Sol–gel derived dye-bridged hybrid materials for white luminescence

Novel yellow and blue emissive dyes have been synthesized using 2,5-diamino-3,6-dicyanopyrazine and various alkoxysilanes and they are covalently bridged to cycloaliphatic epoxy functional oligosiloxane via non-hydrolytic sol–gel reaction. Dye-bridged hybrid materials (DBH) are fabricated by thermal curing the dye-bridged oligosiloxane. Structure and formation of dyes and siloxane network is studied using analysis method. Four components of red, yellow, green and blue emitting DBH cover entire visible range and white luminescence with high color rendering index is realized by controlling their combinations. We have ensured superior thermal stability DBH at 120 °C for 200 h caused by covalently bridged structure and robust siloxane matrix.     

Comparative Electrochemical and Photophysical Studies of Tetrathiafulvalene‐Annulated Porphyrins and Their ZnII Complexes: The Effect of Metalation and Structural Variation

A series of tetrathiafulvalene (TTF)‐annulated porphyrins, and their corresponding Zn^II complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge‐transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge‐transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square‐wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis‐substituted TTF–porphyrin (both free‐base and Zn^II complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent‐dependent charge‐transfer transitions, it is the π‐acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square‐wave voltammograms. In the case of the free‐base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF ^{? +} generated through one‐electron oxidation, can interact with each other; this gives rise to multiple signals in the square‐wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF ^{. +} and TTF²⁺, are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO–LUMO gap. To probe the excited‐state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF–porphyrins of this study display relatively short excited‐state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge‐transfer effects inferred from the ground‐state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF–porphyrins and support the proposed intramolecular charge‐transfer interactions and π‐acceptor effects.     

Taking apart the two-dimensional infrared vibrational echo spectra: more information and elimination of distortions

Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe the fast structural evolution of molecular systems under thermal equilibrium conditions. Structural dynamics are tracked by observing the time evolution of the 2D-IR spectrum, which is caused by frequency fluctuations of vibrational mode(s) excited during the experiment. However, there are a variety of effects that can produce line shape distortions and prevent the correct determination of the frequency-frequency correlation function (FFCF), which describes the frequency fluctuations and connects the experimental observables to a molecular level depiction of dynamics. In addition, it can be useful to analyze different parts of the 2D spectrum to determine if dynamics are different for subensembles of molecules that have different initial absorption frequencies in the inhomogeneously broadened absorption line. Here, an important extension to a theoretical method for extraction of the FFCF from 2D-IR spectra is described. The experimental observable is the center line slope (CLSomega(m)) of the 2D-IR spectrum. The CLSomega(m) is obtained by taking slices through the 2D spectrum parallel to the detection frequency axis (omega(m)). Each slice is a spectrum. The slope of the line connecting the frequencies of the maxima of the sliced spectra is the CLSomega(m). The change in slope of the CLSomega(m) as a function of time is directly related to the FFCF and can be used to obtain the complete FFCF. CLSomega(m) is immune to line shape distortions caused by destructive interference between bands arising from vibrational echo emission, from the 0-1 vibrational transition (positive), and from the 1-2 vibrational transition (negative) in the 2D-IR spectrum. The immunity to the destructive interference enables the CLSomega(m) method to compare different parts of the bands as well as comparing the 0-1 and 1-2 bands. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect the determination of the FFCF with the CLSomega(m) method. The CLSomega(m) can also provide information on the cross correlation between frequency fluctuations of the 0-1 and 1-2 vibrational transitions.     

Design, chemical synthesis, and biological evaluation of novel triazolyl analogues of taranabant (MK-0364), a cannabinoid-1 receptor inverse agonist

Being obese has various health problems that are related to type 2 diabetes mellitus, cardiovascular disease, hypertension, hyperlipidemia, and fibrinolytic abnormalities. Merck's taranabant (MK-0364), a CB1R inverse agonist, is currently in Phase 3 clinical trials, and is being actively pursued by Merck toward obesity market. Merck intends to file for FDA approval of taranabant in 2008. In order to increase solubility and potency of taranabant, or even possibly improve safety, novel triazole analogues of taranabant have been designed and synthesized. We introduced a pivotal asymmetric center via the Evans chiral auxiliary methodology and set up 1,2,4-triazole via substitution of α-bromoketone. Subsequently, diastereoselective reduction was accomplished to install adjacent essential asymmetric center. This method allowed us to prepare readily sub-gram scale of the target compounds in a convenient way. The synthesized analogues were subjected to biological evaluation involving cannabinoid CB1 receptor binding affinity. While the parent taranabant bears highly potent binding affinity to cannabinoid CB1 receptor, neither of the analog structures improved CB1 receptor binding affinities.     

Substituent effects of pyrazine on construction of crystal structures of Zn(II)-benzoate complexes and their catalytic activities (dinuclear,trinuclear, and pentanuclear to 1-D and 2-D)

Several substituted pyrazine ligands (2,3-dimethylpyrazine, 2,6-dimethylpyrazine, 2,5-dimethylpyrazine, quinoxaline) as well as simple pyrazine have been employed to investigate how the bridging pyrazine ligand influences on construction of Zn-benzoate complexes. Simple pyrazine and 2,5-dimethlpyrazine are used as bridging ligands to form two-dimensional and one-dimensional polymeric compounds, respectively. The other quinoxaline and two dimethyl-substituted pyrazine ligands are used only as terminal ligands to form dinuclear, trinuclear, and pentanuclear complexes. This result indicates that the substituents of pyrazine are very important roles for construction of Zn-benzoate complexes. Interestingly, the compounds 1–5 catalyzed efficiently the transesterification of a variety of esters, and among them, the pentanuclear complex 3 showed the most efficient reactivity. The substrates with the electron-withdrawing substituents have undergone faster transesterification, while those with the electron-donating ones have shown slow reaction. In addition, p-nitrophenyl acetate and p-nitrophenyl benzoate, known to be problematic substrates for the transesterification reaction, were also converted quantitatively to the corresponding products. Selectivity test of primary over secondary alcohol protection in the presence of 3 has provided, exclusively, the primary acetate, propyl acetate, suggesting that this catalytic system can be potentially useful in selecting for primary alcohols.     

Solute-solvent complex switching dynamics of chloroform between acetone and dimethylsulfoxide-two-dimensional IR chemical exchange spectroscopy

Hydrogen bonds formed between C-H and various hydrogen bond acceptors play important roles in the structure of proteins and organic crystals, and the mechanisms of C-H bond cleavage reactions. Chloroform, a C-H hydrogen bond donor, can form weak hydrogen-bonded complexes with acetone and with dimethylsulfoxide (DMSO). When chloroform is dissolved in a mixed solvent consisting of acetone and DMSO, both types of hydrogen-bonded complexes exist. The two complexes, chloroform-acetone and chloroform-DMSO, are in equilibrium, and they rapidly interconvert by chloroform exchanging hydrogen bond acceptors. This fast hydrogen bond acceptor substitution reaction is probed using ultrafast two-dimensional infrared (2D-IR) vibrational echo chemical exchange spectroscopy. Deuterated chloroform is used in the experiments, and the 2D-IR spectrum of the C-D stretching mode is measured. The chemical exchange of the chloroform hydrogen bonding partners is tracked by observing the time-dependent growth of off-diagonal peaks in the 2D-IR spectra. The measured substitution rate is 1/30 ps for an acetone molecule to replace a DMSO molecule in a chloroform-DMSO complex and 1/45 ps for a DMSO molecule to replace an acetone molecule in a chloroform-acetone complex. Free chloroform exists in the mixed solvent, and it acts as a reactive intermediate in the substitution reaction, analogous to a SN1 type reaction. From the measured rates and the equilibrium concentrations of acetone and DMSO, the dissociation rates for the chloroform-DMSO and chloroform-acetone complexes are found to be 1/24 ps and 1/5.5 ps, respectively. The difference between the measured rate for the complete substitution reaction and the rate for complex dissociation corresponds to the diffusion limited rate. The estimated diffusion limited rate agrees well with the result from a Smoluchowski treatment of diffusive reactions.     

A new approach to constructing exponentially many nonisomorphic nonorientable triangular embeddings of complete graphs

We prove a theorem that for an integer s?0, if 12s+7 is a prime number, then the number of nonisomorphic face 3-colorable nonorientable triangular embeddings of K_n, where n=(12s+7)(6s+7), is at least . By some number-theoretic arguments there are an infinite number of integers s satisfying the hypothesis of the theorem. The theorem is the first known example of constructing at least 2^αn?+o(n?), ?>1, nonisomorphic nonorientable triangular embeddings of K_n for n=6t+1, . To prove the theorem, we use a new approach to constructing nonisomorphic triangular embeddings of complete graphs. The approach combines a cut-and-paste technique and the index one current graph technique. A new connection between Steiner triple systems and constructing triangular embeddings of complete graphs is given.     

A Property Satisfying Reducedness over Centers

This article concerns a ring property called pseudo-reduced-over-center that is satisfied by free algebras over commutative reduced rings.The properties of radicals of pseudo-reduced-over-center rings are investigated,especially related to polynomial rings.It is proved that for pseudo-reduced-over-center rings of nonzero characteristic,the centers and the pseudo-reduced-over-center property are preserved through factor rings modulo nil ideals.For a locally finite ring R,it is proved that if R is pseudo-reduced-over-center,then R is commutative and R/J(R) is a commutative regular ring with J(R) nil,where J(R) is the Jacobson radical of R.     

Alkali-metal-ion catalysis and inhibition in the nucleophilic displacement reaction of y-substituted phenyl diphenylphosphinates and diphenylphosphinothioates with alkali-metal ethoxides: effect of changing the electrophilic center from P=O to P=S

A kinetic study of the nucleophilic substitution reaction of Y‐substituted phenyl diphenylphosphinothioates 2 a – g with alkali‐metal ethoxides (MOEt; M=Li, Na, K) in anhydrous ethanol at (25.0±0.1) °C is reported. Plots of pseudo‐first‐order rate constants (k_obsd) versus [MOEt], the alkali ethoxide concentration, show distinct upward (KOEt) and downward (LiOEt) curvatures, respectively, pointing to the importance of ion‐pairing phenomena and a differential reactivity of dissociated EtO^? and ion‐paired MOEt. Based on ion‐pairing treatment of the kinetic data, the k_obsd values were dissected into k and k_MOEt, the second‐order rate constants for the reaction with the dissociated EtO^? and ion‐paired MOEt, respectively. The reactivity of MOEt toward 2 b (Y=4‐NO₂) increases in the order LiOEt?NaOEt>KOEt>EtO^?. The current study based on Yukawa–Tsuno analysis has revealed that the reactions of 2 a – g (P?S) and Y‐substituted phenyl diphenylphosphinates 1 a – g (P?O) with MOEt proceed through the same concerted mechanism, which indicates that the contrasting selectivity patterns are not due to a difference in reaction mechanism. The P?O compounds 1 a – g are approximately 80‐fold more reactive than the P?S compounds 2 a – g toward the dissociated EtO^? (regardless of the electronic nature of substituent Y) but are up to 3.1×10³‐fold more reactive toward ion‐paired LiOEt. The origin of the contrasting selectivity patterns is further discussed on the basis of competing electrostatic effects and solvational requirements as a function of anionic electric field strength and cation size (Eisenman’s theory).