SYNTHESIS AND APPLICATIONS OF β- AND MESOSUBSTITUTED PORPHYRINS

Abstract

Substituted porphyrins have unique electronic and optical properties that empower their use as model compounds of biological importance and potential material applications. The advantage of using porphyrin ligands is that they are conformationally flexible and can adopt a range of nonplanar conformations needed for a variety of biological functions. The structural, spectral and electrochemical redox properties of porphyrins can be tuned by peripheral functionalization of porphyrin ring. Particularly, the β- functionalization of meso-tetraphenylporphyrins exerts much larger steric and electronic effects on the porphyrin π-system. Further, the substituted porphyrins with appropriate binding pockets have been the subject of intensive research to design anion receptors with high selectivity owing to their potential applications in environmental, clinical, chemical and biological sectors. Meso- or β-Substituted ‘push-pull’ porphyrins have been utilised as sensitizers in porphyin-sensitised solar cells (PSSC) and photodynamic therapy (PDT), as chemosensors in anion sensing, as non-linear optical (NLO) material and as catalysts for oxidative transformation of organic substrates due to their outstanding properties such as strong absorption in visible region, flexible architectural modification to tailor physicochemical and optoelectronic parameters and high thermal and chemical stability. The thesis consists of eight different Chapters. First Chapter 1 is introductory one which deals with the recent chemical literature on the meso- and β-pyrrole functionalised porphyrins and their potential applications in the arena of PSSC, PDT, NLO, anion sensing and catalysis. v Chapter 2 describes about the synthesis, characterization and tunable electrochemical redox properties of two new families of porphyrins with mixed substituent pattern viz. 2-nitro-12,13- disubstituted-meso-tetraphenylporphyrins (H2TPP(NO2)X2, X = Ph, phenylethynyl (PE), 2- thienyl (Th), Br and CN) and 2-nitro-7,8,12,13,17,18-hexasubstituted-mesotetraphenylporphyrins (H2TPP(NO2)X6, X = Br, Ph, PE and Th) and their metal (Co(II), Ni(II), Cu(II) and Zn(II)) complexes. Remarkable red-shift in the Soret band (45 - 70 nm) and longest wavelength band, Qx(0,0) (65 - 90 nm) are exhibited by H2TPP(NO2)X6 as compared to H2TPP(NO2). The single crystal structures of MTPP(NO2)X2 (M = Zn(II) and Ni(II); X = Br, Ph and PE) showed quasi-planar conformation whereas H2TPP(NO2)Th2 and NiTPP(NO2)Br6 exhibited moderate and highly nonplanar saddle shape conformations, respectively. The imino proton resonances of H2TPP(NO2)X6 are significantly downfield shifted (Δδ = 1.10 - 1.80 ppm) relative to H2TPP(NO2). Mixed substituted highly nonplanarporphyrins exhibited higher protonation and deprotonation constants relative to H2TPPX8 (X = Cl and Br). The redox tunability was achieved by introducing electron donors (Ph and Th) and acceptor groups (PE, Br and CN) on MTPP(NO2) back bone. The unusual variation in spectral and electrochemical redox properties of mixed substituted porphyrins are interpreted in terms of both an inductive and resonance interactions of susbtituents on porphyrin π-system as well as nonplanarity of the macrocycle. Chapter 3 deals with the synthesis of highly electron deficient β-substitutedNi(II) porphyrins (1-5) and their utilization asnovel sensors for selective rapid visual detection of CN‒ions. This chapter describesthe single crystal X-ray structures, electronic spectraland electrochemical redox properties of these sensors. The ratiometric and vi colorimetric responses of these porphyrins were monitored by change in optical absorption spectra. These sensors were found to be highly selective for cyanide ions with extremely high binding constants (1016- 108M-2) through axial ligation of CN‒ ions and are able to detect < 0.11 ppm of CN‒ ions. 1-5 were recovered from 1-52CN‒ adduct by acid treatment and reused without loss of their sensing ability. CN‒ binding strongly perturbs the redox properties of parent porphyrin π-system. The applicability of 1-5 as a practical visible colorimetric test kits for CN‒ ions in an aqueous and non-aqueous media have also been explored.Single crystal X-ray, spectroscopic studies and DFT calculations were used to establish the mode of binding of CN- ions to these sensors. Chapter 4 considers the photophysical, electrochemical redox, solvatochromism and anion sensing properties exhibited by β-tetra and octa-phenylethynyl substituted mesotetraphenyl- porphyrins and their metal complexes (Co(II), Ni(II), Cu(II) and Zn(II)). H2TPP(PE)8 exhibite remarkable red shift in the Soret (Δλmax= 92 nm) band and longest wavelength band, Qx(0,0) (Δλmax = 117 nm), as compared to H2TPP. Interestingly, MTPP(PE)8 exhibited 450-500 mV anodic shift in first ring reduction potentials as compared to MTPP which is ascribed to electron accepting nature of phenylethynyl groups and extended π- conjugation whereas the first ring oxidation potentials remain unaltered. Free base and Zn(II) porphyrinsexhibits 10-20 times lower fluorescence quantum yields and 2-6 timeslower lifetime valuesthan that of MTPPs. These porphyrins display a strong solvatochromism as reflected by a large red-shift in their absorption and emission maxima upon increasing the solvent polarity. These porphyrins exhibited lower radiative vii rate constants (kr) and enhanced nonradiative rate constants (knr) as compared to MTPP. The decrement in fluorescenece lifetime values, quantum yields, radiative rate constants(kr) and profound solvatochromism with enhanced nonradiative rate constants (knr) have been interpreted in terms of intramolecular charge transfer (ICT) from porphyrin core to phenyethynylmoities. Further, ZnTPP(PE)8 was utilized for the colorimetric “naked-eye” detection of CN‒, F‒, Cl‒, Br‒, H2PO4 ‒ and CH3COO‒ ions through axial coordination of anionsto Zn(II) metal centre.The electron deficient nature of Zn(II) centre of ZnTPP(PE)n was established by axial ligation studies with anions in dichloromethane. Chapter 5 deals with a novel series of ‘push-pull’ Zn(II) porphyrin dyes containing various electron donors such as pyrenyl, 2-thienyl, phenyl, 4ʹ-bromophenyl and 4ʹ- tbutylphenyl and 4ʹ-carboxyphenyl acceptor moieties have been designed and synthesized (in two steps). Their optical absorption spectra, electrochemical redox and photovoltaic properties have been investigated in detail. The overall power conversion efficiencies (η) of DSSCs based on these dyes are in the range of 2.2 to 4.3% and highly dependent upon their donor moiety. The incorporation of 10,20-dimesityl groups is highly beneficial to prevent close π-π aggregation, thus favorably suppressing charge recombination and intermolecular interaction. Among all, pyrenyl appended Zn(II) porphyrin has exhibited higher power conversion efficiencies of 4.3% under 1 sun illumination due to electron donating ability of pyrenyl moiety and extended π-conjugation. viii Chapter 6 describes the synthesis and characterization of 2,3,7,8,12,13,17,18- octachloro-meso-tetraphenylporphyrinato oxovanadium(IV) (VOTPPCl8). VOTPPCl8 exhibited dramatic anodic shift 0.5 V in first ring reduction potential whereas 0.22 V anodic shift in first ring oxidation as compared to VOTPP indicating the electron deficient nonplanar conformation of the porphyrin core. We have optimized the catalytic conditions and found VOTPPCl8 is more selective towards epoxidation with very high yield and turnover frequency (TOF) due to highly electron deficient nature and robust structure. O O O O 0.002 mol% VOTPPCl8, 2 equiv. 30% H2O2 0.4 equiv. NaHCO3, CH3CN/H2O, 60 C, 0.5 h Olefins Epoxides 85 - 100% Conversion 100% Selectivity Chapter 7 considers the facile synthesis of Meso-tetrakis(4’-diethoxyphosphorylphenyl) porphyrin (H2TPhPP) and its metal complexes MTPhPP (M = Cu(II), Ni(II) and Zn(II)) via modified Lindsey method with higher yield (50%) as compared to literature methods. H2TPhPP exhibited 120-140 mV anodic shift in first ring redox potentials as compared to H2TPP. Similarly, MTPhPP (M = Ni(II), Cu(II) and Zn(II)) exhibited 70-210 mV anodic shift in first ring oxidation potentials and 80-120 mV in first ring reduction potentials as compared to corresponding MTPPs due to strong electron withdrawing nature of diethoxyphosphoryl substituents. Chapter 8 summarizes the results and conclusions of the present study