SYNTHESIS, SPECTRAL AND ELECTROCHEMICAL REDOX PROPERTIES OF β-FUNCTIONALIZED PORPHYRINS AND THEIR UTILIZATION IN NONLINEAR OPTICS AND CATALYSIS

Abstract

Porphyrins and its analogues are naturally occurring macrocycles and it involved in a wide variety of important biological processes and material applications. 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. β-Functionalized 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. Herein, we focused on the synthesis of β-functionalized tetra-/octa-arylporphyrins, π-extended porphyrins, Meso-β- Oxo-fused porphyrins/benzoporphyrins derived from β-formyl porphyrin in order to tune their structural, photophysical and electrochemical redox properties and their utilization in a variety of applications like nonlinear optics (NLO), and catalysis. The proposed thesis will be consisting of the following eight chapters. Chapter 1 gives a general introduction about the porphyrins, their synthesis and functionalization mainly β-functionalized porphyrins and their potential applications in nonlinear optics (NLO) and catalysis. Chapter 2 describes the facile synthesis of β-functionalized octaphenylporphyrins and its metal (NiII, CuII and ZnII) complexes. These octaphenylporphyrins were characterized by various spectroscopic techniques viz. UV-Vis, fluorescence and NMR, matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, cyclic voltammetry, and density functional theory (DFT) studies. These porphyrins exhibited interesting photophysical and electrochemical redox properties. All synthesized porphyrins exhibited a bathochromic shift (λmax = 23–38 nm) in the B band and (λmax = 21–83 nm) in the last Q band in comparison to MOPP due to attachment of the bromo and nitro groups on its peripheral positions. The octaphenylporphyrins derivative is highly nonplanar with Δ24 = ±0.533–0.646 Å, and ΔCβ = ±1.133–1.303 Å as observed from DFT. First time octaphenylporphyrins have been investigated for NLO studies. The observed structure-dependent nonlinear refractive index (n2) values are in the range of ~ 0.37–1.68 × 10− 19 m2 W−1, and two-photon absorption coefficient (β) values are in the range of ~ 0.19–4.56 × 10−12 m W−1. These result clearly indicates that octaphenylporphyrins showed excellent NLO materials in photonic devices Chapter 3 describes the synthesis, characterization and catalytic applications in the oxidative bromination of phenol & its derivatives by octasubstituted porphyrin molecules viz. V(IV)O(OPP) (1), V(IV)O(OPP)Br4 (2), and V(IV)O(OPP)Cl4 (3). The porphyrin 2 and 3 showed bathochromic shift in the absorption spectra (both B and Q-band) and anodic shift in the cyclic voltammograms as compared to porphyrin 1. The oxidation state of the vanadium(IV) were confirmed by EPR-spectroscopy and exhibited an axially compression with dxy 1 configuration. The catalytic reactions were carried out in KBr, H2O2, HClO4 mixture. Remarkably, porphyrin 3 bio mimics Vanadium Bromoperoxidase (VBPO) enzyme with very high TOF value (up to 86869 h–1) for oxidative bromination of thymol and other phenolic derivatives. Among them catalyst 3 exhibited higher TOF values and better catalytic activity. Chapter 4 deals the one pot synthesis of 2-cyano-tetraphenylporphyrin H2TPPCN (1) using nucleophilic substitution reaction of tetrabutylammonium cyanide (TBACN) on 2-nitrotetraphenylporphyrin and also synthesized oxidovanadium(IV) complexes V(IV)OTPPCN (2). In the thermogram, V(IV)OTPPCN (2) exhibited high thermal stability up to 348 °C. The V(IV)OTPPCN (2) was utilized for the selective epoxidation of different olefins with good TOF values (1.9 s–1) in good yields even with low catalyst loading in a CH3CN/H2O solvent mixture in the presence H2O2 as an oxidant and NaHCO3 as a promoter. The bromination of phenol using KBr/H2O2/HClO4 in water resulted in 100% conversion with a TOF value as high as 19.6 s–1 in 0.5 h. The catalyst 2 was recovered successfully at the end of the reaction up to 5 cycles and had good thermal stability, indicating its industrial viability and applicability. Chapter 5 deals with the synthesis, spectral and electrochemical redox properties of β- dicyanovinyl (DCV) appended porphyrins with mixed substituted pattern. These mixed β- substituted ‘push-pull’ porphyrins and its metal complexes having phenyl and DCV group present at its β-positions have been synthesized via Suzuki-coupling and Knoevenagel condensation and confirmed by different techniques. The introduction of β-DCV/phenyl entities leads to perturbation in the absorption and redox properties. MTPP(Ph)2MN and MTPP(Ph)4MN derivatives exhibited (5–14 nm) red shift in the B-band and (5–25 nm) in the last Q-band and also cathodically shifted in the first oxidation potential comparison to MTPPMN due to the increasing number of phenyl groups, it indicates increasing π conjugation/delocalisation as well as non-planarity on the porphyrin skeleton as compared to MTPPMN. Chapter 6 describes the synthesis of meso-β-Oxo-fused-porphyrins and its derivatives with different substituents like amino-group, benzo-group and also elucidate the effect of these β- substituents like Oxo-fused, -NH2 and benzo on the macrocyclic ring. Optical absorption spectra exhibit broadening of the Soret band and redshifts in both Soret and Q bands in meso- β-Oxo-fused-porphyrins relative to formyl porphyrins due to delocalization of π-electrons via the double bond or ICT. In cyclic voltammetric studies, meso-β-Oxo-fused-mono-benzo porphyrins (3) showed more anodic shifts in their reduction-potentials relative to others. DFT confirms that meso-β-Oxo-fused-mono-benzo porphyrins (3) exhibited more non-planarity with Δ24 = ±0.162–0.348 and ΔCβ = ±0.198–0.270 Å, because more substituents/repulsion comparative to porphyrin (1) and (2). Chapter 7 deals with the synthesis and characterization of a new family of β-trisubstituted π- Extended TCBD (1,1,4,4-tetracyanobuta-1,3-diene) porphyrins, MCHO(PE)TCBD (M = 2H, Cu(II), Zn(II) and Co(II)) and see the effect of β-substituents like- CHO, PE and TCBD on the porphyrin core by spectral and electrochemical redox properties. A new and easy synthetic route to β-trisubstituted TCBD porphyrins begins with the cycloaddition [2 + 2] of TCNE they follow by retro-electrocyclization reactions with H2CHOPE2. These synthesized porphyrins have high dipole moment, blue-shifted in the absorption spectra and in the cyclic voltammograms three reductions shows in the range of (-0.11 to -1.07) which confirmed TCBD group present on porphyrin. In this case HOMO-LUMO energy gap increases due to stabilization of the HOMO energies. Chapter 8 summarizes the results obtained in the proposed thesis with future perspectives.