Construction of Polycyclic π‐Conjugated Systems Incorporating an Azulene Unit Following the Oxidation of 1,8‐Diphenyl‐9,10‐bis(phenylethynyl)phenanthrene

Abstract

Polycyclic aromatic hydrocarbons (PAHs) that incorporate either heptagons or pentagons consist of non-planar molecular structures with unusual optoelectronic properties, and the design of a relatively simple and efficient method to construct these highly fused π-conjugated systems with odd-membered rings is in high demand. This work describes the use of silver(I) cations to promote the efficient synthesis of azulene-embedded PAH 2, which is a structural isomer of tribenzo[fg,ij,rst]pentaphene 3, via tandem oxidative transannulation between the phenyl and arylethynyl moieties. This method involves a carbophilic interaction of the silver(I) cation with the acetylene units, which facilitates an electron transfer in the initial step. The synthesized PAH 2 and the protonated cation 2 H⋅BF were fully characterized by X-ray crystallographic analysis, electronic absorption, electrochemical measurement, and quantum chemical calculation. The azulene-embedded PAH 2 exhibited a low-energy absorption band and amphoteric redox events, which were characterized as non-alternant characteristics originating from the azulene unit.

Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties

“…This remarkable conformation can be illustrated by the interplanar angles between the five and seven‐membered rings: The rings B and J in 2 form an angle of θ BJ =16.1°, while the rings B and L in 3 form an angle of θ BL =15.1°, which is larger than that in 1 ( θ BJ =13.0°), representing a record‐high twisting degree of the reported azulene units. These parameters are in contrast with those of pristine azulene and other reported azulene‐embedded π‐systems in which the azulene moiety mainly adopts a planar structure …”

“…As shown in Figure f, there is almost no bond length alternation in the azulene unit of 3 since the C−C bonds in the seven‐membered ring (ring B) mainly exhibit C(sp 2 )−C(sp 2 ) bond character. In 2 , however, significant double‐bond features (Figure e) are determined for C31−C32 (1.388 Å), C33−C34 (1.367 Å), and C47−C48 (1.398 Å), which is reminiscent of the structural contribution from a pentafulvene unit . Similar to 2 , the C−C bond lengths of the azulene unit in 1 are between 1.39 and 1.48 Å without bond alternation (Figure d).…”

“…Previous related studies have revealed that incorporation of non‐alternant units into PAHs is an efficient method to tailor their optoelectronic properties . However, embedding the azulene unit into a π‐extended helical system is still only possible to a limited extent, mostly due to the lack of a facile synthetic route and the possible azulene‐to‐naphthalene rearrangement …”

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…There are a very few successful approaches known for synthesizing PAHs with azulene units embedded from precursor monolayers on surfaces . However, to the best of our knowledge, there are only a few examples of soluble azulene‐based PAHs that have been synthesized from solution on a larger scale . One is the example recently published by Konishi and co‐workers, in which the PAH contains besides one azulene core, six more fused six‐membered rings.…”

Contorted Polycyclic Aromatic Hydrocarbons with Two Embedded Azulene Units

“…[16] However,t ot he best of our knowledge,t here are only af ew examples of soluble azulene-based PA Hs that have been synthesized from solution on al arger scale. [17,18] One is the example recently published by Konishi and co-workers, [17] in which the PA H contains besides one azulene core,s ix more fused sixmembered rings.V ery recently,t he same group presented as mall PA Ht hat contains two fused azulene units and two additional peri-fused six-membered rings,g iving overall four rings. [18c] This compound shows significant biradical character.…”