Extreme cross-peak 2D spectroscopy

Researchers have long recognized the value of ultrasfast time-resolved spectroscopy for revealing the mechanism of photophysical and light-induced biophysical processes (1–3). Over recent years, we witnessed leaps in technology that have enabled new and ingenious femtosecond laser experiments to be demonstrated. In PNAS, Oliver et al. report a 2D spectroscopy that correlates electronic transition frequencies in a photo-excitation event with infrared transitions detected by a probe (4). They call this 2D electronic-vibrational (2DEV) spectroscopy.

2D spectroscopies including 2D electronic spectroscopy (2DES) and 2D infrared spectroscopy (2DIR) are femtosecond pump-probe techniques where both pump and probe frequencies are resolved. 2D spectroscopy thus correlates the absorption spectrum (UV, visible, or infrared), enabling line-broadening time scales to be distinguished by their different 2D line shapes, and states with a common origin can be identified by cross-peaks (5–8). 2DES and 2DIR are similar to transient absorption spectroscopy; however, two excitation pulses are used cooperatively to excite the sample, followed by a third “probe-pulse,” which interacts with the sample after the pump-probe time delay, causing a four-wave mixing signal to radiate. By Fourier transforming the signal amplitude with respect to the delay between the two excitation pulses, the excitation frequency axis is obtained for a given time delay. The probe axis is frequency resolved by dispersing signal in the detector, like in normal pump-probe methods.

In 2DEV (Fig. 1), time-resolved infrared spectroscopy is rendered multidimensional. This enables a vibrational spectroscopy probe—sensitive to chemical structure—to uncover how electronic excitation conditions promote and interplay with structural changes accompanying photophysical or photochemical transformations on a femtosecond time scale. Just some of the potential for this intriguing new method is illustrated by considering some thoughts on the work reported by Oliver et al. that complement their interpretations.

Fig. 1.

(A) Depiction of the free energies of the LE and ICT states relative to the ground state and an indication of how a 2DEV experiment probes the ICT reaction. (B) Regions of the 2D spectroscopic map relating 2DES and 2DIR, nominally diagonal experiments, to 2DEV, which is an off-diagonal experiment.

The model reaction studied by Oliver et al. (4) is closely related to the famous twisted intramolecular charge transfer (TICT) phenomenon (9, 10). TICT molecules show duel fluorescence: two fluorescence bands that depend strongly on solvent polarity. One fluorescence band has a smaller Stokes shift from the absorption and indicates the locally excited (LE) state. This LE state undergoes a photophysical transformation in moderately polar solvents to populate an intramolecular charge transfer (ICT) state that, in turn, is revealed by a red-shifted fluorescence that is strongly solvent dependent. This interpretation and the idea that the prototypical TICT molecule, dimethylaminobenzonitrile (DMABN), undergoes a marked twist to stabilize the charge-transfer state was proposed by Grabowski and coworkers (11).

For polar molecules in solution, the change in dipole moment between ground and excited electronic states is stabilized by reorganization of the solvent around the molecule to lower the free energy. Seen as a solvent-dependent (and time-dependent) Stokes shift, this process is called nonequilibrium solvation (12). An interesting aspect of the photophysics of TICT compounds is the interplay between solvent reorganization and structural degrees of freedom in the reaction (13). Therefore, in Fig. 1A, I depict the reaction in terms of free energy that accommodates both ensemble reorganization of the solvent around the LE and ICT states, as well as the potential energy surface for the TICT geometry reorganization.

The intramolecular aspects of the TICT reaction are notable because the degree of structural distortion is much greater than found in typical electron transfer reactions. The extent and precise nature of the structural changes accompanying charge separation in TICT molecules has long been controversial—for example, is it really a full twist of the dimethylamino group (9, 14, 15)? This kind of question lends itself to tools that can probe structure. Time-resolved infrared (16) and Raman (17, 18) experiments have therefore provided essential insights.

To understand, in part, what Oliver et al. are observing, consider the 2D spectroscopy map shown in Fig. 1B. 2DES correlates the UV-visible absorption spectrum: it tells us about pathways for interconversion of electronic states (7, 19). 2DEV resolves pathways along the reaction coordinate for producing a product or intermediate detected by its infrared absorption signature. In the present case, it appears that the infrared (IR) band at 1,480 cm⁻¹ signals such a product, as indicated in Fig. 1A. What is interesting is that the pump frequency (at visible wavelengths) that produces this IR signature depends on the time that the 1,480 cm⁻¹ band is detected. The red-most pump frequency (Fig. 1, red arrow) produces the signal quickly—the product is formed quickly and seen in the 2DEV spectrum as indicated by the red circle, which then decays. The blue excitation more slowly produces the intermediate indicated by the 1,480 cm⁻¹ IR band, so the blue circle rises on a slower time scale in the 2DEV spectrum. This interpretation is rather speculative but instructive to convey principles of the experiment. It suggests that part of the reaction coordinate is sampled by each pump frequency. For example, the red excitation frequencies excite closer to the transition state and therefore produce the ICT product more quickly than the blue wavelengths (the rise time of the blue signal is indicated as T₃, longer than T₁, the rise of the red signal). Such inhomogeneity in reaction kinetics on ultrafast time scales was recently described for DMABN by Joo and coworkers recently (20).

This discussion illustrates some of the potential of future 2DEV experiments. More subtle and particularly interesting is the fact that 2DEV is a correlation spectroscopy and not simply a pump-probe technique. Therefore, the 2D line shapes carry information about correlations between the evolution of electronic and nuclear states—insights that are central for elucidating dynamics not described in the framework of the Born–Oppenheimer approximation. The demonstration and, above all, promise of 2DEV spectroscopy highlight the significance of this new advance in multidimensional nonlinear spectroscopy.