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

Abstract

Porphyrins occur in nature in the form of tetrapyrrolic pigments. The large π-electronic system makes them versatile for large number of applications in the field of medicine and material chemistry which include, photodynamic therapy, nonlinear optics, dye sensitized solar cells, and detection of toxic or useful ions. The eight β-pyrrolic positions and four meso-positions provide a platform for the fine tuning of photophysical and electrochemical redox properties. β Substituents exert larger steric and electronic effects in comparison to meso-substituents as they are in direct conjugation with the porphyrin ring. Herein, we focused on the synthesis of β functionalized chlorins and/or porphyrins derived from 2-nitroporphyrin or 2-formylporphyrin in order to tune their structural, photophysical and electrochemical redox properties and utilized for applications like nonlinear optics. The proposed thesis is organised into following eight chapters. Chapter 1 gives a basic description of porphyrins, laboratory approach for synthesis of porphyrinoids, and their utilization in various applications such as, nonlinear optics (NLO), photodynamic therapy (PDT), dye-sensitized solar cells (DSSCs), chemosensing, and catalysis. Chapter 2 deals with the smooth synthesis of β-TCBD (1,1,4,4-tetracyano-buta-1,3-diene) appended porphyrins, M-TCBD (M = 2H, Cu(II), Ni(II), Co(II), and Zn(II)) from 2,3 diphenylethynyl-12-nitro-meso-tetraphenylporphyrin, H2-PE2 and characterized by various spectroscopic techniques and electrochemical studies. The observed unusual reduction potentials in the cyclic voltammograms of synthesized porphyrins in the range of -0.06 to -0.10 V, was the consequence of TCBD moiety present at the β-position. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H2-PE2) can be attributed to the existence of intramolecular charge transfer in the absorption spectra, and enhanced polarization in M-TCBD series. The single beam femtosecond Z-Scan measurements were performed to elucidate the third-order nonlinear optical properties and the temporal response of these porphyrin molecules was investigated using optical pump-probe spectroscopy to study the excited state absorption dynamics. Z-Scan measurement revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrin. Two-photon absorption coefficient (β) and the imaginary part of third-order NLO susceptibility (χ(3)) were obtained from the open aperture experiment whereas the close aperture experiment delivered the magnitude and the sign of the nonlinear refractive index (n2) and the real part of χ(3). Further, the femtosecond transient absorption spectroscopy revealed faster relaxation dynamics of various absorption v

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processes in picosecond timescale. The excellent optical limiting threshold (1.90-2.33  1015 W/m2) of synthesized porphyrins suggested their usage for laser protection and high-power laser operation. Chapter 3 describes the synthesis of a series of β-trisubstituted “push-pull” porphyrins bearing triphenyl amine (TPA) and dicyanovinyl (DCN) groups at antipodal position and characterized by various techniques including UV/Vis, emission, CV, DFT, elemental analysis, and MALDI TOF-MS. The H2TPP(TPA)2MN was procured by palladium (0) catalysed Suzuki cross coupling of H2TPP(Br)2MN with TPA (triphenyl amine), further H2TPP(TPA)2MN was metalated with metal ions including, CuII, NiII, and ZnII. The MTPP(TPA)2MN (M = 2H, CuII, NiII, and ZnII) exhibited 21-48 nm and 38-80 nm bathochromic shift in B-band and Qx(0,0) band in comparison to MTPPs (M = 2H, CuII, NiII, and ZnII); the results are in consistent with the effect of enhanced resonance and -I effect of dicyano vinyl group. The first reduction of synthesized porphyrins is 0.23 to 0.7 V anodically shifted as compared to the MTPPs reason of which can be attributed to the presence of electron withdrawing group (DCN). The third-order nonlinear optical responses were determined using single beam femtosecond Z-scan technique to retrieve information about the nonlinear absorption, and nonlinear refraction of the samples. The two-photon absorption coefficients (β) are in the range of 0.87 × 10-13 to 4.28 × 10-13 mW-1 and the nonlinear refractive index (n2) in the range of 1.21 × 10-19 to 7.36 × 10-19 m2W-1. The ultrafast absorption dynamics of the ground state bleaching (GSB) and the photo-induced absorption (PIA) are monitored by femtosecond broadband transient absorption studies. The strong nonlinearity of these push-pull porphyrins endowing them as potential candidates for nonlinear optical and photonic device applications. Chapter 4 describes the synthesis of asymmetrically hepta/octa-β-substituted electron-deficient nonplanar porphyrinoids with good to excellent yield and characterized using UV-Vis, Fluorescence, NMR (1H and 13C), CV, MALDI-TOF-MS, SC-XRD and elemental analysis. The CuTPP(MN)Br6 and CuTPC(MN)2Br6 exhibit 18-23 nm hyposchromic shift in their B-band with reference to the CuTPP(NO2)Br6 (precursor) due to replacement of strong electro-withdrawing group (-NO2) by moderate electron-withdrawing group (MN). The synthesized porphyrinoids demonstrate 540 to 840 mV anodic shift in the reduction potential as compared to the MTPPs due to nonplanarity and π-extended conjugation. The nonplanarity of macrocycles has been confirmed by the density functional theory; Δ24 = ±0.391-0.571 Å and ΔCβ = ±0.336-1.131 Å. Notably, CuTPP(MN)Br6, and H2MFP(MN)Br6 stabilizes in saddle-shape conformation whereas CuMFP(MN)Br6, and CuDFC(MN)2Br6 prefers to be in ruffled shape conformation. The third order nonlinear optical properties (nonlinear absorption (2PA) and nonlinear refractive index (n2)) were measured using femtosecond laser pulses at 800 nm as irradiation source using Z-scan technique. The enhanced reverse saturable absorption (RSA) behaviour stems from the strong two-photon absorption (2PA ~ 0.1 to 2.7 x 10-13 m W-1) suggesting potential usage for nonlinear optical applications. The ultrafast transient absorption spectroscopy (TAS) provides the temporal dynamic evolution of strong ground state beaching (430 – 500 nm) of the Soret band along with the photoinduced absorption. Chapter 5 encompasses the synthesis of asymmetrically substituted porphyrins possessing EAA/acac (Ethylacetoacetate/acetylacetone) and six bromine atoms at β-position and their characterization using UV/Vis, Fluorescence, 1H NMR, CV, DFT, MALDI-TOF-MS and elemental analysis. The mechanistic pathway followed the nucleophilic substitution reaction (nucleophile: EAA and acac) with MTPP(NO2)Br6 (M = 2H, Cu(II), and Ni(II)) and the engendered hepta-substituted porphyrins exhibited keto-enol tautomerism, as supported by 1H NMR spectroscopy. The six bulky bromo and EAA/acac groups made the macrocyclic ring highly electron deficient and nonplanar that quantum yield (ϕf = 0.00014 to 0.00022) and fluorescence intensity for H2TPP[EAA]Br6 and H2TPP[acac]Br6 was severely reduced contrary to H2TPP. The poor electron-density and nonplanarity over the porphyrin ring shifted first oxidation potential from 11 to 521 mV anodically for MTPP[X]Br6 (M = 2H, Cu(II), and Ni(II); X = EAA or acac) as compared to corresponding MTPPs. Notably, density functional theory proved the nonplanarity of synthesized porphyrins as Δ24 spans from ±0.546 to ±0.559 Å while ΔCβ stretches from ±0.973 to ±1.162 Å. The third-order nonlinear optical measurements were done using femtosecond pulsed laser Z-scan technique at 800 nm and 1 kHz repetition rate to get insights into nonlinear absorption and nonlinear refraction of the porphyrins. The three-photon absorption coefficients (γ) are in the range of 2.2 × 10-23 to 2.8 × 10-23 cm3/W2 and nonlinear refractive index in the range of 3.7 × 10-16 to 5.1 × 10-16 cm2/W. The higher order nonlinear absorption exhibited by porphyrins helps in improvisation of resolution at depth for various photonic and optoelectronic applications. Chapter 6 presents the synthesis of hepta-substituted porphyrins [MTPP(NO2)X6; M = 2H, CuII, NiII, and ZnII; X = Br, Ph, and PE] and their third-order nonlinear optical properties using single beam Z-scan technique with femtosecond, MHz pulses in visible wavelength. Three-photon absorption (γ), third-order nonlinear optical susceptibility (χ3), three-photon absorption cross section (σ3) and nonlinear refractive index (n2) have been determined from their theoretical fits with experimental results. The sign and magnitude of nonlinear refractive index (n2) has been vii obtained from close-aperture experiment while three-photon absorption coefficient and three photon absorption cross-section (σ3) was determined from open-aperture experiment. The 3PA and σ3 extended from 2.7 to 3.4 (10-23 cm3/W2) and 5.5 to 7 (10-78 cm6s2) respectively. The higher magnitude of nonlinear optical coefficients assures their utility in optical and photonic applications. Chapter 7 includes the utilization of well-known Michael addition mechanism to synthesize indanedione (IND) based porphyrins and/or chlorins and also their corresponding fused porphyrinoids by varying the other six β-substituents viz. bromo (Br), phenyl (Ph), and phenylethynyl (PE) and described by electronic absorption spectra, emission spectra, 1H NMR, CV, DFT, elemental analysis and MALDI-TOF-MS. The synthesized porphyrinoids exhibited red shift (27 to 107 nm) in electronic absorption spectra as compared to their corresponding MTPPs due to the steric and electronic factors originated by β-substitution. Cyclic voltammetry exhibited multiple oxidation and reduction potential owing to the nonplanarity and enhancement in delocalization of π-electron. The first reduction potential transpired in the span of -0.256 to 1.18 V, can be attributed to the poor electron density on the porphyrin ring due to the presence of electron withdrawing groups (Br, and IND) and nonplanarity of porphyrin ring. The density functional theory proves the nonplanar conformation of the macrocycles as the displacement of β-pyrrole carbon atom stretches from ±1.2 to 0.24 Å while mean displacement of 24 core atoms lies in the range of ±0.633 to 0.314 Å. Direct fusion of H2DFC(IND)2X6 (X = Br and PE) from the reaction of H2TPP(NO2)X6 (X = Br and PE) with IND (indanedione) can be confirmed from 1H NMR and MALDI-TOF-MS. Chapter 8 concludes the results obtained in the proposed thesis with future perspectives.