Takamitsu Fukuda

A Discrete Supramolecular Conglomerate Composed of Two Saddle-Distorted Zinc(II)-Phthalocyanine Complexes and a Doubly Protonated Porphyrin with Saddle Distortion Undergoing Efficient Photoinduced Electron Transfer†

Porphyrins (Por) and phthalocyanines (Pc) exhibit lightharvesting efficiency for producing charge-separated states as models of the reaction center in photosynthetic bacteria and photovoltaic cells for energy conversion. The use of supramolecular assemblies to model the functionality of the reaction center is an attractive and fruitful strategy to develop photofunctional materials and devices. Porphyrins exhibit strong Soret bands around 400 to 450 nm, whereas phthalocyanines show strong Q bands around 700 to 800 nm. Thus, the combination of those two p systems can cover nearly the whole range of the visible region and can be a useful strategy for development of photofunctional materials for efficient light-energy conversion. Attempts have so far been made to synthesize covalently linked Por–Pc heterodyad molecules and construct Por–Pc heterosupramolecules. Recently, ZnPor and ZnPc have been reported to form two-dimensional arrays on gold surfaces, and the formation of a cofacial ZnPor–ZnPc coordination tetrad has also been reported. However, a crystal structure determination of a discrete supramolecular assembly composed of both Por and Pc has yet to be reported. In addition, since the Q-band absorption of Pc usually overlaps the wavelength of fluorescence of Por, energy transfer is favored over electron transfer in most heterodyads. We have developed supramolecular assemblies based on a saddle-distorted nonplanar porphyrin, dodecaphenylporphyrin (H2DPP), and its metal complexes. [11–13] The saddle distortion facilitates protonation of pyrrole nitrogen atoms to allow access to a stable diprotonated porphyrin, which can act as an electron acceptor. In addition, the saddle distortion affords higher Lewis acidity at the metal center to maintain axial coordination of ligands, as a result of poor overlap of the pyrrole nitrogen lone pair orbitals with d orbitals of the metal center. In contrast, the Zn complex of the saddle-distorted phthalocyanine 1,4,8,11,15,18,22,25-octaphenylphthalocyanine (H2OPPc) exhibits a lower oxidation potential relative to the corresponding porphyrin complex. To construct supramolecular conglomerates composed of both porphyrin and phthalocyanine in a well-defined manner, we have taken advantage of saddle distortion of both components. Herein, we report formation of a discrete supramolecular assembly composed of H4DPP 2+ and [Zn(OPPc)] connected by 4-pyridinecarboxylate (4-PyCOO ) with coordination and hydrogen bonding (Figure 1). The supramolecular conglomerate [(H4DPP){Zn(OPPc)(k-N-4-PyCOO)}2] (1) was synthesized by reaction of [H4DPP](4-PyCOO)2 (2) and Zn(OPPc) (3) in toluene. We crystallized and isolated 1 in pure form by vapor diffusion of hexanes into solution of the mixture in toluene. X-ray crystallography of 1 unambiguously established its structure (Figure 2a).

Synthesis, spectroscopic properties, and electrochemistry of heteroleptic rare earth double-decker complexes with phthalocyaninato and meso-tetrakis (4-chlorophenyl)porphyrinato ligands

A series of fourteen heteroleptic (phthalocyaninato)(porphyrinato) rare earth(III) double-decker complexes, [MIII(Pc)(TClPP)] [M=Y, La–Lu except Ce and Pm; Pc=phthalocyaninate; TClPP=meso-tetrakis(4-chlorophenyl)porphyrinate] has been prepared by base-promoted cyclization of phthalonitrile on the corresponding [MIII(TClPP)(acac)] (acac=acetylacetonate) as the template in refluxing n-octanol. The yields are highest for the mid-lanthanides, which have the optimum size to balance the stabilization due to π-π interactions and the destabilization due to axial compression of the two π systems. The whole series of double-decker complexes has been characterized by elemental analysis and a wide range of spectroscopic methods. The molecular structure of [GdIII(Pc)(TClPP)] has also been determined. All the electronic absorption bands are dependent on the size of the metal center, suggesting that all the transitions involve molecular orbitals with contributions from both Pc and TClPP ligands. This has been supported by a systematic investigation of the electrochemical properties of the whole series of complexes. All these studies indicate that there are substantial π-π interactions and the hole is mainly localized at the Pc ligand.

Influence of intramolecular f-f interactions on nuclear spin driven quantum tunneling of magnetizations in quadruple-decker phthalocyanine complexes containing two terbium or dysprosium magnetic centers.

Nuclear spin driven quantum tunneling of magnetization (QTM) phenomena, which arise from admixture of more than two orthogonal electronic spin wave functions through the couplings with those of the nuclear spins, are one of the important magnetic relaxation processes in lanthanide single molecule magnets (SMMs) in the low temperature range. Although recent experimental studies have indicated that the presence of the intramolecular f-f interactions affects their magnetic relaxation processes, little attention has been given to their mechanisms and, to the best of our knowledge, no rational theoretical models have been proposed for the interpretations of how the nuclear spin driven QTMs are influenced by the f-f interactions. Since quadruple-decker phthalocyanine complexes with two terbium or dysprosium ions as the magnetic centers show moderate f-f interactions, these are appropriate to investigate the influence of the f-f interactions on the dynamic magnetic relaxation processes. In the present paper, a theoretical model including ligand field (LF) potentials, hyperfine, nuclear quadrupole, magnetic dipolar, and the Zeeman interactions has been constructed to understand the roles of the nuclear spins for the QTM processes, and the resultant Zeeman plots are obtained. The ac susceptibility measurements of the magnetically diluted quadruple-decker monoterbium and diterbium phthalocyanine complexes, [Tb-Y] and [Tb-Tb], have indicated that the presence of the f-f interactions suppresses the QTMs in the absence of the external magnetic field (H(dc)) being consistent with previous reports. On the contrary, the faster magnetic relaxation processes are observed for [Tb-Tb] than [Tb-Y] at H(dc) = 1000 Oe, clearly demonstrating that the QTMs are rather enhanced in the presence of the external magnetic field. Based on the calculated Zeeman diagrams, these observations can be attributed to the enhanced nuclear spin driven QTMs for [Tb-Tb]. At the H(dc) higher than 2000 Oe, the magnetic relaxations become faster with increasing Hdc for both complexes, which are possibly ascribed to the enhanced direct processes. The results on the dysprosium complexes are also discussed as the example of a Kramers system.

Formation of highly ordered porphyrin adlayers induced by electrochemical potential modulation

Molecular self-assembly of porphyrin derivatives formed with intermolecular hydrogen bonding on the surface of Au(111) electrode in acidic solution can be controlled by varying the number of peripheral carboxy groups and the applied electrochemical potential.

Magnetic relaxations arising from spin-phonon interactions in the nonthermally activated temperature range for a double-decker terbium phthalocyanine single molecule magnet.

Magnetic relaxations arising from spin-phonon interactions for a magnetically diluted double-decker terbium phthalocyanine single molecule magnet, dil1, in the nonthermally activated temperature range have been investigated. While the relaxation time, τ, is independent of the external static magnetic field, H(dc), in the high temperature range, where linear relationships between -ln τ and T(-1) are observed in the Arrhenius plot, magnetic field dependences for τ are observed in the lower temperature range. The τ(-1) vs H(dc) plot at 12 K fits the quadric curve when H(dc) < 12 kOe, while linear relationships are observed in the τ(-1) vs T plots in the temperature range of 12-20 K. These results indicate that the direct process is the dominant magnetic relaxation pathway in the nonthermally activated temperature range, while the contribution from the Raman process, if any, is not observable. We emphasize in this paper that the contribution from the thermal relaxation processes and the quantum tunneling of magnetizations (QTMs) to the experimentally observed magnetic relaxations must be evaluated carefully in order to avoid confusion between the thermal and quantum-mechanical relaxation pathways.

Observation of exceptionally low-lying π-π* excited states in oxidized forms of quadruple-decker phthalocyanine complexes.

Spectroscopic investigations have been performed for the oxidized forms of two quadruple-decker phthalocyanine complexes in order to clarify the electronic structures of multiply stacked π-systems. Up to three-electron-oxidized species were isolated by using phenoxathiin hexachloroantimonate as the oxidant. As the oxidations proceed, the Q-bands in the visible region shift bathochromically along with the clear isosbestic points. The one- and three-electron-oxidized species exhibited typical π-radical signals in the ESR spectra, while the neutral and two-electron oxidized species gave no indication of the presence of π-radicals. The electronic transitions observed for the oxidized species reach even into the so-called fingerprint region in IR spectroscopy (~1000 cm(-1)). With the aid of theoretical calculations, these bands can be assigned to the π-π* transitions. Our results provide new insights into π-electronic systems having exceptionally small MO energy gaps.

Metal Effects on Electronic Structures of Directly Linked Tribenzotetraazachlorin-Fullerene Conjugates

Mixed condensation of 1,2-dicyanofullerene (1) and 4,5-dibutyloxyphthalonitrile (2a) in the presence of vanadium(III) chloride (VCl3) in quinoline forms the hexabutyloxy-substituted tribenzotetraazachlorin (TBTAC) - fullerene (C60) vanadyl complex (3). UV-vis absorption and magnetic circular dichroism (MCD) spectroscopy demonstrated that 3 shows two intense Q-band components, whereas the previously reported nickel complex possessing the identical peripheral substituents (4) shows one intense and two medium-intense bands in the 600-800 nm region, indicating that the electronic mixing states of 3 are different from those of the nickel complex (4). The metal effects on electronic structures of the conjugates were interpreted using density functional theory (DFT) calculations.

Phthalocyanine–C60 Fused Conjugates Exhibiting Molecular Orbital Interactions Depending on the Solvent Polarity

New covalently C60-connected zinc phthalocyanine (ZnPc) derivatives have been synthesized by utilizing successive cycloaddition reactions of C60 with a ZnPc derivative containing a pyridazine moiety employing Komatsus method in reaction of C60 with phthalazine. The UV/Vis absorption spectrum of the fused conjugate (5) shows red shifts from the corresponding absorption of ZnPc derivative (8), indicating interactions between the ZnPc and C60 moieties. The DFT calculations under non-polar medium predict that the HOMO and LUMO of 5 localize on the ZnPc moiety, whereas LUMO+1 localizes on the C60 moiety, which reasonably explain the magnetic circular dichroism (MCD) and absorption spectra in toluene. Electrochemical redox potentials of 5 in polar solvents indicate the first-oxidation potential arises from the ZnPc moiety, whereas the first reduction potential is associated with the C60 moiety, suggesting the LUMO localizes on the C60 moiety in polar solvent. This reversal of the LUMO is supported by the ZnPc-fluorescence quenching with a nearby C60 moiety in benzonitrile, which leads to the charge-separation via the excited singlet state of the ZnPc moiety. In toluene on the other hand, such a ZnPc-fluorescence quenching owing to the photoinduced charge separation is not observed as predicted by the DFT-calculated LUMO on the ZnPc moiety.

Synthesis and spectroscopic and electrochemical studies of pyrazine- or pyridine-ring-fused tetraazachlorins, bacteriochlorins, and isobacteriochlorins.

Mixed condensation of tetramethylsuccinonitrile and either 2,3-dicyano-5,6-diethylpyrazine, 2,3-dicyanopyridine, or 3,4-pyridinedicarboximide in the presence of nickel chloride forms novel pyrazine-, 2,3-pyridine-, or 3,4-pyridine-ring-fused tetraazachlorin (TAC), tetraazabacteriochlorin (TABC), and tetraazaisobacteriochlorin (TAiBC) derivatives. All possible structural isomers were separated using repeated thin-layer chromatography and have been investigated by absorption and magnetic circular dichroism spectroscopy. Similarly to previously reported TAC analogues, the TAC and TABC derivatives show large splitting of the Q band, while a single, intense absorption band is observed for the TAiBC derivatives. Although the absorption spectra are practically identical in shape for the separated structural isomers of TACs and TABCs, the Q-band maxima of the TAiBCs depend significantly on their structures. The observed spectroscopic properties were interpreted on the basis of electrochemical data and the results of (time-dependent) DFT calculations.

Cation-induced molecular orbital modulations and supramolecular formation of a 15-crown-5-substituted tribenzotetraazachlorin-C60 conjugate.

A tribenzotetraazachlorin (TBTAC)-C(60) conjugate having three 15-crown-5 units as substituents, , has been synthesized, and the electronic and supramolecular structure modulations in the presence of Na(+) or K(+) have been elucidated by means of massmetric and spectroscopic techniques.