Shu-Hua Cheng

Substituent and axial ligand effects on the electrochemistry of zinc porphyrins

Abstract An absorption spectral and electrochemical study for the zinc(II) complexes of meso -tetraphenylporphyrin dianion (TPP), meso -tetramesitylporphyrin dianion (TMP) and meso -tetra(2,6-dichlorophenyl)porphyrin dianion (TDCPP) in the presence of nitrogeneous bases in CH 2 Cl 2 solution is reported. The Soret and Q bands of the zinc porphyrins were red-shifted in the presence of imidazole bases, however, the formation constants ( K f ) with imidazole or 2-methylimidazole titration were found to be of a similar magnitude. The K f and the formal electrode potentials of zinc porphyrins are in agreement with the electron-donating–withdrawing properties of the substituents in the phenyl groups. K f for complexation of the imidazole to the oxidized and reduced zinc porphyrins was obtained from the electrochemical study. The one-electron oxidation of zinc porphyrins showed greater affinity toward imidazole ligation than the other oxidation states of zinc porphyrins. ZnTDCPP + was found to have the greatest affinity with K f up to 1.35×10 8 . Spectroelectrochemical methods were used to obtain absorption spectra of the complexation of zinc porphyrin cation radical with imidazole. The results showed a slight spectral difference between the complexed and uncomplexed zinc porphyrin cation radical.

Electrochemical characterization of small organic hole-transport molecules based on the triphenylamine unit

A series of substituted triphenylamine-containing organic compounds are synthesized and their hole-transport properties are examined by electrochemical and spectroelectrochemical methods. Several substituted tirphenylamines exhibited irreversible electron-transfer reactions both in the oxidative and reductive scan. On the other hand, the cyclic voltammograms of the p-phenylenediamine series are well defined. N,N 0 -bis(4-nitrophenyl)-N,N 0 -diphenyl-1,4-phenylenediamine (NPD) exhibited two reversible oxidation redox couples at +1.00 and +1.28 V vs. Ag/AgCl in dichloromethane solution. There is one reversible reduction redox couple at )1.12 V and one irreversible wave with Ep;c at )1.87 V. Cyano-substituted p-phenylenediamine (CPD) exhibited similar oxidation redox couples. Amino-substituted p-phenylenediamine (APD) is easier to oxidize than NPD and CPD. APD exhibits two reversible oxidation redox couples at +0.40 and +0.70 V and two extra irreversible oxidation waves at +1.26 and +1.52 V. Optically transparent thin-layer electrode (OTTLE) coupled with UV/Vis/NIR spectroscopy was used to examine the oxidation products of the above reactions. The electrogenerated cation and dication of the substituted p-phenylenediamine are very stable in the spectroelectrochemical studies. Oxidation of the compound APD exhibited a distinguished absorption pattern, which is different from those of compound NPD and compound CPD. 2003 Elsevier Science B.V. All rights reserved.

Reusable electrochemical sensor for bisphenol A based on ionic liquid functionalized conducting polymer platform

The toxicity of bisphenol A (BPA) has attracted considerable attention, and the reported electrochemical sensors for BPA need further improvement in reusability due to serious surface fouling. In this study, a composite film is designed aiming to provide both an accurate and repeatable platform for BPA determination. The conducting poly(3,4-ethylenedioxythiophene) film (PEDOT) and ionic liquid 1-butyl-3-methylimidazolium bromide (BMIMBr) were modified onto screen-printed carbon electrodes (SPCE) by electropolymerization and drop/spin methods, respectively. The surface characteristics of the composite film were characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and surface water contact angle experiments. The composite film-modified electrodes exhibited a linear response to BPA in the range of 0.1-500µM in pH 7.0 phosphate buffer solution (PBS) under optimized flow-injection amperometry. The method sensitivity and detection limit (S/N=3) were 0.2661μA μM(-1) (2.419μA μM(-1)cm(-2)) and 0.02µM, respectively. A relative standard deviation of 1.95% was obtained for 77 successive measurements of 10µM BPA, and the repeatability outperformed previously reported work. The proposed method was applied to detect BPA released from plastic water bottles using the standard addition method, and satisfactory recoveries were obtained. The electrochemical assay was validated by comparison with the chromatographic method, and the results showed good agreement between the two methods.

Electrochemical characterization and electrocatalysis of high valent manganese meso-tetrakis(N-methyl-2-pyridyl)porphyrin

Abstract Electrochemical oxidation of water-soluble manganese(III) meso -tetrakis( N -methyl-2-pyridyl)porphyrin (Mn III (2-TMPyP)) generates stable Mn IV and Mn V porphyrins. Speciation of various oxidation states of the porphyrin are characterized by spectroelectrochemical methods. The acid dissociation constants (p K a s) for Mn III (2-TMPyP)(H 2 O) 2 are 9.6 and 10.7, respectively. Spectroelectrochemical results of the one-electron oxidation of Mn III (2-TMPyP) exhibit different forms of oxomanganese(IV) porphyrin, depending on the pH of the solution and the applied potential. The p K a for OMn IV (2-TMPyP)(H 2 O) is 10.5. The axial oxygen atom ligated to the Mn(IV) center is protonated in acidic solution (p K a 3.4). Further one-electron oxidation generates dioxomanganese(V) porphyrin, (O) 2 Mn V (2-TMPyP), which is stable in alkaline solution at room temperature. No oxidation wave is observed in the cyclic voltammograms, indicating the slow heterogeneous electron transfer rate of these oxidation reactions. The electrogenerated dioxomanganese(V) porphyrin exhibits higher reactivity toward olefin oxidation than oxomanganese(IV) porphyrin in basic solutions.

Electrocatalytic oxidation of styrene by a high valent ruthenium porphyrin cation radical

Abstract A sterically-hindered carbonylruthenium(II) porphyrin Ru II (CO)(TMP) (where TMP= meso -tetramesitylporphyrinato dianion) has been synthesized. Chemical oxidation of Ru II (CO)(TMP) by m -chloroperbenzoic acid ( m -CPBA) gives the dioxoruthenium(VI) porphyrin (Ru VI (O) 2 TMP). Cyclic voltammograms show that Ru VI (O) 2 TMP is reversibly oxidized at E 1/2 =+1.24 V in CH 2 Cl 2 . Thin-layer absorption spectra for oxidation of Ru VI (O) 2 TMP at +1.32 V indicates that the product is a porphyrin cation radical (Ru VI (O) 2 TMP + ). Electrogenerated Ru VI (O) 2 TMP + reacts selectively with styrene to give phenylacetaldehyde (96%) and benzaldehyde (4%). We report the first case of styrene oxidation by high valent ruthenium porphyrin under electrochemical conditions. An electrocatalytic oxidation reaction scheme is proposed.

Novel spectral and electrochemical characteristics of triphenylamine-bound zinc porphyrins and their intramolecular energy and electron transfer

Porphine bearing triphenylamine (TPA) pendant groups and their zinc complexes, zinc meso-tetra-p-(di-p-phenylamino)phenylporphyrin (ZnTDPAPP) and zinc meso-tetra-p-(di-p-tolylamino)phenylporphyrin (ZnTDTAPP) are synthesized and their spectral and electrochemical characteristics are studied. Zinc meso-tetraphenylporphyrin (ZnTPP) and zinc meso-tetra-p-aminophenylporphyrin (ZnTAPP) are also used as reference complexes. The B and Q bands of ZnTDPAPP and ZnTDTAPP are located at higher wavelengths and the bandwidths become broader compared with those of ZnTPP and ZnTAPP, indicating the peripheral TPA affects the electronic configuration of zinc porphyrins. Upon excitation in CH2Cl2 at room temperature, the compounds exhibit intramolecular singlet energy transfer from the TPA to the porphyrin core, and emission from the porphyrins are observed. Both ZnTDPAPP and ZnTDTAPP are easier to be oxidized and harder to be reduced than ZnTPP, in agreement with the strong electron-donating effect of the TPA groups. Extra waves corresponding to the oxidation of TPA substituents are also observed. The cation radical ZnTDTAPP+* exhibits an absorption spectrum very different from the typical spectra for porphyrin cation radicals. The NIR absorption band at 1296 nm indicates the electron transfer occurs intramolecularly. The above results evince the ability of TPA to modulate the electronic structure of zinc porphyrins.

Linking of cytochrome P450cam and putidaredoxin by a co-ordination bridge

Cytochrome P450cam (CYP101) catalyzes the oxidation of D(+)-camphor at the 5 position. The enzyme couples the reduction of dioxygen to the oxidation of the substrate. To transfer electrons from the reductant (NADH) to the cytochrome, two additional proteins are required. These are putidaredoxin (PdX) and putidaredoxin reductase (PdR). We have chemically linked a form of PdX with a histidine tag at the C-terminus to the P450. To accomplish this, we have modified cysteine 334 on P450 with a bipyridinyl group, and co-ordinated the C-terminal histidine tag of PdX by the addition of Ni2+ or Ru3+. The Ru3+ complex was the most stable. The non-linked system gave mostly 5-ketocamphor, a product of two consecutive hydroxylations, and H2O2, a product of 2-electron uncoupling. The Ni2+ complex gave both 5-exo-hydroxycamphor and 5-ketocamphor, but it also uncoupled. The Ru3+ complex gave a single product (5-exo-hydroxycamphor) and did not uncouple at the optimal PdR concentration. Our results are consistent with other studies of this system, in that strong binding of PdX to P450 is crucial for good coupling and for release of 5-exo-hydroxycamphor.

Electrochemical and resonance Raman studies of nitridomanganese(V) porphyrins in nonaqueous solution

The UV-vis spectral, electrochemical and resonance Raman data of [meso-tetrakis(4-pyridyl)porphyrinato]nitridomanganese(V), MnN(TPyP), were investigated. MnN(TTP) (where (TTP)2− = dianion of meso-tetrakis(p-tolyl)porphyrin) and MnN(OEP) (where (OEP)2− = dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin) were also examined in comparison. Cyclic voltammetry of MnN(TPyP) showed an irreversible oxidation wave at +1.22 V and two redox couples at −1.05 and −1.40 V vs. Ag/AgCl, respectively. According to spectroelectrochemical studies, the first and the second reductions involved formation of the porphyrin anion and dianion, respectively, the latter leading to a phlorin. The electron-withdrawing 4-pyridyl substituent shifted the redox potential of the porphyrin ring to a more positive value. However, the bond strength of the Mn-N triple bond was not affected. Resonance Raman studies indicated that the Mn≡N stretching frequencies (νMnN) occurring within 1050 ± 1 cm−1 in various solvents did not change significantly. The hydrogen-bonding between methanol and MnN(TPyP) lowered νMnN to 1040 cm−1. A further lowering of νMnN was found in the presence of nitrogeneous bases such as imidazole (ImH), piperidine (Pip) and pyridine (Py) and may be due to coordination to the manganese center. νMnN is correlated with the pKas of a series of pyridines.

A novel electroanalytical assay for sulfamethazine determination in food samples based on conducting polymer nanocomposite-modified electrodes

The toxicity of sulfa drugs has attracted great attention, and the reported electrochemical methods for sulfa drugs usually employ a high oxidation potential. In this work, a one-pot synthesized conducting polymer nanocomposite containing poly(3,4-ethylenedioxythiophene) (PEDOT) and MnO2 was cast on a screen-printed carbon electrode (SPCE), and the modified electrode showed superior electrochemical activity over a bare electrode for sulfamethazine (SMZ) determination. The SMZ detection was based on the electrochemical oxidation product, which showed an adsorptive property and exhibited a redox couple at 0.39V in pH 3 phosphate buffer solutions (PBS). The electrode surfaces were well characterized by the water contact angle technique, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and cyclic voltammetry. By the use of square wave voltammetry (SWV), a wide linear response to SMZ, from 1.0µM to 500μM, was obtained. The sensitivity and detection limits (S/N = 3) were 0.115μAμM-1 and 0.16μM, respectively. The proposed method and a reference high-performance liquid chromatographic method (HPLC) were applied for the determination of SMZ in two real samples using the standard addition method, and satisfactory recoveries and good agreement were obtained.