Facile Redox-Induced Aromatic-Antiaromatic Interconversion of a ß-Tetracyano-21,23-Dithiaporphyrin under Ambient Conditions.

Facile redox-induced aromatic-antiaromatic interconversions were accomplished by using ß-tetracyano-21,23-dithiaporphyrin (CN4 S2 Por). Introduced cyano groups not only increased the reduction potential of the porphyrin core but also stabilized the antiaromatic isophlorin (CN4 S2 Iph) by π conjugation. The reduction of CN4 S2 Por with hydrazine in polar solvents quantitatively affords CN4 S2 Iph, even under ambient conditions. CN4 S2 Iph retains a nearly planar conformation and exhibits considerable antiaromaticity. Aerobic oxidation of CN4 S2 Iph to CN4 S2 Por occurs in nonpolar solvents. This study was conducted to contribute to the understanding of the structure-antiaromaticity relationship.

Single-walled carbon nanotube absolute-handedness chirality assignment confirmation using metalized porphyrin's supramolecular structures via STM imaging technique.

This work reports confirmation of the experimental assignment of the absolute-handedness chirality of single-walled carbon nanotubes (SWNTs). This was achieved by applying the scanning tunneling microscopy (STM) imaging technique to a supramolecular composite consisting of a metalized porphyrin derivative (nickel-5,15-bisdodecylporphyrin [Ni-BDP]) affixed to the surfaces of chiral-concentrated SWNTs (with right-handed helix P- and left-handed helix M-). On the basis of the handedness chirality, different chiral supramolecular structures of Ni-BDP were observed on the surfaces of the two SWNT enantiomers. The incorporation of a metal center into the porphyrin ring did not significantly affect the SWNT absolute-handedness chirality assignment, the large pi-system porphyrin ring being the crucial factor. These findings will effectively pave the way towards the clear selective synthesis, separation, chemistry, and applications of SWNT enantiomers.

Assignment of the Absolute-Handedness Chirality of Single-Walled Carbon Nanotubes Using Organic Molecule Supramolecular Structures.

Supramolecular structures of organic molecules on planar nanocarbon surfaces, such as highly oriented pyrolytic graphite (HOPG), have been extensively studied and the factors that control them are generally well-established. In contrast, the properties of supramolecular structures on curved nanocarbon surfaces like carbon nanotubes remain challenging to predict and/or to understand. This paper reports an investigation into the first study of the supramolecular structures of 5,15-bisdodecylporphyrin (C12P) on chiral, concentrated single-walled carbon nanotubes (SWNTs; with right-handed helix P- and left-handed helix M-) surfaces using STM. Furthermore, the study is the first of its kind to experimentally assign the absolute-handedness chirality of SWNTs, as well as to understand their effect on the supramolecular structures of organic molecules on their surfaces. Interestingly, these SWNT enantiomers resulted in supramolecular structures of opposite chirality based on the handedness chirality. With molecular modelling, we predicted the absolute-handedness chirality of SWNTs, before demonstrating this experimentally.

Stable Singlet Biradicals of Rare-Earth-Fused Diporphyrin-Triple-Decker Complexes with Low Energy Gaps and Multi-Redox States.

Dinuclear rare-earth (TbIII , YIII ) triple-decker complexes with a fused diporphyrin (FP) and two tetraphenylporphyrin (TPP) ligands were synthesized in neutral, dianionic, and diprotonated forms. The neutral forms have large open shell biradical character, as determined from the temperature dependency of the magnetic susceptibility measurements and theoretical calculations. The coupling value (J=-1.4 kcal mol-1 , -487 cm-1 ) of the radical centers in the neutral form of the YIII complex indicates weak pairing interactions. Theoretical calculations on the neutral form reveal a significant biradical character (y=68 %). Furthermore, the TbIII complex exhibits multi-redox states, having more than eight clear peaks in the voltammetry curves. The optical (3700 nm, 0.33 eV) and electrochemical measurements (3400 nm, 0.36 eV) indicate that the neutral form has very small HOMO-LUMO energy gap. Despite the large biradical character, the neutral complexes are thermally stable and do not decompose on heating at 120 °C. These complexes with unique characteristics are promising candidates for use in molecular electronics, optics, and spintronics.

Dualism of Sensitivity and Selectivity of Porphyrin Dimers in Electroanalysis.

This work uncovers the application of porphyrin dimers for the use in electroanalysis, such as potentiometric determination of ions. It also puts in question a current perception of an occurrence of the super-Nernstian response, as a result of the possible dimerization of single porphyrins within an ion-selective membrane. To study that, four various porphyrin dimers were used as ionophores, namely, freebase-freebase, Zn-Zn, Zn-freebase, and freebase-Zn. Since the Zn-freebase and freebase-Zn porphyrin dimers carried both anion- and cation-sensitive porphyrin units, their application in ISEs was utilized in both anion- and cation-sensitive sensors. With respect to the lipophilic salt added, both porphyrins dimers were found anion- and cation-sensitive. This allowed using a single molecule as novel type of versatile ionophore (anion- and cation-selective), simply by varying the membrane composition. All anion-sensitive sensors were perchlorate-sensitive, while the cation-selective sensors were silver-sensitive. The selectivity of the sensors depended primarily on the porphyrin dimers in the ion-selective membrane. Furthermore, the selectivity of cation-sensitive dimer based sensors was found significantly superior to the ones measured for the single porphyrin unit based sensors (precursors of the porphyrin dimers). Thus, the dimerization of single porphyrins may actually be a factor to increase or modulate porphyrin selectivity. Moreover, in the case of cation-sensitive sensors, the selectivity vastly depended on the order of porphyrin units in the dimer. This opens a new approach of regulating and adjusting sensitivity and selectivity of the sensor through the application of complex porphyrin systems with more than one porphyrin units with mix sensitive porphyrins.

Tuning the electrical property of a single layer graphene nanoribbon by adsorption of planar molecular nanoparticles.

In this study, a simple and fast approach of band gap formation in a single layer graphene nanoribbon (sGNR) is demonstrated by using hexaazatriphenylenehexacarbonitrile (HAT-CN6) as an adsorbate molecule. sGNRs were successfully synthesized through the unzipping of double-walled carbon nanotubes followed by casting HAT-CN6 in acetone solution to alter the electronic properties of the sGNRs. Then, the electrical property of a sGNR was measured using a field effect transistor structure and also by point-contact current imaging atomic force microscopy. The results demonstrate the formation of electron trapping sites with the nanoparticles and the neck structure of the sGNR near the adsorbed region of the molecule. Therefore, the charge carriers on the sGNR can only pass through the neck region, which works similarly to a narrow sGNR. Such a narrow sGNR has a lateral confinement of charge carriers around the neck region; hence, the device becomes semiconducting. The fabricated semiconducting sGNR could be widely used in electronic devices.

Systematic Structural Elucidation for the Protonated Form of Rare Earth Bis(porphyrinato) Double-Decker Complexes: Direct Structural Evidence of the Location of the Attached Proton.

Direct structural evidence of the presence and location of the attached proton in the protonated form of rare earth bis(porphyrinato) double-decker complexes is obtained from an X-ray diffraction study of single crystals for a series of protonated forms of bis(tetraphenylporphyrinato) complexes [M(III)(tpp)(tppH)] (M = Tb, Y, Sm, Nd, and La). When CHCl3 is used as a solvent for crystallization of the complexes, their nondisordered molecular structures are obtained and the attached proton is identified on one of the eight nitrogen atoms. Use of other solvents affords another type of crystal, in which the position of the proton is disordered and thus the molecular structure is averaged. La complex also affords the disordered average structure even when CHCl3 is used for crystallization. A variable-temperature diffraction study for the Tb complex reveals that the dynamics of the proton in the nondisordered crystal is restricted.

Design and synthesis of perpendicularly connected metal porphyrin-imide dyads for two-terminal wired single molecular diodes.

Four different porphyrin-imide dyads bearing different central metals (zinc or rhodium) and different substituents on the porphyrin macrocycles (tert-butyl or methoxy) were synthesized for single molecular diode measurements. The molecules were designed to separate the donor component (porphyrin) from the acceptor component (imide) by bonding in a perpendicular arrangement, thus enhancing the rectification properties. UV/Vis absorption spectra and density functional theory calculations showed that the design was successful and that the molecular orbitals of the dyads were the summation of the two components, with minimal interaction between them. The effect of the central metal was found to be significant, with the lowest energy absorption for the zinc dyads being attributed to the mixed state of charge transfer from porphyrin to imide and the Q band, whereas that of the rhodium dyads indicated insignificant charge-transfer character.

Switching of single-molecule magnetic properties of TbIII -porphyrin double-decker complexes and observation of their supramolecular structures on a carbon surface.

Double-decker complexes based on single-molecule magnets (SMMs) are a class of highly promising molecules for applications in molecular spintronics, wherein control of both the ligand oxidative states and the 2D supramolecular structure on carbon materials is of great importance. This study focuses on the synthesis and study of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP)-Tb(III) double-decker complexes with different electronic structures comprising protonated, anionic, and radical forms. Magnetic susceptibility measurements revealed that only the anionic and radical forms of the OEP-Tb(III) double-decker complexes exhibited SMM properties. The barrier heights for magnetic moment reversal were estimated to be 207 and 215 cm(-1) for the anionic and radical forms, respectively. Scanning tunneling microscopy (STM) investigations revealed that these OEP-Tb(III) complexes form well-ordered monolayers upon simple dropcasting from dilute dichloromethane solutions. All three complexes form an isomorphic pseudo-hexagonal 2D pattern, regardless of the differences in the electronic structures of their porphyrin-Tb cores. This finding is of interest for SMM technology as ultrathin films of these materials undergoing chemical transformations will not require any detrimental reorganization. Finally, we demonstrate self-assembly of the protonated 5,15-bisdodecylporphyrin (BDP)-Tb(III) double-decker complex as an example of successful supramolecular design to achieve controlled alignment of SMM-active sites.

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.

Fast, efficient, narrowband room-temperature phosphorescence from metal-free 1,2-diketones: rational design and the mechanism.

We report metal-free organic 1,2-diketones that exhibit fast and highly efficient room-temperature phosphorescence (RTP) with high colour purity under various conditions, including solutions. RTP quantum yields reached 38.2% in solution under Ar, 54% in a polymer matrix in air, and 50% in crystalline solids in air. Moreover, the narrowband RTP consistently dominated the steady-state emission, regardless of the molecular environment. Detailed mechanistic studies using ultrafast spectroscopy, single-crystal X-ray structure analysis, and theoretical calculations revealed picosecond intersystem crossing (ISC) followed by RTP from a planar conformation. Notably, the phosphorescence rate constant k p was unambiguously established as ∼5000 s-1, which is comparable to that of platinum porphyrins (representative heavy-metal phosphor). This inherently large k p enabled the high-efficiency RTP across diverse molecular environments, thus complementing the streamlined persistent RTP approach. The mechanism behind the photofunction has been elucidated as follows: (1) the large k p is due to efficient intensity borrowing of the T1 state from the bright S3 state, (2) the rapid ISC occurs from the S1 to the T3 state because these states are nearly isoenergetic and have a considerable spin-orbit coupling, and (3) the narrowband emission results from the minimal geometry change between the T1 and S0 states. Such mechanistic understanding based on molecular orbitals, as well as the structure-RTP property relationship study, highlighted design principles embodied by the diketone planar conformer. The fast RTP strategy enables development of organic phosphors with emissions independent of environmental conditions, thereby offering alternatives to precious-metal based phosphors.

Identification of Supramolecular Structures of Porphyrin Polymer on Single-Walled Carbon Nanotube Surface Using Microscopic Imaging Techniques.

Although the supramolecular structure of porphyrin polymers on flat surfaces (i.e., mica and HOPG) has been extensively studied, the self-assembly arrays of porphyrin polymers on the SWNT (as curved nanocarbon surfaces) have yet to be fully identified and/or investigated, especially using microscopic imaging techniques, i.e., scanning tunneling microscopy (STM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). This study reports the identification of the supramolecular structure of poly-[5,15-bis-(3,5-isopentoxyphenyl)-10,20-bis ethynylporphyrinato]-zinc (II) on the SWNT surface using mainly AFM and HR-TEM microscopic imaging techniques. After synthesizing around >900 mer of porphyrin polymer (via Glaser-Hay coupling); the as-prepared porphyrin polymer is then non-covalently adsorbed on SWNT surface. Afterward, the resultant porphyrin/SWNT nanocomposite is then anchored with gold nanoparticles (AuNPs), which are used as a marker, via coordination bonding to produce a porphyrin polymer/AuNPs/SWNT hybrid. The polymer, AuNPs, nanocomposite, and/or nanohybrid are characterized using 1H-NMR, mass spectrometry, UV-visible spectroscopy, AFM, as well as HR-TEM measuring techniques. The self-assembly arrays of porphyrin polymers moieties (marked with AuNPs) prefer to form a coplanar well-ordered, regular, repeated array (rather than wrapping) between neighboring molecules along the polymer chain on the tube surface. This will help with further understanding, designing, and fabricating novel supramolecular architectonics of porphyrin/SWNT-based devices.

Photoinduced crystal melting with luminescence evolution based on conformational isomerisation.

The phenomenon of crystal melting by light irradiation, known as photo-induced crystal-to-liquid transition (PCLT), can dramatically change material properties with high spatiotemporal resolution. However, the diversity of compounds exhibiting PCLT is severely limited, which hampers further functionalisation of PCLT-active materials and the fundamental understandings of PCLT. Here, we report on heteroaromatic 1,2-diketones as the new class of PCLT-active compounds, whose PCLT is based on conformational isomerisation. In particular, one of the diketones demonstrates luminescence evolution prior to crystal melting. Thus, the diketone crystal exhibits dynamic multistep changes in the luminescence colour and intensity during continuous ultraviolet irradiation. This luminescence evolution can be ascribed to the sequential PCLT processes of crystal loosening and conformational isomerisation before macroscopic melting. Single-crystal X-ray structural analysis, thermal analysis, and theoretical calculations of two PCLT-active and one inactive diketones revealed weaker intermolecular interactions for the PCLT-active crystals. In particular, we observed a characteristic packing motif for the PCLT-active crystals, consisting of an ordered layer of diketone core and a disordered layer of triisopropylsilyl moieties. Our results demonstrate the integration of photofunction with PCLT, provide fundamental insights into the melting process of molecular crystals, and will diversify the molecular design of PCLT-active materials beyond classical photochromic scaffolds such as azobenzenes.

Photocurrent and electronic activities of oriented-His-tagged photosynthetic light-harvesting/reaction center core complexes assembled onto a gold electrode.

A polyhistidine (His) tag was fused to the C- or N-terminus of the light-harvesting (LH1)-α chain of the photosynthetic antenna core complex (LH1-RC) from Rhodobacter sphaeroides to allow immobilization of the complex on a solid substrate with defined orientation. His-tagged LH1-RCs were adsorbed onto a gold electrode modified with Ni-NTA. The LH1-RC with the C-terminal His-tag (C-His LH1-RC) on the modified electrode produced a photovoltaic response upon illumination. Electron transfer is unidirectional within the RC and starts when the bacteriochlorophyll a dimer in the RC is activated by light absorbed by LH1. The LH1-RC with the N-terminal His-tag (N-His LH1-RC) produced very little or no photocurrent upon illumination at any wavelength. The conductivity of the His-tagged LH1-RC was measured with point-contact current imaging atomic force microscopy, indicating that 60% of the C-His LH1-RC are correctly oriented (N-His 63%). The oriented C-His LH1-RC or N-His LH1-RC showed semiconductive behavior, that is, had the opposite orientation. These results indicate that the His-tag successfully controlled the orientation of the RC on the solid substrate, and that the RC produced photocurrent depending upon the orientation on the electrode.

Influence of nanoparticle size to the electrical properties of naphthalenediimide on single-walled carbon nanotube wiring.

Nanoparticles of N,N'-bis(n-alkyl)tetracarbonatenaphthalenediimide (NDI) were adsorbed on single-walled carbon nanotube (SWNT) wires dispersed on a SiO(2) substrate. The electrical properties were measured along the long axis of the SWNTs, and in all cases through the nanoparticles showed rectification in semiconducting I-V curve. The plateau width of the I-V curve through the NDI nanoparticles on metallic SWNTs decreased as the particle size increased, while the rectification ratio increased. The conduction mechanism was changed from tunneling conduction to Schottky-like conduction and their boundary is at about 3 nm diameter.

Room-temperature phosphorescence of a supercooled liquid: kinetic stabilisation by desymmetrisation.

Achieving organic room-temperature phosphorescence (RTP) in a solvent-free liquid state is a challenging task because the liquid state provides a less rigid environment than the crystal. Here, we report that an unsymmetrical heteroaromatic 1,2-diketone forms an organic RTP liquid. This diketone exists as a kinetically stable supercooled liquid, which resists crystallisation even under pricking or shearing stresses, and remains as a liquid for several months. The unsymmetrical diketone core is flexible, with eight distinct conformers possible, which prevents nucleation and growth for the liquid-solid transition. Interestingly, the thermodynamically stable crystalline solid-state was non-emissive. Thus, the RTP of the diketone was found to be liquiefaction-induced. Single-crystal X-ray structure analysis revealed that the diminished RTP of the crystal is due to insufficient intermolecular interactions and restricted access to an emissive conformer. Our work demonstrates that flexible unsymmetrical skeletons are promising motifs for bistable liquid-solid molecular systems, which are useful for the further development of stimuli-responsive materials that use phase transitions.

Modulating Room-Temperature Phosphorescence-To-Phosphorescence Mechanochromism by Halogen Exchange.

Modulating the stimulus-responsiveness of a luminescent crystal is challenging owing to the complex interdependent nature of its controlling factors, such as molecular structure, molecular conformation, crystal packing, optical properties, and amorphization behavior. Herein, we demonstrate a halogen-exchange approach that disentangles this problem, thereby realizing the modulation of room-temperature phosphorescence-to-phosphorescence mechanochromism. Replacing the bromine atoms in a brominated thienyl diketone with chlorine atoms afforded isostructural crystals; i.e., molecules with different halogen atoms exhibited the same molecular conformation and crystal packing. Consequently, amorphization behavior toward mechanical stimulation was also the same, and the phosphorescence of amorphous states originated from the same conformer of each diketone. In contrast, the phosphorescence properties of each conformer were modulated differently, which is ascribable to heavy atom effects, resulting in the modulation of the mechanochromism. Thus, halogen exchange is a promising approach for modulating the stimulus-responsive photofunctions of crystals involving spin-forbidden processes.

Mechanoresponsive turn-on phosphorescence by a desymmetrization approach.

The room-temperature phosphorescence (RTP) of metal-free organic crystals is normally quenched by mechanical stimulation. Herein, we demonstrate the opposite mechanoresponse of turn-on RTP. A desymmetrization of a C2-symmetric 1,2-diketone creates space for molecular motion in the crystal, quenching the RTP from the crystal while maintaining that from the amorphous solid.

Supramolecular structures of terbium(iii) porphyrin double-decker complexes on a single-walled carbon nanotube surface.

This work mainly reports the observation of novel supramolecular structures of TbIII-5,15-bisdodecylporphyrin (BDP, C12P) double-decker complexes on the surfaces of single-walled carbon nanotubes (SWNTs) performed by scanning tunneling microscopy under an ultra-high vacuum and low temperature, atomic force microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy, and ultraviolet-visible spectroscopy. The molecules formed a well-ordered self-assembled helix-shaped array with regular periodicity on the tube surface. Additionally, some magnetic properties of the BDP-molecule as well as the resulting BDP-SWNT composites were investigated by superconducting quantum interference measurements. The molecule exhibits single-molecule magnetic (SMM) properties and the composite's magnetization increases almost linearly with decreasing temperature which is possibly due to the coupling between porphyrin molecules and SWNTs. Consequently, this may enable the development of more advanced spintronic devices based on porphyrin-nanocarbon composites.

Three site molecular orbital controlled single-molecule rectifiers based on perpendicularly linked porphyrin-imide dyads.

The original single-molecule rectifier proposed by Aviram and Ratner is based on a donor-σ-acceptor structure, in which σ functions as the insulator to disconnect the π electronic systems of the two parts. However, there have been no reports on experimentally demonstrated highly efficient single-molecule rectifiers based on this mechanism. In this paper, we demonstrate single-molecule rectifiers with perpendicularly connected metal porphyrin-imide dyads. Our proposed molecule rectifiers use hydroxyl groups at both ends as weak anchoring groups. Measurements of the single-molecule current-voltage characteristics of these molecules clearly show that the rectification ratio reached a high value of 14 on average. Moreover, the ratio could be tuned by changing the central metal in the porphyrin core. All of these features can be explained by the energy-level shift of the molecular orbital using a model with three electronic parts.