Cascade Synthesis of Benzotriazulene with Three Embedded Azulene Units and Large Stokes Shifts

Abstract

We report here the one‐pot synthesis of benzo[1,2‐a : 3,4‐a′ : 5,6‐a′′]triazulene (BTA), wherein three azulene units are embedded through a tandem reaction comprising two steps, Suzuki coupling and Knoevenagel condensation, between a readily available triborylated truxene precursor and 8‐bromo‐1‐naphthaldehyde. Its nitration leads to a regioselective trinitrated product, namely, BTA‐NO2. Single‐crystal X‐ray crystallography revealed that the superstructure of BTA consists of a dimer stacked by two enantiomeric helicene conformers, while that of BTA‐NO2 consists of an unprecedented π‐tetramer stacked from two enantiomeric dimers, that is, four distinct helicene conformers. Both compounds show excellent stability and fluorescence with large Stokes shifts of up to 5100 cm−1. In addition, BTA‐NO2 exhibits a unique solvatochromic effect in different solvents and hydrogen‐bonding‐induced emission transfer in different ratios of THF/H2O solutions.

Synthesis of a Benzotrisazulene via Trioxobenzotrisazulene

“…Further purification steps, such as additional chromatographic techniques (including HPLC or GPC), recrystallization or washing of the crude product, did not change the 1 H NMR spectrum significantly. Our observation is in accordance with those from Liu and co‐workers, who reported a “complex mixture” using K 2 CO 3 and Pd(PPh 3 ) 4 [9o] . Nevertheless, we subsequently performed the ring closing reaction using potassium hydroxide powder as base to form benzotrisazulene 3 as a bright orange powder in a total yield of 9 % (Scheme 1).…”

“…Based on the triborylated truxene 1 , [12] we first synthesised the tris(formylnaphthyl)truxene 2 via Suzuki‐Miyaura cross‐coupling with 8‐bromo‐2‐naphthaldehyde, similar to the approach of Liu and co‐workers (Scheme 1). [9o] In contrast to them, who used DMF as solvent, we applied dimethoxyethane (DME) as solvent and aqueous K 2 CO 3 solution as base. Pd(PPh 3 ) 4 was used as catalyst.…”

“…It is worth mentioning that Liu et al. did a base screening and found that Cs 2 CO 3 gives reasonable yields of 3 of 26 % [9o] . However, based on these low yields (9%) we decided to develop a different synthetic route.…”

“…Note: Liu et al. used Cs 2 CO 3 and reported a yield of 26 % [9o] . c) ethyl 8‐bromo‐2‐naphthoate (6 equiv), Pd 2 dba 3 (5 mol %), t Bu 3 PHBF 4 (20 mol %), K 2 CO 3 (2 m ), THF, 80 °C, 48 h, 85 %; d) KO t Bu (9 equiv), THF abs , 75 °C, 72 h, 92 % ( anti / syn : 5 : 1).…”

“…We envisioned to generate a formal trisazulene PAH with our strategy of combining borylation, Suzuki‐Miyaura cross‐coupling, followed by base‐induced condensation, which is described herein [11,9o] . This strategy was used to make larger truxenes, [12] dibenzoperylenes, [13] benzoperylene oligomers, [14] heteroannulated coronenes and their oligomers, [15] or more recent PAH monkey‐saddles; [4g,h] and others have successfully adapted it for PAH synthesis.…”