Enantioselective Paternò-Büchi Reactions: Strategic Application of a Triplet Rebound Mechanism for Asymmetric Photocatalysis.

The Paternò-Büchi reaction is the [2 + 2] photocycloaddition of a carbonyl with an alkene to afford an oxetane. Enantioselective catalysis of this classical photoreaction, however, has proven to be a long-standing challenge. Many of the best-developed strategies for asymmetric photochemistry are not suitable to address this problem because the interaction of carbonyls with Brønsted or Lewis acidic catalysts can alter the electronic structure of their excited state and divert their reactivity toward alternate photoproducts. We show herein that a triplet rebound strategy enables the stereocontrolled reaction of an excited-state carbonyl compound in its native, unbound state. These studies have resulted in the development of the first highly enantioselective catalytic Paternò-Büchi reaction, catalyzed by a novel hydrogen-bonding chiral Ir photocatalyst.

Direct Probe of Vibrational Fingerprint and Combination Band Coupling.

Vibrational fingerprints and combination bands are a direct measure of couplings that control molecular properties. However, most combination bands possess small transition dipoles. Here we use multiple, ultrafast coherent infrared pulses to resolve vibrational coupling between CH3CN fingerprint modes at 918 and 1039 cm-1 and combination bands in the 2750-6100 cm-1 region via doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS). This approach provides a direct probe of vibrational coupling between fingerprint modes and near-infrared combination bands of large and small transition dipoles in a molecular system over a large frequency range.

The yaq project: Standardized software enabling flexible instrumentation.

Modern instrumentation development often involves the incorporation of many dissimilar hardware peripherals into a single unified instrument. The increasing availability of modular hardware has brought greater instrument complexity to small research groups. This complexity stretches the capability of traditional, monolithic orchestration software. In many cases, a lack of software flexibility leads creative researchers to feel frustrated, unable to perform experiments they envision. Herein, we describe Yet Another acQuisition (yaq), a software project defining a new standardized way of communicating with diverse hardware peripherals. yaq encourages a highly modular approach to experimental software development that is well suited to address the experimental flexibility needs of complex instruments. yaq is designed to overcome hardware communication barriers that challenge typical experimental software. A large number of hardware peripherals are already supported, with tooling available to expand support. The yaq standard enables collaboration among multiple research groups, increasing code quality while lowering development effort.

Surface coating effects on the assembly of gold nanospheres.

Optical spectra and atomic force microscopy (AFM) images of individually selected spheres and mechanically assembled silica-coated gold nanosphere pairs were recorded. The shell served as a means of rigid control of the minimum spacing between the metal cores. The spectra of the assembled spheres were simulated using classical electrodynamics. The observed spectra resulted in superior characterization of the particle assembly geometry, relative to the AFM data. Experimental investigations regarding less-rigid polyvinylpyrrolidone (PVP) sphere coatings were also performed and some comparisons were made.

Combined apertureless near-field optical second-harmonic generation/atomic force microscopy imaging and nanoscale limit of detection.

A dual function atomic force/near-field scanning optical microscope (AFM/NSOM) with an ultrafast laser excitation source was used to investigate apertureless, tip enhanced second-harmonic generation (SHG) of ZnO nanowires with spatial resolution below the optical diffraction limit. Single-wire SHG spectra show little to no contribution from bandgap or other emission. Polarization data established values for chi(33)/chi(31) close to previous estimates and confirm the SHG process. Experimental results indicate that the SHG signal was reduced for nanowires after exposure to an atmosphere of carbon dioxide and water vapor. An equation was derived for estimating the minimum chi(2) detectable using apertureless SHG NSOM.

Combined chemical and topographic imaging at atmospheric pressure via microprobe laser desorption/ionization mass spectrometry-atomic force microscopy.

The operational characteristics and imaging performance are described for a new instrument comprising an atomic force microscope coupled with a pulsed laser and a linear ion trap mass spectrometer. The operating mode of the atomic force microscope is used to produce topographic surface images having sub-micrometer spatial and height resolution. Spatially resolved mass spectra of ions, produced from the same surface via microprobe-mode laser desorption/ionization at atmospheric pressure, are also used to create a 100 x 100 microm chemical image. The effective spatial resolution of the image (approximately 2 microm) was constrained by the limit of detection (estimated to be 10(9)-10(10) molecules) rather than by the diameter of the focused laser spot or the step size of the sample stage. The instrument has the potential to be particularly useful for surface analysis scenarios in which chemical analysis of targeted topographic features is desired; consequently, it should have extensive application in a number of scientific areas. Because the number density of desorbed neutral species in laser desorption/ionization is known to be orders-of-magnitude greater than that of ions, it is expected that improvements in imaging performance can be realized by implementation of post-ionization methods.

Computational and experimental evaluation of nanoparticle coupling.

We present theoretical and experimental studies on the optical properties of dimers composed of octahedron-shaped, gold nanoparticles. The experimental measurements show that the photoluminescence varies quite dramatically as two octahedra are brought into close proximity. AFM images and optical emission have been recorded for dimers in uncoupled and strongly coupled configurations. The former displays a single emission peak, while the latter shows two peaks with the new feature at longer wavelengths. Calculations indicate that the red-shifted peak originates from a strongly coupled plasmon state that oscillates along the extended axis of the dimer. Theoretically, we investigate the distances over which the dimers couple and find this to be particularly plasmon mode dependent. The anisotropic morphology and sharp apexes contribute significantly to the orientational dependence of the interparticle couplings and field properties.

Optical absorption measurements with parametric down-converted photons.

It has been known for some time that correlated detection of pairs of photons generated by parametric down-conversion can eliminate several sources of error that occur in single-beam measurements. In the correlated photon measurements, the down-converted photons are separated into two beams with one photon of a pair in each beam. The absolute detection efficiency of a detector in one beam can be determined from the count rate of a detector in the other beam and the coincidence rate for the two detectors. These ideas can be used to measure the optical absorbance of a sample placed in front of one of the detectors. Errors due to stray light and dark counts are substantially reduced and fluctuations in pump intensity largely eliminated.

Development of a scanning surface probe for nanoscale tip-enhanced desorption/ablation.

We report on the development of a versatile scanning apparatus for nanoscale surface sampling that utilizes the interaction of laser radiation at a sharp probe tip to effect desorption/ablation on opaque substrates. The process, which currently yields surface craters as small as approximately 50 nm diameterx5 nm deep, has been demonstrated with both metal-coated and bare silicon tips. Desorption/ablation under the tip occurs at illumination intensities below the corresponding optical far-field threshold, suggesting that the latter process should not degrade the spatial resolution attainable for proposed chemical imaging methods based on the scanning surface probe.

Mixed frequency/time domain optical analogues of heteronuclear multidimensional NMR.

Ultrafast spectroscopy is dominated by time domain methods such as pump-probe and, more recently, 2D-IR spectroscopies. In this paper, we demonstrate that a mixed frequency/time domain ultrafast four wave mixing (FWM) approach not only provides similar capabilities, but it also provides optical analogues of multiple- and zero-quantum heteronuclear nuclear magnetic resonance (NMR). The method requires phase coherence between the excitation pulses only over the dephasing time of the coherences. It uses twelve coherence pathways that include four with populations, four with zero-quantum coherences, and four with double-quantum coherences. Each pathway provides different capabilities. The population pathways correspond to those of two-dimensional (2D) time domain spectroscopies, while the double- and zero-quantum coherence pathways access the coherent dynamics of coupled quantum states. The three spectral and two temporal dimensions enable the isolation and characterization of the spectral correlations between different vibrational and/or electronic states, coherence and population relaxation rates, and coupling strengths. Quantum-level interference between the direct and free-induction decay components gives a spectral resolution that exceeds that of the excitation pulses. Appropriate parameter choices allow isolation of individual coherence pathways. The mixed frequency/time domain approach allows one to access any set of quantum states with coherent multidimensional spectroscopy.