Soret Band Region Research Articles

Electroabsorption and Electrophotoluminescence Spectra of Porphyrin Supramolecules in a Polymer Film

Abstract Electroabsorption and electrophotoluminescence spectra of supramolecular porphyrin dimers in a solid film of poly(methyl methacrylate), PMMA, have been examined. The electroabsorption spectra in the Soret band region are fitted by the deconvolution of the corresponding absorption spectrum into four components, indicating that electroabsorption measurements are useful to identify bands that are hard to distinguish in absorption spectra. The values of the changes in molecular polarizability and dipole moment following photoexcitation are evaluated at each absorption band.

Synthesis and spectral properties of a dihydroxo(phthalocyaninato)antimony(V) complex

The title complex cation, [ Sb ( pc )( OH )2]+, (pc = phthalocyaninate, C 32 H 16 N 82-) has been prepared by oxidizing [ Sb ( pc )] I 3 with tert-butyl perbenzoate and isolated as hexafluorophosphate and triflate salts. These salts are relatively well-soluble in polar solvents (e.g. CH 2 Cl 2, acetonitrile, acetone) without detectable aggregation. In particular, they dissolve relatively well in ethanol and methanol in which [ Sb ( pc ) Br 2]+ or [ Sb ( pc ) Cl 2]+ salts are poorly soluble whereas they have extremely low solubility in 1-chloronaphthalene and CHCl 3, unlike the bromo or chloro analogs. The spectral properties of [ Sb ( pc )( OH )2]+ have been investigated by optical absorption and magnetic circular dichroism spectroscopy. Although its Q band tends to appear at a longer wavelength in a solvent with a larger refractive index, as is the case for its Br and Cl analogs, the Q band positions in donor solvents deviate considerably from linear correlation between Onsager's solvent polarity function. In the Soret band region, spectra of [ Sb ( pc )( OH )2]+ are different in CH 2 Cl 2 and acetonitrile, particularly around the B1/B2 bands, unlike the bromo and chloro analogs. The solvent dependence of absorption spectra and the difference in solubility suggests that the presence of the axial hydroxyl groups endows the phthalocyanine macrocycle with considerable hydrophilicity. It also suggests that there are some specific chemical interactions between the axial hydroxyl groups and the surrounding donor solvent molecules.

Photoelectrochemical Properties of Doubly β-Functionalized Porphyrin Sensitizers for Dye-Sensitized Nanocrystalline-TiO2 Solar Cells

Functionalized porphyrins at meso- and β-positions with different carboxylic acid groups were prepared to investigate electronic and photovoltaic properties as dye-sensitized nanocrystalline-TiO2 solar cells. The electronic structures of the porphyrin macrocyclic core are strongly coupled with olefinic side chains so that the absorption spectrum exhibits largely broad and red-shifted Soret and Q-bands, especially up to 475 nm at the Soret band in a porphyrin doubly functionalized with malonic diacid groups. Among porphyrin derivatives prepared in this study, 2b-bdta-Zn exhibits the maximum overall conversion efficiency of 3.03% and the maximum incident photon to current efficiency of 60.1% in the Soret band region, superior to the others. From such photovoltaic performances, we can suggest that multiple pathways through olefinic side chains at two β-positions enhance the overall electron injection efficiency and the moderate distance between the porphyrin sensitizer and the TiO2 semiconductor layer is imp...

A Soret Marker Band for Four-Coordinate Ferric Heme Proteins from Absorption Spectra of Isolated Fe(III)-Heme+ and Fe(III)-Heme+(His) Ions in Vacuo

In this work, we report the absorption spectra in the Soret band region of isolated Fe(III)-heme+ and Fe(III)-heme+(His) ions in vacuo from action spectroscopy. Fe(III)-heme+ refers to iron(III) coordinated by the dianion of protoporphyrin IX. We find that the absorption of the five-coordinate complex is similar to that of pentacoordinate metmyoglobin variants with hydrophobic binding pockets except for an overall blueshift of about 16 nm. In the case of four-coordinate iron(III), the Soret band is similar to that of five-coordinate iron(III) but much narrower. These spectra serve as a benchmark for theoretical modeling and also serve to identify the coordination state of ferric heme proteins. To our knowledge this is the first unequivocal spectroscopic characterization of isolated 4c ferric heme in the gas phase.

Synthesis and Characterization of meso-Triarylsubporphyrins

The synthesis of several meso-triaryl-subporphyrins, based on utilizing tripyrrolylborane as a precursor in a reaction with arylaldehydes (where aryl = phenyl, 4- and 3-pyridyl, tolyl, 4-methoxyphenyl, and 4-(trifluoromethyl)phenyl) under reflux in propionic acid, is reported. All of the compounds have been successfully characterized by 1H-, 13C-, and 13C-1H 2D NMR, electronic absorption, magnetic circular dichroism (MCD), IR, and fluorescence spectroscopy, together with cyclic (CV) and differential pulse (DPV) voltammetry. The X-ray structure of tris{(trifluoromethyl)phenyl}subporphyrin was found to be slightly domed and similar to that of the recently reported hexaethylsubtriazaporphyrin (Inorg. Chem. 2006, 45, 6148). The electronic absorption spectra of all of the subporphyrins contain intense Soret bands in the 370-380 nm region and weaker Q bands in the 420-550 nm region, which are at shorter wavelengths than those observed (at ca. 400-420 nm and ca. 450-650 nm, respectively) for tetrapyrrole porphyrins. The intensity of the Q00 band decreases as the meso-aryl groups become more electron withdrawing. These characteristics can be rationalized by using Gouterman's four-orbital model as a conceptual framework. The MCD bands observed in the Q band region of the subporphyrins (subPs) spectra consistently show a -ve/+ve intensity pattern in ascending energy, while, in contrast, the sign sequence of the bands observed in the Soret band region change dramatically depending on the nature of the aryl groups: from +ve/-ve in the case of the electron withdrawing 4-pyridyl, 4-(trifluoromethyl)phenyl and 3-pyridyl groups to -ve only for phenyl and -ve/+ve for the electron donating 4-tolyl and 4-methoxyphenyl groups. S1 fluorescence emission was observed in the 490-620 nm region. The quantum yields (phiF) in benzene (phiF = 0.10-0.12) are similar to that of metal-free tetraphenylporphyrin (H2TPP) (phiF = 0.11) but are somewhat lower in the case of ethanol (phiF = 0.06-0.07) due to the higher polarity. The redox potential differences observed between the first oxidation and reduction steps are in the 2.52-2.64 V range, which is larger than that of normal porphyrins (ca. 2.25 V). Molecular orbital (MO) calculations of these compounds help to provide an enhanced understanding of the spectroscopic and electrochemical properties. A byproduct of the synthesis was characterized using X-ray crystallography and a range of spectroscopic techniques. A subporphyrin mu-oxo dimer was prepared and characterized.

Supramolecular assemblies of a new class of nonplanar cationic metalloporphyrins and anionic metallophthalocyanines

The heteroaggregation behavior between a new class of nonplanar cationic β-octabrominated meso-alkylpyridinium zinc(II)-porphyrins (β-Br 8(ZnP)) and anionic tetrasulfonated metallophthalocyanines (MTSPc, M = Ni(II) and Cu(II)) has been studied by UV–Vis electronic spectroscopy, in dimethylsulfoxide (DMSO) solution. The heteroaggregate stoichiometry and the association constants were determined by means of Job plots. Dimers and unexpected trimers, taking into account the existence of axially coordinated DMSO molecules to the central metal in both β-Br 8(ZnP) and MTSPc complexes, are formed in solution. The spectroscopic properties of the heteroaggregates are markedly different from those observed in the correspondent planar cationic derivatives, the heteroaggregates showing major changes predominantly in the β-Br 8(ZnP) Soret band region and minor effects in the MTSPc Q bands. The observed changes in the Soret band region (red/blue shifts, decrease in the absorption intensities) depend on the nature of the alkyl substituent attached to the meso-pyridinium group. The greater versatility of the nonplanar porphyrins accommodating the meso-substituents in out-of-plane and in-plane conformations is proposed to explain the observed stoichiometries and the differences on the heteroaggregates spectroscopic properties for each β-Br 8ZnP compound. The likely conformations assumed by the meso-substituents in these β-Br 8(ZnP) compounds and its spectroscopic characteristics are in accordance with the participation of the substituents as the main factor on the extent of the observed red-shifted spectra in nonplanar porphyrins. The obtained association constants ( K IP) for the dimers and trimers are lower than those previously found for the similar planar cationic porphyrin systems, due to the lack of extensive π–π interactions and to the less effective approximation between the ionic groups, resulting in loosened heteroaggregates, particularly for the trimeric systems. Furthermore, the experimental results suggest that the NiTSPc is more distorted in DMSO solution than the CuTSPc derivative, favoring the interaction with the nonplanar β-Br 8(ZnP) compounds.

Porphyrins as spectroscopic sensors for conformational studies of DNA

Abstract Molecular systems containing two or more interacting porphyrins show remarkable spectroscopic features that allow for a very sensitive detection of conformational changes on the microscale level by different methods, such as fluorescence and electronic circular dichroism (ECD). Covalent porphyrin-DNA assemblies can provide a CD profile (exciton couplet) within the porphyrin Soret band region which is very diagnostic for DNA conformational changes. Additionally, covalently linked porphyrins have been shown to function as DNA molecular caps and to stabilize the non-self-complementary non-Watson-Crick guanine-adenine DNA sequence via their strong π-π stacking.

Photochemically-induced ligand exchange reactions of ethoxy-oxo-molybdenum(V) tetraphenylporphyrin in chlorinated solvents

Photochemically-induced ligand exchange reactions of ethoxy-oxo-molybdenum(V) 5,10,15,20-tetraphenylporphyrin, Mo(V)O(TPP)OEt, under irradiation at the Soret band region, were investigated. The reactions were performed in chlorinated solvents and followed with ultraviolet–visible (UV–vis) spectroscopy, whereas the products were analyzed with Fourier transformed infrared (FTIR) spectroscopy, electron spin resonance (ESR) spectroscopy and gas chromatography (GC). The chloro-oxo(TPP)Mo(V) complex was obtained as the final product, where the chlorine came from the solvent. Nevertheless, these reactions were not photocatalytic, due to the photochemical inertness of the formed chloro-oxo complex, and an excess of ethanol could not initiate a new photocatalytic cycle unless water was added as well, resulting in the formation of a biphasic system. On the other hand, addition of ethanol, water and NaOH to the reaction medium led to the formation of the corresponding oxo Mo(IV) complex. The studied photoassisted reactions of oxo Mo(V) porphyrins appear attractive for possible applications in the detection of chlorinated pollutants in sensing devices, but also for the dechlorination of pollutants upon suitable optimization of processing conditions.

Strong Two-Photon Absorption in New Asymmetrically Substituted Porphyrins: Interference between Charge-Transfer and Intermediate-Resonance Pathways

We study two-photon absorption (2PA) in two series of new free-base porphyrins with 4-(diphenylamino)stilbene or 4,4'-bis-(diphenylamino)stilbene (BDPAS) attached via pi-conjugating linkers at the porphyrin meso-position. We show that this new substitution modality increases the 2PA cross section in the Soret band region (excitation wavelength 750-900 nm) of the core porphyrin by nearly 2 orders of magnitude, from sigma(2) approximately 10 GM for the meso-phenyl-substituted analogue to sigma(2) approximately 10(3) GM for the ethynyl-linked BDPAS-porphyrin dyad. The 2PA properties are quantitatively described by considering two different and interfering 2PA quantum transition pathways. The first path involves virtual transition via intermediate one-photon resonance. The second path bypasses the intermediate resonance and occurs due to a large permanent dipole moment difference between the ground and the final electronic states. To our best knowledge, this is the first experimental observation of the combined effect of these two pathways on one particular two-photon transition, resulting in quantum-interference-modulated 2PA strength.

Investigation of human serum albumin (HSA) binding specificity of certain photosensitizers related to pyropheophorbide-a and bacteriopurpurinimide by circular dichroism spectroscopy and its correlation with in vivo photosensitizing efficacy

A series of pyropheophorbide-a and bacteriopurpurinimides were investigated to understand the correlation between HSA (site II) binding affinity and in vivo photosensitizing activity. In our study, photosensitizers that bound to site II of HSA produced a significant difference in the circular dichroism spectra of the corresponding complexes, especially at Soret band region of the photosensitizers. Our results suggest that CD spectroscopy of the photosensitizer–HSA complexes could be a valuable tool in screening new photosensitizers before evaluating them for in vivo efficacy.

Effect of Tris on catalytic activity of MP-11

The effect of tris(hydroxymethyl)aminomethane (Tris) on the catalytic activity and microstructure of heme undecapeptide, microperoxidase-11 (MP-11) in the aqueous solution was investigated using cyclic voltammetry, circular dichroism (CD) spectroscopy, UV–vis absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found for the first time that Tris would inhibit the catalytic activity and electrochemical reaction of MP-11 at the glassy carbon (GC) electrode. This is mainly due to the fact that Tris would induce more α-helix and β-turn conformations from the random coil conformation of MP-11, cause the asymmetric split-up in the Soret band region of MP-11, increase the non-planarity of the heme of MP-11, and change the electron densities of N, O and S atoms of MP-11. Meanwhile, It was found that the electrochemical reaction of MP-11 with Tris at GC electrode is diffusion-controlled, and the diffusion coefficient of MP-11 and the rate constant for the heterogeneous electron transfer of MP-11 in the presence of Tris are decreased by 19% and 16%, respectively. Further experiments showed that the electrocatalytic current of MP-11 on the reduction of H 2O 2 is decreased by about 25% after the addition of Tris to the MP-11 solution.

Vibrational and electronic circular dichroism and absorption spectral study of the DNA–5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin interaction

Vibrational circular dichroism (VCD), electronic circular dichroism (ECD) and IR and UV–Vis absorption spectra were measured for complexes of DNA with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) at the [DNA]/[TMPyP] ratios 2/1–10/1. The nitrogen base and phosphate spectral regions in IR and the Soret band and DNA regions were studied in UV–Vis. The VCD spectra indicate the priority of intercalation to external minor groove binding and the saturation of the intercalating sites for the [DNA]/[TMPyP]=4/1–5/1. The combination of spectroscopic results proved that all three binding modes, intercalation, minor groove binding and external binding to the phosphate backbone, take place for the ratio 3/1. It was shown that the melting point of the DNA–TMPyP complex is increased compared to pure DNA. The relative temperature stability of minor groove binding is smaller than of intercalation. The AT base-pair hydrogen bonds were disrupted at low pH for DNA–TMPyP complex contrary to pure DNA. Combination of VCD and ECD spectroscopy proved to be effective tool of the conformational study of the DNA–TMPyP complexes and its variations caused by different physico-chemical conditions in solution.

Structural Differences Between Light and Heavy Rare Earth Element Binding Chlorophylls in Naturally Grown Fern: Dicranopteris linearis

Chloroplasts and chlorophylls were isolated from the leaves of Dicranopteris linearis, a natural perennial fern sampled at rare earth element (REE) mining areas in the South-Jiangxi region (southern China). The inductively coupled plasma-mass spectrometry (ICP-MS) results indicated that REEs were present in the chloroplasts and chlorophylls of D. linearis. The in vivo coordination environment of light REE (lanthanum) or heavy REE (yttrium) ions in D. linearis chlorophyll-a was determined by the extended X-ray absorption fine structure (EXAFS). Results revealed that there were eight nitrogen atoms in the first coordination shell of the lanthanum atom, whereas there were four nitrogen atoms in the first coordination shell of yttrium. It was postulated that the lanthanum-chlorophyll-a complex might have a double-layer sandwich-like structure, but yttrium-binding chlorophyll-a might be in a single-layer form. Because the content of REE-binding chlorophylls in D. linearis chlorophylls was very low, it is impossible to obtain structural characteristics of REE-binding chlorophylls by direct analysis of the Fourier transform infrared (FTIR) and ultraviolet (UV)-visible spectra of D. linearis chlorophylls. In order to acquire more structural information of REE-binding chlorophyll-a in D. linearis, lanthanum - and yttrium-chlorophyll-a complexes were in vitro synthesized in acetone solution. Element analyses and EXAFS results indicated that REE ions (lanthanum or yttrium) of REE-chlorophyll-a possessed the same coordination environment whether in vivo or in vitro. The FTIR spectra of the REE-chlorophyll-a complexes indicated that REEs were bound to the porphyrin rings of chlorophylls. UV-visible results showed that the intensity ratios of Soret to the Q-band of REE-chlorophyll-a complexes were higher than those of standard chlorophyll-a and pheophytin-a, indicating that REE-chlorophyll-a might have a much stronger ability to absorb the ultraviolet light. The MCD spectrum in the Soret band region of lanthanum-chlorophyll-a showed a special peak, but yttrium-chlorophyll-a did not have this special peak, corresponding well to their double-layer and single-layer structure, respectively.

Thermodynamic analysis of porphyrin binding to Momordica charantia (bitter gourd) lectin.

Owing to the use of porphyrins in photodynamic therapy for the treatment of malignant tumors, and the preferential interaction of lectins with tumor cells, studies on lectin-porphyrin interaction are of significant interest. In this study, the interaction of several free-base and metalloporphyrins with Momordica charantia (bitter gourd) lectin (MCL) was investigated by absorption spectroscopy. Difference absorption spectra revealed that significant changes occur in the Soret band region of the porphyrins on binding to MCL. These changes were monitored to obtain association constants (Ka) and stoichiometry of binding. The tetrameric MCL binds four porphyrin molecules, and the stoichiometry was unaffected by the presence of the specific sugar, lactose. In addition, the agglutination activity of MCL was unaffected by the presence of the porphyrins used in this study, clearly indicating that porphyrin and carbohydrate ligands bind at different sites. Both cationic and anionic porphyrins bind to the lectin with comparable affinity (Ka =10(3)-10(5) m(-1)). The thermodynamic parameters associated with the interaction of several porphyrins, obtained from the temperature dependence of the Ka values, were found to be in the range: DeltaH degrees = -98.1 to -54.4 kJ.mol(-1) and DeltaS degrees =-243.9 to -90.8 J.mol(-1).K(-1). These results indicate that porphyrin binding to MCL is governed by enthalpic forces and that the contribution from binding entropy is negative. Enthalpy-entropy compensation was observed in the interaction of different porphyrins with MCL, underscoring the role of water structure in the overall binding process. Analysis of CD spectra of MCL indicates that this protein contains about 13%alpha-helix, 36%beta-sheet, 21%beta-turn, and the rest unordered structures. Binding of porphyrins does not significantly alter the secondary and tertiary structures of MCL.

Quasi-line vibronic spectra of 2,3,12,13-tetramethyldibenzoporphine and their interpretation

The quasi-line fluorescence and fluorescence excitation spectra of 2,3,12,13-tetrame-thyldibenzoporphine have been obtained at 77 K in n-octane and analyzed. The normal-coordinate analysis of this molecule has been performed, and the spectra have been interpreted in detail. In the Soret band region the fluorescence excitation quasi-line spectra clearly reveal two 0-0 components. The problem of the nature of the Soret band of free-base porphyrins is discussed.

Thermodynamic and kinetic analysis of porphyrin binding to Trichosanthes cucumerina seed lectin.

The interaction of several metallo-porphyrins with the galactose-specific lectin from Trichosanthes cucumeirna (TCSL) has been investigated. Difference absorption spectroscopy revealed that significant changes occur in the Soret band region of the porphyrins upon binding to TCSL and these changes have been monitored to obtain association constants (Ka) and stoichiometry of binding (n). The dimeric lectin binds two porphyrin molecules and the presence of the specific saccharide lactose did not affect porphyrin binding significantly, indicating that the sugar and the porphyrin bind at different sites. The Ka values obtained for the binding of different porphyrins with TCSL at 25 degrees C were in the range of 2 x 10(3)-5 x 10(5) m(-1). Association constants for meso-tetra(4-sulphonatophenyl)porphyrinato copper(II) (CuTPPS), a porphyrin bearing four negative charges and meso-tetra(4-methylpyridinium)porphyrinato copper(II) (CuTMPyP), a porphyrin with four positive charges, were determined at several temperatures; from the temperature dependence of the association constants, the thermodynamic parameters change in enthalpy (DeltaH degrees ) and change in entropy (DeltaS degrees ) associated with the binding process were estimated. The thermodynamic data indicate that porphyrin binding to TCSL is driven largely by a favourable entropic contribution; the enthalpic contribution is very small, suggesting that the binding process is governed primarily by hydrophobic forces. Stopped-flow spectroscopic measurements show that binding of CuTMPyP to TCSL takes place by a single-step process and at 20 degrees C, the association and dissociation rate constants were 1.89 x 10(4) m(-1).s(-1) and 0.29 s(-1), respectively.

PARAFAC for Spectral Curve Resolution: A Case Study Using Total Luminescence in Human Dental Tartar

Chromophore identification in biological samples often requires the physical separation of the compounds, which can be difficult. Although there are several advantages to hyphenated spectroscopic techniques for identification of substances, complex mixtures of chromophores presenting overlapped spectra cannot be identified directly through this method. This work presents an application of chemometrics to compound identification in biological samples by a spectroscopic hyphenated technique using a curve resolution method. The PARAllel FACtor analysis model (PARAFAC), which has no rotational indeterminacy, was used for curve resolution of excitation-emission spectra of human dental tartars. PARAFAC was applied under constraints (i.e., unimodality and non-negativity) and evaluated with a validation procedure. The resolved profiles are porphyrinic-like spectra presenting excitation band maxima at 407, 416, and 431 nm in the Soret band region (390–440 nm) of these substances.

ATP Induces a Conformational Change in Lipid-bound Cytochrome c

Resonance energy transfer studies using a pyrene-labeled phospholipid derivative 1-palmitoyl-2-[10-(pyren-1-yl)decanoyl]-sn-glycero-3-phosphoglycerol (donor) and the heme (acceptor) of cytochrome c (cyt c) have indicated that ATP causes changes in the conformation of the lipid-bound protein (Rytömaa, M., Mustonen, P., and Kinnunen, P. K. J. (1992) J. Biol. Chem. 267, 22243-22248). Accordingly, after binding cyt c via its so called C-site to neat phosphatidylglycerol liposomes (mole fraction of PG = 1.0) has commenced, further quenching of donor fluorescence is caused by ATP, saturating at 2 mm nucleotide. ATP-induced conformational changes in liposome-associated cyt c could be directly demonstrated by CD in the Soret band region (380-460 nm). The latter data were further supported by time-resolved spectroscopy using the fluorescent cyt c analog with a Zn(2+)-substituted heme moiety. A high affinity ATP-binding site has been demonstrated in cyt c (Craig, D. B., and Wallace, C. J. A. (1993) Protein Sci. 2, 966-976) that is compromised by replacing the invariant Arg(91) to norleucine. Although no major effects on conformation and function of cyt c were concluded due to the modification, a significantly reduced effect by ATP on the lipid-bound [Nle(91)]cyt c was evident, implying that this modulation is mediated via the Arg(91)-containing binding site.

Single Site Electronic Spectroscopy of Zinc and Magnesium Chlorin in n-Octane Matrixes at 7 K

The high resolution, single site fluorescence and absorption spectra of zinc and magnesium chlorin in n-octane matrixes at 7 K are reported. Both the Q and Soret band regions have been investigated...

Transient absorption anisotropy study of ultrafast energy transfer in porphyrin monomer, its direct meso–meso coupled dimer and trimer

Transient absorption anisotropies of the Zn(II)porphyrin monomer, its direct meso–meso coupled dimer, and trimer are measured in the B (Soret) band region by sub-30 fs laser pulses. It is shown that detailed information on the electronic structures and energy transfer dynamics can be obtained from the anisotropy and the magic angle data. The anisotropies of all three molecules exhibit remarkable behaviors in the first 200 fs region. Experimental observations can be accounted for adequately by treating the transient absorption signal as an explicit sum of ground state bleach and excited state stimulated emission contributions. In the monomer, the anisotropy decay denotes an oscillatory feature followed by a 100 fs time constant exponential decay. It is argued that the Bx and By transitions of the porphyrin monomer are nondegenerate with an energy splitting of ∼170 cm−1. Furthermore, equilibration of the excitation energy within the B states is slightly underdamped. Excitation of the monomerlike B band of the dimer (trimer) leads to ultrafast ∼30 fs (60 fs) anisotropy decay and a subsequent rise with ∼60 fs (70 fs) time constant. It is concluded that the anisotropy decay is due to the ultrafast energy transfer to the low-energy exciton split B band, while the rise is due to the redshift of the excited state stimulated emission spectrum thereby decreasing the contribution of the excited state stimulated emission component in the overall transient absorption signal.