Aromatic pathways in thieno-bridged porphyrins: Understanding the influence of the direction of the thiophene ring on the aromatic character

Magnetically induced current densities have been investigated for some recently synthesised thieno-bridged porphyrins. The aim of the study is to understand the influence of the direction of the thieno bridge on the aromatic character of the molecules. The calculated ring-current susceptibilities for two tautomers of 2,3-and 3,4-thieno-bridged porphyrins as well as of the corresponding Zn-containing compounds show that the molecules are all aromatic according to the ring current criterion. The ring-current susceptibilities of 16.2 nA/T and 20.2 nA/T around the porphyrin ring of the two tautomers of 2,3-thieno-bridged porphyrin are somewhat weaker than the ring-current susceptibilities of 22.8 nA/T and 23.8 nA/T obtained for the two tautomers of 3,4-thieno-bridged porphyrin. For 2,3-thieno-bridged porphyrin, the positions of the inner hydrogens affect the ring-current susceptibility more than for 3,4-thieno-bridged porphyrin. The current pathways of the Zn porphyrinoids are a superposition of the pathways of the two trans tautomers of the corresponding free-base porphyrinoids. The current-density calculations explain the different aromatic character of 2,3-thieno-bridged and 3,4-thieno-bridged porphyrins. Current-density calculations on tetra-2,3-thieno-bridged and tetra-3,4-thieno-bridged porphyrins yielded similar current pathways as obtained for mono-substituted thieno-bridged porphyrins. Tetra-2,3-thieno-bridged porphyrin is practically non-aromatic since it sustains a very weak ring current of 2.6 nA/T. The calculated ring-current susceptibilities for tetra-3,4-thieno-bridged porphyrin is 13.2 nA/T, which can be compared to the ring-current susceptibilities of 11.8 and 27.5 nA/T for benzene and free-base porphyrin, respectively.