Modular Synthesis of Structurally Diverse Azulene‐Embedded Polycyclic Aromatic Hydrocarbons by Knoevenagel‐Type Condensation

Abstract

The research interest in azulene‐embedded polycyclic aromatic hydrocarbons (PAHs) has significantly increased recently, but the lack of efficient synthetic strategies impedes the investigation of their structure‐property relationships and further opto‐electronic applications. Here we report a modular synthetic strategy towards diverse azulene‐embedded PAHs by a tandem Suzuki coupling and base‐promoted Knoevenagel‐type condensation with good yields and great structural versatility, including non‐alternant thiophene‐rich PAHs, butterfly‐ or Z‐shaped PAHs bearing two azulene units, and the first example of a two‐azulene‐embedded double [5]helicene. The structural topology, aromaticity and photophysical properties were investigated by NMR, X‐ray crystallography analysis and UV/Vis absorption spectroscopy assisted by DFT calculations. This strategy provides a new platform for rapidly synthesizing unexplored non‐alternant PAHs or even graphene nanoribbons with multiple azulene units.

Structural Expansion of Cyclohepta[def]fluorene towards Azulene‐Embedded Non‐Benzenoid Nanographenes

“…Similarly to 1, a sharp kink at the apex, highlighted with a pink arrow in Figure 3b can be attributed to a non-planar conformation of the molecule due to steric hindrance between two adjacent H atoms. [11,12,41] The nc-AFM inset shown in Figure 3c further confirms our assumption. This nonplanarity is clearly visible in the DFT-optimized structure (Figure 3d) of 3 on Au(111), which shows how a segment of the molecule is bent out of the plane (dihedral angle of 10.9°, with an adsorption height of 3.4 Å with respect to the underlying surface).…”

“…High‐resolution STM and nc‐AFM images acquired with a CO‐functionalized tip allow us to determine the structure of 3 . Similarly to 1 , a sharp kink at the apex, highlighted with a pink arrow in Figure 3b can be attributed to a non‐planar conformation of the molecule due to steric hindrance between two adjacent H atoms [11,12,41] . The nc‐AFM inset shown in Figure 3c further confirms our assumption.…”

“…[1][2][3] Among them, the introduction of nonalternant units (i. e., azulene or heptalene) into non-benzenoid NGs could cause local changes in strain and molecular symmetry, thus significantly impacting their physicochemical properties. [4] Moreover, the topological transformation induced by these non-alternant moieties could also determine the electronic ground state of NGs, [5][6][7][8][9][10][11][12][13][14][15] affording the potential open-shell character and promising applications in organic spintronics. [16,17] As a representative example, pyrene (a) can be transformed into one of its non-alternant isomers, that is, cyclohepta[def]fluorene (b) (namely bis-periazulene), by replacing the inner naphthalene unit with azulene (Figure 1a), which is predicted to be an open-shell Kekulé structure with a low triplet ground state.…”

Exploring C–C bond formation reactions for expanding azulene derivatives linked at the 2- and/or 6-positions

“…A recent example of this approach is the modular synthesis of azulene-embedded polycyclic aromatic hydrocarbons (PAH) by Knoevenagel-type condensation, as demonstrated by Liu and colleagues. 12 More recently, Langer and coworkers used a cycloisomerization reaction on the five-membered ring of azulene to synthesize aryl-substituted naphtho[2,1- a ]azulenes. 13 In their study, the use of methanesulfonic acid (MSA) as the Brønsted acid provided various fused azulene derivatives in moderate to good yields.…”

Ni-Catalyzed 1,5-Sigmatropic Ester Shift on Cyclopentadiene Rings: Regioselective Conversion of 5,5-Disubstituted Cyclopentadienes to CH2–Cyclopentadienes

“…Owing to their instability induced by Diels–Alder dimerization and 1,5-sigmatropic hydrogen migration, general and selective synthetic methods for densely functionalized cyclopentadienes are still challenging. , Further, substituted cyclopentadienes obtained by the newly emerged transition metal catalyzed or organocatalytic methods often bear an all-carbon quaternary center . Although such 5,5-disubstitution helps to stabilize the cyclopentadiene ring, it also excludes the accessibility of the relevant Cp anions for further derivations, such as transition metal complexation or condensation with electrophiles …”