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Impact of Ground‐ and Excited‐State Aromaticity on Cyclopentadiene and Silole Excitation Energies and Excited‐State Polarities
Author(s) -
Jorner Kjell,
Emanuelsson Rikard,
Dahlstrand Christian,
Tong Hui,
Denisova Aleksandra V.,
Ottosson Henrik
Publication year - 2014
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.201402577
Subject(s) - aromaticity , antiaromaticity , excited state , singlet state , excitation , ground state , chemistry , cyclopentadiene , computational chemistry , atomic physics , chemical physics , molecule , physics , organic chemistry , quantum mechanics , catalysis
A new qualitative model for estimating the properties of substituted cyclopentadienes and siloles in their lowest ππ* excited states is introduced and confirmed through quantum chemical calculations, and then applied to explain earlier reported experimental excitation energies. According to our model, which is based on excited‐state aromaticity and antiaromaticity, siloles and cyclopentadienes are cross‐hyperconjugated “aromatic chameleons” that adapt their electronic structures to conform to the various aromaticity rules in different electronic states (Hückel’s rule in the π 2 electronic ground state (S 0 ) and Baird’s rule in the lowest ππ* excited singlet and triplet states (S 1 and T 1 )). By using pen‐and‐paper arguments, one can explain polarity changes upon excitation of substituted cyclopentadienes and siloles, and one can tune their lowest excitation energies by combined considerations of ground‐ and excited‐state aromaticity/antiaromaticity effects. Finally, the “aromatic chameleon” model can be extended to other monocyclic compound classes of potential use in organic electronics, thereby providing a unified view of the S 0 , T 1 , and S 1 states of a range of different cyclic cross‐π‐conjugated and cross‐hyperconjugated compound classes.