Ruben D. Abellon

Supercrystals of CdSe Quantum Dots with High Charge Mobility and Efficient Electron Transfer to TiO2

Thermal annealing of thin films of CdSe/CdS core/shell quantum dots induces superordering of the nanocrystals and a significant reduction of the interparticle spacing. This results in a drastic enhancement of the quantum yield for charge carrier photogeneration and the charge carrier mobility. The mobile electrons have a mobility as high as 0.1 cm(2)/(V x s), which represents an increase of 4 orders of magnitude over non-annealed QD films and exceeds existing literature data on the electron mobility in CdSe quantum dot films. The lifetime of mobile electrons is longer than that of the exciton. A fraction of the mobile electrons gets trapped at levels below the conduction band of the CdSe nanocrystals. These electrons slowly diffuse over 50-300 nm on longer times up to 20 micros and undergo transfer to a TiO2 substrate. The yield for electron injection in TiO2 from both mobile and trapped electrons is found to be >16%.

Hydrogen-bond stabilized columnar discotic benzenetrisamides with pendant triphenylene groups

A series of 1,3,5-benzenetrisamide derivatives with three hexaalkoxytriphenylene pendant groups were prepared, in which the triphenylene groups are connected to the central 1,3,5-benzenetrisamide core through a flexible spacer. The length of this spacer, as well as the size of the ortho-substituent at the triphenylene core, influences the columnar packing of these molecules. All compounds show liquid crystalline behavior with high isotropization temperatures. Different columnar hexagonal phases (Colh, Colhp, Colobl and Colr) have been identified using optical polarization microscopy, differential scanning calorimetry and X-ray diffraction. A chiral 1,3,5-benzenetrisamide derivative forms columnar stacks with a single helical sense, both in an apolar solvent and in a film, as observed by circular dichroism studies. Pulse radiolysis time-resolved microwave conductivity (PR-TRMC) studies show that the high ordering in a Colhp phase results in high charge carrier mobilities. On the other hand, the highest mobility was observed for the chiral compound, which has a Colr ordering.

Monitoring the radiation-induced bulk polymerisation of methyl methacrylate with N-(1-pyrene)maleimide

Abstract The fluorogenic probe N-(1-pyrene)maleimide, MPy, exhibits a fluorescence response directly correlated to the monomer conversion in the radiation-induced polymerisation of methyl methacrylate (MMA). In the low-dose region, the ratio between propagation rate constants for the cross-over reaction in which a MMA radical chain-end reacts with MPy relative to the homopolymerisation reaction of MMA is found to be 1.38±0.15.

Radiation-induced polymerization monitored in situ by time-resolved fluorescence of probe molecules in methyl methacrylate

A technique is presented for monitoring radiation-induced polymerizations in situ based on the measurement of the fluorescence lifetime of molecular probes dissolved in the polymerizing medium. This method is illustrated with results on methyl methacrylate (MMA) using two fluorogenic probe molecules; N-(2-anthracene)methacrylamide (AnMA) and maleimido-fluoroprobe (MFP), a molecule which has a highly dipolar excited state.

Steady-state and pulsed studies of the radiation-induced polymerization of methyl methacrylate

In order to quantify our fluorogenic molecular probe studies on the radiation-induced polymerization of methyl methacrylate (MMA), measurements have been made of the monomer conversion, CM, and polymer molecular weight distribution as a function of dose for bulk MMA polymerized by steady-state (60Co γ-rays) and nanosecond-pulsed (3 MeV electrons) radiation. In all cases, CM was found to increase close to linearly with dose up to ca. 30%. Above this conversion, autoacceleration of polymerization (the gel or Trommsdorff effect) occurs in the γ-irradiated samples. From the low-conversion steady-state data, using dose-rates of 0.21 and 2.7 Gy/s, the parameter 〈kp〉 2G(R.)/2〈kt〉 = 0.011 and 0.015 lmol-1 s-1 (100 eV)-1 respectively have been determined (with 〈kp〉 and 〈kt〉 the average rate coefficients for bimolecular propagation and termination respectively, and G(R.) the yield of the chain-initiating free radicals per 100 eV (G = 1 (100 eV)-1 0.1036 μ mol J-1)). From the 5 Hz repetitive pulse data the value of 〈kp〉 G(R.) = 1700 lmol-1 s-1 (100 eV)-1 has been determined. Taking 〈kp〉 = 342 lmol-1 s-1 from the literature results in G(R.) = 5.0 (100 eV)-1 and 〈kt〉 = 2.7 × 107 lmol-1 s-1.