The novel dimerisation of radical anions generated electrochemically in aqueous solution from maleimide

The one-electron reduction of maleimide yields the novel compound 1,2,3,4-butane-tetracarboxylic 1,2:3,4 diimide exclusively. Substrate-substrate hydrogen bonding was found to have a profound influence upon the pathways and kinetics of this unusual reaction.     

LIGHT ABSORPTION AND ENERGY TRANSFER IN POLYENE-PORPHYRIN ESTERS*

Two examples of a new class of compounds, polyenes covalently linked to meso-tetraarylporphyrins, have been prepared. Optical studies revealed that energy transfer from a 26 π electron polyene moiety to the porphyrin was zero, whereas energy transfer efficiency from a polyene containing 18 π electrons was ?80%. The results demonstrate that the polyene moiety can function as an efficient light gathering antenna for possible application to porphyrin-based solar energy conversion schemes. In addition, such polyene-porphyrin esters may serve as probes of the structural requisites for light harvesting by carotenoids in the photosynthetic membranes of green plants.     

Grain Boundary Channel Formation During Interdiffusion in Alloys

We present results of an experimental study of a new phenomenon accompanying grain boundary (GB) interdiffusion: the hole channel formation along GBs. The objects for study were plates of a homogeneous Cu-5 at.% Sn alloy, which were annealed at 800°C in purified hydrogen. Porous zones were found with GB hole channels perpendicular to the surface and practically equidistant from one another. The porous zone propagation and the average pore size growth at early stages of annealing obey a parabolic law. The observed processes are caused by nucleation and growth of the Cu₃Sn phase at the free surface. The new phase works as diffusion pump pumping out Sn atoms from the alloy towards the growing compound layer. The GB channel formation has been described as a relaxation process accompanying GB interdiffusion of Sn and Cu atoms with unequal partial diffusion coefficients (D _Sn>D _Cu). Excessive vacancies appearing at the GBs due to the inequality between D _Sn and D _Cu are absorbed by bulk and GB sinks, and tensile stresses appear near the GBs stimulating hole channels or groove formation.     

Investigations on the Decomposition of Carboplatin in Infusion Solutions II. Effect of 1,1-Cyclobutanedicarboxylic Acid Admixture

 Eleven infusion solutions containing carboplatin ( ≈ 10 mg/ml) stored for 5 to 78 months at room temperature were investigated for the content of carboplatin and the profile of decomposition products by means of atomic absorption spectrophotometry (AAS) and high performance liquid chromatography (HPLC). Carboplatin loss was between 3.09% and 7.28%. Carboplatin degradation was exponential and depended only on the time of storage. For seven of the investigated products an admixture of 1,1-cyclobutane-dicarboxylic acid (CBDCA) (0.75%–2%) was shown. This admixture did not suppress the break-down of carboplatin, but changed the quantity of certain degradation products. Besides hydrophilic aqua- or hydroxyplatinum(II)-species also hydrophobic side products were detectable. Especially in infusion solutions with CBDCA admixture a side product (SP) developed after the 55^th week of storage. The peak area of this SP amounted to more than 1% of the peak area of carboplatin by the time of expiry. Received December 11, 2001; accepted February 27, 2002; published online July 15, 2002     

Phase Characterization of Diffusion Soldered Ni/Al/Ni Interconnections

The formation and growth of intermetallic phases in the Ni-Al system during a novel joining process for Ni/Ni interconnections based on diffusion soldering has been studied. The Ni/Al/Ni bonds were accomplished by isothermal solidification and subsequent interdiffusion of Ni and Al in the Ni/Al/Ni joints held at a temperature of 720°C. Optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction analysis were used to characterize the microstructural changes as a function of the reaction time. The following phases appeared sequentially: liquid Al → Al₃Ni → Al₃Ni₂ → AlNi (stoichiometric) → AlNi (Ni-rich) → AlNi₃. At intermediate stages two to four phases coexisted. The NiAl phase occurred in two variants, namely a Ni-rich AlNi (60 at.% Ni) and stoichiometric AlNi. The joining process was completed after 30 h of reaction. Then only AlNi₃ was present in the Ni/Al/Ni interconnection zone. The quality of the resultant bond and the high melting point of the AlNi₃ phase (1360°C) indicate a great potential of the diffusion soldering for the joining of heat dissipating devices used in electronics and electrotechnics.     

Evolution of Pore Ensembles Located at the Grain Boundaries in Cu and Cu-5 at.% Sn

The kinetics of pore coalescence in ensembles located at the grain boundaries in Cu and Cu-5 at. % Sn has been studied by optical and scanning electron microscopy at 500 and 800°C. In the case of pure Cu the evolution of the pore ensemble followed the kinetical law ¯4 t, where ¯ is the average pore radius and t is the ageing time. This law is typical for a grain boundary diffusion controlled process. However, in the case of the Cu-5 at. % Sn alloy the kinetical law obeyed the ¯3 t relationship, which is characteristic for a reaction controlled process. The reaction in question is the generation of vacancies on the surfaces of the pores. The importance of the surface reaction controlled mass transfer process in alloys is discussed. The grain boundary self-diffusivity and the kinetical coefficients of the reaction are determined and analyzed.     

Molecular mechanical and quantum chemical study of the species involved in the hydrolysis ofcis-diamminedichloroplatinum(II) and substitutedbis(ethylenediamine)-dichloroplatinum(II) complexes II. Simulated transition states

Summary cis-Diamminedichloroplatinum(II) (cisplatin) and its substituted ethylenediamine derivativescis-PtCl₂(R ₂ en) (en=ethylenediamine,R=H,Ph,2-,3-, and 4-PhOH) have been investigated with respect to the possible structures of the hypothetical Transition State Complexes (TSC) of the hydrolytic S_N2 reaction in which one Cl is replaced by H₂O.TSCs withtrigonal bipyramid (TBP) andsquare pyramid (SP) geometry (coordination number 5), have been studied by Molecular Mechanics (MM) and ExtendedHückel (EH) methods. TheEH andMM energies as well as the number of occurrence (entropy factor) for the cisplatinum compound point to a preferredTBP TSC geometry with NH₃ and Cl in axial positions. However, foren and substituteden compounds,TSCs withSP geometries (Cl in apical position) are preferred. The calculatedEH andMM energies of theTBP andSP structures do not differ significantly andTBP SP interconversions may play an essential role inTSC formation. To improve the discrimination, theMM-optimized geometries were treated in terms of displacement coordinates for D_3h (TBP) and C_4v (SP) by calculating the total distortion vectors (DV).DV identified once again theTBP with NH₃ and Cl in axial position as the least-distorted conformer, but it also revealed the combinations of displacement coordinates which shape theTSC geometry.

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Molekularmechanische und quantenchemische Untersuchung der bei der Hydrolyse voncis-Diammindichlorplatin(II) und substituiertenBis(ethylendiamin)dichlorplatin(II)-Komplexen auftretenden Spezies, 2. Mitt. Simulierte Übergangszustände

Zusammenfassung cis-Diammindichlorplatin(II) (Cisplatin) und seine substituierten Ethylendiaminderivatecis-PtCl₂(R ₂ en) (en=Ethylendiamin,R=H, Ph, 2-, 3- und 4-PhOH) wurden im Hinblick auf mögliche Strukturen der hypothetischen Übergangszustandkomplexe (TSC) der hydrolytische S_N2-Reaktion (Substitution eines Cl-Atoms durch H₂O) untersucht.TSCs mit trigonalbipyramidalen (TBP) und quadratisch-pyramidalen (SP) Geometrien (Koordinationszahl 5) wurden mit molekularmechanischen (MM) und Extended-Hückel-Methoden (EH) behandelt.EH- undMM-Energien sowie entropische Faktoren weisen für Cisplatin auf eine trigonale Bipyramide mit NH₃ und Cl in axialen Positionen als bevorzugteTSC-Geometrie hin, während für Komplexe miten-LigandenSP-Geometrien mit Cl in der apicalen Position energetisch begünstigt sind. Da die berechnetenEH- undMM- Energien fürTBP- undSP-Geometrien sehr ähnlich sind, spielen möglicherweiseTBP-SP-Umwandlungen eine wesentliche Rolle bei der Bildung derTSCs. Zur Verbesserung der Unterscheidung wurden für dieMM-optimierten Geometrien die Verschiebungsvektoren (DV) bezüglich D_3h (TBP) und C_4v (SP) berechnet. Daraus resultierte erneut die trigonale Bipyramide mit NH₃ und Cl in den axialen Positionen als das am wenigsten gespannte Konformere; des weiteren konnten mit dieser Methode die Kombinationen der Verschiebungskoordinaten erhalten werden, die für die Ausbildung derTSC-Geometrie verantwortlich sind.

     

Molecule-based photonically switched half-adder

A molecule-based binary half-adder with optical inputs and outputs has been demonstrated. The half-adder consists of two photochromic organic molecules in solution and a third-harmonic-generating crystal. One substance acts as an AND Boolean logic gate and the other as an XOR gate. Inputs are laser pulses at 1064 or 532 nm that initiate photoisomerization reactions. Outputs are the optical absorbance of a fullerene radical anion (AND gate) and fluorescence of a porphyrin (XOR gate). The system carries out binary addition based on the laser input pulses. Half-adders in combination are capable of carrying out all mathematical operations necessary for digital computing.     

The gold porphyrin first excited singlet state

Gold porphyrins are often used as electron-accepting chromophores in artificial photosynthetic constructs. Because of the heavy atom effect, the gold porphyrin first-excited singlet state undergoes rapid intersystem crossing to form the triplet state. The lowest triplet state can undergo a reduction by electron donation from a nearby porphyrin or another moiety. In addition, it can be involved in triplet-triplet energy transfer interactions with other chromophores. In contrast, little has been known about the short-lived singlet excited state. In this work, ultrafast time-resolved absorption spectroscopy has been used to investigate the singlet excited state of Au(III) 5,15-bis(3,5-di-t-butylphenyl)-2,8,12,18,-tetraethyl-3,7,13,17-tetramethylporphyrin in ethanol solution. The excited singlet state is found to form with the laser pulse and decay with a time constant of 240 fs to give the triplet state. The triplet returns to the ground state with a life-time of 400 ps. The lifetime of the singlet state is comparable with the time constants for energy and photoinduced electron transfer in some model and natural photosynthetic systems. Thus, it is kinetically competent to take part in such processes in suitably designed supermolecular systems.     

Photochromic control of photoinduced electron transfer. Molecular double-throw switch

A molecular double-throw switch that employs a photochromic moiety to direct photoinduced electron transfer from an excited state donor down either of two pathways has been prepared. The molecular triad consists of a free base porphyrin (P) linked to both a C(60) electron acceptor and a dihydroindolizine (DHI) photochrome. Excitation of the porphyrin moiety of DHI-P-C(60) results in photoinduced electron transfer with a time constant of 2.3 ns to give the DHI-P(*)(+)-C(60)(*)(-) charge-separated state with a quantum yield of 82%. UV (366 nm) light photoisomerizes the DHI moiety to the betaine (BT) form, which has a higher reduction potential than DHI. Excitation of the porphyrin of BT-P-C(60) is followed by photoinduced electron transfer with a time constant of 56 ps to produce BT(*)(-)-P(*)(+)-C(60) in 99% yield. Isomerization of BT-P-C(60) back to DHI-P-C(60) may be achieved with visible light, or thermally. Thus, photoinduced charge separation originating from the porphyrin is reversibly directed down either of two different pathways by photoisomerization of the dihydroindolizine. The switch may be cycled many times.