Coherent femtosecond multidimensional probes of molecular vibrations.

F emtosecond visible and IR analogues of multiple-pulse NMR techniques provide snapshot probes of molecular structure and vibrational motions, interactions, and relaxation processes (1). The work of Larsen et al. (2) in a recent issue of PNAS makes a novel application of 2D IR spectroscopy to a rotaxane system whose mechanically interlocked architecture is often used in nanoscale molecular machines to provide large-amplitude controllable, reversible mechanical motion. Intermolecular noncovalent (e.g. hydrogen) bonding interactions provide multiple bonding sites between the macrocycle and thread, with the hysteretic character necessary for the operation of bistable switching devices in many applications, including nanocomputing and molecular electronics (3). The study reports a weak 3-cm 1 coupling between the 1,600-cm 1 carbonyl stretching modes of the thread and macrocycle that is very difficult to measure in any other way. This coupling, combined with cross-peak anistropy, allowed the extraction of the distance and angle of the two interlocked groups. Similar modes (the amide I vibrations) have been widely used in earlier studies of peptides and proteins (4–7). An exciting potential application of this technique to the real-time probing of molecular devices should be possible by combining it with a fast triggering (8). Virtually all of the important developments in the field of nonlinear laser spectroscopy have followed the footsteps of NMR work done 20–30 years earlier. The celebrated work of Feynman, Vernon, and Hellwarth (9) had established the equivalence of spin 1 2 with an optically driven two-level system; coherent transients observed optically could then be associated with their NMR analogues. The spin-echo discovered by E. Hahn (24), a remarkable example of interference and time reversal, was followed by the photon echo observed in the nanosecond, picosecond, and eventually in the femtosecond regimes (10). These are only a few examples of NMR techniques that have been extended to optics. A new era began in the 1970s with the introduction of multidimensional NMR techniques by Richard Ernst (see ref. 25). The response of a system of coupled spins to a series of up to a few hundred pulses, when properly processed, gives extremely valuable information about complex molecular structure and dynamics (11). These techniques have now been extended to study molecular vibrations by using IR and visible pulses (12). Spins are elementary quantum systems whose Hamiltonian depends on a few universal parameters. Vibrational and electronic motions are far more complex than spins and require new concepts and tools for designing of pulse sequences and extracting the desired information. Inversion algorithms (13) that could yield the interchromophore couplings and ultimately the distribution of structures directly from the signals may be used to produce real-time movies of dynamical events. Optical four-wave mixing (Fig. 1) is the generic analogue of multidimensional NMR. Three pulses with wavevectors k1, k2, k3 interact with the sample to generate a signal in one of the directions ks k1 k2 k3, which is then detected by mixing it with a fourth (heterodyne) pulse.