White light emission from a single component system: remarkable concentration effects on the fluorescence of 1,3-diaroylmethanatoboron difluoride

Concentration effects on the fluorescence (FL) emission of 1,3-dibenzoylmethanatoboron difluoride (1aBF₂) and its diisopropyl derivative (1bBF₂) in KBr and CH₂Cl₂ were investigated. Powder samples of 1aBF₂ and 1bBF₂ in KBr exhibit yellow and white FL emissions, respectively, whose intensities and wavelengths are not significantly affected by concentration. In contrast, remarkable concentration effects on FL properties of these compounds in CH₂Cl₂ solutions were observed. Increases in the concentrations of 1aBF₂ and 1bBF₂ from 1 × 10⁻⁷ to ca. 2 × 10⁻¹ M lead to dramatic changes in the FL colors from blue (398 and 411 nm, respectively) to yellow (548 and 558 nm) via white. Careful analysis of the FL spectra, involving lifetime determinations and wave deconvolutions, reveals that emissions from 1BF₂ involve two FL domains, corresponding to an excited monomer and an excimer, and that concentration increases promote a continuous change from the former to the latter major FL domain. Thus, white FL of 1aBF₂ and 1bBF₂ is achieved by modulation of the dual FL of the excited monomer (blue) and excimer (yellow). These findings indicate that 1,3-diaroylmethanatoboron difluoride (1BF₂) represents a new white emitting material that has advantageous features which arise from the fact that it is an easily prepared, low molecular weight, single component system not containing a heavy metal atom.     

Nuclear Moment Studies with Polarized Radioactive Nuclear Beams

Magnetic dipole and electric quadrupole moments of nuclei in the light-mass neutron-rich region have been studied by taking advantage of spin-polarized radioactive nuclear beams that have been obtained from the projectile fragmentation reaction. Analyses of the results reveal a few interesting phenomena characteristic of nuclear structures in this region. In particular, we report in some detail the latest result on the magnetic moment of the ¹⁷C ground state. The distinctly small value of the g-factor obtained, |g(¹⁷C)|=0.5054±0.0025, clearly excludes a I ^π=1/2⁺ candidate for the spin-parity assignment of this marginally bound nucleus, providing a reasonable account of the non-halo nature reported in recent breakup reaction experiments. Finally, future plans at the upcoming radioactive beam facility presently under construction at RIKEN are briefly mentioned. This revised version was published online in September 2006 with corrections to the Cover Date.     

Long RNA dangling end has large energetic contribution to duplex stability

Long terminal unpaired nucleotides known as dangling ends play interesting roles in biological systems. Previous studies, however, only dealt with the energy contributions of single dangling bases. The energy contributions of long dangling ends on the stability of duplexes have not been systematically studied. We now report a quantitative increase in stability of RNA-RNA and DNA-DNA duplexes containing a long dangling end. We found a larger enhancement of the stability by the long RNA dangling end of the RNA-RNA duplex than has been observed for the DNA duplexes. It is also found that structural stabilizations by long dangling ends seem to originate from the single-stranded stacking interactions of nucleotides. These results indicate that RNA stability can be achieved by increasing the length of the dangling end. The thermodynamic parameters of the long dangling ends are useful for designing ribozymes and antisense oligonucleotides, and for the prediction of the RNA secondary structure like the pseudoknot.     

Electrochiroptical response of 2,2'-(2,2-diarylethenyl)binaphthyl-type electron donors that undergo reversible C-C bond formation/breaking upon two-electron transfer

2,2'-[2,2-Bis(4-dimethylaminophenyl)ethenyl]biphenyl (1) is a strong electron donor that undergoes oxidative C-C bond formation to give a stable dication rac-2(2+), the 9,10-dihydrophenanthrene derivative substituted with two bis(4-dimethylaminophenyl)methylium chromophores. This dication salt regenerates the starting diolefin 1 by reductive C-C bond breaking, thus realizing a new electrochromic system with high electrochemical bistability and a vivid change in color from yellow to deep blue. Similarly, the binaphthylic diolefin rac-3 and the helicene-type dication rac-4(2+) are interconvertible upon two-electron transfer. Both the UV-vis and CD spectra changed drastically upon electrochemical transformation between optically pure 3 and 4(2+), which represents a new electrochiroptical system.     

Applications of the model potential method to transition metal compounds

Model potential parameters and basis sets, presented previously for the transition metal atoms Sc through Hg, are tested in calculations of the transition metal compounds (CuF, CuCl, Cu₂, TiCl₄, ZrCl₄, CoF₆^3?, CoF₆^2?, AgH, AuH, CrF₆, ScO, ZrO, Cr₂, Mo₂). Calculated values of the bond distances, vibrational frequencies, and some transition energies (for Cu₂ and CoF₆^2?) are compared with those given by all-electron calculations with basis sets of high quality. Singlet-triplet splittings in Cu₂ and correlation energies in CrF₆^n? (n = 0, 1, and 2) are also examined. The satisfactory results obtained by these calculations strongly support the contention that the model potential method is a reliable and economical alternative to the ab initio Hartree-Fock-Roothaan method.     

Fluorescence studies on nyctinasty which suggest the existence of genus-specific receptors for leaf-movement factor

Periodic leaf-movement of legumes is called nyctinasty and has been known since the age of Alexander the Great. We found that nyctinasty is controlled by a periodic change of the internal concentration of leaf-opening and leaf-closing substances in the plant body. Now, we have developed novel fluorescent probes (1) based on the structure of cis-p-coumaroylagmatine (3), which was isolated as a leaf-opening substance of Albizzia juribrissin Durazz. Binding experiments using probe 1 showed that Albizza plants have receptors for a leaf-opening substance in their motor cells. By using probes 1 we then found that genus-specific receptors are involved in nyctinasty.     

Inner-valence states of N2+ and the dissociation dynamics studied by threshold photoelectron spectroscopy and configuration interaction calculation

The N2(+) states lying in the ionization region of 26-45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2(+) states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N2(2+). Dissociation products formed from the inner-valence N2(+) states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.