Magnetic field effect on the fluorescence of glyoxal

Fluorescence from the ¹A_u state of glyoxal is quenched by a magnetic field and the pre-exponential factor of phosphorescence from the ³A_u state increases with increasing magnetic field. These findings are explained by the enhancement of intramolecular intersystem crossing by the magnetic field.     

A theoretical study on the third-order nonlinear optical properties of pi-conjugated linear porphyrin arrays

By using the Pariser-Parr-Pople (PPP) theory, the second hyperpolarizabilities (gamma) have been calculated for various pi-conjugated porphyrin arrays including "porphyrin tapes": the meso-beta doubly linked porphyrin array Dn and the meso-meso, beta-beta, beta-beta triply linked array Tn. The validity of the PPP theory is checked via a comparison with both the ab initio Hartree-Fock and the B3LYP theories in the case of porphyrin monomers and dimers. It is found that Dn and especially Tn exhibit much more remarkable evolution of gamma/n along with an increasing number of porphyrin units n compared with the butadiyne-bridged array, Yn. As a result, the static third-order susceptibilities chi((3)) of Dn and Tn are expected to be 1 and 3 orders of magnitude larger than that of Yn in the limit n --> infinity, and these advantages of porphyrin tapes become more prominent by taking into account geometrical relaxations of porphyrin units in the arrays. The structure-property relationship in various conjugated polymers including porphyrin arrays is interpreted on the basis of the scaling behaviors of chi((1)) and chi((3)) with the effective conjugation length (ECL) as well as the reciprocal HOMO-LUMO energy gap (1/E(g)). In particular, from the master plot of chi((3)) (and even chi((1))) versus 1/E(g), the pi-conjugation of Tn is noted to indeed be exceptional, because its large susceptibilities cannot be expected from the scaling behavior of ordinary one-dimensional conjugated systems. We also point out that the theory of scaling relationship, chi((3)) approximately 1/E(g)(x)(), is significantly improved by taking into account electron-electron interactions based on the comparison with experiments.     

Syntheses, crystal and solution structures, ligand-exchange, and ligand-coupling reactions of mixed pentaarylantimony compounds

All possible combinations of mixed pentaarylantimony compounds bearing p-methylphenyl and p-trifluoromethylphenyl groups were synthesized; ArnTol5-nSb (n=0-5: Ar=p-CF3C6H4, Tol=p-CH3C6H4): Tol5Sb (1), ArTol4Sb (2), Ar2Tol3Sb (3), Ar3Tol2Sb (4), Ar4TolSb (5), and Ar5Sb (6). Compounds 2-5 are the first well-characterized examples of mixed acyclic pentaarylantimony species. The structures of 2-6 were determined by X-ray crystallography to feature trigonal-bipyramidal (TBP) geometry with the more electronegative p-trifluoromethylphenyl substituents selectively occupying the apical positions. Consideration of the chemical shifts of the ipso carbons of the aryl and tolyl groups suggested that the solution structures of 1-6 were also TBP, although their pseudorotation could not be frozen even at -80 degrees C. Ligand-exchange reactions (LERs) took place between 1 and 6 at approximately 60 degrees C in [D6]benzene and all six species 1-6 were found in the equilibrium mixture. The relative stabilities of 1-6 were determined quantitatively by comparison of the observed molar ratios of 1-6 in equilibrium with calculated statistical molar ratios, and Ar2Tol3Sb (3) was found to be the most stable. The ligand-coupling reactions (LCRs) of 2-5 in solution were greatly accelerated by adding Cu(acac)2 or Li+TFPB- (TFPB: [3,5-(CF3)2 C6H3]4 B), whereby the rate becomes comparable to the LER. The use of flash vacuum thermolysis (FVT) allowed the LCR to occur with very little ligand-exchange; the exception ArTol4Sb had very fast ligand-exchange. The selectivities of the LCRs were calculated from the yield of the biaryls synthesized by using FVT. These results were highly consistent with reactions catalyzed in solution, in which bitolyl was not obtained at all. The experimental results suggested that the LCR of pentaarylantimony compounds proceeds in the manner of apical-apical coupling.     

Detection of Single Molecule on Water Surface by Confocal Fluorescence Microscopy

In the past decade,several research groups have succeeded in observing single molecule in liquid, and more recently at silica surface in the near-field mode as well as at polymer-air interface in far-field mode. Due to the significance of air-water interface in surface chemistry, we prospect that research works on single molecule detection (SMD) of the air-water surface could open a new era in surface photochemistry and photophysics.     

Laser Spectroscopy on Water Surface

The water surface has unique properties, which are different from those in the bulk and have exceptional scientific and environmental importance. Surface tension has been measured, but is not versatile for a molecular point of view. Recently laser spectroscopy has appeared as a sensitive tool to investigate water surface on a molecular basis. There are three major laser techniques to study the water surface:the photoionization, the second-harmonic and sum-frequency generation, and the laser-induced fluorescence. Recent progresses in these fields are overviewed based mainly on investigations carried out in my group.     

Semisynthesis of isetexane diterpenoid analogues and their cytotoxic activity

Isetexane diterpene analogues were semisynthesized from demethylsalvicanol isolated from Perovskia abrotanoides (Labiatae). The structure and cytotoxic activity relationships (SAR) of the natural parent diterpene, demethylsalvicanol, and its semisynthetic analogues were studied by using P388 murine leukemia cells.     

Inclusion relations between the Bers embeddings of Teichmüller spaces

We prove that if the Bers embeddings of the Teichmüller spaces of infinitely generated Fuchsian groups are coincident, then these Fuchsian groups are the same.     

Quantum-chemical investigation of the electroabsorption spectra of directly meso-meso-linked porphyrin arrays: essential role of charge-transfer excited states accidentally overlapping with soret bands

The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.     

Stereoregularity of poly(2-vinylpyridine) and poly-(4-vinylpyridine)

In order to determine the stereoregularity of poly(4-vinylpyridine), 4-vinylpyridine-β,β-d₂ was synthesized from 4-acetylpyridine. The ¹H-NMR spectra of the deuterated and nondeuterated polymers were measured and analyzed. From the ¹H-NMR spectra of poly(4-vinylpyridine-β,β-d₂), triad tacticity can be obtained, while the ¹H-NMR spectra of nondeuterated poly(4-vinylpyridine) give the fraction of isotactic triad. The ¹³C-NMR spectra of poly(4-vinylpyridine) were also observed, and the spectra of C₄ carbon of polymers were assigned by the pentad tacticities. The fraction of isotactic triad of poly(2-vinylpyridine) and poly(4-vinylpyridine) obtained under various polymerization conditions were determined. The radical polymerization and anionic polymerizations with phenylmagnesium bromide and n-butyllithium as catalysts of 4-vinylpyridine gave atactic polymers.