β/MESO-FUNCTIONALIZED PORPHYRINS: SYNTHESES AND THEIR UTILIZATION IN ANION SENSING, NLO AND COMPLEXATION WITH FULLERENES

Abstract

Porphyrins are naturally occurring tetrapyrrolic pigments. They are highly π-conjugated system which makes them interesting and versatile. In living systems, porphyrins are very vital to biomolecules and many enzymes which play significantly important physiological process like redox reaction, photosynthesis, gas transport and so on. Porphyrins are chemically versatile molecule due to chemical and thermal stabilities and conformational flexibility. Porphyrins have optical, photophysical and electrochemical redox properties which make them robust precursor for obtaining medicinally, scientifically and industrially suitable materials. The photophysical and electrochemical redox properties of porphyrins can be altered by fine tuning in structural modification and electronic nature of meso/β-substituents. β-Substituents exerts much larger electronic and steric effects in comparison to substitution at meso-positions since they are in direct conjugation with porphyrin π-system. Various β-substituted porphyrins are utilized for NLO, PDT, ion sensing, catalytic and DSSCs applications. Herein, we report the synthesis of meso/β-functionalized porphyrins and their utilization in various application such as anion sensing, NLO and complexation with fullerene. We have organized this thesis into following 8 chapters. Chapter 1 describes the basic introduction about porphyrins, their synthetic methods and their applications in different arena such as catalysis, NLO, DSSC, anion sensing and PDT. Chapter 2 includes the synthesis of mixed β-substituted arylaminoporphyrins and N-fused porphyrins (MTPP(NHPh)X2 and MTPP(N-fused)X2, X = H, Br, Ph, PE and M = 2H, Co(II), Ni(II), Cu(II), Zn(II)). These porphyrins exhibited interesting photophysical and electrochemical redox properties. Single crystal X-ray study revealed twist conformation of H2TPP(N-fusedPh)Br2.Photoinduced electron transfer study of ZnTPP(N-fused)X2, X = H, Br, Ph, PE was carried out via axial coordination of C60Im and C60Py which exhibited 1:1 supramolecular dyads formation. These porphyrins exhibited very low oxidation potential due to intramolecular charge-transfer from β-arylamine group to porphyrin core. Efficient fluorescent quenching was observed for Zn(II) N-fused porphyrins while titrating with C60 derivatives due to axial coordination of C60Im/C60Py to Zn(II) porphyrin. The first oxidation potential of supramolecular dyads are anodically shifted (~0.10 V) as compared to C60Im/C60Py free Zn(II)porphyrins which exhibited supramoleccular interaction between Zn(II) porphyrins and C60Im/C60Py system in ground state. iv Chapter 3 describes the synthesis of tetrabenzoquinone appended Ni(II) and Cu(II) porphyrins. These porphyrins exhibited strong solvatochromism behavior in nitrogenous base through axial coordination. We studied the F¯ and CN¯ anion sensing properties of these porphyrins. The electrochemical studies revealed that tetrabenzoquinone appended metalloporphyrin is electron deficient by ~1.1 V as compared to TDtBPPM (M = Cu, and Ni) and nearly 0.94 V as compared to oxoporphyrinogen respectively and therefore binds to less basic F¯ ions also in addition to highly basic CN¯ ions through axial ligation mechanism. They act as F¯ and CN¯ chemosensor in nonaqueous media and selective CN¯ sensor in aqueous media due to high solvation of F¯ ions in aqueous media. Chapter 4 describes the synthesis of two new series of unsymmetrically nonplanar mixed β-octasubstituted porphyrins (MTPP(Ph2)Br5X, X = NO2, Br and M = 2H, Co(II), Ni(II), Cu(II), Zn(II)) and studied their structural, photophysical and electrochemical redox properties. Single crystal X-ray study of H2TPP(NO2)Ph2Br5 exhibited saddle shape nonplanar conformation with deviation of 24 core atoms (Δ24 = 0.558 Å) and displacement of β-carbons (ΔCβ = 1.23 Å) from the mean plane of porphyrin core. So H2TPP(NO2)Ph2Br5 exhibited highly nonplanar conformation as compared to precursor (H2TPP(NO2)(Ph)2) (ΔCβ =  . and Δ24 =  . 2 ) due to the effect of five β-bromo substituents at periphery of the porphyrin. H2TPP(Ph2)Br5X, X = NO2, Br exhibited 53-61 nm and 90-95 nm bathochromic shift in the Soret band and Qx(0,0) band, respectively as compared to H2TPP. They are exhibited higher protonation and deprotonation constant due nonplanar conformation of macrocyclic core and electronic nature of β-substituents. HOMO-LUMO energy gap of CuTPP(Ph2)Br6 and CuTPP(NO2)Ph2Br5 decreased to 0.55 V and 0.62 V as compared to CuTPP, respectively . Chapter 5 presents the synthesis of meso-tetraalkylporphyrin and their Zn(II) derivatives. Single crystal X-ray structure of meso-tetrapropylporphyin revealed the orientation of alkyl chains and planar conformation of porphyrin macrocycle. Photophysical, spectroscopic and electrochemical redox properties of self-assembled donor-acceptor dyads formed by meso-tetraalkylporphyrin and fullerene C60 were investigated. The binding of C60 with porphyrins (H2TMeP, H2TEtP and H2TPrP) and their Zn derivatives was determined by UV-Vis, fluorescence and 1H NMR analyses. The stoichiometry of complexation between C60 and porpnyrin was found to be 1:1. The determined association constants of (K) follw the order: H2TMeP > H2TEtP > H2TPrP H2THexP > H2TPP. The effect of alkyl chain length on porphyrin-fullerene complexation were v investigated. The oxidation potentials of dyads anodically shifted (20-100 mV) as compared to corresponding meso-tetraalkylporphyrins indicating supramolecular interaction between the constituents in ground state. Optimized geometry of H2TMeP:C60 revealed the formation of supramolecular dyads and charge transfer interaction between porphyrin host and fullerene (C60) guest. Chapter 6 encompasses the synthesis of mixed β-trisubstituted (MTPP(TPA)2X, (where M = Co, Ni, Cu, Zn and X = NO2/CHO) porphyrins with their photophysical, electrochemical redox properties and DFT studies. H2TPP(TPA)2X, (X = NO2/CHO) exhibited ~22 nm and 31-39 nm red shift in Soret and Qx(0,0) bands, respectively than H2TPP due to effect of β-substituents and nonplanarity of macrocycle. The push-pull effect of NO2/CHO and TPA affect the HOMO-LUMO energy gap. H2TPP(TPA)2NO2 and H2TPP(TPA)2CHO exhibited large resultant dipole moment values 7.62 D and 4.55 D, respectively as compared to H2TPP (0.052 D). Chapter 7 presents the synthesis of antipodal β-tetrasubstituted porphyrins and characterized by using various spectroscopic techniques. Due to β-tetrasubstitution they exhibited tunable redox potentials and moderate nonplanar conformation. These porphyrins are easy to oxidize as compared to MTPPs. NiTPP(TPA)4 exhibited 0.15-0.29 V and 0.03-0.04 V cathodic shift in first oxidation potential and first reduction potential, respectively as compared to NiTPP and NiOPP due to destabilization of HOMOs. Chapter 8 summarizes the results obtained from the present thesis with future perspectives