Scandium(III)-catalyzed oxidation of meso-meso-linked zinc(II)-porphyrin arrays (up to dodecamers) with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) led to efficient formation of triply meso-meso-, beta-beta-, and beta-beta-linked zinc(II)-oligoiporphyrins with 62 to 91% yields. These fused tape-shaped porphyrin arrays display extremely red-shifted absorption bands that reflect extensively pi-conjugated electronic systems and a low excitation gap. The lowest electronic absorption bands become increasingly intensified and red-shifted upon the increase in the number of porphyrins and eventually reach a peak electronic excitation for the dodecamer at approximately 3500 wavenumber. The one-electron oxidation potentials also decreased progressively upon the increase in the number of porphyrins. These properties in long and rigid molecular shapes suggest their potential use as molecular wires.

Conjugated porphyrin arrays that possess delocalised electronic networks have, for the most part, been assembled by using alkene or alkyne type bridging units or by directly connecting individual porphyrin chromophores with multiple bonds to form fused porphyrin arrays. Throughout the last two decades, such conjugated porphyrin arrays have been actively explored due to their attractive electronic, optical and electrochemical properties. This review aims to cover the multitude of synthetic methodologies that have been developed for the construction of conjugated porphyrin arrays as well as to summarise their structure-property relationships and use in various applications such as near infrared (NIR) dyes, nonlinear optical materials and electron-conducting molecular wires.

The chemistry of expanded porphyrins, which are higher homologues of porphyrins, has been intensively explored for the last three decades. Expanded porphyrins exhibit structures, electronic properties, coordination chemistry, and reactivities that are entirely different from those of porphyrins. Through these studies, it has become increasingly apparent that expanded porphyrins are attractive in views of aromaticity and multimetal coordination, or as functional dyes, nonlinear optical materials, ion receptors, or stable organic radicals. As such, we have continuously witnessed the emergence of expanded porphyrins that exhibit unprecedented structures and properties, as is highlighted by the facile realization of Möbius aromatic and even antiaromatic systems with twisted molecular structures. In this Review, the recent progress of the chemistry of expanded porphyrins after the seminal Review by Sessler and Seidel in 2003 is presented.

Owing to the central importance of multiporphyrin assemblies in photosynthetic reaction centers and light-harvesting antenna complexes, considerable attention has been focused on the development of covalently linked porphyrin arrays as effective biomimetic models. Systematic studies on these models will aid also in the development of new photonic materials and molecular devices.[' In these model studies a variety of bridges that connect porphyrin chromophores have been examined in their abilities to facilitate electron and photon connectivity or to perturb the spectral properties of the porphyrin chromophores. As an extension. a bridgeless, directly coupled porphyrin array may be a fascinating. ultimate candidate for exploring novel connectivi ty.Here we report on a novel, very facile oxidative coupling reaction of zinc(1r) 5,15-di(3,5-di-tert-butylphenyl)porphyrin (1) to give porphyrin dimer 2 and porphyrin trimer 3 as the first example of meso,meso-coupled multiporphyrins. Treatment of a chloroform solution of 1 with AgPF, (0.5 equiv) dissolved in acetonitrile for 5 h gave 2 and 3; NMR analysis revealed the distribution of products: 1 (47%), 2 (27%), and 3 (4%). Although these products were difficult to separate over standard silica gel columns, they were separated by size exclusion chrom a t~g r a p h y ,~~] by which the products are eluted in the order of molecular weight: first, higher oligomers and trace amounts of tetramer 4 > 3 > 2 > 1. The yields of isolated compounds were 25% for 2 and 4 % for 3, based on the amount of 1 used.Reaction of 2 under analogous conditions (0.5 equiv AgPF,) gave 4 in 23% yield. The regioselectivty of the coupling was quite high.'']Compounds 2-4 were fully characterized by 'H NMR spectroscopy (500 MHz) and FAB mass spectrometry. 'HNMR analysis revealed the presence of one meso-and two, four, six, and eight &protons in 1-4, respectively (Scheme 1). In the spec-1 2 9.20 8.72 8.l7 8.29 n m I r kr 3 8.84 9.21 Ar Ar 4 Ar 5 Scheme 1. Structures of 1-5. NMR chemical shifts of the mesoand fi-protons (CDCI,) are indicated. Ar = 3,5-tBu,C,H3.trum of 2 the signals of the inner P-protons are shifted upfield by A6 = 1.33 and 0.46 ppm relative to those of 1, reflecting the ring current effect of the second porphyrin ring, while the signals of the outer P-protons appear nearly at the same chemical shifts. Similar features are observed for 3 and 4. These data, analyzed in terms of the ring current model, suggest an approximate perpendicular arrangement of the neighboring porphyrins. AgCIO, and AgBF, were similarly effective in this coupling, but the use of AgNO, led to meso-nitration, giving 5 almost quantitatively. The coupling reaction was accelerated by addition of I,; under these conditions the reaction was complete within 5-10 min.['] In the I,-promoted reactions use of one or more equivalents of the Ag' salt and I, led to lower yields of 2 and 3, under otherwise the same reaction conditions."'] Most probably, the initial generation of radical cation 1" followed by nucleophilic attack by ...

Since the discovery of its facile synthesis in 2001, meso-aryl-substituted expanded porphyrins have been developed as a new class of azaannulenes in light of their facile redox interconversions, conformational flexibilities involving flipping of the constitutional pyrroles, rich metal coordination behaviors, unprecedented chemical reactivities, effective platforms to realize versatile electronic states including Möbius aromatic and antiaromatic species, and abilities to stabilize organic radicals. In this Review, the syntheses, structures, and optical, electronic, and magnetic properties of meso-aryl-substituted expanded porphyrins and their metal complexes have been updated with a particular focus on the relationship between "aromaticity and molecular twist (molecular topology)". While the importance of the interplay of these two characteristics has been long recognized from the theoretical viewpoint, meso-aryl-substituted expanded porphyrins offered solid experimental evidence to provide Möbius aromatic and antiaromatic molecules with distinct diatropic and paratropic ring currents, respectively. This attribute is not shared with β-alkylated expanded porphyrin counterparts, underlining the importance and uniqueness of meso-aryl-substituted expanded porphyrins.

Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.

Considerable attention has been focused on the synthesis of monodisperse macromolecular rods of precise length and constitution in light of their potential application as molecular-scale electronics, optical devices, sensors, and for conversion of solar energy.[1±3] Discrete molecular rods of known structure are used to position two active centers at a known distance, and the resulting assemblies are of interest as potent electronic or photonic molecular wires. Recently, the length of linear, monodisperse, p-conjugated oligomers have reached the range of approximately 10 nm. [1,4] Yet it still remains a great synthetic challenge to explore discrete, finite functional supramolecules with well-defined structures far beyond these achievements.One of the most attractive building blocks for supramolecular rods are porphyrins, since they offer a variety of desirable features such as a rigid, planar geometry, high stability, an intense electronic absorption, a strong fluorescence emission, a small HOMO ± LUMO energy gap, as well as flexible tunability of their optical and redox properties by appropriate metalation.[5] Recent efforts on the preparation of supramolecular porphyrin arrays have become increasingly focused on the realization of various molecular devices. [5,6] However, these studies are often hampered by poor solubility, difficult separations, and demanding characterizations. Therefore, high solubilities, easy separations, and reliable characterizations of the arrays are of prime importance in devising a larger molecular system. Recently we found that the Ag I -promoted meso ± mesocoupling reaction of Zn II 5,15-diarylporphyrins has several advantages; [7,8] 1) the regioselectivity of the meso ± meso coupling is quite high, 2) the porphyrin arrays have essentially the same linear rodlike shape, 3) the porphyrin arrays are highly soluble, presumably because of orthogonal conformations arising from steric hindrance around the meso ± meso linkage, 4) the separation of the coupling products is easy by recycling preparative GPC-HPLC chromatography as a result of large differences in molecular weight, and finally 5) the long coupling products still bear two free meso positions that are available for the next reaction.Here we report the synthesis and characterization of meso ± meso-coupled porphyrin oligomers up to 128-mer, which is, to the best of our knowledge, the longest (ca. 106 nm) monodisperse, rodlike molecule prepared so far. Previously we employed Zn II 3,5-di-tert-butylphenylporphyrin as a building block, but we encountered a serious solubility problem at the stage of the porphyrin 8-mer. In order to circumvent the solubility problem, we employed the more soluble Zn II 3,5-dioctyloxyphenylporphyrin Z1 (here we denote the meso ± meso-coupled Zn II porphyrin arrays as Zn where n represents the number of porphyrins; Ar 3,5-dioctyloxyphenyl). Chain elongation can be achieved quite simply by repeating the dimerization reactions from Z1 to Z2, Z2 to Z4, Z4 to Z8, Z8 to Z16, and Z16 to Z32. The yields of the di...

To elucidate the dynamics and mechanisms of radiationless transitions from higher excited electronic states as well as the ultrafast intramolecular vibronic relaxation in porphyrin derivatives, we have studied the fluorescence dynamics of Zn-tetraphenylporphyrin (ZnTPP) and Zn-diphenylporphyrin derivatives (ZnDPP) in fs-ps time regimes by means of fluorescence up-conversion technique. Detailed measurements on ZnTPP in ethanol have demonstrated fluorescence dynamics over the whole spectral range from 430 to 620 nm when excited to the S 2 state. The time constant (∼2.3 ps) of the single-exponential decay of S 2 fluorescence around 430 nm agreed with that of the single-exponential rise of S 1 fluorescence around 600 nm (wavelength of 0-0 transition in the stationary spectrum), indicating that the relaxation by the ultrafast vibronic redistribution immediately after S 2 f S 1 internal conversion mainly gives lower vibronic states near the bottom of the S 1 state. However, we have observed the dynamics of weak hot fluorescence probably from the nonrelaxed vibronic state immediately after internal conversion and also higher vibronic states in S 1 formed in competition with the main product of the vibronic redistribution, all over the wavelength region between S 2 and S 1 . Preliminary results of our studies on ZnDPP were very similar to those of ZnTPP.