COMMENTARY
COMMENTARY
Extreme cross-peak 2D spectroscopy Gregory D. Scholes1 Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6; and Department of Chemistry, Princeton University, Princeton, NJ 08544
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 pumpprobe techniques where both pump and probe frequencies are resolved. 2D spectroscopy thus correlates the absorption spectrum (UV, visible, or infrared), enabling linebroadening 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.
LE
B IR probe at 1480 cm-1
slow fast
Visible pump pulse
ICT
Detection (probe) wavelength
Reaction free energy F
A
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 pumpprobe 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. 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
T3 >T3 >T1
2DEV
2DIR
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
2DES Reaction coordinate
Author contributions: G.D.S. wrote the paper.
Excitation (pump) wavelength
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.
www.pnas.org/cgi/doi/10.1073/pnas.1410105111
The authors declare no conflict of interest. See companion article on page 10061. 1
Email: [email protected].
PNAS | July 15, 2014 | vol. 111 | no. 28 | 10031–10032
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
10032 | www.pnas.org/cgi/doi/10.1073/pnas.1410105111
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.
1 Fleming GR, Morris JM, Robinson GW (1976) Direct observation of rotational diffusion by picosecond spectroscopy. Chem Phys 17(1):91–100. 2 Porter G (1978) The Bakerian Lecture, 1977: In vitro models for photosynthesis. Proc R Soc Lond A Math Phys Sci 362(1710):281–303. 3 Zewail AH (1996) Femtochemistry: Recent progress in studies of dynamics and control of reactions and their transition states. J Phys Chem 100(31):12701–12724. 4 Oliver TAA, Lewis NHC, Fleming GR (2014) Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy. Proc Natl Acad Sci USA 111:10061–10066. 5 Hamm P, Lim MH, Hochstrasser RM (1998) Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy. J Phys Chem B 102(31):6123–6138. 6 Jonas DM (2003) Two-dimensional femtosecond spectroscopy. Annu Rev Phys Chem 54:425–463. 7 Brixner T, et al. (2005) Two-dimensional spectroscopy of electronic couplings in photosynthesis. Nature 434(7033):625–628. 8 Mukamel S (2000) Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations. Annu Rev Phys Chem 51:691–729. 9 Grabowski ZR, Rotkiewicz K, Rettig W (2003) Structural changes accompanying intramolecular electron transfer: Focus on twisted intramolecular charge-transfer states and structures. Chem Rev 103(10):3899–4032. 10 Rettig W (1986) Charge separation in excited-states of decoupled systems: TICT compounds and implications regarding the development of new laser-dyes and the primary processes of vision and photosynthesis. Angew Chem Int Ed Engl 25(11):971–988. 11 Rotkiewicz K, Grellman KH, Grabowski ZR (1973) Reinterpretation of anomalous fluorescence of para-N,Ndimethylamino-benzonitrile. Chem Phys Lett 19(3):315–318.
12 van der Zwan G, Hynes JT (1985) Time-dependent fluorescence solvent shifts, dielectric friction, and noneqilibrium solvation in polar solvents. J Phys Chem 89(20):4181–4188. 13 Scholes GD, Fournier T, Parker AW, Phillips D (1999) Solvation and intramolecular reorganization in 9,9′-bianthryl: Analysis of resonance Raman excitation profiles and ab initio molecular orbital calculations. J Chem Phys 111(13):5999–6010. 14 Zachariasse KA, et al. (1996) Intramolecular charge transfer in the excited state. Kinetics and configurational changes. J Photochem Photobiol Chem 102(1):59–70. 15 Sobolewski AL, Domcke W (1996) Charge transfer in aminobenzonitriles: Do they twist? Chem Phys Lett 250(3-4):428–436. 16 Hashimoto M, Hamaguchi H (1995) Structure of the twistedintramolecular-charge-transfer excited singlet and triplet-states of 4-(dimethylamino)benzonitrile as studied by nanosecond timeresolved infrared-spectroscopy. J Phys Chem 99(20):7875–7877. 17 Kwok WM, et al. (2003) Further time-resolved spectroscopic investigations on the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) and its derivatives, 4-diethylaminobenzonitrile (DEABN) and 4-dimethylamino-3,5dimethylbenzonitrile (TMABN). Phys Chem Chem Phys 5(6):1043–1050. 18 Kwok WM, et al. (2001) A determination of the structure of the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) by time-resolved resonance Raman spectroscopy. J Phys Chem A 105(6):984–990. 19 Ostroumov EE, Mulvaney RM, Cogdell RJ, Scholes GD (2013) Broadband 2D electronic spectroscopy reveals a carotenoid dark state in purple bacteria. Science 340(6128):52–56. 20 Park M, Kim CH, Joo T (2013) Multifaceted ultrafast intramolecular charge transfer dynamics of 4-(dimethylamino) benzonitrile (DMABN). J Phys Chem A 117(2):370–377.
Scholes
COMMENTARY
Extreme cross-peak 2D spectroscopy Gregory D. Scholes1 Department of Chemistry, University of Toronto, Toronto, ON, Canada M5S 3H6; and Department of Chemistry, Princeton University, Princeton, NJ 08544
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 pumpprobe techniques where both pump and probe frequencies are resolved. 2D spectroscopy thus correlates the absorption spectrum (UV, visible, or infrared), enabling linebroadening 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.
LE
B IR probe at 1480 cm-1
slow fast
Visible pump pulse
ICT
Detection (probe) wavelength
Reaction free energy F
A
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 pumpprobe 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. 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
T3 >T3 >T1
2DEV
2DIR
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
2DES Reaction coordinate
Author contributions: G.D.S. wrote the paper.
Excitation (pump) wavelength
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.
www.pnas.org/cgi/doi/10.1073/pnas.1410105111
The authors declare no conflict of interest. See companion article on page 10061. 1
Email: [email protected].
PNAS | July 15, 2014 | vol. 111 | no. 28 | 10031–10032
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
10032 | www.pnas.org/cgi/doi/10.1073/pnas.1410105111
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.
1 Fleming GR, Morris JM, Robinson GW (1976) Direct observation of rotational diffusion by picosecond spectroscopy. Chem Phys 17(1):91–100. 2 Porter G (1978) The Bakerian Lecture, 1977: In vitro models for photosynthesis. Proc R Soc Lond A Math Phys Sci 362(1710):281–303. 3 Zewail AH (1996) Femtochemistry: Recent progress in studies of dynamics and control of reactions and their transition states. J Phys Chem 100(31):12701–12724. 4 Oliver TAA, Lewis NHC, Fleming GR (2014) Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy. Proc Natl Acad Sci USA 111:10061–10066. 5 Hamm P, Lim MH, Hochstrasser RM (1998) Structure of the amide I band of peptides measured by femtosecond nonlinear-infrared spectroscopy. J Phys Chem B 102(31):6123–6138. 6 Jonas DM (2003) Two-dimensional femtosecond spectroscopy. Annu Rev Phys Chem 54:425–463. 7 Brixner T, et al. (2005) Two-dimensional spectroscopy of electronic couplings in photosynthesis. Nature 434(7033):625–628. 8 Mukamel S (2000) Multidimensional femtosecond correlation spectroscopies of electronic and vibrational excitations. Annu Rev Phys Chem 51:691–729. 9 Grabowski ZR, Rotkiewicz K, Rettig W (2003) Structural changes accompanying intramolecular electron transfer: Focus on twisted intramolecular charge-transfer states and structures. Chem Rev 103(10):3899–4032. 10 Rettig W (1986) Charge separation in excited-states of decoupled systems: TICT compounds and implications regarding the development of new laser-dyes and the primary processes of vision and photosynthesis. Angew Chem Int Ed Engl 25(11):971–988. 11 Rotkiewicz K, Grellman KH, Grabowski ZR (1973) Reinterpretation of anomalous fluorescence of para-N,Ndimethylamino-benzonitrile. Chem Phys Lett 19(3):315–318.
12 van der Zwan G, Hynes JT (1985) Time-dependent fluorescence solvent shifts, dielectric friction, and noneqilibrium solvation in polar solvents. J Phys Chem 89(20):4181–4188. 13 Scholes GD, Fournier T, Parker AW, Phillips D (1999) Solvation and intramolecular reorganization in 9,9′-bianthryl: Analysis of resonance Raman excitation profiles and ab initio molecular orbital calculations. J Chem Phys 111(13):5999–6010. 14 Zachariasse KA, et al. (1996) Intramolecular charge transfer in the excited state. Kinetics and configurational changes. J Photochem Photobiol Chem 102(1):59–70. 15 Sobolewski AL, Domcke W (1996) Charge transfer in aminobenzonitriles: Do they twist? Chem Phys Lett 250(3-4):428–436. 16 Hashimoto M, Hamaguchi H (1995) Structure of the twistedintramolecular-charge-transfer excited singlet and triplet-states of 4-(dimethylamino)benzonitrile as studied by nanosecond timeresolved infrared-spectroscopy. J Phys Chem 99(20):7875–7877. 17 Kwok WM, et al. (2003) Further time-resolved spectroscopic investigations on the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) and its derivatives, 4-diethylaminobenzonitrile (DEABN) and 4-dimethylamino-3,5dimethylbenzonitrile (TMABN). Phys Chem Chem Phys 5(6):1043–1050. 18 Kwok WM, et al. (2001) A determination of the structure of the intramolecular charge transfer state of 4-dimethylaminobenzonitrile (DMABN) by time-resolved resonance Raman spectroscopy. J Phys Chem A 105(6):984–990. 19 Ostroumov EE, Mulvaney RM, Cogdell RJ, Scholes GD (2013) Broadband 2D electronic spectroscopy reveals a carotenoid dark state in purple bacteria. Science 340(6128):52–56. 20 Park M, Kim CH, Joo T (2013) Multifaceted ultrafast intramolecular charge transfer dynamics of 4-(dimethylamino) benzonitrile (DMABN). J Phys Chem A 117(2):370–377.
Scholes
