A Unified Mechanism on the Formation of Acenes, Helicenes, and Phenacenes in the Gas Phase

Abstract

A unified low‐temperature reaction mechanism on the formation of acenes, phenacenes, and helicenes—polycyclic aromatic hydrocarbons (PAHs) that are distinct via the linear, zigzag, and ortho‐condensed arrangements of fused benzene rings—is revealed. This mechanism is mediated through a barrierless, vinylacetylene mediated gas‐phase chemistry utilizing tetracene, [4]phenacene, and [4]helicene as benchmarks contesting established ideas that molecular mass growth processes to PAHs transpire at elevated temperatures. This mechanism opens up an isomer‐selective route to aromatic structures involving submerged reaction barriers, resonantly stabilized free‐radical intermediates, and systematic ring annulation potentially yielding molecular wires along with racemic mixtures of helicenes in deep space. Connecting helicene templates to the Origins of Life ultimately changes our hypothesis on interstellar carbon chemistry.

Curious Case of Singlet Triplet Gaps in Nonlinear Polyaromatic Hydrocarbons

“…17−22 The successful synthesis of helicenes and phenacenes has been done, and these species can often be more stable than the polyacenes. 23,24 While the electronic properties of polyacenes have long been studied and understood, 2,11−13 relatively little is known about the electronic structure of helicenes and phenacenes. 25−28 This is even more crucial in the context of the recent synthesis of planar and nonplanar structures of circulenes 29,30 enes, 31 which are a combination of linear and nonlinear arrangements of benzene rings.…”

“…The kinked (nonlinear) structures thus formed depending upon the direction of the nonlinearity are called helicenes or phenacenes (shown in Figure ). They can be exploited in semiconductors, organic light-emitting diodes (OLEDs), and organic solar cells and for chiral sensing, chemical sensing, nonlinear optical materials, etc. − The successful synthesis of helicenes and phenacenes has been done, and these species can often be more stable than the polyacenes. , While the electronic properties of polyacenes have long been studied and understood, ,− relatively little is known about the electronic structure of helicenes and phenacenes. − This is even more crucial in the context of the recent synthesis of planar and nonplanar structures of circulenes , and phenylenes, which are a combination of linear and nonlinear arrangements of benzene rings.…”

The Curious Case of Low-lying States in Non-linear Pol- yaromatic Hydrocarbons

“…16 The successful synthesis of helicenes and phenacenes has been done and these species can often be more stable than the polyacenes. 17,18 While the electronic properties of polyacenes have long been studied and understood, 2,[11][12][13] relatively little is known about the electronic structure of helicenes and phenacenes. [19][20][21][22] This is even more crucial in the context of the recent synthesis of planar and non-planar structures of circulenes 23,24 and phenylenes 25 , which are a combination of linear and non-linear arrangements of benzene rings.…”

Formation of phenanthrene via H‐assisted isomerization of 2‐ethynylbiphenyl produced in the reaction of phenyl with phenylacetylene

“…Molecules and radicals other than acetylene, including aromatics themselves, 8‐10,20 methyl, 11,12,21‐23 vinyl 24 and resonantly‐stabilized propargyl 25,26 and allyl 27,28 radicals, and vinylacetylene, 29‐37 have been proposed to complement acetylene‐based HACA, especially in the production of purely benzenoid PAHs. In particular, Shukla et al 21,38 and Xiong et al 39 proposed a mechanism termed PAC (phenyl addition/dehydrocyclization) in which phenyl radical adds to a bay area (armchair edge) of a PAH molecule thus resulting in its extension by two six‐member rings, see, for example, the reaction of phenanthrene with C 6 H 5 producing benzo[e]pyrene 21 .…”