Circularly polarized phosphorescence with a large dissymmetry factor from a helical platinum(II) complex.

A chiral platinum(II) complex with a helical Schiff-base [4]helicene ligand exhibits intense red circularly polarized phosphorescence (CPP) with a glum of 0.010 in the dilute solution state. The intense CPP was caused by a change in the electronic transition character based on the induction of the helical structure.

Assembling of a Water-Soluble N

We report an unprecedented result of self-aggregation of [Pt(L1 )Cl] (HL1 =1,3-di(5-carboxy-2-pyridyl)benzene) triggered by CO2 in basic aqueous solution. The color of basic aqueous solution containing [Pt(L1 )Cl] changes from yellow to blue-green during the aggregation resulted from a reaction with CO2 in air. Upon CO2 gas bubbling, strong and broad absorption bands of aggregate assigned to the metal-metal-to-ligand charge-transfer transition appeared at 701 and 1152 nm. Recrystallization of [Pt(L1 )Cl] from Na2 CO3 aqueous solution afforded polymorphic crystals of red and blue-green forms. A single X-ray crystallography revealed that the red form of crystal consists of a Pt-Pt stacked dimer bridged by CO3 2- ion and one of the carboxy groups of L1 is deprotonated. An elemental analysis provided evidence that the blue-green crystal is constructed by linear array consisting of the [Pt(L2 )(CO3 )]3- (HL2 =1,3-di(5-carboxylate-2-pyridyl)benzene) units. The formation process of blue-green aggregate in aqueous solution was monitored through a transient absorption spectrum, and the absorption of aggregates involved in the spectral change were examined by a global analysis. A singular value decomposition and kinetic analysis provide that there are four species resulted from the self-assembling reaction in the solution and the maximal degree of aggregation is at least 32-mer.

Kinetic Formation of Pt-Pt Dimers of Cationic and Neutral Platinum(II) Complexes under Rapid Freeze Conditions in Solution.

We report the formation of M-M dimers (M = Pt or Pd) of cationic [M(dpb)(CH3CN)]+ [dpbH = 1,3-di(2-pyridyl)benzene] and neutral [M(dpb)Cl] complexes resulting from the rapid freezing of solutions. Dimers based on M-M dz2 overlap were found to preferentially form rather than the thermodynamically favored head-to-tail π-stacking structures typically observed in the crystalline state. Kinetic dimers in glassy frozen solutions generated broad metal-metal-to-ligand charge-transfer emissions within the range of 600-800 nm at 77 K. These emissions were red-shifted relative to monomer emissions. As expected, the degree of aggregation of these complexes was affected by the concentration in each solution. Photoexcitation evidently accelerated Pt-Pt dimerization even at ambient temperature. Electrostatic attraction between [Pt(dpb)Cl]+ and [Pt(dpb)Cl]- ions resulting from disproportionation due to photoinduced electron transfer is thought to have driven excimer formation. [Pt(dpb)(CH3CN)]OTf (OTf- = trifluoromethanesulfonate ion) and its Pd(II) analogue were determined to have isostructural crystals, but a Pd-Pd stacked polymorph was not observed and the photophysics of the two complexes are evidently different.

Luminescence color change of [3,4-difluoro-2,6-bis(5-methyl-2-pyridyl)phenyl-κ3N,C1,N']cyanidoplatinum(II) by aggregation.

We have investigated the color and luminescence color changes of novel Pt(L)CN (L = 4,6-difluoro-1,3-di(2-(4-methyl)pyridyl)benzene) in solution and crystalline states that resulted from aggregation-induced emission (AIE). In the solution state, the AIE results from excimer and trimer formation in the excited states at high concentrations. We determined the emission lifetimes of the excimer and trimer to be τE = 1.72 µs and τT = 0.43 µs, respectively, and the emission quantum yields to be ϕE = 20% and ϕT = 12%, respectively, which are slightly smaller yet comparable to τM = 8.85 µs and ϕM = 67% of the monomeric species. In the crystalline state, the purple color of Pt(L)CN with no solvent of crystallization changes to red upon exposure to chloroform vapor, and the invisible emission turns to bright red emission. This phenomenon can be applied to inexpensive devices for the fast chloroform detection. The exposure of purple crystals to dichloromethane vapor causes a further redshift of the invisible emission and blue coloration, which suggests the capability of the discrimination of chloroform from dichloromethane.

Vapochromism of an iridium(III) bis-terpyridine complex based on the modulation of halide-to-ligand charge transfer transition.

We report, for the first time, a color change originating from the shift of the halide-to-ligand charge transfer (XLCT) band of the Ir(III) bis-terpyridine complex crystal in response to the sorption/desorption of water of crystallization. Red and orange coloration reversibly takes place by heat and cool treatments, respectively. Single X-ray crystallography shows that the Ir(III) complex possesses two waters of crystallization constructing a dimer structure, rO-O = 2.911 Å, by hydrogen bonding. It was found that the water dimer connects to one of the iodide ions with rO-I = 3.664 Å by hydrogen bonding and comes into contact with another iodide ion with rO-I = 3.747 Å, suggesting that water desorption from the crystal easily changes the XLCT transition arising from the interaction between the iodide ion 5p orbital and tpy π* orbital. Thermogravimetry measurement reveals the stepwise water desorption from the crystal, and powder X-ray diffraction shows the robustness of the Ir(III) complex crystal framework during the sorption/desorption cycles. Nitrogen gas flow or presence of a polar organic solvent contributes to the red-shift of the XLCT absorption band due to the desorption of water molecules resulted by the shift of equilibrium between water molecules in the crystal and vaporized water molecules. Exposure to ammonia vapor from 25% aqueous ammonia is found not to contribute to the color change of the Ir(III) complex crystal.

Reversible colour/luminescence colour changes of tetracyanoruthenium(II) complexes by sorption/desorption of water molecules in crystals.

We report colour/luminescence colour changes of M[Ru(bpy)(CN)4] crystal (M2+ = Ca2+, Sr2+, and Ba2+; bpy = 2,2'-bipyridine). The X-ray crystallographic study revealed that the crystals are constructed from linear-chains of {[Ru(bpy)(CN)4][Ca(H2O)5]}n, {[Ru(bpy)(CN)4][Sr(H2O)6]}n, and {[Ru(bpy)(CN)4]2[Ba(H2O)5]2(µ-H2O)2}n, respectively. Ru(II) complex linear chains and the hydrophilic channels composed of M2+ ion and water along them enable reversible water sorption/desorption without collapse of crystals responsible for the colour change. The emission spectra of Ca2+ and Sr2+ salts are remarkably shifted to the red side when the temperature was increased from 296 to 500 K, while Ba2+ salt shows a slight shift in the emission spectrum during the heating. The change in the interaction of M2+ ion to the equatorial CN ligand depending on the number of hydrated water molecules effectively contributes to the luminescence colour change for Ca2+ and Sr2+ salts. FT-IR spectra after heating at 473 K show the high-frequency shifts in the CN stretching mode for Sr2+ salt, while no remarkable peak shifts are observed for Ca2+ and Ba2+ salts. Thermogravimetry results indicate that heating over 470 K leads to the desorption of 5H2O from all salts, resulting in {[Ru(bpy)(CN)4][Ca]}n, {[Ru(bpy)(CN)4][Sr(H2O)]}n, and {[Ru(bpy)(CN)4]2[Ba]2(µ-H2O)2}n for linear chains. The change in the hydration structure for M2+ ions regulates the shift of CN stretching modes.

Emission Intensity Enhancement for Iridium(III) Complex in Dimethyl Sulfoxide under Photoirradiation.

We found emission intensity enhancement for fac-Ir(ppy)3 (ppy = 2-(2'-phenyl)pyridine) in aerated dimethyl sulfoxide (DMSO) during photoirradiation for the first time. This phenomenon was concluded to be responsible for the consumption of 3O2 dissolved in DMSO through dimethyl sulfone production by photosensitized reaction using fac-Ir(ppy)3. A 3O2 adduct of DMSO molecule was detected by UV absorption measurement and theoretical calculation. We proposed a mechanism for the emission enhancement reaction including 1,3O2 molecules and 1,3O2-DMSO adducts and validated it through a simulation of emission intensity change using an ordinary differential equation solver.

Vortex-Induced Harmonic Light Scattering of Porphyrin J-Aggregates.

An understanding of macroscopic vortex-induced chirality can provide insights into the origin of the homochirality of life. While circular dichroism measurements in stirred solutions are useful for the analysis of chiral supramolecular structures induced by vortex motion, there are no reports on the application of other spectroscopic methods. To obtain a deeper understanding of macroscopic vortex-induced chirality, it is essential to develop novel in situ spectroscopic methods that provide information about changes in both the size and chirality in stirred solutions. Here, we report the first observation by harmonic light scattering of the mirror-symmetry-breaking process of porphyrin J-aggregates under the rotation of a magnetic stirrer. The chiral supramolecular structure observed during stirring is likely due to the formation of a chiral aggregate that consists of porphyrin J-aggregates. The dissociation of the structure proceeds in two steps (a fast step and a slow step), as indicated by the signal decay rate when stirring was stopped. This novel method is useful for analyzing the supramolecular structural changes of chiral aggregates induced by external stimuli.

Dual emission from an iridium(III) complex/counter anion ion pair.

[Ir(tpy)2](PF6)3 (tpy = 2,2':6',2''-terpyridine) dissolved in CH3CN was found to exhibit dual color luminescent emission depending on the excitation wavelength. Specifically, blue and green emissions were obtained with excitation at 350 and 410 nm, respectively. Because the associated emission spectra were consistent with those of [Ir(tpy)2]Cl3 in water and [Ir(tpy)2](PF6)3 in the crystalline state, respectively, this dual emission is attributed to emissions from the [Ir(tpy)2]3+ cation and its ion pair [Ir(tpy)2]3+·PF6-. The emission is assigned to the 3π-π* transition of the ligands based on time-dependent density functional theory (TD-DFT) calculations. Conversely, [Ir(tpy)2]I3 in CH3CN shows emission due to [Ir(tpy)2]3+ but not [Ir(tpy)2]3+·I-, while crystalline [Ir(tpy)2]I3 emits red luminescence at 77 K that is inconsistent with that from [Ir(tpy)2]3+. Since the emission energies of crystalline [Ir(tpy)2]X3 (X- = Cl-, Br- or I-) show a good correlation with the electron affinity of X, the emissions are assigned to a counter anion to complex ion charge-transfer transition. This hypothesis is supported by TD-DFT calculations regarding [Ir(tpy)2]3+·X-.

Discovery of Amorphous Iron Hydrides via Novel Quiescent Reaction in Aqueous Solution.

Novel amorphous iron hydrides (AIHs) are synthesized for the first time under ambient conditions by employing novel "quiescent reaction", without stirring for mixing solutions, during a conventional aqueous reduction-precipitation process. The kind and morphology of AIHs are dependent on the processing condition, where two types are found, with one form consisting of a tangle of uniform nanowires and the other being granular in nature. Both AIHs undergo transformation to crystalline α-Fe by heat treatment at 600 °C. The nanowire AIH exhibits the hydrogen content of 0.10 wt%, while the granular AIH of 0.22 wt%. Their magnetic and thermal properties are accordingly different, and the non-diffusive hydrogen contributes to stability of AIHs. It is strongly suggested that, by use of quiescent reaction, iron-hydrogen clusters are formed and preserved at an early stage of precipitation reaction, and subsequently aggregated into novel AIHs, preventing α-Fe crystallization. Hence, the AIHs would be categorized as metastable hydrides stabilized with iron-hydrogen clusters. In addition, newly discovered quiescent reaction in aqueous solution, from which unprecedented AIHs are derived, sheds new light on fundamental and essential aqueous reaction.

H/D solvent isotope effects on the photoracemization reaction of enantiomeric the tris(2,2'-bipyridine)ruthenium(ii) complex and its analogues.

This work assessed solvent isotope effects on the photoracemization rate and emission lifetime for [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) in water. An analysis of the effects of temperature on photoracemization rate and emission lifetime demonstrated that the transition from one enantiomer to the other is unaffected by the isotopic composition of the solvent. The results also showed that deactivation from the metal-to-ligand charge-transfer (3MLCT) excited state to the ground state is responsible for the solvent isotope effect on the photoracemization rate. The photoracemization reaction was found to proceed via a bond-breaking mechanism. In this process, a five-coordinated species produced through breaking of the Ru-N bond in the 3d-d* state undergoes a structural change to produce an achiral five-coordinated species. An analysis of the effect of temperature on emission lifetime, excluding the activation to the 3d-d* state that leads to the structural change, showed that the solvent isotopic composition affects deactivation from the 4th MLCT state.

Highly Selective Aluminum(III) Ion Sensing with Luminescent Iridium(III) Complexes Bearing a Distorted 2,2'-Bipyridine-3,3'-diol Moiety Utilizing a Rigidified Seven-Membered Chelate Ring.

To create an ion sensor utilizing a rigidified seven-membered chelate ring, we developed two Ir(III) complexes with 2,2'-bipyridine-3,3'-diol (bpy(OH)2, bpydL) ligands as reaction centers, namely Ir1 ([Ir(ppy)2{bpy(O-)(OH)}], ppy = 2-phenylpyridine) and Ir2 ([Ir(bzq)2{bpy(O-)(OH)}], bzq = benzo[h]quinoline), and evaluated their reactivities toward metal ions by spectrophotometry. When they are reacted with Al3+, these complexes exhibit dramatic enhancements in emission intensity (775-fold for Ir1 and 51.0-fold for Ir2) and distinct orange to green changes in emission color. The reactions of Ir1 and Ir2 with Al3+ were found to barely be affected by nearly all common metal ions. We conclude that these high selectivities arise from the high affinities of the (O,O) atoms in bpydL for hard metal ions and the increased strain of the seven-membered chelate ring due to the coordination of bpydL to the Ir(III) center in each complex, which excludes large metal ions out of the chelate ring.

A stochastic analysis based on a one-dimensional random walk model of the persistent phosphorescence of Mn2+ ions doped in zinc magnesium phosphate.

We investigate the mechanism of the persistent phosphorescence (PP) of ß-Zn3(PO4)2:Mn2+ and γ-(Zn2+,Mg2+)3(PO4)2:Mn2+ systems via stochastic analysis. An electron hopping mechanism, recombining electrons with Mn3+ ions, is proposed to elucidate the long-tailed decay feature that represents a non-exponential decay curve for the PP intensity change over time. A simulation using a one-dimensional random walk for the electron hopping in the host material successfully reproduces the PP decay curve. In the simulation, electrons, as charge carriers, are obliged to move on by a number of linearly ordered traps, to step forward or backward to the next trap with a probability of 1/2, and to keep hopping until reaching the emitting center. Furthermore, we present simulations of PP results depending on the temperature, Mn2+ ion concentration in ß-Zn3(PO4)2:Mn2+, and crystal lattice size in γ-(Zn2+,Mg2+)3(PO4)2:Mn2+. These results allow us to know that the length of the trap array can control the features of the PP time profile. Consequently, we conclude that a large number of patterns for electron hopping that recombine electrons with Mn3+ ions are responsible for the long-tailed PP decay curve.

Optical Resolution, Determination of Absolute Configuration, and Photoracemization of cis-RuL2(CN)2 (L = 2,2'-Bipyridine and Its Analogues).

We synthesized neutral Ru(II) complexes cis-Ru(bpy)2(CN)2 (bpy = 2,2'-bipyridine), cis-Ru(dmb)2(CN)2 (dmb = 4,4'-dimethyl-2,2'-bipyridine), cis-Ru(dbb)2(CN)2 (dbb = 4,4'-di-tert-butyl-2,2'-bipyridine), and cis-Ru(phen)2(CN)2 (phen = 1,10-phenanthroline) and optically resolved them into respective enantiomers using high-performance liquid chromatography with a chiral column. The absolute configuration of enantiomer of cis-Ru(dbb)2(CN)2 was determined by an X-ray crystallography. Upon photoirradiation, the entire enantiomers of the complexes underwent the racemization with considerably slow rates (k = 1 × 10(-6) to 1 × 10(-5) s(-1)) and small quantum yields (ϕ = 1 × 10(-6) to 1 × 10(-5)). The photoracemization was concluded to proceed via a five-coordinate pyramidal intermediate with the base plane composed of Ru, bidentate polypyridine, and two cyanides and the axial ligand of monodentate polypyridine. We derived the equations for photoracemization rate and quantum yield by a kinetics analysis of the photoracemization reaction that depended on polypyridine ligand, solvent, temperature, wavelength and intensity of irradiation light, and emission lifetime. From the temperature-dependent photoracemization reaction, the energy gap between (3)MLCT (metal-to-ligand charge transfer) and (3)d-d* states was estimated as ΔE = 4000-5000 cm(-1), and the energy of invisible (3)d-d* state was estimated to be ca. 20 500 cm(-1), which was in good agreement with that of [Ru(bpy)3](2+).

Circularly Polarized Luminescence of Chiral Pt(pppb)Cl (pppbH=1-pyridyl-3-(4,5-pinenopyridyl)benzene) Aggregate in the Excited State.

We prepared enantiomers of chiral Pt(II) complexes, Pt(pppb)Cl and Pt(pppb)CN (pppbH=1-pyridyl-3-(4,5-pinenopyridyl)benzene), and measured their CPL (circularly polarized luminescence) spectra for excimer and trimer emission. The contribution of the pinene moiety to CPL was considerably low for the π-π* emission of the monomer but large for MMLCT (metal-metal-to-ligand charge-transfer) of the excimer and trimer which had a helical structure induced in a face-to-face stacking fashion. The trimer CPL for (+)-Pt(pppb)Cl was larger in intensity than that of excimer CPL; on the other hand, that for (+)-Pt(pppb)CN was opposite in sign compared with that of excimer CPL. We conclude that differences in the excited-state structure of the aggregate between Pt(pppb)Cl and Pt(pppb)CN account for the variation in the CPL spectra. By the aid of TD-DFT calculations it was predicted that the dihedral angle θ(Cl-Pt-Pt-Cl) was 50-60° or 110-140° for Pt(pppb)Cl aggregates and 160° for Pt(pppb)CN aggregates.

Intermolecular interactions and aggregation of fac-tris(2-phenylpyridinato-C2,N)iridium(III) in nonpolar solvents.

The intermolecular interaction and aggregation of the neutral complex fac-tris(2-phenylpyridinato-C(2),N)iridium(III) (fac-Ir(ppy)3) in solution was investigated. Intermolecular interactions were found to effectively decrease the luminescence lifetime via self-quenching with increasing fac-Ir(ppy)3 concentrations. A Stern-Volmer plot for quenching in acetonitrile was linear, due to bimolecular self-quenching, but curved in toluene as the result of excimer formation. (1)H NMR spectra demonstrated a monomer-aggregate equilibrium which resulted in spectral shifts depending on solvent polarity. X-ray crystallography provided structural information concerning the aggregate, which is based on a tetramer consisting of two Δ-fac-Ir(ppy)3-Λ-fac-Ir(ppy)3 pairs. Offset π-π stacking of ppy ligands and electrostatic dipole-dipole interactions between complex molecules play an important role in the formation of these molecular pairs.

Luminescence change by the solvent of crystallization, solvent reorganization, and vapochromism of neutral dicyanoruthenium(II) complex in the solid state.

The "solvent effect" on the solid-state luminescence of a neutral complex, [Ru(dbb)(2)(CN)(2)] (dbb = 4,4'-di-tert-butyl-2,2'-bipyridine), was presented. The crystals of this complex showed a variety of luminescence color from orange to dark-red, depending on the acceptor number of the solvent included in the crystal as a solvent of crystallization. The luminescence change was very similar to the solvatochromism in solution, which was attributed to the local donor-acceptor interaction between the CN group and the solvent molecules. The dynamic shift observed in the transient emission spectrum of the crystalline powder was accounted for by the solvent molecule reorganization. X-ray crystallography of [Ru(dbb)(2)(CN)(2)].3(CH(3))(2)CO showed the complex molecule having an approximate C(2) symmetry and very weak interactions between the acetone molecules and the CN groups. A three-dimensional network constructed by acetone molecules was observed in the hydrophobic space consisting of t-butyl groups in dbb ligands. A thin film of the complex showed vapochromic behavior such that the luminescence changed depending on the solvent of crystallization. This suggests a capability for organic molecule discrimination using the complex in the solid state.

Luminescence color change of a platinum(II) complex solid upon mechanical grinding.

The mechanochemical behavior of Pt(5dpb)Cl (5dpbH = 1,3-di(5-methyl-2-pyridyl)benzene) was investigated in terms of solid-state luminescence. The yellow luminescence of the crystalline complex changed to orange when grinding into fine powder on a glass substrate with a spatula. A broad emission band, which was not detected for the crystal, was observed at around 670 nm for the powder. The powder X-ray diffraction (XRD) pattern was the same as that calculated from X-ray crystallographic data of the single crystal. A broad band appeared within 100 ns after laser excitation accompanied by quenching of the s(pi,pi*) emission of Pt(5dpb)Cl, which was then weakened with decreasing temperature and disappeared below 120 K. The phenomenon was very similar to the excimer formation observed in solution. A related complex, Pt(dpb)Cl (dpbH = 1,3-di(2-pyridyl)benzene), also exhibited luminescent mechanochromism. However, the broad emission that appeared upon grinding still remained at 77 K, and XRD showed that the ground sample of Pt(dpb)Cl was amorphous.

[2,6-Bis(5-methyl-2-pyridyl)phenyl-kappa(3)N,C(1),N']chloridoplatinum(II).

In the title compound, [Pt(C(18)H(15)N(2))Cl], the Pt(II) centre adopts a distorted square-planar coordination geometry due to the pincer-type monoanionic N-C-N tridentate ligand. The planar complexes stack via pi-pi interactions to form two-dimensional accumulated sheets. This packing pattern is in contrast to that in related pincer-type N-C-N complexes, which exhibit a one-dimensional columnar stacking.

Quantitative analysis of long-range interactions between adsorbed dipolar molecules on Cu(111).

Highly dispersed superstructures of a dipolar iridium complex are formed on a Cu(111) surface. We show that the dilute superstructures with density-controlled intermolecular separations are stabilized by the strong and long-range repulsive intermolecular interactions. The repulsive intermolecular interactions are quantitatively evaluated by using low-temperature scanning tunneling microscopy, which are characterized by the surface-enhanced dipole-dipole interactions.