Specific Chiral Sensing of Amino Acids Using Induced Circularly Polarized Luminescence of Bis(diimine)dicarboxylic Acid Europium(III) Complexes

The circularly polarized luminescence (CPL) from [Eu­(pda)₂]⁻ (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [Eu­(bda)₂]⁻ (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) in aqueous solutions containing various amino acids was investigated. The europium­(III) complexes exhibited bright-red luminescence assignable to the f–f transition of the Eu^III ion when irradiated with UV light. Although the luminescence was not circularly polarized in the solid state or in aqueous solutions, in accordance with the achiral crystal structure, the complexes exhibited detectable induced CPL (iCPL) in aqueous solutions containing chiral amino acids. In the presence of l-pyrrolidonecarboxylic acid, both [Eu­(pda)₂]⁻ and [Eu­(bda)₂]⁻ showed similar iCPL intensity (<i>g</i>_lum ∼ 0.03 for the ⁵D₀ → ⁷F₁ transition at 1 mol·dm^–3 of the amino acid). On the other hand, in the presence of l-histidine or l-arginine, [Eu­(pda)₂]⁻ exhibited intense CPL (<i>g</i>_lum ∼ 0.08 for the ⁵D₀ → ⁷F₁ transition at 0.10 mol·dm^–3 of the amino acid), whereas quite weak CPL was observed for [Eu­(bda)₂]⁻ under the same conditions (<i>g</i>_lum < 0.01). On the basis of analysis of the iCPL intensities in the presence of 12 amino acids, [Eu­(pda)₂]⁻ was found to be a good chiral CPL probe with high sensitivity (about 10^–2 mol·dm^–3) and high selectivity for l-histidine at pH 3 and for l-arginine at pH 7. The mechanism of iCPL was evaluated by analysis of the fine structures in the luminescence spectra and the amino acid concentration dependence of <i>g</i>_lum. For the [Eu­(pda)₂]⁻–histidine/arginine systems, the europium­(III) complexes possess coordination structures similar to that in the crystal with slight distortion to form a chiral structure due to specific interaction with two zwitterionic amino acids. This mechanism was in stark contrast to that of the europium­(III) complex–pyrrolidonecarboxylic acid system in which one amino acid coordinates to the Eu^III ion to yield an achiral coordination structure

Real-Time Observation of Tight Au–Au Bond Formation and Relevant Coherent Motion upon Photoexcitation of [Au(CN)₂^–] Oligomers

Structural dynamics involving tight Au–Au bond formation of excited-state oligomers [Au­(CN)₂^–]_<i>n</i> was studied using picosecond/femtosecond time-resolved emission and absorption spectroscopy. With selective excitation of the trimer ([Au­(CN)₂^–]₃) in aqueous solutions, transient absorption due to the excited-state trimer was observed around 600 nm. This transient exhibited a significant intensity increase (τ = 2.1 ps) with a blue shift in the early picosecond time region. Density functional theory (DFT) and time-dependent DFT calculations revealed that the observed spectral changes can be ascribed to a structural change from a bent to a linear staggered structure in the triplet excited-state trimer. The transient absorption also exhibited a clear modulation of the peak position, reflecting coherent nuclear wave packet motion induced by photoexcitation. The frequencies of the coherent motions are 66 and 87 cm^–1, in very good accord with the frequencies of two Au–Au stretch vibrations in the excited state of the trimer calculated by DFT. Time-resolved emission spectra in the subnanosecond time region showed that association of the excited-state trimer with the ground-state monomer proceeds with τ = 2.0 ns, yielding the excited-state tetramer

Chiral Sensing Using an Achiral Europium(III) Complex by Induced Circularly Polarized Luminescence

[Eu­(bda)₂]⁻ (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) produces intense circularly polarized luminescence (CPL) in aqueous solutions in the presence of (<i>S</i>)-2-pyrrolidone-5-carboxylic acid upon UV irradiation, although the molecular structure of the europium­(III) complex is achiral. The mechanism for the induction of CPL was preliminarily attributed to distortions induced by association with an amino acid to generate chirality in the achiral complex. The optical anisotropy factor (<i>g</i>_lum value) for the ⁵D₀ → ⁷F₁ transition was 0.03 in the presence of 1.0 mol dm^–3 of the amino acid. Analysis of the CPL intensity as a function of the amino acid concentration gave an association constant between those of [Eu­(bda)₂]⁻ and the amino acid, <i>K</i>_aso = 0.55 ± 0.09 mol^–1 dm³. These results demonstrate the potential of [Eu­(bda)₂]⁻ to act as a luminescent chiral-sensing reagent in microscopic spectroscopy

Considerable Enhancement of Emission Yields of [Au(CN)₂^–] Oligomers in Aqueous Solutions by Coexisting Cations

The photophysical properties of [Au­(CN)₂^–] oligomers in aqueous solutions were investigated as functions of coexisting cations as well as the viscosity and temperature of solutions. A solution of [Au­(CN)₂^–] in the concentration range of 0.03–0.2 mol/dm³ exhibited emission peaks at 460–480 nm because of the presence of oligomers larger than trimers. Although the emission yields (ϕ_em) of K­[Au­(CN)₂] solutions were <1%, it considerably increased to 43% when 1.0 mol/dm³ tetraethylammonium chloride (Et₄NCl) was added. The lifetimes of the main emission bands were also significantly varied with additional salts, e.g., KCl, 15 ns; Et₄NCl, 520 ns. The time-resolved emission measurements of [Au­(CN)₂^–] in a water/glycerol mixture indicated that the lifetimes were almost directly proportional to the inverse of the viscosity of the solution. On the other hand, the intrinsic lifetimes of dimers and trimers with weak emission in shorter wavelength regions were very short and independent of the viscosity of the solutions and coexisting cations (dimer, ∼25 ps; trimer, ∼2 ns). These results indicated that the deactivation of the excited-state [Au­(CN)₂^–]_<i>n</i> oligomers (<i>n</i> ≥ 4) was dominated by the dissociation of the oligomers to a shorter species (dimer or trimer). The hydrophobic interactions between tetraalkylammonium cations and CN ligands remarkably stabilized the larger oligomers and suppressed the dissociation of the excited-state oligomers, which enhanced the emission yield of the oligomers. This work provides a new method of “exciplex tuning” by changing the environment of excited-state [Au­(CN)₂^–]_<i>n</i> oligomers

Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand

An NAD/NADH-functionalized ligand, benzo­[<i>b</i>]­pyrido­[3,2-<i>f</i>]­[1,7]-phenanthroline (bpp), was newly synthesized. A Ru compound containing the bpp ligand, [Ru­(bpp)­(bpy)₂]²⁺, underwent 2e^– and 2H⁺ reduction, generating the NADH form of the compound, [Ru­(bppHH)­(bpy)₂]²⁺, in response to visible light irradiation in CH₃CN/TEA/H₂O (8/1/1). The UV–vis and fluorescent spectra of both [Ru­(bpp)­(bpy)₂]²⁺ and [Ru­(bppHH)­(bpy)₂]²⁺ resembled the spectra of [Ru­(bpy)₃]²⁺. Both complexes exhibited strong emission, with quantum yields of 0.086 and 0.031, respectively; values that are much higher than those obtained from the NAD/NADH-functionalized complexes [Ru­(pbn)­(bpy)₂]²⁺ and [Ru­(pbnHH)­(bpy)₂]²⁺ (pbn = (2-(2-pyridyl)­benzo­[<i>b</i>]-1.5-naphthyridine, pbnHH = hydrogenated form of pbn). The reduction potential of the bpp ligand in [Ru­(bpp)­(bpy)₂]²⁺ (−1.28 V vs SCE) is much more negative than that of the pbn ligand in [Ru­(pbn)­(bpy)₂]²⁺ (−0.74 V), although the oxidation potentials of bppHH and pbnHH are essentially equal (0.95 V). These results indicate that the electrochemical oxidation of the dihydropyridine moiety in the NADH-type ligand was independent of the π system, including the Ru polypyridyl framework. [Ru­(bppHH)­(bpy)₂]²⁺ allowed the photoreduction of oxygen, generating H₂O₂ in 92% yield based on [Ru­(bppHH)­(bpy)₂]²⁺. H₂O₂ production took place via singlet oxygen generated by the energy transfer from excited [Ru­(bppHH)­(bpy)₂]²⁺ to triplet oxygen

Synthesis, Structures, and Luminescence Properties of Interconvertible Au^I ₂Zn^II and Au^I ₃Zn^II Complexes with Mixed Bis(diphenylphosphino)methane and d‑Penicillaminate

The reaction of the digold­(I) complex [Au₂(dppm) ­(d-pen)₂]^2– ([<b>1</b>]^2–; dppm = bis­(diphenylphosphino)­methane and d-pen = d-penicillaminate) with Zn²⁺ in a 1:1 ratio gave the heterometallic Au^I ₂Zn^II trinuclear complex [Au₂Zn­(dppm)­(d-pen)₂] ([<b>3</b>]), in which the Zn²⁺ ion is coordinated by [<b>1</b>]^2– in an N₂O₂S₂ octahedral geometry with the trans­(O) configuration, forming an 8-membered Au₂ZnS₂P₂C metalloring. A similar reaction using the newly prepared and crystallographically characterized trigold­(I) complex [Au₃(dppm)₂(d-pen)₂]⁻ ([<b>2</b>]⁻) produced the Au^I ₃Zn^II tetranuclear complex [Au₃Zn­(dppm)₂(d-pen)₂]⁺ ([<b>4</b>]⁺), in which the Zn²⁺ ion is coordinated by [<b>2</b>]⁻ in a similar octahedral geometry to form a Au₃ZnS₂P₄C₂ 12-membered metalloring. Complex [<b>3</b>] was converted to [<b>4</b>]⁺ by treatment with [Au₂(dppm)₂]²⁺ in a 2:1 ratio, whereas [<b>4</b>]⁺ reverted to [<b>3</b>] upon treatment with a mixture of [Au­(d-pen)₂]^2– and Zn²⁺ in a 1:1 ratio, indicative of the facile insertion/removal of the [Au­(dppm)]⁺ moiety with retention of the geometry of the <i>trans</i>(<i>O</i>)-[Zn­(d-pen-<i>N</i>,<i>O</i>,<i>S</i>)₂]^2– unit. An analogous interconversion that requires the insertion/removal of the [Au­(dppm)]⁺ moiety was also recognized between [<b>1</b>]^2– and [<b>2</b>]⁻. NMR spectroscopy revealed that [<b>4</b>]⁺ is in equilibrium with [<b>3</b>] and [Au₂(dppm)₂]²⁺ in solution, the ratio of which is largely dependent on the solvent polarity. The luminescence properties of these complexes were also investigated, revealing the importance of the intramolecular aurophilic interaction, as well as the Zn^II coordination, for enhancement of the emission quantum efficiencies

Preparation, Structure, and Properties of Tetranuclear Vanadium(III) and (IV) Complexes Bridged by Diphenyl Phosphate or Phosphate

Three novel tetranuclear vanadium­(III) or (IV) complexes bridged by diphenyl phosphate or phosphate were prepared and their structures characterized by X-ray crystallography. The novel complexes are [{V­(III)₂(μ-hpnbpda)}₂{μ-(C₆H₅O)₂PO₂}₂(μ-O)₂]·6CH₃OH (<b>1</b>), [{V­(III)₂(μ-tphpn)­(μ-η³-HPO₄)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·4.5H₂O (<b>2</b>), and [{(V­(IV)­O)₂(μ-tphpn)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·H₂O (<b>3</b>), where hpnbpda and tphpn are alkoxo-bridging dinucleating ligands. H₃hpnbpda represents 2-hydroxypropane-1,3-diamino-<i>N,N′</i>-bis­(2-pyridylmethyl)-<i>N,N′</i>-diacetic acid, and Htphpn represents <i>N,N,N′,N′</i>-tetrakis­(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine. A dinuclear vanadium­(IV) complex without a phosphate bridge, [(VO)₂(μ-tphpn)­(H₂O)₂]­(ClO₄)₃·2H₂O (<b>4</b>), was also prepared and structurally characterized for comparison. The vanadium­(III) center in <b>1</b> adopts a hexacoordinate structure while that in <b>2</b> adopts a heptacoordinate structure. In <b>1</b>, the two dinuclear vanadium­(III) units bridged by the alkoxo group of hpnbpda are further linked by two diphenylphosphato and two oxo groups, resulting in a dimer-of-dimers. In <b>2</b>, the two vanadium­(III) units bridged by tphpn are further bridged by three phosphate ions with two different coordination modes. Complex <b>2</b> is oxidized in aerobic solution to yield complex <b>3</b>, in which two of the three phosphate groups in <b>2</b> are substituted by oxo groups

Preparation, Structure, and Properties of Tetranuclear Vanadium(III) and (IV) Complexes Bridged by Diphenyl Phosphate or Phosphate

Three novel tetranuclear vanadium­(III) or (IV) complexes bridged by diphenyl phosphate or phosphate were prepared and their structures characterized by X-ray crystallography. The novel complexes are [{V­(III)₂(μ-hpnbpda)}₂{μ-(C₆H₅O)₂PO₂}₂(μ-O)₂]·6CH₃OH (<b>1</b>), [{V­(III)₂(μ-tphpn)­(μ-η³-HPO₄)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·4.5H₂O (<b>2</b>), and [{(V­(IV)­O)₂(μ-tphpn)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·H₂O (<b>3</b>), where hpnbpda and tphpn are alkoxo-bridging dinucleating ligands. H₃hpnbpda represents 2-hydroxypropane-1,3-diamino-<i>N,N′</i>-bis­(2-pyridylmethyl)-<i>N,N′</i>-diacetic acid, and Htphpn represents <i>N,N,N′,N′</i>-tetrakis­(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine. A dinuclear vanadium­(IV) complex without a phosphate bridge, [(VO)₂(μ-tphpn)­(H₂O)₂]­(ClO₄)₃·2H₂O (<b>4</b>), was also prepared and structurally characterized for comparison. The vanadium­(III) center in <b>1</b> adopts a hexacoordinate structure while that in <b>2</b> adopts a heptacoordinate structure. In <b>1</b>, the two dinuclear vanadium­(III) units bridged by the alkoxo group of hpnbpda are further linked by two diphenylphosphato and two oxo groups, resulting in a dimer-of-dimers. In <b>2</b>, the two vanadium­(III) units bridged by tphpn are further bridged by three phosphate ions with two different coordination modes. Complex <b>2</b> is oxidized in aerobic solution to yield complex <b>3</b>, in which two of the three phosphate groups in <b>2</b> are substituted by oxo groups

Preparation, Structure, and Properties of Tetranuclear Vanadium(III) and (IV) Complexes Bridged by Diphenyl Phosphate or Phosphate

Three novel tetranuclear vanadium­(III) or (IV) complexes bridged by diphenyl phosphate or phosphate were prepared and their structures characterized by X-ray crystallography. The novel complexes are [{V­(III)₂(μ-hpnbpda)}₂{μ-(C₆H₅O)₂PO₂}₂(μ-O)₂]·6CH₃OH (<b>1</b>), [{V­(III)₂(μ-tphpn)­(μ-η³-HPO₄)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·4.5H₂O (<b>2</b>), and [{(V­(IV)­O)₂(μ-tphpn)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·H₂O (<b>3</b>), where hpnbpda and tphpn are alkoxo-bridging dinucleating ligands. H₃hpnbpda represents 2-hydroxypropane-1,3-diamino-<i>N,N′</i>-bis­(2-pyridylmethyl)-<i>N,N′</i>-diacetic acid, and Htphpn represents <i>N,N,N′,N′</i>-tetrakis­(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine. A dinuclear vanadium­(IV) complex without a phosphate bridge, [(VO)₂(μ-tphpn)­(H₂O)₂]­(ClO₄)₃·2H₂O (<b>4</b>), was also prepared and structurally characterized for comparison. The vanadium­(III) center in <b>1</b> adopts a hexacoordinate structure while that in <b>2</b> adopts a heptacoordinate structure. In <b>1</b>, the two dinuclear vanadium­(III) units bridged by the alkoxo group of hpnbpda are further linked by two diphenylphosphato and two oxo groups, resulting in a dimer-of-dimers. In <b>2</b>, the two vanadium­(III) units bridged by tphpn are further bridged by three phosphate ions with two different coordination modes. Complex <b>2</b> is oxidized in aerobic solution to yield complex <b>3</b>, in which two of the three phosphate groups in <b>2</b> are substituted by oxo groups

Preparation, Structure, and Properties of Tetranuclear Vanadium(III) and (IV) Complexes Bridged by Diphenyl Phosphate or Phosphate

Three novel tetranuclear vanadium­(III) or (IV) complexes bridged by diphenyl phosphate or phosphate were prepared and their structures characterized by X-ray crystallography. The novel complexes are [{V­(III)₂(μ-hpnbpda)}₂{μ-(C₆H₅O)₂PO₂}₂(μ-O)₂]·6CH₃OH (<b>1</b>), [{V­(III)₂(μ-tphpn)­(μ-η³-HPO₄)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·4.5H₂O (<b>2</b>), and [{(V­(IV)­O)₂(μ-tphpn)}₂(μ-η⁴-PO₄)]­(ClO₄)₃·H₂O (<b>3</b>), where hpnbpda and tphpn are alkoxo-bridging dinucleating ligands. H₃hpnbpda represents 2-hydroxypropane-1,3-diamino-<i>N,N′</i>-bis­(2-pyridylmethyl)-<i>N,N′</i>-diacetic acid, and Htphpn represents <i>N,N,N′,N′</i>-tetrakis­(2-pyridylmethyl)-2-hydroxy-1,3-propanediamine. A dinuclear vanadium­(IV) complex without a phosphate bridge, [(VO)₂(μ-tphpn)­(H₂O)₂]­(ClO₄)₃·2H₂O (<b>4</b>), was also prepared and structurally characterized for comparison. The vanadium­(III) center in <b>1</b> adopts a hexacoordinate structure while that in <b>2</b> adopts a heptacoordinate structure. In <b>1</b>, the two dinuclear vanadium­(III) units bridged by the alkoxo group of hpnbpda are further linked by two diphenylphosphato and two oxo groups, resulting in a dimer-of-dimers. In <b>2</b>, the two vanadium­(III) units bridged by tphpn are further bridged by three phosphate ions with two different coordination modes. Complex <b>2</b> is oxidized in aerobic solution to yield complex <b>3</b>, in which two of the three phosphate groups in <b>2</b> are substituted by oxo groups