Broadband stimulated Raman scattering (SRS) is often observed in applications that use nonlinear spectroscopy to probe the composition or dynamics of complex systems. Whether the SRS response is measured intentionally or unintentionally, as a background signal, the relative scattering intensities provide a quantitative measure of the population profile of target molecules. Solvent scattering is a valuable internal reference for determining absolute concentrations in these applications, but accurate cross sections have been reported for only a limited number of transitions in select solvents and were measured using spontaneous Raman scattering with narrowband continuous wave or nanosecond light sources. This work reports the measurement and analysis of absolute Raman scattering cross sections spanning the frequency range of 500–4000 cm–1 for cyclohexane, DMSO, acetonitrile, methanol, water, benzene, and toluene using broadband SRS with femtosecond and picosecond Raman pump pulses at 488 nm. Varying the duration of the Raman pump pulses from ∼80 fs to >1 ps confirms that the cross sections are independent of the spectral bandwidth across the range of ∼250 to <20 cm–1. The cross sections and depolarization ratios measured using broadband SRS agree with the limited number of previously reported values, after accounting for overlapping transitions in the lower-resolution femtosecond and picosecond spectra. The SRS cross sections reported here can be used with confidence as internal reference standards for a wide range of applications, including nonlinear spectroscopy and coherent microscopy measurements using ultrafast lasers.

The utility of a chiral Ru–prolinamide catalytic system has been demonstrated in one-pot synthesis of optically active β-triazolylethanol and β-hydroxy sulfone derivatives. The said methodology proceeds through asymmetric transfer hydrogenation of in situ formed ketones of the corresponding chiral products. Various chiral prolinamide ligands were screened, and ligand L6 with isopropyl groups substituted at the ortho position has shown excellent activity at 60 °C in aqueous medium producing up to 95% yield and 99.9% enantioselectivity.

The incorporation of trifluoromethyl groups into organic molecules such as agrochemicals and pharmaceuticals has attracted a significant amount of interest because they will impact the binding ability, lipophilicity, metabolic stability, and chemical stability of the resulting molecules. Over the past few years, photocatalytic trifluoromethylation of aryl alkenes has been reported, which typically requires precious Ru/Ir-containing photocatalytic systems. Herein, we report a metal-free organic photocatalyst composed of a bibenzothiazole core and two iminebridged methoxyphenyl substituents (dBIP−OMe), which is able to drive trifluoromethylative bifunctionalization of alkenes photocatalytically without the use of any sacrificial reagents. Mechanistic studies reveal two consecutive single-electron-transfer steps between the excited dBIP−OMe*, the CF 3 precursor (Umemoto's reagent), and the alkene substrate. Substrate scope studies demonstrated that our trifluoromethylative bifunctionalization strategy using dBIP−OMe is applicable for both aryl and aliphatic alkenes. Furthermore, a variety of nucleophiles, such as H 2 O, acetate, cyanide, azide, etc., can be readily incorporated into the carbocation intermediate once the foremost trifluoromethylation step is accomplished, substantially broadening the application scope of this photocatalytic method. Finally, taking advantage of the two-photon absorption capability of dBIP−OMe in the nearinfrared region, we demonstrated that the hydroxytrifluoromethylation of styrene could be achieved using an inexpensive 740 nm LED as the sole light source.

No abstract

Nonlinear spectroscopy provides important information about chemical systems that is not available from linear spectra alone. However, nonlinear spectroscopies have rarely been used for quantitative measurements due to the difficulty in obtaining accurate cross-sections. To overcome this limitation, we have been developing new methods to obtain accurate two-photon absorption (2PA) and stimulated Raman scattering (SRS) cross-sections of liquids and liquid solutions using ultrafast pump-probe spectroscopy. Here, we describe recent advances in measuring absolute SRS cross-sections of common liquid solvents based on a simple setup using single-wavelength pump and broadband probe pulses derived from a tunable repetition rate Yb:KGW laser. We obtain a two-dimensional transient absorption spectrum that is integrated over the time delay between the pump and the probe to obtain the Raman response as a frequency-dependent SRS spectrum. We model the overlap between the pump and probe beams in terms of the beam diameter, crossing angle, and refractive index of the solvent in order to determine accurate values for the SRS cross-sections based on the pump-probe signal strength. Specifically, we report absolute cross-sections for the C-H stretching modes of cyclohexane, dimethyl sulfoxide, acetonitrile, methanol, benzene, and toluene using Raman excitation wavelengths of 1030 nm (anti-Stokes), 515 nm (Stokes and anti-Stokes), and 343 nm (Stokes). The measured cross-sections follow the expected ∼ ν 4 frequency dependence. Our measurements also allow for direct comparison of the absolute SRS cross-sections for Stokes and anti-Stokes scattering from a single (515 nm) Raman excitation laser, which may be important for quantitative measurements using coherent nonlinear spectroscopies. For example, the SRS cross-sections that we measure for these solvents provide valuable internal reference standards for the determination of accurate 2PA cross-sections of solutes.