Synthesis, Conformational Dynamics and Spectro-electrochemical Charecteristics of Core-Modified Expanded Isophlorinoids

Abstract

Porphyrin and its expanded congeners display a variety of non-planar structures and tend to deviate from aromatic characteristics. They are also redox active species and hence exhibit facile proton coupled electron transfer (PCET). In contrast, non-pyrrolic macrocycles prefer to adopt isophlorin-like framework and undergo facile one/two electron ring oxidation. However, thiophene based giant porphyrinoids are expected to adopt novel topological conformations owing to the bulky sulphur atoms in the core of the macrocycle. In this thesis, design, synthesis, structural diversity and spectro-electrochemcial properties of thiophene based porphyrinoids will be discussed in detail. A variety of thiophene based isophlorinoids varying from 22π to 80π electrons have been synthesized. They have been characterized by spectroscopic methods to study their aromatic characteristics. Invariably, their structural features control the aromatic character in solution state. Large macrocycles are fluxional in nature and hence they can possibly exist in more than one conformation in solution state. Unexpectedly, some of these macrocycles display structural isomerism in the solid state. The smaller macrocycles are more rigid and hence retain distinct structure in solution and solid states. Further, their structural diversity and polymorphic features were studied by single crystal X-ray diffraction. It was observed that non-aromatic macrocycles were also redox active akin to anti-aromatic porphyrinoids. Cyclic voltammetry and spectro-electrochemical studies were performed to identify their reversible redox properties. These findings could be equally substantiated by chemical redox with appropriate reagents. Suitable quantum chemical calculations were also employed to buttress the observed experimental results. All these studies provide adequate support to establish that anti-aromatic and non-aromatic porphyrinoids display facile reversible redox properties in contrast to aromatic congeners.