Synthesis and Photophysical Properties of Functional Tetracoordinate Boron Containing Organic Compounds

Abstract

ABSTRACT Over the past few decades, exploring pathways to access the triplet excited states of organic chromophores has been an appealing area of research. In this regard, tetracoordinate boron containing organic compounds have emerged as a fascinating class of luminescent molecules and photosensitizers. Overlap of the empty p-orbital of boron with π- conjugated core in tetracoordinate boron containing organic compounds leads to the delocalization of the electron cloud and planarization of the π-systems thereby resulting in interesting photophysical properties. Boron coordination also results in red shifted absorption and emission as compared to the corresponding chelating units. BODIPYs are renowned boron containing fluorescent dyes with strong and tuneable absorption in the visible region, high thermal and photo-stability and exceptional fluorescence quantum yields, and they have been turned into efficient triplet photosensitizers by appropriate design strategy with strong absorption in the visible to NIR region. A major drawback with BODIPYs is low reactions yields and the precise reaction conditions that are necessary for their synthesis. This issue could be overcome by chelating boron to simple organic ligands. Among them, N,O-chelated salicylideneimine-boron difluoride complexes, popularly known as boranils, are interesting as they can be easily synthesized in good yields via one pot synthesis using simple, commercially available starting materials like amines and aldehydes. This methodology enables the synthesis of a wide variety of molecules whose properties can be easily tuned across the entire range of electromagnetic spectrum with suitable derivatisation. While they are popularly known for their luminescence properties, their photosensitisation properties are largely unexplored to date. Considering these facts, this thesis dissertation is devoted to the design and synthesis of novel tetracoordinate boron-containing organic compounds with the objective of tuning their photophysical properties through systematic variations in their chemical structures. Chapter 1 discusses the importance of triplet states and photosensitized generation of singlet oxygen for applications in various fields. An extensive overview of BODIPYs as photosensitizers has been presented from the literature. Common strategies adopted to achieve populated triplet states with a special focus on halogenation and transition metal complex incorporation in BODIPYs are also discussed. We have also thrown some light on the structural features and the associated photophysical properties of a few boranil derivatives. Chapter 2 describes the synthesis and characterization of appropriately functionalized BODIPY and boranil derivatives. The compounds synthesized include various boron difluoride complexes ranging from heavy metal and transition metal complex incorporated BODIPYs A- C to N,O-chelated salicylideneimine boron difluoride complexes D-G to diiodosalicylideneimine-boron difluoride functionalized polyethyleneimine H. xiD A C B E G F H Chapter 3 discusses the photophysical properties of these compounds with respect to their absorption, emission and singlet oxygen generation abilities and our results underline the importance of appropriate functionalisation on the BODIPY/boranil skeletons. The ease of synthetic functionalization at meso and pyrrolic positions of the BODIPY resulted in diverse structural and photophysical features. High CO release efficiency along with singlet oxygen generation under biologically relevant visible light were obtained from single component photoCORMs A and B whereas the peculiar structure of hexabrominated BODIPY C exhibited drastic changes in the luminescence and photosensitizing abilities upon aggregation. On the other hand, the boranil derivatives typically exhibited bright emission in the blue region with good photostabilities and high singlet oxygen generation quantum yields. Chapter 4 focuses on the potential applications of the synthesized molecules for degradation of water contaminants, as antimicrobial coatings and as antibacterial photodynamic agents. BODIPY complexes A and B were incorporated into non-woven fabrics using PMMA and the fabrics showed excellent biocompatibility to l929 cell lines and were strongly cytotoxic to c6 cancer cell lines and E. coli bacteria under light exposure. PMMA films incorporating the iodo-functionalized boranil G were capable of serving as a reusable heterogeneous medium for the photosensitized degradation of organic water pollutants. Furthermore, multilayer nanofilms incorporating boron-functionalized polymer H grown inside the surface of the glass vials generated reactive oxygen species upon exposure to visible light. The coated glass vials showed exceptional anti-microbial action against both gram- positive and gram-negative bacteria, functioning as self-cleaning vials.