Synthesis,Characterization and Redox Properties of Expanded Isophlorinoids Containing Six, Seven, Eight, Nine and Ten Heterocyclic Units

Abstract

Aromatic molecules as stated by Huckel are planar, conjugated, (4n+2)π electronic systems. Benzene is the simplest known example for the same. On the other hand, 4nπ systems are anti-aromatic and not so stable. Isophlorinoids are typical examples for stable anti-aromatic systems. Such π conjugated macrocycles could be stabilized by changing the core of a porphyrin by substituting pyrrole with other heterocyclic units such as furan or thiophene. Signature paratropic ring current effects from NMR spectroscopy is crucial to evaluate the extent of planarity and antiaromaticity of these macrocycles. Such molecules are vulnerable to redox reactions similar to metal ions. However, large expanded porphyrins adopt figure-of-eight topology owing to their structural flexibility. In this thesis, synthesis, characterization and redox properties of novel core modified expanded isophlorinoids with six, seven, eight, nine and ten heterocyclic units will be discussed. In attempts to stabilize a planar 32π heptaphyrin, a novel asymmetric tristhiophene was designed to yield the macrocycle with four bridging carbons. By virtue of its anti-aromaticity, it undergoes two-electron reversible oxidation to yield the 30π dicationic species. Adopting a similar approach to synthesize a 40π octaphyrin resulted in figure of eight conformation of the macrocycle, hence rendering it non-antiaromatic in nature. However, this molecule also displayed reversible oxidation and stable aromatic 38π dication could be isolated. This strategy was modified to synthesize for 38π octaphyrin and 46π decaphyrin by reducing the number of bridging carbon atoms. Interestingly two structural isomers of a 38π octaphyrin were isolated from the same reaction. Details of synthesis, characterization, redox properties and quantum chemical calculations will be discussed in detail.