Effect of Meso-Thienyl and Meso-Furyl Groups on the Electronic Properties of Aromatic 14π Triphyrin(2.1.1)s

A series of triphyrin(2.1.1)s containing five membered heterocycles such as thienyl and furyl groups in place of six membered aryl groups at two or four meso-positions were synthesized by [2+1] condensation of appropriate dipyrroethene diols with pyrrole under acid catalyzed conditions. The meso-thienyl and meso-furyl triphyrin(2.1.1)s were characterized and studied by HR-MS, 1D & 2D NMR spectroscopy, X-ray crystallography, absorption, cyclic voltammetry and DFT/TD-DFT studies. The X-ray structure obtained for one of the meso-thienyl trirphyrins indicated that the macrocycle was nearly planar and showed strong intramolecular hydrogen bonding. The spectral and electrochemical studies revealed that in meso-thienyl and meso-furyl triphyrin(2.1.1)s, the resonances of triphyrin core protons downfield shifted in 1H NMR, the absorption bands bathochromically shifted and redox potentials shifted cathodically and anodically compared to meso-tetra(p-tolyl) triphyrin(2.1.1). These observations indicated that the thienyl/furyl groups at meso-positions alter the electronic properties of triphyrin(2.1.1)s significantly and the maximum effects were noted in meso-tetra(2-thienyl) triphyrin(2.1.1) and meso-tetra(2-furyl) triphyrin(2.1.1)s. The DFT and TD-DFT studies corroborated experimental observations.