QUINODIZATION REACTION OF NAPHTHALENE DIIMIDES TO AFFORD NOVEL P-NAPHTHOQUINODIMETHANE LIGHT-HARVESTING CHROMOPHORES: AROMATICITY, PHOTOPHYSICS AND APPLICATION FOR PHOTON UPCONVERSION

ABSTRACT The chemistry and aromaticity of polycyclic benzenoid hydrocarbons have been a topic of intensive investigations since the early 19th century. The concept of aromaticity has played a crucial role in organic chemistry and related fields; and it expanded very rapidly for its ability to explain the stability, electronic structures and physical properties of π-conjugated molecules. Besides benzenoid structures, chemists have also investigated quinoidal systems, which have been shown to exhibit unique aromaticity and physical properties in the ground state. Notably, it is customary to draw quinoidal polycycles as a mixture of closed– and open–shell structures. While closed–shell quinoids do exhibit interesting physical and electronic properties, the open–shell forms can be related to benzenoids with the presence of radicals or unpaired electrons. In this context, it has been demonstrated that open–shell quinoids can have both singlet and triplet multiplicity in their ground state. In this thesis, I will demonstrate that this feature can be tuned/suppressed through clever design of the molecular framework that contains a quinoidal moiety. Often, quinoids can be labeled as aromatic like benzenoids or (anti)/pro-aromatic depending on the molecular framework. Before labeling a quinoidal compound as aromatic or (anti)/pro-aromatic, it is important to define the different aromaticity rules and their influence on the corresponding physical and electronic properties of the system(s) of interest. Chemists have relied on two main aromaticity rules (Hückel vs. Baird) to predict the nature, electronic, and physical properties of quinoidal structures and related systems. We proposed and synthesized novel quinoidal naphthalene derivatives (QDM) that exhibit “compromised” aromaticity in the ground state. A combination of computation and photophysical methods were employed to: i) decipher the reaction mechanism during the formation of the new poly-heterocyclic structures, ii) investigate their ground state and excited state opto-electronic properties, and iii) explore the nature and aromaticity of the corresponding excited state species. Additionally, the QDM was used as light-harvesting triplet sensitizers to achieve photon upconversion. This dissertation will first present a historical perspective in the investigation of various polycyclic aromatic systems that exhibit interesting aromaticity. Then, in chapter 2, I will detail the synthetic route and corresponding mechanism during the preparation of thio-QDM. The characterization and photophysical properties including the transient absorption spectra and excited state dynamics and kinetics of these chromophores will also be discussed. In chapter 3, I will discuss the use of QDM as light-harvesting energy donor to sensitize perylene and achieve triplet-triplet annihilation photon upconversion. The mechanism of this non-linear photophysical process will also be highlighted. Finally, the conclusion chapter will introduce future perspectives in the synthesis of derivatives of QDM