Emission from charge recombination during the pulse radiolysis of arylethynylpyrenes

Emission from several 1-(arylethynyl)pyrenes with a substituent on the aryl group (REPy, R = phenyl (PEPy), 4-dimethylaminophenyl (NPEPy), 4-isopropoxyphenyl (OPEPy), 2-quinonyl (QEPy), and 9-(10-cyanoanthracenyl) (AEPy)) was studied with time-resolved fluorescence measurements during pulse radiolysis in benzene. NPEPy and AEPy showed only monomer emission, while PEPy, OPEPy, and QEPy showed both monomer and excimer emissions during pulse radiolysis. In addition, REPy's also showed long-lived emissions with very weak intensities in the absence of oxygen, which were assigned to the "P-type" delayed fluorescence derived from the triplet-triplet annihilation. The formation of REPy's in the singlet excited state (1REPy*) can be interpreted as the charge recombination between the REPy radical cation and anion (REPy*+ and REPy*-, respectively), which are initially generated from the radiolytic reaction in benzene. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PEPy are localized on the 1-pyrenyl (Py) moiety, while the HOMO of REPy's with an electron donating or withdrawing substituent on the benzene ring (R(D)EPy such as NPEPy and OPEPy or R(A)EPy such as QEPy and AEPy) is mainly localized on the donor moieties (R(D) or Py) and the LUMO on the acceptor ones (Py or R(A), respectively). Therefore, it is suggested that the one-electron oxidation and reduction of REPy's can occur from the donor and acceptor moieties, respectively. This scheme reasonably explains the relationship between the annihilation enthalpy changes (-Delta H' degrees) for the charge recombination of REPy*+ and REPy*- and the singlet excitation energies (E'(S1) of the REPy's. The results are compared with those in electrogenerated chemiluminescence.     

Modelling of nano-alloying and structural evolution of bimetallic core-shell nanoparticles obtained via the microemulsion route

A Monte Carlo model has been developed to describe the formation of bimetallic nanoparticles via the microemulsion route. The motivation stems from the need to understand the kinetics of nanoparticle formation in microemulsion droplets in order to determine the best experimental conditions to synthesize a nanoparticle with a given structure. We focus our study on the influence of the homogeneous and heterogeneous critical nucleus sizes of both metals on nanoparticle structure, as well as the role played by the surfactant film flexibility. The study reveals that the final structure is sensitive to changes in the critical nucleus numbers, because these parameters determine the rate of nucleation. An increase in the difference between nucleation rates of both metals gives rise to a better segregation of metals in the final nanoparticle. Likewise, as long as the formation of heterogeneous seeds is faster, the degree of alloying is greater. Finally, a fast material intermicellar exchange leads to a better mixture of metals, so the influence of the critical nucleus sizes on nanoparticle structure becomes less pronounced as the flexibility of surfactant film is increased.     

Ethylene glycol‐based ionic liquids via azide/alkyne click chemistry

A facile synthetic route for the preparation of dicationic ethylene glycol based‐ionic liquids (ILs) via the azide/alkyne “click” reaction is presented. The copper(I) catalyzed, microwave‐assisted azide/alkyne “click” reaction between diazido‐ethylene glycols and the corresponding alkyne containing IL‐head group enables a simple preparation of different sets of poly(ethylene glycol)‐based ILs. Beside tetra‐ and hexa(ethylene glycol)‐based ILs, also oligomeric (M_n = 400 g/mol) and polymeric ILs (M_n up to 1550 g/mol) could be prepared in good yield and with full conversion of the ionic head group. The prepared ILs were extensively characterized via NMR spectroscopy and ESI‐time‐of‐flight (TOF) mass spectroscopy, revealing the formation of multiply charged ions in the negative mode. Thermal stability proved to be exceptionally high (up to 300 °C) together with low glass‐transition temperatures. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Effect of hydrophobic amino acids on the conformational change of decapeptides in micellar environments

A series of peptides containing various hydrophobic amino acids [methionine (Met), leucine (Leu), norleucine (Nle), phenylalanine (Phe), 2-aminooctanoic acid (Aoc), and 2-aminodecanoic acid (Ade)] were synthesized and their conformations were studied using circular dichroism (CD) spectroscopy in different solvents such as water, methanol, and aqueous solution of ammonium tetradecanesulfonate. Peptides containing hydrophobic amino acids with linear side chains formed -sheets in water and methanol. Electrostatic interaction between the charged side chain (lysine) and a micelle consisting of an anionic surfactant, ammonium tetradecanesulfonate, is necessary for the formation of -helices in micellar environments. The conformational transition from -helix to -sheet structure required moderate hydrophobicity and linear side chains. This conformational transition depended on the surfactant concentration.     

Specific heat on Sr<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> and Sr<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>7</Subscript>

Summary Specific heats on the single crystals of Sr₂Nb₂O₇, Sr₂Ta₂O₇ and (Sr_1-xBa_x)₂Nb₂O₇ were measured in a wide temperature range of 2-600 K. Heat anomalies of a λ-type were observed at the incommensurate phase transition of T_INC (=495 K) on Sr₂Nb₂O₇ and at the super-lattice phase transition of T_SL (=443 K) on Sr₂Ta₂O₇; the transition enthalpies and the transition entropies were estimated. Furthermore, a small heat anomaly was observed at the low temperature ferroelectric phase transition of T_LOW (=95 K) on Sr₂Nb₂O₇. The transition temperature T_LOW decreases with increasing Ba content x and it vanishes for samples of x>2%.     

Excess Heat Capacity in Yttria Stabilized Zirconia

The heat capacity of 9.70 and 11.35 mol% yttria stabilized zirconia ((ZrO₂)_1–x(Y₂O₃)_x; x=0.0970, 0.1135) was measured by adiabatic calorimetry between 13 and 300 K, and some thermodynamic functions were calculated and given in a table. A large excess heat capacity extending from the lowest temperature to room temperature with a broad maximum at about 75 K was found in comparison with the heat capacity calculated from those of pure zirconia and yttria on the basis of simple additivity rule. The shape of the excess heat capacity is very similar to the Schottky anomaly, which may be attributed to a softening of lattice vibration. The amount of the excess heat capacity decreased with increasing yttria doping, while the maximum temperature did not vary. The relationships among the excess heat capacity, defect structure and interatomic force constants, and also the role of oxygen vacancy were discussed.This revised version was published online in November 2005 with corrections to the Cover Date.