Synthesis, characterization, and preliminary intramolecular energy transfer studies of rigid, emissive, rhenium-linked porphyrin dimers

The reaction of pyridyl functionalized porphyrins with Re(CO)(5)Cl in THF results in the formation of porphyrin dimers which, despite incorporation of rhenium into the assemblies, remain fluorescent. The rigid compounds provide an efficient geometry and/or orbital pathway for singlet energy transfer, rendering these compounds suitable, in principle, for the study of both through-bond and through-space energy transfer. Derivatives containing both metallated and freebase porphyrins connected via the metal corners display efficient porphyrin-porphyrin energy transfer. The photophysical properties of the assemblies have been studied by both steady-state and time-resolved fluorescence techniques, yielding approximate rates and efficiencies for porphyrin-porphyrin energy transfer.     

A comparative study of heavy-atom effects on the room-temperature phosphorescence of biologically important purines

Room-temperature phosphorescence (r.t.p.) of nine biologically important purines on filter paper is studied. The heavy-atom effect leads to a significant enhancement of r.t.p. signals, with the trend being Tl⁺ > Pb²⁺ ? I — ? Sm³⁺ for most purines. Absolute limits of detection range from 40 pg (purine) to 19 ng (theophylline).     

Synthesis and formulation of vinyl-containing polyacids for improved light-cured glass-ionomer cements

Vinyl-containing poly(acrylic acid-co-itaconic acid) copolymers were synthesized and used to formulate light-curable cements containing reactive glass fillers (Fuji II LC). The conditions for light curing were studied and optimized. Effects of molecular weight (MW), grafting ratio, comonomer, liquid composition, powder/liquid (P/L) ratio, glass powder and aging were evaluated. The results show that the vinyl-containing glass-ionomer cements (GICs) prepared in this study exhibit higher compressive strength (CS, 225.6 MPa), diametral tensile strength (DTS, 28.4 MPa) and much higher flexural strength (FS, 116.4 MPa), as compared to commercial Fuji II LC GIC (186.6 in CS, 19.1 in DTS and 57.1 in FS). The optimal light-exposure time was found to be around 10 min, and concentrations of CQ and DC were 0.5% (by weight) and 1.0%, respectively. Effects of MW, grafting ratio, P/L ratio and content of polymer in the liquid formulation were significant. The highest strengths were found for the optimal formulations where the MW was 15,000 (weight average), grafting ratio 25 mol%, P/L ratio 2.7 and liquid composition 50:20:30. During aging, the cement showed an increase of strength over the first week and then no change for a month. SEM analysis suggests that more integrated microstructures and smaller glass particles can lead to higher FS and higher polymer content in GICs leads to tough fracture surface and plastic deformation.     

An Investigation of Inclusion Complexes of Cyclodextrins with Phenylurea Herbicides by Photochemically-Induced Fluorescence. Analytical Applications

The effect of -cyclodextrin (-CD) and hydroxypropyl--cyclodextrin (HP--CD) upon the photochemically-induced fluorescence (PIF) properties of four phenylurea herbicides, including linuron, diuron, isoproturon and neburon has been studied. Photochemical conversion of these nonfluorescent herbicides into strongly fluorescent photoproducts was shown to occur in -CD and HP--CD aqueous media. The influence of pH, UV irradiation time and photoproduct stability on the fluorescence intensity was also investigated. In addition, the stoichiometry and formation constants of the complexes formed between herbicides and -cyclodextrin (-CD) or 2-hydroxypropyl--cyclodextrin (HP--CD) were determined. The formation constant values, ranging from 184 ± 40 to 1498 ± 245 M^-1, were calculated by applying the iterative nonlinear regression (NLR) approach to the PIF data. Linear calibrations graphs were established in the interval 1–12 g/mL, for diuron, linuron and neburon. The IUPAC limits of detection ranged between 580 and 700 ng/mL, according to the compound. Application to the analysis of phenylurea herbicides in spiked river water was also described.     

Quantification of several atmospheric gases from high resolution infrared solar spectra obtained at the south pole in 1980 and 1986

Simultaneous total column amounts of a number of minor and trace atmospheric gases above the South Pole in December 1980 and December 1986 have been deduced from analysis of high resolution solar absorption spectra recorded (by F. J. M. and F. H. M.) from Amundsen-Scott South Pole Station. These spectra also contain some limited information on the vertical profiles of the observed atmospheric gases.The data sets were recorded with a Bomem Michelson-type interferometer and analyzed with a spectral least-squares fitting procedure, utilizing the best available spectroscopic line parameters and absorption cross sections. Because the same instrument, line parameters, and analysis method have been used in analyzing the December 1980 and December 1986 data sets, the precision in comparing the column amounts from these two dates is rather high, about 10–20% for the stronger absorbing gases. For this reason, it has been possible to quantify or determine upper limits for differences between the December 1980 and December 1986 total column amounts, of a number of atmospheric gases including O₃, N₂O, HNO₃, CO₂, CH₄, and CF₂C_{1 2} (CFC 12). In addition, vertical column amounts for a number of atmospheric gases covered only in the December 1986 observations have been derived, including HC1, NO, NO₂, and C₂H₆. Some of these results will be discussed here. The HC1 measurements are especially interesting since the observed amounts are higher than expected from observations made at lower latitudes in the northern and southern hemispheres.     

Photochemically-induced fluorescence determination of sulfonylurea herbicides using micellar media

An analytical method based on the use of UV irradiation to produce fluorescent derivatives from four non-fluorescent sulfonylurea herbicides, including chlorsulfuron, metsulfuron methyl, 3-rimsulfuron and sulfometuron methyl is described. Their photochemically-induced fluorescence (PIF) properties in several solvents (water, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), acetonitrile, methanol, ethanol, propan-2-ol and their binary mixtures with water) and micellar solutions of sodium dodecyl sulfate (SDS), and cetyltrimethylammonium chloride (CTAC) are reported. Physicochemical variable influencing the sensitivity of the method have been optimized. A PIF method is developed for the determination of the four herbicides under study. Micellar media are found to provide the best analytical figures of merits. Linear dynamic ranges are established over about two orders of magnitude. The limit of detection (LOD) range from 0.2 to 6 ng ml(-1) according to the compound, with relative standard deviation (RSD) between 1.2 and 3.9%. Examples of applications to the analysis of these herbicides in spiked river water samples are given. The mean recoveries range from 80 to 104%.     

Thermodynamic study of the three isomers of bromo and iodobenzoic acids. Part II

Using our technique of combustion of small amount of a substance, we determined by calorimetry the standard molar enthalpy of formation in the condensed state and atT=298.15 K of the three isomers of bromo and iodobenzoic acids. Associating to these values their standard molar enthalpies of sublimation previously measured, it was possible to determine their standard molar enthalpies of formation in the gaseous state and atT=298.15 K. The experimental values of the thermodynamic properties _f H _m ^o (cr, 298.15 K), _f H _m ^o (cr, 298.15 K), _sub H _m ^o (298.15 K), and _f H _m ^o (g, 298.15 K) are given for the two series. From the experimental value of the standard molar enthalpy of atomization, it was possible to determine an enthalpy value for the C_b-Br and C_b-I bonds. The experimental and theoretical values of the resonance energy of bromo and iodobenzoic acids are compatible. The relative stability of some monosubstituted derivatives of benzoic acid studied in our laboratory is also discussed.Part I is concerned with Ref. 22 (for bromobenzoic acids) and with Ref. 23 (for iodobenzoic acids).     

Mapping the influence of molecular structure on rates of electron transfer using direct measurements of the electron spin-spin exchange interaction

The spin-spin exchange interaction, 2J, in a radical ion pair produced by a photoinduced electron transfer reaction can provide a direct measure of the electronic coupling matrix element, V, for the subsequent charge recombination reaction. We have developed a series of dyad and triad donor-acceptor molecules in which 2J is measured directly as a function of incremental changes in their structures. In the dyads the chromophoric electron donors 4-(N-pyrrolidinyl)- and 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, 5ANI and 6ANI, respectively, and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked to the meta positions of a phenyl spacer to yield 5ANI-Ph-NI and 6ANI-Ph-NI. In the triads the same structure is used, except that the piperidine in 6ANI is replaced by a piperazine in which a para-X-phenyl, where X = H, F, Cl, MeO, and Me(2)N, is attached to the N' nitrogen to form a para-X-aniline (XAn) donor to give XAn-6ANI-Ph-NI. Photoexcitation yields the respective 5ANI(+)-Ph-NI(-), 6ANI(+)-Ph-NI(-), and XAn(+)-6ANI-Ph-NI(-) singlet radical ion pair states, which undergo subsequent radical pair intersystem crossing followed by charge recombination to yield (3)NI. The radical ion pair distances within the dyads are about 11-12 A, whereas those in the triads are about approximately 16-19 A. The degree of delocalization of charge (and spin) density onto the aniline, and therefore the average distance between the radical ion pairs, is modulated by the para substituent. The (3)NI yields monitored spectroscopically exhibit resonances as a function of magnetic field, which directly yield 2J for the radical ion pairs. A plot of ln 2J versus r(DA), the distance between the centroids of the spin distributions of the two radicals that comprise the pair, yields a slope of -0.5 +/- 0.1. Since both 2J and k(CR), the rate of radical ion pair recombination, are directly proportional to V(2), the observed distance dependence of 2J shows directly that the recombination rates in these molecules obey an exponential distance dependence with beta = 0.5 +/- 0.1 A(-)(1). This technique is very sensitive to small changes in the electronic interaction between the two radicals and can be used to probe subtle structural differences between radical ion pairs produced from photoinduced electron transfer reactions.     

Intermolecular alkyne hydroaminations involving 1,1-disubstituted hydrazines

[reaction: see text] Two readily prepared catalysts have been developed for the hydroamination of alkynes by 1,1-disubstituted hydrazines. The catalyses are facile with terminal alkynes and some internal alkynes. If the hydrazine bears an aryl group, Fischer cyclization can occur in a one-pot procedure. In addition, reactions with acetylene to produce a plethora of hydrazones are described. Catalytic reactions involving acetylene and substituted hydrazines are complete in less than 2 h at room temperature and 1 atm of pressure.