Aromaticity in the Electronic Ground and Lowest Triplet States of Molecules with Fused Thiophene Rings

Abstract

Analysis of the variations of the off‐nucleus isotropic magnetic shielding, σiso(r), around thiophene, thienothiophenes, dithienothiophenes and sulflowers in their electronic ground (S0) and lowest triplet (T1) states reveals that some of the features of aromaticity and bonding in these molecules do not fit in with predictions based on the popular Hückel’s and Baird’s rules. Despite having 4n π electrons, the S0 states of the sulflowers are shown to be aromatic, due to the local aromaticities of the thiophene rings. To reduce T1 antiaromaticity, the geometry of thiophene changes between S0 and T1: In addition to losing planarity, the carbon‐carbon two ‘double’ and one ‘single’ bonds in S0 turn into two ‘single’ and one ‘double’ bonds in T1. Well‐defined Baird‐style aromaticity reversals are observed between the S0 and T1 states of only three of the twelve thiophene‐based compounds investigated in this work; in contrast, the sulflower with six thiophene rings which is weakly aromatic in S0 becomes more aromatic in T1. The results suggest that the change in aromaticity between the S0 and T1 states in chains of fused rings is likely to affect mostly the central ring(s); rings sufficiently far away from the central ring(s) can retain aromatic character.