Femtosecond excitation energy transport in triarylamine dendrimers

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.     

Critical surface of the ising model with first-neighbor,second-neighbor,and four-spin interactions

Exact values are obtained for the slopesK ₁ ^c (0, 0)/K ₂,K ₁ ^c (0, 0)/K ₄ of the critical surface of paramagnetic-ferromagnetic transitionsK ₁ ^c (K ₂,K ₄) for the two-dimensional Ising model on a square lattice with first-neighbor, second-neighbor, and four-spin couplingsK ₁,K ₂, andK ₄, respectively. The results are obtained using universality arguments to relate the slopes to known spin-spin correlation functions forK ₂=K ₄=0. The equivalence of different expressions for the slopes in terms of correlation functions yields sum rules for the divergent part of certain sums over the second-neighbor and four-spin energy-energy correlation functions.     

Deprotonated and non-deprotonated cobalt(II) complexes of the bis-amide tetradentate ligand N,N′-(dipicolyl)-1,8-naphthylene diamine. Three different modes of coordination by the ligand

The preparation and some properties of the cobalt(II) complexes Co(LH₂)Cl₂·2H₂O, Co(LH₂(NCS)₂ and CoL·H₂O (whereLH₂=N,N-(dipicolyl)-1,8-naphthylenediamine) are reported. On the basis of magnetic moments, visible reflectance and IR data, the structure is proposed to be pseudo-octahedral for Co(LH₂)Cl₂·2 H₂O, pseudo-tetrahedral for Co(LH₂)(NCS)₂ and square planar for CoL·H₂O.

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Deprotonierte und Nicht-deprotonierte Co(II)-Komplexe des vierzähnigen Bisamid-Liganden N,N-(Dipicolyl)-1,8-naphthyldiamin. Drei verschiedene Koordinationstypen

Zusammenfassung Es werden die Darstellung und einige Eigenschaften der Kobalt(II)-Komplexe Co(LH₂)Cl₂·2 H₂O, Co(LH₂)(NCS)₂ und CoL·H₂O [LH₂=N,N-(dipicolyl)-1,8-naphthylendiamin] diskutiert. Auf der Grundlage von magnetischen Momenten, von Daten der sichtbaren Reflektions-und IR-Spektren wird eine pseudooctaedrische Struktur für Co(LH₂)Cl₂·2H₂O, eine pseudotetraedrische für Co(LH₂)(NCS)₂ und eine planar-quadratische für CoL·H₂O vorgeschlagen.

     

The correlation of vibrational broadening of core lines in x-ray photoelectron spectra with valence bond resonance structures

When core ionization of an atom in a molecule causes significant changes in bond orders, the core-hole ion is formed in a strained configuration. This strain causes vibrational broadening of the core line. The core-hole ion can be represented as an ordinary chemical species by applying the equivalent cores approximation. Then simple rules of classical valence bond theory can be used to predict changes in the weighting of resonance structures and corresponding changes in bond orders. Thus qualitative changes in relative linewidths can be predicted.     

The stoichiometric hydrogenation of 9-methylidenefluorene and related compounds with hydridocobalt tetracarbonyl

9-Methylideneflourene (IIa) reacts rapidly with HCo(CO)₄ at -67° C to give a quantitative yield of 9-methylfluorene (IIIa); k₂  (13.4 ± 0.5) × 10^-2 l mol^-1_S^-1. Although the internal olefin, 9-ethylidenefluorene (IIb) reacts more slowly than IIa, it is hydrogenated about 2.5 times as fast as the terminal olefin, 1,1-diphenylethylene (I). Measurement of the rate of the reaction of IIb with DCo(CO)₄ and comparison with HCo(CO)₄ shows a very large inverse isotope effect k_H/k_D of 0.43.     

5α-androstano [3,2-b]pyridines, 5α-androstano[17,16-b]pyridines and androst-4 and 5-eno[17,16-b]pyridines

A new synthesis of 5α-androstano[3,2-b]pyridin-17β-ol acetate (VIa) and 17-methyl-5α-androstano[3,2-b]pyridin-17β-ol (VIb), first reported by Shimizu, Ohta, Ueno, and Takegoshi, was achieved. The analogous 5α - androstano[17,16-b]pyridin-3β-ol (XII), 5α-androstano[17,16-b]pyridin-3-one (XIVa), and androst-4-eno[17,16-b]pyridin-3-one (XIVb) were also prepared. An illustration of the method follows. Condensation of 3β-hydroxy-5α-androstan-17-one (VIIa) with 3-(2-furyl)acrolein afforded 16-[3-(2-furyl)-2-propenylidene]-3β-hydroxy-5α-androstan-17-one (VIIIa), the oxime (IXa) of which was thermally cyclized to 5α-androstano[17,16-b]-6′-(2-furyl)pyridin-3β-ol (Xa). 3β-Hydroxy-5α-androstano[17,16-b]pyridine-6′-carboxylic acid (XI) was obtained by ozonolysis of Xa. Thermal decarboxylation of XI gave XII. Cinnamaldehyde was used in place of 3-(2-furyl)acrolein to give the corresponding phenylpyridines.     

Polymeric thiols as enzyme activators of serum creatine phosphokinase

Several hydrophilic polymeric thiols were prepared from aminoactivated polymeric supports by reaction with N-acetylhomocysteinethiolactone. Supports include agaroses, cellulose, Glycophase™ controlled-pore glass, and Matrex™ acrylic beads. Thiol content in these polymers was 3–72 μmol SH/g dry polymer. Several were effective solid-phase activators of the sulfhydryl-dependent enzyme creatine phosphokinase at concentrations comparable to that of monomeric thiol required for enzyme activation. The kinetic activation curves for the polymeric and the monomeric (thioglucose) activators were similar, suggesting unhindered interaction of the enzyme with the polymeric activator.     

The preparation of dimolybdenum(V,V) complexes from molybdenum quadruply bonded metal-metal dimers

Oxidation of quadruply bonded metal-metal dimers in the presence of good π-accepting ligands results in the formation of Mo^V---Mo^V compounds of the type [MO₂(μ-X)₂(Y)(Y′)]²⁺ (X = O or S; Y,Y′ = O,O; S,S; O,S). Reaction of MO₂(O₂CCH₃)₄ with oxygen in the presence of Na₂mnt (mnt = 1,2-dicyanoethylene-2,2-dithiolate) gives [MO₂(μ-S)₂(O)(S)(mnt)₂]²⁻ (1). The compound crystallizes in the monoclinic space group P2₁/c, with cell dimensions a = 19.547(4), b = 15.210(4), c = 18.754(6) Å, β = 101.69(2)°, V= 5460(2) ų, and Z = 4. Similarly, oxidation of o-dichlorobenzene solutions of Mo₂Cl₄(CH₃CN)₄ and 4,4′-dimethyl-2,2′-dipyridyl (dmpby) or, more directly, the reaction of Mo₂Cl₄(dmbpy)₂ with oxygen leads to the formation of a red solid, which was characterized by X-ray crystallography to be Mo₂(μ-O)₂(O)₂(Cl)₂(dmbpy)₂ (2). Red diamond crystals, prepared by slow evaporation of CH₃CN solutions of 2, are trigonal and in the space group P3₁21 with cell dimensions a = 16.135(4), b = 16.135(4), c = 10.709(3) Å, V = 2414.4(13) ų and Z = 3. In both structures, the geometry about each of the molybdenum atoms is a distorted square pyramid with terminal oxygen or sulphur atoms at the apices and in a syn conformation. The molybdenum-molybdenum bond distances of 2.858(1) Å and 2.562(2) Å in structures of 1 and 2, respectively, are typical of other Mo^V---Mo^V dimers and indicative of a single Mo---Mo bond.     

Ascorbic Acid/Iodine and Triphenylphosphine/Iodine as Reducing Agents for the As(V)[dbnd]O Group

The scope of ascorbic acid/iodine and triphenylphosphine/iodine in methanol for the direct reduction of arsenic(V) compounds having the As[dbnd]O group has been investigated. Ascorbic acid/iodine reduces arsonic acids, diphenylarsinic acid (but not dimethylarsinic acid), and triphenylarsine oxide. The rates of reduction depend on the electronic effects of the ligands bound to arsenic and on the hydrogen-bonding strength of the species, when present. When the As(V) compound has an [sbnd]NH ₂ or an [sbnd]NH ₃ ⁺ group, the reduction product reacts with a ketonic form of dehydroascorbic acid, giving condensation product(s). Triphenylphosphine/iodine reduced slowly the zwitterionic o-aminophenylarsonic acid but reduced faster the hydrochloric acid salt of the same acid. It reduced dimethylarsinic acid as well because the powerful electron-withdrawing Ph ₃ P ⁺coordinated to As[dbnd]O seems to outweigh the electronic and hydrogen bonding effects.