The Pascal group has synthesized dodecaphenyltetracene 1.¹

While this paper has little computational work, it is of interest to readers of this blog since I have discussed many aspect of aromaticity. This new tetracene is notable for its large twisting along the tetracene axis: about 97° in the x-ray structure. I have optimized the structure of 1 at B3LYP-D3(BJ)/6-311G(d) and its structure is shown in Figure 1. It is twisted by about 94°. The computed and x-ray structures are quite similar, as seen in Figure 2. Here the x-ray structure is shown with red balls, the computed structure with gray balls, and hydrogens have been removed for clarity.

Figure 1. B3LYP-D3(BJ)/6-311G(d) optimized structure of 1.

Figure 2. Comparison of the x-ray (red) and computed (gray) structures of 1. (Hydrogens omitted for clarity.)

The authors note that this molecule is chiral, having near D₂ symmetry. (The optimized structure has D₂ symmetry.) They performed AM1 computations to estimate a very low barrier for racemization of only 17.3 kcal mol^-1, leading to a half-life of less than one second at RT.

A notable aspect of the molecule is that aromaticity can adapt to significant twisting yet retain aromatic character. For example, the molecule is stable even surviving boiling off of chloroform (61 °C) to form crystals and the majority of the C-C bonds in the tetracene portion have distances typical of aromatic systems (~1.4 Å).

References

1) Xiao, Y.; Mague, J. T.; Schmehl, R. H.; Haque, F. M.; Pascal Jr., R. A., “Dodecaphenyltetracene.” Angew. Chem. Int. Ed. 2019, 58, 2831-2833, DOI: 10.1002/anie.201812418.

InChIs

1: InChI=1S/C90H60/c1-13-37-61(38-14-1)73-74(62-39-15-2-16-40-62)78(66-47-23-6-24-48-66)86-82(70-55-31-10-32-56-70)90-84(72-59-35-12-36-60-72)88-80(68-51-27-8-28-52-68)76(64-43-19-4-20-44-64)75(63-41-17-3-18-42-63)79(67-49-25-7-26-50-67)87(88)83(71-57-33-11-34-58-71)89(90)81(69-53-29-9-30-54-69)85(86)77(73)65-45-21-5-22-46-65/h1-60H
InChIKey=NJQABVWYMCSFNE-UHFFFAOYSA-N