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A highly distorted binuclear rhodium(I) complex, 2Rh, was successfully synthesized from hexaphyrin(2.1.2.1.2.1) containing dimethylvinylene-bridges between dipyrrin units. IR spectroscopy, 1H NMR spectroscopy, and X-ray crystallography revealed that the complex 2Rh consists of two rhodium(I) ions coordinated to two dipyrrin units. Rh complexation induced a transformation from a trans-/cis-/trans- to trans-/cis-/cis-conformation on the dimethylvinylene-bridges. This is the first example of rhodium(I)-ion-induced cis-/trans-isomerization in the porphyrin derivatives. Theoretical calculations of 2Rh predicted the presence of intramolecular charge-transfer absorption due to the distorted molecular structure.

Cyclic butadiyne-linked porphyrin(2.1.2.1) oligomers are synthesized from 5,16-diethynylporphyrin(2.1.2.1) by Glaser–Hay coupling. Porphyrin(2.1.2.1) forms a bent structure which gives advantages for making cyclic structure without templating molecules. We isolated cyclic trimer and tetramer and characterized them by MALDI-TOF-MS and [Formula: see text]H NMR spectroscopy, theoretical calculations, UV-vis absorption and fluorescence spectra and cyclic voltammetry. The cyclic structure mainly affects the reduction potentials because of expansion of [Formula: see text]-conjugations through butadiyne-linkages to stabilize their LUMOs.

Tetrabenzoporphyrin (BP) holds attractive characteristics for optoelectronic applications, such as the large π-conjugated framework and high photoabsorption capability. However, its use in organic solar cells (OSCs) has been limited because of the extremely low solubility that hampers direct solution processing and also the high frontier-orbital energies that lead to low open-circuit voltage (VOC). Herein, we examine BP derivatives equipped with multiple strongly electron-withdrawing groups for photovoltaic applications. The derivatives are generated in thin films through a thermal precursor approach, wherein the corresponding bicyclo[2.2.2]octadiene-fused porphyrin derivatives are solution-cast, and then annealed to carry out the in situ retro-Diels-Alder reaction. The frontier-orbital energies of the resulting derivatives are effectively stabilized as compared to pristine BP to such a degree that they afford high VOC of up to 0.94 V when used as a donor or can even work as a new class of nonfullerene acceptor in OSCs. Single-crystal X-ray diffraction analyses demonstrate that the conformation of the BP framework largely varies from being near planar to highly curved depending on its substituents. The morphology of polymer:BP-derivative bulk-heterojunction films prepared by the thermal precursor approach also varies between the BP derivatives. These results can greatly extend the scope of both molecular design and morphology control for utilization of the BP chromophore toward achieving viable organic optoelectronic devices.

Vinylene-bridged hexaphyrin(2.1.2.1.2.1) was synthesized from dipyrrolyl diphenylethenes by acid-catalyzed condensation reactions. Freebase hexaphyrin(2.1.2.1.2.1) forms a distorted structure with non-aromatic characteristics. The aromaticity and molecular configuration of non-planar hexaphyrin(2.1.2.1.2.1) can be controlled by insertion of metal ions. Freebase and zinc complexes show a distorted structure without macrocyclic aromaticity, whereas copper complexes show a figure-of-eight structure with macrocyclic aromaticity. It is the first example of aromaticity conversion of a distorted expanded porphyrin involving vinylene bridges.

We have investigated conformational structures of meso- meso linked porphyrin arrays (Z n) by single molecule fluorescence spectroscopy. Modulation depths ( M values) were measured by excitation polarization fluorescence spectroscopy. The M value decreases from 0.85 to 0.46 as the number of porphyrin units increases from 3 to 128, indicating that longer arrays exhibit coiled structures. Such conformational changes depending on the length have been confirmed by coarse-grained simulation. The histograms of M values and traces of centroid position of emitting sites by localization microscopy showed that the structures of longer arrays changed to more stretched after solvent vapor annealing with tetrahydrofuran.

A general design of a meso-aryl porphyrin for the formation of a 1D columnar stacking structure in the single crystal state is proposed. We found that 5,10-diphenyl- and 5,10-bis(pentafluorophenyl)porphyrin metal complexes (M = Zn, Ni) can stack in a complementary manner to form a 1D columnar stacking structure in the single crystals, while the corresponding freebase porphyrins displayed herringbone or dimeric orthogonal packing forms. The structural details of the obtained 1D columnar packing structures are discussed in comparison with precedented examples including 5,10-diphenylporphyrin Cu(II) and Ag(II) complexes, 5,10-diphenyl [Formula: see text]-confused porphyrin Ag(III) complex and 5,10-bis(pentafluorophenyl)corrole.

This Letter details a simple and effective method to synthesize a porphyrin(2.1.2.1)-based nanobelt NBNi3 via condensation reactions of 1,2,4,5-tetra(pyrrol-2-yl)benzene and benzaldehydes. NBNi3 consists of three arch-shaped porphyrins(2.1.2.1) and benzene linkages with a C3h-symmteric structure. The concave-shaped cavity of NBNi3 behaves as a C60 receptor by capturing two C60 cooperatively. Additionally, NBNi3 exhibits reversible oxidation and reduction peaks with five electrons in each peak, indicating that the shape of the nanobelt can stabilize multicationic and anionic states.

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...

meso-to-meso 2,5-Pyrrolylene bridged zig-zag porphyrin arrays have been synthesized via a stepwise Suzuki–Miyaura cross coupling reaction. Both the dimeric and trimeric structures of ZnII porphyrin were confirmed by X-ray diffraction analysis. DFT calculations indicate that the porphyrin oligomers are all in zig-zag conformations. The UV/Vis absorption and emission spectra of these porphyrin oligomers were red-shifted as the number of their porphyrin units increased.

A 1,3-phenylene-bridged hexameric Zn(II) porphyrin wheel was synthesized by a Suzuki-Miyaura coupling reaction through a one-pot or a stepwise route. The hexameric wheel structure was revealed by using X-ray diffraction analysis. The porphyrin wheel exhibits a split Soret band due to effective exciton coupling and displays efficient excitation energy transfer along the wheel. Measurements of fluorescence anisotropy decay and pump-power-dependent decay reveal a rapid excitation energy hopping along the wheel with a rate of 1.4 ps.

β-to-β 2,5-Pyrrolylene linked cyclic porphyrin oligomers have been synthesized by Suzuki-Miyaura coupling of 2,5-diborylpyrrole and 3,7-dibromoporphyrin. The cyclic porphyrin oligomers exhibit roughly coplanar structures, strong excitonic coupling, small electrochemical HOMO-LUMO gaps, and ultrafast excitation energy transfer between the neighboring porphyrins via the pyrrolylene bridge.

By utilizing single-molecule defocused wide-field fluorescence microscopy, we have investigated the molecular structural properties such as transition dipole moment orientations and the angular relationship among chromophores, as well as structural distortions and flexibilities depending on the ring size, in a series of cyclic porphyrin arrays bearing close likeness in overall architectures to the LH2 complexes in purple bacterial photosynthetic systems. Furthermore, comparing the experimental results with molecular dynamics simulations, we ascertained site selection for fluorescent trapping sites. Collectively, these experimental and computational results provide the basis for structure–property relationships and energy hopping/emitting processes in an important class of artificial light-harvesting molecular systems widely used in molecular electronics technology.

The reactions at the benzylic positions of meso-alkyl-substituted porphyrins have been achieved via the reaction with DDQ in the presence of alcohol or water for the first time, and ethenyl porphyrins from methoxylated porphyrins and an acetoxy porphyrin from the hydroxyl porphyrin were prepared.

Hybrid porphyrin tapes 3 and 4, consisting of a mixture of 3,5-di-tert-butylphenyl-substituted donor-type Zn(II)-porphyrins and pentafluorophenyl-substituted acceptor-type Zn(II)-porphyrins, were prepared by a synthetic route involving cross-condensation reaction of a Ni(II)-porphyrinyldipyrromethane and pentafluorophenyldipyrromethane with pentafluorobenzaldehyde followed by appropriate demetalation, remetalation, and oxidative ring-closure reaction. The Ni(II)-substituted porphyrin tapes 5 (Ni-Zn-Ni) and 6 (Ni-H(2)-Ni) were also prepared through similar routes. The hybrid porphyrin tapes 3 and 4 are more soluble and more stable than normal porphyrin tapes 1 and 2 consisting of only donor-type Zn(II)-porphyrins. The solid-state and crystal packing structures of 3, 4, and 5 were elucidated by single-crystal X-ray diffraction analysis. Singly meso-meso-linked hybrid porphyrin arrays 12 and 14 exhibit redox potentials that roughly correspond to each constituent porphyrin segments, while the redox potentials of the hybrid porphyrin tapes 3 and 4 are positively shifted as a whole. The two-photon absorption (TPA) values of 1-6 were measured by using a wavelength-scanning open aperture Z-scan method and found to be 1900, 21,000, 2200, 27,000, 24,000, and 26,000 GM, respectively. These results illustrate an important effect of elongation of π-electron conjugation for the enhancement of TPA values. The hybrid porphyrin tapes show slightly larger TPA values than the parent ones.

A novel NCN type palladium pincer complex based on the porphyrin backbone was synthesized. The palladium atom bound to the meso-carbon of the porphyrin core was effectively supported by two imidazolyl groups. The product was characterized by spectroscopic and X-ray diffraction analysis. The structure of the porphyrinic macrocycle was revealed to be highly distorted by the coordination of the outer palladium center. The degree of deformation was found to be even larger than the analogous palladium pincer complexes in which pyridine groups were employed as the supporting ligands. The complex exhibited high catalytic activity in the Mizoroki-Heck reaction of iodobenzene with butyl acrylate with 0.005% of the catalyst loading.

meso-β doubly linked Zn(II) porphyrin arrays and their indene-fused analogues were prepared by Suzuki-Miyaura coupling and subsequent oxidation with DDQ-Sc(OTf)(3). The excited state properties of these conjugated porphyrin arrays have been investigated by steady-state and time-resolved spectroscopy along with quantum mechanical calculations to reveal their molecular shape dependencies with respect to syn- versus anti-connectivity as well as nonfused versus fused structures. Nonfused syn-trimer exhibits nearly the same S(1)-state lifetime and two-photon absorption (TPA) value as the nonfused dimer, while the nonfused anti-trimer exhibits different values. The TD-DFT calculations also demonstrate that the nonfused anti-trimer has a different electronic structure from the corresponding syn-trimer. Indene-fused dimers and trimers exhibit the enhanced TPA values and shortened S(1)-state lifetime because of the elongated π-conjugation and the perturbation of electronic structures. Our research provides further insight into the molecular-shape-dependent electronic properties of meso-β doubly linked Zn(II) porphyrin arrays.

Covalently linked multiAin arrays have been extensively explored for their numerous applications in molecular devices such as models of photosynthetic systems, sensors, nonlinear optical (NLO) devices, and others. [1–3] The electronic coupling between the constituent porphyrin chromophores is a key parameter that determines the various important properties of porphyrin arrays. [4] Covalent construction of multiporphyrin arrays is often superior to a noncovalent strategy in terms of fine-tuning of the electronic coupling, since a variety of spacers varying in size, shape, and electronic structure can be inserted between porphyrins. Oligothiophenes are also important as potential functional organic molecules that have been used in various applications such as field effect transistors (FETs), electrical conductors, and NLO devices. [5] In addition, the high conductivity of oligothiophenes make them attractive candidates for molecular wires and many synthetic efforts have been made along this line. [6, 7] Recently, we reported b-to-b 2,5-thienylene-bridged cyclic porphyrin arrays which exhibited large two-photon absorption cross-section values owing to effective electronic delocalization through thienylene linkages. [8] As an extension of these studies, we report here the synthesis and characterizations of novel cyclic porphyrin arrays 3, 4, and 5 bridged by meso-to-meso 2,2’-bithiophene spacers. Interestingly, we have found that Zn II complexes of these cyclic arrays are particularly suitable for multicharge storage and hence molecular information storage systems. [9] The synthetic scheme of 3H - 5H is shown in Scheme 1. 5,10-Diaryl-15,20-dibromoporphyrin 1 [10] was coupled with 5,5’-diboryl-2,2’-bithiophene 2 [11] under Suzuki–Miyaura cross-coupling conditions. Separation by gel permeation chromatography (GPC) and silica gel chromatography afforded the bithiophene-bridged cyclic porphyrin 3-mer 3H (4.8 %), 4-mer 4H (3.2 %), and 5-mer 5H (3.3 %). It is noteworthy that these large cyclic arrays were obtained in onepot reaction from porphyrin monomers without any template molecules, as a rare case. [12] Matrix-assisted laser de

Since the crystal structure of LH2 was elucidated to be circularly arranged chromophoric assemblies, [1–3] much effort has been devoted towards the synthesis of cyclic porphyrin arrays to study the excitation energy transfer (EET) and electronic coupling along the wheel. [4] These cyclic porphyrin arrays are also interesting in host–guest chemistry, [5] single-molecule photochemistry, [6] nonlinear optical (NLO) materials [7] and so on. [8–12] Cyclic porphyrin arrays are constructed by means of covalent bonds, noncovalent bonds, or metal coordination bonds. [8–12] Although the last two are beneficial to construct cyclic porphyrin arrays, those are often unstable toward solvent change or addition of other competing species. The covalently bonded arrays are structurally robust, but often difficult to synthesize. In addition, the final macrocyclization steps are the most tedious and generally need assistance from a suitable template. [5] Although there are some reports on covalently bonded cyclic porphyrin arrays, most of them were constructed through meso-to-meso bridging methods. As rare examples, we recently reported two b,b’-bridged cyclic porphyrin arrays with a 1,3-butadiyne or a 2,5-thienyl spacer. [13] Herein, we wish to report the efficient synthesis of 2,6-pyridylene-bridged b-tob porphyrin nanorings by Suzuki–Miyaura coupling, which is particularly effective for medium to large porphyrin rings. We have achieved the synthesis of 2,5-thienylene-bridged cyclic porphyrin dimers and trimers, but it was difficult to expand the size of the macrocycle beyond trimers. We then examined the Suzuki–Miyaura coupling of b,b’-diborylated Ni II porphyrin 1 with 2,6-dibromopyridine, which provided linear oligomers as the main products. However, we noticed that bromopyridyl-terminated oligomers were selectively obtained with the use of an excess amount of 2,6-dibromopyridine. For instance, b,b’-diborylporphyrin 1–Ni II[14] was crosscoupled with 2,6-dibromopyridine (5 equiv) under standard conditions to give 1–Br in 80 % yield, along with 2–Br (10 %) and 3–Br (2 %) (Scheme 1). It is worth noting that deborylated products were hardly detected in this reaction. Compound 2–Br was also obtained in 42 % yield through the three-component coupling of 1 (1 equiv), 1–Br (2.5 equiv), and 2,6-dibromopyridine (2.5 equiv). Further cross-coupling of 1 (1 equiv) with 1–Br (5 equiv) or 2–Br (5 equiv) afforded 3–Br or 5–Br in 50 or 45 % yields, respectively. Compound 7–Br was prepared in 50 % yield from 1 and 3–Br. With the linear precursors 2–Br, 3–Br, 5–Br, and 7–Br in hand, we examined the cyclization with 1 through a 1:1 Suzuki–Miyaura coupling reaction, which worked very effectively to give porphyrin rings 3–Ni, 4–Ni, 6–Ni, and 8–Ni in 60, 58, 55, and 55 % yields, respectively, even without any template. Treatment of 3–Ni, 4–Ni, 6–Ni, and 8–Ni with sulfuric acid in chloroform at room temperature induced Ni II demetalation to provide 3–H, 4–H, 6–H, and 8–H quantitatively, which were all converted into 3–Zn, 4–Zn, 6–Zn, and 8–Zn upon treatment with ZnA2 in quantitative yields. These newly synthesized porphyrin rings were fully characterized by high-resolution mass spectrometry and

A series of zinc porphyrin arrays comprised of a meso-meso linked porphyrin dimer, a meta-phenylene linked dimer, gable-like tetramers consisting of the meso-meso linked dimers bridged via a meta-phenylene linker, and a dodecameric ring composed of this alternating dimeric pattern were singly oxidized, and intramolecular hole hopping between the porphyrin moieties was probed using electron paramagnetic resonance (EPR) spectroscopy. Electron nuclear double resonance (ENDOR) spectroscopy was also used to probe hole hopping within the dimers. Rapid hole hopping occurs between both porphyrins within both dimers and among three porphyrins of the tetramers with rates10(7) s(-1) at 290 K. Additionally, the hole hops among 8-12 porphyrins in the dodecameric ring with a rate that is10(7) s(-1) at 290 K, but hopping is slow at 180 K. These results show that hole hopping is rapid even though the meta-phenyl bridges and direct meso-meso linkages do not provide optimal electronic coupling between the porphyrins within these multiporphyrin arrays. This greatly expands the scope of possible structures that can be employed to tailor the design of long distance charge transport systems.

The importance of photosynthesis has driven researchers to seek ways to mimic its fundamental features in simplified systems. The absorption of a photon by light-harvesting (antenna) complexes made up of a large number of protein-embedded pigments initiates photosynthesis. Subsequently the many pigments within the antenna system shuttle that photon via an efficient excitation energy transfer (EET) until it encounters a reaction center. Since the 1995 discovery of the circularly arranged chromophoric assemblies in the crystal structure of light-harvesting antenna complex LH2 of purple bacteria Rps. Acidophila, many designs of light-harvesting antenna systems have focused on cyclic porphyrin wheels that allow for efficient EET. In this Account, we review recent research in our laboratories in the synthesis of covalently and noncovalently linked discrete cyclic porphyrin arrays as models of the photosynthetic light-harvesting antenna complexes. On the basis of the silver(I)-promoted oxidative coupling strategy, we have prepared a series of extremely long yet discrete meso-meso-linked porphyrin arrays and covalently linked large porphyrin rings. We examined the photophysical properties of these molecules using steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related to the EET processes within the porphyrin rings. Within these structures, the exciton-exciton annihilation time and the polarization anisotropy rise time are well-described in terms of the Forster-type incoherent energy hopping model. In noncoordinating solvents such as CHCl(3), meso-pyridine-appended zinc(II) porphyrins and their meso-meso-linked dimers spontaneously assemble to form tetrameric porphyrin squares and porphyrin boxes, respectively. In the latter case, we have demonstrated the rigorous homochiral self-sorting process and efficient EET along these cyclic porphyrin arrays. The meso-cinchomeronimide appended zinc(II) porphyrin forms a cyclic trimer. We have also shown that the corresponding meso-meso-linked diporphyrins undergo high-fidelity self-sorting assembling to form discrete cyclic trimer, tetramer, and pentamer with large association constants through perfect discrimination of enantiomeric and conformational differences of the meso-cinchomeronimide substituents. Collectively, these studies of covalently and noncovalently linked discrete cyclic porphyrin arrays aid in the understanding of the structural requirements for such very fast EET in natural light-harvesting complexes.

A series of covalently linked cyclic porphyrin arrays CNZ that consist of N/2 of meso-meso directly linked zinc(II) porphyrin dimer subunits Z2 bridged by 1,3-phenylene spacers have been prepared by Ag(I)-promoted oxidative coupling reaction. We have investigated the excitation energy migration processes of CNZ in toluene by using femtosecond transient absorption anisotropy decay measurements by taking 2Z2 composed of two Z2 units linked by 1,3-phenylene as a reference molecule. On the basis of the excitation energy transfer rate determined for 2Z2, we have revealed the excitation energy hopping rates in the cyclic arrays CNZ by using a regular polygon model. The number of excitation energy hopping sites N(flat) calculated by using a regular polygon model is close to the observed N(expt) value obtained from the transient absorption anisotropy decays for C12Z-C18Z with circular and well-ordered structures. On the other hand, a large discrepancy between N(flat) and N(expt) was found for smaller or larger arrays (C10Z, C24Z, and C32Z). In the case of C10Z, m-phenylene linked 2Z2 motif with the interchromophoric angle of 120 degrees is not well suited to make a cyclic pentagonal array C10Z based on planar pentagonal structure. This geometrical factor inevitably causes a structural distortion in C10Z, leading to a discrepancy between N(expt) and N(flat) values. On the contrary, the presence of highly distorted conformers such as figure-eight structures reduces the number of effective hopping sites N(expt) in large cyclic arrays C24Z and C32Z. Thus, our study demonstrates that not only the large number of porphyrin chromophores in the cyclic arrays CNZ but the overall rigidity and three-dimensional orientation in molecular architectures is a key factor to be considered in the preparation of artificial light harvesting porphyrin arrays.

We have investigated the relationship between the photophysical properties and structures of a series of directly linked zinc(II) porphyrin dimers (mmZ2, mbZ2, and bbZ2) using time-resolved spectroscopic measurements and theoretical calculations. Their unique characters such as CT nature and torsional motion are caused by interporphyrin interactions and steric effects, respectively, which can be fully understood in terms of three structural factors: linking position, dihedral angle, and linkage length. The orthogonal geometry and heterolinking of mmZ2 and mbZ2 induce the localized MOs and electron unbalance in the constituent porphyrin units, respectively, and consequently lead to distinct CT characters in spite of their different origin. On the other hand, a small interporphyrin steric hindrance in bbZ2 makes a torsional motion possible around the direct beta-beta' linkage in the excited state, which is correlated with the solvent dependence of the fast S(1) fluorescence decay component. On the basis of this work, we can gain further insight into the effect of individual structural factors on the photophysical properties, which provides a firm basis for further understanding of the photophysical properties mainly determined by the structural factors in multiporphyrin systems.

meso-meso-Linked diporphyrins fabricated with multiple unsaturated carboxylic acid groups are efficiently synthesized by the use of transition-metal catalysis. Iridium-catalyzed direct borylation of meso-meso-linked diporphyrins furnish multi-borylated diporphyrins with high regioselectivity. Then, the introduction of alpha,beta- or alpha,beta,gamma,delta-unsaturated ester functions to the diporphyrins is achieved through rhodium-catalyzed Heck-type addition of the borylated diporphyrins to acrylate or 2,4-pentadienoate esters. Saponification of the esters to the corresponding acids is accomplished in high yields. The post-modification of diporphyrin 6 smoothly provides a water-soluble multiporphyrin array 1 in 84% total yield in 3 steps. This functionalization significantly affects the electronic properties of the porphyrins, resulting in different energy levels of each porphyrin unit. The absorption and fluorescence spectra indicate efficient energy transfer in the porphyrin dimers.

Facile synthesis of 1,2-phenylene-bridged porphyrin dimers via Pd-catalyzed cross-coupling reaction and their photophysical properties only moderately perturbed by inter-porphyrin electronic interactions are reported.

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.

Free base porphyrin sheet 5 was prepared by demetalation of zinc complex 1 , which was now more conveniently prepared in 30% yield by oxidation of a mixture of tetraporphyrins, 8 , 9 , and 11 . The 1 H NMR spectrum of 5 shows no indication of an aromatic ring current for the porphyrin rings and evidences the freezing of the pyrrolic NH protons at the most inner and outer corner positions, both of which contrast sharply with strong aromatic ring currents and rapid NH tautomerism of normal porphyrins. DFT calculations supported the experimental results, suggesting that the enforced planar COT core causes these unique properties. The free base 5 was transformed into Cu(II) complex 6 that exhibits antiferromagnetic interaction among the Cu(II) ions with J =−1.16 cm −1 .

We report the synthesis and characterization of L- and T-shaped porphyrin tapes as extensible structural motifs of two-dimensionally extended porphyrin tapes. The two-photon absorption (TPA) cross-section values (sigma((2))) for L- and T-shaped porphyrin tapes as well as those for linear trimeric and tetrameric porphyrin tapes were measured by an open-aperture Z-scan method at 2300 nm, a wavelength at which the one-photon absorption contribution is either zero or almost negligible. Under these conditions, the sigma((2)) values for the linear porphyrin tape trimer and tetramer were determined to be 18 500 and 41 200 GM, respectively. The sigma((2)) value for the L-shaped trimer was determined to be 8700 GM, which is only half that of the linear trimer, whereas the sigma((2)) value for the T-shaped tetramer was measured to be 35 700 GM. These results clearly indicate the dependence of the TPA cross-section on the molecular shape, which underscores the importance of directionality in the pi-conjugation pathway for the enhancement of TPA cross- section.

Thermal reaction of meso – meso , β–β, β′–β′ triply linked diporphyrins with an azomethine ylide produced seven-membered cycloadducts via formal [3+4] cycloaddition at the bay-area. Bischlorin structures of the cycloadducts are characterized on the basis of spectroscopic data and confirmed by single crystal X-ray diffraction analysis. Interestingly, the stability of the cycloadducts depends on the central metal ions in the porphyrin core.

Starting from 1,3-phenylene linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag I-promoted coupling reactions provided linear oligomers from 2nZA up to 128ZA. Of these zigzag shaped porphyrin arrays, the Ag I-promoted intramolecular cyclization reaction of 2 nZA (n=5, 6, 8, 9, 12, and 16) under dilute conditions gave the corresponding cyclic porphyrin wheels C2nZA (n=5, 6, 8, 9, 12, and 16), whereas large arrays 2nZA (n=24, 32, and 48) did not provide cyclic porphyrin products. These large discrete porphyrin arrays and wheels were fully characterized by means of 1H NMR spectroscopy, MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, GPC-HPLC analysis, and the scanning tunneling microscopy (STM) technique. The STM images of C12ZA and C18ZA reveal their large circular structures. In the cyclic structures of C2nZA in solution, however, the gradual decrease in fluorescence quantum yields and fluorescence lifetimes are observed, reflecting some conformational heterogeneities. Collectively, the present work provides an important contribution to the construction of fully covalently linked large cyclic arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.

We have explored a variety of self-assembled cyclic porphyrin arrays mainly as biomimetic models of light harvesting antenna in photosynthetic systems. The key reaction is Ag(I)-promoted coupling reaction of 5,15-diaryl zinc(II) porphyrin that provides a meso – meso linked diporphyrin. meso -Pyridine-appended zinc(II) porphyrins M1 – M3 and their meso – meso linked dimers D1 – D3 assemble spontaneously in non-coordinating solvents into tetrameric porphyrin squares S1 – S3 and porphyrin boxes B1 – B3 , respectively. In the latter case, it has been demonstrated that four molecules of R -diporphyrin assemble into R -box and four molecules of S -diporphyrin assemble into S -box via rigorous homochiral self-sorting process. Efficient excitation energy transfer along these cyclic porphyrin arrays has been revealed by the time-resolved transient absorption and fluorescence measurements. We have also demonstrated that the meso -cinchomeronimide appended Zn(II) porphyrin CIM forms a cyclic trimer, while the corresponding diporphyrins CID exhibit high-fidelity self-sorting assembling to form discrete cyclic trimer, tetramer, and pentamer with large association constants, through perfect discrimination of enantiomeric and conformational differences of the meso -cinchomeronimide substituents.

We have investigated the perturbed electronic states and energy relaxation dynamics of a series of phenylene bridged ZnII porphyrin dimers to reveal the effects of phenylene bridges by using computational and time-resolved spectroscopic methods. The electronic states of ZnII porphyrin dimers are largely perturbed by the interchromophore interactions that can be controlled by the phenylene bridges; linking position and number of phenyl rings. On the basis of our observations, we can gain further insight into the dipole−dipole interactions and through-bond/through-space electronic exchange interactions between the neighboring porphyrin moieties, which provides a firm basis for further understanding the modification of photophysical properties caused by the phenylene bridge in multiporphyrin assemblies as artificial light-harvesting apparatus.

The synthesis and photophysical properties of three-dimensionally arranged porphyrin arrays with through-space electronic communication are reported. 1,3,5-Trioxamethylphenylene bridged Zn(II) porphyrin trimer 3 was coupled by Ag(I)-promoted oxidative coupling reaction to give porphyrin cage 5 comprising three meso–meso linked diporphyrins, which was then transformed by oxidation with DDQ and Sc(OTf)3 into porphyrin cage 7 comprising three fused diporphyrins. Intramolecular meso–meso coupling reaction was applied to porphyrin pentamer 11 to provide porphyrin array 12 consisting of a porphyrin core flanked by two meso–meso linked diporphyrins. Further oxidation of 12 with DDQ and Sc(OTf)3 afforded triply stacked porphyrin array 13 that is comprised of a porphyrin core flanked by two porphyrin tapes. UV–vis–NIR absorption and fluorescence spectra of 5, 7, 12, and 13 showed their distorted conformations and electronic interaction within the stacked porphyrin arrays.

A belt-shaped hexagonal cyclic porphyrin array 2 that comprises of six meso-meso, beta-beta, beta-beta triply linked diporphyrins 3 bridged by 1,3-phenylene spacers is prepared by oxidation from cyclic dodecameric array 1 consisting of six meso-meso directly linked diporphyrins 4 with DDQ and Sc(OTf)3. The absorption spectrum of 2 is similar to that of the constituent subunit 3 but shows a slight red-shift for the Q-bands in near-infrared (NIR) region, indicating the exciton coupling between the neighboring diporphyrin chromophores. Observed total exciton coupling energies in the absorption spectra were largely matched with the calculated values based on point-dipole exciton coupling approximation. It was found that the experimental exciton coupling strength (292 cm(-1)) of the Q-band in 2 is slightly larger than the calculated one (99 cm(-1)), indicating that the electronic communications are enhanced through 1,3-phenylene linkers in hexameric macromolecule. A rate of the excitation energy hopping (EEH) that occurs in 2 at the lowest excited singlet state in the near-infrared region has been determined to be (1.8 ps)(-1) on the basis of the pump-power dependent femtosecond transient absorption (TA) and the transient absorption anisotropy (TAA) decay measurements. The 2 times faster EEH rate of 2 than that of 1 (4.0 ps)(-1) mainly comes from involving through-bond energy transfer among diporphyrin subunits via 1,3-phenylene bridges as well as Förster-type through-space EEH processes. STM measurement of 2 in the Cu(100) surface revealed that it takes several discrete conformations with respect to the relative orientation of neighboring diporphyrins. Collectively, an effective EEH in the NIR region is realized in 2 due largely to the intensified oscillator strength in the S(1) state (Q-band) and the close proximity held by 1,3-phenylene spacers.

Conformations of two dodecameric porphyrin wheels adsorbed on a Cu(1 0 0) were probed by using scanning tunneling microscopy (STM). Whereas a wheel consisting of six meso – meso linked diporphyrins was detected as uniform ring structure, several different images with three discrete molecular heights were detected for a wheel consisting of six meso – meso , β-β,β-β triply-linked planar diporphyrins. These results indicate that the former has a conformation similar to that in a free space, while the latter has various conformations with respect to orientation of planar diporphyrin units toward the metal surface. Several discrete STM images of the latter have been interpreted in terms of possible eight conformations, which vary as to relative orientation of neighboring diporphyrin units.

Covalently linked cyclic porphyrin arrays have been synthesized to mimic natural light-harvesting apparatuses and to investigate the highly efficient energy migration processes occurring in these systems for future applications in molecular photonics. To avoid an ensemble-averaged picture, we performed a single-molecule spectroscopic study on the energy migration processes of cyclic porphyrin arrays and a linear model compound embedded in a rigid polymer matrix by recording fluorescence intensity trajectories, by performing coincidence measurements, and by doing wide-field defocused imaging. Our study demonstrates efficient energy migration within the cyclic porphyrin arrays at the single-molecule level. By comparison with the data of the linear model compound, we could pinpoint the role of the dipole-dipole coupling between diporphyrin subunits and the rigidity of the cyclic structures on the energy transfer processes.

We have explored a variety of covalently and non-covalently assembled cyclic porphyrin arrays mainly as biomimetic models of light harvesting antenna in photosynthetic systems. The key reaction is Ag(I)-promoted coupling reaction of 5,15-diaryl zinc(II) porphyrin that provides a meso–meso linked diporphyrin. An advantage of this coupling reaction is its extremely easy extension to higher porphyrin arrays, since longer porphyrin arrays have practically the same reactivity as that of the monomer. On the basis of this strategy, we have prepared cyclic porphyrin arrays including directly meso–meso linked porphyrin rings CZ4 – CZ8 , large porphyrin wheels C12ZA and C24ZB , and three-dimensional porphyrin boxes D1 – D3 . Efficient excitation energy transfer along these cyclic porphyrin arrays has been revealed by the time-resolved transient absorption and fluorescence measurements.

1,10-Dioxydecamethylene doubly strapped ZnII-porphyrin S1 was prepared and treated with AgPF6 to give meso–meso-linked porphyrin oligomers Sn (n = 2, 3, 4, 6, 8, and 12), which were converted to triply linked porphyrin tapes TSn by meso,meso′-dibromo meso–meso-linked porphyrin arrays BSn and meso,meso′-diphenyl meso–meso-linked porphyrin arrays PSn. The structures of S1 and S2 have been determined by single-crystal X-ray diffraction analysis. Characteristically, Sn exhibit sharp Q(0,0) absorption and fluorescence bands. Low energy Q-band-like absorption bands of TSn are progressively red-shifted with an increase in the number of porphyrins without saturation behavior of conjugation. The double straps suppress π–π stacking to some extent as seen from partial preservation of vibration structures in the Q-band-like bands of TS4 and TS6 and improve the chemical stabilities of longer tapes such as TS8 and TS12. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

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.

The excitation energy hopping (EEH) in a dodecameric porphyrin box (TB2) prepared by self-assembly of four meso–meso linked zinc(II) triporphyrin (T2)s is explored by picosecond fluorescence and femtosecond transient absorption (TA) spectroscopy. The exciton–exciton annihilation and anisotropy depolarization observed in both fluorescence and TA provide a consistent EEH rate, i.e. 300 ± 20 ps−1, assuming Forster energy hopping among four T2 units in TB2. The observed energy hopping time 300 ± 20 ps−1 is slower than the previous 98 ± 3 ps−1 of B2 (an octameric porphyrin box), even though the size of hopping unit is larger in TB2.

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 for a coherent tunneling process. It suggests that the electrical conduction through porphyrin arrays is mainly due to the thermally activated hopping transport. 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, independently of molecular conformation and length.

On the basis of the Ag I -pro- moted coupling reaction of zinc(ii)- 5,15-bis(3,5-dioctyloxyphenyl)porphy- rin Z1, chain elongation has been at- tempted by using a stepwise doubling approach, which provides Z2, Z4, Z8, Z16, Z32, Z64, Z128, Z256, Z384, and Z512. The porphyrin arrays up to Z128 are sufficiently soluble in CHCl3 and THF despite their very long molecular lengths and rodlike structures, while the arrays over Z128 show a significant drop in solubility and stability. The dis- crete porphyrin arrays thus isolated were characterized by means of 1 H NMR spectroscopy, matrix-assisted laser desorption ionization time-of- flight (MALDI-TOF) mass spectrome- try, UV/Vis spectroscopy, gel-permea- tion chromatography (GPC), cyclic vol- tammetry (CV), single-crystal X-ray crystallography, scanning tunneling mi- croscopy (STM), and atomic force mi- croscopy (AFM). Contrary to expected linear conformations of the arrays Z n (where n is the number of porphyrins), the single molecular images of Z128, Z256, and Z512 revealed largely bent structures; this finding indicates the substantial conformational flexibility of Z n. We also exploited an effective syn- thetic route by means of which Z n can be fabricated with a thiol-protected aryl group to provide Z n S2 through Z n Br2, by bromination with N-bromo- succinimide and subsequent Pd-cata- lyzed Suzuki-Miyaura arylation. Final- ly, the reaction of Z256 provided Z512, Z768, and Z1024. Collectively, this work provides an important milestone in the preparation of sub-microscale discrete organic molecules and the fab- rication of molecular-based materials, hence significantly contributing to device applications.

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.

During the last decade, the exploration of nanoscale device and circuitry based on molecules has gained increasing interest. In parallel with this, considerable effort is being devoted to the development of molecular photonic/electronic materials based on various porphyrin arrays. This involves light as an input/output signal and excitation energy migration as a mechanism for signal transmission. Absorption of a photon at the light collector end of the porphyrin array yields the excited state, which migrates among the intervening pigments until reaching the emitter, whereupon another photon is emitted. As a consequence, it is relevant to understand the excitation energy transfer (EET) processes occurring in various forms of porphyrin arrays for the applications as artificial light harvesting arrays and molecular photonic/electronic wires. Since the excitonic (dipole) and electronic (conjugation) couplings between the adjacent porphyrin moieties in porphyrin arrays govern the EET processes, we have characterized the EET rates of various forms of multiporphyrin arrays (linear, cyclic, and box) based on various time-resolved spectroscopic measurements. We believe that our observations provide a platform for further development of molecular photonic/electronic materials based on porphyrin arrays.

1,4-Phenylene-bridged zinc(II) meso–meso linked diporphyrin arrays were prepared by Ag(I)-promoted coupling reaction in a regioselective manner.

Excitation-energy transport (EET) phenomena in meso-meso directly linked Zn(II)porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady-state and time-resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II)porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm-1, and that of the Q bands is about 570 cm-1. The coherence length in the S1 state of the Zn(II)porphyrin arrays was determined to be 4-5 porphyrin units, which is comparable to that of the well-ordered two-dimensional circular structure B850 in the peripheral light-harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II)porphyrin arrays are well suited for mimicking natural light-harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 microseconds-1, and the very weak coupling between the triplet states (ca. 0.003 cm-1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.