| Abstract
| - A quantum chemical study has been performed to assess changes in aromaticity along the T1 stateZ/E-isomerization pathways of annulenyl-substituted olefins. It is argued that the point on the T1energy surface with highest substituent aromaticity corresponds to the minimum. According toBaird (J. Am. Chem. Soc. 1972, 94, 4941), aromaticity and antiaromaticity are interchanged whengoing from S0 to T1. Thus, olefins with S0 aromatic substituents (set A olefins) will be partiallyantiaromatic in T1 and vice versa for olefins with S0 antiaromatic substituents (set B olefins). Twistof the CC bond to a structure with a perpendicular orientation of the 2p(C) orbitals (3p*) in T1should lead to regaining substituent aromaticity in set A and loss of aromaticity in set B olefins.This hypothesis is verified through quantum chemical calculations of T1 energies, geometries (bondlengths and harmonic oscillator measure of aromaticity), spin densities, and nucleus independentchemical shifts whose differences along the T1 PES display zigzag dependencies on the number ofπ-electrons in the annulenyl substituent of the olefin. Aromaticity changes are reflected in theprofiles of the T1 potential energy surfaces (T1 PESs) for Z/E-isomerizations because olefins in setA have minima at 3p* whereas those in set B have maxima at such structures. The propercombination (fusion) of the substituents of set A and B olefins could allow for design of novel opticalswitch compounds that isomerize adiabatically with high isomerization quantum yields.
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