ below the initial reactants.

Statistical calculations

It should be also noted that isomerization channels connecting the first and second groups of the initial complexes represented, e.g., by the tail addition of propargyl to the ring (i1, ipso) and to the tail of fulvenallenyl (i18) are only weakly interconnected. Although pathways between the two groups exist, they are not kinetically competitive. We carried out product branching ratio calculations for these two distinct entrance channel groups at various temperatures and pressures. Fig. S16† illustrates the results for the pressure of 0.01 atm representative for the reactive zone of the microreactor. One can see that for the propargyl tail addition to the ring or its head addition to the tail of fulvenallenyl at high temperatures relevant to the conditions in the microreactor, naphthalene (p1) is the prevailing product followed by 5-methylene-5H-indene (p4), fulvalene (p3) and azulene (p2). Alternatively, the second group of entrance channels (tail-to-tail and propargyl head to fulvenallenyl ring) is expected to predominantly form fulvalene. The experimental branching ratio of 5.7 ± 0.7% (azulene) versus 94.3 ± 11.3% (naphthalene) at 998 ± 10 K favors the gas-phase formation of the naphthalene isomer. Considering only the primary reaction products, the calculated branching ratio of azulene to naphthalene at 1000 K for the case of head-to-tail and tail-to-ring group of additions is 1.8% vs. 98.2%, whereas the tail-to-tail and head-to-ring additions should produce negligibly small amounts of both naphthalene and azulene. In the meantime, neither fulvalene nor 5-methylene-5H-indene (IE = 7.34 eV, Section 6†) could be detected experimentally. It is likely that these products are rapidly converted to naphthalene and/or azulene via a hydrogen assisted isomerization.^70–72 Besides, it was reported that fulvalene is preferred in the high-temperature condition over 1000 K.^67,73 In particular, considering the reaction kinetics on the PES explored in the previous theoretical studies,⁶⁷ once can anticipated that the hydrogen atom plus fulvalene reaction would predominantly produce naphthalene plus atomic hydrogen via the spiran mechanism, whereas atomic hydrogen plus 5-methylene-5H-indene would preferentially form azulene and a hydrogen atom by methylene insertion into the six-membered ring. Within this framework, the overall azulene to naphthalene branching ratio can be determined taking account both the primary reaction and secondary hydrogen atom assisted isomerization and considering the branching between the two groups of entrance channels as a variable parameter. Taking this entrance channel group branching as 50/50, we obtain the azulene/naphthalene yields as 10.9/89.1; the experimental branching ratio can be reproduced with the entrance channel branching of 31/69. Accurate calculations of the branching of various entrance channels and absolute temperature- and pressure-depending rate constants for the formation of various products is a demanding task which can be achieved using variable reaction coordinate transition state theory (VRC-TST) and thus is left for a future study.

Conclusion & outlook

The experimental and computational exploration of the reaction of two resonantly stabilized free radicals – fulvenallenyl and propargyl – provided persuasive evidence of the very first gas-phase synthesis of the 10π aromatic azulene molecule (C₁₀H₈) via the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism. Eight distinct entrance channels have been identified including head (CH₂) additions of propargyl to the tail (terminal CH) of fulvenallenyl and tail (CH) additions of propargyl to the ring of fulvenallenyl as well as tail-to-tail and head-to-ring additions. The additions from the first group preferentially form naphthalene along with 5-methylene-5H-indene, fulvalene and azulene, whereas the second group of additions predominantly forms fulvalene under high temperature conditions. Under the experimental conditions, 5-methylene-5H-indene and fulvalene can be rapidly converted to azulene and naphthalene, respectively, via hydrogen-assisted isomerization. Within this framework, the experimental azulene-to-naphthalene branching ratio can be reproduced assuming a nearly 2 : 1 branching between the entrance channel tail-to-tail plus head-to-ring and head-to-tail plus tail-to-ring groups. Note that a previous study of the fulvenallenyl – propargyl system exploiting infrared (IR)/ultraviolet (UV) ion dip spectroscopy only detected the naphthalene isomer, but not azulene.⁵⁶ The unsuccessful identification of azulene by Horsch et al.⁷⁴ is likely the direct consequence of the IR/UV detection method, which makes an identification of products at a level of less than about 10% very challenging.⁷⁵

The reaction mechanism elucidated here provides a versatile concept introducing an azulene moiety, i.e., a seven-membered ring adjacent to a five membered ring, into polycyclic aromatic systems via molecular mass growth processes involving the reaction of a fulvenallenyl moiety with propargyl radicals (Fig. 6). Commencing with the bimolecular reaction of a PAH-embedded benzene ring with a methylidyne radical (CH), a six-membered ring can be effectively converted into a fulvenallene moiety as long as the embedded benzene ring has three adjacent hydrogen atoms demonstrated in the crossed molecular beam reaction of benzene (C₆H₆) with methylidyne (CH).⁵¹ Hydrogen atom loss from the exocyclic CCH₂ functional group through photodissociation or abstraction leads to the PAH-embedded fulvenallenyl unit, which can then react with a propargyl radical to the azulene moiety embedded in an aromatic system. It is important to highlight that the last step requires a third body collisional stabilization – as provided in our microreactor by the helium buffer gas – to transfer the internal energy to the third body collider or a potential radiative stabilization under interstellar conditions. Therefore, hydrocarbon-rich atmospheres of planets and their moons such as Titan present ideal natural laboratories to allow barrierless and exoergic reactions to, e.g., azulene, which are terminated by stabilization through collision with a bath molecule such as molecular nitrogen in case of Titan. Even in combustion systems, extensive molecular mass growth processes could lead to PAHs carrying seven-membered rings as a critical prerequisite of exotic ‘saddle-shaped’ aromatic systems with both a positive and negative curvature of the carbon backbone. Overall, the isomer selective identification of aromatic molecules formed via the reaction of (resonantly stabilized) hydrocarbon radicals exploiting tunable synchrotron light has just scratched the surface and is expected to enhance our fundamental knowledge on molecular mass growth processes to aromatic structures on the fundamental, microscopic level in our Universe.

Fig. 6 Schematic representation of versatile molecular mass growth processes to PAH-embedded azulene units involving the novel Fulvenallenyl Addition Cyclization Aromatization (FACA) reaction mechanism. The waved lines symbolize the chemical bonding to extended aromatic systems.

Data availability

Essential data are provided in the main text and the ESI.† Additional data can be available from the corresponding author upon reasonable request.

Author contributions

L. Z. and N. H. designed the experiments; W. L. carried out the experimental measurements in NSRL and performed the data analysis; D. C. and M. S. M. carried out the experimental measurements at Sandia and performed the data analysis. L. Z., J. Y. and N. H. supervised the experiments; A. N. M. and A. M. M. carried out the theoretical analysis; all authors discussed the data. L. Z., R. I. K., N. H., and A. M. M. wrote the manuscript.

Conflicts of interest

The authors declare no competing interest.

Acknowledgements

The authors are grateful for the funding supports from National Natural Science Foundation of China (22173091). RIK acknowledges support from the U.S. Department of Energy, Basic Energy Sciences DE-FG02-03ER15411. ANM and AMM acknowledge support from the U.S. Department of Energy, Basic Energy Sciences DE-FG02-04ER15570. This works is based on user proposal 2022-HLS-PT-004982 (RIK, LZ). DC, MSM, NH acknowledge support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. MSM was supported by the Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internships Program (SULI). Sandia National Laboratories is a multimission laboratory managed and operated by the National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DENA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. DOE or the U.S. Government.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section (Fig. S1–S3); additional PIE curves (Fig. S4–S8); phthalide pyrolysis (Fig. S9–S11); absolute photoionization cross-section (PICS) of azulene (Fig. S12); fit of PIE at m/z = 128 (Fig. S13–S14); computational methods (Section 6); potential energy diagram illustrated with the structures of representative transition states (Fig. S15); product branching ratios (Fig. S16). See DOI: https://doi.org/10.1039/d3sc03231k

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