3 years ago

Cyclopropyl Group: An Excited-State Aromaticity Indicator?

Cyclopropyl Group: An Excited-State Aromaticity Indicator?
Burkhard Zietz, Kjell Jorner, Henrik Ottosson, Raffaello Papadakis, Rabia Ayub
The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T1 and S1). Baird's rule says that the π-electron counts for aromaticity and antiaromaticity in the T1 and S1 states are opposite to Hückel's rule in the ground state (S0). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4nπ-electron heterocycles with two or more heteroatoms represent limitations. So exciting! The cyclopropyl (cPr) group, which is a known probe for detecting radical character at atoms to which it is connected, was tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T1 and S1; see scheme). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701404

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