Porphyrin Nanorods Research Articles

Surface assembly of porphyrin nanorods with one-dimensional zinc–oxygen spinal cords

We have built long-range ordered, one-dimensional (1D) nanorods by self-assembly of zinc porphyrin derivatives through axial coordination with oxygenated ligands on different noble-metal surfaces. The structures were studied by a combination of Variable-Temperature Scanning-Tunnelling Microscopy (VT-STM), X-Ray Photoelectron Spectroscopy (XPS) and Density Functional Theory (DFT) calculations. The combined morphological, chemical and theoretical results demonstrate that the zinc atoms at neighbouring porphyrin molecules are coordinatively linked through oxygen-containing species, most probably water, leading to an adsorption geometry in which the porphyrin planes are perpendicular to the substrate plane, and the polymers are lying parallel to the surface.

Effects of O2, Xe, and Gating on the Photoconductivity and Persistent Photoconductivity of Porphyrin Nanorods

The photoconductivity of samples including both individual nanorods and bundles self-assembled from meso-tetra(4-sulfonato-phenyl)porphine grows over several days of illumination by more than a factor of 15. It also shows a transition from an initially nonpersistent response (in which the conductivity goes to zero when illumination is removed) to a primarily persistent response (in which the conductivity decays slowly when illumination is removed). Application of a gate voltage results in an initial n-type response, most of which decays away slowly, even though the gate voltage is held constant. The addition of O2 to the inert atmosphere surrounding the samples drastically reduces the persistent photoconductivity. When Xe is used instead of O2, there is at most a slight reduction of conductivity. A qualitative model is presented, in which the conductivity changes are attributed to light-induced and gate-voltage-induced adsorption and desorption of O2.

Ionic self-assembly of porphyrin nanostructures on the surface of charge-altered track-etched membranes

Track-etched polymer membranes are typically used as templates in the synthesis of various nanowires or nanotubes arrays. The unique advantages of track-etched membranes, such as uniform pore structure, excellent porosity, easily tailored pore sizes, and a well characterized surface chemistry, may find use in self-assembly strategies where colloidal nanostructures can be tethered to a suitable substrate to produce devices of interest. Meso-tetrakis(4-phenylsulfonicacid)porphyrin dihydrochloride and Sn(IV) tetrakis(4-pyridyl)porphyrin were used to synthesize ionic self-assembled porphyrin nanorods. The track-etched membranes surface charge was changed from negative to positive using polyethyleneimine. The porphyrin nanorods were either filtered through or self-assembled onto the surface of track-etched membranes. Comparisons were made with track-etched membranes modified with, and without, polyethyleneimine. Assembly of the porphyrin nanotubes only occurred on the surface of positively charged track-etched membranes, and filtration of the porphyrin nanorods produced a mesh-like structure on the surface of the membrane irrespective of the track-etched membrane pore diameter. In each case the characteristic absorbance profiles of the porphyrin nanorods was maintained. Transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and UV-vis spectroscopy were used to characterize the various systems.

Spectroscopic studies of nanostructures of negatively charged free base porphyrin and positively charged tin porphyrins

Spectroscopic studies were carried out on the homoaggregates of negatively charged free base meso-tetraphenylsulfonated porphyrin ([H 2TPPS 4] 4−) and heteroaggregates of a mixture of protonated ([H 4TPPS 4] 2−) and tin meso-tetra (N-methyl-4-pyridyl) porphyrin ([SnTMPyP] 4+). The spectroscopic studies were done to determine the optimal conditions required for the fabrication of porphyrin nanorods by ionic self assembly of two oppositely charged porphyrins. In addition, the various spectral changes of [H 4TPPS 4] 2− with concurrent change in pH and concentration are also investigated. In acid media at pH <3, and at concentrations >1 × 10 −5 M, [H 4TPPS 4] 2− molecules form J aggregates. A mixture of [H 4TPPS 4] 2− and [SnTMPyP] 4+ forms heteroaggregates of the J type in acid media. At pH’s 2 to 3, the optimum ratio for the formation of J aggregates is 3:1 and for pH 1, the optimum ratio is 2:1. Transmission electron microscope images of the nanostructures formed show that they are of cylindrical shape.

Preparation and Photophysical and Photoelectrochemical Properties of Supramolecular Porphyrin Nanorods Structurally Controlled by Encapsulated Fullerene Derivatives

A new class of porphyrin nanorods structurally controlled by encapsulated fullerene derivatives is prepared via a solvent mixture technique. These nanorods, composed of fullerenes (C60, C60 derivatives and C70) and zinc meso-tetra(4-pyridyl)porphyrin [ZnP(Py)4], are formed with the aid of a surfactant, cetyltrimethylammonium bromide (CTAB), in a DMF/acetonitrile mixture. In scanning electron microscopy (SEM) measurement, ZnP(Py)4 pristine hexagonal nanotubes with a large hollow structure [denoted as ZnP(Py)4 tube] are observed, whereas the hollow hole is completely closed in the case of nanorods composed of fullerenes (C60 and C70) and ZnP(Py)4 [fullerene−ZnP(Py)4 rod]. In C60 derivative−ZnP(Py)4 rods, the distorted polygonal columnar structures with large diameter and length are formed, as compared to the hexagonal structures of C60−ZnP(Py)4 and C70−ZnP(Py)4 rods. X-ray diffraction (XRD) analyses also reveals that ZnP(Py)4 alignment in the nanorod is based on the stacked assemblies of ZnP(Py)4 coordinated hexagonal formations. Elemental analysis and titration experiment by absorption measurement were also performed to quantitatively check the relative molecular ratio between porphyrins and fullerenes. Steady-state and time-resolved fluorescence spectra show efficient fluorescence quenching, suggesting the forward electron-transfer process from the singlet excited state of ZnP(Py)4 to fullerenes. Moreover, the back electron-transfer processes are detected by nanosecond transient absorption measurements. The forward and back electron-transfer rate constants are largely dependent on the structures of the nanorods. To construct photoelectrochemical solar cells, fullerene−ZnP(Py)4 rods are deposited onto nanostructured SnO2 films (OTE/SnO2). Fullerene−ZnP(Py)4 rod-modified electrodes exhibited efficient light energy conversion properties, such as a power conversion efficiency (η) of 0.63% and an incident photon to current conversion efficiency (IPCE) of 35%, which are much larger than those of ZnP(Py)4 tube.

Structural Control of Fullerene-Encapsulated Porphyrin Nanorods and their Role in Light Energy Conversion

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Effect of hypergravity created by strong magnetic force on orientation of porphyrin nanorods

An effect of hypergravity created by strong downward magnetic force on orientation of a nanometer-sized substance suspended in water, which is likely to be prevented because of ambient thermal energy, was studied by using cylindrical aggregates (nanorods) made of tetrakis(4-sulfonatophenyl)porphine (TPPS). Some of the rods were arranged in a direction parallel to a vertical magnetic field of 12 T under the hypergravity while no magnetic orientation took place even at a stronger magnetic field of 15 T where the usual gravity existed. This phenomenon was not explained by the diamagnetic susceptibility anisotropy of the rod. At present it was interpreted as an orientation accomplished hydrodynamically in a process of the rod precipitation accelerated by the downward magnetic force under the hypergravity.

A simple fabrication method of nanogap electrodes for top-contacted geometry: applicationto porphyrin nanorods and a DNA network

We have developed a fabrication method for nanogap electrodes without employingphoto- or electron-beam lithography to measure the electrical characteristics ofnanostructured molecules. This angle-controlled shadow-masking method enables us toconstruct nanogap electrodes without a wet process after the molecules are positioned onthe substrate. The proposed method makes it possible to measure electrical characteristicswithout structurally deforming or denaturing the molecules due either to the step edgeof an electrode or to the organic solvents used in the wet process. The resultsdemonstrate that a gap length between the electrodes of less than 100 nm can befabricated reproducibly. We have measured the electrical characteristics of lambdaDNA (λ-DNA) networks and molecular nanorods made of porphyrin-derivative molecules (TPPS:5,10,15,20-tetraphenyl-21H,23H-porphyrine tetrasulfonic acid) in which J-aggregates areformed inside. Experimental findings reveal that the electrical conductivity ofλ-DNA decreased under a vacuum condition, whereas that of TPPS nanorods decreasedunder oxygen and nitrogen gas-purged conditions.

Photoconductivity of Self-Assembled Porphyrin Nanorods

The photoconductivity of nanorods self-assembled from meso-tetrakis(4-sulfonatophenyl)porphine is described. The nanorods are insulating in the dark. Upon illumination with 488 nm light, the nanorods become photoconductive, exhibiting a rapid turn on/off (<100 ms) of the current when the light is turned on/off. This photoconductivity grows over hundreds of seconds with light exposure and decays slowly when the light is off. The nanorods can be trained via an applied bias to exhibit a short-circuit photocurrent (with corresponding open-circuit photovoltage) that flows in the direction opposite that of the training bias. A qualitative model is proposed, in which conduction occurs through the tightly coupled LUMOs of close-packed porphyrin molecules.