Porphyrin Monolayer Research Articles

Minimising monolayer collapse on Langmuir troughs

The stability (area loss as a function of time) of dipalmitoylphosphatidylcholine (DPPC) at the air–water interface is an important property in the context of studying its behaviour as a major component of synthetic lung surfactant. However, some Langmuir troughs cannot support a stable monolayer of lung surfactant for long periods of time owing to adhesion between the surfactant and the materials that compose the trough. In this work, the stability has been measured as a function of surface pressure, Wilhelmy plate length and width, compression barrier material and the height of the water surface relative to the edge of the compression barriers on a Nima 601M Langmuir trough. Octadecanoic acid (stearic acid) and a non-amphiphilic porphyrin, 5,10,15,20-tetrakis(3,4-bis[ethylhexyloxy]phenyl)-porphyrinatozinc (Zn EHO) have also been investigated in order to identify the features of area loss which are surfactant material dependent and those which are affected by the properties of the trough itself. Stability of the highly amphiphilic monolayers was greatly improved when using Delrin barriers instead of PTFE barriers but had little effect on the porphyrin monolayers.

Synthesis of Thioacetate‐Functionalized Cobalt(II) Porphyrins and Their Immobilization on Gold Surface – Characterization by X‐ray Photoelectron Spectroscopy

AbstractCobalt tetraarylporphyrins 1‐Co and 2‐Co with thioacetate‐functionalized carbon chains on the aryl groups were synthesized. The cobalt porphyrin 2‐Co was immobilized on a gold surface after deprotection of the S‐acetyl group. The immobilized porphyrin was studied by X‐ray Photoelectron Spectroscopy (XPS) and the results suggest that a complete monolayer of porphyrins is formed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis and characterization of silica microspheres functionalized with porphyrin monolayer

A new type of inorganic and organic composite (SiO2-MPS-TAPPI), oppositely charged meso-tetra-(4-trimethylaminophenyl)porphyrin iodide (TAPPI) bonded on the surface of silica microspheres coated with mercaptopropyltrimethoxysilane (MPS), was prepared by self-assembly method. The composite was tested by TEM, XRD and TG. The TEM images show that SiO2-MPS microspheres in the composite are uniform. There are linkages among some different SiO2-MPS microspheres. The XRD measurement bears out that the composite structure is the same with SiO2-MPS. TG curves show that thermal stability of TAPPI assembled onto SiO2-MPS microspheres is higher than that of TAPPI. Solid UV diffusion reflection spectra and fluorescence spectra are used to investigate the photophysical properties of the SiO2-MPS-TAPPI composite. It is showed that the spectra of the composite exhibit significant enhancement of the spectra range. The strong interaction between TAPPI and SiO2-MPS in the composite is responsible for the different spectra.

Migration and transfer of excitation energy in homogeneously dispersed porphyrin monolayers prepared from amphiphilic copolymers

Photophysical processes of tetraphenylporphyrin (TPP) units incorporated into a polymer monolayer have been studied by fluorescence spectroscopy with emphasis on energy migration and aggregate formation of TPPs in the two-dimensional sheet. Although UV–VIS absorption spectra showed few of TPP aggregates, the fluorescence was largely quenched in the high-TPP samples due to efficient energy migration and following energy transfer to the aggregates. With the aid of computer simulation, time-resolving decay analysis revealed the large diffusion coefficient of excitation energy and a low concentration of aggregate, suggesting that the TPP moieties were placed in the plane not with a random distribution, but with an ordered arrangement, probably due to some mutual repulsive forces which prevented overlap of the TPP units at the air–water interface. These results suggest that the polymer monolayer containing TPP units can be utilized as a component of molecular assemblies for designing efficient energy harvesting systems.

Self-Organization of Porphyrins and Fullerenes for Molecular Photoelectrochemical Devices

We have constructed a variety of molecular assemblies of porphyrin as a donor and fullerene as an acceptor on electrodes for molecular photoelectrochemical devices. Highly efficient energy- and electron-transfer processes have been realized at gold electrodes modified with self-assembled monolayers of porphyrin- or fullerene linked systems mimicking light-harvesting and charge separation in bacterial photosynthesis. Highly ordered organization of porphyrins and fullerenes has also been achieved using step-by-step self-assembly of porphyrin and fullerene units by association with gold nanoparticles or dendrimers on tin oxide electrodes, which exhibit high power-conversion efficiency of up to 1.5%. These results will provide valuable information on the design of donor-acceptor type molecular assemblies that can be tailored to construct highly efficient organic solar cells.

Zwitterionic vs porphyrin free-base structures in 4-phenylsulfonic acid meso-substituted porphyrins

The 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TPPS4) and 5-phenyl-10,15,20-tris(4-sulfophenyl)porphyrin (TPPS3)] were obtained for the first time as the pure sulfonic acid derivatives and characterized for their constitutional structure in the solid state and in water solution. The zwitterionic species in water solutions, above a critical micellar concentration, are stabilized by side-to-side homoassociation to J-aggregates (B-band: 490 nm), but in solids they only give the J-aggregates when water is present in enough amount. Severe dried solids show a zwitterionic species, stabilized by side-to-side interactions, that probably corresponds to the dimer (B-band: 455 nm). The monomeric zwitterion (B-band: 434 nm) is detected in dilute water solutions and its acid/base properties (concentration pK a (1/2) ≈ 5.0) are not significantly different from those of the acidified solutions of their sodium sulfonate derivatives. However, pure water solutions of TPPS n , in contrast to those of their sodium salts, show important interactions with hydrophilic surfaces: e.g. TPPS4 adsorbs on fused quartz forming a monolayer of the free-base porphyrin together with its counter cations. Dilution of the title porphyrins in a solid matrix (e.g. KBr ) leads to a free-base porphyrin species with a red-shifted B-band (424 nm) that points to a side-to-side homoassociation, which is in contrast with the typical π-stacked aggregates of the corresponding alkaline metal sulfonato salts.

Covalent Bonding of Alkene and Alkyne Reagents to Graphitic Carbon Surfaces

Various aromatic and aliphatic alkynes and one alkene were covalently bonded to sp(2)-hybridized carbon surfaces by heat treatment in an argon atmosphere. X-ray photoelectron spectroscopy, Raman, and FTIR spectra of the modified surfaces showed that the molecules were intact after the 400 degrees C heat treatment but that the alkyne group had reacted with the surface to form a covalent bond. Alkynes with ferrocene and porphyrin centers exhibited chemically reversible voltammetric waves that could be cycled many times. Atomic force microscopy of the modified surfaces indicated a thickness of the molecular layer consistent with monolayer coverage, and surface coverage determined by voltammetry was also in the monolayer range. Raman spectroscopy of the porphyrin monolayers formed from a porphyrin alkyne showed no evidence for dimer formation, although multilayer formation may occur at undetected levels. FTIR spectra of the porphyrin-modified carbon surfaces were well-defined, similar to the parent molecule, and indicative of an average tilt angle between the porphyrin plane and the surface normal of 37 degrees . The bond between the molecular monolayer and the carbon surface was quite stable, withstanding sonication in tetrahydrofuran, mild aqueous acid and base, and repeated voltammetric cycling in propylene carbonate electrolyte. Heat treatment of alkynes and alkenes appears to be a generally useful method for modifying carbon surfaces, which can be applied to both aromatic and aliphatic molecules.

Self-assembled monolayers of porphyrin–perylenetetracarboxylic diimide–[60] fullerene onindium tin oxide electrodes: enhancement of light harvesting in the visible lightregion

The porphyrin–perylenetetracarboxylic diimide–fullerene triad has been designedto act as an efficient light-to-current converter in molecular devices. Lightabsorption by the perylenetetracarboxylic diimide–porphyrin dyad occurs incomplementary regions, giving extended absorption in the visible light region. Thisdyad of perylenetetracarboxylic diimide–porphyrin was covalently linked withC60, which can act as an electron acceptor, to construct porphyrin–perylenetetracarboxylicdiimide–fullerene (FPP) self-assembled monolayers (SAMs) on the indium tin oxide (ITO)surface. The relative strong photocurrent generation of the ITO/FPP/ methyl viologen(MV2+)/Pt system in the continuous visible light region (from 400 to 650 nm) indicated that thissystem is highly promising as regards light–energy conversion.

Optical pH Meter by Means of a Porphyrin Monolayer Covalently Assembled on a Molecularly Engineered Silica Surface

The evolution of the UV−vis absorbance spectra of a new porphyrin covalently assembled monolayer, upon immersion in increasing [H3O+] solutions, showed the disappearing of the Soret band and the progressive increase of a new band. A few seconds of H2O washing and drying under a N2 gas stream totally recovers the starting Soret. Therefore, the system represents a well-suited fast and reversible solid optical pH meter.

Redox and photochemical behaviour of a porphyrin monolayer on an indium-tin oxide electrode

In order to investigate photoluminescence behaviour of an ordered molecular porphyrin monolayer and its quenching properties by oxygen gas, a porphyrin with long alkyl chains, 5,10,15,20-tetrakis[4-(11-carboxylundecane-1-oxy)phenyl]porphyrin ( 4), was synthesized and adsorbed onto an indium-tin oxide (ITO) substrate by a chemical dipping method. Cyclic voltammetry was used to analyze the ITO electrode coated with 4. The peak current of the first oxidation was proportional to the sweep rate, and the surface coverage was estimated to be 2.3–2.5 × 10 −10 mol cm −2. The UV–vis spectrum of the monolayer showed a broadened Soret band, which shifted to longer wavelength. These features suggest that the porphyrin moieties of 4 are packed to form a J-type structure. The oxygen quenching ratio of the porphyrin 4 monolayer on the ITO electrode, I 0/ I 100, was estimated to be 1.25, where I 0 and I 100 are, respectively, luminescence intensity values in 100% argon and 100% oxygen. On repeated step cycling between 100% argon and 100% oxygen atmospheres, the response times of luminescence quenching were 10 s (argon to oxygen) and 23 s (oxygen to argon). These findings suggest that a monolayer of sensing dye is applicable for oxygen sensing system without deterioration of size-accuracy of models.

Self-assembled monolayers of a thiol-derivatized porphyrin on gold electrode: Film formation and electrocatalytic dioxygen reaction

A novel thiol-derivative porphyrin (5-[p-(mercaptopropyloxy)-phenyl]-10,15,20-triphenylporphine shortly as H 2MPTPP) and its cobalt(II) complex (MPTPPCo(II) ) have been successfully immobilized on gold surface by self-assembly technique. The surface coverages ( Γ) of MPTPPCo(II) was calculated to be 4.86 × 10 − 11 mol/cm 2 by investigating the reductive desorption of S–Au which demonstrated that the monolayer was not packed well and had many interval pinholes. The gold electrodes modified with MPTPPCo(II) and MPTPPCo(II) + thiolglycolic acid show good activity to the two-electron catalytic reduction of O 2 to H 2O 2. Nanoscopic structures gained by atomic force microscopy (AFM) not only provided further evidence for the successful formation and immobilization of such monolayers, but also elucidated the potential reasons for the electrocatalytic activity of such monolayers to dissolved oxygen, which was proved to be very interesting and significant that the mixed monolayer could enhance the electroreduction and proceed more readily for the electroreduction of O 2.

Structural and Electron-Transfer Characteristics of O-, S-, and Se-Tethered Porphyrin Monolayers on Si(100) [J. Am. Chem. Soc. 2004, 126, 15603−15612

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionStructural and Electron-Transfer Characteristics of O-, S-, and Se-Tethered Porphyrin Monolayers on Si(100) [J. Am. Chem. Soc. 2004, 126, 15603−15612].Amir A. Yasseri, Dennis Syomin, Robert S. Loewe, Jonathan S. Lindsey, Francisco Zaera, and David F. BocianCite this: J. Am. Chem. Soc. 2005, 127, 25, 9308Publication Date (Web):June 2, 2005Publication History Published online2 June 2005Published inissue 1 June 2005https://doi.org/10.1021/ja053081dCopyright © 2005 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views404Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (10 KB) Get e-Alerts Get e-Alerts

Photoinduced Electron Transfer in Self-Assembled Monolayers of Porphyrin−Fullerene Dyads on ITO

Two porphyrin-fullerene dyads were synthesized to form self-assembled monolayers (SAMs) on indium-tin oxide (ITO) electrode, with either ITO-porphyrin-fullerene or ITO-fullerene-porphyrin orientations. The dyads contain two linkers for connecting the porphyrin and fullerene moieties and enforcing them essentially to similar geometries of the donor-acceptor pair, and two linkers to ensure the attachment of the dyads to the ITO surface with two desired opposite orientations. The transient photovoltage responses (Maxwell displacement charge) were measured for the dyad films covered by insulating LB films, thus ensuring that the dyads interact only with the ITO electrode. The direction of the electron transfer was from the photoexcited dyad to ITO independent of the dyad orientation. The response amplitude for the ITO-fullerene-porphyrin structure, where the primary intramolecular electron-transfer direction coincides with the direction of the final electron transfer from the dyad to ITO, was 25 times stronger than that for the opposite ITO-porphyrin-fullerene orientation of the dyad. Static photocurrent measurements in a liquid electrochemical cell, however, show only a minor orientation effect, indicating that the photocurrent generation is controlled by the processes at the SAM-liquid interface.

Photoinduced Electron Transfer in Langmuir−Blodgett Monolayers of Porphyrin−Fullerene Dyads

A series of electron donor-acceptor (DA) dyads, composed of a porphyrin donor and a fullerene acceptor covalently linked with two molecular chains, were used to fabricate solid molecular films with the Langmuir-Blodgett (LB) technique. By means of the LB technique, the DA molecules can be oriented perpendicular to the plane of the substrate. In DHD6ee and its zinc derivative hydrophilic groups are attached to the phenyl moieties in the porphyrin end of the molecule; while in the other three dyads, TBD6a, TBD6hp, and TBD4hp, the hydrophilic groups are in the fullerene end of the molecule. This makes it possible to alternate the orientation of the molecules in two opposite directions with respect to the air-water interface and to fabricate molecular assemblies in which the direction of the primary photoinduced vectorial electron transfer can be controlled both by the deposition direction of the LB monolayer and by the selection of the used DA molecule. This was proved by the time-resolved Maxwell displacement charge measurements. The spectroscopic properties of the DA films were studied with the steady-state absorption and fluorescence methods. In addition, the time correlated single photon counting technique was used to determine the fluorescence properties of the dyad films.

Structural and Electron-Transfer Characteristics of Carbon-Tethered Porphyrin Monolayers on Si(100)

Structural and electron-transfer characteristics are reported for two classes of zinc porphyrin monolayers attached to Si(100) surfaces via Si-C bonds. One class, designated ZnP(CH(2))(n)- (n = 2-4), contains an alkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being p-tolyl groups. The other, designated ZnPPh(CH(2))(n)- (n = 0-3), contains a phenyl or phenylalkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being mesityl groups. Both classes of zinc porphyrin monolayers on Si(100) were examined using Fourier transform infrared spectroscopy and various electrochemical methods. The studies reveal the following: (1) The structural and electron-transfer characteristics of the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers are generally similar to those of monolayers formed from porphyrins with analogous linkers, but anchored with an O, a S, or a Se atom. (2) The ZnP(CH(2))(n)-, ZnPPh-, and ZnPPhCH(2)- monolayers exhibit lower saturation coverages and have their porphyrin ring more tilted with respect to the surface normal than the ZnPPh(CH(2))(2)- and ZnPPh(CH(2))(3)- monolayers. (3) The electron-transfer rates for both the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- classes of monolayers monotonically decrease as the length of the linker increases. (4) For all the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers, both electron-transfer rates and charge-dissipation rates decrease monotonically as the surface coverage increases. Collectively, the studies reported herein provide a detailed picture of how the linker type influences the structural and electron-transfer characteristics of these general classes of monolayers.

Observation of co-existence of ‘face-on’ and ‘edge-on’ stacking styles in a porphyrin monolayer

We report a direct observation adopting ‘face-on’ and ‘edge-on’ stacking styles in monolayer of a novel porphyrin derivative on highly ordered pyrolytic graphite (HOPG) using scanning tunneling microscope (STM). The porphyrin derivative, 5-hydroxyphenyl-10, 15, 20-trikis(4-dodecyloxyphenyl)porphyrinatozinc(II) (ZnDPPOH) is synthesized and assembled on HOPG. In the STM images, we observed two kinds of ordered domains. One consists of ZnDPPOH dimmers, adopting ‘face-on’ stacking style, the other exhibits a segmented columnar structure, like piled-up ‘armchairs’, lying down on the substrate, adopting an ‘edge-on’ stacking style. In the latter case, the conjugated and highly delocalized π-systems play key roles in stabilizing the monolayer.

Characterization, Optical Recognition Behavior, Sensitivity, and Selectivity of Silica Surfaces Functionalized with a Porphyrin Monolayer

The optical gas recognition capability of a covalently self-assembled monolayer of 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin molecules on silica substrates was studied. The following analytes have been investigated: NO2, CO, CH4, H2, NH3, HCl, CHCl3, C2H5OH, CH3OH, pyridine, tetrahydrofurane, triethylamine, and dimethylformamide. The self-assembled porphyrin monolayer appears highly sensitive to 1 ppm of NO2 in both anhydrous and humid conditions. The selectivity of the self-assembled porphyrin monolayer with respect to other analytes was also examined and present data show that the presence of CO, CH4, H2, and NH3 does not influence its UV−vis spectrum. Many common solvents slowly affect the position of the Soret band. The presence of HCl vapors results in a broad band extending over the entire 440−500 nm range while the starting Soret disappears. UV−vis measurements on a n-hexane 1.0·10-5 solution of the 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine, that does not contain phenyl substituents in the meso positions, show no changes in the optical spectrum upon NO2 interaction. It emerges that aromatic substituents in the meso positions play a crucial role in determining the optical sensing properties.

Preparation, Characterization, and Electrical Properties of a Self-Assembled meso-Pyridyl Porphyrin Monolayer on Gold Surfaces

A meso-pyridyl porphyrin, 5-(4-(2-(4-(S-acetylthiomethyl)phenyl)ethynyl)phenyl)-10,15,20-tris(4-pyridyl) porphyrin was synthesized by coupling of 5-{4-ethynyl}phenyl-10,15,20-tris(4-pyridyl) porphyrin with 4-(S-acetylthiomethyl)-4-iodobenzene under mild palladium coupling conditions. The meso-pyridyl porphyrin was used for the preparation of self assembled monolayers on gold substrates. The gold substrates were made by the thermal evaporation of gold on oxidized Si(100) surfaces. The self-assembled monolayers (SAMs) were characterized using ground state UV absorption and X-ray photoelectron spectroscopic (XPS) techniques and also by scanning probe microscopy (SPM) techniques. The UV-absorption spectra of the porphyrin monolayer exhibited a 10 nm red shift in the Soret band compared to the porphyrin in CHCl3 solution. This indicates that the porphyrin molecules are aligned on the gold substrate in a side-by-side orientation. The typical shifts in the binding energy by XPS revealed that the chemisorption of the thiolate species of the porphyrin to the surface occurs through a strong sulfur–gold bonding mechanism. Basic hexagonal (√3×√3) R 30° well ordered self-assembled monolayers of meso-pyridyl porphyrin was observed by molecular-resolution atomic force microscopy (AFM). Room temperature current–voltage (I–V) spectra by scanning tunneling spectroscopy (STS) at varying set point current were collected to study the electronic transport properties of the monolayers on the gold surface.

Photoinduced electron transfer in multilayer self-assembled structures of porphyrins and porphyrin–fullerene dyads on ITO

A new strategy for constructing well-ordered, self-assembled multilayer structures of photoactive donor–acceptor systems has been developed. In this approach indium–tin oxide (ITO) electrodes were modified with successive self-assembled monolayers of Zn porphyrins (ZnP) and free-base porphyrin–fullerene (H2P–C60) dyads to obtain oriented triad configurations. All the layers were attached with two molecular linkers to achieve an organized composition. The multilayer structures were found to enhance the photocurrent and photovoltage generation compared to monolayers of the same compounds. Particularly, combining a densely packed ZnP layer with the highly oriented H2P–C60 dyad layer resulted in 280 times higher photovoltage response than measured for a monolayer of the dyad. Furthermore, both photovoltage and photocurrent were generated in the desired direction pursued with the sample orientations and, as a result of the organization, almost ideal current–voltage curves were formed.

Amphiphilic meso-Disubstituted Porphyrins: Synthesis and the Effect of the Hydrophilic Group on Absorption Spectra at the Air−Water Interface

Five amphiphilic meso-disubstituted porphyrins bearing one polar group were synthesized, and their monolayer films were prepared. Their limiting molecular areas obtained from pi-A isotherms were 0.5-0.6 nm2, which were smaller than those of the corresponding meso-tetrasubstituted porphyrins. At the air-water interface, the disubstituted porphyrins showed a broad band in visible absorption spectra compared with the solution state, and the red shift of the Soret band exhibited a significant dependence on the kind of hydrophilic groups at the meso positions. Interestingly, the monolayer of the disubstituted porphyrin bearing a carboxyphenyl group (1-CO2H) showed a blue shift of the Soret band by adding cadmium chloride to the subphase, and the spectra varied upon multilayer deposition. The observed phenomena were interpreted by using the exciton theory. The effect of hydrophilic substituents on the absorption spectra of disubstituted porphyrin monolayers is discussed.