DIVERSE FUNCTIONALIZED PORPHYRINS FOR CATALYTIC, EET AND SENSOR APPLICATIONS

Abstract

Porphyrins and their metal derivatives are of considerable importance owing to their applications in various fields such as catalysis, sensors, non-linear optics, light emitting diodes and dye sensitized solar cells (DSSCs). In living systems, porphyrins are very vital to many enzymes and biomolecules which perform significantly important physiological processes like photosynthesis, gas transport, redox reactions and so on. Porphyrins are chemically versatile molecules due to their conformational flexibility and rich thermal and chemical stabilities. They also have remarkable optical, photophysical and electrochemical redox properties which make them robust precursors for obtaining industrially, medicinally and scientifically suitable chemicals. On account of the above properties, porphyrins are the most studied molecules in chemistry and are made to undergo innumerable amount of reactions. Herein, we report the synthesis and characterization of meso- and/or β-functionalized porphyrins and their utilization in a variety of applications like catalysis, energy transfer and sensing of anions and cations. The proposed thesis will consist of the following chapters. Chapter 1 gives a general introduction to the field of porphyrins, their synthesis and different functionalization procedures undertaken to mould them into desirable target molecules. It also gives a peek into the material applications of porphyrins including dye sensitized solar cells, photodynamic therapy, catalysis, molecular sensing and nonlinear optics. Chapter 2 describes the synthesis of vanadyl-5,10,15,20-tetrakis(3,5-dimethoxyphenyl) porphyrin and its hexadecabromo derivative through facile synthetic procedures. The synthesized compounds were characterized by various spectroscopic techniques including UV-Vis., FTIR, EPR spectroscopy, MALDI-TOF mass spectrometry and single crystal X-ray diffraction analysis. The hexadecabromo derivative is highly nonplanar as observed from its crystal structures and electrochemical studies. Catalytic studies (alkene epoxidation) were successfully carried out in CH3CN/H2O solvent mixture and ambient temperature resulting in significantly enhanced TOF numbers even with low catalyst loading. Remarkably, the hexadecabromo compound biomimics Vanadium Bromoperoxidase (VBrPO) enzyme with very high TOF value vi for oxidative bromination of thymol. Both the catalysts were successfully recovered at the end of the reactions indicating their viability and industrial applicability. Chapter 3 describes the synthesis, characterization and catalytic applications in the epoxidation of alkenes by tetrasubstituted porphyrin molecules viz. vanandyl tetrabromotetraphenylporphyrin and vanadyl tetracyanotetraphenylporphyrin. The synthesized porphyrins are functionalized with oppositely natured ligands and were analyzed for their catalytic application in the epoxidation of differently structured alkenes. The catalytic reactions were carried out in CH3CN/H2O mixture in 3:1 ratio. Both displayed good efficiency in terms of mild reaction conditions, lower temperature and little catalyst amounts. Both the catalysts exhibited excellent selectivity, high conversion efficiency and huge TOF numbers in significantly less reaction times of 0.5 h. This work highlights that nature of the substituent (electron withdrawing or electron releasing) affects the efficiency of the catalyst whereas the ligand strength (strong or weak) does not influence it. Chapter 4 describes the synthesis of a new series of aryloxyporphyrins bearing benzyl and naphthyl substituents viz., Zn(II)tetrabenzylporphyrin (ZnTBPP), Zn(II)tetranaphthylporphyrin (ZnTNPP), and Zn(II)octanaphthylporphyrin (ZnONPP). The synthesized porphyrins have been utilized as donors to interact with fullerene C60 acceptor to form 1:1 complex in toluene at 298 K. The subsequent investigation into fluorescence quenching measurements with concomitant increase in fullerene concentration revealed effective quenching constants. The calculated association constants were in the order of 103 M‒1. However, ZnTNPP exhibited higher binding constant as compared to other analogues due to effective π-π interactions. ZnONPP exhibited 3−4 folds lower association constant as compared to ZnTNPP due to steric hindrance offered by meso-3,5-dinapthyloxyphenyl groups. The geometric and electronic structure of Zn(II) porphyrin-fullerene dyad was optimized by DFT calculations which suggested the possibility of charge transfer from meso-aryloxyporphyrin core to fullerene C60. Chapter 5 describes the synthesis of pyrenyl porphyrin and its metal complexes. Pyrene is a polyaromatic hydrocarbon (PAH) and PAHs have made a lasting impression in the field of molecular electronics and organic semiconductors. They have an extended π-electron cloud which reduces their HOMO−LUMO gap and makes them more suited for such devices. They also have very high charge mobility of the order of 10 cm2V-1S-1 in certain cases. The synthesized porphyrins were studied for their relative electrochemistry with tetraphenyl vii porphyrins. DFT optimized geometries revealed that the porphyrin core and the pyrenyl substituents remain essentially orthogonal to each other in both free base and metallated porphyrins. Fӧrster energy transfer studies in toluene displayed efficient energy transfer of the order of 80 to 85 percent in case of free base pyrenylporphyrin and zinc pyrenylporphyrin. Furthermore, it was observed that energy transfer occurs mainly via through-bond (TB) interaction and very less via through-space (TS) interaction. Chapter 6 describes the facile synthesis of nitrovanillin porphyrin. The hydroxyl group bearing porphyrin was synthesized in good yield and successfully utilized in colorimetric “naked eye” detection of toxic CN‒ and F‒ ions. The synthesized probe detects CN‒ and F‒ ions via anion induced deprotonation mechanism, which brings about drastic spectral and colorimetric changes. Other commonly known interfering anions such as OAc‒, H2PO4‒, Cl‒, Br‒, I‒, ClO4‒, HSO4‒ and PF6‒ failed to bring such changes. The toxic CN‒ and F‒ ions interact selectively with the –OH protons as was established via 1H NMR, UV-Visible and fluorescence spectroscopic methods. The sensor could detect F‒ ions up to ~1.1 ppm and CN‒ ions up to ~1.34 ppm which is very appreciable and promising. Chapter 7 descirbes the synthesis of Ni(II) porphyrins having fused –NH groups. The synthesized porphyrins were used to detect species of opposite polarity. Fused –NH porphyrin was used to sense toxic anions viz. cyanide and fluoride ions, whereas fused –CHO porphyrin was used for the detection of some selective metal ions including toxic mercury(II) ions. Acidic –NH protons detect anions via hydrogen bonding interactions which leads to deprotonation. On the other hand, sensing of metal species takes place via weak charge transfer interactions between oxygen atom of formyl group and the metal ions. Water washing reverses the interactions and regenerates the original porphyrins which makes them reversible probes. The detection limit (LOD) was found to be 2.13 ppm for cyanide and 3.15 ppm for fluoride ions, respectively. Similarly, the detection limit was found to be 0.930 ppm, 2.231 ppm and 0.718 ppm for Cu(II), Fe(III) and Hg(II) ions respectively. Chapter 8 serves as the concluding chapter summarizing the results of this work and also the future perspectives.