On the longest circuit in an alterable digraph

Analterable digraph is a digraph with a subset of its edges marked alterable and their orientations left undecided. We say that an alterable digraph has an invariant ofk on the length of the longest circuit if it has a circuit of length at leastk regardless of the orientations over its alterable edges. Computing the maximum invariant on the length of the longest circuit in an alterable digraph is aglobal optimization problem. We show that it is hard to approximate the global optimal solution for the maximum invariant problem.Research supported in part by NSF grant CCR 9121472.     

Novel carbazole/fluorene hybrids: host materials for blue phosphorescent OLEDs

[reaction: see text] A series of carbazole/fluorene (CBZm-Fn) hybrids were effectively synthesized through Friedel-Crafts-type substitution of the carbazole rings. These compounds were thermally and morphologically stable host materials for OLED applications. Efficient blue phosphorescent OLEDs were obtained when employing CBZ1-F2 as the host and FIrpic as the guest.     

Improved synthesis of 2,2'-dibromo-9,9'-spirobifluorene and its 2,2'-bisdonor-7,7'-bisacceptor-substituted fluorescent derivatives

Pure 2,2'-Dibromo-9,9'-spirobifluorene was synthesized by a method that did not involve troublesome dibromination of 9,9'-spirobifluorene or Sandmeyer reaction of 2,2'-diamino-9,9'-spirobifluorene. A series of donor-acceptor orthogonally substituted 9,9'-spirobifluorene was subsequently prepared showing rich variation of fluorescence in solution and in solid state. [reaction: see text]     

Ortho-substituent effect on fluorescence and electroluminescence of arylamino-substituted coumarin and stilbene

[structure: see text] Newly synthesized arylamino-substituted coumarins and stilbenes show enhanced fluorescence emission (bluer and brighter) both in solution and in solid film. Pure blue efficient electroluminescence with 2.7% and 4.1% of external quantum efficiency can be achieved with MeC1 and XTPS, respectively.     

Solid-state highly fluorescent diphenylaminospirobifluorenylfumaronitrile red emitters for non-doped organic light-emitting diodes

Bright (maximum 11,000 cd m(-2) and 500 cd m(-2) at 20 mA cm(-2)) and efficient (maximum external quantum efficiency of 3.1% at 1 mA cm(-2)) red (CIE, x = 0.66, y = 0.34) organic light-emitting diodes (OLEDs) employ arylaminospirobifluorene-substituted fumaronitriles as the novel non-dopant red emitter.     

A Study on the Thermal Behaviour of Solder Glass

Vitreous solder glasses, such as Mansol #40 and FEG-2002, are commercialized solder glasses, which are compression sealing glasses that can be used to solder materials with expansions between 55-68⊙10^-7°C^-1, such as Al₂O₃. In order to understand and tailor the thermal behaviour of solder glasses, cylindrical-like glasses were first carefully ground with a stainless steel mortar and pestle. Initially, no exothermic or endothermic data were obtained from the DTA/DSC curves except those relating to melting. However, exothermic peaks appeared after the glass samples were re-melted. In this work, kinetic parameters such as the activation energy, and the morphology of the devitrification mechanisms for two kinds of solder glasses were also investigated, using non-isothermal DTA techniques. The activation energies ranged from 220 to 235 kJ mol^-1 and the devitrification mechanism parameters were close to 1. This indicates that the devitrification mechanisms of the two kinds of solder glasses involve surface nuclei. This revised version was published online in July 2006 with corrections to the Cover Date.