Photophysical Properties of Pyrazinocoumarin Derivatives

Pyrazinocoumarin (PzC), 7-methoxypyrazinocoumarin (7-MOPzC) and 5,7-dimethoxypyrazinocoumarin (5,7-DMPzC) show the similar UV-visible absorption spectra as the corresponding coumarins except for more distinct shoulders in the near UV absorption band (300–350 nm), probably due to n * transition. Fluorescence excitation spectra of PzC derivatives in nonpolar solvents have the lowest energy maxima in the region of the shoulders of UV-visible absorption, indicating the lowest fluorescent state to have the same energy as these shoulders. Fluorescence quantum yields (ρF) of pyrazinocoumarins are on the order of 10 2-10-4, much lower than the corresponding coumarins. The activation energies (Ea(nr)) for nonradiative processes (internal conversion, intersystem crossing, etc.) determined by the temperature effect on the fluorescence are 10.2, 6.35 and 6.57 kj/mol for PzC, 7-MOPzC and 5,7-DMPzC, respectively. Phosphorescence emission spectra of PzC and 7-MOPzC are structureless with φP of 0.12 and 0.17, respectively     

Subtle role of polyatomic anions in molecular construction: structures and properties of AgX bearing 2,4'-thiobis(pyridine) (X(-) = NO(3)(-), BF(4)(-), ClO(4)(-), PF(6)(-), CF(3)CO(2)(-), and CF(3)SO(3)(-))

Studies on the subtle effects and roles of polyatomic anions in the self-assembly of a series of AgX complexes with 2,4'-Py(2)S (X(-) = NO(3)(-), BF(4)(-), ClO(4)(-), PF(6)(-), CF(3)CO(2)(-), and CF(3)SO(3)(-); 2,4'-Py(2)S = 2,4'-thiobis(pyridine)) have been carried out. The formation of products appears to be primarily associated with a suitable combination of the skewed conformers of 2,4'-Py(2)S and a variety of coordination geometries of Ag(I) ions. The molecular construction via self-assembly is delicately dependent upon the nature of the anions. Coordinating anions afford the 1:1 adducts [Ag(2,4'-Py(2)S)X] (X(-) = NO(3)(-) and CF(3)CO(2)(-)), whereas noncoordinating anions form the 3:4 adducts [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = ClO(4)(-) and PF(6)(-)). Each structure seems to be constructed by competition between pi-pi interactions of 2,4'-Py(2)S spacers vs Ag.X interactions. For ClO(4)(-) and PF(6)(-), an anion-free network consisting of linear Ag(I) and trigonal Ag(I) in a 1:2 ratio has been obtained whereas, for the coordinating anions NO(3)(-) and CF(3)CO(2)(-), an anion-bridged helix sheet and an anion-bridged cyclic dimer chain, respectively, have been assembled. For a moderately coordinating anion, CF(3)SO(3)(-), the 3:4 adduct [Ag(3)(2,4'-Py(2)S)(4)](CF(3)SO(3))(3) has been obtained similarly to the noncoordinating anions, but its structure is a double strand via both face-to-face (pi-pi) stackings and Ag.Ag interactions, in contrast to the noncoordinating anions. The anion exchanges of [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = BF(4)(-), ClO(4)(-), and PF(6)(-)) with BF(4)(-), ClO(4)(-), and PF(6)(-) in aqueous media indicate that a [BF(4)(-)] analogue is isostructural with [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = ClO(4)(-) and PF(6)(-)). Furthermore, the anion exchangeability for the noncoordinating anion compounds and the X-ray data for the coordinating anion compounds establish the coordinating order to be NO(3)(-) > CF(3)CO(2)(-) > CF(3)SO(3)(-) > PF(6)(-) > ClO(4)(-) > BF(4)(-).     

Photophysical Properties of Pyrazinopsoralen,A New Monofunctional Psoralen*

Abstract— Pyrazinopsoralen (PzPs), a new monofunctional psoralen, has a UV absorption spectrum similar to other psoralens except that it absorbs more strongly in the long-UVA than 8-methoxypsoralen. The solvent effects on the UV absorption and fluorescence emission spectra indicate that the lowest excited singlet state is the π,π* state like other psoralen derivatives. It shows a much lower fluorescence quantum yield (0.0008 in ethanol at room temperature) than the other psoralens as expected by the increased proximity effect (vibronic perturbation) due to close ¹(n,π*) to ¹(π,π*) states. The fluorescence lifetime was 1.05 ns in methylcyclohexane with a single exponential decay, while more than two components were observed in other solvents with the short-lived component being the major (>95%). The triplet state of PzPs could not be detected by phosphorescence, laser flash excitation (T-T absorption) and singlet oxygen formation probably due to very low φ_isc, or short lifetime of the triplet state (τ_T) caused by the fast T₁→ S₀ intersystem crossing.     

Facile palladium-catalysed synthesis of 1-aryl-1H-indazoles from 2-bromobenzaldehydes and arylhydrazines

2-Bromobenzaldehydes react with arylhydrazines in toluene at 100 [degree]C in the presence of a catalytic amount of a palladium catalyst and phosphorus chelating ligands such as 1,1[prime or minute]-bis(diphenylphosphino)ferrocene and 1,3-bis(diphenylphosphino)propane along with NaO-t-Bu to afford 1-aryl-1H-indazoles in good yields.     

Studies on pore structures and surface functional groups of pitch-based activated carbon fibers

The present study concerns the physical activation and chemical oxidation of pitch-based activated carbon fibers (ACFs) as ways to improve the adsorption properties. The surface oxides of the ACFs studied were determined by Boehm's titration and the pore structures were studied by the BET method with N(2)/77 K adsorption. Also, the adsorption properties of the ACFs were investigated with chromium ion adsorption by different adsorption models. As a result, it was observed that carboxyl groups were largely created after nitric acid treatment on ACFs. The affinity for chromium ions increases with increasing specific surface area, micropore volume, and surface functionalities of ACFs as the activation time increases.     

ISOLATION and CHARACTERIZATION OF THE PHOTOADDUCTS OF 5,7-DIMETHOXYCOUMARIN and ADENOSINE

The isolation and structural characterization of the photoadducts of 5,7-dimethoxycoumarin and adenosine are described. Two of the major photoadducts were isolated by preparative column chromatography and reverse-phase high performance liquid chromatography. Structure of the products was determined by UV, FT-IR, mass spectrometry, ¹H NMR and ¹³C NMR studies, including the homonuclear decoupling, COSY method and DEPT experiments. The photoadducts were not C₄-cycloadducts but simple addition products in contrast to pyrimidine base adducts. Covalent bonds were formed between the carbon-3 or carbon-4 of the pyrone ring of 5,7-dimethoxycoumarin and the carbon-5'of ribose ring in adenosine.     

Synthesis and characterisation of macrocyclic palladium(II)-sodium(I) complexes: generation of an unusual metal-mediated electron delocalisation

Reaction of sodium perchlorate-crown ether derivative (LH2) complex [Na2LH2](ClO4)2 (1) with palladium acetate afforded two related compounds of macrocyclic palladium(II)-sodium(I) dimeric tetranuclear complexes, [Pd2Na2L2(mu-OH2)2](ClO4)2(CH2Cl2)3 (2) and [Pd2Na2(L-)2](CH3CN)2(C3H6O)2 (3) and their structures were characterised by IR, NMR, mass and X-ray analysis; the latter was revealed as an unusual metal-mediated electron delocalised complex.     

Time-Resolved Spectroscopic Study on Photoinduced Electron-Transfer Processes in Zn(II)porphyrin–Zn(II)chlorin–Fullerene Triad

Femtosecond time-resolved transient absorption studies have been performed to investigate the photoinduced energy and electron-transfer processes in Zn(II )porphyrin–Zn(II )chlorin–fullerene triad in which energy and oxidation potential gradients are directed along the donor–acceptor-linked arrays. Fast energy transfer (≈450 fs) from photoexcited Zn(II )porphyrin to Zn(II )chlorin was observed upon selective photoexcitation of Zn(II )porphyrin unit in the triad. In a nonpolar solvent such as toluene, the energy transfer from the excited singlet state of Zn(II )chlorin to fullerene occurs and is followed by the formation of an intermediate state with a time constant of nanoseconds, which was attributed to the intramolecular exciplex between Zn(II )chlorin and fullerene. In benzonitrile, on the other hand, the photoexcitation of the triad results in the fast electron transfer (<1 ps) from photoexcited Zn(II )chlorin to fullerene. The generated charge-separated species recombine with a time constant of ≈12 ps. The relatively fast charge separation and charge recombination rates imply that the strong electronic coupling between Zn(II )chlorin and fullerene moieties is probably induced by the folded conformation between Zn(II )chlorin and fullerene moieties which enhances direct through-space interaction between the proximately contacted π systems.     

Synthesis and characterization of fluorene‐based copolymers containing siloxane or distilbene moieties on their main chain

Two novel types of polyfluorene copolymers containing siloxane linkages or distilbene moieties on their main‐chains were synthesized by Ni(0)‐mediated Yamamoto coupling polymerization. These polymers, designated P2Silo05, P2Silo15, PF‐P02, and PF‐P05 were prepared by copolymerization between 2,7‐dibromo‐9,9′‐dihexylfluorene and bis(bromobenzene)‐terminated disiloxane monomer (for P2Silo05 and P2Silo15) or dibromodistilbene monomer (for PF‐P02 and PF‐P05). All of the polymers were highly soluble in common organic solvents such as chloroform, toluene, and p‐xylene. The glass transition temperatures of the polymers were between 92 and 113 °C, and the decomposition temperatures for a 5% weight loss (T_d) were above 420 °C for all of the polymers, demonstrating high thermal stability. The molecular weight (M_w) of the polymers ranged from 4.2 × 10⁴ to 8.8 × 10⁴. The blue shift of the maximum in the UV‐visible absorption was greater in polymers with a higher molar percentage of siloxane linkages or distilbene moieties than in homo poly (dihexylfluorene) (PDHF). However, the photoluminescence spectra of the polymers were similar to those of PDHF in terms of the onsets and patterns. Single‐layer light‐emitting diodes were fabricated with a configuration of ITO/PEDOT:PSS/polymers/Ca/Al. The maximum electroluminescence emission wavelengths of the polymers were 425–450 nm, corresponding to pure blue light. The CIE co‐ordinates of the polyfluorenes containing siloxane linkages or distilbene moieties ranged from (0.21, 0.21) to (0.17, 0.10), indicating deeper blue light than that of PDHF {CIE co‐ordinates of (0.25, 0.29)}, with P2Silo15 giving the deepest blue‐light {CIE co‐ordinates of (0.17, 0.10)}. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1595–1608, 2009