Syntheses and properties of conjugated polycyclic molecules containing heptazethrene, indacene, and azulene cores

Abstract

The investigation of π-conjugated systems has gained significant attention in recent times due to the intriguing optoelectronic properties and tunable ground state characteristics. These systems have a wide range of applications in various fields, such as organic electronics, spintronics, photovoltaics, and sensors. Understanding the concepts of aromaticity, proaromaticity, and anti-aromaticity is crucial and has a significant impact on the stability and ground-state electronic properties of these systems. Chapter 1 presents an introduction to the background of zethrene and its higher homologues, specifically heptazethrene. The subsequent discussion is related to indacene- and azulene-embedded polycyclic arenes (PAs) and polycyclic heteroarenes (PHAs). Based on the aforementioned topics, a series of novel PAs and PHAs are designed, wherein the heptazethrene, indacene, and azulene cores are incorporated into the π-backbone. Higher-order zethrenes exhibit diradical character in the ground state, which can only be modulated by π-ring annulation, whereas substitution did not affect the electronic ground state of the zethrene molecules. In Chapter 2, we have shown the isolation of a dibenzoheptazethrene derivative in the closed-shell ground state bearing push-pull substituents, which results in a zero diradical character and a low-lying zwitterionic form. In Chapter 3, attempts were made to isolate highly reactive dicyclopenta[b,d]thiophene (DCPT) core by fusion of naphthalene and phenanthrene units. However, unexpected decomposition products were obtained. DCPT is a heteroatom-modified as-indacene in which one of the central Csp 2−Csp 2 double bonds of the as-indacene unit is replaced with an isoelectronic sulfur atom. Extending this indacene chemistry to the Chapter 4, we reported a formally aromatic dicyclopenta[c]fluorenothiophene, which exhibits a dominant antiaromatic character of the as-indacene subunit due to the inclusion of thiophene unit in the π-backbone. Finally, in Chapter 5, a tribenzo-extended non-alternat isomer of peri-acenoacene was reported with a formal azulene unit embedded in the polycyclic backbone, making it the newest nonhexagonal nanographene accessed through a Scholl-type oxidation approach. All the reported molecules exhibit remarkable photophysical properties with tunable (anti)aromatic or diradical characters that make them promising materials for applications in organic electronics and photonics.