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

Analysis of the variations of the off-nucleus isotropic magnetic shielding, σ_iso(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 individual thiophene rings. To reduce its T₁ antiaromaticity, the geometry of thiophene changes considerably 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 longer chains of fused rings is likely to affect mostly the central ring (or the pair of central rings); rings sufficiently far away from the central ring(s) can retain aromatic character.