Photophysics of 9,9-Dimethylacridan-Substituted Phenylstyrylpyrimidines Exhibiting Long-Lived Intramolecular Charge-Transfer Fluorescence and Aggregation-Induced Emission Characteristics

Six pyrimidine-based push–pull systems substituted at positions C2 and C4/6 with phenylacridan and styryl moieties, employing methoxy or N,N-diphenylamino donors, have been designed and synthesized through cross-coupling and Knoevenagel reactions. X-ray analysis confirmed that the molecular structure featured the acridan moiety arranged perpendicularly to the residual π system. Photophysical studies revealed significant differences between the methoxy and N,N-diphenylamino chromophores. Solvatochromic studies revealed that the methoxy derivatives showed dual emission in polar solvents. Time-resolved spectroscopy revealed that the higher energy band involved very fast (<80 ps) fluorescence, whereas the lower energy one included long components (≈30 ns) due to long-lived intramolecular charge-transfer fluorescence. In contrast to N,N-diphenylamino chromophores, the methoxy derivatives also showed aggregation-induced emission in mixtures of THF/water, as well as dual emission in thin films, covering almost the whole visible spectrum with corresponding chromaticity coordinates not far from that of pure white light. These properties render the methoxy derivatives as very promising organic materials for white organic light-emitting diodes.     

Detection and Characterization of a Novel Copper-Dependent Intermediate in a Lytic Polysaccharide Monooxygenase

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes capable of oxidizing crystalline cellulose which have large practical application in the process of refining biomass. The catalytic mechanism of LPMOs still remains debated despite several proposed reaction mechanisms. Here, we report a long-lived intermediate (t_1/2=6–8 minutes) observed in an LPMO from Thermoascus aurantiacus (TaLPMO9A). The intermediate with a strong absorption around 420 nm is formed when reduced LPMO-Cu^I reacts with sub-equimolar amounts of H₂O₂. UV/Vis absorption spectroscopy, electron paramagnetic resonance, resonance Raman and stopped-flow spectroscopy suggest that the observed long-lived intermediate involves the copper center and a nearby tyrosine (Tyr175). Additionally, activity assays in the presence of sub-equimolar amounts of H₂O₂ showed an increase in the LPMO oxidation of phosphoric acid swollen cellulose. Accordingly, this suggests that the long-lived copper-dependent intermediate could be part of the catalytic mechanism for LPMOs. The observed intermediate offers a new perspective into the oxidative reaction mechanism of TaLPMO9A and hence for the biomass oxidation and the reactivity of copper in biological systems.     

Dispersion coefficients for first hyperpolarizabilities using coupled cluster quadratic response theory

The frequency dependence of third-order properties can in the normal dispersion region be expanded in a Taylor series in the frequency arguments. The dispersion coefficients thus obtained provide an efficient way of expressing the dispersion of frequency-dependent properties and are transferable between different optical processes. We derive analytic expressions for the dispersion coefficients of third-order properties in coupled cluster quadratic response theory and report an implementation for the three coupled cluster models CCS, CC2, and CCSD. Calculations are performed for the first hyperpolarizability of the NH₃ molecule. The convergence of the dispersion expansion with the order of the coefficients is examined and we find good convergence up to about half the frequency at which the first pole in the hyperpolarizability occurs. Padé approximants improve the convergence dramatically and extend the application range of the dispersion expansion to frequencies close to the first pole. The sensitivity of the dispersion coefficients on the dynamic correlation treatment and on the choice of the one-electron basis set is investigated. The results demonstrate that, contrary to presumptions in the literature, the dispersion coefficients are sensitive to basis set effects and correlation treatment similar to the static hyperpolarizabilities. Received: 26 March 1998 / Accepted: 21 July 1998 / Published online: 19 October 1998     

Factor analysis of deuterium isotope effects on 13C NMR chemical shifts in Schiff bases

We have analyzed deuterium isotope effects on (13)C chemical shifts in a series of o-hydroxy Schiff bases by applying factor analysis. Two orthogonal factors were obtained that explain about 80 and 10 % of the variance of the data. The numerical values of these factors can be related to 1H NMR chemical shifts of the proton involved in the intramolecular bonds delta(XH) (X = O or N). Such a relation allows one to identify clusters of compounds with different tautomeric forms of hydrogen bonding. Application of a similar approach to solution 13C NMR chemical shifts produces three important factors, which have a different structure to factors describing isotope effects. This illustrates well the different nature of chemical shifts and isotope effects. The three factors explain about 54, 15, and 13 % of variance. They can be rationalized and are strongly related to the electronic properties and location of substituents.     

Short and long term stability of the elemental composition of human body fluid reference materials and their use as master lots

Summary Studies have been made of both short and long term stability of trace elements in lyophilized human body fluid reference materials, as well as the stability of mercury in reconstituted urine solutions. No detectable concentration changes for mercury, lead and aluminium occurred during the 5-year period. There are large differences in the amounts of mercury loss among different reconstituted materials. Addition of traces of gold to the solutions minimized the mercury loss and increased the useable time of the reconstituted material from hours to 8 days. A certification process based on direct determinations by reference laboratories and by comparison against master lots of the same material and against similar certified reference materials is presented. Values for calcium, copper and mercury obtained from reference laboratories and using the data transfer principle have been assigned in new batches of urine and serum.     

Synthesis of hydroxymethyl branched [3.2.0]bicyclic nucleosides using a regioselective oxetane ring-formation

Two [3.2.0]bicyclic nucleosides, 35 and 34, with one and two hydroxymethyl substituents, respectively, have been efficiently synthesized. A protected (3'-C-vinyl-beta-D-allofuranosyl)thymine derivative 28 was easily prepared from diacetone-D-glucose and the thymine moiety was protected with a BOM-group. After the introduction of a leaving group in the 2'-position, the subsequent nucleoside 31 was used as the substrate for a stereoselective dihydroxylation and a regioselective oxetane ring-formation to give after deprotection the bicyclic nucleoside 34. The surprisingly efficient formation of an oxetane was first discovered by serendipity on a corresponding methylfuranoside derivative. The allo-configured bicyclic nucleoside 34 was easily shortened to a ribo-configured analogue 35 by a diol-cleaving reaction and subsequent reduction. Both 34 and 35 are conformationally restricted in the important intermediate 04'-endo conformation.     

Triplet—triplet absorption spectra of alternant hydrocarbons within the grand canonical time-dependent hartree—fock approximation

Triplet—triplet excitation energies and transition moments have been calculated for some alternant hydrocarbons by the grand canonical time-dependent Hartree—Fock approximation within the Pariser—Parr—Pople model. The main features of the experimental spectra are found to be reproduced more consistently in this scheme than in previously applied models.     

Fabrication of rutile rod-like particle by hydrothermal method: an insight into HNO3 peptization

In this work, well-crystallized and well-dispersed rod-like TiO(2) rutile particles were prepared by hydrothermally treating acid-peptized TiO(2) sols at relatively low temperatures of 200 and 240 degrees C. Raman spectra, transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to characterize the peptized sols before and after hydrothermal treatment. The results showed that HNO(3) peptization of amorphous TiO(2) was able to promote, at room temperature, the formation of crystalline phases of anatase or rutile, at low (HNO(3)/Ti=1) or at high (HNO(3)/Ti=4) concentrations of peptizer, respectively. However, after hydrothermal treatment, well-crystalline rutile developed independent of the starting concentration of the peptizer. The formation of well-dispersed rutile particles is attributed to high long-range electrostatic forces between particles in the presence of the high concentration of the peptizer. The acid peptization would easily break the oxolation bonds between triple bond Ti-O-Ti triple bond, promote the formation of titanium species with fewer oxolation bonds depending on the amount of acid, and create conditions for the formation of rutile nuclei after structural rearrangements.     

Pyridinethiones. XV. 3-Methylthio-2-pentene-1,5-diones as synthons for 4-methylthio-2(1H)-pyridinethiones,and synthesis of 4-methylene-1,4-dihydropyridines

Starting from α-oxoketene dithioacetals the 3-methyithio-1,5-pentenedione enolates 4 obtained from ketones 3 give 4-methylthio-2(1H)-pyridinethiones with isothiocyanates. Enolates 4 can be alkylated with methyl iodide at C-2, giving 5-methyl-4-methylthio-2-(1H)-pyridmethiones with isothiocyanates. The S-alkylated pyridinethiones react with the anion of malodinitrile, giving 4-(1,1-dicyanomethylene)-1,4-dihydropyridines.     

Novel optical and statistical methods reveal colloid-wall interactions inconsistent with DLVO and Lifshitz theories

We present an experimental method based on video microscopy to perform nanometer scale position detection of a micrometer bead in the direction along the propagation of the detection light. Using the same bead for calibration and detection significantly improves the in depth resolution in comparison to video microscopy methods from literature. This method is used together with an optical trap to measure interaction potentials between a glass surface and colloids made of polystyrene or silica at different electrolyte concentrations. The results are confirmed by an independent method where the optical trap is used in connection with a quadrant photodiode. Also, we present a maximum likelihood analysis method which considerably improves the spatial resolution of interaction potentials by optimizing the underlying potential function to fit all observed position distributions. The measured interaction potentials agree well with DLVO theory for small electrolyte concentrations; however, for larger electrolyte concentrations the potentials differ qualitatively from both DLVO and Lifshitz theory.