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 Kekul & eacute; 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 Kekul & eacute; structures. Complementary computations based either on magnetic or electronic criteria for aromaticity indicate that in long helicenes, the Clar and Kekul & eacute; rules of preferred sextet localization do not hold. image