Muniappan Sankar

Synthesis, Spectral, and Electrochemical Studies of Electronically Tunable β-Substituted Porphyrins with Mixed Substituent Pattern

Two new families of porphyrins with mixed substituent pattern, viz. 2-nitro-12,13-disubstituted-meso-tetraphenylporphyrins (H2TPP(NO2)X2, X = Ph, phenylethynyl (PE), 2-thienyl (Th), Br, and CN) and 2-nitro-7,8,12,13,17,18-hexasubstituted-meso-tetraphenylporphyrins (H2TPP(NO2)X6, X = Br, Ph, PE, and Th), and their metal (Co(II), Ni(II), Cu(II), and Zn(II)) complexes have been synthesized and characterized. H2TPP(NO2)X6 exhibited remarkable red shift in the Soret band (45-70 nm) and longest wavelength band, Qx(0,0) (65-90 nm), as compared to H2TPP(NO2). Single-crystal structures of MTPP(NO2)X2 (M = Zn(II) and Ni(II); X = Br, Ph, and PE) showed quasi-planar conformation, whereas H2TPP(NO2)Th2 and NiTPP(NO2)Br6 exhibited moderate and highly nonplanar saddle-shape conformations, respectively. Further, DFT fully optimized geometries of H2TPP(NO2)X2 and H2TPP(NO2)X6 revealed moderate and severe saddle-shape nonplanar conformations, respectively. The imino proton resonances of H2TPP(NO2)X6 are significantly downfield shifted (Δδ = 1.10-1.80 ppm) relative to H2TPP(NO2). Mixed substituted highly nonplanar porphyrins exhibited higher protonation and deprotonation constants relative to H2TPPX8 (X = Cl and Br). The redox tunability was achieved by introducing electron donor (Ph and Th) and acceptor (PE, Br, and CN) groups on the MTPP(NO2) backbone. The unusual variation in the spectral and electrochemical redox properties of mixed substituted porphyrins are interpreted in terms of both an inductive and a resonance interaction of substituents on the porphyrin π-system as well as nonplanarity of the macrocycle.

Porphyrin chemodosimeters: synthesis, electrochemical redox properties and selective ‘naked-eye’ detection of cyanide ions

Dicyanovinyl appended β-pyrrole substituted porphyrins have been synthesized and utilized as chemodosimeters for ratiometric and colorimetric sensing of cyanide ions. In addition to meso-tetraphenylporphyrin (TPP) based system, we have designed and synthesized octaphenylporphyrin (OPP) based moderately nonplanar systems. These systems also depicted effective π-conjugation between the pyrrole ring and its olefinic substituent which is reflected in the UV-vis absorption spectra as the red-shifting of both Soret and Q-bands and possibly the splitting of the absorption Soret band. This article describes the single crystal X-ray structure, electronic spectral and electrochemical redox properties of these porphyrin chemodosimeters. The stable anionic-porphyrin species formed after addition of CN− ions has a blue shifted unperturbed absorption spectrum due to the disruption of electronic resonance and charge-transfer interactions between the β-dicyanovinyl acceptor and porphyrin core. The probes exhibited excellent selectivity towards CN− over other anions such as F−, Cl−, Br−, NO3−, H2PO4−, AcO−, I−, HSO4−, ClO4− and PF6−. The action of the chemodosimeters were investigated by 1H-NMR titrations, ESI-mass spectrometry, electrochemical studies and DFT calculations which supported the formation of cyanide addition adduct (porphyrin–CN)−. These chemodosimeters depicted lowest detection limit (LOD) of 0.023–0.082 ppm, which is very much lower than the permissible limit (0.2 ppm) and other porphyrinoids reported till date. These porphyrins are also able to detect cyanide ions in (9:1, v/v) CH3CN:H2O mixture with low detection limits thereby providing practical applicability of these chemodosimeters.

Colorimetric “naked eye” detection of CN−, F−, CH3COO− and H2PO4− ions by highly nonplanar electron deficient perhaloporphyrins

Highly nonplanar perhaloporphyrin-based receptors have been synthesized and characterized by various spectroscopic techniques. Nonplanar and electron deficient nature was authenticated by electronic spectral, electrochemical redox and protonation–deprotonation studies. The colorimetric “naked eye” anion sensing ability of these sensors was probed by spectroscopic and electrochemical studies. These receptors were found to be highly selective and sensitive for the basic anions, such as CN−, F−, OAc− and H2PO4−, over the tested anions and were able to detect these anions even at nanomolar concentrations via anion induced deprotonation. These perhaloporphyrin receptors have been recovered from receptor–anion host–guest complexes formed during sensing events by acid treatment and were reused for the detection of basic anions without loss of their sensing ability.

1,8-Naphthyridinic fluorescent ‘turn-on’ and ‘turn-off’ chemosensors for detection of F− and Hg2+ ions mimicking INHIBIT molecular logic behaviour

Two new receptors viz. 2-bromoacetamido-5,7-dimethyl-1,8-naphthyridine (1a) and 2-chloroacetamido-5,7-dimethyl-1,8-naphthyridine (1b) containing 1,8-naphthyridine as fluorophore unit have been designed and synthesised. These receptors exhibited very high selectivity for F− ions among the various anions tested such as H2PO4−, OAc−, Cl−, Br−, I−, HSO4−, PF6−, ClO4−, BF4− and NO3− owing to anion-induced deprotonation of the amide NH by F− ions. Among the various cations tested (Na+, Ag+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Mn3+), 1a exhibited selective detection of Hg2+ ions by binding with N1 and N8 atoms of the naphthyridine moiety. Receptor 1a has been explored as a dual-ion detector for F− and Hg2+ ions in two contrasting modes (fluorescence ‘turn-on’ and ‘turn-off’) which is supported by spectroscopic and lifetime measurements. 1a exhibited lower detection limits for F− (0.4 ppm) and Hg2+ (2.4 ppm) ions. These receptors were recovered from anionic species of 1a formed after addition of F− ions by treating with protic solvents such as CH3OH. The mode of binding was also confirmed by spectroscopic and DFT studies. The chemical inputs of F− and Hg2+ ions generate an output which satisfies the condition of two-input INHIBIT logic operators.

Co(II)-porphyrin-decorated carbon nanotubes as catalysts for oxygen reduction reactions: an approach for fuel cell improvement

The development of high-performance and cost-effective catalysts for the oxygen reduction reaction (ORR) is essential for the advancement of fuel cells. In this work, three different functionalized cobalt porphyrins, meso-tetraphenylporphyrinatocobalt(II) (CoTPP), meso-tetrakis(4′-hydroxyphenyl)porphyrinatocobalt(II) (CoTHPP) and meso-tetrakis(4′-carboxy-phenyl)porphyrinatocobalt(II) (CoTCPP), are prepared. These porphyrins are immobilized non-covalently on multiwalled carbon nanotubes (MWCNTs) and used for the ORR in 0.1 M HClO4, 0.1 M phosphate buffer solution (pH 7.0) and 0.1 M KOH media. The composite materials are characterized by using spectroscopic and electrochemical techniques and their oxygen reduction efficiencies are compared in different media. Kinetic interpretations and hydrodynamic voltammetry (in three media) studies demonstrated that the MWCNT–CoTPP, MWCNT–CoTHPP and MWCNT–CoTCPP composite materials exhibit significant efficiency with decreased overpotential, considerable methanol tolerance and long term operational stability (up to 3000 cycles) for the ORR similar to commercially available platinum carbon (Pt–C) catalysts. These results reveal that the new MWCNT–cobalt porphyrin composite materials can be a potential alternative to the expensive Pt–C catalysts or other commercial cathode materials in fuel cells.

1,8-Naphthyridine-based fluorescent receptors for picric acid detection in aqueous media

We utilized the potential applications of 1,8-naphthyridine-based sensors for the detection of various nitroaromatics such as picric acid (PA), 3-nitrophenol (NP), 2,4-dinitrotoluene (DNT), 1,3-dinitrobenzene (DNB), 4-nitrotoluene (NT), nitrobenzene (NB) and nitromethane (NM). Among the various nitroaromatic analogues, nap-OH (1) and nap-Cl (2) sense picric acid (PA) much more efficiently and with very low detection limits (1.12 and 0.96 ppm, respectively), owing to favourable electron and/or energy transfer mechanisms along with potential electrostatic interactions. Interestingly, both sensors 1 and 2 detect PA in aqueous media (H2O/CH3OH, 8 : 2) with the same quenching efficiency as in neat CH3OH, resulting in cheap, sensitive and environmentally friendly detection methodology. Therefore, use of these molecules for picric acid detection in aqueous and methanolic solutions, having low detection limits along with exceptional quenching constants (kSV) meets the present need. In addition, sensors 1 and 2 exhibit instant visualization of trace amounts of PA in both the solid-state and when using test strips.

Unusual solvent dependent electronic absorption spectral properties of nickel(II) and copper(II) perhaloporphyrins

Electronic absorption spectra of divalent metal (Ni(II) and Cu(II)) complexes of 2,3,7,8,12,13,17,18-octa(bromo/chloro)-5,10,15,20-tetraphenylporphyrins (MOXTPP; X = Br, Cl) were examined in various solvents. M(II) perhaloporphyrins exhibited dramatic shifts in their optical absorption spectral features relative to the corresponding metallotetraphenylporphyrins, MTPPs. Copper(II) perhaloporphyrins show significant red-shifts of the absorption bands in coordinating solvents relative to that observed for nickel(II) perhaloporphyrins. The large red-shift of the electronic absorption bands and the gain in intensity of the longest wavelength band (Q(0,0)), of Cu(II) perhaloporphyrins in certain coordinating solvents is comparable to that found in meso-tetraphenylporphinatozinc(II), ZnTPP. The solvent dependent spectral features of M(II) perhaloporphyrins are attributed to a coordinative interaction of the solvent with the core metal ion induced by the electron deficient porphyrin macrocycle.

Synthesis, spectroscopic, electrochemical redox, solvatochromism and anion binding properties of β-tetra- and -octaphenylethynyl substituted meso-tetraphenylporphyrins

β-Tetra/octa-phenylethynyl substituted porphyrins and their metal (Co(II), Ni(II), Cu(II) and Zn(II)) complexes have been synthesized and characterized. H2TPP(PE)8 exhibited a remarkable red shift in the Soret (Δλmax = 92 nm) band and longest wavelength band, Qx(0,0) (Δλmax = 117 nm), as compared to H2TPP. Interestingly, MTPP(PE)8 exhibited a 450–500 mV anodic shift in the first ring reduction potentials as compared to MTPP which is ascribed to the electron accepting nature of phenylethynyl groups and extended π-conjugation whereas the first ring oxidation potentials remain unaltered. Hence, we observed a significant reduction in the HOMO–LUMO gap. The fluorescence quantum yields and lifetime values of the free base and Zn(II) porphyrins are found to be 10–20 and 2–6 times, respectively lower than MTPPs. These porphyrins display a strong solvatochromism that is reflected by a large red-shift in their absorption and emission maxima upon increasing the solvent polarity. These porphyrins exhibited lower radiative rate constants (kr) and enhanced nonradiative rate constants (knr) as compared to MTPPs. The decrement in fluorescence lifetime values, quantum yields, radiative rate constants (kr) and profound solvatochromism with enhanced nonradiative rate constants (knr) have been interpreted in terms of intramolecular charge transfer (ICT) from the porphyrin core to phenylethynyl moieties and extended π-conjugation. Further, ZnTPP(PE)8 was utilized for the colorimetric “naked-eye” sensing of CN−, F−, Cl−, Br−, H2PO4− and CH3COO− ions through axial coordination of anions to Zn(II) metal centre. The electron deficient nature of the Zn(II) centre of ZnTPP(PE)8 was further proved by axial ligation studies with anions in toluene. ZnTPP(PE)8 exhibited extremely high binding constants (1012 to 107 M−2) with anions.

Mono- and tri-β-substituted unsymmetrical metalloporphyrins: synthesis, structural, spectral and electrochemical properties

Mono-/tri-β-substituted metalloporphyrins, viz. MTPP(X)Y2 (X = CHO, CH2OH, COOH; Y = H, Br, Ph; M = 2H, Co(II), Ni(II), Cu(II), Zn(II)) have been synthesized and characterized. This work examines the influence of β-substitution on the structural, spectral and electrochemical redox properties of MTPP(X) and MTPP(CHO)Y2. The redox tunability was achieved by introducing electron donors (CH2OH and Ph) and acceptors (CHO, COOH and Br) on the MTPP skeleton. Dramatic reduction in the HOMO–LUMO gap with considerable increment in Δa1u was observed as the number of electron withdrawing groups increased. The spectral and electrochemical redox potentials are influenced by the peripheral β-substituents and electronegativity of the core metal ion. These porphyrins exhibited tunable electronic spectral and redox properties with modulated frontier orbitals by means of mono- and tri-β-substituents which are in direct conjugation with the porphyrin π-system. DFT studies of these porphyrins revealed that mono-substituted porphyrins are nearly planar whereas tri-substituted porphyrins have a moderate nonplanar conformation.

Synthesis and characterization of simple cost-effective trans-A2BC porphyrins with various donor groups for dye-sensitized solar cells

We have designed and synthesized a series of simple cost-effective ‘push–pull’ Zn(II) porphyrin dyes containing various electron donors such as 2-thienyl, pyrenyl, phenyl, 4′-bromophenyl, 4′-tbutylphenyl and 4′-carboxyphenyl acceptor moieties in three steps. Their optical absorption spectra, electrochemical redox and photovoltaic properties have been investigated in detail. The overall power conversion efficiencies (η) of DSSCs based on these dyes are in the range of 2.1 to 4.2% and highly depend on their donor moiety. The incorporation of trans-10,20-dimesityl groups is highly beneficial for preventing the π–π aggregation among the porphyrin moieties, thus favorably suppressing the charge recombination and intermolecular interaction. Among all, pyrenyl appended Zn(II) porphyrin has exhibited a higher power conversion efficiency of 4.2% under 1 sun illumination due to the extended π-conjugation and electron donating ability of the pyrenyl moiety.