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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.

Novel tetraazachlorin (TAC)-fullerene (C60) conjugates (TAC-C60) and their analogues (TAiBC-C60 and TABC-C60 where TAiBC = tetraazaisobacteriochlorin and TABC = tetraazabacteriochlorin) have been synthesized by condensing 1,2-dicyanofullerene (1) and phthalonitrile derivatives (2) in the presence of nickel chloride in quinoline, and fully characterized using mass spectrometry and 1H and 13C NMR. By arranging the TAC and C60 units at the minimum distance, and taking also into account the molecular symmetry, the resultant conjugates show on-off electronic communication behavior, depending on the push-pull properties of the peripheral substituents in the TAC moiety. Consequently, the UV-vis absorption spectrum of the electron-releasing butyloxy-substituted TAC-C60 (3a) contains an unusual group of three absorption bands in the Q-band region (500-900 nm) as a result of a strong electronic communication between the two moieties. On the other hand, the absorption spectrum of the electron-withdrawing butylsulfonyl-substituted TAC-C60 (3b) comprises a typically normal TAC spectrum with markedly split two-peak Q-bands. A similar phenomenon is observed between alkoxy-substituted TAiBC-C60 (4a) and butylsulfonyl-substituted TAiBC-C60 (4b). This study reveals that the electron-donating or -withdrawing nature of the peripheral substituents on an azachlorin moiety has an important effect on the electronic structures of our novel azachlorin-C60 conjugates, although the linking carbon atoms are aliphatic sp3 carbon atoms which generally do not contribute to aromaticity. The electronic structures of the conjugates have been investigated in detail using spectroscopic and electrochemical techniques with the aid of DFT calculations.

A method which preferentially produces adjacently di-aromatic ring-substituted tetraazaporphyrins (TAPs) has been developed, and their electrochemical and spectroscopic properties have been studied and compared with those of the corresponding series of mono-aromatic ring-fused TAPs. Mono-aromatic ring-fused TAPs show a split Q-band, and the splitting energy increases with increasing size of the aromatic ring. In addition, for the split Q-bands, the relative intensity of the band at longer wavelength decreases with increasing molecular size of the fused aromatics, compared with the shorter wavelength band. In the di-aromatic ring-fused TAPs, this kind of splitting is not seen, and only a shift of the band is observed. The intensity and band position of the split or unsplit Q-bands are quantitatively evaluated by simultaneous band deconvolution analysis, using both electronic absorption and magnetic circular dichroism spectra. The preparation of these TAP compounds has made it possible to adjust the Q-band position in a stepwise manner between ca. 600 and 750 nm. The first reduction and oxidation potentials of the TAP ring shift negatively with increasing number and size of the fused aromatics. The extent of the shift is found to be very small for the LUMOs but significant for the HOMOs. These spectroscopic and electrochemical properties are almost perfectly reproduced by molecular orbital calculations within the framework of the Pariser-Parr-Pople approximation. In particular, a small variation of the LUMO level and large destabilization of the HOMO level on ring expansion are rationalized from the extent of stretch of molecular orbitals: i.e., since the LUMOs are localized in the central TAP moiety irrespective of the molecular size, while the HOMOs have appreciable coefficients even over the fused aromatics, the HOMO level destabilizes while the LUMO level remains constant with increasing molecular size. In one CoTAP derivative, Co(III/II) and the first ligand oxidation couples occur experimentally at the same potential.

The reaction of a double-decker phthalocyanine (Pc) complex, bis(phthalocyaninato)lutetium ([LuPc(2)](-)), with cadmium acetate at 400 degrees C for 30 min under argon provided the first example of a discrete quadruple-decker Pc complex, 1. The structure of 1 was unambiguously determined through the use of high-resolution mass spectrometry, elemental analysis, and NMR techniques such as (1)H-(1)H COSY, (13)C-(1)H COSY, and NOE experiments. The absorption and magnetic circular dichroism spectra of 1 resembled those of [LuPc(2)](-) and [Lu(2)Pc(3)] rather than those of the neutral double-decker species [LuPc(2)](0), since the formal charge on each Pc moiety is 2-. Electrochemical measurements exhibited a stacking effect based on the delocalization of the holes or electrons introduced by oxidation or reduction, respectively, over the entire complex.