Matthias Lütgens

Hydrogen bonding in ionic liquids probed by linear and nonlinear vibrational spectroscopy

Three imidazolium-based ionic liquids of the type [Cnmim][NTf2] with different alkyl chain lengths (n?=?1, 2 and 8) at the first position of the imidazolium ring were studied applying infrared, linear Raman and multiplex coherent anti-Stokes Raman scattering spectroscopy. The focus has been on the CH-stretching region of the imidazolium ring, which is supposed to carry information about a possible hydrogen bonding network in the ionic liquid. The measurements are compared with calculations of the corresponding anharmonic vibrational spectra for a cluster of [C2mim][NTf2] consisting of four ion pairs. The results support the hypothesis of weak hydrogen bonding involving the C(4)?H and C(5)?H groups and somewhat stronger hydrogen bonds of the C(2)?H groups.

Coherent anti-Stokes Raman scattering with broadband excitation and narrowband probe

An improved CARS setup based on noncollinear optical parametric amplifiers (NOPAs) is presented which combines broad tunability and a wide excitation bandwidth with good spectral and temporal resolution. Picosecond Raman pump and probe pulses are generated by a modified narrowband NOPA. Combining them with sub-50 fs Stokes pulses results in highly time resolved CARS spectra with line widths down to 20 cm-1. The determination of a vibrational decoherence time is demonstrated for chloroform. Beating phenomena in case of overlapping Raman bands and an increase of spectral structure for coalescing bands are observed for cyclohexane and an ionic liquid respectively.

Vibrational Dephasing in Ionic Liquids as a Signature of Hydrogen Bonding

Understanding both structure and dynamics is crucial for producing tailor-made ionic liquids (ILs). We studied the vibrational and structural dynamics of medium versus weakly hydrogen-bonded CH groups of the imidazolium ring in ILs of the type [1-alkyl-3-methylimidazolium][bis(trifluoromethanesulfonyl)imide] ([Cn mim][NTf2 ]), with n=1, 2, and 8, by time-resolved coherent anti-Stokes Raman scattering (CARS) and quantum-classical hybrid (QCH) simulations. From the time series of the CARS spectra, dephasing times were extracted by modeling the full nonlinear response. From the QCH calculations, pure dephasing times were obtained by analyzing the distribution of transition frequencies. Experiments and calculations reveal larger dephasing rates for the vibrational stretching modes of C(2)H compared with the more weakly hydrogen-bonded C(4,5)H. This finding can be understood in terms of different H-bonding motifs and the fast interconversion between them. Differences in population relaxation rates are attributed to Fermi resonance interactions.

Ultrafast Energy Transfer in Dinuclear Complexes with Bridging 1,10-Phenanthroline-5,6-Dithiolate

We report herein the preparation and characterization of dinuclear complexes with the bridging ligand 1,10-phenanthroline-5,6-dithiolate (phendt2-) bearing Ru(bpy)2 or Ir(ppy)2 at the diimine moiety and Ni(dppe), Ni(dppf), CoCp, RhCp*, and Ru( p-Me-iPr-benzene) at the dithiolate unit. In comparison with the mononuclear precursors used in the synthesis, all dinuclear complexes were characterized by absorption and photoluminescence spectroscopy as well as cyclic voltammetry. Because of the beneficial spectral and electrochemical properties of the Ir/Co complex for a light-driven charge separation, this complex was investigated in detail by time-resolved luminescence {nanosecond (ns)-resolution} and transient absorption spectroscopy {femtosecond (fs)-resolution}. All measurements supported by DFT calculations show that the observed effective luminescence quenching by the dithiolate coordinated metal is caused by an ultrafast singlet-singlet Dexter energy transfer.

Structural Motifs of Liquid Acetic Acid from Ultrafast CARS Spectroscopy

The carbonyl vibration of acetic acid is analyzed by spontaneous and ultrafast coherent anti-Stokes Raman spectroscopy. The complex band is decomposed into four contributions from different structural motifs and the cyclic dimer signature is extracted.