Time-Resolved Vibrational Spectroscopy

In complex molecular systems, conformational changes take place on time scales ranging from less than a picosecond to microseconds and longer. Structural dynamics taking place on such vastly different time scales are difficult to investigate with conventional structure-resolving methods, but they can be probed directly using time-resolved vibrational spectroscopy, which combines structural sensitivity at the level of specific chemical bonds with subpicosecond temporal resolution. Recently developed multi-dimensional vibrational spectroscopies improve the structural sensitivity even further, by making it possible to probe distances and orientations of chemical bonds with respect to each other from their vibrational interactions.

In the Molecular Photonics group, time-resolved vibrational spectroscopy is used to investigate a wide range of molecular processes, including peptide and protein folding, chemical reactions in (photo)catalytic transition-metal complexes, the transport of protons in liquid water, and the motion of molecular machines.

Two Ti:sapphire-based laser setups are used for the experiments. In both, subpicosecond infrared pulses are generated using optical parametric amplification and difference-frequency generation. Molecular processes are triggered by UV, visible or IR pulses obtained either from the Ti:sapphire output (for time delays from fs to ns) or from independent nanosecond lasers (for time delays from ns to s) that are electronically synchronized to the Ti:sapphire system.

Recent highlights include the discovery of co-operative vibrational relaxation in hydrogen-bonded liquids, and the unraveling of the operation mechanism of a synthetic molecular machine (see figure).

Contact: Sander Woutersen, e-mail: s.woutersen@uva.nl
Molecular Photonics (UvA)
www.science.uva.nl/research/molphot/trvs
 Woutersen

Translational motion in a rotaxane-based molecular machine. Left: chemical structure. Right: transient infrared absorption change for increasing delay time. The departure of the macrocycle from the succinamide (succ) station is observed as the appearance of the free succ CO-stretch mode, and the disappearance of the hydrogen-bonded (HB) succ CO-stretch mode; the arrival at the naphthalimide (ni-) station as the appearance of the hydrogen-bonded ni- CO-stretch mode, and the disappearance of the free ni- CO-stretch mode (Science 2010, 328, 1255-1258).