The role of aromaticity and the pi-conjugated framework in multiporphyrinic systems as single-molecule switches

Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

On-Surface Synthesis of Square-Type Porphyrin Tetramers with Central Antiaromatic Cyclooctatetraene Moiety

The synthesis of two-dimensionally extended polycyclic heteroatomic molecules keeps attracting considerable attention. In particular, frameworks bearing planar cyclooctatetraenes (COT) moieties can display intriguing properties, including antiaromaticity. Here, we present an on-surface chemistry route to square-type porphyrin tetramers with a central COT ring, coexisting with other oligomers. This approach employing temperature-induced dehydrogenative porphyrin homocoupling in an ultrahigh vacuum environment provides access to surface-supported, unsubstituted porphyrin tetramers that are not easily achievable by conventional synthesis means. Specifically, monomeric free-base (2H-P) and Zn-metalated (Zn-P) porphines (P) were employed to form square-type free-base and Zn-functionalized tetramers on Ag(100). An atomic-level characterization by bond-resolved atomic force microscopy and scanning tunneling microscopy and spectroscopy is provided, identifying the molecular structures. Complemented by density functional theory modeling, the electronic structure is elucidated, indeed revealing antiaromaticity induced by the COT moiety. The present study thus gives access, and insights, to a porphyrin oligomer, representing both a model system for directly fused porphyrins and a potential building block for conjugated, extended two-dimensional porphyrin sheets.

FeII/III and MnII complexes based on 2,4,6-tris(2-pyridyl)-triazine: synthesis, structures, magnetic and biological properties

Reaction of the polypyridyl ligand 2,4,6-tris(2-pyridyl)-s-triazine (tpt) with mono- and polynuclear Fe(iii) or Mn(ii) precursors, specifically tri- or hexanuclear Fe(iii) pivalates (piv^-), [Mn₆O₂(piv)₁₀(Hpiv)₄] and Mn(ii) isobutyrate (ib^-), in various solvents and under various reaction conditions showcase the ligand's surprising coordination characteristics. The reactions result in mononuclear [Fe^III(tpt)(tptH)][Fe^IIICl₄]₄·2(thf)·0.23(H₂O) (1), [Fe^III(piv)(tpt)Cl₂] (2), [Fe^II(tpt)Cl₂]·2(H₂O) (3a), dinuclear [Fe^III ₂O(tpt)₂Cl₄] (3), and heptanuclear [Fe^III ₇O₄(piv)₁₂(tpt-O)]·A [A = MeCN (4a) or 4(dioxane) (4b)] and [Fe^III ₇O₄(piv)₁₁(tpt-O)(i-PrO)(i-PrOH)]·0.75(i-PrOH) (5), as well as the mononuclear compounds [Mn^II(tpt)(NO₃)(H₂O)₂](NO₃) (6) and [Mn^II(tpt)(ib)(Cl)(MeOH)]·MeOH (7). Single-crystal X-ray diffraction analyses identify tpt as a tridentate NNN donor ligand that forms two five-membered metallocycles in 1-3, 6 and 7, whereas in 4 and 5 five tpt N atoms form coordinative bonds accompanied by an unusual metal-induced oxidation of one of the carbon atoms of the central triazine core. Magnetic properties of Fe(iii)-tpt (2-5), Fe(ii)-tpt (3a), and Mn(ii)-tpt (7) compounds show dominant antiferromagnetic coupling for polynuclear coordination cluster compounds. The Mn(ii)-tpt complexes 6 and 7 exhibit efficient catalytic properties in the production of enzymes by microorganisms, which concerns the synthesis of exocellular proteases in Fusarium gibbosum CNMN FD 12 or Trichoderma koningii Oudemans CNMN FD 15 fungi strains. Thus, compounds 6 and 7 can be used for producing proteolytic enzymes with wide applications including in the food, detergents and pharmaceutical industries.

Organic Spintronics: A Theoretical Investigation of a Graphene-Porphyrin Based Nanodevice

Spintronics is one of the most exciting applications of graphene-based devices. In this work Density Functional Theory is used to study a nanojunction consisting of two semi-infinite graphene electrodes contacted with an iron-porphyrin (FeP) molecule, which plays the role of spin filter for the incoming unpolarized electrons. The graphene-FeP contact closely resembles the recently synthesized porphyrin-decorated graphene [He et al., Nat. Chem. 2017, 9, 33–38]. The analysis of the spectral properties of the system shows a variation of the orbital occupancy with respect to the isolated FeP molecule and an hybridization with the delocalized states of the substrate, while the overall magnetic moment remains unchanged. Doping the electrodes with boron or nitrogen atoms induces a relevant rearrangement in the electronic structure of the junction. Upon B doping the current becomes significantly spin polarized, while N doping induces a marked Negative Differential Resistivity effect. We have also investigated the possible exploitation of the FeP junction as a gas sensor device. We demonstrate that the interaction of CO and O2 molecules with the Fe atom, while being strong enough to be stable at room temperature (2.0 eV and 1.1 eV, respectively), induces only minor effects on the electronic properties of the junction. Interestingly, a quenching of the spin polarization of the current is observed in the B-doped system.

The enhanced spin-polarized transport behaviors through cobalt benzene-porphyrin-benzene molecular junctions: the effect of functional groups

The modification effects of the groups amino (NH₂) and nitro (NO₂) on the spin polarized transport properties of the cobalt benzene-porphyrin-benzene (Co-BPB) molecule coupled to gold (Au) nanowire electrodes are investigated by the nonequilibrium Green's function method combined with the density functional theory. The calculation results show that functional groups can lead to the significant spin-filter effect, enhanced low-bias negative differential resistance (NDR) behavior and novel reverse rectifying effect in Co-BPB molecular junction. The locations and types of functional groups have distinct influences on spin-polarized transport performances. The configuration with NH₂ group substituting H atom in central porphyrin ring has larger spin-down current compared to that with NO₂ substitution. And Co-BPB molecule junction with NH₂ group substituting H atom in side benzene ring shows reverse rectifying effect. Detailed analyses confirm that NH₂ and NO₂ group substitution change the spin-polarized transferred charge, which makes the highest occupied molecular orbitals (HOMO) of spin-down channel of Co-BPB closer to the Fermi level. And the shift of HOMO strengthens the spin-polarized coupling between the molecular orbitals and the electrodes, leading to the enhanced spin-polarized behavior. Our findings might be useful in the design of multi-functional molecular devices in the future.

Synthesis of a series of Zinc(II)/freebase porphyrin dimers and trimers with programmable sequences from a common key molecule

We have developed a new methodology that enables the synthesis of any sequence of metal porphyrin arrays starting from a common key molecule. Using this method, we prepared porphyrin dimers and trimers with varying component unit sequences via consecutive Suzuki coupling reactions using the same key porphyrin compound under the same reaction conditions. The key porphyrin compound was synthesized on a gram scale in one batch, and the coupling reactions afforded the desired oligomers in good yields. Thus, the prepared porphyrin arrays showed unique physical properties depending on the sequence of the central metals. The reaction is potentially applicable for the automated synthesis of porphyrin arrays.

Insight into the electronic structure, optical properties, and redox behavior of the hybrid phthalocyaninoclathrochelates from experimental and density functional theory approaches

An insight into the electronic structure of several hafnium(IV), zirconium(IV), and lutetium(III) phthalocyaninoclathrochelates has been discussed on the basis of experimental UV-vis, MCD, electro- and spectroelectrochemical data as well as density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. On the basis of UV-vis and MCD spectroscopy as well as theoretical predictions, it was concluded that the electronic structure of the phthalocyninoclathrochelates can be described in the first approximation as a superposition of the weakly interacting phthalocyanine and clathrochelate substituents. Spectroelectrochemical data and DFT calculations clearly confirm that the highest occupied molecular orbital (HOMO) in all tested complexes is localized on the phthalocyanine ligand. X-ray crystallography on zirconium(IV) and earlier reported hafnium(IV) phthalocyaninoclathrochelate complexes revealed a slightly distorted phthalocyanine conformation with seven-coordinated metal center positioned ∼1 Å above macrocyclic cavity. The geometry of the encapsulated iron(II) ion in the clathrochelate fragment was found to be between trigonal-prismatic and trigonal-antiprismatic.

Electron transport characteristics of organic molecule encapsulated carbon nanotubes

One-dimensional carbon nanotube (CNT) junctions with interesting device characteristics have been designed by encapsulating p- and n-type organic molecules into CNTs with electrophilic tetracyano-p-quinodimethane (TCNQ) and nucleophilic tetrakis(dimethylamino)ethylene (TDAE) molecules in order to explore the effect of encapsulation of organic molecules and rectifying behaviors of the designed one-dimensional CNT p-n junctions. Our results show that p- and n-type doping of CNTs and their associated charge transfer play an important role in determining the electron transport characteristics and lead to materials with unique properties, p-n junction diode, i.e. Zener-like diode. Furthermore, we show that the operational device characteristics of non-covalently doped CNT junctions originate from the distinct response of intrinsic transmission peaks of pure CNTs according to the type of dopant and the applied bias. We believe that the results give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.

Electron transport characteristics of one-dimensional heterojunctions with multi-nitrogen-doped capped carbon nanotubes

We present a systematic analysis of electron transport characteristics for one-dimensional heterojunctions with two multi-nitrogen-doped (multi-N-doped) capped carbon nanotubes (CNTs) facing one another at different numbers of nitrogen atoms and conformations. Our results show that the modification of the molecular orbitals by the nitrogen dopants generates conducting channels in the designed heterojunctions inducing multi-switching behavior with sequential negative differential resistance (NDR). The NDR behavior significantly depends on the doping site and conformation of doped nitrogen atoms. Furthermore, we provide a clear interpretation for the NDR behavior by a rigid shift model of the HOMO- and LUMO-filtered energy levels in the left and right electrodes under the applied biases. We believe that our results will give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.

Designing nanogadgetry for nanoelectronic devices with nitrogen-doped capped carbon nanotubes

A systematic analysis of electron transport characteristics for 1D heterojunctions with two nitrogen-doped (N-doped) capped carbon nanotubes (CNTs) facing one another at different conformations is presented considering the chirality of CNTs (armchair(5,5) and zigzag(9,0)) and spatial arrangement of N-dopants. The results show that the modification of the molecular orbitals by the N-dopants generates a conducting channel in the designed CNT junctions, inducing a negative differential resistance (NDR) behavior, which is a characteristic feature of the Esaki-like diode, that is, tunneling diode. The NDR behavior significantly depends on the N-doping site and the facing conformations of the N-doped capped CNT junctions. Furthermore, a clear interpretation is presented for the NDR behavior by a rigid shift model of the HOMO- and LUMO-filtered energy levels in the left and right electrodes under the applied biases. These results give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.