Single Porphyrin Molecules Research Articles

Density Functional Theory Calculations of Optoelectronic Properties of Individual and Encapsulated Magnesium Porphyrin in Carbon Nanotubes for Organic Nanohybrid Solar Cells

This research discusses incorporating a single magnesium porphyrin molecule into a semiconducting single‐walled carbon nanotube (SWCNT) for solar cell applications. Using density functional theory (DFT), the study examines the optical and electronic properties of the isolated magnesium porphyrin molecule and two configurations of the hybrid system. Results show structural stability due to charge transfer between the molecule and the nanotube. Different exchange‐correlation functionals (GGA and HSE06) yield varied bandgap results, affecting light absorption. Integration of the molecule into the SWCNT reduces the bandgap. Encapsulation of the molecule influences absorption and stability under irradiation. These encapsulated systems exhibit type II heterojunction characteristics, making them promising for organic solar cells based on SWCNTs, offering potential for highly efficient solar cells.

Single Molecular Semi-Sliding Ferroelectricity/Multiferroicity.

In recent years, the unique mechanism of sliding ferroelectricity with ultralow switching barriers has been experimentally verified in a series of 2-dimensional (2D) materials. However, its practical applications are hindered by the low polarizations, the challenges in synthesis of ferroelectric phases limited in specific stacking configurations, and the low density for data storage since the switching process involves large-area simultaneous sliding of a whole layer. Herein, through first-principles calculations, we propose a type of semi-sliding ferroelectricity in the single metal porphyrin molecule intercalated in 2D bilayers. An enhanced vertical polarization can be formed independent on stacking configurations and switched via sliding of the molecule accompanied by the vertical displacements of its metal ion anchored from the upper layer to the lower layer. Such semi-sliding ferroelectricity enables each molecule to store 1 bit data independently, and the density for data storage can be greatly enhanced. When the bilayer exhibits intralayer ferromagnetism and interlayer antiferromagnetic coupling, a considerable difference in Curie temperature between 2 layers and a switchable net magnetization can be formed due to the vertical polarization. At a certain range of temperature, the exchange of paramagnetic-ferromagnetic phases between 2 layers is accompanied by ferroelectric switching, leading to a hitherto unreported type of multiferroic coupling that is long-sought for efficient "magnetic reading + electric writing".

Gating and photogalvanic effect tuning the spin transport in single chromium porphyrin from first principles

Magnetic molecules are of crucial importance for miniaturizing future spintronic devices. In this work, we investigated the possibility of modulating spin transport in a single chromium porphyrin molecule using the gate voltage and the photogalvanic effect. It was found that, under a certain bias voltage, the gate voltage can effectively regulate the spin filtering effect. Moreover, when the molecule is asymmetrically connected to two gold nanowire electrodes, fully polarized and even pure spin currents can be generated under the irradiation of linearly polarized light. Our findings offer potential pathways for designing single-molecule spintronics and optoelectronic devices.

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.

Charge transport pathways in metal porphyrin as interplay between long and short range scattering processes

We have investigated the ballistic transport for Mn, Fe–porphyrin molecules in contact with Au(111) electrodes by using density functional theory. We show that the information resulted from the projected density of electronic states does not provide a complete picture of the transport mechanism. Instead, we propose a methodology based on the concept of the orbital projected transmissions for selected groups of atoms. We have found that the transmission channels for occupied states can be assigned to short range scattering processes at gold-molecule interface, while for the states above Fermi level the transmission takes place via long range scattering processes. The interplay between these two conduction mechanisms is responsible for the transport path in metal–porphyrin; our model is in qualitative agreement with the existing experimental data on transport pathway on single porphyrin molecules.

Tunneling Electron Induced Fluorescence from Single Porphyrin Molecules Decoupled by Striped-Phase Octanethiol Self-assembled Monolayer

We investigate tunneling electron induced luminescence from isolated single porphyrin molecules that are decoupled by striped-phase self-assembled monolayer of octanethiol from the underneath Au(111) substrate. Intrinsic single-molecule electroluminescence has been realized by such decoupling at both bias polarities. The photon emission intensity acquired from the molecular lobe is found stronger than that from the molecular center. These results provide useful information on the understanding of electroluminescent behavior and mechanism in molecular tunnel junctions.

Single-Molecule Tribology: Force Microscopy Manipulation of a Porphyrin Derivative on a Copper Surface

The low-temperature mechanical response of a single porphyrin molecule attached to the apex of an atomic force microscope (AFM) tip during vertical and lateral manipulations is studied. We find that approach-retraction cycles as well as surface scanning with the terminated tip result in atomic-scale friction patterns induced by the internal reorientations of the molecule. With a joint experimental and computational effort, we identify the dicyanophenyl side groups of the molecule interacting with the surface as the dominant factor determining the observed frictional behavior. To this end, we developed a generalized Prandtl-Tomlinson model parametrized using density functional theory calculations that includes the internal degrees of freedom of the side group with respect to the core and its interactions with the underlying surface. We demonstrate that the friction pattern results from the variations of the bond length and bond angles between the dicyanophenyl side group and the porphyrin backbone as well as those of the CN group facing the surface during the lateral and vertical motion of the AFM tip.

Highly conductive single-molecule wires with controlled orientation by coordination of metalloporphyrins.

Porphyrin-based molecular wires are promising candidates for nanoelectronic and photovoltaic devices due to the porphyrin chemical stability and unique optoelectronic properties. An important aim toward exploiting single porphyrin molecules in nanoscale devices is to possess the ability to control the electrical pathways across them. Herein, we demonstrate a method to build single-molecule wires with metalloporphyrins via their central metal ion by chemically modifying both an STM tip and surface electrodes with pyridin-4-yl-methanethiol, a molecule that has strong affinity for coordination with the metal ion of the porphyrin. The new flat configuration resulted in single-molecule junctions of exceedingly high lifetime and of conductance 3 orders of magnitude larger than that obtained previously for similar porphyrin molecules but wired from either end of the porphyrin ring. This work presents a new concept of building highly efficient single-molecule electrical contacts by exploiting metal coordination chemistry.

Single Molecule Force Spectroscopy (Invited)

Recently, intramolecular resolution is studied on a variety of molecules. A further challenge is the manipulation of molecules on surfaces, including the controlled rotation, which means that the direction of rotation of the molecule can be chosen by the experimentalist [1]. The control of the probing tip is of central importance for a quantitative understanding of nc-AFM imaging and force spectroscopy. Progress has been made by a variety of preparation procedures: sputtering, indentation or pick-up of molecules, such as CO. The attachement of a single molecule to the end of the tip is shown for the case of a functionalized porphyrin, which can used as a stable probing tip for imaging in the attractive as well as the repulsive regime. [1] R. Pawlak, S. Fremy, S. Kawai, T. Glatzel, H. Fang, L.-A. Fendt, F. Diederich, and E. Meyer , Directed rotations of single porphyrin molecules controlled by localized force spectroscopy, ACS Nano, 6, (2012), 6318–6324

Single-Molecule Femtochemistry: Molecular Imaging at the Space-Time Limit

Through a combination of light and electron probes, it may be possible to record single-molecule dynamics with simultaneous sub-Ångstrom spatial and femtosecond temporal resolution. Single-molecule femtochemistry is becoming a realistic prospect through a melding of laser spectroscopy and electron microscopy techniques. The paper by Lee et al. in this issue of ACS Nano takes a significant step toward chemical imaging at the space-time limit of chemical processes. By imaging electroluminescence spectra of single porphyrin molecules with submolecular resolution, the authors extract the implicit femtosecond dynamics of the coupled electron orbital-molecular skeletal motion triggered by a reduction-oxidation scattering process.

Porphyrin Assemblies and Their Scaffolds

The chlorophyll and heme molecules of chloroplasts and mitochondria are brought to life by "the global fold of the protein scaffolds". Proteins in hydrophobic cell regions touch the dye platelets from both sides, pushing and orienting them according to their life-spending activities in light and electron transfers. The conjugated π-electron systems or planarity of the porphyrin macrocycles are never disturbed. Most artificial porphyrin assemblies contain meso-tetraphenylporphyrins (TPPs), because the four phenyl groups rotate freely and carry their substituents above or below the macrocycle. A single porphyrin molecule can, for example, be attached to an anionic surface with ammonium groups on its 2,3-carbons, be located within a hydrophobic membrane with its alkyl chains on the 4-position, and then fixate a cationic polymer with 4,5-sulphonates. Charged TPPs also show unique spectroscopic changes at different pH values and a reversible loss of the macrocycle's planarity. On smooth silicate, graphite, or gold scaffolds TPPs have been attached irreversibly as single molecules, as extended non-covalent H or J aggregates as well as acetylene or thiophene-linked polymers. Soft, mobile porphyrin ladders conduct excited electrons ("excitons") better than rigid porphyrin wires ("polarons").

Self-Decoupled Porphyrin with a Tripodal Anchor for Molecular-Scale Electroluminescence

A self-decoupled porphyrin with a tripodal anchor has been synthesized and deposited on Au(111) using different wet-chemistry methods. Nanoscale electroluminescence from single porphyrin molecules or aggregates on Au(111) has been realized by tunneling electron excitation. The molecular origin of the luminescence is established by the vibrationally resolved fluorescence spectra observed. The rigid tripodal anchor not only acts as a decoupling spacer but also controls the orientation of the molecule. Intense molecular electroluminescence can be obtained from the emission enhancement provided by a good coupling between the molecular transition dipole and the axial nanocavity plasmon. The unipolar performance of the electroluminescence from the designed tripodal molecule suggests that the porphyrin molecule is likely to be excited by the injection of hot electrons, and then the excited state decays radiatively through Franck-Condon π*-π transitions. These results open up a new route to generating electrically driven nanoscale light sources.

Localized interaction of single porphyrin molecules with oxygen vacancies on TiO2(110)

Alterations in the electronic structure of adsorbed zinc(II) etioporphyrin I (ZnEtio) through interaction with bridging oxygen vacancies on TiO(2)(110) are studied by scanning tunneling microscopy and spectroscopy at cryogenic temperatures under ultrahigh vacuum (UHV) conditions. Upon lateral manipulation of ZnEtio molecules above surface oxygen vacancies, the highest occupied molecular orbital shifts away from the Fermi level. The magnitude of the shift rapidly decreases with increasing distance of the molecule from the vacancy, indicating a highly localized interaction.

Directed Rotations of Single Porphyrin Molecules Controlled by Localized Force Spectroscopy

Directed molecular repositioning is a key step toward the build up of molecular machines. To artificially generate and control the motion of molecules on a surface, excitations by light, chemical, or electrical energy have been demonstrated. Here, the application of local mechanical forces is implemented to achieve directed rotations of molecules. Three-dimensional force spectroscopy with sub-Ångström precision is used to characterize porphyrin derivatives with peripheral carbonitrile groups. Extremely small areas on these molecules (≈ 100 × 100 pm(2)) are revealed which can be used to control rotations. In response to the local mechanical forces, the molecular structure elastically deforms and then changes its conformation, which leads to its rotation. Depending on the selection of one of four submolecular areas, the molecule is either rotated clockwise or counterclockwise.

Determining Charge Transport Pathways through Single Porphyrin Molecules Using Scanning Tunneling Microscopy Break Junctions

Charge transport in a porphyrin with four identical pyridyl substituents, 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP), was investigated using the scanning tunneling microscopy break junction method. To determine the dominant pathway, we studied two structurally similar porphyrins, o-DPyP and p-DPyP. Our experiments reveal that charge transport through TPyP in a break junction configuration does not follow the traditional assumption, i.e., the shortest path between the neighboring side groups. Instead, the charge transport pathway was dominated by the farthest anchoring groups. Furthermore, these single molecule experiments can distinguish between the two structural isomers, which is important in molecular discrimination, porphyrin chemistry, and molecular electronics.

Asymmetric saddling of single porphyrin molecules on Au(111)

The interaction of organometallic molecules with metal surfaces affects their conformation and accordingly their functionality. Surprising conformational complexity is revealed in single Au-porphyrin molecules on Au(111) exhibiting a variety of different asymmetric conformers. A combination of low-temperature scanning tunneling microscopy and spectroscopy experiments with density functional theory calculations discloses different registries of the pyrroles with the substrate lattice (determined and quantified with atomic precision) that cause asymmetric sub-\AA{} buckling of the macrocycle and, furthermore, a redshift of the frontier-orbital energy gap.

Optical imaging of excited-state tautomerization in single molecules

Tautomerism process of single fluorescent molecules was studied by means of confocal microscopy in combination with azimuthally or radially polarized laser beams. During a tautomerism process the transition dipole moment (TDM) of a molecule changes its orientation which can be visualized by the fluorescence excitation image of the molecule. We present experimental and theoretical studies of two porphyrazine-type molecules and one type of porphyrin molecule: a symmetrically substituted metal-free phthalocyanine and porphyrin, and nonsymmetrically substituted porphyrazine. In the case of phthalocyanine the fluorescence excitation patterns show that the angle between the transition dipole moments of the two tautomeric forms is near 90°, in agreement with quantum chemical calculations. For porphyrazine we find that the orientation change of the TDM is less than 60° or larger than 120°, as theoretically predicted. Most of the porphyrin molecules show no photoinduced tautomerization, while for 7% of the total number of investigated molecules we observed excitation patterns of two different trans forms of the same single molecule. We demonstrate for the first time that a molecule, undergoing a tautomerism process stays in one tautomeric trans conformation during a time comparable with the acquisition time of one excitation pattern. This allowed us to visualize the existence of each of the two trans forms of one single porphyrin molecule, as well as the sudden switching between these tautomers.

Single azopyridine-substituted porphyrin molecules for configurational and electronic switching

A new azopyridine functionalized Ni-porphyrin was synthesized as a model compound for deposition and switch on surfaces. Two geometrically and electronically different states of single molecules on Au(111) were found by scanning tunneling microscopy/spectroscopy and analyzed with density functional calculations.

A robust boehmite-supported cobalt tetraphenylporphyrin catalyst for aerobic oxidation of cyclohexane

Using a method of physically adsorbing and chemically coordinating cobalt porphyrin on to boehmite, a new boehmite-supported cobalt tetraphenylporphyrin [Co II TPP/BM] catalyst has been prepared and characterized by UV–Vis, FT-IR, XRD, TEM and TG techniques. The supported catalyst was used for aerobic oxidation of cyclohexane under mild conditions and in the absence of any solvent and additive. The experimental results indicate that, after 10 consecutive uses for the oxidation, a quantity of catalyst containing only 1 mg of cobalt porphyrin, can not only provide, on average, 9.2% cyclohexane conversion, 92.1% selectivity for K/A oil and a catalyst turnover number of 1.14 × 10 5, but also can maintain continuously the original catalytic activity with promotion by beomite (BM). A proposed reason for the good reusability and stability of the supported catalyst for aerobic oxidation of cyclohexane was that the anchored form of the single cobalt porphyrin molecules were highly dispersed and immobilized on the boehmite.

STM Investigation of Temperature-Dependent Two-Dimensional Supramolecular Architectures of C60 and Amino-tetraphenylporphyrin on Ag(110)

Multicomponent supramolecular self-assemblies of exceptional long-range order and low defectivity are obtained if C(60) and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on Ag(110) by sequential evaporation in the submonolayer range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. A (+/-2 -3, 6 +/- 3) array consisting of supramolecular stripes of a 1:1 C(60)/TPP-NH2 2D adduct develops at 410 K (the low temperature, LT, phase). If the LT phase is annealed at 470 K, then a 3:1 fullerene/TPP-NH2 (+/-3 -5, 5 +/- 5) nanoporous array (the HT phase) forms, with each pore containing a single porphyrin molecule. Phase separation occurs by annealing the HT phase at 520 K. Structural models are proposed and discussed on the basis of the experimental scanning tunneling microscopy results.