Mitsuhiko Morisue

Tandem Cofacial Stacks of Porphyrin–Phthalocyanine Dyads through Complementary Coordination

A novel straightforward methodology to organize discrete heterogeneous stacks of porphyrin and phthalocyanine employed an imidazolyl-to-zinc complementary coordination protocol for a Zn(II) phthalocyanine that contains an imidazolyl terminal with an ethynylporphyrin as a coplanar spacer. Structural elucidation was performed by means of size-exclusion chromatography, spectral titration, and NMR spectroscopy. The association constants for the complementary coordination of the heterogeneous slipped-cofacial tetrads reached extremely high values, in the order of 10(14) M(-1). Close contact of the porphyrin and phthalocyanine planes led to a strong shielding of the cofacial protons, which were split due to the slipped-cofacial heterogeneous environment. In variable-temperature NMR spectroscopy, the split signals remained in the aromatic region, a result suggesting structural robustness. Addition of trifluoroacetic acid dissociated the coordination structure to unify the split signals. The stacked tetrads showed unique electronic structures, such as strong exciton coupling and charge-transfer properties between the porphyrin and phthalocyanine units, which were modulated by the peripheral substituents of the phthalocyanine subunit and by the solvent. Interconversion between the coordination tetrad and the corresponding dyad was observed upon addition of an axial ligand.

Synthesis and electrochemical properties of slipped-cofacial porphyrin dimers of ferrocene-functionalized Zn-imidazolyl-porphyrins as potential terminal electron donors in photosynthetic models

A systematic series of ferrocene-functionalized Zn-imidazolyl-porphyrins were synthesized to assemble into the slipped-cofacial porphyrin dimers through imidazolyl-to-zinc complementary coordination as artificial photosynthetic models. Direct substitution at the meso position of the porphyrin ring with ferrocence and octamethylferrocene leads to the characteristic electronic structures, while the ferrocene substituents through phenylene-ethenylene and phenylene-ethylene spacers mitigate the electronic communications. Bathochromic shift of Q band, fluorescence quenching, and redox potentials of porphyrin ring are rationalized by the degree of electron-donating ability of the terminal ferrocenes.

Strong two-photon and three-photon absorptions in the antiparallel dimer of a porphyrin–phthalocyanine tandem

Imidazolyl-appended phthalocyaninatozinc(II) containing an ethynylporphyrin linkage was stacked by complementary coordination into an antiparallel dimer that displayed strong nonlinear optical properties involving two-photon and three-photon absorptions.

Highly Fluorescent Slipped-Cofacial Phthalocyanine Dimer as a Shallow Inclusion Complex with α-Cyclodextrin

Supramolecular control of the π-stacked configuration of aqueous phthalocyanine (Zn[Pc(SO(3))(4)]) was achieved, allowing organization of a J-type slipped-cofacial dimer with per-O-methylated α-cyclodextrin (TMe-α-CDx) by the aid of host-guest interactions. Pristine Zn[Pc(SO(3))(4)] forms nonfluorescent face-to-face aggregates in water. The π-stacked configuration was controlled in the slipped-cofacial dimer, which was formed as a shallow inclusion complex with TMe-α-CDx, giving remarkably enhanced fluorescence with a very small Stokes shift. Organization of the J-type slipped-cofacial dimer as a 2:2 Zn[Pc(SO(3))(4)]-TMe-α-CDx complex was achieved through π-stacking of the unencapsulated segment of Zn[Pc(SO(3))(4)] shallowly encapsulated by a small TMe-α-CDx cavity.

Group 14 Dithienometallole-Linked Ethynylene-Conjugated Porphyrin Dimers

The considerably conjugated π systems of the group 14 dithienometallole-linked ethynylene-conjugated porphyrin dimers (1Ms) were described based on comprehensive experimental and theoretical studies. The electronic absorption spectra of 1M displayed a large splitting in the Soret band and a red-shifted Q-band, indicating that the dithienometallole spacer was effective in facilitating the porphyrin-porphyrin electronic coupling. Torsional planarization behaviors of 1M were observed in the time-resolved fluorescence spectra. Density functional theory (DFT) calculations revealed that the dithienometallole spacer is an ideal partner for the ethynylene-conjugated porphyrin to produce fully delocalized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels due to their similar HOMO and LUMO levels. Finally, 1M exhibited a strong propensity for the quinoidal-cummulenic conjugation in the dithienometallole spacer when in a photoexcited state.

A Tightly Stretched Ultralong Supramolecular Multiporphyrin Array Propagated by Double-Strand Formation.

A shape-programmed linearity through supramolecular polymerization is demonstrated by a step-growth double-strand formation of a telechelic oligomeric porphyrin array in which two alternating pyridyl-porphyrin sequenced units are held together by self-complementary ligand-to-metal coordination. The stiff rod-like structure and sufficiently large binding constant of the double-strand unit considerably extended a supramolecular array in the one dimension, which produced a tightly stretched string with a length that exceeded several micrometers.

Fine-tuning of a ferrocene|porphyrin|ITO redox cascade for efficient sequential electron transfer commenced by an S2 photoexcited special-pair mimic

A systematic series of ferrocene/porphyrin redox cascade architectures was assembled through a slipped-cofacial porphyrin dimer on ITO electrode in optimizing the anodic photocurrent generation to perform the highest quantum yield compared to reported values on ITO electrodes.

Amorphous porphyrin glasses exhibit near-infrared excimer luminescence

The amorphous nature of a series of zinc–porphyrins bearing two 3,4,5-tri((S)-3,7-dimethyloctyloxy)phenyl groups at the meso-positions, named “porphyrin glass”, were tolerant of π-conjugation engineering in ethynylene-linked dimers. The butadiyne-linked dimeric porphyrin glass formed an intermolecular excimer, which exhibited bright and exceptionally long-lived, near-infrared (NIR) luminescence at approximately 970 nm in the solid state. Therefore, porphyrin glasses overcame a general bottleneck for NIR-luminescence, such as an undesired π-stacked aggregation of a large porphyrin plane in addition to the energy gap law. The formation of amorphous molecular glasses from a series of meso-ethynylene-conjugated zinc–porphyrins, named “porphyrin glass”, is described. The butadiyne-linked dimeric porphyrin glass formed an intermolecular excimer, which exhibited solid-state, near-infrared (NIR) luminescence at approximately 970 nm.

Preferential Solvation Unveiled by Anomalous Conformational Equilibration of Porphyrin Dimers: Nucleation Growth of Solvent–Solvent Segregation

Preferential solvation was explored using ethynylene- or butadiynylene-linked porphyrin dimers bearing 3,4,5-tri(( S)-3,7-dimethyloctyloxy)phenyl groups at the meso positions in binary hexafluorobenzene (C6F6) and cyclohexane (C6H12) mixture, expecting contrasting solvent affinity of the porphyrin core and the alkyl side chains toward the individual solvent component. Although the solvent polarity remained nearly constant along with the continuous variation of the solvent composition, the porphyrin dimer showed dramatic change in spectroscopic signatures, indicating the occurrence of preferential solvation. Because of small rotational barrier around the ethynylene and butadiynylene linkage, the torsional conformations of the porphyrin dimers varied from orthogonal to planar due to continuous variation of molar fraction of C6H12-C6F6 mixture. Thorough thermodynamic analyses inferred that nucleation as the enthalpic component and phase segregation as the entropic component operated preferential solvation. The porphyrin dimer nucleated the C6H12-C6F6 segregation, and the torsional conformation was diagnostic of the inversion of the interfacial curvature of the solvent segregation along with the continuous variation.

Fully Conjugated Porphyrin Glass: Collective Light-Harvesting Antenna for Near-Infrared Fluorescence beyond 1 μm

Expanded π-systems with a narrow highest occupied molecular orbital–lowest unoccupied molecular orbital band gap encounter deactivation of excitons due to the “energy gap law” and undesired aggregation. This dilemma generally thwarts the near-infrared (NIR) luminescence of organic π-systems. A sophisticated cofacially stacked π-system is known to involve exponentially tailed disorder, which displays exceptionally red-shifted fluorescence even as only a marginal emission component. Enhancement of the tail-state fluorescence might be advantageous to achieve NIR photoluminescence with an expected collective light-harvesting antenna effect as follows: (i) efficient light-harvesting capacity due to intense electronic absorption, (ii) a long-distance exciton migration into the tail state based on a high spatial density of the chromophore site, and (iii) substantial transmission of NIR emission to circumvent the inner filter effect. Suppression of aggregation-induced quenching of fluorescence could realize collective light-harvesting antenna for NIR-luminescence materials. This study discloses an enhanced tail-state NIR fluorescence of a self-standing porphyrin film at 1138 nm with a moderate quantum efficiency based on a fully π-conjugated porphyrin that adopts an amorphous form, called “porphyrin glass”.