Structure‐Dependent Photoinduced Electron Transfer in Fullerodendrimers with Light‐Harvesting Oligophenylenevinylene Terminals

Oligophenylenevinylene (OPV)‐terminated phenylenevinylene dendrons G1 – G4 with one, two, four, and eight “side‐arms”, respectively, were prepared and attached to C₆₀ by a 1,3‐dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N‐methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C₆₀G1 – C₆₀G4 , reaching a 99:1 ratio for C₆₀G4 (antenna effect). UV/Vis and near‐IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C₆₀G1 – C₆₀G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free‐energy change for electron transfer is the same along the series owing to the invariability of the donor–acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV→C₆₀ singlet energy transfer. In CH₂Cl₂, the OPV→C₆₀ electron transfer from the lowest fullerene singlet level (¹C₆₀*) is slightly exergonic (ΔG_CS≈0.07 eV), but is observed, to an increasing extent, only in the largest systems C₆₀G2 – C₆₀G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV→C₆₀ electron transfer observed for C₆₀G3 and C₆₀G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer.     

A precise PKPβ+ measurement at Z = 55: (I). The mass of 130Cs

The Q-value for the ¹²⁹Xe(³He, d)¹³⁰Cs reaction is measured to be +5 ± 20 keV. By combining this result with the known neutron separation energy of ¹³⁰Xe, we derive the total decay energy of ¹³⁰Cs

¹³⁰Xe to beQ_EC = 2974 ± 20 keV. This value agrees well with two previous positron end-point measurements but disagrees with the corresponding value derived from the 1977 atomic mass evaluation. This has significance in testing the accuracy of the calculated ratio for ¹³⁰Cs decay. The mass excess of ¹³⁰Cs is derived to be ?86908 ± 14 keV. An excited state in ¹³⁰Cs. was also identified at 121 ± 15 keV with J < 3 and positive parity. The Q-value for ¹³⁶Xe(³He, d)¹³⁷Cs was measured to be 1918 ± 12 keV.     

Irreversible enzyme inhibitors. CXI. Candidate active-site-directed irreversible inhibitors of dihydrofolic reductase derived from 4,6-diamino-1,2-dihydro-l-phenyl-s-triazines. V.

Condensation of 5-(p-nitrophenyl)-2-pentanone with phenylbiguanide hydrochloride (V) gave a 2-methyl-2-(p-nitrophenylpropyl)-1,2-dihydro-s-triazine (IX); hydrogenation of the nitro group to amino followed by bromoacetylation afforded the candidate irreversible inhibitor of dihydrofolic reductase, namely, 2-(p-bromoacetamidophenylpropyl)-4,6-diamino-1,2-dihydro-2-methyl-s-triazine hydrochloride (VIII). Similarly, the o, m, and p-isomers of 5-nitrophenoxy-2-pentanone were converted to the corresponding 2-(bromoacetamidophenoxypropyl)-1,2-dihydro-s-triazines (XI). The four candidate irreversible inhibitors were evaluated on the dihydrofolic reductases from pigeon liver, Walker-256 rat tumor, and L-1210/FR8 mouse leukemia. Only VIII was an irreversible inhibitor; VIII slowly inactivated the L-121-/FR8 mouse leukemia enzyme with a half-life of 2–3 hours at 37°, but VIII showed no inactivation of the other two dihydrofolic reductases—a species specific inactivation.     

An unexpected symmetry of the coulomb potential in the debye-smoluchowski equation

Generalised diffusion processes are discussed using the theory of stochastic processes and several elementary results are proved for the survival probability of a pair of particles with an arbitrary potential. The reaction probability conditioned on reaction ultimately occurring is considered and its backward equation is determined. In the case of the Coulomb potential the probability is shown analytically to be identical in both the attractive and repulsive cases for a given absolute value of r_c. The utility of this result in numerical solutions of the Debye—Smoluchowski equation is discussed.     

Electron-impact induced fragmentation of partially fluorinated β-diketones

The ionic fragmentation of twelve partially fluorinated β-diketones, RCOCH₂COCF₃ was studied with a medium resolution mass spectrometer. In addition to the anticipated fragmentation by β-cleavage, a number of interesting rearrangements are observed. [CF₃COCH₂CO]⁺., an ion common to all of the compounds investigated, decomposes predominantly by elimination of HF. When R is an alkyl group containing hydrogen γ to the adgacent carbonyl, the McLafferty rearrangement occurs. The [RCOCH₂CO]⁺.ion eliminates neutral RH when R is phenyl, thienyl, or cyclopropyl. An intense metastable peak, absent when R is an alkyl substituent, accompanies the rearrangement in these three compounds. A number of fragments characteristic of the R substituent are also observed in many cases. This is interpreted as indicating a considerable degree of charge localization on the R moiety.     

Finite-size scaling study of a first-order temperature-driven symmetry-breaking structural phase transition

We present a study of finite-size effects in a model exhibiting a first-order temperature-driven symmetry-breaking structural phase transition in theL × cylindrical geometry in theL limit. Exact studies demonstrate the applicability of our scaling ansatz even in the one-dimensional limit, making this model ideal for studying finite-size effects. The scaling ansatz, similar to the previously developed ansatz for field-driven transitions, demonstrates that latent heat is crucial in driving these transitions. This ansatz is supported by a 2×2 phenomenological transfer matrix based upon the symmetries of the system; this produces an analytic free energy which has the scaling form. Order parameter probability distributions show that the high- and low-temperature phases coexist only in a small finite-size-affected regime near the bulk transition temperature; this regime vanishes exponentially fast asL diverges.     

Incoherent inelastic neutron scattering studies of potassium bicarbonate and caesium hydrogen dinitrate

Inelastic neutron scattering spectra of KHCO₃ and CsH(NO₃)₂ have been obtained in the region 400 → 2400 and 400 → 2800 cm^?1 respectively. The in- and out-of-phase bending vibrations of the hydrogen bonds have been observed and assigned. For CsH(NO₃)₂ the two bending modes are closer in frequency than in KHCO₃ and they are not resolved from the antisymmetric stretch.     

A direct hydrogen-1 and phosphorus-31 nuclear magnetic resonance cation solvation study of Al(ClO4)3, Ga(ClO4)3, In(ClO4)3, UO2(ClO4)2, and UO2(NO3)2 in water-acetone-dimethylsulfoxide and water-acetone-hexamethylphosphoramide mixtures

A competitive solvation study of Al(ClO₄)₃, Ga(ClO₄)₃, In(ClO₄)₃, UO₂(ClO₄)₂, and UO₂(NO₃)₂ in water-acetone-dimethylsulfoxide (DMSO) and water-acetone-hexamethylphosphoramide (HMPT) mixtures has been carried out by direct H¹ and P³¹ nuclear magnetic resonance (NMR) techniques. At low temperature, proton and ligand exchange are slow enough in these systems to permit the observation of signals for bulk and coordinated molecules of water and the organic bases (DMSO and HMPT). Both DMSO and HMPT compete effectively with water for coordination sites in the Al³⁺, Ga³⁺, and In³⁺ systems, with steric effects dominating the HMPT results. Both Al³⁺ and In³⁺ are able to bind a maximum of two to three HMPT molecules, for example. In contrast, UO²⁺ is solvated selectively by the organic molecules to the allowed maximum of 4 molecules per cation. H¹ and P³¹ NMR spectral results support the formation of only the mono-, tri-, and tetra-HMPT solvation complexes.     

Polymerizable siloxane precursors having in-chain perfluoroalkyl groups

The synthesis and polymerization of several silphenylene siloxane polymer precursors containing a perfluoroalkylsegment in the backbone was carried out. The molecular weight characteristics of polymers from 1,3-bis[p(-hydroxydimethylsilyl)phenyl]hexafluoropropane and 1,3-bis[p(-dimethylaminodimethylsilyl)phenyl]hexafluoropropane were studied as a function of polymerization conditions. Polymers containing the dodecafluorohexane chain segment were also prepared. Polymers having inherent viscosities of 0.55 to 0.9 were obtained. The polymers crosslinked at room temperature to thermoset elastomers which were characterized by improved thermal and oxidative stability over dimethylsilicones. Room temperature swelling of the experimental polymers hydrocarbon solvents was also much lower than that of dimethylsilicones. The polymers containing the (CF₂)₃ and (CF₂)₆ linkages had glass transition points of ?12°C and ?34°C, respectively.