Junpei Yuasa

Noncovalent Ligand-to-Ligand Interactions Alter Sense of Optical Chirality in Luminescent Tris(β-diketonate) Lanthanide(III) Complexes Containing a Chiral Bis(oxazolinyl) Pyridine Ligand

Highly luminescent tris[β-diketonate (HFA, 1,1,1,5,5,5-hexafluoropentane-2,4-dione)] europium(III) complexes containing a chiral bis(oxazolinyl) pyridine (pybox) ligand--[(Eu(III)(R)-Ph-pybox)(HFA)(3)], [(Eu(III)(R)-i-Pr-pybox)(HFA)(3)], and [(Eu(III)(R)-Me-Ph-pybox)(HFA)(3)])--exhibit strong circularly polarized luminescence (CPL) at the magnetic-dipole ((5)D(0) → (7)F(1)) transition, where the [(Eu(III)(R)-Ph-pybox)(HFA)(3)] complexes show virtually opposite CPL spectra as compared to those with the same chirality of [(Eu(III)(R)-i-Pr-pybox)(HFA)(3)] and [(Eu(III)(R)-Me-Ph-pybox)(HFA)(3)]. Similarly, the [(Tb(III)(R)-Ph-pybox)(HFA)(3)] complexes were found to exhibit CPL signals almost opposite to those of [(Tb(III)(R)-i-Pr-pybox)(HFA)(3)] and [(Tb(III)(R)-Me-Ph-pybox)(HFA)(3)] complexes with the same pybox chirality. Single-crystal X-ray structural analysis revealed ligand-ligand interactions between the pybox ligand and the HFA ligand in each lanthanide(III) complex: π-π stacking interactions in the Eu(III) and Tb(III) complexes with the Ph-pybox ligand, CH/F interactions in those with the i-Pr-pybox ligand, and CH/π interactions in those with the Me-Ph-pybox ligand. The ligand-ligand interactions between the achiral HFA ligands and the chiral pybox results in an asymmetric arrangement of three HFA ligands around the metal center. The metal center geometry varies depending on the types of ligand-ligand interaction.

Self-Discriminating Termination of Chiral Supramolecular Polymerization: Tuning the Length of Nanofibers†

Directing the supramolecular polymerization towards a preferred type of organization is extremely important in the design of functional soft materials. Proposed herein is a simple methodology to tune the length and optical chirality of supramolecular polymers formed from a chiral bichromophoric binaphthalene by the control of enantiomeric excess (ee). The enantiopure compound gave thin fibers longer than a few microns, while the racemic mixture favored the formation of nanoparticles. The thermodynamic study unveils that the heterochiral assembly gets preference over the homochiral assembly. The stronger heterochiral binding over homochiral one terminated the elongation of fibrous assembly, thus leading to a control over the length of fibers in the nonracemic mixtures. The supramolecular polymerization driven by π-π interactions highlights the effect of the geometry of a twisted π-core on this self-sorting assembly.

Nona-Coordinated Chiral Eu(III) Complexes with Stereoselective Ligand–Ligand Noncovalent Interactions for Enhanced Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) of chiral Eu(III) complexes with nona- and octa-coordinated structures, [Eu(R/S-iPr-Pybox)(D-facam)(3)] (1-R/1-S; R/S-iPr-Pybox, 2,6-bis(4R/4S-isopropyl-2-oxazolin-2-yl)pyridine; D-facam, 3-trifluoroacetyl-d-camphor), [Eu(S,S-Me-Ph-Pybox)(D-facam)(3)] (2-SS; S,S-Me-Ph-Pybox, 2,6-bis(4S-methyl-5S-phenyl-2-oxazolin-2-yl)pyridine), and [Eu(Phen)(D-facam)(3)] (3; Phen, 1,10-phenanthroline) are reported, and their structural features are discussed on the basis of X-ray crystallographic analyses. These chiral Eu(III) complexes showed relatively intense photoluminescence due to their (5)D(0) → (7)F(1) (magnetic-dipole) and (5)D(0) → (7)F(2) (electric-dipole) transition. The dissymmetry factors of CPL (g(CPL)) at the former band of 1-R and 1-S were as large as -1.0 and -0.8, respectively, while the g(CPL) of 3 at the (5)D(0) → (7)F(1) transition was relatively small (g(CPL) = -0.46). X-ray crystallographic data indicated specific ligand-ligand hydrogen bonding in these compounds which was expected to stabilize their chiral structures even in solution phase. CPL properties of 1-R and 1-S were discussed in terms of transition nature of lanthanide luminescence.

One-Step versus Stepwise Mechanism in Protonated Amino Acid-Promoted Electron-Transfer Reduction of a Quinone by Electron Donors and Two-Electron Reduction by a Dihydronicotinamide Adenine Dinucleotide Analogue. Interplay between Electron Transfer and Hydrogen Bonding

Semiquinone radical anion of 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ(*-)) forms a strong hydrogen bond with protonated histidine (TolSQ(*-)/His x 2 H(+)), which was successfully detected by electron spin resonance. Strong hydrogen bonding between TolSQ(*-) and His x 2 H(+) results in acceleration of electron transfer (ET) from ferrocenes [R2Fc, R = C5H5, C5H4(n-Bu), C5H4Me] to TolSQ, when the one-electron reduction potential of TolSQ is largely shifted to the positive direction in the presence of His x 2 H(+). The rates of His x 2 H(+)-promoted ET from R2Fc to TolSQ exhibit deuterium kinetic isotope effects due to partial dissociation of the N-H bond in His x 2 H(+) at the transition state, when His x 2 H(+) is replaced by the deuterated compound (His x 2 D(+)-d6). The observed deuterium kinetic isotope effect (kH/kD) decreases continuously with increasing the driving force of ET to approach kH/kD = 1.0. On the other hand, His x 2 H(+) also promotes a hydride reduction of TolSQ by an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH2). The hydride reduction proceeds via the one-step hydride-transfer pathway. In such a case, a large deuterium kinetic isotope effect is observed in the rate of the hydride transfer, when AcrH2 is replaced by the dideuterated compound (AcrD2). In sharp contrast to this, no deuterium kinetic isotope effect is observed, when His x 2 H(+) is replaced by His x 2 D(+)-d6. On the other hand, direct protonation of TolSQ and 9,10-phenanthrenequinone (PQ) also results in efficient reductions of TolSQH(+) and PQH(+) by AcrH2, respectively. In this case, however, the hydride-transfer reactions occur via the ET pathway, that is, ET from AcrH2 to TolSQH(+) and PQH(+) occurs in preference to direct hydride transfer from AcrH2 to TolSQH(+) and PQH(+), respectively. The AcrH2(*+) produced by the ET oxidation of AcrH2 by TolSQH(+) and PQH(+) was directly detected by using a stopped-flow technique.

Heteroleptic [Bis(oxazoline)](dipyrrinato)zinc(II) Complexes: Bright and Circularly Polarized Luminescence from an Originally Achiral Dipyrrinato Ligand

Heteroleptic zinc(II) complexes synthesized using achiral dipyrrinato and chiral bis(oxazoline) ligands show bright fluorescence with quantum efficiencies of up to 0.70. The fluorescence originates from the (1)π-π* photoexcited state localized exclusively on the dipyrrinato ligand. Furthermore, the luminescence is circularly polarized despite the achirality of the dipyrrinato ligand. Single-crystal X-ray structure analysis discloses that the chiral bis(oxazoline) ligand undergoes intramolecular π-π stacking with the dipyrrinato ligand, inducing axial chirality in the dipyrrinato moiety.

Effects of Counter Anions on Intense Photoluminescence of 1-D Chain Gold(I) Complexes

A series of cationic carbene complexes of Au(I) with different types of counteranions, [Au{C(OMe)NHMe}(2)](+)(X(-)) (X(-) = CF(3)SO(3)(-), PF(6)(-), CF(3)CO(2)(-), ClO(4)(-), and I(-)), was synthesized, and their intense photoluminescence was studied. These complexes crystallize in the same space group, and the X-ray crystallographic data of these samples revealed the short Au(I)-Au(I) aurophilic distances (3.263-3.335 A), where planar carbene Au(I) complexes are organized into linear stacks. The aurophilic Au(I)-Au(I) interactions found in the crystalline state give rise to phosphorescence with relatively high emission quantum efficiencies, whereas [Au{C(OMe)NHMe}(2)](+)(X(-)) complexes show no appreciable emission in solutions. The Au(I)-Au(I) distances in the crystal of [Au{C(OMe)NHMe}(2)](+)(X(-)) vary depending on the type of counteranions because the carbene ligands of Au(I) cations are linked through hydrogen bonds with adjacent counteranions. The effects of counteranions on the Au(I)-Au(I) aurophilic interactions allow one to modulate the intense photoluminescence color from blue to yellow by adjusting the counteranions of [Au{C(OMe)NHMe}(2)](+)(X(-)) complexes.

Circularly Polarized Light from Chiral Lanthanide(III) Complexes in Single Crystals

A circularly polarized emission (CPE) microscope system was designed for measuring circularly polarized light (CPL) from small single crystals of lanthanide(III) complexes with different crystal structures. CPL under excitation with 370-nm laser light was successfully observed from the crystals, and the CPL spectra were significantly different from those observed in solution. Dependence of the CPL spectra on the lattice plane of the crystals was also demonstrated.

Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand

This work demonstrates sign reversal of large circularly polarized luminescence (CPL) signal based on the hinge-like twisting motion of a bidentate ligand, 3,3-bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO), in a cis-trans isomerization of chiral europium(III) complexes. X-ray diffraction analysis revealed that twisting motion of BIPYPO provides s-cis and s-trans geometries of a chiral Eu(III) complex containing either tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (D-1) or tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate] (D-2). The s-cis Eu(III) complexes show eight-coordinate geometry around the Eu(III) ion, in which the chelate between the phosphoryl oxygen and the Eu(III) ion forces the s-cis geometry of BIPYPO. In contrast, the phosphorus-nitrogen interaction provides a conformational lock for the s-trans geometry of the BIPYPO ligand, inducing a quasi-seven-coordinate Eu(III) complex. The difference in coordination geometry causes the sign change of the CPL signals between the s-cis and s-trans isomers, whereby the s-cis and s-trans isomers of Eu(III) complexes exhibit the positive and negative CPL signals, respectively, for the (5) D0 →(7) F1 transition. The proportion of the s-trans-D-1 against s-cis-D-1 increases upon changing the solvent from [D3 ]acetonitrile to [D6 ]acetone, inducing a sign change of the CPL signals. The complexes D-1 and D-2 show a biexponential decay with two different lifetimes, suggesting two emitting species, that is, the s-cis and s-trans isomers of Eu(III) complexes. In both cases, the proportions of the longer lifetime components (τ1 ) decrease and instead the shorter lifetime components (τ2 ) increase upon changing the solvent from [D3 ]acetonitrile to [D6 ]acetone.

π–π* Emission from a tetrazine derivative complexed with zinc ion in aqueous solution: a unique water-soluble fluorophore

A 3,6-bis(5-amino-2-pyridyl)-1,2,4,5-tetrazine exhibits strong π-π* fluoresce in the presence of zinc ion (Zn(2+)) in aqueous solution, whereas it is not fluorescent in the absence of Zn(2+) as well as in the presence of competing metal ions.

Detection of a Radical Cation of an NADH Analogue in Two‐Electron Reduction of a Protonated p‐Quinone Derivative by an NADH Analogue

Dihydronicotinamide coenzyme (NADH) plays a vital role as a source of two electrons and a proton (equivalent to a hydride ion) in a number of biological redox processes. On the other hand, quinones (Q) act as biological electron acceptors that can undergo either oneor two-electron reductions coupled with protonation to afford the corresponding semiquinones (QHC) and hydroquinones (QH2), respectively. Two mechanisms are possible in hydride transfer from NADH and analogues to Q: one-step hydride transfer (NADH + Q!NAD + QH ) and electron transfer (ET) followed by proton/electron (or hydrogen) transfer (NADH + Q!NADHC + QC !NADC + QHC!NAD + QH ). In contrast to the one-step hydride-transfer pathway, which proceeds without an intermediate, the ET pathway would produce radical cations of NADH and its analogues as reaction intermediates. Such one-step versus multistep pathways of hydride-transfer reaction of NADH and analogues, particularly with inclusion of the effect of metal cations and acids, have been extensively studied because of the essential role of acid catalysis in the enzymatic reduction of carbonyl compounds by NADH. However, the resulting NADHC or its analogue in the ET pathway has never been detected directly in two-electron reduction of carbonyl compounds by NADH or its analogues. We report herein the successful detection of a radical cation of an NADH analogue, namely, 10-methyl-9,10dihydroacridine (AcrH2), in two-electron reduction of the protonated p-quinone derivative 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) by AcrH2. This is the first direct evidence that hydride transfer from an NADH analogue to a hydride acceptor actually proceeds via an ET pathway. AcrH2 and TolSQ were chosen as an acid-stable NADH model compound and a p-quinone derivative that can be readily protonated, respectively. This study reveals how electron transfer from AcrH2 to TolSQH + occurs in preference to direct hydride transfer from AcrH2 to TolSQH . Efficient reduction of TolSQ by AcrH2 occurs to yield AcrH and TolSQH2 in the presence of perchloric acid (HClO4) [Eq. (1)], [26] whereas no reaction occurs between AcrH2 and TolSQ in the absence of HClO4.