The influence of solvent polarity and metalation on energy and electron transfer in porphyrin-phthalocyanine heterotrimers

Heteroporphyrin and -phthalocyanine arrays represent an attractive class of light harvesters and charge-separation systems exhibiting an easy route of synthesis and high chemical stability. In the present work, we report the results of photophysical investigations of two novel non-sandwich-type porphyrin-phthalocyanine heterotriads, in which two meso-tetraphenylporphyrin rings (H2TPP or ZnTPP) are linked to the central silicon atom of a silicon(IV) phthalocyanine core. It was found that the photophysical properties of the triads (H2Tr and ZnTr) in N,N-dimethylformamide (DMF) and toluene are strongly affected by two different types of interaction between the porphyrin (P) and the phthalocyanine (Pc) parts, namely excitation energy transfer (EET) and photoinduced charge transfer. The first process results in appearance of the Pc fluorescence when the P-part is initially excited, and plays a dominant role in fast depopulation of the first excited singlet state of the P moiety. If the first excited singlet state of the Pc-part is populated (either directly or via EET), it undergoes fast depopulation by hole transfer (HT) to the charge-separated (CS) state. In polar DMF, the CS state is the lowest excited state, and the charge recombination occurs directly to the ground state. Using transient absorption spectroscopy, the lifetime of the CS state was estimated to be 30 and 20 ps for H2Tr and ZnTr, respectively. In nonpolar toluene, the energy gap between the first excited singlet state of the Pc-part and the CS state is very small, and back HT occurs in both triads, resulting in appearance of "delayed fluorescence" of the Pc-part with a decay time similar to the lifetime of the CS state (190 and 280 ps for H2Tr and ZnTr, respectively). Since the energy of the CS state of ZnTr in toluene is lower than that of H2Tr, the probability of back HT for ZnTr is lower. This was clearly proved by decay-associated fluorescence spectral measurements.     

Functional insights gained from structural analyses of DNA duplexes that contain UV-damaged photoproducts

Ultraviolet photolesions endow DNA with distinct structural and dynamic properties. Biophysical studies of photoproduct-containing DNA have shown that these lesions affect the mutagenic properties of DNA and damage recognition by DNA repair systems. Recently obtained high-resolution cocrystal structures of damaged DNA bound to either DNA polymerase or DNA repair enzymes have enriched our understanding of the mechanisms by which DNA lesions are bypassed or recognized by DNA metabolizing proteins. Here, we summarize the results of these structural studies and discuss their implications for DNA metabolism.     

Expeditious synthesis of indolizine derivatives via iodine mediated 5-endo-dig cyclization

Iodine-mediated 5-endo-dig cyclization of propargylic esters 2 at room temperature proceeded smoothly to give highly functionalized indolizines 3 in excellent yields. A pyridine group was employed as a nucleophilic partner in this facile process for the first time.     

Post-harvest HPLC determination of chlorfluazuron residues in pears treated with different programs

The present study was conducted to monitor the level of chlorfluazuron residues in pear samples in order to assess the risk posed by the presence of such residues to the consumer. Chlorfluazuron was sprayed onto pear trees at the recommended dose rate at two different times at 30 and 21 days prior to harvesting in one treatment, at 21 and 14 days prior to harvesting in another treatment, and three times at 30, 21 and 14 days prior to harvesting in a third treatment. Chlorfluazuron residues were extracted with acetonitrile and partitioned into ethyl acetate. The residue determination was performed on an Apollo C(18) column using HPLC with a UV detection of 254 nm following the clean-up of the extract by open column chromatography with Florisil. The versatility of this method was evidenced by its good linearity (>0.995) in the concentration range between 0.2 and 10 microg/mL. The majority of the mean recoveries at two different fortification levels, 0.05 and 0.25 ppm, ranged from 84.9 +/- 3.2 to 94.3 +/- 10.6, and the repeatability (as the relative standard deviation) from three repetitive determinations of recovery was between 3.8 and 11%. The calculated limit of detection (LOD) was 0.008 ppm and the limit of quantitation was 0.03 ppm. Trace amounts of chlorfluazuron were detectable when it was applied onto the pear trees at two or three times prior to harvesting; however, the levels of chlorfluazuron were not quantified. The excellent sensitivity and selectivity of this method allowed for quantitation and identification at low levels with a run time of less than 12 min. Chlorfluazuron can be used safely to protect pears when sprayed two or three times at 14 days prior to harvesting.     

Kinetics of hydrogen atom transfer from (eta(5)-C(5)H(5))Cr(CO)(3)H to various olefins: influence of olefin structure

Treating (etha(5)-C(5)H(5))Cr(CO)3H (1) or (etha(5)-C(5)H(5))Cr(CO)3D (1-d(1)) with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives kH and kD, the rate constants for H* (D*) transfer from (etha(5)-C(5)H(5))Cr(CO)(3)H (D) to various olefins. The values of kH and kD vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H* more readily than do carbomethoxy-substituted olefins, although the latter accept H* more readily than do alkyl-substituted olefins. A methyl substituent on the incipient radical site increases k(H) at 323 K by a factor between 5 and 50. A methyl substituent on the carbon to which the H* is being transferred decreases kH substantially. On the whole, the rate constants for H* transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.     

Rapid vapor-phase fabrication of organic-inorganic hybrid superlattices with monolayer precision

We report a new layer-by-layer growth method of self-assembled organic multilayer thin films based on gas-phase reactions. In the present molecular layer deposition (MLD) process, alkylsiloxane self-assembled multilayers (SAMs) were grown under vacuum by repeated sequential adsorptions of C=C-terminated alkylsilane and titanium hydroxide. The MLD method is a self- limiting layer-by-layer growth process, and is perfectly compatible with the atomic layer deposition (ALD) method. The SAMs films prepared exhibited good thermal and mechanical stability, and various unique electrical properties. The MLD method, combined with ALD, was applied to the preparation of organic-inorganic hybrid nanolaminate films in the ALD chamber. The organic-inorganic hybrid superlattices were then used as active mediums for two-terminal electrical bistable devices. The advantages of the MLD method with ALD include accurate control of film thickness, large-scale uniformity, highly conformal layering, sharp interfaces, and a vast library of possible materials. The MLD method with ALD is an ideal fabrication technique for various organic-inorganic hybrid superlattices.     

A degradable hyperbranched poly(ester amine) based on poloxamer diacrylate and polyethylenimine as a gene carrier

Polyethylenimine (PEI) is a well-known cationic polymer which has high transfection efficiency due to its buffering effect. However, nondegradability, cytotoxicity, aggregation, and short-circulation time in vivo still need to be overcome for a successful gene delivery. Degradable, hyperbranched poly(ester amine)s (PEAs) based on poloxamer diacrylate and low molecular weight branched PEI, were successfully synthesized and evaluated as a nonviral gene carrier. The PEAs were obtained in significant yields through Michael type addition reaction of diacrylate monomers and low molecular weight branched PEI. Analysis of degradation products by the reduction in molecular weight demonstrated that PEAs degrade in a controlled fashion. The PEA showed good DNA binding ability and the sizes of complexes under physiological condition were below 150 nm, implicating its potential for intracellular delivery. It showed lower cytotoxicity in three different cell lines (A549, 293T, and HepG2) compared with PEI 25K. PEAs showed much higher transfection efficiencies in three cell lines compared with PEI 25K and PEI 1.8K, and revealed little serum dependency in A549 cell line when the content of poloxamer in the PEA was increased up to 30%.     

Frequency-dependent interpolation rules using first derivatives for oscillatory functions

We construct frequency-dependent rules to interpolate oscillatory functions y(x)$y(x)$ with frequency ω$\omega$ of the form,

y(x)=_f1(x)cos(ωx)+_f2(x)sin(ωx),$y(x)=f_1(x)\cos (\omega x)+f_2(x)\sin (\omega x)\text{,}$

Relation between the permeability and the degree of coil overlap of polymer in the bulk state

Based on the expression of the index of coil interpenetration (i.e.,the degree of coil overlap,ξ_b) of amorphous polymers in the bulk state proposed by Qian,theξ_b was rewritten asξ_b=ρ_b[η]_θ/1.66 via Flory's intrinsic viscosity equation at them conditions.Theξ_b ofpoly (vinyl chloride) (PVC) dense membranes with different molecular weights and cellulose diacetate (CA) dense membranes prepared using different solvents were determined.The permeability,permeation coefficient (P) of nitrogen and permeation flux (J) of liquid through both membranes,respectively,was measured.P and J decreased exponentially with increasingξ_b for PVC and CA membranes were revealed.