Helicene Aromaticity Deviates from the Clar Rule—On the Electronic Dissimilarity of Large Isomeric Fibonacenes

This combined experimental and theoretical study illustrates the profound consequences of non‐planarity on the electronic properties of polycyclic arenes. Three isomeric [10]fibonacene tetraesters were synthesized through a robust and regiocontrolled Perkin/Mallory approach: a nearly planar [10]phenacene derivative, a moderately twisted [10]semicircle derivative, and a 3D non‐planar [10]helicene derivative. The photophysical properties of the 3D [10]helicene isomer were found to be dramatically different from the comparable ones of the [10]phenacene and [10]semicircle isomers. The aromatic properties of the [10]phenacene and [10]semicircle isomers conform well with their predictive Kekulé and Clar analyses, but the [10]helicene isomer deviates from these general topological rules, which appears to be a general phenomenon for [n]fibonacenes with n≥9. Long isomeric fibonacenes—a phenacene, a helicene and a croissant‐shaped isomer—are found to show strikingly different absorption and emission spectra, despite their equivalent Kekulé structures. Complementary computations based either on magnetic or electronic criteria for aromaticity indicate that in long helicenes, the Clar and Kekulé rules of preferred sextet localization do not hold.