FUNCTIONALIZED PORPHYRINS AND THEIR APPLICATION IN DSSC, ELECTROCATALYSIS AND ANION SENSING

Abstract

Porphyrins are a class of naturally occurring macrocyles involved in a wide variety of important biological processes and material applications. Porphyrin based biological molecules include heme, chlorophylls, vitamin B12 and co-enzyme F430. β-functionalization of porphyrins exhibit more altered physiochemical, electronic spectral and redox changes as compared to mesofunctionalization. The proposed thesis describes the syntheses of β- and meso-functionalized porphyrins and their application in dye-sensitized solar cells (DSSCs), electrocatalysis and anion sensing. Chapter 1 deals with the basic introduction to porphyrins and their application in dye-sensitised solar cells, photodynamic therapy, catalysis, nonlinear optics and anion sensing. Chapter 2 describes the synthesis of β-susbtituted porphyrins for dye-sensitized solar cells application. Three new β-substituted “push-pull” Zn(II) porphyrin dyes with various electron donors at meso- and cyanoacetic acid as acceptor at β-positions have been synthesized and characterized. The Soret and Q bands of Zn(II) porphyrin dyes were found to be red-shifted (30- 35 nm) as compared to ZnTPP. The fluorescence quenching and the decrement in quantum yield and lifetime suggest intramolecular charge transfer from donor to acceptor. These Zn(II) porphyrins exhibited anodic shift in their first redox potentials (0.03-0.11 V) as compared to ZnTPP. These dyes displayed power conversion efficiency (PCE) of η = 1.72-3.13%. ZnT(Mes)P(CN-COOH) has been co-sensitized with N719 dye to improve the PCE efficiency and the resulted co-sensitized ZnT(Mes)P(CN-COOH)(N719) dye gave maximum PCE efficiency up to 5.35%, with a Jsc of 11.8 mA cm-2, a Voc of 630 mV and a fill factor (FF) of 72% due to better light harvesting capacity. Chapter 3 demonstrates the facile Pd free high yield synthesis of trans-A2BC/A2B2 Zn(II) porphyrin sensitizers with various donor groups (pyrene, bisthiophene, N,Ndimethylaminophenyl, triphenylamine, carbazole and phenothiazine) involving lesser number of synthetic steps. The maximum power conversion efficiencies (η) of DSSCs based on these dyes are in the range of 1.71% to 7.11% under 1 sun illumination and highly depend on donating strength of appended moieties. Among all, phenothiazine appended trans-A2BC porphyrin exhibited highest power conversion efficiency (η) of 7.1%. It exhibited butterfly conformation in the ground state which can impede the intermolecular aggregation. This dye shows a broader absorption on TiO2 surface and more significantly improved IPCE values in Soret and Q-bands region as compared to the other dyes, which ensured a good light-harvesting ability and high short-circuit current density of 14.2 mAcm-2. Co-sensitization of trans-A2B2 porphyrin with N719 dye drastically improved their photovoltaic performance from 1.7-5.5% to 4.9-8.9%. The electrochemical impedance spectroscopy results were also in good agreement with photovoltaic performance of the porphyrin dyes. Chapter 4 describes the synthesis of borylated porphyrin and their deprotection-protection stratagy for anion sensing. Free-base borylated porphyrin (1) and its metal (Zn(II) (1a) and Cu(II) (1b)) complexes were synthesized and characterized by various spectroscopic techniques. The deprotection of borylated porphyrins was carried out by two different methods. The first one involves the use of trifluroacetic acid (TFA) and H2SO4 mixture to afford tetraphenylboronic acid porphyrin (3) whereas second one describes the formation of tetrabutylammonium decorated porphyrins (2-2b) via anion induced deprotection using TBAX (X = F-, CN-, H2PO4 - and OAc-) followed by TFA/H2SO4 treatment. Due to the presence of strong acids, deprotection and demetallation processes proceed simultaneously for metal complexes (1a-1b and 2a-2b) which formed 3 as a final product. Addition of neopentyl glycol to 2-2b and 3 regenerated the original porphyrins (1-1b) in quantitative yield with colorimetric changes. Further, Zn(II) borylated porphyrin (1a) was utilized as a qualitative tool for anion sensing as it shows colorimetric response and distinct spectral (UV-Vis and fluorescence) changes towards various anions such as F‒, CN‒, H2PO4 ‒ and OAc‒ and can be reused for numerous cycles. The sensing ability of 1a is more specific for H2PO4 ‒ whereas ZnTPP doesn’t show any spectral and colorimetric change with H2PO4 ‒. Chapter 5 describes the regioselective synthesis of β-substituted monoarylaminodibromoporphyrins from free base β-tetrabromoporphyrin through Pd(II) catalysed coupling reaction and their structural, photophysical and electrochemical redox properties. We have appendeded different electrondonating and electronwithdrawing arylamine susbtituent at the β-position of porphyrin core. The β-arylamino porphyrins exhibit 10-23 nm bathochromic shift in absorption spectra as compared to H2TPPBr4 with one Soret band and reduce number of Q bands. The absorption as well as emission spectra are broader which depicts vii charge transfer from arylamine susbtituent to porphyrin moiety. The quantum yield and lifetime values of these porphyrins are comparatively lower due to the heavy atom effect of bromo atoms. The electrochemical studies of β-arylaminoporphyrin revealed the multiple oxidation and reduction due to the presence of arylamine susbtituents. Chapter 6 deals with mono/tri/hepta-β-substituted porphyrins and their application in electrocatalytic reduction of molecular oxygen (O2). This work describes the influence of β- substitution on structural, electronic spectral and redox properties of MTPP(X) and MTPP(CHO)Y2. DFT studies revealed that mono-substituted porphyrins are quasi-planar whereas trisubstituted porphyrins (H2TPP(CHO)Br2 and H2TPP(CHO)(Ph)2) show moderate nonplanar conformation. Dramatic reduction in HOMO-LUMO gap with considerable increment in Δa1u was observed as the number of electron withdrawing groups increased. Conclusively, the electronic and redox tunability was achieved by introducing electron donors (CH2OH and Ph) and acceptors (CHO, COOH and Br) on MTPP skeleton. Tri- and hepta-β-substituted nitroporphyrins (MTPP(NO2)X2Y4) have been examined for electrocatalytic activity for molecular oxygen reduction. These Co(II) complexes of β-substituted nitroporphyrins exhibited better oxygen reduction and follows the two electron pathway for electrochemical reduction process of O2. Chapter 7 involves the synthesis of π-conjugated mono β-functionalized donor-acceptor porphyrin dyads (1-4) with various donors and their metal complexes (M = Zn, Cu, Co, Ni). Horner-Emmons-Width reaction was performed to synthesize ethenyl linked porphyrin dyads in higher yield. A bathochromic shift was observed in electronic properties (UV-vis and emission spectra) of porphyrin dyads as compared to MTPP(CHO). Quantum yield and lifetime measurements confirmed the effective intramolecular energy transfer from donor moiety to porphyrin core in anthracene and pyrene appended porphyrin dyads. Time resolved fluorescence studies and Förster energy transfer calculation revealed that energy transfer occurs through bond as well as through space also and more efficient further for 4 and 4a as compared to 3 and 3a. All porphyrin dyads exhibited cathodic shift in their redox potential which suggest the facile oxidation of porphyrin core due the presence of electro-donating group. The DFT and TD-DFT studied revealed the planar structure of porphyrin dyads where donor substituent either in plane or perpendicular the porphyrin core. The distribution of electron density on HOMOs and LUMOs signified the EET mechanism from donor moiety to porphyrin acceptor core. Chapter 8 summarizes the results obtained in the proposed thesis with future perspectives