Zinc Porphyrin Molecules Research Articles

Theoretical study of the thermoelectric properties through a single-molecule junction of Zinc Porphyrin

A theoretical analysis was performed to investigate the transport properties of the Zinc Porphyrin molecule and its response to thermal fluctuations. The renormalization method was used for this analysis, using the Green’s functions technique through the Dyson equation, focusing on first-neighbor interactions. This study produced various results, including energy-dependent transmission probability, current-voltage characteristic curves, electrical and thermal conductance assessments, Seebeck coefficient, and figure of merit (ZT). In order to validate the method used, both the profile results in the transmission probability and the I–V curves were compared with theoretical (DFT) and experimental results, respectively, obtaining significant agreements in the HOMO and LUMO of the molecule, as well as a characteristic behavior of a rectifying device. Complementing the above by calculating the ZT through the molecular system Zn(5,15-di(p-thiolphenyl)-10,20-di(p-tolyl)porphyrin) (ZnTPPdT), it was demonstrated that said molecule could be used as an efficient thermoelectric device that allows thermal energy to be converted into electrical energy.

Connections to the Electrodes Control the Transport Mechanism in Single-Molecule Transistors.

When designing a molecular electronic device for a specific function, it is necessary to control whether the charge-transport mechanism is phase-coherent transmission or particle-like hopping. Here we report a systematic study of charge transport through single zinc-porphyrin molecules embedded in graphene nanogaps to form transistors, and show that the transport mechanism depends on the chemistry of the molecule-electrode interfaces. We show that van der Waals interactions between molecular anchoring groups and graphene yield transport characteristic of Coulomb blockade with incoherent sequential hopping, whereas covalent molecule-electrode amide bonds give intermediately or strongly coupled single-molecule devices that display coherent transmission. These findings demonstrate the importance of interfacial engineering in molecular electronic circuits.

Influence of Diketopyropyrole spacer on the Ultrafast Non-Linear Optical Properties and Excited state dynamics of Dimeric Zinc Porphyrin Molecules

Zinc porphyrins (ZnP) are an important class of small molecule donors that have gained significant attention in the domain of organic photovoltaics. In this work, we investigate and correlate various...

Plasmon-enhanced S2 electroluminescence from the high-lying excited state of a single porphyrin molecule

We demonstrate the B-band electroluminescence from the high-lying S2 excited state of a single zinc porphyrin molecule with the scanning tunneling microscope-induced luminescence technique by using an aluminum tip. The nanocavity plasmon mode is found to be critical for the occurrence of S2 electroluminescence. When using a silver tip to excite the molecule electronically decoupled from the Ag(100) substrate by an ultrathin sodium chloride spacer, we only observe the Q-band electroluminescence originating from the radiative decay of the S1 first excited state, without any B-band emission due to the lack of effective plasmonic enhancement for the B-band. However, when the nanocavity plasmon resonance is tuned to a bluer range by using an aluminum tip, the S2 electroluminescence from a single zinc porphyrin shows up because the nanocavity plasmon mode can now spectrally overlap with the B-band emission to generate efficient plasmonic enhancement for the radiative decay directly from the S2 state. Interestingly, the excitation mechanisms for these two types of emission are found to be different. While the Q-band emission is attributed mainly to a carrier-injection mechanism, the B-band electroluminescence is found to be excited via an inelastic electron scattering process. Our results open a route to investigate the photophysical property and dynamic behavior of isolated molecules in their excited states.

Temperature dependent electron transport and inelastic electron tunneling spectroscopy of porphyrin molecular junctions

Abstract We report electron transport measurements through a metal-molecule-metal junction of free base or zinc porphyrin molecules. Junctions are formed by zig-zag electromigration of a gold nanowire. Inelastic electron tunneling spectroscopy measurements were performed at 4.3 K to confirm the presence of molecules in the junction and to measure the vibrational modes of the molecular junction. Temperature dependent current/voltage measurements are performed in order to determine that the electron conduction mechanism through these molecular junctions is direct tunneling. The electron attenuation coefficient ( β 0 ) was also calculated; the average β 0 for free base and zinc porphyrin was 0.231 ± 0.133 A−1 and 0.188 ± 0.049 A−1, respectively. The barrier height was experimentally found to be 1.6 eV and 1.1 eV for FBP and Zn-P SAMs on Au, respectively.

Time-Resolved Continuous-Wave and Pulse EPR Investigation of Photoinduced States of Zinc Porphyrin Linked with an Ethylenediamine Copper Complex

Electron spin dynamics of the photoexcited state of a new porphyrin system built of a zinc porphyrin molecule with a linked complex of copper ion (ZnPCu) was studied using electron paramagnetic resonance (EPR) methods. The time-resolved continuous-wave and echo-detected EPR data show that electron spins of the ZnPCu system are polarized. The shape of the EPR spectrum and the echo-detected nutation spectroscopy data indicate that there is a relatively small spin–spin interaction between two subunits, namely, less than the zero-field splitting parameter of the excited triplet ZnP molecule. Results have been interpreted assuming that the S = 1 Zn porphyrin subunit electron spins are polarized via the triplet mechanism and this polarization is transferred to the S = 1/2 copper subunit via a flip-flop process induced by the exchange interaction between these subunits. The formation of the integral spin polarization in the triplet–doublet system as a function of the value and type of the triplet–doublet interaction was analyzed.

Electron-transport properties of ethyne-bridged diphenyl zinc-porphyrin molecules

We investigate the electron-transport properties of ethyne-bridged diphenyl zinc-porphyrin molecules suspended between gold (111) electrodes by first-principles calculations within the framework of density functional theory. It is found that the conductance of a molecular junction in which phenyl and porphyrin rings are perpendicular is reduced by three orders of magnitude compared with that of a junction in which the phenyl and porphyrin rings are coplanar. In the coplanar configuration, electrons are transmitted through π states, which extend over the whole molecule. In the perpendicular configuration, the conductance is suppressed because of the reduction of electron hopping between π states of the phenyl ring and σ states of the porphyrin ring.

Enhancement of the Yield of Photoinduced Charge Separation in Zinc Porphyrin–Quantum Dot Complexes by a Bis(dithiocarbamate) Linkage

This paper describes the use of a phenyl bis(dithiocarbamate) (PBTC) linker to enhance the quantum yield of photoinduced electron transfer (eT) from a zinc porphyrin (ZnP) molecule (donor) to a CdSe quantum dot (QD) (acceptor), where quantum yield is defined as the fraction of photoexcited ZnP molecules in the sample that donate an electron to the QD. The PBTC ligand links the ZnP to the QD by coordinating to Cd2+ on the surface of the QD and the Zn metal center in ZnP via its dithiocarbamate groups. Compared with the donor–acceptor complex formed in the absence of PBTC linkers, where the ZnP molecule adsorbs to the QD through its carboxylate moiety, the PBTC linkage increases the binding affinity between ZnP molecules and QDs by an order of magnitude, from 1.0 × 105 ± (0.7 × 104) M–1 to 1.0 × 106 ± (1.0 × 105) M–1, and thereby increases the eT quantum yield by, for example, a factor of 4 (from 8% to 38%) within mixtures where the molar ratio ZnP:QD = 1:1.

Nanostructured TiO2 modified with acetohydrazide zinc porphyrin well-arrays for supramolecular solar cells

In this work, employing the metal-mediated assembling strategy for organizing acetohydrazide zinc porphyrin (ZnP) and different organic acid (Ai) ligands on the nanostructured TiO2 electrode surfaces have been prepared, wherein the ZnP and Ai units are bound by a series of differently bivalent metal ions (M, M=Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+), resulting in a ZnP–M–Ai assembled approach. The photovoltaic performances of these assemblies-sensitized solar cells were performed under irradiance of 100mWcm−2 AM 1.5G sunlight. A significantly improved conversion efficiency of ZnP–Mn–A5 and ZnP–Cd–A5 devices occurs, indicating well-ordered arrangements of ZnP molecules on the TiO2 electrode surfaces for ZnP–Mn–A5 and ZnP–Cd–A5, where A5 is a pyridine–monocarboxylic acid ligand bearing longer arm. The results were also verified by the HOMO–LUMO gaps, UV–vis and fluorescence spectra.

Gate-dependent photoconductivity of single layer graphene grafted with metalloporphyrin molecules

We present the gate-dependent photoconductivity measurements of single layer graphene ribbons grafted with zinc porphyrin molecules Zn(OEP). The Zn(OEP)-graphene showed a maximum 610% increase in its photo-sensitivity compared to the bare graphene samples. Furthermore, the measured photocurrent exhibited strong dependence on the gate bias, light power, and light wavelength. These dependences showed clear evidence of the excitation of the carriers in Zn(OEP) and their energy transfer to graphene.

Amine-sensing properties of a covalently linked calix[4]arene–porphyrin ("calixporph") multilayered film

Four zinc porphyrin molecules have been linked covalently to a central amphiphilic calix[4]arene molecule to form a zinc "calixporph", ZnCP , which displays enhanced sensitivity to dibutylamine compared to both the discrete zinc porphyrin and two-component mixtures of zinc porphyrin with calixarene. Langmuir-Blodgett films (20 monolayers) of ZnCP display UV-visible spectra which are modified upon exposure to dibutylamine in the concentration range 0.09–0.45 ppt (in N2) with a t50response time of 8.8 s. The dense packing of the monomeric porphyrin moieties leads to a high absorbance per layer and thus a large sensing signal compared to other LB films capable of amine-detection. This, coupled to the very fast t50value, leads to a high figure of merit. This verifies the adopted strategy of using a covalently linked calixporph in order to avoid the potential problems of phase separation associated with two-component mixtures. Furthermore, the ZnCP is selectively responsive to the secondary amine, dibutylamine, compared to the primary and tertiary compounds.

Excitation Energies of Zinc Porphyrin in Aqueous Solution Using Long-Range Corrected Time-Dependent Density Functional Theory

We study the low-lying excited states of the zinc porphyrin molecule in aqueous solution using long-range corrected TDDFT. We report results using the CAM-B3LYP and CAM-PBE0 functionals and compare them with previously reported excited states based on high-level coupled cluster (CC) methods. The aqueous environment is treated via a QM/MM approach.

Thin films of porphyrin molecules imaged with the atomic force microscope

The assembly of two-dimensional molecular structures of zinc porphyrin molecules arising from the dewetting of a porphyrin solution on mica and graphite is investigated using atomic force microscopy. Both a near equilibrium nucleation and growth process, and a far from equilibrium spinodal dewetting process are observed. Nucleation and growth around pre-existing surface defects on mica produces single layer disks, ∼ 10 μm in diameter, of densely packed molecules. Spinodal dewetting gives rise to the formation of much smaller, single layer, molecular islands of various sizes on both mica and graphite.

Large-scale parallel calculations with combined coupled cluster and molecular mechanics formalism: Excitation energies of zinc–porphyrin in aqueous solution

The need for combined QM/MM approaches with high-level coupled cluster (CC) methods in the QM core is unquestionable. In an effort to make these methods widely applicable, the scalability of CC approaches across hundreds/thousands of CPUs will play a central role. In this Letter we report the performance of our CC codes in calculations of vertical excitation energies of low-lying excited states of the zinc–porphyrin molecule in aqueous solution. Coupled cluster description of excited states is based on equation-of-motion approach with singles and doubles (EOMCCSD) as well as its non-iterative extension for triply excited configurations. These results are compared with those obtained with time-dependent density functional theory (TD-DFT).

Self-assembled two-dimensional supramolecular architectures of zinc porphyrin molecules on mica

The assembly of two-dimensional molecular structures of zinc porphyrin molecules arising from the dewetting of a porphyrin solution on a mica substrate is investigated using the atomic force microscope (AFM). Both a near equilibrium nucleation and growth process, and a far from equilibrium spinodal dewetting process are observed. By choosing a zinc porphyrin molecule with a prefabricated structure, we are able to control the morphology of the single layer molecular films formed on the substrate.

Needles and clusters of zinc porphyrin molecules on mica

Molecular monolayers of zinc porphyrins deposited from a liquid solution on mica substrates have been characterised using atomic force microscopy (AFM). At room temperature, needle-shaped single layer porphyrin islands are formed due to an anisotropic growth rate. The average island size depends on the concentration of the porphyrin solution, but the length to width aspect ratio is rather independent of the island size. Annealing the porphyrin monolayers to 450 K leads to the reduction of the size of the monolayer islands in two different ways depending on the heating rate. With slow heating rate, the island size was found to shrink by losing molecules from the edge of the island. With high heating rate, nanoscale molecular clusters were found to form.

Zinc(II) Porphyrins at the Air−Water Interface As Studied by Polarized Total-Reflection X-ray Absorption Fine Structure

The polarized total-reflection X-ray absorption fine structure method was applied to characterize zinc porphyrins at the air-water interface. The X-ray absorption near edge structure exhibited a significant difference depending on the polarization of the X-ray. A shoulder peak of the Zn K-edge corresponding to the 1s-4p(z) transition for a square planar metal complex without axial coordination(s) was observed at 9662 eV, which indicates that the axial coordination sites of zinc porphyrin molecules examined are not fully hydrated at the air-water interface. The molecular orientation of zinc porphyrins was determined by analyzing the polarization dependence of the transition peak intensity. The meso-substituted porphyrin derivative 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) orients rather parallel to the solution surface. In contrast to ZnTPP, the zinc(II) protoporphyrin IX (ZnPP) with hydrophilic carboxyl groups at one side of the molecule stands up with respect to the solution surface, and the average tilting angle of the porphyrin plane to the surface was evaluated to be between 57 degrees and 43 degrees. In addition, the axial coordination of ZnPP is modified depending on the surface concentration, in which the axial hydration to the zinc center is effectively inhibited in the compressed surface layer.

Liquid phase deposition of supramolecular monolayers of zinc porphyrin molecules ongraphite

Molecular monolayers of zinc porphyrins deposited from liquid sources on graphitesubstrates have been characterized using atomic force microscopy. The morphology of themolecular layers shows a strong dependence on both the structure of the individualmolecules and the properties of the solvent used for the deposition. By controlling theconcentration of the starting solution and the amount of liquid applied to the graphitesurface, single layer high molecular weight films have been successfully grown with a varietyof zinc porphyrin molecules. The nucleation and growth, similar to those commonlyobserved in vapour deposition, have been found to depend on the strength of theintermolecular interaction as well as the level of interaction between the solvent moleculesand the substrate.

Self-aggregation of synthetic zinc 2 1-hydroxy-12 1/13 1-oxo-porphyrins

Zinc complexes of 2 1-hydroxy-12 1/13 1-oxo-porphyrins were prepared by modifying octaethylporphyrin as models of self-aggregative chlorophylls found in extramembranous antennas of green photosynthetic bacteria. Their visible absorption spectra showed that less hindered interactive groups, such as CH 2OH and CHO in a synthetic zinc porphyrin molecule were necessary for their self-aggregation in non-polar organic solvents. Zinc 2-hydroxymethyl-12-formyl-porphyrin self-aggregated to form larger oligomers with red-shifted broad visible bands than the 2,13-regioisomer, indicating that the linear location of OH, Zn and CO in a molecule is requisite for similar self-aggregation with natural systems.

Potential-Independent Electron Transfer Rate at the Liquid/Liquid Interface

Electron transfer (ET) from neutral zinc porphyrin (ZnPor) molecules in various organic solvents to aqueous redox species (e.g., Ru(CN)63-) was probed by the scanning electrochemical microscope (SECM). Unlike previous studies in which organic redox species were charged, the ET rate was found to be essentially independent of the potential drop across the interfacial boundary. This difference is explained by diffuse double-layer effect predicted by known models of the liquid/liquid interface. The effect of solvent dynamics on the rate of ET at the liquid/liquid interface was observed for the first time. The high precision and sensitivity of the SECM allow one to detect complexity in an interfacial reaction that may not be obvious from other electrochemical measurements and select a suitable system for checking the ET theory.