Linear Porphyrin Arrays Research Articles

ChemInform Abstract: Exploration of Giant Functional Porphyrin Arrays

AbstractReview: [143 refs.

Exploration of Giant Functional Porphyrin Arrays

Abstract Directly linked linear porphyrin arrays were synthesized and their photophysical properties depending on the length of the arrays were investigated. Ag(I)-promoted oxidative coupling of 5,15-diaryl Zn(II) porphyrins has proven to be effective for preparations of very long meso–meso-linked Zn(II) porphyrin arrays (Zn). A stepwise doubling strategy with strict purification at each doubling step allowed for preparation of gigantic but discrete linear arrays in an unprecedented manner. Large cyclic porphyrin wheels were also synthesized by final meso–meso coupling intramolecular cyclization. Crucial to this success are high reactivity and high regioselectivity of the Ag(I)-promoted oxidative coupling. Equally important are high solubilities of Zn despite their large molecular weights, which arise from their perpendicularly linked structures. Another advantage of this doubling strategy is a large difference in the molecular weights among reactants and products that increases as the reactant array becomes larger, allowing separation of coupling products by preparative GPC-HPLC. As an alternative protocol, Pd-catalyzed Suzuki–Miyaura cross-coupling has been extensively used for synthesis of cyclic porphyrin arrays. This strategy gave novel linear and cyclic porphyrin arrays from β-borylated porphyrins that were prepared by Ir-catalyzed borylation of meso-free porphyrins. As an interesting example, a porphyrin nanotube consisting of four porphyrins doubly bridged by 2,6-pyridinylene spacers was synthesized and revealed to possess a C60-encapsulation capability. Exciton coupling, effective energy hopping, coherent length, and excitation energy transfer of meso–meso-linked linear and cyclic porphyrin arrays are discussed. STM and AFM detections of the large porphyrin arrays are also presented. We believe that these studies expand the range of organic molecules we can manipulate and open a door to a new area of nanoscaled and sub-microscaled functional porphyrin materials.

The Role of Electronic Coupling in Linear Porphyrin Arrays Probed by Single‐Molecule Fluorescence Spectroscopy

Single-molecule photophysical properties of two families of linear porphyrin arrays have been investigated by single-molecule fluorescence detection techniques. Butadiyne-linked arrays (Z(N)B) with extensive π-conjugation perform as photostable one-quantum systems. This demonstration has been suggested by the long-lasting initial emissive state and subsequent discrete one-step photobleaching in the fluorescence intensity trajectories (FITs). As in the behavior of a one-quantum system, Z(N)B shows anti-bunching data in the coincidence measurements. On the other hand, in directly-linked arrays (Z(N)) with strong dipole coupling, each porphyrin moiety keeps individual character in photobleaching dynamics. The stepwise photobleachings in the FITs account for this explanation. Most of the FITs of Z(N) do not carry momentary cessation of fluorescence emission, which has been explained by the strongly bound electron-hole pair of Frenkel exciton that suppresses charge transfer between the molecule and surrounding polymers. These results give insight into the influences of interchromophorinc interactions between porphyrin moieties in the multiporphyrin arrays on their fluorescence dynamics at the single-molecule level.

Determination of the Superradiance Coherence Length of Directly Linked Linear Porphyrin Arrays at the Single‐Molecule Level

AbstractDer Verlauf des Photobleichens zeigt an, dass die Excitonendelokalisierungslänge in direkt verknüpften Porphyrinketten (siehe Bild) ungefähr vier bis fünf Porphyrineinheiten beträgt. Dieses Resultat gibt Einblick in den Mechanismus der Lichtübertragung im Festkörper und belegt, dass Porphyrinanordnungen als molekulare photonische Drähte fungieren könnten.magnified image

Determination of the Superradiance Coherence Length of Directly Linked Linear Porphyrin Arrays at the Single‐Molecule Level

Down to the wire: Photobleaching dynamics show the exciton delocalization length of directly linked porphyrin arrays (see picture) to be about four or five porphyrin units at the single-molecule level. This result provides a better understanding of how light-signal transmission occurs in the solid state and gives a perspective for the porphyrin arrays to be used as single-molecule photonic wires.

Synthesis, solid-state molecular structure and polymerization of a trans-substituted meso-porphyrin with thienyl pendant arms

A new trans-substituted meso-porphyrin with two thienyl arms at the meso-positions has been synthesized and characterized. The molecular structure of this porphyrin has been determined from X-ray single crystal diffraction data. This di- meso-substituted porphyrin shows a very planar conformation suggesting that the peripheral arms of the porphyrin do not affect the solid architecture. Some physico-chemical properties (NMR, UV–visible, electrochemistry and luminescence) of the dithienyl porphyrin are also discussed. Such a porphyrin might constitute a key building block in the synthesis of linear porphyrin arrays.

A Theoretical Study on the Third-Order Nonlinear Optical Properties of π-Conjugated Linear Porphyrin Arrays

By using the Pariser-Parr-Pople (PPP) theory, the second hyperpolarizabilities (gamma) have been calculated for various pi-conjugated porphyrin arrays including "porphyrin tapes": the meso-beta doubly linked porphyrin array Dn and the meso-meso, beta-beta, beta-beta triply linked array Tn. The validity of the PPP theory is checked via a comparison with both the ab initio Hartree-Fock and the B3LYP theories in the case of porphyrin monomers and dimers. It is found that Dn and especially Tn exhibit much more remarkable evolution of gamma/n along with an increasing number of porphyrin units n compared with the butadiyne-bridged array, Yn. As a result, the static third-order susceptibilities chi((3)) of Dn and Tn are expected to be 1 and 3 orders of magnitude larger than that of Yn in the limit n --> infinity, and these advantages of porphyrin tapes become more prominent by taking into account geometrical relaxations of porphyrin units in the arrays. The structure-property relationship in various conjugated polymers including porphyrin arrays is interpreted on the basis of the scaling behaviors of chi((1)) and chi((3)) with the effective conjugation length (ECL) as well as the reciprocal HOMO-LUMO energy gap (1/E(g)). In particular, from the master plot of chi((3)) (and even chi((1))) versus 1/E(g), the pi-conjugation of Tn is noted to indeed be exceptional, because its large susceptibilities cannot be expected from the scaling behavior of ordinary one-dimensional conjugated systems. We also point out that the theory of scaling relationship, chi((3)) approximately 1/E(g)(x)(), is significantly improved by taking into account electron-electron interactions based on the comparison with experiments.

Electrical transport properties and their reproducibility for linear porphyrin arrays

We present an investigation of electrical transport through fused and orthogonal dithiolated porphyrin arrays of different molecular lengths. Length dependence measurements show that conductance decreases much more slowly with molecular length than the exponential dependence expected from a simple theoretical model. From the temperature dependence of I SD − V SD curves, the thermal activation energy E a is estimated to be about 0.35 eV at zero-bias voltage, independent of molecular conformation and length. At high-bias voltages, however, the decrease in E a as a function of bias voltage is more significant for the longer porphyrin arrays. We have also studied the thermal cycling effects. After thermal cycling, dithiolated porphyrin arrays are found to aggregate into clusters between nanoelectrodes. This is probably due to the diffusion of porphyrin arrays on the SiO 2 substrate at low temperatures, which enhances the conductance.

Effect of Conformational Heterogeneity on Excitation Energy Transfer Efficiency in Directly meso−meso Linked Zn(II) Porphyrin Arrays

We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.

Photophysical Properties of Directly Linked Linear Porphyrin Arrays

A variety of porphyrin arrays connected by diverse linkers have been envisaged and prepared for the applications in molecular photonics and electronics. From a viewpoint of operational requirements, the porphyrin arrays should have the very regular pigment arrangements which allow a facile light energy or charge flow along the arrays but do not result in the alteration of individual properties of the constituent pigments leading to formation of so-called energy or charge sink. In these respects, the directly coupled (orthogonal and fused) porphyrin arrays without any linkers are ideal, because the conformational heterogeneity mainly arising from a dihedral angle distribution between the neighboring porphyrin moieties should be minimized. In addition, the electronic effect of the linker can be disregarded in design strategy of molecular photonic devices, because the linker can also be considered as a transmission element in electronic communication. Considering these features, these types (orthogonal vs fu...

Electrical conduction through linear porphyrin arrays.

Electrical conduction measurements were made on two extreme types of directly linked porphyrin arrays by using nanoelectrodes. One type is the directly linked Zn(II)porphyrin arrays, consisting of 48 Zn(II)porphyrin moieties (Z48), and the other type is the completely flat, tape-shaped Zn(II)porphyrin arrays, consisting of eight Zn(II)porphyrin units (T8). The I-V curve for Z48 exhibits the diode-like behavior and the hysteresis depending on the voltage sweep direction presumably due to the conformational heterogeneity arising from the dihedral angle distribution in Z48. On the other hand, the I-V curve for T8 is nearly symmetric without any hysteresis, leading to the higher conductivity and the smaller band gap. These results illustrate that the stronger pi-electron conjugation in T8, as compared with that of Z48, results in better electrical conduction.

Self-organization of self-assembled photonic materials into functional devices: photo-switched conductors.

Linear porphyrin arrays self-assembled by either hydrogen bonding or metal ion coordination self-organize into lipid bilayer membranes. The length of the transmembrane assemblies is determined both by the thermodynamics of the intermolecular interactions in the supermolecule and by the dimension and physical chemical properties of the bilayer. Thus, the size of the porphyrin assembly can self-adjust to the thickness of the bilayer. An aqueous electron acceptor is placed on one side of the membrane and an electron donor is placed on the opposite side. When illuminated with white light, substantial photocurrents are observed. Only the assembled structures give rise to the photocurrent, as no current is observed from any of the component molecules. The fabrication of this photogated molecular electronic conductor from simple molecular components exploits several levels of self-assembly and self-organization.

A New Strategy for Construction of Covalently Linked Giant Porphyrin Arrays with One, Two, and Three Dimensionally Arranged Architectures

Abstract Synthesis, structural characteristics, and optical and electrochemical properties of various meso–meso-linked porphyrin arrays were described. The AgI-salt-promoted meso–meso coupling reaction of 5,15-diaryl and 5,10,15-triaryl ZnII-porphyrins is advantageous in light of its high regioselectivity as well as its easy extension to large porphyrin arrays. A similar coupling reaction is also possible by electrochemical oxidations for ZnII- and MgII-porphyrins, while the electrochemical oxidation of NiII-, PdII, and CuII-porphyrins led to formation of meso-β linked biporphyrins with high regio-selectivity. As an extension of the above AgI-promoted coupling reaction, 1,4-phenylene-bridged linear porphyrin arrays were coupled to give orthogonally arranged windmill-like porphyrin arrays, which in turn were further coupled to afford grid-like porphyrin arrays of giant size. In all the ZnII-metallated windmill arrays, the intramolecular singlet–singlet excitation energy transfer proceeds efficiently from the peripheral porphyrins to the central biporphyrin core. The doubling reaction was repeated up to the synthesis of 128-mer. During these attempts, many meso–meso-linked porphyrin arrays were isolated in a discrete form by repetitive gel-permeation chromatography (GPC)–HPLC, all exhibiting high solubility in common organic solvents in spite of their giant molecular sizes. Finally, the oxidation with strong oxidant tris(4-bromophenyl)aminium hexachloroantimonate allowed the coupling of 5,10,15-triaryl NiII- and PdII-porphyrins to meso-β doubly-linked fused biporphyrins and the smooth conversion of meso–meso-linked CuII-biporphyrin to meso–meso β–β β–β triply-linked biporphyrin. These fused biporphyrins exhibit full π-conjugation over the arrays.

Three-Dimensionally Arranged Windmill and Grid Porphyrin Arrays by AgI-Promoted meso–meso Block Oligomerization

The syntheses of soluble windmill and grid porphyrin arrays through the AgI-promoted coupling reaction of 1,4-phenylene-bridged linear porphyrin arrays, which are comprised of a central ZnII β-free porphyrin and flanking peripheral NiII β-octaalkylporphyrins, are described. The coupling reaction is advantageous in light of its high regioselectivity occurring only at the meso-position of the ZnII β-free porphyrin as well as its easy extension to large porphyrin arrays. The windmill porphyrin arrays in turn serve as an effective substrate for further coupling reactions, to give three-dimensionally arranged grid porphyrin arrays. Further the grid porphyrin 12-mer (a tetramer of the linear porphyrin trimer) was also coupled to afford grid porphyrins (24-mer, 36-mer, and 48-mer). These porphyrin arrays were isolated in a discrete form by repetitive GPC/HPLC (GPC=gel-permiation chromatography). Competitive experiments with three linear porphyrin trimers bearing different peripheral metalloporphyrins (ZnII, NiII, and CuII), and the trapping experiment of the radical cation at the peripheral porphyrin with AgNO2, suggested that an initial one-electron oxidation of the easily oxidizable peripheral ZnII β-octaalkylporphyrin with an AgI ion and a subsequent endothermic hole transfer assist the generation of the radical cation at the central ZnII β-free porphyrin. In all ZnII-metallated windmill porphyrin arrays, the energy level of the S1 state of the meso–meso-linked diporphyrin core is lower than that of the peripheral porphyrins, thereby allowing an energy flow from the peripheral porphyrins to the central diporphyrin core; this has been confirmed by measurements of fluorescence lifetimes and picosecond time-resolved fluorescence spectra. The excitation energy transfer in the arrays encourages their potential use as an light-harvesting antenna.

Three-dimensionally arranged windmill and grid porphyrin arrays by AgI-promoted meso-meso block oligomerization

The syntheses of soluble windmill and grid porphyrin arrays through the AgI-promoted coupling reaction of 1,4-phenylene-bridged linear porphyrin arrays, which are comprised of a central ZnII beta-free porphyrin and flanking peripheral NiII beta-octaalkylporphyrins, are described. The coupling reaction is advantageous in light of its high regioselectivity occurring only at the meso-position of the ZnII beta-free porphyrin as well as its easy extension to large porphyrin arrays. The windmill porphyrin arrays in turn serve as an effective substrate for further coupling reactions, to give three-dimensionally arranged grid porphyrin arrays. Further the grid porphyrin 12-mer (a tetramer of the linear porphyrin trimer) was also coupled to afford grid porphyrins (24-mer, 36-mer, and 48-mer). These porphyrin arrays were isolated in a discrete form by repetitive GPC/HPLC (GPC= gel-permiation chromatography). Competitive experiments with three linear porphyrin trimers bearing different peripheral metalloporphyrins (ZnII, NiII, and Cull), and the trapping experiment of the radical cation at the peripheral porphyrin with AgNO2, suggested that an initial one-electron oxidation of the easily oxidizable peripheral ZnII beta-octaalkylporphyrin with an AgI ion and a subsequent endothermic hole transfer assist the generation of the radical cation at the central ZnII beta-free porphyrin. In all ZnII-metallated windmill porphyrin arrays, the energy level of the S1 state of the meso-meso-linked diporphyrin core is lower than that of the peripheral porphyrins, thereby allowing an energy flow from the peripheral porphyrins to the central diporphyrin core; this has been confirmed by measurements of fluorescence lifetimes and picosecond time-resolved fluorescence spectra. The excitation energy transfer in the arrays encourages their potential use as an light-harvesting antenna.

Synthesis of 1,4-phenylene-bridged linear porphyrin arrays

1,4-Phenylene-bridged, linear porphyrin arrays including the dimer, trimer, pentamer, heptamer and nonamer have been synthesized by acid-catalysed condensation of formyl-substituted porphyrins with bis(3-ethyl-4-methylpyrrol-2-yl)methane in moderate yields. The molecular length of the nonamer is estimated to be as long as ca. 122 Å. The Soret bands of both zinc complexes and doubly protonated free base porphyrins are split depending upon the number of the porphyrins, which can be explained in terms of exciton coupling theory. With an increase in the number of the porphyrins, the fluorescence spectra become broader and red-shifted without significant decrease in the fluorescence quantum yield, reflecting their stretched conformations that do not allow the formation of a singlet-excitation-energy trapping site.

One-flask synthesis of meso-substituted dipyrromethanes and their application in the synthesis of trans-substituted porphyrin building blocks

The reaction at room temperature of an aldehyde with excess pyrrole in the absence of solvent affords the meso-substituted dipyrromethane. The reaction is catalyzed with trifluoroacetic acid or with BF 3ṡO(Et) 2. The dipyrromethane is purified by crystallization or by flash chromatography on silica with eluants containing 1% triethylamine. The reaction is compatible with aliphatic or aromatic aldehydes, including 2,6-disubstituted benzaldehydes. Nine dipyrromethanes have been prepared in this manner in yields of 47–86%, indicating the broad scope of the reaction. The dipyrromethanes are stable in the purified form in the absence of light and air. Similar reaction with terephthalaldehyde and pyrrole affords the corresponding bis-dipyrromethane. The reaction of a meso-substituted dipyrromethane with an aldehyde under the conditions of the two-step one-flask porphyrin synthesis affords a direct route to trans-substituted meso-porphyrins. Acidolysis of the dipyrromethane is negligible under the conditions of the porphyrin-forming reaction. Four porphyrins bearing peripheral functional groups and facially-encumbering groups have been prepared which serve as key building blocks in the synthesis of linear porphyrin arrays.

An Efficient Synthesis of Linear Porphyrin Arrays

Abstract Synthesis of linear porphyrin arrays of unusual length was reported. 1,4-Phenylene-bridged porphyrin oligomers up to a nonamer (ca. 122 Å length) were synthesized by acid catalyzed condensation of formyl-substituted porphyrins with dipyrrylmethane. The absorption and fluorescence spectra of these molecules exhibit systematic changes upon the increase in the number of the porphyrins.