Time-Gated Raman Spectroscopy for Quantitative Determination of Solid-State Forms of Fluorescent Pharmaceuticals.

Raman spectroscopy is widely used for quantitative pharmaceutical analysis, but a common obstacle to its use is sample fluorescence masking the Raman signal. Time-gating provides an instrument-based method for rejecting fluorescence through temporal resolution of the spectral signal and allows Raman spectra of fluorescent materials to be obtained. An additional practical advantage is that analysis is possible in ambient lighting. This study assesses the efficacy of time-gated Raman spectroscopy for the quantitative measurement of fluorescent pharmaceuticals. Time-gated Raman spectroscopy with a 128 × (2) × 4 CMOS SPAD detector was applied for quantitative analysis of ternary mixtures of solid-state forms of the model drug, piroxicam (PRX). Partial least-squares (PLS) regression allowed quantification, with Raman-active time domain selection (based on visual inspection) improving performance. Model performance was further improved by using kernel-based regularized least-squares (RLS) regression with greedy feature selection in which the data use in both the Raman shift and time dimensions was statistically optimized. Overall, time-gated Raman spectroscopy, especially with optimized data analysis in both the spectral and time dimensions, shows potential for sensitive and relatively routine quantitative analysis of photoluminescent pharmaceuticals during drug development and manufacturing.

Optical Forging of Graphene into Three-Dimensional Shapes.

Atomically thin materials, such as graphene, are the ultimate building blocks for nanoscale devices. But although their synthesis and handling today are routine, all efforts thus far have been restricted to flat natural geometries, since the means to control their three-dimensional (3D) morphology has remained elusive. Here we show that, just as a blacksmith uses a hammer to forge a metal sheet into 3D shapes, a pulsed laser beam can forge a graphene sheet into controlled 3D shapes in the nanoscale. The forging mechanism is based on laser-induced local expansion of graphene, as confirmed by computer simulations using thin sheet elasticity theory.

Time-Resolved Coherent Anti-Stokes Raman Scattering of Graphene: Dephasing Dynamics of Optical Phonon.

We report dynamics of the G-mode in graphene probed with time-resolved coherent anti-Stokes Raman scattering measurements. By applying BOXCARS excitation geometry with three different excitation wavelengths, various nonlinear processes can be selectively detected due to energy and momentum conservation and temporal sequence of the pulses. The Raman signal due to resonant coherent excitation of the G-mode shows exponential decay with lifetime of ∼325 ± 50 fs. This decay time is shorter than expected based on the line width of the G-mode in the Raman spectrum. We propose that the unexpectedly short dephasing time is a result of dynamic variation of nonadiabatic coupling of the photoexcited electron distribution and the G-mode.

Patterning and tuning of electrical and optical properties of graphene by laser induced two-photon oxidation.

We demonstrate a simple all-optical patterning method for graphene, based on laser induced two-photon oxidation. By tuning the intensity and dose of irradiation, the level of oxidation is controlled, the band gap is introduced and electrical and optical properties are continuously tuned. Complex patterning is performed for air-suspended monolayer graphene and for graphene on substrates. The presented concept allows development of all-graphene electronic and optoelectronic devices with an all-optical method.

Counterintuitive mechanisms of the addition of hydrogen and simple olefins to heavy group 13 alkene analogues.

The mechanism of the reaction of olefins and hydrogen with dimetallenes ArMMAr (Ar = aromatic group; M = Al or Ga) was studied by density functional theory calculations and experimental methods. The digallenes, for which the most experimental data are available, are extensively dissociated to gallanediyl monomers, :GaAr, in hydrocarbon solution, but the calculations and experimental data showed also that they react with simple olefins, such as ethylene, as intact ArGaGaAr dimers via stepwise [2 + 2 + 2] cycloadditions due to their considerably lower activation barriers vis-à-vis the gallanediyl monomers, :GaAr. This pathway was preferred over the [2 + 2] cycloaddition of olefin to monomeric :GaAr to form a gallacyclopropane ring with subsequent dimerization to yield the 1,2-digallacyclobutane intermediate and, subsequently, the 1,4-digallacyclohexane product. The calculations showed also that the addition of H(2) to digallene proceeds by a different mechanism involving the initial addition of one equivalent of H(2) to form a 1,2-dihydride intermediate. This reacts with a second equivalent of H(2) to give two ArGaH(2) fragments which recombine to give the observed product with terminal and bridging H-atoms, Ar(H)Ga(µ-H)(2)Ga(H)Ar. The computations agree with the experimental observation that the :GaAr(iPr(8)) (Ar(iPr(8)) = C(6)H-2,6-(C(6)H(3)-2,4,6-(i)Pr(3))(2)-3,5-(i)Pr(2)), which does not associate even in the solid state, does not react with ethylene or hydrogen. Calculations on the reaction of propene with ArAlAlAr show that, in contrast to the digallenes, addition involves an open-shell transition state consistent with the higher singlet diradical character of dialuminenes.