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.

Directly fused diporphyrins display the extensive pi conjugation as evinced by highly perturbed electronic absorption spectra as well as lowered and largely split first oxidation potentials. Such diporphyrins prepared include meso-beta doubly linked diporphyrins 7, meso-meso beta-beta beta-beta triply linked diporphyrins 8, and meso-meso beta-beta doubly linked diporphyrins 9. Oxidation of 5,15-diaryl-substituted and 5,10,15-triaryl-substituted Ni(II)-, Cu(II)-, and Pd(II)-porphyrins with tris(4-bromophenyl)aminium hexachloroantimonate (BAHA) in CHCl(3) afforded 7, and triply linked Cu(II)-diporphyrins 8a and 8g were respectively prepared by the oxidation of meso-meso singly linked Cu(II)-diporphyrins 5c and 5f with BAHA. Meso-meso beta-beta doubly linked Ni(II)-diporphyrin 9a was isolated along with triply linked Ni(II)-diporphyrin 8e from the similar oxidation of meso-meso singly linked Ni(II)-diporphyrin 5a. Doubly linked diporphyrins 7 and 9a both exhibit significantly perturbed electronic absorption spectra, in which the Soret-like bands are largely split at around 405-418 and 500-616 nm and the Q-bandlike absorption bands are substantially intensified and red-shifted at 748-820 nm, probably as a consequence of symmetry lowering. Triply linked diporphyrins 8 display more strongly perturbed electronic absorption spectra with split Soret-like bands at 408-419 and 567-582 nm and Q-bandlike absorption bands reaching far-infrared region. Structures of three types of fused diporphyrins 7b and 7c, 8g and 8j, and 9a have been unambiguously determined by X-ray crystallography to be nearly coplanar. Both the triply linked diporphyrins 8g and 8j exhibit very flat structures, whereas the doubly linked diporphyrins 7b and 7c exhibit ruffled structures. The doubly linked diporphyrin 9a shows a helically twisted conformation with larger ruffling toward the opposite directions and has been actually separated into two enantiomers, which display strong Cotton effects in the CD spectra. The first oxidation potentials (E(OX1)) decrease in the order of 5 > 7 > or = 9 > 8, indicating lift-up of HOMO orbital in this order, and split potential differences DeltaE = E(OX1) - E(OX2), in turn, increase in the reverse order of 5 < 7< or = 9 < 8. The (1)H NMR spectra have indicated that the aromatic porphyrin ring current becomes weakened in the order of 5 > 7 > 8. Collectively, the electronic interactions between the diporphyrins have been concluded to increase in the other of 5 << 7 < or = 9 < 8.

Which way around? A J‐aggregated zinc porphyrin dendrimer can be used to detect the macroscopic chirality of a vortex. The sign of the circular dichroism response changes quickly upon switching from clockwise (CW) to counterclockwise (CCW) stirring (see picture). The observed chiroptical activity most likely arises from a macroscopic helical alignment of nanofibers formed from the polymeric J‐aggregate.

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meso-Pyridine-appended zinc(II) porphyrins Mn and their meso-meso-linked dimers Dn assemble spontaneously, in noncoordinating solvents such as CHCl3, into tetrameric porphyrin squares Sn and porphyrin boxes Bn, respectively. Interestingly, formation of Bn from Dn proceeds via homochiral self-sorting assembly, which has been verified by optical separations of B1 and B2. Optically pure enantiomers of B1 and B2 display strong Cotton effects in the CD spectra, which reflect the length of the pyridyl arm, thus providing evidence for the exciton coupling between the noncovalent neighboring porphyrin rings. Excitation energy migration processes within Bn have been investigated by steady-state and time-resolved spectroscopic methods in conjunction with polarization anisotropy measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Förster-type incoherent energy hopping model by assuming a number of hopping sites of N = 4 and an exciton coherence length of L = 2. Consequently, the excitation energy hopping rates between the zinc(II) diporphyrin units have been estimated for B1 (48 ps)(-1), B2 (98 +/- 3 ps)(-1), and B3 (361 +/- 6 ps)(-1). Overall, the self-assembled porphyrin boxes Bn serve as a well-defined three-dimensional model for the light-harvesting complex.

Well-ordered architectures of self-assembling porphyrins [1] have been attracting considerable interest in light of potential applications in material science, [2] template-directed synthesis, [3] reaction catalysis, [4] and duplication of photosynthetic functions of light harvesting and charge separation. [5] Among these, the coordination interaction between zinc porphyrin and pyridine groups have been often used to construct selfassembled porphyrin architectures both in infinite and discrete forms. [1±6] Fleischer and Shachter have reported a linear infinite polymeric aggregate from 5-p-pyridyl-10,15,20-triphenylporphyrinato zinc(ii), [6] whereas Hunter and co-workers reported the formation of a cyclic tetramer from a zinc(ii) porphyrin bearing a p-(iso-nicotinamide)phenyl group [7] and Imamura and co-workers reported a similar cyclic tetramer from 5-p-pyridyl-10,15,20triaryl porphyrinato ruthenium(ii), [8] both without X-ray crystal structures. Herein, we report a self-assembling cyclic tetramer of 5-p-pyridyl-15-(3,5-di-tert-butylphenyl)porphyrinato zinc(ii) (Z1) with its X-ray structure and a self-assembling box-shape tetramer of meso ± meso-linked bis{5-p-pyridyl-15-(3,5-di-octyloxyphenyl)porphyrinato zinc(ii)} (Z2; Scheme 1). In the latter case, a very stable supramoleuclar aggregate is constructed as a result of many cooperative interactions with simultaneous structural rigidification, as suggested by its unique absorption and fluorescence spectra. 5-p-Pyridyl-15-(3,5-di-tert-butylphenyl)porphyrin (H1) was prepared by condensation of 4-pyridinecarboxaldehyde and 3,5-di-tert-butylbenzaldehyde with 2,2'-dipyrrylmethane in 8 % yield, and subsequent zinc insertion with Zn(OAc) 2 afforded Z1 quantitatively. The 1 H NMR spectrum of Z1 in CDCl 3 is concentration independent at > 3 mm and exhibits large upfield shifts for the pyridyl protons at d 6.18 and 2.17 ppm in comparison to those of H1 at d 8.29 and 7.81 ppm, which indicates the coordination of the pyridyl group to zinc(ii) porphyrin. As judged from the chemical shifts of the pyridyl group, Z1 forms a similar aggregate in C 6 D 6 (d 5.94 and 2.59 ppm) but exists in a monomeric form in [D 8 ]THF (d 8.97 and 8.31 ppm). The latter result suggested the coordination of THF to the zinc center, thus preventing self-assembling. Vapor pressure osmometry (VPO) in CHCl 3 afforded average molecular weights of 2760 AE 370 g mol À1 for Z1 and 550 AE 20 gmol À1 for H1 in a range of 3.0 ± 13.0 Â 10 À3 m, which correspond to (Z1) 4 and monomeric H1, respectively. The absorption spectrum of Z1 in CHCl 3 is concentration dependent in an range of roughly 10 À6 ± 10 À5 m and shows the Q-band at 540 nm at < 10 À6 m and at 549 nm at > about 10 À5 m (Figure 1), which correspond, respectively, to four-coordinate and five-coordinate zinc porphyrin units. A good fit for the observed sigmoidal curve is obtained by assuming porphyrin tetramer formation ((Z1) 4 ), which gives an association constant of K 4 1.4 Â 10 15 m À3 . [9]

Two-photon absorption (TPA) properties and energy relaxation dynamics of meso-β doubly linked Ni(II) porphyrin arrays (DLNi n , n = 2−5) in CHCl3 have been investigated. DLNi n exhibits near-IR-extended absorption bands up to 1000 nm and enlarged TPA cross-section values without any one-photon contribution as the number of subunits increases mainly due to an elongation of effective π-conjugation length. Especially, in the case of pentamer, DLNi5 had a TPA cross-section value of 41 000 GM, which is the largest value per subunit (σ(2)/n ∼ 8000 GM) among the conjugated porphyrins ever studied. Fast energy relaxation processes within a few picoseconds were observed by femtosecond transient absorption measurements, indicating that various charge transfer and (d,d) states of the central Ni(II) ion are involved in deactivation pathways as the ladder states in DLNi n .

A zinc complex of (3-pyridyl)porphyrin with an alkynyl sidearm (1) preferentially formed a cyclic tetramer in toluene, which showed supramolecular thermochromism at ambient temperatures of 0-100 degrees C. Because of an appropriate extension of pi-electronic conjugation of the porphyrin chromophore, the thermal-induced self-assembling dynamics of 1 can be detected as a vivid color change from red to yellow to green. In sharp contrast, 2 without any alkynyl group, on heating, developed only a small color change from orange to pink, while 3 bearing two alkynyl groups stayed green and did not display any thermochromism.