Benjamin J. Schwartz

Semiconducting polymers : A new class of solid-state laser materials

Gain narrowing in optically pumped thin films, both neat and undiluted, of luminescent conjugated polymers with different molecular structures was demonstrated. These results indicate that the polymers studied have large cross sections for stimulated emission, that population inversion can be achieved at low pump energies, and that the emitted photons travel distances greater than the gain length within the gain medium. The use of simple waveguide structures is sufficient to cause low gain narrowing thresholds in submicrometer-thick films.

Conjugated polymer aggregates in solution: Control of interchain interactions

We present evidence that the degree of interchain interactions and morphology in conjugated polymer films can be controlled by altering the chain conformation in the solution from which the film is cast. Light scattering experiments show that the physical size of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) chains can vary by a factor of 2 in different solvents such as chlorobenzene (CB) or tetrahydrofuran (THF). Photoluminescence and wavelength-dependent excitation indicate that MEH-PPV forms aggregate species with an absorption and luminescence spectra that are distinctly red-shifted from the intrachain exciton. The degree of aggregation is both concentration and solvent dependent; for solutions with concentrations typical of those used in spin casting, aggregates comprise a significant fraction of the total number of excited state species. The overall photoluminescence quantum yield is found to depend both on how restricted the polymer conformation is due to the choice of solv...

Mutation of A677 in histone methyltransferase EZH2 in human B-cell lymphoma promotes hypertrimethylation of histone H3 on lysine 27 (H3K27)

Trimethylation of histone H3 on lysine 27 (H3K27me3) is a repressive posttranslational modification mediated by the histone methyltransferase EZH2. EZH2 is a component of the polycomb repressive complex 2 and is overexpressed in many cancers. In B-cell lymphomas, its substrate preference is frequently altered through somatic mutation of the EZH2 Y641 residue. Herein, we identify mutation of EZH2 A677 to a glycine (A677G) among lymphoma cell lines and primary tumor specimens. Similar to Y641 mutant cell lines, an A677G mutant cell line revealed aberrantly elevated H3K27me3 and decreased monomethylated H3K27 (H3K27me1) and dimethylated H3K27 (H3K27me2). A677G EZH2 possessed catalytic activity with a substrate specificity that was distinct from those of both WT EZH2 and Y641 mutants. Whereas WT EZH2 displayed a preference for substrates with less methylation [unmethylated H3K27 (H3K27me0):me1:me2 kcat/Km ratio = 9:6:1] and Y641 mutants preferred substrates with greater methylation (H3K27me0:me1:me2 kcat/Km ratio = 1:2:13), the A677G EZH2 demonstrated nearly equal efficiency for all three substrates (H3K27me0:me1:me2 kcat/Km ratio = 1.1:0.6:1). When transiently expressed in cells, A677G EZH2, but not WT EZH2, increased global H3K27me3 and decreased H3K27me2. Structural modeling of WT and mutant EZH2 suggested that the A677G mutation acquires the ability to methylate H3K27me2 through enlargement of the lysine tunnel while preserving activity with H3K27me0/me1 substrates through retention of the Y641 residue that is crucial for orientation of these smaller substrates. This mutation highlights the interplay between Y641 and A677 residues in the substrate specificity of EZH2 and identifies another lymphoma patient population that harbors an activating mutation of EZH2.

Laser emission from solutions and films containing semiconducting polymer and titanium dioxide nanocrystals

Abstract We report laser emission from solutions and dilute blend films containing a semiconducting polymer, poly(2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV), and titanium dioxide (TiO2) nanocrystals under pulsed optical excitation at 532 and 435 nm. The TiO2 nanoparticles multiply scatter photons in the active polymer medium such that gain exceeds loss above a critical excitation threshold. Above threshold, the emission spectrum narrows significantly. Solid state lasing is observed from free standing films of MEH-PPV and TiO2 in polystyrene. This is the first demonstration of lasing with a semiconducting polymer in the solid state as the active medium.

Quantum decoherence and the isotope effect in condensed phase nonadiabatic molecular dynamics simulations

In this paper, we explore in detail the way in which quantum decoherence is treated in different mixed quantum‐classical molecular dynamics algorithms. The quantum decoherence time proves to be a key ingredient in the production of accurate nonadiabatic dynamics from computer simulations. Based on a short time expansion to a semiclassical golden rule expression due to Neria and Nitzan [J. Chem. Phys. 99, 1109 (1993)], we develop a new computationally efficient method for estimating the decay of quantum coherence in condensed phase molecular simulations. Using the hydrated electron as an example, application of this method finds that quantum decoherence times are on the order of a few femtoseconds for condensed phase chemical systems and that they play a direct role in determining nonadiabatic transition rates. The decay of quantum coherence for the solvated electron is found to take ≊50% longer in D2O than in H2O, providing a rationalization for a long standing puzzle concerning the lack of experimentally...

Aqueous solvation dynamics with a quantum mechanical Solute: Computer simulation studies of the photoexcited hydrated electron

We have used molecular dynamics simulation to explore aqueous solvation dynamics with a realistic quantum mechanical solute, the hydrated electron. The simulations take full account of the quantum charge distribution of the solute coupled to the dielectric and mechanical response of the solvent, providing a molecular‐level description of the response of the quantum eigenstates following photoexcitation. The solvent response function is found to be characterized by a 25 fs Gaussian inertial component (40%) and a 250 fs exponential decay (60%). Despite the high sensitivity of the electronic eigenstates to solvent fluctuations and the enormous fractional Stokes’ shift following photoexcitation, the solvent response is found to fall within the linear regime. The relaxation of the quantum energy gap due to solvation is shown to play a direct role in the nonradiative decay dynamics of the excited state electron, as well as in the differing relaxation physics observed between electron photoinjection and transient hole‐burning (photoexcitation) experiments. A microscopic examination of the solvation response finds that low frequency translational motions of the solvent play an important role in both the inertial and diffusive portions of the relaxation. Much of the local change in solvation structure is associated with a significant change in size and shape of the electron upon excitation. These results are compared in detail both to previous studies of aqueous solvation dynamics and to ultrafast transient spectroscopic work on the hydrated electron.

Improving the performance of conjugated polymer-based devices by control of interchain interactions and polymer film morphology

Interchain interactions in conjugated polymer films promote good carrier transport but also reduce the luminescence quantum yield, leading to a fundamental trade-off in optimizing film morphology for device performance. We present two methods to improve the efficiency of light-emitting diodes (LEDs) based on poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) by altering film morphology without changing device architecture. First, “trilayer” LEDs, which use a central MEH-PPV layer with reduced interchain interactions between layers of highly aggregated MEH-PPV near the electrodes, have a higher efficiency than single-layer devices. Second, device efficiency can be improved by annealing MEH-PPV films, so that the reduced emission upon increasing interchain interactions is overcome by more balanced charge injection.

Single-shot two-photon exposure of commercial photoresist for the production of three-dimensional structures

We report the use of an amplified femtosecond laser for single-shot two-photon exposure of the commercial photoresist SU-8. By scanning of the focal volume through the interior of the resist, three-dimensional (3-D) structures are fabricated on a shot-by-shot basis. The 800-nm two-photon exposure and damage thresholds are 3.2 and 8.1TW/cm(2), respectively. The nonlinear nature of the two-photon process allows the production of features that are smaller than the diffraction limit. Preliminary results suggest that Ti:sapphire oscillators can achieve single-shot two-photon exposure with thresholds as low as 1.6TW/cm(2) at 700 nm, allowing 3-D structures to be constructed at megahertz repetition rates.

Does the Hydrated Electron Occupy a Cavity

Filling a Cavity Unlike liquid ammonia, water cannot sustain a steady concentration of isolated electrons. Nonetheless, high-energy irradiation can introduce a small number of free charges that engage in potent reductive chemistry and have clear spectroscopic signatures. The manner in which water solubilizes these hydrated electrons has remained uncertain, but the general consensus has been that repulsive interactions drive the nearest water molecules away, leaving the electron in a nearly spherical empty cavity. Larsen et al. (p. 65; see the Perspective by Jordan and Johnson) upend this consensus with simulations based on a more thorough potential function for modeling the competing attractions and repulsions between the electron and surrounding water. The calculations suggest that the hydrated electron actually draws water in, occupying a region denser than the pure bulk liquid. The model reproduces experimental spectral and dynamic observations as effectively as, and in some cases better than, the cavity framework. A long-standing model of the solvent geometry surrounding a free charge in water is questioned by new numerical simulations. Since the discovery of the hydrated electron more than 40 years ago, a general consensus has emerged that the hydrated electron occupies a quasispherical cavity in liquid water. We simulated the electronic structure and dynamics of the hydrated electron using a rigorously derived pseudopotential to treat the electron-water interaction, which incorporates attractive oxygen and repulsive hydrogen features that have not been included in previous pseudopotentials. What emerged was a hydrated electron that did not reside in a cavity but instead occupied a ~1-nanometer-diameter region of enhanced water density. Both the calculated ground-state absorption spectrum and the excited-state spectral dynamics after simulated photoexcitation of this noncavity hydrated electron showed excellent agreement with experiment. The relaxation pathway involves a rapid internal conversion followed by slow ground-state cooling, the opposite of the mechanism implicated by simulations in which the hydrated electron occupies a cavity.

Plastic lasers: Semiconducting polymers as a new class of solid-state laser materials

Abstract We demonstrate optically pumped lasing in submicron thick films, neat and undiluted, of photoluminescent conjugated polymers. Lasing is evidenced by a dramatic collapse of the emission line width (to as little as 7 nm) at very low pump energy thresholds (~10 μJ/cm 2 ). Laser action is found in over a dozen different conjugated polymers representing a variety of molecular structures, including poly( p -phenylenevinylene), poly( p -phenylene) and polyfluorene derivatives; lasing wavelengths in these materials span the visible spectrum. The short gain lengths in these conjugated polymers are attributed to the high density of chromophores, the large density of states associated with the interband (π-π * ) transition in quasi-one-dimensional systems, and the Stokes shift which minimizes self-absorption and enhances stimulated emission in the absence of excited state absorption. The observation of lasing in this new class of solid-state laser materials is explained in terms of simple planar waveguiding structures which allow the distance traveled by emitted photons to readily exceed the short intrinsic gain lengths. The dependence of the threshold and the gain narrowed line width on the solvent from which the film is spin cast suggests that chain packing can be used to control lasing in some of these materials. The prospects for producing electrically pumped solid state polymer diode lasers using this class of materials are discussed in the context of the lowthreshold gain narrowing in submicron films.