Exciton-coupled charge-transfer dynamics in a porphyrin J-aggregate/TiO(2) complex

Exciton-coupled charge-transfer (CT) dynamics in TiO(2) nanoparticles (NP) sensitized with porphyrin J-aggregates has been studied by femtosecond time-resolved transient absorption spectroscopy. J-aggregates of 5,10,15-triphenyl-20-(3,4-dihydroxyphenyl) porphyrin (TPPcat) form CT complexes on TiO(2) NP surfaces. Catechol-mediated strong CT coupling between J-aggregate and TiO(2) NP facilitates interfacial exciton dissociation for electron injection into the conduction band of the TiO(2) nanoparticle in pulse width limited time (<80 fs). Here, the electron-transfer (<80 fs) process dominates over the intrinsic exciton-relaxation process (J-aggregates: ca. 200 fs) on account of exciton-coupled CT interaction. The parent hole on J-aggregates is delocalized through J-aggregate excitonic coherence. As a result, holes immobilized on J-aggregates are spatially less accessible to electrons injected into TiO(2) , and thus the back electron transfer (BET) process is slower than that of the monomer/TiO(2) system. The J-aggregate/porphyrin system shows exciton spectral and temporal properties for better charge separation in strongly coupled composite systems.     

Manganese(III) complexes of bis(hydroxyphenyl)dipyrromethenes are potent orally active peroxynitrite scavengers

We report a new series of biscyclohexano-fused Mn(III) complexes of bis(hydroxyphenyl)dipyrromethenes, 4a-c, as potent and orally active peroxynitrite scavengers. Complexes 4a-c are shown to reduce peroxynitrite through a two-electron mechanism, thereby forming the corresponding Mn(V)O species, which were characterized by UV, NMR, and LC-MS methods. Mn(III) complex 4b and its strained BODIPY analogue 9b were analyzed by X-ray crystallography. Finally, complex 4a is shown to be an orally active and potent analgesic in a model carrageenan-induced hyperalgesia known to be driven by the overproduction of peroxynitrite.     

Tautomerization in gas-phase ion chemistry of isomeric C-8 deoxyguanosine adducts from phenol-induced DNA damage

Collision-induced dissociation (CID) of 8-(4'-hydroxyphenyl)-2'-deoxyguanosine and 8-(2'-hydroxyphenyl)-2'-deoxyguanosine was investigated using sequential tandem mass spectrometry. These adducts represent biomarkers of DNA damage linked to phenolic radicals and were investigated to gain insight into the effects of chemical structure of a C-8 modification on fragmentation pathways of modified 2'-deoxyguanosine (dG). CID in MS(2) of the deprotonated molecules of both the isomers generated the same product ion having the same m/z values. CID in MS(3) of the product ion at m/z 242 and CID in MS(4) experiments carried out on the selected product ions at m/z 225 and m/z 218 afford distinct fragmentation patterns. The conformational properties of isomeric product ions from CID showed that the ortho-isomers possess the unique ability to tautomerize through an intramolecular proton transfer between the phenolic OH group and the imine nitrogen (N7). Tautomerization of ortho-isomers to their keto-tautomers led to differences in their system of conjugated double bonds compared with either their enol-tautomer or the para-isomer. The charge redistribution through the N-7 site on the imidazole ring is a critical step in guanosine adduct fragmentation which is disrupted by the formation of the keto-tautomer. For this reason, different reaction pathways are observed for 8-(4'-hydroxyphenyl)-2'-deoxyguanosine and 8-(2'-hydroxyphenyl)-2'-deoxyguanosine. We present herein the dissociation and the gas-phase ion-molecule reactions for highly conjugated ions involved in the CID ion chemistry of the investigated adducts. These will be useful for those using tandem mass spectrometry for structural elucidation of C-8 modified dG adducts. This study demonstrates that the modification at the C-8 site of dG has the potential to significantly alter the reactivity of adducts. We also show the ability of tandem mass spectrometry to completely differentiate between the isomeric dG adducts investigated.     

New iron(II) α-iminopyridine complexes and their catalytic activity in the oxidation of activated methylene groups and secondary alcohols to ketones

A set of iron(II) complexes of the general formula [Fe(OTf)(2)L(2)] was synthesized in 32 to 78% isolated yields, where L represents a bidentate α-iminopyridine ligand. Four of the iron complexes were characterized structurally, revealing a rich coordination chemistry, because the coordination geometry of the iron complexes strongly depends on the substitution pattern exhibited by the ligands L. The catalytic activity of the new complexes was demonstrated in the oxidation of cyclohexane, activated methylene groups and secondary alcohols to the corresponding ketones utilizing H(2)O(2) and t-BuOOH as the oxidants. The oxidation of activated methylene groups and secondary alcohols to the corresponding ketones with t-BuOOH gave isolated yields between 22 and 91% (4 h, room temperature, 3% catalyst load). The influence of the structure of the ligand on the activity of the corresponding metal complex is also reported. Furthermore, UV-vis experiments were performed which provided evidence for the formation of an [Fe-O-O-t-Bu] intermediate.     

A tetraalkylated pyrene building block for the synthesis of pyrene-fused azaacenes with enhanced solubility

The synthesis and characterisation of a soluble pyrene-fused tetraazaoctacene derivative has been achieved by developing a key pyrene-based building block with four solubilising groups.     

Characterization of an unprecedented organomercury adduct via Hg(II)-mediated cyclization of N9-propargylguanine

This communication describes mercury(II)-mediated cyclization of N9-propargylguanine under hydrothermal conditions. The structural investigations reported here confirm cyclization and reveal simultaneous formation of a unique organomercury adduct of a tricyclic guanine derivative.     

An isolable, metastable, geometrically unique manganese(IV) trihydrazide complex poised for reactivity

A metastable Mn(IV) tris(N,N')diphenylhydrazide complex, Li(2)Mn(κ(2)-N(2)Ph(2))(3) (1) has been prepared, and characterized by X-ray diffraction analysis and spectroscopic techniques. Mn precursors are reacted with LiNH(t)Bu to form an intermediate manganese-imido mixture, which reacts with N,N'-diphenylhydrazine, oxidizing the metal, and forming 1. Despite ease of formation, 1 is poised for reactivity, as loss of ether ligand triggers reductive elimination of azobenzene.     

Surface‐State‐Mediated Charge‐Transfer Dynamics in CdTe/CdSe Core–Shell Quantum Dots

Herein, we report the synthesis of aqueous CdTe/CdSe type‐II core–shell quantum dots (QDs) in which 3‐mercaptopropionic acid is used as the capping agent. The CdTe QDs and CdTe/CdSe core–shell QDs are characterized by X‐ray diffraction (XRD), high‐resolution transmission electron microscopy (HR‐TEM), steady‐state absorption, and emission spectroscopy. A red shift in the steady‐state absorption and emission bands is observed with increasing CdSe shell thickness over CdTe QDs. The XRD pattern indicates that the peaks are shifted to higher angles after growth of the CdSe shell on the CdTe QDs. HR‐TEM images of both CdTe and CdTe/CdSe QDs indicate that the particles are spherical, with a good shape homogeneity, and that the particle size increases by about 2 nm after shell formation. In the time‐resolved emission studies, we observe that the average emission lifetime (τ_av) increases to 23.5 ns for CdTe/CdSe (for the thickest shell) as compared to CdTe QDs (τ_av=12 ns). The twofold increment in the average emission lifetime indicates an efficient charge separation in type‐II CdTe/CdSe core–shell QDs. Transient absorption studies suggest that both the carrier cooling and the charge‐transfer dynamics are affected by the presence of traps in the CdTe QDs and CdTe/CdSe core–shell QDs. Carrier quenching experiments indicate that hole traps strongly affect the carrier cooling dynamics in CdTe/CdSe core–shell QDs.     

Hamming weight distributions of multi-twisted codes over finite fields

Designs, Codes and Cryptography - Let $${\mathbb {F}}_q$$ denote the finite field of order q,&nbsp; and let $$n = m_1+m_2+\cdots +m_\ell ,$$ where $$m_1,m_2,\ldots ,m_\ell $$ are arbitrary...     

Pb（NO3）2 Mediated Synthesis of Pyrazine

Lead nitrate was used as an efficient catalyst in the oxidation and condensation reaction of hydroxy ketone with diamine leading to the formation of pyrazine derivatives in excellent yields.