Transient 2D-IR spectroscopy瞬态二维红外光谱

Transient 2D-IR spectroscopy瞬态二维红外光谱 Transient 2D-IR Spectroscopy: Towards Measuring Ultrafast Structural Dynamics 1121Jens Bredenbeck, Jan Helbing, Josef Wachtveitl and Peter Hamm 1Physikalisch Chemisches Institut, Universität Zürich, Winterthurer Strasse 190, CH-8057 Zürich, Switzerland 2Institut für Physikalische und Theoretische Chemie, Marie-Curie-Strasse 11, Johann Wolfgang Goethe Universität Frankfurt am Main, D-60439 Frankfurt am Main, Germany Two dimensional infrared (2D-IR) spectroscopy measures the ultrafast vibrational response of a molecule as a function of IR excitation frequency, providing a wealth of information that cannot be obtained from linear spectroscopy. Here we introduce the technique of transient two dimensional infrared (T2D-IR) spectroscopy, which extends the advantages of 2D-IR spectroscopy to the investigation of transient species after triggering a photoreaction with picosecond time resolution. The T2D-IR experiment is essentially a UV-pump-IR-(narrow band)-pump-IR-(broad band)-probe scheme. The experimental setup and the measurement procedure, as well as first applications, are described. The T2D-IR experiment offers a large number of degrees of freedom (timing of all pulses involved, polarizations, etc.). To learn about the dependence of the spectra on these parameters we investigated the rhenium carbonyl complex [Re(CH-bpy(CO)Cl] as a model 33 system. Upon excitation of the bpy (2,2’-bipyridine) chormorphore with 380 nm light, metal- to-ligand charge transfer occurs and a dynamic vibrational Stokes shift is observed for the three coupled, spectrally narrow and well separated carbonyl stretching modes. The evolution of the transient ensemble in the long-lived excited electronic state is investigated by T2D-IR spectroscopy while it equilibrates on a timescale of tens of picoseconds. The results demonstrate that T2D-IR spectroscopy provides an ultrafast non-linear probe for measuring ensemble inhomogeneity and structural parameters such as bond angles and intramolecular coupling of transient species with picosecond time resolution. As a second application we investigate the conformational transition of a small peptide. The conformational change is triggered by photo-isomerization of an azobenzene based 1omega amino acid integrated into the peptide backbone. For a cyclic peptide, T2D-IR spectra are measured at 3 ps, 20 ps, 200 ps and 1.7 ns after triggering. Substantial changes are found in the transient 2D-IR spectra where the transient IR-absorption (T1D-IR) spectra only show minor changes in time, illustrating the information gain provided by 2D-IR spectroscopy. A 1 J. Bredenbeck, J. Helbing, A. Sieg, T.Schrader, W. Zinth, J. Wachtveitl, C. Renner, R. Behrendt, L. Moroder , P. Hamm, Picosecond conformational transition and equilibration of a cyclic peptide, Proc. Nat. Acad. Sci., in press simple model for the time dependence of the diagonal part of the T2D-IR spectra is introduced. With the help of this model, the homogeneous line width of the amide-I band is identified as the main parameter causing the time dependence in the T2D-IR spectra. The homogeneous line width reflects the structural fluctuations of the peptide on a < 1 ps time scale. It is changing by about 25% during the conformational transition. Possible causes for a variation of this parameter that are discussed: a change in temperature, directed motion in conformation space as well as the properties of the energy landscape far away from the global 2minimum. 2 J. Bredenbeck, J. Helbing, R. Behrendt, C. Renner, L. Moroder, J. Wachtveitl, P. Hamm, Transient 2D-IR spectroscopy -- Snapshots of the non-equilibrium ensemble during the picosecond confor-mational transition of a small peptide, J. Phys. Chem., submitted