Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks leading to carbonaceous nanoparticles identified in combustion systems and extraterrestrial environments. However, the understanding of their formation and growth in these high temperature environments has remained elusive. We present a mechanism through laboratory experiments and computations revealing how the prototype PAH—naphthalene—can be efficiently formed via a rapid 1-indenyl radical—methyl radical reaction. This versatile route converts five- to six-membered rings and provides a detailed view of high temperature mass growth processes that can eventually lead to graphene-type PAHs and two-dimensional nanostructures providing a radical new view about the transformations of carbon in our universe.

Incorporation of dicarboxylate nanocrystalline cellulose (DCNC) and dialdehyde nanocrystalline cellulose (DANC) into poly(acrylic acid) (PAA) hydrogel remarkably improved the mechanical performance.

A synthetic route to racemic helicenes via a vinylacetylene mediated gas phase chemistry involving elementary reactions with aryl radicals is presented. In contrast to traditional synthetic routes involving solution chemistry and ionic reaction intermediates, the gas phase synthesis involves a targeted ring annulation involving free radical intermediates. Exploiting the simplest helicene as a benchmark, we show that the gas phase reaction of the 4-phenanthrenyl radical ([C₁₄H₉]^•) with vinylacetylene (C₄H₄) yields [4]-helicene (C₁₈H₁₂) along with atomic hydrogen via a low-barrier mechanism through a resonance-stabilized free radical intermediate (C₁₈H₁₃). This pathway may represent a versatile mechanism to build up even more complex polycyclic aromatic hydrocarbons such as [5]- and [6]-helicene via stepwise ring annulation through bimolecular gas phase reactions in circumstellar envelopes of carbon-rich stars, whereas secondary reactions involving hydrogen atom assisted isomerization of thermodynamically less stable isomers of [4]-helicene might be important in combustion flames as well.

The three-ring polycyclic aromatic hydrocarbons (PAHs) 3H-benz[e]indene (C₁₃H₁₀) and 1H-benz[f]indene (C₁₃H₁₀) along with their naphthalene-based isomers 2-(prop-2-yn-1-yl)naphthalene (C₁₃H₁₀), 2-(prop-1-yn-1-yl)naphthalene (C₁₃H₁₀), and 2-(propa-1,2-dien-1-yl)naphthalene (C₁₃H₁₀) were formed through a “directed synthesis” via a high temperature chemical micro reactor under combustion-like conditions (1300 ± 35 K) through the reactions of the 2-naphthyl isomer (C₁₀H₇˙) with allene (C₃H₄) and methylacetylene (C₃H₄). The isomer distributions were probed utilizing tunable vacuum ultraviolet radiation from the Advanced Light Source (ALS) by recording the photoionization efficiency curves at mass-to-charge of m/z = 166 (C₁₃H₁₀) and 167 (¹³CC₁₂H₁₀) of the products in a supersonic molecular beam. Complemented by electronic structure calculations, our study reveals critical mass growth processes via annulation of a five-membered ring from the reaction between aryl radicals and distinct C₃H₄ isomers at elevated temperatures as present in combustion processes and in circumstellar envelopes of carbon stars. The underlying reaction mechanisms proceed through the initial addition of the 2-naphthyl radical with its radical center to the π-electron density of the allene and methylacetylene reactants via entrance barriers between 8 and 14 kJ mol⁻¹, followed by isomerization (hydrogen shifts, ring closure), and termination via atomic hydrogen losses accompanied by aromatization. The reaction mechanisms reflect the formation of indene – the prototype PAH carrying a single five- and a single six-membered ring – synthesized through the reaction of the phenyl radical (C₆H₅˙) with allene and methylacetylene. This leads us to predict that aryl radicals – upon reaction with allene/methylacetylene – may undergo molecular mass growth processes via ring annulation and de facto addition of a five-membered ring to form molecular building blocks essential to transit planar PAHs out of the plane.