Sachiko Tojo

Synthesis of ODNs Containing 4-Methylamino-1,8-naphthalimide as a fluorescence probe in DNA

Synthesis and fluorescence properties of oligodeoxynucleotides containing 4-methylamino-1,8-naphthalimide (NI) have been described. NI was successfully incorporated into DNA without significant destabilization of DNA whilst retaining its high fluorescence quantum yield. The attachment site of the NI greatly affected its property as an energy acceptor in FRET analysis.

Radical Ions of Cycloparaphenylenes: Size Dependence Contrary to the Neutral Molecules.

Cycloparaphenylenes (CPPs) have attracted wide attention because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals. For application of CPPs, information on their charged states (radical cation and radical anion) is essential. Here, we measured absorption spectra of the radical cations and the radical anions of CPPs with various ring sizes over a wide spectral region by means of radiation chemical methods. The peak position of the near-IR bands for both the radical cation and the radical anion shifted to lower energies with an increase in the ring size. This trend is contrary to what is observed for transitions between the HOMO and LUMO of the neutral CPP. The observed spectra of the CPP radical ions were reasonably assigned based on time-dependent density functional theory. These results indicate that the next HOMO and the next LUMO levels are important in the electronic transitions of radical ions.

Delocalization of Positive Charge in π-Stacked Multi-benzene Rings in Multilayered Cyclophanes

In the present study, delocalization of a positive charge in π-stacked multi-benzene rings in multilayered para- and meta-cyclophanes, in which benzene rings are connected by propyl chains to form a chromophore array with the face-to-face structure, was investigated by means of transient absorption spectroscopy during the pulse radiolysis using dichloroethane as a solvent. The local excitation and charge resonance (CR) bands were successfully observed. It was revealed that the CR band shifted to the longer wavelength side with the number of the benzene rings. The stabilization energy estimated from the peak position of the CR band showed the efficient charge delocalization over the cyclophanes. Furthermore, the CR bands showed the slight spectral change attributable to the change in distribution of the conformers. The substantially long lifetime of the CR band can be explained on the basis of the smaller charge distribution on the outer layers of the multilayered cyclophanes.

Electron Transfer in the Supramolecular Donor-Acceptor Dyad of Zinc Porphycene

The electron transfer processes of Zn octaethylporphycene (ZnPcn), a structural isomer of Zn octaethylporphyrin, have been investigated mainly using transient absorption spectroscopy. To form a supramolecular donor-acceptor dyad, imide compounds bearing a pyridine group at the N position of the imides have been used as an acceptor. The N atom of the pyridine ring can coordinate to the central Zn ion of ZnPcn. Formation of a supramolecular donor-acceptor dyad, that is, pentacoordinated ZnPcn, was confirmed by steady-state absorption spectroscopy using toluene as a solvent. Charge separation upon excitation of ZnPcn was indicated by efficient fluorescence quenching, especially when pyromellitic diimide was used as the acceptor. Electron transfer processes were confirmed by subpicosecond transient absorption spectroscopy, in which generation of a radical anion of the acceptor and a radical cation of ZnPcn, which was identified by means of gamma-ray radiolysis, was confirmed. It became clear that the charge separation rate was smaller than that of the corresponding supramolecular dyads of Zn tetraphenylporphyrin and Zn octaethylporphyrin despite a similar driving force. This observation indicates a larger internal reorganization energy and a smaller coupling element of the ZnPcn dyad.

Stilbene Dimer Radical Cations in the Radiolyses of Stilbenes and 1,2,3,4-Tetraphenylcyclobutanes

The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.

Intermolecular and Intramolecular Electron Transfer Processes from Excited Naphthalene Diimide Radical Anions

Excited radical ions are interesting reactive intermediates owing to powerful redox reactivities, which are applicable to various reactions. Although their reactivities have been examined for many years, their dynamics are not well-defined. In this study, we examined intermolecular and intramolecular electron transfer (ET) processes from excited radical anions of naphthalene-1,4,5,8-tetracarboxydiimide (NDI(•-)*). Intermolecular ET processes between NDI(•-)* and various electron acceptors were confirmed by transient absorption measurements during laser flash photolysis of NDI(•-) generated by pulse radiolysis. Although three different imide compounds were employed as acceptors for NDI(•-)*, the bimolecular ET rate constants were similar in each acceptor, indicating that ET is not the rate-determining step. Intramolecular ET processes were examined by applying femtosecond laser flash photolysis to two series of dyad compounds, where NDI was selectively reduced chemically. The distance dependence of the ET rate constants was described by a β value of 0.3 Å(-1), which is similar or slightly smaller than the reported values for donor-acceptor dyads with phenylene spacers. Furthermore, by applying the Marcus theory to the driving force dependence of the ET rate constants, the electronic coupling for the present ET processes was determined.

Effects of substituents on aryl groups during the reaction of triarylphosphine radical cation and oxygen

In a previous report (S. Yasui, S. Tojo and T. Majima, J. Org. Chem., 2005, 70, 1276), we presented the results from the laser flash photolysis (LFP) and product analysis of the 9,10-dicyanoanthracene (DCA)-photosensitized oxidation of triarylphosphine (Ar(3)P) in acetonitrile under air, which showed that the photoreaction results in the oxidation of Ar(3)P to give the corresponding phosphine oxide (Ar(3)P=O) in a nearly quantitative yield, and that the reaction is initiated by the electron transfer (ET) from Ar(3)P to DCA in the singlet excited state ((1)DCA*), producing the triarylphosphine radical cation Ar(3)P (+). This radical cation decays through radical coupling with O(2) to afford the peroxy radical cation Ar(3)P(+)-O-O*, which we proposed to be the intermediate leading to the product Ar(3)P=O. We now examined this photoreaction in more detail using ten kinds of Ar(3)P with various electronic and steric characteristics. The decay rate of Ar(3)P*(+) measured by the LFP was only slightly affected by the substituents on the aryl groups of Ar(3)P. During the photolysis of trimesitylphosphine (Mes(3)P), the peroxy radical cation intermediate (Mes(3)P(+)-O-O*) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar(3)P=O increased with either electron-withdrawing or -releasing substituents on the aryl groups, suggesting that a radical center is developed on the phosphorus atom during the step when the quantum yield is determined. In addition, the o-methyl substituents in Ar(3)P decreased the quantum yield. These results clearly indicated that Ar(3)P(+)-O-O* undergoes radical attack upon the parent phosphine Ar(3)P that eventually produces the final product, Ar(3)P=O.

Formation and decay of pyrene radical cation and pyrene dimer radical cation in the absence and presence of cyclodextrins during resonant two-photon ionization of pyrene and sodium 1-pyrene sulfonate

Formation and decay of pyrene radical cation and pyrene dimer radical cation during resonant two-photon ionizations (TPI) of pyrene (Py) and sodium 1-pyrene sulfonate (NaPySA) in the absence and presence of cyclodextrins were studied with visible and near IR transient absorption measurement using 355-nm laser flash photolysis technique. Ionization of Py and 1-pyrene sulfonate anion (PySA−) occurred within the laser flash (5 ns) to give Py radical cation (Py˙+) and PySA− radical cation (Py˙+SA−) having Py˙+ chromophore. Dimerizations of Py˙+ and Py and of Py˙+SA− and PySA− were observed to give the dimer radical cations (Py2˙+ and Py2˙+(SA−)2) based on time-resolved measurements of the charge resonance (CR) band. The TPI of PySA− was also examined in the presence of β- and γ-cyclodextrins (βCD and γCD, respectively). Py2˙+(SA−)2 in the cavity of two γCDs having fully overlapped structure showed the CR band around 1700 nm, while Py2˙+(SA−)2 in the cavity of two βCD having partially and fully overlapped structures indicated the CR bands around 1500 or 1700 nm, respectively. On the basis of formation and decay rates of Py2˙+(SA−)2, bimolecular formation of Py2˙+(SA−)2 and the neutralization are inhibited in the cavities of CDs. Selective formation of the fully overlapped structure of Py2˙+(SA−)2 in the cavity of two γCDs within a laser flash is explained by direct TPI of two PySA−s with the parallel structure of two Py chromophores in two PySA−s in the cavity of two γCDs.

Radical cation of star-shaped condensed oligofluorenes having isotruxene as a core: importance of rigid planar structure on charge delocalization.

Because of their excellent optical and electronic properties, oligofluorenes and polyfluorenes have been investigated for years. Recently developed star-shaped oligomers bearing a truxene or isotruxene core are interesting two-dimensional oligomers. Since employment of a condensed ring system will be effective in further extension of π-conjugation system, we studied electronic and vibrational properties of radical cation of CITFn, star-shaped condensed oligomer with isotruxene core and fluorene unit, by means of the radiation chemical methods. Absorption spectra of radical cation of CITFn were measured in the wide spectral range, which revealed extended π-conjugation of CITFn. Furthermore, time-resolved resonance Raman spectra during pulse radiolysis revealed that the oxidation of CITFn induced structural change to enhance quinoidal character. The Raman data and theoretical calculation indicated that the rigid framework of the present star-shaped oligomer which makes the oligomer a planar structure is quite important in extension of the conjugation pathway.

Hole transfer in DNA: DNA as a scaffold for hole transfer between two organic molecules

Abstract Hole transfer process in ODNs conjugated with two organic molecules, pyrene (Py) and phenothiazine (Ptz) was investigated with the pulse radiolysis measurements. Monitoring the transient absorption of Py + and Ptz +, it was shown that the hole transfer rate was dependent on the distance and sequence between Py and Ptz.