FACILE SYNTHESIS, STRUCTURAL, SPECTRAL AND ELECTROCHEMICAL STUDIES OF π-EXTENDED NIR ABSORBING PORPHYRINS AND THEIR UTILIZATION IN NONLINEAR OPTICS AND ELECTROCATALYSIS

Abstract

Porphyrins are naturally occurring tetrapyrroles, known for their versatile structure and intriguing properties, and they serve as the cornerstone of a wide array of functional materials and bioactive compounds. Peripheral functionalization of the porphyrin ring enables tuning its structural, spectral and electrochemical redox properties. Modulating the degree of π-conjugation and incorporating appropriate substituents at the β-positions lead to electronic spectral features shifting towards longer wavelengths, a reduced HOMO-LUMO gap and high ground state dipole moment, essential for enhanced nonlinear optical behavior. Extension of π-conjugation induced by intramolecular fusion has attracted much attention because of its unique photophysical and fascinating electrochemical properties, making them good candidates for an immense variety of applications, including photovoltaics, two-photon absorption and catalysis. The introduction of fused ring in the periphery of porphyrin by meso-β and β-β’ fusion afford porphyrins with enhanced chemical and thermal stability, near-infrared optical absorption and altered molecular orbitals renders them promising material with applications ranging from optoelectronics to sustainable energy conversion. Through a combination of synthetic methodologies, spectroscopic characterization and application-driven studies, this thesis contributes to a journey of π-extended NIR absorbing porphyrins and their utilization in nonlinear optics and electrocatalysis. The proposed thesis consists of the following chapters. Chapter 1 provides a comprehensive overview of tetrapyrroles, including laboratory synthesis of porphyrins and β-functionalization of porphyrins. It explores their potential applications in the arena of nonlinear optics (NLO), catalysis, photodynamic therapy (PDT) and dye-sensitized solar cells (DSSCs). Chapter 2 describes the synthesis, characterization and NLO studies of unsymmetrically β-functionalized π-extended porphyrins. The disubstituted porphyrins having two different functional groups at antipodal 2,12-position (MTPP(NO2)MA) and mono-nitro mono-benzo porphyrins (MTPP(NO2)MB) have shown a significant bathochromic shift in their Soret and Q-bands compared to precursor due to the asymmetric electronic distribution and extended π-conjugation. The single crystal X-ray analysis revealed that ZnTPP(NO2)MA and ZnTPP(NO2)MB adopted saddle-shape conformation. Synthesized MTPP(NO2)MA and MTPP(NO2)MB have exhibited an anodic shift in reduction and oxidation potentials compared to the corresponding MTPPs due to electron-withdrawing side arms and extended π-conjugation Interestingly, NiTPP(NO2)MA has shown an additional NiII/NiIII oxidation potential observed at 2.05 V. Furthermore, the NLO studies of all the synthesized porphyrins revealed that intense reverse-saturable absorption behavior and the self-focusing behavior with positive nonlinear refractive index. Among all, zinc porphyrins have shown the highest two-photon absorption coefficient (β) and cross-section (σTPA) values 95 × 10−12 m/W and 19 × 104 GM, respectively. The detailed NLO investigation suggests that these porphyrins stand superior to recent literature reports as promising NLO materials.Chapter 3 deals with the synthesis, characterization and electrochemical studies of mono meso- β fused porphyrins and their metal complexes and explores their third-order nonlinearities. The mono-malononitrile (MN) substituted porphyrins are prepared as major products under controlled reaction conditions from corresponding 2-nitro porphyrins following the Michael addition reaction. Then oxidative fusion was performed using DDQ to synthesize β-to-orthophenyl monofused porphyrins. The single crystal structural analysis revealed that the core adopted saddled geometry. A significant redshift in the absorption and emission spectral features is observed due to the peripheral fusion and extended π-conjugation. The optimized structures showed high dipole moment values in the range of 3.71-8.26 Debye (D). Further, the zinc complexes of each series exhibited intense reverse saturable absorption (RSA) and selfdefocusing behavior with a negative nonlinear refractive index. The extracted NLO data suggests that the mono-fused porphyrins showed promising nonlinear optical properties.Chapter 4 includes facile synthesis, spectroscopic characterization and intriguing electrochemical redox properties of novel π-extended nonplanar porphyrins and their excellent nonlinear optical properties. Two new series of nonplanar π-extended porphyrins having meso-β fused vinyl cyanide (VCN) units with two antipodal phenylethynyl (PE) side arms (M(VCN)2(PE)2) and one PE and one tetracyanobutadiene (TCBD) units (MTCBD) where M = 2H, CoII, NiII, CuII and ZnII and have been synthesized in good yield. In each case, the panchromatic absorption spectral features are observed due to the tunning of the energy levels through extended π-conjugation that pushes the low energy band to the NIR region. The effect of the extension of π-network is reflected in the cyclic voltammograms of the synthesized porphyrins. As a result, easier reductions and reduced HOMO−LUMO energy gaps are witnessed for all the investigated porphyrins than for the related parent TPPs. Interestingly, it is possible to accomplish a tetra anionic species in the case of MTCBD. The first reduction process is very facile and the peaks appear in the range of −0.11 V to −0.13 V, which is assigned to the reduction of the TCBD unit (TCBD0/−1). Subsequently, the second and third redox couples are assigned to the ring-centered reduction. Notably, all the optimized structures showed high ground state dipole moment values in the range of 10.8−15.2 D. The NLO studies of zinc complexes showed intense reverse saturable absorption and self-defocusing behavior with a negative nonlinear refractive index. An exceptionally high two-photon absorption coefficient (β) and cross-section (σTPA) values in the range of 428−523 × 10−11 m/W and 88−108 ×105 GM, respectively, are observed for both Zn(VCN)2(PE)2 and ZnTCBD.Chapter 5 describes the highly substituted nonplanar π-extended Ni(II) porphyrins: synthesis, characterization, and their utilization as a promising NLO material. β-to-ortho-phenyl monofused (2,3) and difused (5-7) hexa-substituted π-extended nonplanar Ni(II) porphyrins have been synthesized with moderate to good yield. Three compounds 2, 4 and 7 were unambiguously confirmed and characterized by single-crystal XRD analysis. Each structure (2, 4 and 7) was crystallized in a triclinic crystal system with the P1̅space group acquired nonplanar saddledshaped geometry. Notably, 7 displayed the highest twist angle of 35.60° compared to the related meso−β fused porphyrins. Porphyrins 6 and 7 showed panchromatic absorption spectra extending to the NIR region due to altered molecular orbitals and extended π-conjugation. β-substituents, peripheral fusion and extended π-conjugation substantially tune the oxidation and reduction potentials. The difused porphyrins show high dipole moment values in the range of 9.828−10.928 D compared to monofused porphyrins (4.865−6.767 D). The intensity dependent femtosecond third-order NLO studies revealed that NiDFP(VCN)2Br6 (6) shows the highest two-photon absorption coefficient (β ~ 22-33 × 10-10 m/W) and cross-section (σTPA ~ 3.77-6.95 × 106 GM) values among all.Chapter 6 deals with meso-β, β-β’ trifused porphyrins: synthesis, characterization and thirdorder nonlinear optical properties. The single crystal X-ray diffraction analysis confirmed the β- β’, meso-β fusion. The crystal structure of ZnTFPMB(MN)2 crystallizes in a triclinic crystal system with the P1̅space group having two molecules in the asymmetric unit. The structure adopted nonplanar ruffled geometry with ‘armchair’ conformation. Installing the peripheral fusion to the porphyrin macrocycles causes dramatic changes in the electronic spectral properties. Interestingly, the split Soret and broad Q bands were observed for the π-extended porphyrins MTFPMB(VCN)2 due to the peripheral fusion that shifts the optical spectra to the NIR region. The electrochemistry of the trifused porphyrins is highly affected by the fusion and extended π- conjugation. Notably, the HOMO-LUMO energy gap for the π-extended porphyrins has been tuned to ≤ 1.5 V due to the extended π-conjugation induced by peripheral fusion. DFT studies revealed that the trifused porphyrins are highly nonplanar with high ground state dipole moment values in the range of 8.6−11.85 D. The NLO studies of the investigated trifused porphyrins showed intense reverse saturable absorption (RSA) and self-focusing behavior with a positive nonlinear refractive index. Furthermore, the synthesized trifused porphyrins exhibited an efficient third-order nonlinearity.Chapter 7 describes π-extended nonplanar cobalt porphyrins immobilized on MWCNTs as efficient electrocatalysts for selective oxygen reduction reaction. Two new meso-β, β-β’ fused π- extended cobalt porphyrins (2Co and 3Co) have been synthesized and one of them is crystallographically characterized. The nanocomposites of nonplanar curved porphyrins immobilized multi-walled carbon nanotubes were thoroughly characterized spectroscopically and microscopically. Both the nanocomposites showed a ~200 mV positive shift in the oxygen reduction peak potential in aqueous media and a ~100 mV shift in the onset potential of the O2 reduction relative to the control meso-tetraphenylporphyrinatocobalt(II) nanocomposite. In each case, the nanocomposites efficiently catalyze the selective 4e−/4H+ oxygen reduction under ambient conditions with excellent methanol tolerance and high stability due to π-π interaction, which could be an alternative for expensive Pt-based cathode materials in fuel cells.Chapter 8 concludes the results obtained in the proposed thesis with future perspectives.