Synthesis, Structural Characterization and Tuning Electronic Properties in Planar and Large Porphyrinoids

Abstract

The thesis is entitled as “Synthesis, Structural Characterization and Tuning Electronic Properties in Planar and Large Porphyrinoids”. These larger porphyrinoids have a greater number of heterocyclic units and the meso positions in compare to porphyrin ring. The replacement of the pyrrole unit with thiophene/furan/selenophene is result the core modification of these porphyrinoids. Therefore, expansion and modification of the ring affect the electronic and structural properties of these macrocycle. This thesis mainly focuses on the expanded planar and core modified macrocycles including hexaphyrin, octaphyrins and decaphyrins. These macrocycles were synthesized through multistep synthesis utilizing small precursor units. As the macrocycle's conjugated backbone enhances its redox nature, the oxidation of these macrocycles was thoroughly addressed. The isolation of these macrocycles were done with column chromatography and characterized with the mass spectroscopy, NMR spectroscopy, UV-vis absorption spectroscopy, X-ray single crystal diffractometer, cyclic voltammetry and spectroelectrochemistry. Quantum chemical calculation provides additional support for all of the experimental findings. This thesis comprises five chapters, the first chapter include the literature overview of porphyrins and isophlorins, including its structural and electronic properties. Subsequently, the reports on expanded porphyrinoids and discussion related to their synthesis and structural properties. It includes the strategic modification in expanded porphyrinoids to study structure dynamics. This chapter well describe the properties of porphyrinoids to stabilised the radical species and further its enhanced two -photon absorption values of these porphyrinoids. The second chapter of thesis states the intriguing properties of expanded porphyrinoids as Huckel and Mobius aromaticity. It describes the transformation of Huckel to Mobius macrocycle with the effect of pH, solvent, kinetic and thermodynamics conditions. It includes the synthesis of planar and core-modified hexaphyrin with 30π electrons, incorporated the two pyrrole and four furan rings in the conjugated ring, and expressed as Huckel aromatic macrocycle. Unexpectedly, the two electrons reversible transition of Huckel aromatic hexaphyrins led the formation of Mobius aromatic hexaphyrin, with the predominately ring oxidation. Along with NMR and single crystal studies, computational analysis of both hexaphyrins provide insight into their aromatic behavior through AICD plots and NICS values.. This aromatic transition was accomplished via an unstable intermediate radical cation, as confirmed by spectro-electrochemical experiments. The synthesis of four and eight membered isophlorinoids, including all of the furan units in the ring is thoroughly described in the third chapter of the thesis. These antiaromatic macrocycles include 20π and 40π electrons in the conjugated backbone. The octaphyrin exhibits the intriguing characteristic of solvatomorphism, presenting different solid-state conformations depending on the solvent of crystallization. In contrast, the functionalized meso-position, i.e., p-Br-phenyl, in these isophlorins makes them active in coupling reactions, and thus they could form the porphyrin-isophlorin dimer when coupled with borylated porphyrins. The fourth chapter of the thesis describes the synthesis of larger and planar decaphyrins with 44π and 46π electrons, which were synthesized with lesser meso-position and non-pyrrolic heterocyclic units. These are the first reports of decaphyrins with planar topology that have been studied using single crystal x-ray diffractometers.. Experimental studies of 46π decaphyrins demonstrate its radical character with a g value of 2.0054 in EPR measurements, while computational studies reveals a small ΔE(S-T) gap and 69% diradical character for the macrocycle. The synthesised decaphyrins reversibly show the 2e- oxidation processes and all the possible electronic stages of these decaphyrins were characterized with the spectroelectrochemistry. The 5th chapter explains the well-versed strategy to synthesis porphyrin-isophlorin dimer through the suzukii-miyuara cross coupling of functionalized isophlorin and porphyrins. The electronic coupling between 20π isophlorin and 18π porphyrin was studied with UV-vis absorption and NMR experiments. Interestingly the isophlorin-porphyrin dimer undergo the oxidation and form dication of the dimer. This dimer could act as captivating materials for investigating electron transfer mechanism in artificial photosynthesis and also in optoelectronis. Hence the thesis demonstrates the synthesis of aromatic and antiaromatic models of expanded porphyrinoids. The tailoring of electronic and structural properties of expanded porphyrinoids discovers the possibility of diradical character and solvatomorphism in these expanded derivatives.