Two-Dimensional Electronic Spectroscopy in the Ultraviolet

Perspective pubs.acs.org/JPCL

Two-Dimensional Electronic Spectroscopy in the Ultraviolet Wavelength Range Brantley A. West† and Andrew M. Moran*,‡ †

Department of Physics and Astronomy and ‡Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States ABSTRACT: Coherent two-dimensional (2D) spectroscopies conducted at visible and infrared wavelengths are having a transformative impact on the understanding of numerous processes in condensed phases. The extension of 2D spectroscopy to the ultraviolet spectral range (2DUV) must contend with several challenges, including the attainment of adequate laser bandwidth, interferometric phase stability, and the suppression of undesired nonlinearities in the sample medium. Solutions to these problems are motivated by the study of a wide range of biological systems whose lowest-frequency electronic resonances are found in the UV. The development of 2DUV spectroscopy also makes possible the attainment of new insights into elementary chemical reaction dynamics (e.g., electrocyclic ring opening in cycloalkenes). Substantial progress has been made in both the implementation and application of 2DUV spectroscopy in the past several years. In this Perspective, we discuss 2DUV methodology, review recent applications, and speculate on what the future will hold.

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ltrafast laser spectroscopies have long been central to the understanding of chemical dynamics in condensed phases.1 The range of accessible scientific questions has grown steadily with experimental innovations and continuing improvements in time resolution. Principal among modern experimental techniques are Fourier transform two-dimensional (2D) spectroscopies, which were developed with inspiration from multidimensional NMR experiments (e.g., COSY, NOESY).2−5 2D spectroscopies were first applied and matured most rapidly in the infrared spectral range.6−8 Technical advances have since enabled the routine application of 2D measurements at visible wavelengths.9,10 In the past decade, such experiments have yielded transformative insights into processes ranging from chemical exchange equilibrium in liquids to energy transfer in photosynthesis.11−16 Further progress “from NMR to X-rays”17 must contend with several challenges encountered in the ultraviolet (UV) spectral region, including the attainment of adequate laser bandwidth, interferometric phase stability, and the suppression of solute and solvent ionization. Solutions to these problems are motivated by the study of numerous biological systems whose lowest-frequency electronic resonances are found in the UV (e.g., DNA, amino acids).18 2DUV spectroscopy will also provide valuable new insights into elementary chemical dynamics such as ring opening of cycloalkenes. In this Perspective, we discuss 2DUV methodology, review recent applications, and speculate on what the future will hold. Only a small number of Fourier transform 2DUV experiments have been reported to date.19−23 Instrument development was naturally the focus of the earliest work,19,20 whereas more recent investigations have examined DNA nucleobases.21−23 2DUV studies in our laboratory have leveraged exceptionally © 2012 American Chemical Society

The extension of 2D spectroscopy to the ultraviolet spectral range (2DUV) must contend with several challenges, including the attainment of adequate laser bandwidth, interferometric phase stability, and the suppression of undesired nonlinearities in the sample medium. broad laser bandwidths to uncover new insights into photoinduced relaxation processes in DNA components at temperatures ranging from 100 to 300 K.22 We discuss these applications in addition to more recent work on electrocyclic ring-opening reactions below. Our discussion of technical issues pertains specifically to the deep-UV near 265 nm, where the optical response should be considered on a different footing than that in the visible-to-near-UV wavelength regions. The deep UV spectral range is particularly challenged by twophoton ionization processes and preresonance effects associated with electronic transitions in common solvents (e.g., water, methanol).24,25 The short period of the radiation at 265 nm (0.9 fs) is also an important consideration. Among the principle advantages of 2D spectroscopy is the attainment of time and frequency resolution limited only by the Received: July 27, 2012 Accepted: August 29, 2012 Published: August 29, 2012 2575

dx.doi.org/10.1021/jz301048n | J. Phys. Chem. Lett. 2012, 3, 2575−2581

The Journal of Physical Chemistry Letters

Perspective

or (ii) a passively phase-stabilized interferometer employing four pulses (where one pulse is used as a reference field for interferometric detection).19,22,23 The basic configuration of our diffractive -optic-based interferometer is displayed in Figure 2. The key to interferometric phase stability is the production

Our discussion of technical issues pertains specifically to the deepUV near 265 nm, where the optical response should be considered on a different footing than that in the visible-to-near-UV wavelength regions. line widths of the system’s resonances. Consider, for example, the simplest realization of a pump−probe experiment shown in Figure 1. The transmission of a probe pulse is monitored as a

Figure 2. (a) Schematic of the diffractive-optic-based interferometer used to conduct 2DUV experiments in our laboratory. (b) Pulse sequence used in 2DUV spectroscopy. The system absorbs light during the delay τ, nonradiative dynamics occur in the delay T, and the signal is emitted in the interval t. 2DUV shows how correlations in the excitation and emission frequencies, ωτ and ωt, vary with respect to T.

of two pairs of phase-related laser pulses at the diffractive optic.26,27 In contrast with alternative approaches in which all four pulses are phase-related,28 we align two separate laser beams into the experiment because the delay between excitation and detection, T, can then be scanned to hundreds of picoseconds in a transient grating mode. One limitation of this design is the accumulation of dispersion in the prism wedges, which are used to control the delay between the two excitation pulses, τ. An innovative all-reflective 2DUV setup has been developed to address this issue.19 We prefer the present approach because the delay between pulse 3 and the reference field remains conveniently fixed throughout the experiment. It is ultimately the line widths of the system under investigation that determine the appropriate level of engineering. For example, the signal fully decays by |τ| < 40 fs in the systems that we have investigated so far.22,23 The amount of glass associated with a delay of 40 fs stretches a 20 fs laser pulse at 267 nm by only 0.01 fs and therefore has a negligible effect on the 2D line shape. Primary Technical Challenges. We next discuss three of the primary technical issues confronting 2DUV experiments. Notably, these challenges affect essentially all femtosecond UV spectroscopies. Therefore, solutions to many of these problems were developed in other contexts and have been adapted to 2DUV. The three points outlined below are illustrated in Figure 3. (1) Laser Bandwidth. It is generally desirable in 2D experiments to make the laser bandwidth as broad as possible. The challenge in the deep UV is that sufficiently short pulse durations (20 fs) are not readily produced using nonlinear optical crystals. To overcome this limitation, physical chemists have borrowed techniques from the optical physics community, wherein argon gas is used as a nonlinear medium.29 Bradforth and co-workers demonstrated that 25 fs pulse durations can be

Figure 1. (a) In a pump−probe experiment with dispersed detection, a trade-off between time and frequency resolution is made for the pump pulse, whereas the emission frequency is resolved without compromising the time resolution. (b) The delay between two pump pulses is scanned, and a numerical Fourier transform is carried out with respect to τ in 2D spectroscopy. The line widths of the system’s resonances ultimately limit the time and frequency resolution corresponding to both the excitation and detection dimensions.

function of the delay with respect to a pump pulse. In this configuration, time resolution comes at the expense of frequency resolution because short laser pulses necessarily have broad bandwidths. This trade-off between time and frequency resolution is readily eliminated in the probe dimension by dispersing the signal pulse in a spectrometer after the sample; the spectrometer effectively Fourier transforms the signal pulse from the time to frequency domain. In 2D spectroscopy, a Fourier transform technique is similarly carried out for the excitation process. The delay between a pair of “pump” pulses is scanned, and a numerical Fourier transformation is taken, thereby yielding the spectrum of the absorbed pump radiation. Qualitatively, 2D spectroscopy can be viewed as a special case of a pump−probe experiment with dispersed excitation and dispersed detection. This dual Fourier transform method is inherently sensitive to the full range of coherent dynamics initiated within the laser bandwidth. Although such an approach has other merits, it should be emphasized that equivalent information is not derived by combining dispersed signal detection with spectral tuning of a narrow-band pump pulse. The few approaches to Fourier transform 2DUV reported to date use either (i) a two-beam geometry based on a pulse shaper, which closely resembles the schematic in Figure 1b20,21 2576



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Figure 3. 2DUV spectroscopy is challenged by these three technical issues. (a) The 20 fs laser pulses are generated using cross-modal phase matching in an argon-filled hollow-core fiber. (b) Experiments are carried out at extremely low fluences to suppress undesired photoionization processes. The peak powers associated with these transient grating data are given in the figure legend. (c) The dynamics of interest cannot be probed when the three laser pulses are overlapped in the sample because undesired nonlinearities dominate the response.

improvements have enabled the first observations of quantum beats in 2DUV spectra. (1) Toward 2DUV Spectroscopy of DNA. In recent years, 2D electronic spectroscopies conducted in the visible spectral range have provided many new and surprising insights into energy-transfer mechanisms in molecular aggregates and lightharvesting proteins.13−16 These systems possess what is known as Frenkel exciton electronic structure. Essentially, this means that the electron and hole involved in the electronic excitation are strongly bound. It is then natural to envision the excitation as a single entity that is exchanged between molecular sites. Inspired by these investigations of exciton dynamics, many of the first 2DUV experiments have targeted DNA where the electronic resonances of the subunits similarly transform into collective excitations in the macromolecule.20−23 The nature of these delocalized states and their role in photoprotection is now a subject of significant interest.32,33 Mounting evidence suggests that light absorption produces fairly localized Frenkel excitons (2−3 units) in DNA, which self-trap into excimers on the 100 fs time scale.34 2DUV holds great potential for uncovering these physics.

achieved in an argon-filled hollow-core waveguide pumped by a 100 fs Ti:Sapphire laser system.30 In our laboratory, we have additionally employed cross-modal phase matching in a similar waveguide to further enhance the UV bandwidth. These extraordinary UV bandwidths can now be compressed with commercially available mirrors imparting negative group delay dispersion.31 (2) Photoionization. Photoionization of the solute and solvent is an insidious problem in the deep-UV because the ionization efficiencies increase as the laser pulse duration becomes shorter.23 Fortunately, the peak powers of the laser pulses scale more steeply with the light intensity than does the desired four-wave mixing signal. Therefore, photoionization can be suppressed by carrying out the experiments at extremely low fluences. The key is to keep the peak power in the range of a few GW/cm2, which unfortunately translates into a miniscule signal strength. Consider, for example, the four-wave mixing signal radiated by the thymine nucleobase. For a 20 fs laser pulse with a peak power of 1.4 GW/cm2, the associated fluence of 3.8 × 1013 photons/cm2 produces electronic excitations in only 0.2% of the solute molecules. We find the background-free laser beam geometry shown in Figure 2 to be essential for the detection of such weak nonlinearities. Our signal-to-noise ratios typically exceed 100 under these low fluences at room temperature.23 (3) Coherence Spike. The last issue to be considered is the coherence spike observed when all three incoming laser pulses are overlapped in the sample.23 This undesired nonlinearity has contributions from both two-photon absorption and the electronic polarizability of the solvent. It is a problem of particular importance because the amplitude of the spike in aqueous solution is often an order of magnitude larger than the signal of interest. With 20 fs pulses, we find that this undesired nonlinearity contributes at delay times as large as T = 80 fs in aqueous solutions. Thus, the coherence spike potentially masks interesting electronic and nuclear relaxation processes (e.g., solvation, exciton self-trapping in DNA). Applications to DNA Components and Cycloalkenes. Here, we discuss the two classes of systems presently under investigation in our laboratory. 2DUV experiments conducted on DNA nucleobases are first presented. Signatures of internal conversion and vibrational cooling dynamics are examined in these systems at temperatures ranging from 100 to 300 K. The second area of application investigates photoinduced ringopening dynamics in cyclohexadiene (CHD) and its derivatives. Notably, this new work makes use of recent upgrades in our time resolution and detection sensitivity. These technical

2DUV holds great potential for uncovering the nature of collective electronic excitations in DNA. Our initial 2DUV experiments examined DNA nucleobases with the goal of first establishing signatures of relaxation processes in the individual units before tackling larger systems, where signal interpretation will pose more significant challenges.22,23 It was shown that the study of solvation dynamics at room temperature is challenged by the contributions from undesired nonlinearities in the region of pulse overlap. 2DUV experiments carried out by Weinacht and co-workers reached similar conclusions.21 Essentially, all evidence of correlation between the excitation, ωτ, and detection, ωt, frequencies vanishes in this crucial period of time, where the coherence spike dominates the response. As shown in Figure 4, we have also demonstrated that correlation between ωτ and ωt is maintained for several picoseconds at 100 K for both thymine and 9-methyladenine. Such long-lived correlations reflect the suppression of thermal motions in the frozen samples. These studies at cryogenic temperatures revealed interesting and unanticipated behaviors, which are now being investigated with experiments applied to a wider variety of systems (e.g., thymidine, thymine dinucleotide). 2577



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Figure 4. Absorptive 2DUV spectra measured at T = 0.6 ps for (top row) thymine and (bottom row) 9-methyladenine in an 85:15 mixture of methanol/water at (a,d) 100 and (b,e) 300 K. (c,f) Ratios in the antidiagonal and diagonal line widths, Γad/Γd, are plotted with respect to the (logarithmic) pulse delay at 100 (blue) and 300 K (green). Signals acquired for thymine are adapted from ref 22 with permission from the American Institute of Physics.

Figure 5. The real part of the rephasing 2DUV spectrum (lower left) is measured for CHD in cyclohexane at a sequence of delay times between excitation and detection, T. Vibrational coherences are revealed by monitoring the signal amplitude in particular regions of the 2DUV spectrum. The beats near 105 and 801 cm−1 correspond to CHD and cyclohexane, respectively.

resolution.36 In contrast, the sub-100 fs time scale has never been directly monitored in solution because of insufficient time resolution. The quantum yield of ring opening approaches 100% in the gas phase, whereas it is only 40% in solution.35 This discrepancy in quantum yields motivates an understanding of the influence that solute−solvent interactions have on the photoinduced relaxation processes in CHD. We are now using 2DUV experiments to explore the extraordinary physics governing electronic relaxation in CHD and its derivatives. With new upgrades to our experimental

(2) Electrocyclic Ring Opening in Cycloalkenes. Photoinduced electrocyclic ring-opening reactions in conjugated cycloalkenes are among the most elementary processes in organic chemistry. One of the most well-known reactions transforms CHD into hexatriene.35−37 It has long been established that a sequence of extremely fast internal conversion processes precedes bond breaking in CHD and some of its derivatives.35 These excited-state dynamics have recently come into focus in photoionization mass spectrometry measurements carried out on CHD in the gas phase with 13 fs 2578



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setup, these systems can now be interrogated at delay times as early as T = 40 fs. Nonradiative transitions on the sub-100 fs time scale hold interesting implications in condensed phases because equilibrium is generally not established in the surrounding environment. Conventional kinetic theories do not apply in this regime of coincident electronic and nuclear dynamics.38 The key issue is that, in traditional models, the correlation functions governing nonradiative transitions generally do not fully decay in less than 100 fs because a sufficient number of phase-randomizing collisions has not yet occurred. CHD is an excellent model for studies of such non-Markovian processes. Moreover, nowhere are the physics of internal conversion more interesting than at conical intersections between electronic states (such as those that mediate ring opening).39 Figure 5 presents the results of a 2DUV experiment recently conducted on CHD in our laboratory. 2DUV spectra are obtained at a number of delay times, T, following excitation with a 20 fs laser pulse at 267 nm. Photoexcitation of CHD in this wavelength range is known to initiate ring opening.35−37 Interestingly, the frequencies and phases of the vibrational coherences induced by photoexcitation depend on the region of the 2D spectrum. The 801 cm−1 mode of the cyclohexane solvent is detected below the diagonal, whereas a 105 cm−1 intramolecular mode of CHD is found at higher detection frequencies, ωt. In addition, a phase shift of rougly 180° is observed for the 105 cm−1 mode with respect to the excitation frequency, ωτ. We are now considering a mechanism in which the 180° phase shift in the 105 cm−1 mode originates in the interference between signal components associated with two classes of wavepacket motions. In this interpretation, one pathway directly produces a ground-state wavepacket through a stimulated Raman process. The second nonlinearity involves a sequence wherein a wavepacket initiated on the ππ* potential energy surface accumulates a phase shift when internal conversion returns the system to the ground electronic state. This possibility will be investigated in future work. Such knowledge of the vibronic couplings in CHD will be particularly important because its excited-state nuclear motions are thought to be driven by steep potential energy gradients.40,41 The signs of these gradients govern the directions in which the excited-state wavepackets are initiated, thereby controlling the photochemical yields. Outlook and Future Directions. In the past several years, substantial progress has been made in both the implementation and application of 2DUV spectroscopy. Our recent work on photoinduced ring-opening reactions in cycloalkenes confirms that 2DUV has already matured to a level at which valuable new insights can be uncovered in these systems and perhaps many others. In contrast, significant technical issues still stand in the way of 2DUV studies of collective excitations in DNA. Below, we discuss prospects for the application of 2DUV spectroscopies to DNA because related work is now being conducted in several laboratories, and the results are likely to generate broad interest. Overall, previous 2DUV experiments involving DNA components at ambient temperatures suggest that much broader laser bandwidths will be required to obtain data that is substantially more informative than that provided by conventional transient absorption experiments. One viable solution was recently reported by Chergui and co-workers, who developed an apparatus in which both the excitation and

Our recent work on photoinduced ring-opening reactions in cycloalkenes confirms that 2DUV has already matured to a level at which valuable new insights can be uncovered in these systems and perhaps many others. detection bandwidths exceed 8000 cm−1.42 In this setup, a 2D spectrum is constructed by combining a large number of transient absorption signals acquired with tunable UV pump pulses. The extremely large and unprecedented spectral range makes this approach very powerful. Couplings between resonances with well-separated frequencies can be examined on time scales greater than 150 fs. The one limitation of this method is the compromise made between time and frequency resolution in the excitation process, which will prevent the study of sub-100 fs dynamics (e.g., exciton self-trapping in DNA). It is our view that 2DUV spectroscopies conducted at liquid helium temperatures are a particularly viable route to resolving the short-lived exciton electronic structure in DNA. Indeed, a long history of research involving light-harvesting proteins and molecular crystals suggests that excitonic effects are most evident at temperatures less than 10 K.43,44 We believe that beautiful physics can be explored in DNA by using temperature to control the interplay between the exciton sizes and thermal fluctuation amplitudes. In a four-wave mixing implementation of 2DUV, the primary challenge will be to retain high optical quality at liquid helium temperatures. Unfortunately, our experience suggests that scattered light will be very difficult to suppress in the frozen samples. 2D experiments in which fluorescence is detected may hold the key to studies of sub-100 fs dynamics in DNA (in both solid and liquid phases) because the signals are immune to light scattering.45,46 These experiments operate best at high repetition rates (at least hundreds of kilohertz), where 20 fs deep-UV laser pulses are not readily produced due to lower pulse energies. It is likely that advances in laser technology will soon eliminate this obstacle and enable such applications.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. Biographies Brantley A. West earned his B.S. in Physics from Davidson College in 2008. He is currently a graduate student in the Department of Physics and Astronomy at the University of North Carolina, where he develops and applies femtosecond four-wave mixing spectroscopies in the deep-UV spectral range. Andrew M. Moran received his Ph.D. in Physical Chemistry from Kansas State University in 2002. He has been an Assistant Professor of Chemistry at the University of North Carolina since 2007. Andrew’s research group uses nonlinear spectroscopies to investigate systems including DNA components, light-harvesting proteins, and molecule− semiconductor interfaces. Website: http://www.chem.unc.edu/ people/faculty/moran/group/ 2579



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ACKNOWLEDGMENTS This work is supported by the National Science Foundation under CHE-0952439.

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(21) Tseng, C.-H.; Sándor, P.; Kotur, M.; Weinacht, T. C.; Matsika, S. Two-Dimensional Fourier Transform Spectroscopy of Adenine and Uracil Using Shaped Ultrafast Laser Pulses in the Deep UV. J. Phys. Chem. A 2012, 116, 2654−2661. (22) West, B. A.; Womick, J. M.; Moran, A. M. Influence of Temperature on Thymine-to-Solvent Vibrational Energy Transfer. J. Chem. Phys. 2011, 135, 114505/1−114505/9. (23) West, B. A.; Womick, J. M.; Moran, A. M. Probing Ultrafast Dynamics in Adenine With Mid-UV Four-Wave Mixing Spectroscopies. J. Phys. Chem. A 2011, 115, 8630−8637. (24) Zimdars, D.; Francis, R. S.; Ferrante, C.; Fayer, M. D. Electronic Dephasing in Nonpolar Room Temperature Liquids: UV Photon Echo Pulse Duration Dependent Measurements. J. Chem. Phys. 1997, 106, 7498−7511. (25) Ajdarzadeh Oskouei, A.; Bräm, O.; Cannizzo, A.; van Mourik, F.; Tortschanoff, A.; Chergui, M. Photon Echo Peak Shift Experiments in the UV: p-Terphenyl in Different Solvents. J. Mol. Liq. 2008, 141, 118−123. (26) Goodno, G. D.; Dadusc, G.; Miller, R. J. D. Ultrafast Heterodyne-Detected Transient-Grating Spectroscopy Using Diffractive Optics. J. Opt. Soc. Am. B 1998, 15, 1791−1794. (27) Maznev, A. A.; Nelson, K. A.; Rogers, J. A. Optical Heterodyne Detection of Laser-Induced Gratings. Opt. Lett. 1998, 23, 1319−1321. (28) Nemeth, A.; Sperling, J.; Hauer, J.; Kauffmann, H. F.; Milota, F. Compact Phase-Stable Design for Single- and Double-Quantum TwoDimensional Electronic Spectroscopy. Opt. Lett. 2009, 34, 3301−3303. (29) Durfee, C. G., III; Backus, S.; Murnane, M. M.; Kapteyn, H. C. Ultrabroadband Phase-Matched Optical Parametric Generation in the Ultraviolet by Use of Guided Waves. Opt. Lett. 1997, 22, 1565−1567. (30) Jailaubekov, A. E.; Bradforth, S. E. Tunable 30-fs Pulses Across the Deep Ultraviolet. Appl. Phys. Lett. 2005, 87, 021107/1−021107/3. (31) Rivera, C. A.; Bradforth, S. E.; Tempea, G. Gires-Tournois Interferometer Type Negative Dispersion Mirrors for Deep Ultraviolet Pulse Compression. Opt. Express 2010, 18, 18615−18624. (32) Kohler, B. Nonradiative Decay Mechanisms in DNA Model Systems. J. Phys. Chem. Lett. 2010, 1, 2047−2053. (33) Markovitsi, D.; Gustavsson, T.; Vayá, I. Fluorescence of DNA Duplexes: From Model Helices to Natural DNA. J. Phys. Chem. Lett. 2010, 1, 3271−3276. (34) Middleton, C. T.; de La Harpe, K.; Su, C.; Law, Y. K.; CrespoHernández, C. E.; Kohler, B. DNA Excited-State Dynamics: From Single Bases to the Double Helix. Annu. Rev. Phys. Chem. 2009, 60, 217−239. (35) Deb, S.; Weber, P. M. The Ultrafast Pathway of Photo-Induced Electrocyclic Ring-Opening Reactions: The Case of 1,3-Cyclohexadiene. Annu. Rev. Phys. Chem. 2011, 62, 19−39. (36) Kosma, K.; Trushin, S. A.; FuB, W.; Schmid, W. E. Cyclohexadiene Ring Opening Observed With 13 fs Resolution: Coherent Oscillations Confirm the Reaction Path. Phys. Chem. Chem. Phys. 2009, 11, 172−181. (37) Pullen, S. H.; Anderson, N. A.; Walker, L. A.; Sension, R. J. The Ultrafast Photochemical Ring-Opening Reaction of 1,3-Cyclohexadiene in Cyclohexane. J. Chem. Phys. 1998, 108, 556−563. (38) Nitzan, A. Chemical Dynamics in Condensed Phases; Oxford University Press: Oxford, U.K., 2006. (39) Farrow, D. A.; Qian, W.; Smith, E. R.; Ferro, A. A.; Jonas, D. M. Polarized Pump−Probe Measurements of Electronic Motion Via a Conical Intersection. J. Chem. Phys. 2008, 128, 144510. (40) Garavelli, M.; Page, C. S.; Celani, P.; Olivucci, M.; Schmid, W. E.; Trushin, S. A.; Fuss, W. Reaction Path of a Sub-200 fs Photochemical Electrocyclic Reaction. J. Phys. Chem. A 2001, 105, 4458−4469. (41) Trulson, M. O.; Dollinger, G. D.; Mathies, R. A. Excited State Structure and Femtosecond Ring-Opening Dynamics of 1,3-Cyclohexadiene from Absolute Resonance Raman Intensities. J. Chem. Phys. 1989, 90, 4274−4281. (42) Auböck, G.; Consani, C.; van Mourik, F.; Chergui, M. Ultrabroadband Femtosecond Two-Dimensional Ultraviolet Transient Absorption. Opt. Lett. 2012, 37, 2337−2339.

REFERENCES

(1) Fleming, G. R. Chemical Applications of Ultrafast Spectroscopy; Oxford University Press: New York, 1986. (2) Jonas, D. M. Two-Dimensional Femtosecond Spectroscopy. Annu. Rev. Phys. Chem. 2003, 54, 425−463. (3) Mukamel, S. Multidimensional Femtosecond Correlation Spectroscopies of Electronic and Vibrational Excitations. Annu. Rev. Phys. Chem. 2000, 51, 691−729. (4) Hamm, P.; Zanni, M. T. Concepts and Methods of 2D Infrared Spectroscopy; Cambridge University Press: Cambridge, U.K., 2011. (5) Cho, M. Coherent Two-Dimensional Optical Spectroscopy. Chem. Rev. 2008, 108, 1331−1418. (6) Hybl, J. D.; Albrecht, A. W.; Gallagher Faeder, S. M.; Jonas, D. M. Two-Dimensional Electronic Spectroscopy. Chem. Phys. Lett. 1998, 297, 307−313. (7) Asplund, M. C.; Zanni, M. T.; Hochstrasser, R. M. TwoDimensional Infrared Spectroscopy of Peptides by Phase-Controlled Femtosecond Vibrational Photon Echoes. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 8219−8224. (8) Golonzka, O.; Khalil, M.; Demirdöven, N.; Tokmakoff, A. Vibrational Anharmonicities Revealed by Coherent Two-Dimensional Infrared Spectroscopy. Phys. Rev. Lett. 2000, 86, 2154−2157. (9) Cowan, M. L.; Ogilvie, J. P.; Miller, R. J. D. Two-Dimensional Spectroscopy Using Diffractive Optics Based Phase-Locked Photon Echoes. Chem. Phys. Lett. 2004, 386, 184−189. (10) Brixner, T.; Mancal, T.; Stiopkin, I. V.; Fleming, G. R. PhaseStabilized Two-Dimensional Electronic Spectroscopy. J. Chem. Phys. 2004, 121, 4221−4236. (11) Fayer, M. D. Dynamics of Liquids, Molecules, and Proteins Measured with Ultrafast 2D IR Vibrational Echo Chemical Exchange Spectroscopy. Annu. Rev. Phys. Chem. 2009, 60, 21−38. (12) Anna, J. M.; Ross, M. R.; Kubarych, K. J. Dissecting Enthalpic and Entropic Barriers to Ultrafast Equilibrium Isomerization of a Flexible Molecule Using 2DIR Chemical Exchange Spectroscopy. J. Phys. Chem. A 2009, 113, 6544−6547. (13) Collini, E.; Wong, C. Y.; Wilk, K. E.; Curmi, P. M. G.; Brumer, P.; Scholes, G. D. Coherently Wired Light-Harvesting in Photosynthetic Marine Algae at Ambient Temperature. Nature 2010, 463, 644−647. (14) Lewis, K. L. M.; Ogilvie, J. P. Probing Photosynthetic Energy and Charge Transfer with Two-Dimensional Electronic Spectroscopy. J. Phys. Chem. Lett. 2012, 3, 503−510. (15) Panitchayangkoon, G.; Hayes, D.; Fransted, K. A.; Caram, J. R.; Harel, E.; Wen, J.; Blankenship, R. E.; Engel, G. S. Long-Lived Quantum Coherence in Photosynthetic Complexes at Physiological Temperature. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 12766−12770. (16) Brixner, T.; Stenger, J.; Vaswani, H. M.; Cho, M.; Blankenship, R. E.; Fleming, G. R. Two-Dimensional Spectroscopy of Electronic Couplings in Photosynthesis. Nature 2005, 434, 625−628. (17) Mukamel, S.; Abramavicius, D.; Yang, L.; Zhuang, W.; Schweigert, I. V.; Voronine, D. V. Coherent Multidimensional Optical Probes for Electron Correlations and Exciton Dynamics: From NMR to X-rays. Acc. Chem. Res. 2009, 42, 553−562. (18) Abramavicius, D.; Jiang, J.; Bulheller, B. M.; Hirst, J. D.; Mukamel, S. Simulation Study of Chiral Two Dimensional Ultraviolet Spectroscopy of the Protein Backbone. J. Am. Chem. Soc. 2010, 132, 7769−7775. (19) Selig, U.; Schleussner, C.-F.; Foerster, M.; Langhojer, F.; Nuernberger, P.; Brixner, T. Coherent Two-Dimensional Ultraviolet Spectroscopy in Fully Noncollinear Geometry. Opt. Lett. 2010, 35, 4178−4180. (20) Tseng, C.-H.; Matsika, S.; Weinacht, T. C. Two-Dimensional Ultrafast Fourier Transform Spectroscopy in the Deep Ultraviolet. Opt. Express 2009, 17, 18788−18793. 2580



Perspective

(43) Schwoerer, M.; Wolf, H. C. Organic Molecular Solids; WileyVCH: Weinheim, Germany, 2007. (44) Jankowiak, R. Probing Electron-Transfer Times in Photosynthetic Reaction Centers by Hole-Burning Spectroscopy. J. Phys. Chem. Lett. 2012, 3, 1684−1694. (45) Tian, P.; Keusters, D.; Suzaki, Y.; Warren, W. S. Femtosecond Phase-Coherent Two-Dimensional Spectroscopy. Science 2003, 300, 1553−1555. (46) Lott, G. A.; Perdomo-Ortiz, A.; Utterback, J. K.; Widom, J. R.; Aspuru-Guzik, A.; Marcus, A. H. Conformation of Self-Assembled Porphyrin Dimers in Liposome Vesicles by Phase-Modulation 2D Fluorescence Spectroscopy. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 16521−16526.

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Two-Dimensional Electronic Spectroscopy in the Ultraviolet

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