Transient two-dimensional infrared spectroscopy – towards measuring ultrafast structural dynamics

Ultrafast two-dimensional infrared spectroscopy (2D-IR) is a promising tool for the investigation of molecular structures [1-3] and their equilibrium fluctuations [2, 4]. Similar to 2D-NMR spectroscopy [5], cross-peaks between coupled states emerge in 2D-IR spectra, and contain structural information [2, 6]. The outstanding feature of 2D-IR spectroscopy is the combination of its structure resolution power with an intrinsic sub-picosecond time resolution, freezing in all but the fastest molecular motions. This makes it particularly suited for application to fast dynamical processes, offering means to resolve equilibrium distributions of fast interconverting structures [7]. Most promising is the extension of the 2D-IR technique to the non-equilibrium regime, where its high time resolution can be put to full use [8]. This is the topic of this contribution. In principle, changes of molecular structures can thereby be followed from the sub-ps timescale up to milliseconds. We have recently reported first steps in this direction, employing transient 2D-IR spectroscopy (T2D-IR) to study the picosecond conformational transition of a photo-switchable cyclic peptide [9,10]. Here we demonstrate the application of T2D-IR to an electronically excited state, the metal-to-ligand chargetransfer (MLCT) state in rhenium(I) carbonyle complex [Re(CO)3Cl(dmbpy)] (dmbpy=4,4'dimethyl-2,2'bipyridine). MLCT in rhenium(I) polypyridyl carbonyles and other metal carbonyles has been studied in considerable detail by time resolved IR spectroscopy (see [1113] and references therein). Upon excitation at 390 nm MLCT occurs and a large frequency shift of the CO vibrations is observed (see the FTIR and UV-IR pump-probe spectra in Fig. 1 a and d). This change in vibrational frequency can be attributed to an ultrafast change in the electronic structure and the subsequent vibrational stokes shift due to solvation, which takes place on a picosecond timescale [11, 12]. We present two dimensional infrared spectra of the transient species in the course of the solvation process, with characteristic cross peaks that depend on the relative orientations and couplings between the three carboxyl groups of the complex.