Azulene‐Embedded [n]Helicenes (n=5, 6 and 7)

Abstract

Azulene is a non‐benzenoid aromatic building block with unique chemical structure and physicochemical properties. By using the “bottom‐up” synthetic strategy, we synthesized three azulene‐embedded [n]helicenes ([n]AzHs, n=5, 6 and 7), in which one terminal azulene subunit was fused with n‐2 benzene rings. P‐ and M‐enantiomers were observed in the packing diagrams of [5]‐, and [6]AzHs. However, P‐ and M‐[7]AzHs could be isolated by recrystallization of the racemic mixture. These [n]AzHs were endowed with new properties through the azulene moiety such as low‐lying first electric state (S1), small optical energy gap and anti‐Kasha emission. [6]‐, and [7]AzHs exhibit strong chiroptical responses with high absorption dissymmetry factor (gabs) maxima of about 0.02, which is among the highest |gabs| values of helicenes in the visible range. These azulene‐embedded [n]helicenes contribute to the non‐benzenoid helicene library and allow the structure–property relationships to be better understood.

The road to bis‐periazulene (cyclohepta[def]fluorene): Realizing one of the long‐standing dreams in nonalternant hydrocarbons

“…Azulene‐embedded carbohelicenes 33 – 35 that correspond to the nonalternant isomers of [5]helicene, [6]helicene, and [7]helicene, respectively, were successfully synthesized and characterized. [ 55 ] The helicenes 34 and 35 exhibited strong chiroptical responses with high absorption dissymmetry factor ( g abs ) maxima of approximately 0.02, which is among the highest | g abs | value of helicenes in the visible range. The unsymmetric frontier orbitals derived from the azulene moiety might account for the observed high | g abs | values.…”

Linear Nonalternant Isomers of Acenes Fusing Multiple Azulene Units

“…There are two general approaches to introduce azulene, namely, i) starting from commercially available azulene to prepare azulene‐containing PAHs and ii) in situ constructing azulene skeleton from nonazulene substrates. Although the former can build a wide range of PAHs containing σ‐bonded [7–10] and fused azulene [11–19] units, the high cost of azulene limits its use. By contrast, the latter provides an opportunity to build azulene from simple nonazulene precursors with diverse structural modifications which facilitates the incorporation of azulene unit into polymer backbones [20, 21] and fused PAHs [22–31] .…”

Curious Case of Singlet Triplet Gaps in Nonlinear Polyaromatic Hydrocarbons

“…14,15 The alternative approach to tune the ST gap is to introduce nonplanarity and nonlinearity in the arrangement of the fused benzene rings. 16 The kinked (nonlinear) structures thus formed depending upon the direction of the nonlinearity are called helicenes or phenacenes (shown in Figure 1). 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.…”