COMMENTARY COMMENTARY Extreme cross-peak 2D spectroscopy Gregory D. Scholes1 polar solvents to populate an intramolecu- Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6; lar charge transfer (ICT) state that, in turn, and Department of Chemistry, Princeton University, Princeton, NJ 08544 is revealed by a red-shifted fluorescence that is strongly solvent dependent. This in- Researchers have long recognized the value of By Fourier transforming the signal amplitude terpretation and the idea that the prototyp- ultrasfast time-resolved spectroscopy for re- with respect to the delay between the two ical TICT molecule, dimethylaminobenzo- vealing the mechanism of photophysical and excitation pulses, the excitation frequency nitrile (DMABN), undergoes a marked twist to stabilize the charge-transfer state was pro- light-induced biophysical processes (1–3). axis is obtained for a given time delay. The posed by Grabowski and coworkers (11). Over recent years, we witnessed leaps in tech- probe axis is frequency resolved by dispersing Forpolarmoleculesinsolution,thechange nology that have enabled new and ingenious signal in the detector, like in normal pump- in dipole moment between ground and ex- femtosecond laser experiments to be demon- probe methods. cited electronic states is stabilized by re- strated. In PNAS, Oliver et al. report a 2D In 2DEV (Fig. 1), time-resolved infrared organization of the solvent around the mol- spectroscopy that correlates electronic transi- spectroscopy is rendered multidimensional. ecule to lower the free energy. Seen as a tion frequencies in a photo-excitation event This enables a vibrational spectroscopy solvent-dependent (and time-dependent) with infrared transitions detected by a probe probe—sensitive to chemical structure—to Stokes shift, this process is called nonequilib- (4). They call this 2D electronic-vibrational uncover how electronic excitation condi- rium solvation (12). An interesting aspect of (2DEV) spectroscopy. tions promote and interplay with structural the photophysics of TICT compounds is the 2D spectroscopies including 2D electronic changes accompanying photophysical or interplay between solvent reorganization and spectroscopy (2DES) and 2D infrared spec- photochemical transformations on a femto- structural degrees of freedom in the reaction troscopy (2DIR) are femtosecond pump- second time scale. Just some of the potential (13). Therefore, in Fig. 1A, I depict the re- probe techniques where both pump and for this intriguing new method is illustrated action in terms of free energy that accommo- probe frequencies are resolved. 2D spectros- by considering some thoughts on the work dates both ensemble reorganization of the copy thus correlates the absorption spectrum reported by Oliver et al. that complement solvent around the LE and ICT states, as well (UV, visible, or infrared), enabling line- their interpretations. as the potential energy surface for the TICT broadening time scales to be distinguished The model reaction studied by Oliver et al. geometry reorganization. by their different 2D line shapes, and states (4) is closely related to the famous twisted The intramolecular aspects of the TICT with a common origin can be identified by intramolecular charge transfer (TICT) phe- reaction are notable because the degree of – cross-peaks (5 8). 2DES and 2DIR are sim- nomenon (9, 10). TICT molecules show duel structural distortion is much greater than ilar to transient absorption spectroscopy; fluorescence: two fluorescence bands that de- found in typical electron transfer reactions. however, two excitation pulses are used co- pend strongly on solvent polarity. One fluo- The extent and precise nature of the struc- operatively to excite the sample, followed by rescencebandhasasmallerStokesshiftfrom tural changes accompanying charge separa- “ ” a third probe-pulse, which interacts with the absorption and indicates the locally ex- tion in TICT molecules has long been the sample after the pump-probe time delay, cited (LE) state. This LE state undergoes a controversial—for example, is it really a full causing a four-wave mixing signal to radiate. photophysical transformation in moderately twist of the dimethylamino group (9, 14, 15)? This kind of question lends itself to tools that can probe structure. Time-resolved infrared A B (16) and Raman (17, 18) experiments have LE slow IR probe at therefore provided essential insights. 1480 cm-1 T3 >T3 >T1 To understand, in part, what Oliver et al. fast ICT are observing, consider the 2D spectroscopy F map shown in Fig. 1B.2DEScorrelatesthe 2DEV 2DIR UV-visible absorption spectrum: it tells us Visible about pathways for interconversion of elec- pump pulse tronic states (7, 19). 2DEV resolves pathways along the reaction coordinate for producing a product or intermediate detected by its in- Reaction free energy frared absorption signature. In the present Detection (probe) wavelength case, it appears that the infrared (IR) band 2DES Reaction coordinate Excitation (pump) wavelength Author contributions: G.D.S. wrote the paper. The authors declare no conflict of interest. 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, See companion article on page 10061. nominally diagonal experiments, to 2DEV, which is an off-diagonal experiment. 1Email: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1410105111 PNAS | July 15, 2014 | vol. 111 | no. 28 | 10031–10032 Downloaded by guest on September 23, 2021 − at 1,480 cm 1 signals such a product, as in- not simply a pump-probe technique. There- in the framework of the Born–Oppenheimer dicated in Fig. 1A. What is interesting is that fore, the 2D line shapes carry information approximation. The demonstration and, the pump frequency (at visible wavelengths) about correlations between the evolution of above all, promise of 2DEV spectroscopy that produces this IR signature depends on electronic and nuclear states—insights that are highlight the significance of this new advance − thetimethatthe1,480cm 1 band is detected. central for elucidating dynamics not described in multidimensional nonlinear spectroscopy. The red-most pump frequency (Fig. 1, red arrow) produces the signal quickly—the product is formed quickly and seen in the 1 Fleming GR, Morris JM, Robinson GW (1976) Direct observation of 12 van der Zwan G, Hynes JT (1985) Time-dependent fluorescence 2DEV spectrum as indicated by the red circle, rotational diffusion by picosecond spectroscopy. Chem Phys 17(1):91–100. solvent shifts, dielectric friction, and noneqilibrium solvation in polar whichthendecays.Theblueexcitationmore 2 Porter G (1978) The Bakerian Lecture, 1977: In vitro models for solvents. 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