β-FUNCTIONALIZED CHLORINS AND PORPHYRINS: SYNTHESIS, SPECTRAL AND REDOX PROPERTIES

Abstract

Porphyrins are naturally occurring tetrapyrroles and utilized as model compounds for many biological and material applications. Due to their conformational flexibility, porphyrins can adopt a range of nonplanar conformations which is needed for a variety of biological functions. β-Functionalization of porphyrins results into varying degrees of porphyrin core confirmation, intriguing photophysical and electrochemical redox properties. Herein, we focused on the synthesis of β-functionalized novel chlorins, porphyrins and benzoporphyrins derived from 2- nitroporphyrin in order to tune their structural, photophysical and electrochemical redox properties. The proposed thesis will be consisting of the following chapters. Chapter 1 deals with the general introduction to tetrapyrroles, laboratory synthesis of porphyrin, β-functionalized porphyrinoids and their potential applications in dye-sensitized solar cells (DSSCs), photodynamic therapy (PDT), chemosensing, nonlinear optics (NLO) and catalysis. Chapter 2 describes about the versatile synthetic route for β-functionalized chlorins and porphyrins by varying the size of Michael donors. Size dependent approach was applied for the fine-tuning of product formation from porphyrins to chlorins. Notably, we were able to isolate mono/trisubstituted porphyrin and/or di/tetra-substituted chlorin from one-pot synthesis. Singlecrystal X-ray diffraction analysis revealed the quasiplanar to moderate nonplanar conformation of chlorins due to trans orientation of the β-substituents, whereas porphyrins exhibited higher mean plane deviation of 24-atom core (Δ24) as compared to chlorins. β-Functionalized chlorins exhibited lower protonation constants and much higher deprotonation constants as compared to porphyrins revealing the combined effect of the conformation of macrocyclic core and the electronic nature of β-substituents. Facile synthesis of porphyrins and/or chlorins based on the size of Michael donor employed and in turn resulted in tunable photophysical and electrochemical redox properties are the significant features of the present work. Chapter 3 describes the facile selective conversion of planar trans-chlorins into highly twisted doubly fused-porphyrins or -chlorins via oxidative fusion. β-to-ortho-phenyl doubly fusedporphyrins (DFPs) or -chlorins (DFCs) were regioselectively synthesized in good to excellent yields (70-92%) under mild reaction conditions with high atom economy. The product selectivity was controlled by the presence or absence of Ni(II) ion in the macrocyclic core. Ni(II) transchlorins selectively yielded Ni(II) difused porphyrins NiDFPs whereas free base trans-chlorins vi afforded only free base difused chlorins H2DFCs. The synthesized fused porphyrinoids exhibited significantly red-shifted spectral features (Δλmax = 16-53 nm) of Soret band due to extended π- conjugation and highly twisted macrocyclic conformation (twist angle ~20-34º) with tunable electrochemical redox properties. Inner core NHs of fused chlorins exhibited tremendous downfield shift (Δδ = 1.71-2.02 ppm) as compared to their precursors. The overall protonation constants for H2DFC(IND)(X)2 (X = H, Br, Ph) were profoundly higher (~40-60 folds) as compared to H2DFC(MN)(X)2 (X = H, Br, Ph) due to the combined effect of electronic nature of the substituents and nonplanarity of the macrocyclic core. Overall this chapter describes the facile synthetic routes to electronically tunable β-modified porphyrins and chlorins. Chapter 4 deals with nickel acetate catalyzed autoxidation of free base trans-chlorins into monofused Ni(II) porphyrins. One flask synthesis of β-to-ortho-phenyl monofused porphyrins with indanedione functionalities has been achieved via metal mediated oxidative fusion of free base trans-chlorins. Extended π-conjugation of porphyrins aromatic circuit was accomplished by connecting β-pyrrolic indanedione groups with meso-ortho-phenyls. Compared to the unfused precursors i.e. trans-chlorins, these monofused porphyrins and their Co(II), Ni(II), Cu(II) and Zn(II) complexes have shown a spectacular bathochromic shift of Soret as well as the longest wavelength bands in the absorption spectra. NiMFP(IND)R2 (where R = H and Ph) were best described as typically ruffled due to their high Δ24 and ΔCβ as compared to the Zinc(II) complexes. Notably, Ni(II) complexes NiMFP(IND) (1) and NiMFP(IND)Ph2 (3) exhibited metal centered oxidation (NiII/NiIII) due to extended π-conjugation and electronic nature of β- substituents. Monofused Ni(II) complexes (Ni(II)MFP(IND)s) have shown slightly higher HOMO-LUMO gap (80-150 mV) as compared to corresponding difused porphyrins (Ni(II)DFP(IND)2s). Chapter 5 describes the synthesis of 2-nitro-benzoporphyrin using Heck coupling reaction of NiTPP(NO2)Br2 which further treated with active methylene compounds (malononitrile and indane-1,3-dione) to obtain the corresponding trans-benzochlorins. The oxidative fusion of benzochlorins using DDQ resulted into triply fused porphyrins. These doubly fused benzoporphyrins exhibited broadened and red-shifted UV-Vis spectra as compared to simple difused systems due to extended π-conjugation and enhanced nonplanarity provided by fused benzene ring at β-β‟ position. In contrast to planar trans-chlorins or simple doubly fused porphyrins, a nice alteration in Q/B ratio was observed for antipodal β-β‟ fused porphyrins. The vii values for Q/B ratio have been found very high in theses meso-β, β-β’ fused porphyrins which again described the impact of enhanced nonplanarity on the electronic properties of the macrocyclic skeleton. Chapter 6 deals with the synthesis of β-trisubstituted “push-pull” porphyrins. NiTPP(NO2)R2 (where R = Br and Ph) have shown nitroalkene type reactivity towards active methylene compounds (acetylacetone and ethyl acetoacetate) for Michael addition reactions to obtain corresponding porphyrins. Notably, in case of ethyl acetoacetate appended porphyrin, the high temperature favors the rapid cleavage of C-C bond which resulted into the dissociation of COCH3 group and formation of ester appended porphyrins instead of desired product. The diketone or ester has little influence on the absorption and fluorescence profiles inducing small bathochromic shifts and slight decrement in fluorescence intensity and quantum yield as compared to the precursor 2-nitroporphyrin whereas a noticeable effect of acetylacetone group on the redox properties have been observed. Diketone appended porphyrins have shown multiple oxidation and reduction due to the presence of keto-enol tautomerism. The impact of the antipodal β-substituent on the redox potentials has also been observed. Chapter 7 descirbes about the facile synthesis of perchloro-2-nitro-meso-tetraphenylporphyrin and its metal complexes (CoII, NiII, CuII and ZnII). The data compared with homosubstituted porphyrins i.e. MTPPCl8 to highlight the effect of nitro group over their conformation as well as their spectral properties. Nitro bearing porphyrins exhibited 10-15 nm red shifts in the electronic spectra and dramatic anodic shift in the reduction potentials. Free base perhaloporphyrins exhibited colorimetric responses toward highly basic anions such as CN˗, F˗, CH3COO˗ and H2PO4 ˗ ions and being able to detect these anions in nanomolar concentration. The red-shifted electronic spectral features, the higher β2 values for deprotonation and anion recognition were interpreted in terms of enhanced nonplanarity and electron withdrawing effect of NO2 and/or halo substituents. The large anodic shift in voltammetric studies and disappearance of 1H NMR signals of imino protons strongly support the anion induced deprotonation. The electron deficient Co(II) perhaloporphyrins were utilized as sensors for the selective rapid visual detection of cyanide ions for the first time in porphyrin chemistry. Chapter 8 concludes the results obtained in the proposed thesis with future perspectives.