Structurally influenced optical nonlinearities and ultrafast dynamics in ß-acroleyl- and ß-dicyanobutadienyl-appended cobalt corroles.

The strong two-photon induced nonlinear absorption and self-focusing type positive nonlinear refraction are pronounced by the structural engineering in ß-functionalized cobalt corroles.

Third-order nonlinear optical properties of highly electron deficient, nonplanar push-pull porphyrins: ß-nitro-hexa-substituted porphyrins bearing bromo, phenyl, and phenylethynyl groups.

ß-Heptasubstituted porphyrins [MTPP(NO2)X6; M = 2H, NiII, CuII, and ZnII; X = Br, Ph, and PE] were synthesized and their third-order nonlinear optical (NLO) properties explored using the single-beam Z-scan technique with femtosecond, MHz pulses in the visible range. The 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 theoretical fits with experimental results. The sign and magnitude of the nonlinear refractive index (n2) have been obtained from the closed-aperture experiment while the three-photon absorption coefficient and three-photon absorption cross-section were determined from the open-aperture experiment. The magnitudes of the 3PA and σ3 extended in the range of (2.7-3.4) × 10-23 cm3 W-2 and (5.5-7.0) × 10-78 cm6 s2, respectively. The higher magnitude of the NLO coefficients ensures their utility in optical and photonic applications.

π-Extended nonplanar cobalt porphyrins immobilized on MWCNTs as efficient electrocatalysts for selective oxygen reduction reaction.

Two π-extended cobalt porphyrins are synthesized and one of them is crystallographically characterized. The nanocomposites of nonplanar (curved) porphyrin immobilized multi-walled carbon nanotubes were thoroughly characterized spectroscopically and microscopically, showing ∼200 mV positive shift in the O2 reduction peak potential in aqueous media and ∼100 mV shift in the onset potential of the O2 reduction relative to the control meso-tetraphenylporphyrinatocobalt(II) nanocomposite. Both the π-extended cobalt porphyrin immobilized nanocomposites efficiently catalyze selective 4e-/4H+ O2 reduction under ambient conditions with excellent methanol tolerance and high stability due to effective π-π interactions, and could be an alternative for expensive Pt-based cathode materials in fuel cells.

Panchromatic and Perturbed Absorption Spectral Features and Multiredox Properties of Dicyanovinyl- and Dicyanobutadienyl-Appended Cobalt Corroles.

Four new ß-functionalized π-extended cobalt corroles with one and two dicyanovinyl (DCV) or dicyanobutadienyl (DCBD) moieties at the 3- and 3,17-positions have been synthesized and characterized by various spectroscopic techniques. Interestingly, the synthesized DCV- and DCBD-appended cobalt corroles displayed panchromatic and near-infrared absorption in the range 300-1100 nm in CH2Cl2 and pyridine solvents. (MN)2-(Cor)Co and A2MN2-(Cor)Co exhibited 8-9 times enhancement in the molar absorptivity of the Q band compared to the parent corrole ((Cor)Co). The unique absorption spectral features of these ß-functionalized cobalt corroles are splitting, broadening, and red-shifting in the Soret and Q bands. One DCV unit brings a 30-46 nm red shift, whereas one DCBD unit brings a 40-75 nm red shift in the Q band compared to the corresponding precursors. This is rare that the intensity of the longest Q band is greater than or equal to the Soret-like bands. These corrole derivatives exhibit UV-vis spectral features similar to those of chlorophyll a. A 220 mV positive shift per DCV group and 160 mV positive shift per DCBD group were observed in the first oxidation potentials compared to (Cor)Co in the desired direction for the utility of these cobalt complexes in electrocatalysis. DFT studies revealed that HOMO and LUMO were stabilized after appending DCV and DCBD groups on the corrole macrocycle and exhibited a "push-pull" behavior leading to promising material applications in nonlinear optics (NLO) and catalysis.

Insights into the mechanism of binding of doxorubicin and a chlorin compound with 22-mer c-Myc G quadruplex.

BACKGROUND: The interaction of small molecules with G quadruplexes is in focus due to its role in molecular recognition and therapeutic drug design. Stabilization of G-quadruplex structures in the promoter regions of oncogenes by small molecule binding has been demonstrated as a potential approach for cancer therapy. METHODS: In this study, electronic spectroscopy (ultraviolet-visible, fluorescence, circular dichroism), differential scanning calorimetry, and molecular modeling were employed to explore the interactions between the chemotherapy drug doxorubicin and a chlorin compound 5,10,15,20-tetraphenyl-[2,3]-[bis(carboxy)-methano]chlorin (H2TPC(DAC)), and the c-Myc 22-mer G quadruplex DNA. RESULTS: Spectroscopic studies indicated external binding of the compounds with partial stacking at the end quartets. Calorimetric studies and temperature dependent circular dichroism data displayed increased melting temperatures of G quadruplex structure on binding with the compounds. Circular dichroism spectra indicated that the G quadruplex structure is intact upon ligand binding. Both the compounds showed binding affinities of the order of 106 M-1. Fluorescence lifetime studies revealed static quenching as major mechanism for fluorescence quenching. Polymerase chain reaction stop assay hinted that binding of both ligands under study could inhibit the amplification of the DNA sequence. CONCLUSION: Results show that doxorubicin and H2TPC(DAC) bind to the 22-mer c-Myc quadruplex structure with good affinity and induce stability. SIGNIFICANCE: Doxorubicin and H2TPC(DAC) have demonstrated their affinity towards c-Myc G quadruplex DNA, stabilizing it and inhibiting expression and polymerization. The results can be of practical use in designing new analogs for the two compounds, which can become potent anti-cancer agents targeting the c-Myc GQ structure.

Enhanced Femtosecond Nonlinearities and Multiphoton Absorptions in Discrete Bands of Porphyrins.

Optical nonlinearities of discrete absorption energy levels of one of the typical heterocyclic aromatic molecules, free-base porphyrins, have been probed over a broad spectral region (400-1600 nm) utilizing intense femtosecond pulses. A wide range of strong one- and multiphoton-induced nonlinear absorptions of both the blue-end Soret (B) band (au → b1g) and red-end orbital mixing split quasi-allowed Q-bands (Qx(0,0; 0,1), Qy(0,0; 0,1), au → eg) are critically probed and reported. During the resonant excitation within B- (400 nm) and Q-bands (600-750 nm), the nonlinear absorption has become predominant by the saturation of absorption (SA) of the one-photon absorption (1PA) process due to ground-state bleaching. At nonresonant wavelengths, it is dominated by the reverse saturation of absorption (RSA), involving various nonlinear processes of two-, three-, and four-photon (2PA, 3PA, and 4PA) absorptions, either to B- or Q-bands (1100-1600 nm). The laser intensity-dependent nonresonant (2PA, 800 nm) excitations for the prominent B-band show a distinct cross-over from SA to RSA, contributed by the excited-state absorption (ESA) utilizing a three-photon induced (3PA) process, whereas resonant (1PA, 400 nm) excitation reveals a systematic strong SA process. Both wavelength- and intensity-dependent nonlinear refractive index studies exhibit positive electronic Kerr-based self-focusing effects, with prominent contributions of nonlinear absorption and higher-order effects. The spectrally discrete, highly intense laser probing of individual energy bands and the consequent variety of nonlinearities can be broadly generalized for many free-base porphyrins and metalloporphyrins. The present studies provide a strong foundation and new insight into the broad categories of macrocycles, such as porphyrins and phthalocyanines, for myriad applications in nonlinear optics and bio/optophotonics.

Co(II) Porphyrin-MWCNT Nanoconjugate as an Efficient and Durable Electrocatalyst for Oxygen Reduction Reaction.

Recently, researchers are seeking alternatives to replace Pt-based oxygen reduction reaction (ORR) catalysts used in fuel cells due to their high cost and certain stability and selectivity issues. For this purpose, we have synthesized a nanoconjugate, cobalt(II) porphyrin (5,10,15-triphenyl-20-(4-aminophenyl)porphyrinatocobalt(II), CoTPP-NH2) covalently attached to the acid-functionalized multiwalled carbon nanotubes and characterized by various techniques including UV-vis spectroscopy, FTIR, TGA, FESEM, TEM, and Raman spectroscopy. The oxygen reduction performance of the nanoconjugate is checked in basic medium. The ORR onset potential of the nanoconjugate-modified electrode is nearly the same as that of the state-of-the-art platinum-carbon electrode and stable for more than 3000 CV cycles with a 20 mV difference in the onset potential before and after the 3000 CV cycles. The above extrapolations reveal that the nanoconjugate has efficient performance for the ORR in basic medium.

Synthesis, Spectral, Redox, and Sensing Studies of ß-Dicyanovinyl-Appended Corroles and Their Metal Complexes.

A new family of ß-dicyanovinyl (DCV)-appended corroles represented as MTPC(MN) (where M = 3H, Cu, Ag, and Co(PPh3) and MN = malononitrile and TPC = 5,10,15-triphenylcorrole) were synthesized starting from the free base mono ß-formyl corrole, H3TPC(CHO), and characterized along with their respective MTPC(CHO) and MTPC complexes as to their spectroscopic and electrochemical properties in nonaqueous media. Comparisons between the two series of corroles demonstrate a pronounced substituent effect of the ß-DCV group on the physicochemical properties making the MTPC(MN) derivatives substantially easier to reduce and more difficult to oxidize than the formyl or unsubstituted corroles. In addition, the colorimetric and spectral detection of 11 different anions (X) in the form of tetrabutylammonium salts (TBAX, X = PF6-, OAc-, H2PO4-, CN-, HSO4-, NO3-, ClO4-, F-, Cl-, Br-, and I-) were also investigated in nonaqueous media. Of the investigated anions, only CN- was found to induce changes in the UV-vis and 1H NMR spectra of the ß-DCV metallocorroles. This data revealed that CuTPC(MN) and AgTPC(MN) act as chemodosimeters for selective cyanide ion detection via a nucleophilic attack at the vinylic carbon of the DCV substituent, while (PPh3)CoTPC(MN) acts as a chemosensor for cyanide ion sensing via axial coordination to the cobalt metal center. A low-limit detection of cyanide ions was observed at 1.69 ppm for CuTPC(MN) and 1.17 ppm for AgTPC(MN) in toluene.

Symmetrically Functionalized Copper and Silver Corrole-Bis-Tetracyanobutadiene Push-Pull Conjugates: Efficient Population of Triplet States via Charge Transfer.

Copper and silver tritolylcorroles (TTC) are symmetrically functionalized to carry two tetracyanobutadiene (TCBD) entities via [2+2] cycloaddition-retroeletrocyclization reaction involving ethynyl functionalized corroles with an electron acceptor, tetracyanoethylene (TCNE) in excellent yields, as the first examples of corrole-TCBD push-pull systems. The strong push-pull effect resulted in charge polarization in the ground state resulting in a considerable hypsochromic shift of the spectrum extending it into the near-IR region. Electrochemical studies coupled with computational studies revealed considerable interactions between the two TCBD entities via the corrole π-system and the degree of such interactions was found to depend on the metal ion present in the corrole cavity. Energy considerations suggested charge transfer (CT) from the S2 or vibrationally hot S1 state but not the relaxed S1 state in the case of CuTTC(TCBD)2 while CT to occur from all these states in the case of AgTTC(TCBD)2 . Additionally, the high-energy CT states populate the low-lying triplet states. Systematic femtosecond pump-probe studies provided the ultimate proof for the occurrence of excited CT as a function of excitation wavelength followed by the efficient population of the triplet states. The present study brings out the significance of charge transfer in efficiently populating the triplet states in rather unusual copper and silver corroles carrying two TCBD entities.

Ethyl acetoacetate and acetylacetone appended hexabromo porphyrins: synthesis, spectral, electrochemical, and femtosecond third-order nonlinear optical studies.

Asymmetrically substituted porphyrins possessing ethyl acetoacetate or acetylacetone (EAA or acac) with six bromine atoms at ß-positions were synthesized and then characterized by various spectroscopic techniques, such as UV-Vis, fluorescence and NMR, and also by 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 resultant ß-heptasubstituted 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 such that the quantum yield and fluorescence intensity for H2TPP[EAA]Br6 and H2TPP[acac]Br6 were severely reduced in contrast to those values for H2TPP. The poor electron density and nonplanarity over the porphyrin ring shifted the 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 the synthesized porphyrins as Δ24 spans from ±0.546 to ± 0.559 Å while ΔCß stretches from ±0.973 to ±1.162 Å. The third-order nonlinear optical measurements were performed using the femtosecond pulsed laser Z-scan technique at 800 nm and 1 kHz repetition rate to acquire insights into nonlinear absorption and nonlinear refraction of the porphyrins. The three-photon absorption coefficients (γ) are in the range of 2.2 × 10-23-2.8 × 10-23 cm3 W-2 and the nonlinear refractive index values were in the range of 3.7 × 10-16-5.1 × 10-16 cm2 W-1. The higher-order nonlinear absorption exhibited by porphyrins helps improve resolution at depth for various photonic and optoelectronic applications.

Oxido-Molybdenum(V) Corroles as Robust Catalysts for Oxidative Bromination and Selective Epoxidation Reactions in Aqueous Media under Mild Conditions.

Two new meso-substituted oxido-molybdenum corroles were synthesized and characterized by various spectroscopic techniques. In the thermogram, MoO[TTC] (1) exhibited excellent thermal stability up to 491 °C while MoO[TNPC] (2) exhibited good stability up to 318 °C. The oxidation states of the molybdenum(V) were verified by electron paramagnetic resonance (EPR) spectroscopy and exhibited an axial compression with dxy1 configuration. Oxido-molybdenum(V) complexes were utilized for the selective epoxidation of various olefins with high TOF values (2066-3287 h-1) in good yields in a CH3CN/H2O (3:2, v/v) mixture in the presence of hydrogen peroxide as a green oxidant and NaHCO3 as a promoter. The oxidative bromination catalytic activity of oxido-molybdenum(V) complexes in an aqueous medium has been reported for the first time. Surprisingly, MoO[TNPC] (2) biomimics of the vanadium bromoperoxidase (VBPO) enzyme activity exhibited remarkably high TOF values (36 988-61 646 h-1) for the selective oxidative bromination of p-cresol and other phenol derivatives. Catalyst MoO[TNPC] (2) exhibited higher TOF values and better catalytic activity than catalyst MoO[TTC] (1) due to the presence of electron-withdrawing nitro groups evident from cyclic voltammetric studies.

Facile Synthesis of ß-Tetracyano Vanadyl Porphyrin from Its Tetrabromo Analogue and Its Excellent Catalytic Activity for Bromination and Epoxidation Reactions.

Complex 2,3,12,13-tetracyano-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[VIVOTPP(CN)4], 2} has been prepared by nucleophilic substitution of ß-bromo groups of the corresponding 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[VIVOTPP(Br)4], 1} using CuCN in quinoline. Both complexes show biomimetic catalytic activity similar to enzyme haloperoxidases and efficiently brominate various phenol derivatives in the presence of KBr, H2O2, and HClO4 in the aqueous medium. Between these two complexes, 2 exhibits excellent catalytic activity with high turnover frequency (35.5-43.3 s-1) due to the strong electron-withdrawing nature of the cyano groups attached at ß-positions and its moderate nonplanar structure as compared to 1 (TOF = 22.1-27.4 s-1). Notably, this is the highest turnover frequency value observed for any porphyrin system. The selective epoxidation of various terminal alkenes using complex 2 has also been carried out, and the results are good, specifying the importance of electron-withdrawing cyano groups. Catalysts 1 and 2 are recyclable, and the catalytic activity proceeds through the corresponding [VVO(OH)TPP(Br)4] and [VVO(OH)TPP(CN)4] intermediates, respectively.

Ultrafast Dynamics and Strong Two-Photon Absorption Properties of Nonplanar ß-Functionalized "Push-Pull" Copper Corroles with a Mixed Substituent Pattern.

A new series of nonplanar and unsymmetrically ß-functionalized "push-pull" copper corroles, CuTPC(CHO)R7 [R = H, Br, Ph, Me, or 2-thienyl (Th)], were synthesized and characterized to elucidate the effect of ß-functionalization and nonplanarity on the photophysical, redox, and nonlinear optical (NLO) properties on the corrole ring. The synthetic route to unsymmetrically ß-octasubstituted copper corroles includes bromination of CuTPC(CHO) to get CuTPC(CHO)Br7 in 80% yield, which was further subjected to the Pd-catalyzed Suzuki reaction. CuTPC(CHO)Br7 exhibited a large red shift in the Soret band (Δλmax = 35-40 nm) and both the Q bands (Δλmax = 10-50 nm), as compared to CuTPC and CuTPC(CHO). CuTPC(CHO)Br7 was 510 and 290 mV anodically shifted in the first oxidation and the first reduction compared to CuTPC owing to the strong -I effect of CHO and Br groups. Density functional theory studies revealed that all the ß-octasubstituted copper corroles exhibited highly nonplanar saddle-shape conformation of the corrole ring. Very high torsional saddling was observed for CuTPC(CHO)Th7 (79-83°) than that for CuTPC (49-53°), even larger than that for CuTPCBr8 (67-70°). Femtosecond laser-induced third-order NLO studies from these copper corroles showed strong two-photon absorption cross-sections (0.48-6.98 × 104 GM) and self-focusing-type positive nonlinear refraction behavior. The observed structure-dependent two-photon absorption coefficients (ß) are in the range of ∼2.7-20.9 × 10-12 m/W, and the n2 values are in the range of ∼0.64-6.45 × 10-18 m2/W. The present results may facilitate a new window for these copper corroles in nonlinear optical devices, femtosecond optical limiters, and many other ultrafast photonic applications.

Efficient charge transfer from organometal lead halide perovskite nanocrystals to free base meso-tetraphenylporphyrins.

The efficient charge transfer from methylammonium lead halide, MAPbX3 (X = Br, I), perovskite nanocrystals (PNCs) to 5,10,15,20-tetraphenylporphyrin (TPP) molecules has been investigated in detail. The hydrophobically-capped MAPbX3 PNCs exhibited bright fluorescence in the solution state. However, in the presence of TPP, the fluorescence intensity was quenched, which is ascribed to the electron transfer from the PNCs to TPP. Photoluminescence (PL) spectroscopy and absolute quantum yield measurements were used to evaluate the fluorescence quenching. This efficient fluorescence quenching leads to an increase in the quenching efficiency value. The quenching of fluorescence intensity is not attributed to the change in lifetime, as evidenced by time-correlated single-photon counting (TCSPC) measurements, suggesting a static electron transfer from the PNCs to TPP molecules. Such a static fluorescence quenching corresponds to the adsorption of TPP onto the surface of hydrophobic PNCs, and has been examined via transmission electron microscopy (TEM). Cyclic voltammetry (CV) studies were used to compare the PNCs and PNCs@TPP nanocomposites, revealing that the electron transfer process takes place from the PNCs to the organic acceptor TPP molecules.

Unsymmetrically ß-Functionalized π-Extended Porphyrins: Synthesis, Spectral, Electrochemical Redox Properties, and Their Utilization as Efficient Two-Photon Absorbers.

Two new series of unsymmetrically ß-functionalized porphyrins, MTPP(NO2)MA (1M), (MA = methyl acrylate) and MTPP(NO2)MB (2M) (MB = mono-benzo) (where M = 2H, Co(II), Ni(II), Cu(II) and Zn(II)), were synthesized and characterized by various spectroscopic techniques. The saddle shape conformation of ZnTPP(NO2)MAPy and ZnTPP(NO2)MB was confirmed by single-crystal X-ray analysis. Density functional theory (DFT) calculation revealed that NiTPP(NO2)MB has a severe nonplanar geometry possessing a high magnitude of ΔCß = ±0.727 Å and Δ24 = ±0.422 Å values among all other porphyrins. Synthesized ß-substituted porphyrins exhibited red-shifted B- and Q-bands corresponding to their parent molecule due to the electron-withdrawing peripheral substituents. Notable redshift (Δλmax = 50-60 nm) in electronic spectral features and with weak-intensity emission spectral features were observed for the free-base porphyrins and Zn(II) complexes compared to H2TPP and ZnTPP, respectively. The first-ring reduction potential of MTPP(NO2)MA (1M) exhibited 0.21-0.5 V anodic shift, whereas 0.18-0.23 V anodic shift was observed in the first-ring oxidation potential compared to the corresponding MTPPs due to the presence of electron-withdrawing ß-substituents at the periphery of the macrocycle. Interestingly, NiTPP(NO2)MA (1Ni) has shown an additional NiII/NiIII oxidation potential observed at 2.05 V along with two ring-centered oxidations. The first-ring reduction and oxidation potentials of MTPP(NO2)MB (2M) have shown 0.39-0.46 and 0.19-0.27 V anodic shifts with respect to their corresponding MTPPs. The nonlinear optical (NLO) properties of all of the porphyrins were investigated, and the extracted nonlinear optical parameters revealed intense reverse-saturable absorption (RSA) behavior and the self-focusing behavior with positive nonlinear refractive index in the range of (0.19-1.75) × 10-17 m2/W. Zn(II) complexes exhibited the highest two-photon absorption coefficient (ß) and cross section (σTPA) of ∼95 × 10-12 m/W and 19.66 × 104 GM, respectively, among all of the metal complexes.

Synthesis and the spectral, electrochemical, and nonlinear optical properties of ß-dicyanovinyl-appended 'push-pull' porphyrins.

A series of "push-pull" porphyrins, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII), having triphenylamine (TPA) and dicyanovinyl (DCN) groups at antipodal positions were synthesized and characterised by UV-Vis, fluorescence and NMR spectroscopic techniques, MALDI-TOF mass spectrometry, cyclic voltammetry, DFT, and elemental analysis, which were then further utilized for third-order nonlinear optical measurements under mild conditions using femtosecond laser pulses. Remarkably, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII) exhibited 21-48 nm and 38-80 nm bathochromic shifts in B and Qx(0,0) bands as compared to the corresponding MTPPs (M = 2H, CuII, NiII, and ZnII); the results are consistent with the effect of enhanced resonance due to TPA and -I effect of DCN moieties. In cyclic voltammetry, the push-pull porphyrins exhibited a cathodic shift (0.13-0.51 V) in their first oxidation potential as compared to the precursor owing to the presence of electron-donating TPA groups. The third-order nonlinear optical responses were recorded using a single-beam femtosecond Z-scan technique to retrieve information about the nonlinear absorption and nonlinear refraction of the samples. The two-photon absorption coefficients (ß) are in the range of 0.87 × 10-13 to 4.28 × 10-13 m W-1 and the nonlinear refractive index (n2) in the range of 1.21 × 10-19 to 7.36 × 10-19 m2 W-1. The ultrafast absorption dynamics of the ground-state bleaching (GSB) and photo-induced absorption (PIA) are monitored by femtosecond broadband transient absorption studies. The strong nonlinearity of these push-pull porphyrins makes them potential candidates for nonlinear optical and photonic device applications.

ß-Tetracyanobutadiene-Appended Porphyrins: Facile Synthesis, Spectral and Electrochemical Redox Properties, and Their Utilization as Excellent Optical Limiters.

A series of ß-TCBD (1,1,4,4-tetracyano-buta-1,3-diene)-appended porphyrins, M-TCBD (M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)), was synthesized from 2,3-diphenylethynyl-12-nitro-meso-tetraphenylporphyrin, H2-PE2, and characterized by various spectroscopic techniques and electrochemical studies. The reaction proceeds via [2 + 2] cycloaddition and retroelectrocyclization reactions of tetracyanoethylene (TCNE) with H2-PE2. The observed unusual reduction potentials in the cyclic voltammograms of the synthesized porphyrins in the range of -0.06 to -0.10 V are the consequence of the TCBD moiety present at the ß-position of the porphyrin macrocycle. Notably, these porphyrins exhibited three porphyrin ring-centered reductions due to extended π-conjugation. The higher nonlinear optical response exhibited by the M-TCBD series as compared to the precursor (H2-PE2) was attributed to the existence of intramolecular charge transfer and enhanced polarization in the 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 measurements revealed that Co-TCBD exhibited a higher nonlinear optical response as compared to free base porphyrins. The two-photon absorption coefficient (ß) and the imaginary part of third-order nonlinear optical 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). Furthermore, the femtosecond transient absorption spectroscopy revealed a faster relaxation dynamics of various absorption processes in a picosecond timescale. The excellent optical limiting threshold (1.90-2.33 × 1015 W/m2) of the synthesized porphyrins makes them good materials for laser protection and high-power laser operation.

Nickel(II) monobenzoporphyrins and chlorins: synthesis, electrochemistry and anion sensing properties.

A series of nickel(II) monobenzochlorins (MBCs) and monobenzoporphyrins (MBPs) containing ß-appended or meso,ß-fused indanedione (IND) or malononitrile (MN) groups were synthesized and characterized for their physicochemical, electrochemical and anion sensing properties. Each investigated compound contained four meso-phenyl rings and a single ß,ß'-fused 4,5-di(methoxycarbonyl)benzene ring, with the chlorins represented as NiMBC(Y)2(R)4 and the porphyrins as NiMBP(YF)2, where Y is an indanedione (IND) or malononitrile (MN) group, R = H or Br and YF is a meso,ß-fused IND or MN substituent. One of the investigated compounds, NiMBP(IND)2, was structurally characterized and shown to possess a ruffled macrocyclic conformation. The monobenzochlorins, NiMBC(IND)2, NiMBC(IND)2Br4 and NiMBC(MN)2, reversibly respond to basic anions such as CN-, F-, OAc- and H2PO4- through a visible color change assigned to the deprotonation of the vicinal proton on the appended IND or MN substituents. The malononitrile-fused NiII monobenzoporphyrin, NiMBP(MN)2, exhibited a selective but irreversible visual detection of cyanide ions (LOD = 2.23 ppm). This reaction afforded a tri-fused π-extended monobenzoporphyrin product represented as NiMBP(VCN)2 (where VCN = meso,ß-fused vinyl cyanide) in non-aqueous media and proceeded via anion induced electron transfer (AIET). The in situ generated π-extended porphyrin was also isolated and characterized as to its physicochemical and electrochemical properties and found to possess a narrow electrochemical HOMO-LUMO gap of 1.46 V along with a near-IR (NIR) absorption band located at 861 nm.

Strong two-photon absorption and ultrafast dynamics of meso-functionalized "push-pull"trans-A2BC porphyrins.

A new series of "push-pull"meso-substituted trans-A2BC porphyrins, where A = mesityl, B = phenothiazine (push) and C = o/p-nitrophenyl moiety (pull) and M = 2H, Ni(ii), Cu(ii), and Zn(ii), were synthesized. These trans-A2BC porphyrins were characterized by various techniques viz. UV-Vis, fluorescence and NMR spectroscopy, matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, cyclic voltammetry, single-crystal X-ray diffraction analysis and density functional theory (DFT) studies. The single crystal structure of the newly synthesized free base trans-H2A2BC porphyrin (2) revealed the orientation of meso-substituents and the planar conformation of the porphyrin core. All newly synthesized free base trans-H2A2BC porphyrins (1 or 2) and their metal complexes exhibited a sharp Soret band. The first reduction potential of all synthesized trans-MA2BC porphyrins showed an anodic shift as compared with that of meso-tetraphenylporphyrins (MTPPs) due to the strong electron withdrawing nature of the nitrophenyl group. In general, the first oxidation potential of trans-MA2BC porphyrins shows a cathodic shift due to the electron donating phenothiazine moiety as compared to that of MTPPs. p-Nitrophenyl appended trans-A2BC porphyrins exhibited large ground state dipole moment values (8.59-9.64 D) as compared to MTPPs (0.0013-0.052 D) owing to the polarized "push-pull" effect of meso-substituents such as phenothiazine (push) and nitrophenyl (pull) moieties. Femtosecond nonlinear optical (NLO) studies performed with kHz pulses at 800 nm revealed strong two-photon absorption coefficients (∼0.082-0.0953 cm GW-1) and cross-sections (∼1.71-1.95 × 103 GM) for these "push-pull" porphyrins. We have also attempted to understand the NLO coefficients in terms of the structural changes in these porphyrin derivatives. A comparison that has been accomplished with similar porphyrin molecules and under similar experimental conditions revealed the superior performance of the title molecules. Furthermore, femtosecond transient absorption studies demonstrated several ultrafast processes from various excited states in these porphyrins, useful for identifying the processes relevant to optical switching applications.

Fused Nickel(II) Porphyrins-Sensing of Toxic Anions and Selected Metal Ions Through Supramolecular Interactions.

Ni(II) porphyrins having fused -NH group were synthesized and characterized by various spectroscopic techniques. The fused porphyrins 1 and 2 were used to detect species of opposite polarity. 1 was used to sense toxic anions viz. cyanide and fluoride ions whereas 2 was used for detecting some selective metal ions including toxic mercury(II) ions. 1 is having acidic -NH proton, which detects anions via hydrogen bonding interactions followed by anion-induced deprotonation. On the other hand, 2 senses the metal species via weak charge transfer interactions from oxygen atom of the formyl group to the added metal ions. The limit of detection was calculated in case of 1 as 2.13 and 3.15 ppm for cyanide and fluoride ions, respectively. Similarly, the detection limit was found to be 0.930, 2.231, and 0.718 ppm for Cu(II), Fe(III), and Hg(II) ions, respectively, for probe 2. The probes were recovered and reused for several cycles.