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Symmetry 2010, 2(4), 1846-1924; doi:10.3390/sym2041846

Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

Dipartimento di Chimica, Università di Firenze, via della Lastruccia 3, 50019 Sesto Fiorentino (Firenze), Italy
Author to whom correspondence should be addressed.
Received: 23 September 2010 / Revised: 6 October 2010 / Accepted: 19 October 2010 / Published: 5 November 2010
(This article belongs to the Special Issue Aromaticity and Molecular Symmetry)
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The paper introduces general considerations on structural properties of aromatic, antiaromatic and non-aromatic conjugated systems in terms of potential energy along bond length alternation and distortion coordinates, taking as examples benzene, cyclobutadiene and cyclooctatetraene. Pentalene, formally derived from cyclooctatetraene by cross linking, is also considered as a typical antiaromatic system. The main interest is concerned with [n]annulenes and model [n]annulene molecular systems, n ranging from 10 to 18. The rich variety of conformational and  configurational isomers and of dynamical processes among them is described. Specific attention is devoted to bridged [10]- and [14]annulenes in the ground and lowest excited states as well as to s-indacene and biphenylene. Experimental data obtained from vibrational and electronic spectroscopies are discussed and compared with ab initio calculation results. Finally, porphyrin, tetraoxaporphyrin dication and diprotonated porphyrin are presented as annulene structures adopting planar/non-planar geometries depending on the steric hindrance in the inner macrocycle ring. Radiative and non-radiative relaxation processes from excited state levels have been observed by means of time-resolved fluorescence and femtosecond transient absorption spectroscopy. A short account is also given of porphycene, the structural isomer of porphyrin, and of porphycene properties.
Keywords: annulenes; aromaticity; antiaromaticity; symmetry; porphyrins annulenes; aromaticity; antiaromaticity; symmetry; porphyrins
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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MDPI and ACS Style

Gellini, C.; Salvi, P.R. Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States. Symmetry 2010, 2, 1846-1924.

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