Porphyrin Layers Research Articles

Photocurrent Enhancement by Multilayered Porphyrin Sensitizers in a Photoelectrochemical Cell

Multilayer Zn(II) tetraphenylporphyrin chromophores, assembled using copper-catalyzed azide-alkyne cycloaddition (CuAAC), provide a new sensitization scheme that could be useful in dye-sensitized solar cells (DSSCs). We report on the photoelectrochemical responses of multilayer films of Zn(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin (1) assembled on planar ITO substrates operating as a p-type DSSC using three different redox mediators. The traditional I(-)/I3(-) redox couple results in the greatest short circuit current densities (JSC) but very low open circuit potentials (VOC). The use of cobalt sepulchrate ([Co(sep)](2+/3+)) and cobalt tris-bipyridine ([Co(bpy)3](2+/3+)) as redox mediators generates higher VOC values, but at the expense of lower photocurrents. These results highlight the inherent differences in the interactions between the redox mediator and Zn(II) tetraphenylporphyrin multilayer films. Increasing the porphyrin content through multilayer growth proved to be effective in increasing the performance of photoelectrochemical cells with all three redox mediators. Cells using I(-)/I3(-) reached maximum performance (power output) at five porphyrin layers, [Co(bpy)3](2+/3+) at five layers, and [Co(sep)](2+/3+) at three layers. For all mediators, JSC increases with the addition of porphyrin layers beyond a monolayer. However, JSC reaches a maximum value at a point greater than one layer after which it decreases, presumably due to exciton diffusion limitations and the insulating effects of the multilayer film. Similarly, all cells also reach a maximum VOC beyond one porphyrin layer. We show that porphyrin arrays assembled using newly developed CuAAC layer-by-layer growth may be useful as a multilayer sensitization scheme for use in photoelectrochemical cells.

The effects of urea, guanidinium chloride and sorbitol on porphyrin aggregation: Molecular dynamics simulation

This paper compares the inhibition effect of porphyrin aggregation in the presence of urea, guanidinium chloride (Gdn) and sorbitol by molecular dynamics simulation. It demonstrates that porphyrin aggregation increases in sorbitol, but decreases towards addition of urea and Gdn. It shows that urea, Gdn and sorbitol can have a large effect — positive or negative, depending on the concentration — on the aggregation of the porphyrin. The effect of urea, Gdn and sorbitol on porphyrin aggregation has been inferred from the effect of these solutes on the hydration layer of porphyrin. It appears that the Gdn is more suitable than urea for decreasing the hydration layer of porphyrin while several osmolites like sorbitol are known to increase hydration layer and thus might stabilize the porphyrin aggregation. Results of radial distribution function (RDF), distributed atoms or molecules around target species, indicated that the increase and exclusion of solvent around porphyrin by osmolytes and Gdn would affect significantly on porphyrin aggregation. There was a sizeable difference in potency between the Gdn and urea, with the urea being less potent to decrease hydration layer and porphyrin aggregation. A comparative study of the effect of urea, guanidinium chloride (Gdn) and sorbitol on porphyrin aggregation was carried out by molecular dynamics simulation. The aggregation increases in sorbitol, but decreases upon addition of urea and Gdn.

Enhanced magneto-optical SPR platform for amine sensing based on Zn porphyrin dimers

Ethane-bridged Zn porphyrins dimers (ZnPP) have been deposited by Langmuir–Schäfer (LS) deposition technique onto proper transducer layers for surface plasmon resonance (SPR) and magneto-optical surface plasmon resonance (MO-SPR) characterization techniques performed in controlled atmosphere. This last tool has emerged as a novel and very performing sensing technique using as transducer layers a combination of noble and magnetic layers deposited onto glass substrates. A magnetic actuation allows recording a magneto optical SPR signal which ensures best gas sensing performances in terms of signal to noise ratio, sensitivity and limit of detection parameters. Primary and secondary amines in vapour phase have been used as sensing analytes and a possible explanation of the mechanism as well as of the dynamics of the interaction with the sensing Zn Porphyrin layers is provided.

Microstructures and photovoltaic properties of C60 based solar cells with copper oxides, CuInS2, phthalocyanines, porphyrin, PVK, nanodiamond, germanium and exciton diffusion blocking layers

Organic solar cells have a potential for use in lightweight, flexible, inexpensive and large scale solar cells. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor–acceptor proximity in the devices, which are called bulk heterojunction solar cells. Bulk heterojunction is an efficient method to generate free charge carriers, and the charge transfer is possible at the semiconductor interface. The purpose of the present work is to fabricate and characterise C60 based solar cells with copper oxides, CuInS2, phthalocyanines, porphyrin, poly-vinylcarbazole, nanodiamond, germanium and exciton diffusion blocking layers. In the present work, C60 and fullerenol [C60(OH)10–12] were used for n-type semiconductors, and metal copper oxides, metal phthalocyanine derivative, porphyrin and poly-vinylcarbazole were used for p-type semiconductors. In addition, nanodiamond and germanium based molecules were added into the active layers of the solar cells. The novel aspect of the research is to investigate the relation between properties and microstructures of the solar cells using transmission electron microscopy, X-ray diffraction and electronic structure calculation. The impact of the research concerns the study of organic solar cells by means of microstructural analysis, property measurements and theoretical calculations.

Exploring dicyano-ferriprotoporphyrin as a novel electrocatalytic material for selective H2S gasotransmitter detection

Dicyano-ferriprotoporphyrin supported on Pt microelectrodes was investigated as a possible electrocatalytic material capable of selectively oxidizing the gasotransmitter H2S over interfering gasotransmitters CO and NO. The modified microelectrodes were able to detect H2S concentrations as low as 50nM at a potential of 0.15V vs. Ag/AgCl and demonstrated a 100–1000× higher sensitivity toward H2S than CO and NO, respectively. However, H2S detection was affected by the presence of CO and NO due, at least in part, to imperfect coverage of the underlying Pt microelectrode by the electrocatalyst. Incomplete CN− coordination and questionable stability of the porphyrin layer are also discussed as possible causes of CO and NO interference. Work is on-going to alleviate these limitations in order to fully develop the material into a practical electrocatalyst capable of selective H2S oxidation.

Self-assembly of metal free porphyrin layers at copper-electrolyte interfaces: Dependence on substrate symmetry

Combined Cyclic Voltammetry (CV) and in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) studies were employed to compare the self-assembly of redox-active 5,10,15,20-Tetrakis (4-trimethyl ammonium phenyl) porphyrin tetra (p-toluenesulfonate), abbreviated as [H2TTMAPP]+4 on Cu(100) and Cu(111) electrode surfaces in 10mM HCl solution. Under these conditions, both surfaces are chloride pre-covered with a c(2×2) Cl- and cp×3 Cl-layer on Cu(100) and Cu(111), respectively. On both surfaces highly ordered [H2TTMAPP]+4 layers are spontaneously formed. The short range molecular arrangement is quadratic in nature on both surfaces which can be described by a 58×58 R23° unit cell on the c(2×2) Cl-Cu(100), and a (3×4) unit cell on the cp×3 Cl-Cu(111) substrate. Large scale domains are formed on both surfaces the orientation of which reflects the symmetry of the substrate, namely on Cu(100) [Cu(111)] equivalent domains are rotated by 90° [120°], respectively. Thus, the different symmetry of the respective substrate surface has an influence on the otherwise self-assembled organic molecules.

Comparison of nitric oxide binding to different pure and mixed protoporphyrin IX monolayers

The nitric oxide (NO) binding properties of monolayers of four different protoporphyrins IX adsorbed on aluminum oxide surfaces have been investigated. XPS and AFM results are consistent with the presence of a monolayer of porphyrins, bound to the surface by their carboxylic acid groups and with the porphine ring oriented more or less perpendicular to the surface. Kelvin probe measurements were done in which the samples were exposed to varying NO concentrations. NO binds strongly to all porphyrins, but protoporphyrin IX zinc(II) (Zn-PP) shows the most promising response. The response rate could be increased by adding spacer molecules to the porphyrin layer. Low ppb levels of NO were easily detected in ambient conditions. The NO binding properties of Zn-PP compare favorably to those of other porphyrins which were previously used for NO sensing. This makes Zn-PP a new and promising candidate for this application.

Directed electron transfer in Langmuir–Schäfer layers of porphyrin–fullerene and phthalocyanine–fullerene dyads in inverted organic solar cells

In this study double linked porphyrin-fullerene and phthalocyanine-fullerene dyads and a single linked phthalocyanine-fullerene dyad were studied as components in inverted organic solar cells (OSCs) equipped with the well known P3HT:PCBM bulk heterojunction as the photoactive layer. The dyad monolayers were deposited onto a surface of P3HT:PCBM by using the Langmuir-Schäfer method, therefore forming oriented monolayers in which the electron donor (D) and the acceptor (A) exist as a close proximity pair in a 1:1 molar ratio. As a result of this structure short circuit current density (J(sc)), open circuit voltage (V(oc)), and power conversion efficiency (η) increased, while the fill factor (FF) remained the same. The devices which contained dyads with double linkage produced higher efficiencies than the one with a single linked dyad. This result can be explained in terms of molecular orientation. It was also verified that the prepared OSC devices have promising long term air stability.

Effects of protonation of pyridine moieties on the 2D assembly of porphyrin layers on Au(111) at electrochemical interfaces

Unique molecular assemblies of a porphyrin derivative are prepared on Au(111) by controlling the protonation/unprotonation of the pyridine groups. The porphyrin derivative, driven by the protonation of the pyridine groups, can provide characteristic assemblies with specific molecular conformations on an Au(111) surface at the electrochemical interface. In situ scanning tunneling microscopy images revealed clear differences in the adlayer structures for the unprotonated and the protonated forms of the molecules that depended upon the electrochemical potential.

Molecules for Charge-Based Information Storage

The inexorable drive to miniaturize information storage and processing devices has fueled the dreams of scientists pursuing molecular electronics: researchers in the field envisage exquisitely tailored molecular materials fulfilling the functions now carried out by semiconductors. A bottom-up assembly of such all-molecular devices would complement, if not supplant, the present top-down lithographic procedures of modern semiconductor fabrication. Short of these grand aspirations, a more near-term objective is to construct hybrid architectures wherein molecules are incorporated in semiconductor-based devices. Such a combined approach exploits the advantages of molecules for selected device functions while retaining the well-developed lithographic approaches for fabrication of the overall chip. In this Account, we survey more than a decade of results from our research programs to employ porphyrin molecules as charge-storage elements in hybrid semiconductor-molecular dynamic random access memory. Porphyrins are attractive for a variety of reasons: they meet the stability criteria for use in real-world applications, they are readily prepared and tailored synthetically, they undergo read-write processes at low potential, and they store charge for extended periods (up to minutes) in the absence of applied potential. Porphyrins typically exhibit two cationic redox states. Molecular architectures with greater than two cationic redox states are achieved by combinations of porphyrins in a variety of structures (for example, dyads, wherein the porphyrins have distinct potentials, triple deckers, and dyads of triple deckers). The incorporation of porphyrins in hybrid architectures has also required diverse tethers (alkyl, alkenyl, alkynyl, aryl, and combinations thereof) and attachment groups (alcohol, thiol, selenol, phosphonate, and hydrocarbon) for linkage to a variety of surfaces (Au, Si, SiO(2), TiN, Ge, and so forth). The porphyrins as monolayers exhibit high charge density and are robust to high-temperature excursions (400 °C for 30 min) under inert atmosphere conditions. Even higher charge densities, which are invaluable for device applications, were achieved by in situ formation of porphyrin polymers or by stepwise growth of porphyrin-imide oligomers. The various molecular architectures have been investigated by diverse surface characterization methods, including ellipsometry, atomic force microscopy, FTIR spectroscopy, and X-ray photoelectron spectroscopy, as well as a variety of electrochemical methods. These studies have further revealed that the porphyrin layers are robust under conditions of deposition of a top metal contact. The results to date indicate the superior features of selected molecular architectures for molecular electronics applications. The near-term utilization of such materials depends on further work for appropriate integration in semiconductor-based devices, whereas ultimate adoption may depend on advances that remain far afield, such as the development of fully bottom-up assembly processes.

Electrophoretically-Assisted Deposition of Mesoporphyrin IX on Nanoparticulate TiO2 Films for Constructing Efficient Dye-Sensitized Solar Cells

The successful fabrication of efficient dye-sensitized solar cells containing electrophoretically-deposited mesoporphyrin IX in semiconductor films based on sintered TiO2 nanoparticles is presented in this work. For comparison purposes, photocells utilizing mesoporphyrin IX-sensitized TiO2 electrodes were also constructed following the traditional dip-coating deposition technique. Electrochemical responses obtained from these modified surfaces revealed that the electrophoretic methodology generated perpendicularly-arranged porphyrin layers on the sintered oxide nanoparticles, while the dip-coating deposition technique resulted in parallel layers of porphyrin. It was demonstrated that the photovoltaic performance of cells containing electrophoretically-dyed TiO2 films was three times higher than that of devices having photoelectrodes constructed according to the dip-coating protocol. To better understand this behavior, both types of solar cells were further investigated by means of photoelectrochemical impedance spectroscopy.

Carbon nanotube films as a platform to transduce molecular recognition events inmetalloporphyrins

Porphyrins have been widely used for many years as functional materials for chemicalsensors. Their outstanding chemical features are balanced by some restrictionsin terms of transduction techniques. In particular, porphyrin layers are barelyconductive, with the consequence that the fabrication of porphyrin based chemiresistorsis not possible, except in few rare cases. On the other hand, carbon nanotubes(CNTs) have superior electric properties ranging from metallic to semiconductorin character. Although the conductivity of CNTs is very sensitive to adsorbedmolecules, it should be considered that the adsorption onto carbon structures isalso scarcely selective and cannot be modified unless other molecular recognitionsystems are coupled with the CNTs. Following this approach, in this paper weinvestigated the sensing properties of hybrid CNT–porphyrin films to explore thepossibility of transducing the adsorption events occurring in a porphyrin layer intoresistance changes of the CNT layers. The results obtained indicate that thepresence of the porphyrin films increases the sensitivity of the electric resistanceof the CNTs to the concentration of volatile compounds. This enhancement isprobably due to the catalytic effect of the metalloporphyrin in conveying the chargetransfer from the adsorbate molecule to the CNTs substrate. This property ofmetalloporphyrins may introduce a further differentiation between porphyrinbased sensors that could be positively utilized in sensor array configurations.

Porphyrin nanotubes film for optical gas sensing

The purpose of this work was to develop a novel morphology of gas sensing layer based on porphyrin sheets and nanotubes and to investigate the physical-chemistry and sensing properties of the porphyrin layer. In the present work the porphyrins nanoparticles were synthesized by ionic selfassembly in aqueous solutions of two oppositely charged non-metal porphyrins, namely, the anionic Meso-tetra (4-sulfonatophenyl) porphine dihydrochloride (TPPS4) and cationic Meso-tetra (4-pyridyl) porphine (T4MPyP) and an ethanol was not added to the solution. We investigated in details the nanosheets and nanotubes formation and behaviors of aggregation induced by mixing an anionic TPPS4 and cationic T4MPyP acidic solutions by means of the UV–vis spectroscopy and aggregates structure and morphology by transmission electron microscopy (TEM), scanning electron microscopy (SEM). The sensing capabilities of the samples have been finally evaluated by exposing the films to the different concentration of ethanol atmospheres.

Enhancement of the Open-Circuit Voltage and Hole Conduction of Tetraphenyl Porphyrin/C60 Multilayered Photovoltaic Device by the Insertion of Oxide Hole Collection Layers

We investigated the photovoltaic properties of organic multilayered photovoltaic devices consisting of Indium–tin-oxide (ITO)/oxide/tetraphenyl porphyrin (H2TPP, ZnTPP)/fullerene (C60)/bathocuproine (BCP)/Al structures. The open-circuit voltage VOC increases with the thickness of porphyrin layers between 10 and 30 nm. The upper limit of VOC is attributed to the built-in potential and the energy difference between the highest occupied molecular orbital (HOMO) of H2TPP and the lowest unoccupied molecular orbital (LUMO) of the C60 layer ΔE. The use of oxide hole collection layers, such as NiO and MoO3, is effective for increasing the built-in potential across the organic layers resulting in the improved VOC. The “kink” in the J–V curve observed at approximately VOC for the device with a thick H2TPP layer and the device with and without a MoOx layer is analyzed on the basis of the Poole–Frenkel and Schottky models assuming the amorphous porphyrin layers as dielectrics. The resistance of the organic layers is dominated by the field-dependent bulk resistance of H2TPP films for V<VOC, whereas the kink above VOC was attributed to the relatively high Schottky barrier for holes at the ITO/H2TPP and ITO/MoO3 interfaces.

Current-Voltage Characteristics of Porphyrin/C60 Multi-Layered Organic Photovoltaic Device with and without Hole Collection Oxide Films

We investigated the photovoltaic properties of multi-layered devices consisting of ITO/oxide/Tetraphenyl porphyrin (H2TPP)/Fullerene (C60)/Bathocuproine (BCP)/Al structures. The VOC markedly increases with the insertion of NiO and MoO3 hole collection layers. However, the “kink” behaviors and temperature dependent properties are observed for the devices with and without MoO3 especially for the thick H2TPP film. We demonstrated the analysis of the photovoltaic properties using the Poole-Frenkel and Schottky models based on the dielectric behaviors of porphyrin and MoO3 layers.

ChemInform Abstract: Porphyrin‐Based Nanostructures for Sensing Applications

AbstractReview: 40 refs.

Direct Synthesis of Nickel(II) Tetraphenylporphyrin and Its Interaction with a Au(111) Surface: A Comprehensive Study

Tetraphenylporphyrin (2HTPP) molecules were vapor-deposited onto a gold(111) surface to serve as reactive ligands for the direct synthesis of nickel(II) tetraphenylporphyrin (NiTPP) under ultrahigh vacuum (UHV) conditions. The surface-confined coordination reaction between a 2HTPP monolayer and coadsorbed Ni as well as the structure of 2HTPP multilayer films on Au(111) was characterized in detail using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). It is shown that the vapor-deposited 2HTPP multilayer on Au(111) has a high degree of ordering with a preferential orientation of the molecular plane relative to the substrate. Monolayers of 2HTPP on Au(111) were obtained by annealing the 2HTPP multilayers to 520 K and were found to be thermally stable up to at least 580 K. NiTPP can be synthesized directly on the Au(111) surface through reaction of the 2HTPP monolayer with postadsorbed Ni atoms. The same reaction occurs if Ni is deposited prior to the deposition of 2HTPP. It is proposed that the complete and uniform metalation of the porphyrin layer is possible because the Ni atoms can diffuse easily on the porphyrin-covered surface. Ni 2p XP spectra of NiTPP monolayers indicate that the electronic interaction between the coordinated Ni ion and the Au substrate is weak.

Fabrication and Characterization of Fullerene-Based Bulk Heterojunction Solar Cells with Porphyrin, CuInS2, Diamond and Exciton-Diffusion Blocking Layer

Fullerene-based bulk heterojunction solar cells were fabricated, and the electronic and optical properties were investigated. C60 were used as n-type semiconductors, and porphyrin, CuInS2 and diamond were used as p-type semiconductors. An effect of exciton-diffusion blocking layer of perylene derivative on the solar cells between active layer and metal layer was also investigated. Optimized structures with the exciton-diffusion blocking layer improved conversion efficiencies. Electronic structures of the molecules were investigated by molecular orbital calculation, and energy levels of the solar cells were discussed. Nanostructures of the solar cells were investigated by transmission electron microscopy, electron diffraction and X-ray diffraction, which indicated formation of mixed nanocrystals.

Effect of substrate piezoelectricity on surface acoustic wave propagation in humidity-sensitive structures with porphyrin layers

The impact of air humidity on surface acoustic wave (SAW) propagation in different piezoelectric substrates (lithium niobate, lithium tantalate, and gallium nitride on sapphire) with nanostructured meso-tetra(4 sulfonatophenyl) porphyrin (TPPS4) overlays has been investigated. The humidity-induced increase in SAW attenuation is proportional to the electromechanical coupling constant of the substrate and is attributed to the viscoelastic loss due to interaction of water adsorbing TPPS4 material with electric field of the SAW. The humidity-induced decrease in SAW velocity does not depend on the substrate piezoelectric properties and is explained by the changes in TPPS4 layer mechanical properties due to adsorbed moisture.

Self-Assembly of Porphyrins on a Single Crystalline Organic Substrate

Layers of a free base porphyrin have been deposited from n-heptane solution onto single crystalline potassium acid phthalate substrates and the self-assembled structures have been characterized by atomic force microscopy and X-ray diffraction. Depending on the concentration of the porphyrin solution, different anisotropic structures are found that are epitaxially related to the substrate, with the length axis parallel to the [001] direction of the (010) substrate surface. For undersaturated solutions, islands, cellular structures, and a hole pattern of monolayers of approximately 2.5 nm height are formed, which correspond to porphyrin molecules stacking approximately perpendicular to the substrate surface. From supersaturated solutions, multilayers are formed. At the highest concentrations, nanosized needles develop, which cover the whole substrate, creating a "monocrystalline" film with a length scale of several millimeters.