Efficiency enhancement in a microcavity solid-state dye laser with Bragg grating reflectors

Bragg grating reflectors placed along microcavity facets can improve the efficiency of a polymer dye laser built with such a microcavity. The impact of different reflector designs on the mode pattern and resonance frequencies of the microcavity is numerically simulated and analyzed. This rigorous physical model is based on solving the Maxwell equations and includes such material properties as absorption, dispersion, fluorescence and optical gain. In certain cases, an asymmetrical layout of the reflectors can be more preferable than the pair of reflectors located on opposite sides of the microcavity as it is implemented for typical design.     

Synthetic and mechanistic aspects of cross-coupling of nitroxyl radicals of 3-imidazoline series with terminal alkynes

Practical synthetic approaches to the new class of acetylenic derivatives of 3-imidazolyl-3-oxide-1-oxyls, including biradicals, were developed through cross-coupling reactions of 3-imidazolyl halides with either terminal alkynes or their copper salts. The presence of nitroxyl functional group as an internal oxidant leads to a competition between the formation of cross-coupling products and the products of oxidative homocoupling. The balance in this competition can be shifted toward the cross-coupling products through the combination of factors that includes nature of the catalyst, reactivity of the halides, and reaction conditions.     

Titanium complexes of dialkanolamine ligands as initiators for living ring‐opening polymerization of ε‐caprolactone

The titanium complexes with one ( 1a , 1b , 1c ) and two ( 2a , 2b ) dialkanolamine ligands were used as initiators in the ring‐opening polymerization (ROP) of ε‐caprolactone. Titanocanes 1a and 1b initiated living ROP of ε‐caprolactone affording polymers whose number‐average molecular weights (M_n) increased in direct proportion to monomer conversion (M_n ≤ 30,000 g mol^?1) in agreement with calculated values, and were inversely proportional to initiator concentration, while the molecular weight distribution stayed narrow throughout the polymerization (M_w/M_n ≤ 1.2 up to 80% monomer conversion). ¹H‐NMR and MALDI‐TOF‐MS studies of the obtained poly(ε‐caprolactone)s revealed the presence of an isopropoxy group originated from the initiator at the polymer termini, indicating that the polymerization takes place exclusively at the Ti–OⁱPr bond of the catalyst. The higher molecular weight polymers (M_n ≤ 70,000 g mol^?1) with reasonable MWD (M_w/M_n ≤ 1.6) were synthesized by living ROP of ε‐caprolactone using spirobititanocanes ( 2a , 2b ) and titanocane 1c as initiators. The latter catalysts, according MALDI‐TOF‐MS data, afford poly(ε‐caprolactone)s with almost equal content of α,ω‐dihydroxyl‐ and α‐hydroxyl‐ω(carboxylic acid)‐terminated chains arising due to monomer insertion into “Ti–O” bond of dialkanolamine ligand and from initiation via traces of water, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1230–1240, 2010     

Controlled cationic polymerization of cyclopentadiene with B(C6F5)3 as a coinitiator in the presence of water

The controlled cationic polymerization of cyclopentadiene (CPD) at 20 °C using 1‐(4‐methoxyphenyl)ethanol (1)/B(C₆F₅)₃ initiating system in the presence of fairly large amount of water is reported. The number–average molecular weights of the obtained polymers increased in direct proportion to monomer conversion in agreement with calculated values and were inversely proportional to initiator concentration, while the molecular weight distribution slightly broadened during the polymerization (M_w/M_n ～ 1.15–1.60). ¹H NMR analyses confirmed that the polymerization proceeds via reversible activation of the C? OH bond derived from the initiator to generate the growing cationic species, although some loss of hydroxyl functionality happened in the course of the polymerization. It was also shown that the enchainment in cationic polymerization of CPD was affected by the nature of the solvent(s): for instance, polymers with high regioselectivity ([1,4] up to 70%) were obtained in acetonitrile, whereas lower values (around 60%) were found in CH₂Cl₂/CH₃CN mixtures. Aqueous suspension polymerization of CPD using the same initiating system was successfully performed and allowed to synthesize primarily hydroxyl‐terminated oligomers (F_n = 0.8–0.9) with M_n ≤ 1000 g mol^?1 and broad MWD (M_w/M_n ～ 2.2). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4734–4747, 2008     

The review of possible interrelations between ionic conductivity, internal friction and the viscosity of glasses and glass forming melts within the framework of Maxwell equations

The review contains the results of application of Maxwell equations for mechanical relaxation and electrical conductivity, to the systematization of large amount of experimental data related to mechanical, viscous, and electrical properties of inorganic glasses and glass forming melts. The generalization of internal friction results shows the existence of characteristic values for the ratios of temperatures, responsible for α-, β- and ionic relaxations; they are independent on the frequency and chemical composition. This is the evidence for the main role of elastic deformations at various corpuscular processes and the existence of characteristic scale of activation barriers predetermined by local volumes of deformation. It is shown the possibility of very precise calculation of the temperature of “ionic” internal friction maximum for one-alkali oxide glasses directly from Maxwell equation and d.c. conductivity experimental data. The volumes of particles overcoming the potential barrier at viscous flow practically coincide with the results of direct structural determinations. The existence of universal relation between d.c. conductivity and viscosity for the extremely wide temperature intervals (Littleton relation) is proved for silicate and phosphate melts. The theory of this dependence is proposed. The results show the effectiveness of the attempts to unite the continual and discrete approaches within the framework of Maxwell equations to obtain the simplest understanding the mechanisms of different types of relaxation. The review comprises many Russian papers unknown in English scientific literature.     

Tuning multiphase amphiphilic rods to direct self-assembly

New methods to direct the self-assembly of particles are highly sought after for multiple applications, including photonics, electronics, and drug delivery. Most techniques, however, are limited to chemical patterning on spherical particles, limiting the range of possible structures. We developed a lithographic technique for fabrication of chemically anisotropic rod-like particles in which we can specify both the size and shape of particles and implement multiple diverse materials to control interfacial interactions. Multiphase rod-like particles, including amphiphilic diblock, triblock, and multiblock were fabricated in the same template mold having a tunable hydrophilic/hydrophobic ratio. Self-assembly of diblock or triblock rods at a water/oil interface led to the formation of bilayer or ribbon-like structures.     

Electrochemistry of platinum(II) porphyrins: effect of substituents and π-extension on redox potentials and site of electron transfer

Fourteen platinum(II) porphyrins with different π-conjugated macrocycles and different electron-donating or electron-withdrawing substituents were investigated as to their electrochemical and spectroscopic properties in nonaqueous media. Eight compounds have the formula (Ar(4)P)Pt(II), where Ar(4)P = the dianion of a tetraarylporphyrin, while six have π-extented macrocycles with four β,β'-fused benzo or naphtho groups and are represented as (TBP)Pt(II) and (TNP)Pt(II) where TBP and TNP are the dianions of tetrabenzoporphyrin and tetranaphthoporphyrin, respectively. Each Pt(II) porphyrin undergoes two reversible one-electron reductions and one to three reversible one-electron oxidations in nonaqueous media. These reactions were characterized by cyclic voltammetry, UV-visible thin-layer spectroelectrochemistry and in some cases by ESR spectroscopy. The two reductions invariably occur at the conjugated π-ring system to yield relatively stable Pt(II) π-anion radicals and dianions. The first oxidation leads to a stable π-cation radical for each investigated porphyrin; but in the case of tetraarylporphyrins containing electron-withdrawing substituents, the product of the second oxidation may undergo an internal electron transfer to give a Pt(IV) porphyrin with an unoxidized macrocycle. The effects of macrocycle structure on UV-visible spectra, oxidation/reduction potentials, and site of electron transfer are discussed.     

Nonadiabatic excited-state molecular dynamics: numerical tests of convergence and parameters

Nonadiabatic molecular dynamics simulations, involving multiple Born-Oppenheimer potential energy surfaces, often require a large number of independent trajectories in order to achieve the desired convergence of the results, and simulation relies on different parameters that should be tested and compared. In addition to influencing the speed of the simulation, the chosen parameters combined with the frequently reduced number of trajectories can sometimes lead to unanticipated changes in the accuracy of the simulated dynamics. We have previously developed a nonadiabatic excited state molecular dynamics methodology employing Tully's fewest switches surface hopping algorithm. In this study, we seek to investigate the impact of the number of trajectories and the various parameters on the simulation of the photoinduced dynamics of distyrylbenzene (a small oligomer of polyphenylene vinylene) within our developed framework. Various user-defined parameters are analyzed: classical and quantum integration time steps, the value of the friction coefficient for Langevin dynamics, and the initial seed used for stochastic thermostat and hopping algorithms. Common approximations such as reduced number of nonadiabatic coupling terms and the classical path approximation are also investigated. Our analysis shows that, at least for the considered molecular system, a minimum of ~400 independent trajectories should be calculated in order to achieve statistical averaging necessary for convergence of the calculated relaxation timescales.