Design, Syntheses and Studies of Supramolecular Systems for Light Harvesting, Photo-controlled Assembly-Disassembly and Sensing Properties

Siva Rama Krishna., V. (2016) Design, Syntheses and Studies of Supramolecular Systems for Light Harvesting, Photo-controlled Assembly-Disassembly and Sensing Properties. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The aim of this thesis is to explore and focus on the development of photophysical and photochemical properties of highly organized supramolecular assemblies and architectures. Chapter 1 presents a general introduction of the subject largely focused on supramolecular photochemistry and a general introduction of the systems used in this work. Supramolecular assemblies such as the light harvesting complexes found in purple bacteria can be found widely in nature. Mimicking these complexes is still challenging because of the precise positioning of the series of the chromophores in the three-dimensional space. To mimic the natural lightharvesting system, a coumarin based tripodal molecule with a 7-diethylaminocoumarin at its centre and three coumarin units at its periphery has been synthesized. The molecule selfaggregates in the water with efficient sequential energy transfer, akin to the light harvesting complex in purple bacteria. This has been discussed in Chapter 2. Construction of a novel class of supramolecular assemblies using self-organization of π-conjugated molecules is an emerging field of research. Light-triggered assembly-disassembly using a dimethyldihydropyrene photochromic π-switch has been demonstrated in chapter 3. There, the photochromic donor molecule (D) and an electron deficient acceptor dye (A) forms an ordered donor-acceptor π-stack using a photo-responsive dimethyldihydropyrene π-switch that disassembles on exposure to visible light. Upon exposure to the UV light, the stacks can be formed again. Chapter 4 deals with a chemosensor for a heavy metal ion. Owing to the toxicity to the environment and biological systems of the heavy metal ions, development receptors for their sensitive and selective detection is an important field research. A tripodal systems based on alkyne-azide 1, 3 dipolar cycloaddition was designed and synthesized. The receptor displayed a selective binding to silver ions. The synthesized triazole tripod was shown to provide a strong binding to Ag⁺ ions. The enhancement of the fluorescence signal was achieved by using a supramolecular cyclodextrin ancillary for the metal-receptor complex as discussed in the chapter. The pyrimidine nucleic bases in RNA/DNA absorb ultra violet (UV) radiation that can trigger harmful photochemical reactions of the formation of photo induced [2+2] cyclobutane pyrimidine dimers in DNA/RNA. In Chapter 5, two derivatives of the nucleotide, uridine, containing a RET donor (D) and an acceptor (A) have been used to form an alternate π-stacked helical assembly of the D and the A molecules. A [2+2] photochemical cycloaddition reaction of the uridine bases resulted in a covalent linked D-A pairs. This brought in structural rigidity that resulted in the loss of the helical structures. The structure and the functions of the supramolecular assembly studied here have been established by several spectroscopic and spectrometric techniques such as NMR, UV-visible, steady state and time-resolved emission spectroscopy, dynamic light scattering (DLS), scanning electron microscopy (SEM) studies and mass spectrometry.

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