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 ‐ NO 2 . 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 ‐ NO 2 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 ‐ NO 2 exhibits a unique solvatochromic effect in different solvents and hydrogen‐bonding‐induced emission transfer in different ratios of THF/H 2 O solutions.