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Cyclopropyl Group: An Excited‐State Aromaticity Indicator?
Author(s) -
Ayub Rabia,
Papadakis Raffaello,
Jorner Kjell,
Zietz Burkhard,
Ottosson Henrik
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201701404
Subject(s) - aromaticity , antiaromaticity , excited state , chemistry , heteroatom , annulene , singlet state , ring (chemistry) , computational chemistry , atomic physics , molecule , physics , organic chemistry
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 (T 1 and S 1 ). Baird's rule says that the π‐electron counts for aromaticity and antiaromaticity in the T 1 and S 1 states are opposite to Hückel's rule in the ground state (S 0 ). 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 [4 n ]annulenes ruin the excited‐state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited‐state aromaticity (e.g., biphenylene), as well as 4 n π‐electron heterocycles with two or more heteroatoms represent limitations.