Ultrafast Raman observation of the perpendicular intermediate phantom state of stilbene photoisomerization.

Trans-cis photoisomerization is generally described by a model in which the reaction proceeds via a common intermediate having a perpendicular conformation around the rotating bond, irrespective of from which isomer the reaction starts. Nevertheless, such an intermediate has yet to be identified unambiguously, and it is often called the 'phantom' state. Here we present the structural identification of the common, perpendicular intermediate of stilbene photoisomerization using ultrafast Raman spectroscopy. Our results reveal ultrafast birth and decay of an identical, short-lived transient that exhibits a vibrational signature characteristic of the perpendicular state upon photoexcitation of the trans and cis forms. In combination with ab initio molecular dynamics simulations, it is shown that the photoexcited trans and cis forms are funnelled off to the ground state through the same, perpendicular intermediate.

Near-white light emission from single crystals of cationic dinuclear gold(I) complexes with bridged diphosphine ligands.

Three cationic dinuclear Au(I) complexes containing acetonitrile (AN) as an ancillary ligand were synthesized: [µ-LMe(AuAN)2]·2BF4 (1), [µ-LEt(AuAN)2]·2BF4 (2), and [µ-LiPr(AuAN)2]·2BF4 (3) (LMe = {1,2-bis[bis(2-methylphenyl)phosphino]benzene}, LEt = {1,2-bis[bis(2-ethylphenyl)phosphino]benzene}, and LiPr = {1,2-bis[bis(2-isopropylphenyl)phosphino]benzene}). The unique structures of complexes 1-3 with two P-Au(I)-AN rods bridged by rigid diphosphine ligands were determined through X-ray analysis. The Au(I)-Au(I) distances observed for complexes 1-3 were as short as 2.9804-3.0457 Å, indicating an aurophilic interaction between two Au(I) atoms. Unlike complexes 2 and 3, complex 1 incorporated CH2Cl2 into the crystals as crystalline solvent molecules. Luminescence studies in the crystalline state revealed that complexes 1 and 2 mainly exhibited bluish-purple phosphorescence (PH) at 293 K: the former had a PH peak wavelength at 415 nm with the photoluminescence quantum yield ΦPL = 0.12, and the latter at 430 nm with ΦPL = 0.13. Meanwhile, complex 3 displayed near-white PH, that is dual PH with two PH bands centered at 425 and 580 nm with ΦPL = 0.44. The PH spectra and lifetimes of complexes 2 and 3 were measured in the temperature range of 77-293 K. The two PH bands observed for complex 3 were suggested to originate from the two emissive excited triplet states, which were in thermal equilibrium. From theoretical calculations, the dual PH observed for complex 3 is explained to occur from the two excited triplet states, T1H and T1L: the former exhibits a high-energy PH band (bluish-purple) and the latter exhibits a low-energy PH band (orange). The T1H state is considered 3ILCT with a structure similar to that of the S0-optimized structure. Conversely, the T1L state is assumed to be a 3MLCT with a T1-optimized structure, which has a short Au(I)-Au(I) bond and two bent rods (Au-AN). The thermal equilibrium between the two excited states is discussed based on computational calculations and photophysical data in the temperature range of 77-293 K. With regard to the crystal of complex 1, we were unable to precisely measure the temperature-dependent emission spectra and lifetimes, particularly at low temperatures, because the cooled crystals became irreversibly turbid over time.

Ultrafast dynamics of an azobenzene-containing molecular shuttle based on a rotaxane.

An ultrafast spectroscopic study was carried out for a photoisomerizable, rotaxane-based molecular shuttle, in which photoisomerization of the azobenzene moiety of the thread-like guest drives a shuttling motion of a cyclodextrin host. Femtosecond upconversion and time-resolved absorption measurements revealed distinct S1 dynamics with time constants of 1.2 and 17 ps. Both time constants are smaller when the cyclodextrin host is absent, implying that, within the S1 state, there are mutiple barriers to the isomerization and subsequent shuttling, due to steric interference from the cyclodextrin.

Photoluminescent properties and molecular structures of dinuclear gold(I) complexes with bridged diphosphine ligands: near-unity phosphorescence from 3XMMCT/3MC.

The gold(i) complexes [µ-LiPr(AuX)2] {X = Cl (1D), Br (2D), and I (3D); LiPr = 1,2-bis[bis(2-isopropylphenyl)phosphino]benzene} were synthesised to investigate the photoluminescence properties of dinuclear Au complexes comprising weak Au(i)-Au(i) bonds. Single crystals of the tetrahydrofuran (THF) adducts 1DOR, 2DOR, and 3DOR were obtained by recrystallisation of 1D, 2D, and 3D from a mixed solution of THF and n-hexane. These THF adducts afford orange emission, with peak wavelengths ranging from 597 to 630 nm, in the crystalline state at 293 K. Recrystallisation of 3D from a mixed solution of acetone and n-hexane afforded single crystals of the acetone adduct 3DGR, which exhibits blue-green emission at 293 K. No crystals of the acetone adduct were obtained from 1D and 2D. The emission spectra and lifetimes of 1DOR, 2DOR, 3DOR, and 3DGR measured in the temperature range 77-293 K indicate that emission from these complexes in the solid state is due to phosphorescence. Notably, although the molecular structure of 3D in the 3DOR crystal is near-similar to that of 3DGR, the phosphorescence spectrum of 3DOR differs markedly from that of 3DGR, with peak wavelengths at 597 and 506 nm, respectively. Theoretical studies revealed that (1) phosphorescence occurs via the electronic transition from the excited triplet state, which is mainly composed of halogen-to-metal-metal charge transfer and metal-centered transitions and (2) the T1-optimised structure of 3D in the 3DGR crystals differs markedly from that in 3DOR, and the differences in the phosphorescence colour observed between 3DGR and 3DOR are ascribed to the differences in their stabilised structures in the excited triplet state.

Luminescence color alteration induced by trapped solvent molecules in crystals of tetrahedral gold(i) complexes: near-unity luminescence mixed with thermally activated delayed fluorescence and phosphorescence.

The tetrahedral gold(i) complex [Au(LtBu)2]Cl 1Cl [LtBu = 1,2-bis[bis(4-tert-butylphenyl)phosphino]benzene], having eight tert-butyl (tBu) groups, was synthesized and characterized. Emission color alteration caused by solvent molecules captured in the crystal lattice of 1Cl was investigated. The recrystallization of 1Cl in mixed solvents of tetrahydrofuran, THF, and various alkanes afforded two single crystals: 1Cl·THF (1BG), presenting intense blue-green luminescence with an emission peak wavelength, λmax, = 507 nm and 1Cl·THF·0.5n-hexane·H2O (1OR), exhibiting weak orange luminescence with λmax = 625 nm. The emission quantum yields, ΦPL, of the two complexes were 0.95 and 0.06, respectively. X-ray structural analysis revealed that all the solvent molecules captured in the crystal were located inside the spaces surrounded by the tBu groups of the 1+ cation. Further, an n-hexane molecule in 1OR was found to be sandwiched between two 1+ cations via CH/π interactions. The structure of 1Cl in 1OR could be distorted by the n-hexane molecule incorporated in the crystal, leading to the red-shift of the emission peak wavelength and the low quantum efficiency of 1OR over that of 1BG. On the other hand, [Au(L)2]Cl [L = 1,2-bis(diphenylphosphino)benzene], having no peripheral tBu groups, was unable to incorporate the solvent molecules in the crystal by recrystallization from the mixed solvents, and thus, the crystal solely yielded intense blue light emission. The results revealed that the tBu groups of the peripheral phenyl units are essential for the luminescence color alteration caused by the incorporation of organic solvent molecules into the crystal lattice. Finally, studies on the emission spectra and quantum yields in the temperature range 77-293 K revealed that luminescence from both 1BG and 1OR was composed of phosphorescence and thermally activated delayed fluorescence.

Near-unity thermally activated delayed fluorescence efficiency in three- and four-coordinate Au(i) complexes with diphosphine ligands.

The synthesis and photoluminescence properties of three-coordinate Au(i) complexes with rigid diphosphine ligands LMe {1,2-bis[bis(2-methylphenyl)phosphino]benzene}, LEt {1,2-bis[bis(2-ethylphenyl)phosphino]benzene}, and LiPr {1,2-bis[bis(2-isopropylphenyl)phosphino]benzene} are investigated. The LMe and LEt ligands afford two types of complexes: dinuclear complexes [µ-LMe(AuCl)2] (1d) and [µ-LEt(AuCl)2] (2d) with an Au(i)-Au(i) bond and mononuclear three-coordinate Au(i) complexes LMeAuCl (1) and LEtAuCl (2). On the other hand, the bulkiest ligand, LiPr, affords three-coordinate Au(i) complexes, LiPrAuCl (3) and LiPrAuI (4), but no dinuclear complexes. X-ray analysis suggests that both 3 and 4 possess a highly distorted trigonal planar geometry. Moreover, luminescence data reveal that at room temperature, 3 and 4 exhibit yellow-green thermally activated delayed fluorescence in the crystalline state with maximum emission wavelengths at 558 and 549 nm, respectively. The emission yields are close to unity. Quantum chemical calculations suggest that the emission of 4 originates from the (σ + X) → π* excited state that possesses strong intraligand charge-transfer character. The luminescent properties of four-coordinate Au(i) complex (5) possessing a tetrahedral geometry are discussed on the basis of the emission spectra and decay times measured in a temperature range of 309-77 K.

Photoluminescence properties of TADF-emitting three-coordinate silver(i) halide complexes with diphosphine ligands: a comparison study with copper(i) complexes.

Synthesis and X-ray structures of silver(i) bromide complexes with diphosphine ligands LMe, LEt, and LiPr are described, where LMe = 1,2-bis[bis(2-methylphenyl)phosphino]benzene, LEt = 1,2-bis[bis(2-ethylphenyl)phosphino]benzene, and LiPr = 1,2-bis[bis(2-isopropylphenyl)phosphino]benzene. Crystals of complex [(LMe)AgBr]2 (1), prepared from LMe and AgBr, showed a tetrahedral bimetallic structure. LEt and LiPr, with bulkier substituents than those of LMe, reacted with AgBr to give crystalline three-coordinate complexes (LEt)AgBr (2) and (LiPr)AgBr (3). Nuclear magnetic resonance (NMR) studies demonstrated that 1 dissociates in solution to yield a monomeric three-coordinate complex (LMe)AgBr. Luminescence studies showed that complexes 1-3 exhibit efficient blue thermally activated delayed fluorescence (TADF) in both solid state and solution. Density functional theory (DFT)/Time-dependent (TD)-DFT calculations revealed that the electronic transition responsible for TADF corresponds to (σ + Br) → π*. The photophysical properties of silver complexes 1-3 are discussed in detail and compared to those of the copper complex (LMe)CuBr.

Application of three-coordinate copper(I) complexes with halide ligands in organic light-emitting diodes that exhibit delayed fluorescence.

A series of three-coordinate copper(I) complexes (L(Me))CuX [X = Cl (1), Br (2), I (3)], (L(Et))CuBr (4), and (L(iPr))CuBr (5) [L(Me) = 1,2-bis[bis(2-methylphenyl)phosphino]benzene, L(Et) = 1,2-bis[bis(2-ethylphenyl)phosphino]benzene, and L(iPr) = 1,2-bis[bis(2-isopropylphenyl)phosphino]benzene] exhibit efficient blue-green emission in the solid state at ambient temperature with peak wavelengths between 473 and 517 nm. The emission quantum yields were 0.38-0.95. The emission lifetimes were measured in the temperature range of 77-295 K using a nanosecond laser technique. The temperature dependence of the emission lifetimes was explained using a model with two excited states: a singlet and a triplet state. The small energy gaps (<830 cm(-1)) between the two states suggest that efficient emission from 1-5 was thermally activated delayed fluorescence (TADF). Alkyl substituents at ortho positions of peripheral phenyl groups were found to have little effect on the electronic excited states. Because the origin of the emission of complexes 2, 4, and 5 was thought to be a (σ + Br)→π* transition, photoluminescence characteristics of these complexes were dominated by the diphosphine ligands. Complexes 2, 4, and 5 had similar emission properties. Complexes 1-5 had efficient green TADF in amorphous films at 293 K with maximum emission wavelengths of 508-520 nm and quantum yields of 0.61-0.71. Organic light-emitting devices that contained complexes 1-5 and exhibited TADF exhibit bright green luminescence with current efficiencies of 55.6-69.4 cd A(-1) and maximum external quantum efficiencies of 18.6-22.5%.

Highly efficient blue-green delayed fluorescence from copper(I) thiolate complexes: luminescence color alteration by orientation change of the aryl ring.

Highly emissive three-coordinate thiolate copper(I) complexes are synthesized and characterized. The Cu(I) complexes emit intense blue-green delayed fluorescence with high photoluminescence quantum yields of approximately 1.0 at both 293 K and 77 K in the solid state, whereas orange emission at 293 K in solution is observed.

Photoluminescence properties, molecular structures, and theoretical study of heteroleptic silver(I) complexes containing diphosphine ligands.

The homoleptic complex [Ag(L)(2)]PF(6) (1) and heteroleptic complexes [Ag(L)(L(Me))]BF(4) (2) and [Ag(L)(L(Et))]BF(4) (3) [L = 1,2-bis(diphenylphosphino)benzene, L(Me) = 1,2-bis[bis(2-methylphenyl)phosphino]benzene, and L(Et) = 1,2-bis[bis(2-ethylphenyl)phosphino]benzene] were synthesized and characterized. X-ray crystallography demonstrated that 1-3 possess tetrahedral structures. Photophysical studies and time-dependent density functional theory calculations of 1-3 revealed that alkyl substituents at the ortho positions of peripheral phenyl groups in the diphosphine ligands have a significant influence on the energy and intensity of phosphorescence of the complex in solution at room temperature. The results can be interpreted in terms of the geometric preferences of each complex in the ground and excited states. The homoleptic complex 1 exhibits weak orange phosphorescence in solution arising from its flat structure in the triplet state, while heteroleptic complexes 2 and 3 show strong green phosphorescence from triplet states with tetrahedral structure. Larger interligand steric interactions in 2 and 3 caused by their bulkier ligands probably inhibit geometric relaxation within the excited-state lifetimes, leading to higher energy phosphorescence than that observed for 1. NMR experiments revealed that 2 and 3 in solution possess structures that are much more immobilized than that of 1; fluxional motion is completely suppressed in 2 and 3. Accordingly, conformational changes of 2 and 3 are expected to be suppressed by the alkyl substituents not only in the ground state but also in excited states. Consequently, nonradiative decay of the excited states of 2 and 3 occurs less efficiently than in 1. As a result, the quantum yields of phosphorescence for 2 and 3 are 6 times larger than that for the homoleptic complex 1.

Highly efficient green organic light-emitting diodes containing luminescent three-coordinate copper(I) complexes.

A series of highly emissive three-coordinate copper(I) complexes, (dtpb)Cu(I)X [X = Cl (1), Br (2), I (3); dtpb =1,2-bis(o-ditolylphosphino)benzene], were synthesized and investigated in prototype organic light-emitting diodes (OLEDs). 1-3 showed excellent photoluminescent performance in both degassed dichloromethane solutions [quantum yield (Φ) = 0.43-0.60; lifetime (τ) = 4.9-6.5 µs] and amorphous films (Φ = 0.57-0.71; τ = 3.2-6.1 µs). Conventional OLEDs containing 2 in the emitting layer exhibited bright green luminescence with a current efficiency of 65.3 cd/A and a maximum external quantum efficiency of 21.3%.

Vapochromic and mechanochromic tetrahedral gold(I) complexes based on the 1,2-bis(diphenylphosphino)benzene ligand.

Tetrahedral gold(I) complexes containing the diphosphane ligand (dppb=1,2-bis(diphenylphosphino)benzene), [Au(dppb)(2)]X [X=Cl (1), Br (2), I (3), NO(3) (4), BF(4) (5), PF(6) (6), B(C(6)H(4)F-4)(4) (7)], and the ethanol and methanol adducts of complex 4, 8, and 9, were prepared to analyze their unique photophysical properties. These complexes are classified into two categories on the basis of their crystal structures. In Category I, the complexes (1-5) have relatively-small counter anions and two dppb ligands are symmetrically coordinated to the central Au(I) atom, and display an intense blue phosphorescence. Alternatively, the complexes (6-9) in Category II have large counter anions and two dppb ligands asymmetrically coordinated to Au(I) atom, and display a yellow or yellow orange phosphorescence. The difference in the phosphorescence color of the complexes between the Category I and II is ascribed to the change in the structure of the cationic moiety in the complex. According to DFT calculations, the symmetry reduction caused by the large counter anion of the complex in Category II gives the destabilization of HOMO (σ*) levels, leading to the red-shift of the emission peak. We have demonstrated that the symmetry reductions are responsible for the phosphorescence color alteration caused by external stimuli (volatile organic compounds and mechanical grinding).

Intra-complex energy transfer of europium(III) complexes containing anthracene and phenanthrene moieties.

Laser excitation and luminescence studies were carried out for an ethanol solution of tris(hexafluoroacetylacetonato)europium(III), Eu(hfac)(3)(H(2)O)(2) (1), and n-hexane solutions of tris(hexafluoroacetylacetonato)europium(III) bis(9-diisopropylphosphorylanthracene), Eu(hfac)(3)(DiPAnO)(2) (2), and tris(hexafluoroacetylacetonato)europium(III) bis(2-diisopropylphosphorylphenanthrene), Eu(hfac)(3)(DiPPheO)(2) (3). The absorption spectra of 2 and 3 in n-hexane are interpreted by assuming that the central Eu(III) ion weakly interacts with the DiPAnO and DiPPheO moieties. The emission spectroscopic studies revealed that (1) Eu(hfac)(3)(DiPAnO)(2) emits only fluorescence from the DiPAnO moiety and (2) Eu(hfac)(3)(DiPPheO)(2) gives luminescence from the central Eu(III) ion, and (3) Eu(hfa)(3)(H(2)O)(2) and Eu(hfac)(3)(DiPPheO)(2) afford emission from both the (5)D(1) and (5)D(0) states of the Eu(III) ion upon 355 nm laser excitation. The intracomplex energy transfer processes are presented on the basis of absorption, emission and laser excitation studies.

Photochemistry and photophysics of the tetrahedral silver(I) complex with diphosphine ligands: [Ag(dppb)2]PF6 (dppb = 1, 2-bis[diphenylphosphino]benzene).

The photophysical and photochemical properties of [Ag(dppb)2]PF6 (1) having a tetrahedral coordination geometry have been described.

Photoluminescent properties and molecular structures of [NaphAu(PPh(3))] and [mu-Naph {Au(PPh(3))}(2)] ClO(4) (Naph = 2-naphthyl).

The complexes [NaphAu(PPh(3))], and [mu-Naph{Au(PPh(3))}(2)]ClO(4), having the Au-C (aromatic) bond have been synthesized and characterized. The unique structure of with two gold atoms bridged by a naphthyl group has been determined by X-ray crystallography. The intramolecular Au-Au separation in is 2.7731(4) A. Upon excitation at 266 nm, both complexes display intraligand phosphorescence at room temperature in solution and in solid state.

Photo-activation of Pd-catalyzed Sonogashira coupling using a Ru/bipyridine complex as energy transfer agent.

The mixed catalyst system, Pd(CH3CN)2Cl2/P(t-Bu)3/[Ru(2,2'-bipyridine)3].2PF6, promotes the copper-free Sonogashira coupling reaction of aryl bromides at room temperature under irradiation of visible light.

Synthesis and characterization of phenanthrylphosphine gold complex: observation of Au-induced blue-green phosphorescence at room temperature.

A new 9-diphenylphosphinophenanthrene ligand (9DPP, 1), its oxide (9DPPO, 2), and its gold complex [(AuCl(9DPP)] (3) were synthesized. The Au(I) complex 3 was found to exhibit intense blue-green, room-temperature phosphorescence (Phip = 0.06 and tauT = 22.7 micros) originating in the locally excited triplet of the phenanthrene moiety (3LE) in degassed 2-methyltetrahydrofuran solution. On the assumption that PhiST = 1.0 for 3, the radiative rate constant (kr) in the triplet state is calculated to be 2.6 x 10(3) s(-1). This value is 4 orders of magnitude larger than the radiative rate constant of the triplet phenanthrene (0.26 s(-1)). Thus, the coordinated Au(I) atom is concluded to have a markedly large heavy-atom effect on kr of the phenanthrene chromophore in 3.

A novel dinuclear cyclometalated iridium complex bridged with 1,4-bis[pyridine-2-yl]benzene: its structure and photophysical properties.

The synthesis, structure, and photophysical properties of a new cyclometalated dinuclear iridium complex, (ppy)2Ir(mu-BPB)Ir(ppy)2 [ppy = 2-phenylpyridine, BPB = 1,4-bis(pyridin-2-yl)benzene], have been investigated.