Article pubs.acs.org/JPCB
Strong Intermolecular Vibrational Coupling through Cyclic Hydrogen-Bonded Structures Revealed by Ultrafast Continuum MidIR Spectroscopy Ashley M. Stingel, Carmella Calabrese, and Poul B. Petersen* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States ABSTRACT: Cyclic hydrogen-bonded structures are common motifs in biological systems, providing structural stability and mediating proton transfer for redox reactions. The mechanism of proton transfer across hydrogen-bonded interfaces depends on the strength of the intermolecular coupling between bridging OH/NH vibrational modes. Here we present a novel ultrafast continuum mid-IR spectroscopy experiment to study the vibrational dynamics of the 7-azaindole−acetic acid (7AI-Ac) heterodimer as a model system for asymmetric cyclic hydrogen-bonded structures. In addition to spreading of the excitation across the whole OH band within the time resolution of the experiment, excitation of a 300 cm−1 region of the ∼1000 cm−1 broad OH stretching mode of the acetic acid monomer leads to a frequency shift in the NH stretching mode of the 7AI monomer. This indicates that the NH and OH stretching modes located on the two monomers are strongly coupled despite being separated by 750 cm−1. The strong coupling further causes the OH and NH bands to decay with a common decay time of ∼2.5 ps. This intermolecular coupling is mediated through the hydrogen-bonded structure of the 7AI-Ac heterodimer and is likely a general property of cyclic hydrogen-bonded structures. Characterizing the vibrational dynamics of and the coupling between the high-frequency OH/NH modes will be important for understanding proton transfer across such molecular interfaces.
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INTRODUCTION Electron transfer and energy transfer in biological systems are frequently linked to proton transfer across hydrogen-bonded interfaces through proton-coupled electron transfer reactions.1,2 Proton transfer is thus integral to biological function and lies at the heart of bioenergetics and biomimetic energy conversion.3−5 Furthermore, excited-state proton transfer within DNA following UV excitation has been invoked to explain the photostability of DNA, but whether such UV excitation leads to proton transfer between DNA base pairs in solution remains controversial.3,6 Cyclic hydrogen-bonded structures, such as those found in DNA and those formed by carboxylic acid− amidine interfaces, are common structural motifs in biological systems and are capable of mediating multiproton transfer. The strength of the coupling between hydrogen-bonded vibrational modes within these cyclical structures influences the mechanism of multiproton transfer reactions. However, the vibrational dynamics and couplings of such strongly hydrogenbonded systems are difficult to study experimentally with conventional ultrafast techniques because of their very broad (>500 cm−1) spectral features in the infrared. While ultrafast visible spectroscopy typically utilizes continuum visible pulses to probe photoinduced spectral changes across the whole visible spectrum in a single laser pulse, ultrafast IR spectroscopy has until recently been limited to observing spectral changes over a window of 100−500 cm−1 in © 2013 American Chemical Society
a single laser shot by employing femtosecond pulses generated in optical parametric amplifiers (OPAs).7−11 The spectral window of the probe pulse can be scanned across the vibrational spectrum by tuning the OPA, but this is a timeconsuming process and is subject to drifts in the beam pointing, complicating experimental implementation.12 We recently demonstrated an IR pump−probe spectroscopic technique utilizing an ultrafast continuum mid-IR pulse that is capable of probing the entire vibrational region (4000 cm−1) by scanning only the detection optics.13 This new technology holds promise to revolutionize transient IR spectroscopy in the same manner as continuum visible pulses revolutionized transient visible spectroscopy. Despite the limitations of OPAs, there have been several studies of the vibrational dynamics of cyclic intermolecular hydrogen-bonded structures.14−24 It has generally been observed that the vibrational dynamics of the high-frequency OH and/or NH stretching modes are modulated by lowfrequency modes that control how strongly the high-frequency modes are hydrogen-bonded. In particular, symmetric hydroSpecial Issue: Michael D. Fayer Festschrift Received: June 29, 2013 Revised: September 8, 2013 Published: September 9, 2013 15714
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recent theoretical investigation of phosphinic acid dimers found multiple peak structures in the 1600−3000 cm−1 spectral region due to Fermi resonances between the strongly hydrogenbonded OH stretch and bend vibrational modes.28 To study the vibrational dynamics of and the intermolecular coupling between the OH and NH modes, we excited the highfrequency part of the 1000 cm−1 broad OH band with a conventional 300 cm−1 broad femtosecond laser pulse from an OPA and probed the 1750 cm−1 broad spectral changes across both the OH and NH bands with a continuum pulse.
gen-bonded systems consisting of carboxylic acid and 7azaindole (7AI) homodimers have been studied in detail.14−20 Both acetic acid and 7AI homodimers display highly structured vibrational features that are ∼500 cm−1 broad and centered around 3000 cm−1, as shown in the top panel of Figure 1. In
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EXPERIMENTAL BACKGROUND For more than a decade, broadband terahertz pulses have been generated by simultaneously focusing 800 and 400 nm femtosecond laser pulses in air, causing breakdown and the formation of a plasma.29−31 In 2007, Fuji and Suzuki32 extended the continuum pulses into the mid-IR by using 25 fs laser pulses and reflective optics to limit dispersion. Petersen and Tokmakoff extended the two-color generation scheme to a three-color scheme by simultaneously focusing 800, 400, and 267 nm pulses collinearly in air.33 The inclusion of the 267 nm pulse results in about a 10-fold increase in the power of the continuum pulse relative to the two-color scheme and the compact collinear geometry minimizes the pulse-to-pulse power fluctuations, making it possible to use the continuum source as a probe pulse in transient IR spectroscopy. This prospect stimulated several groups to pursue mid-IR continuum generation. Baiz and Kubarych34 demonstrated the use of upconversion detection to probe a part of the continuum IR pulse (1800−2600 cm−1) using a visible CCD, while Khalil and co-workers studied the effect of the gas medium in continuum generation using the two-color scheme35 and Fuji and Nomura36 recently reported very short and energetic continuum pulses using the two-color scheme. We recently demonstrated the first implementation of the continuum pulse in transient spectroscopy in pump−continuum mid-IR probe experiments,13 and Tokmakoff and co-workers recently published pump−probe and 2D IR experiments on water using a continuum mid-IR probe pulse.37 We expect further optimization of the continuum IR generation to continue to facilitate its implementation in a wide range of applications.
Figure 1. FTIR spectra and cyclic hydrogen-bonded structures of the 7-azaindole and acetic acid homodimers (top panel) and the 7azaindole−acetic acid heterodimer (bottom panel). The heterodimer spectrum is color-coded with the spectral assignments of the O−H vibrations (red), Fermi resonances (yellow), and N−H vibrations (blue).
these symmetric dimers, the OH/NH modes form symmetric and asymmetric stretching combinations that fall on top of the 0 → 2 overtone transitions of several ring bending and deformation modes, resulting in a multitude of Fermi resonances. The very complex vibrational bands arise from both coupling to low-frequency modes, which contributes mainly to the breadth of the band, and from the Fermi resonances, which cause the substructure.17,18 The vibrational population relaxation times of these systems are very short (sub-picosecond) because of the strong coupling to the bending modes and a modulation due to the low-frequency modes. Elsaesser and co-workers recently studied the asymmetric hydrogen-bonded structures of DNA base pairs in solution as well as hydrated films of DNA using both IR pump−probe and 2D IR spectroscopies.21−24 Like the symmetric model systems, these cyclic hydrogen-bonded biological systems also display structured bands from 2600 to 3400 cm−1 due to Fermi resonances and couplings to lowfrequency modes. The pump−probe experiment again revealed sub-picosecond vibrational population decay times, and the 2D IR spectra resolved several individual NH modes and their couplings. In this work, we applied ultrafast continuum mid-IR spectroscopy to probe the intermolecular coupling between the OH and NH stretching vibrations within the 7-azaindole− acetic acid (7AI-Ac) heterodimer. This heterodimer exhibits a much broader IR spectrum than either homodimer, as shown in Figure 1. The 7AI-Ac heterodimer vibrational spectrum was assigned through chemical modifications and DFT calculations. The sharper feature around 3250 cm−1 is attributed to the NH stretch and the broad double-hump structure from 1800 to 2700 cm−1 to the OH stretch vibration, while the structured region around 3000 cm−1 is due to Fermi resonances.25 The double-hump character of the OH stretching is intriguing but is not unique to this dimer. Similar structures have previously been observed for strongly hydrogen-bonded OH vibrations26,27 and for the 7AI−pyridine dimer.25 Furthermore, a
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EXPERIMENTAL METHODS The ultrafast continuum mid-IR technique has been described previously.13 Briefly, the continuum mid-IR probe pulse generation is driven by 1 mJ, 25 fs, 800 nm laser pulses from a regenerative Ti:sapphire amplifier (Coherent Legend Elite Duo). The three driving laser pulses are produced collinearly by first doubling the 800 nm laser pulses and then summing the fundamental and doubled pulses to generate the 267 nm pulse using thin BBO crystals. The temporal overlap between the 800 and 400 nm pulses and their relative polarization are controlled by a delay plate and dual waveplate, respectively, between the doubling and tripling crystals. The three colors are focused collinearly in air by a 50 cm focal length spherical mirror, and the resulting continuum IR pulse is collimated by a 1 m focal length spherical mirror. Employing 25 fs, 800 nm pulses to drive the continuum generation results in an IR spectrum that extends below 600 cm−1 and up to 5000 cm−1.13 In the present transient experiment, we used a monochromator and a single-element MCT detector to measure the continuum spectrum. The 15715
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measurable spectral range was limited by the detector and grating efficiencies. Using an MCT detector doped for 12 μm and three gratings (300 grooves/mm blazed for 2 μm, 150 grooves/mm blazed for 4 μm, and 75 grooves/mm blazed for 8 μm), we could cover a spectral range of 1000−5000 cm−1. The spectral information was obtained by scanning the monochromator, which was time-consuming and limited our signalto-noise ratio in the current configuration. An MCT array detector would facilitate measuring multiple wavelengths simultaneously, thereby greatly reducing the acquisition time. Current models feature up to 128 detector elements, effectively turning a 5 day experiment into one requiring a single hour or alternatively improving the signal-to-noise ratio by more than an order of magnitude. The pump pulse was generated in an OPA (Coherent OPerA) and was centered at approximately 2500 cm−1 with a 300 cm−1 bandwidth [full width at half-maximum (fwhm)]. The temporal resolution of the experiment was determined to be sub-160 fs using cross-correlation in a 1 mm thick Ge window.13 This measurement likely was limited by propagation effects in the Ge and provides an upper limit to the temporal resolution. The temporal resolution could be increased to less than 50 fs by compressing the probe pulse using a pulse shaper.38,39 Acetic acid and 7-azaindole were obtained from SigmaAldrich and used without further purification. A 0.25 M 7AI-Ac heterodimer sample was prepared in carbon tetrachloride with an acetic acid/7AI molar ratio of approximately 0.95:1. The sample was contained in a 100 μm sandwich cell with 1 mm thick calcium fluoride windows.
Figure 2. FTIR spectrum of the 7AI-Ac heterodimer, overlaid by the pump and probe spectra, and three transient spectra (negative adsorption change) at delays of 0.35, 1.0, and 2.5 ps.
the pump−probe spectrum is much narrower than the probe, as in the present case, the frequency shifts due to vibrational coupling are observed as a reduced absorption at the unperturbed frequency and an induced absorbance at the new frequency. The strongly hydrogen-bonded OH and NH vibrations considered here exhibit very anharmonic potentials and are strongly coupled to low-frequency modes and to Fermi resonances. While we have a general understanding of the FTIR spectrum, a more detailed description requires extensive multidimensional anharmonic calculations.17,18 It is thus difficult to predict the frequencies of the 1 → 2 transitions that sample a higher part of the potential and are accordingly more sensitive to the anharmonicity. Furthermore, because of the strong anharmonicity of the potential, induced absorbances arising from 1 → 3 transitions could be observed. Overtone transitions are typically not observed in transient IR experiments because of their low amplitudes but can be observed for very anharmonic potentials. For example, the 0 → 2 overtone vibration of a shared proton (O···H···O) was recently observed in the 2D IR spectrum of hydroxide solutions.43 The transient spectrum is a result of interfering contributions from broad reduced absorbances for the 0 → 1 transitions and even broader induced absorbances due to the 1 → 2 and possibly the 1 → 3 transitions. In the present experiment, the assignment of the reduced absorbances at 1938 and 2500 cm−1 to the 0 → 1 transitions of two parts of the OH stretching mode is clear. Pending further theoretical investigation, we assign the induced absorbances at 1803 and 2999 cm−1 to the 1 → 2 transition associated with this very anharmonic vibrational mode that extends to either side of the fundamental 0 → 1 transition. Alternatively, the 2999 cm−1 feature could be due to part of the 1 → 3 transition of the OH stretching mode. The reduced absorbance at 3254 cm−1 and the induced absorbance at 3320 cm−1 are attributed to a frequency shift in the NH vibration upon excitation of the OH vibration resulting
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RESULTS AND DISCUSSION In the present pump−probe experiment, we excited the highfrequency part of the OH stretch with a 300 cm−1 fwhm broad, 50 fs pump pulse, resulting in a transient spectrum spanning from 1650 to 3400 cm−1. Transient spectra at 0.35, 1.0, and 2.5 ps are shown in Figure 2 along with the FTIR spectrum of 7AIAc and the pump and probe pulse spectra. To more easily make the comparison between the transient and FTIR spectra, we have plotted the negative absorption changes (−ΔA). The transient spectrum is dominated by reduced absorbances at 1938, ∼2500, and 3254 cm−1, corresponding to the two humps of the OH band and the NH transition seen in the FTIR spectrum. In addition, we observe three weaker induced absorbances at 1803, 2999, and 3320 cm−1. These new spectral features are more difficult to definitively assign, as will be discussed below. In third-order transient vibrational spectroscopy, reduced absorbances are observed at the vibrational frequencies that are excited, accompanied by induced absorbances corresponding to the associated 1 → 2 vibrational transitions.40−42 These induced absorbances are commonly red-shifted with respect to the 0 → 1 transition because of the anharmonicity of the potential. Additionally, spectral changes can be due to a shift in one vibrational frequency upon excitation of another mode caused by anharmonic coupling between the modes. In 2D IR spectroscopy these frequency shifts result in cross-peaks that are typically much weaker than the diagonal peaks.40−42 A pump−probe spectrum can be viewed as an integrated version of a 2D IR spectrum by collapsing the pump-frequency axis. In the degenerate pump−probe case, where the pump and probe pulse spectra are identical, the cross-peak features are usually hidden underneath the stronger diagonal peak features. When 15716
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from a strong coupling between the two modes as described above. This shift to higher frequency is likely caused by a weakening of the hydrogen bond between the NH and the acetic acid carbonyl oxygen as a result of excitation of the OH vibration. Alternatively, the reduced absorbance at the NH frequency could be due to excitation transfer from the OH mode to the NH mode resulting from the strong coupling. However, such a 750 cm−1 uphill energy transfer would have to be accompanied by the loss of several quanta of low-frequency vibrations, which is unlikely. Also, in this picture the induced absorbance at 3254 cm−1 is difficult to assign and could be due to either a blue-shifted 1 → 2 transition or a 1 → 3 transition of the NH stretching mode. We are currently performing extensive DFT calculations to model the 0 → 1, 0 → 2, 1 → 2, and 1 → 3 transitions, which will be the subject of a future publication. Independent of the spectral assignment, the main conclusions remain clear: the NH and OH vibrational modes are very strongly intermolecularly coupled, and this strong coupling is due to the cyclic structure of the hydrogen-bonded interface. The transient spectrum obtained at each time delay was modeled by fitting to a sum of six Gaussian peaks corresponding to the three reduced absorbances and the three induced absorbances described above. The transient spectra and fits at time delays of 0.35, 1.0, and 2.5 ps are shown in Figure 2. In view of the anharmonicity of the vibrations, the transitions are likely not true Gaussians, but within the current level of analysis, Gaussian peaks are sufficient and reproduce the observed spectral changes well. Among these features, the high-frequency part of the 0 → 1 transition of the OH stretching vibration at 2500 cm−1 was directly excited in this experiment. The other transitions are due to energy transfer, vibrational coupling, or excited state absorption, as described above. The directly excited mode at 2500 cm−1 exhibits a frequency shift and narrowing following the excitation due to spectral diffusion dynamics, while the other five transient features exhibit only amplitude changes following excitation. To reduce the number of free-floating fitting parameters, the center frequencies and peak widths of the five static Gaussian peaks were fixed, and only their amplitudes were allowed to vary as a function of time delay. The center frequencies and peak widths of these five Gaussian peaks were determined by first floating all of the parameters and then averaging the center frequencies and peak widths from the spectral fits at each time point from 0.2 to 2.5 ps. All three parameters of the sixth Gaussian peak, corresponding to the region of the OH transition that was directly excited in the experiment, were allowed to vary as a function of time delay. In addition to the representative fits of the transient spectra shown in Figure 2, the full data set and fit are shown in Figure 3 along with the residuals. The fitting parameters are shown in Figures 4 and 5 and are further discussed below. When the pump and probe pulses are overlapped in time, a coherent artifact due to the nonresonant signal from the sample cell windows is produced. The fwhm time response of the experiment was sub-160 fs. In the following analysis, we focus on the data starting at 300 fs, well after the coherent artifact. The fitted amplitudes as a function of time delay are shown in Figure 4, which also indicates the center frequencies of the five fixed peaks and the average center frequency of the floating peak. All of the peak amplitudes decay to zero over a few picoseconds. Within the signal-to-noise ratio, the amplitudes of all five fixed Gaussians could be fitted with single-exponential
Figure 3. Full IR pump−CIR probe data set (negative adsorption change, bottom) spanning a frequency range of 1750 cm−1 in 25 cm−1 steps and a time delay of −1.0 to 2.75 ps in 50 fs steps. The spectral fit (middle) and residuals (top) are shown from 0.2 to 2.750 ps, denoted by the dotted line in the data. Each plot contains 17 evenly spaced contours from −1.35 (darkest blue) to 1.35 (darkest red) mOD.
Figure 4. Fitted amplitudes of the six Gaussian peaks. The legend gives the five fixed center frequencies (1795, 1939, 3000, 3260, and 3320 cm−1) and the average center frequency value of the floating peak (2450 cm−1). The dashed vertical line at 0.3 ps indicates the start of the exponential fit, as described in the text.
decays with a common time constant of ∼2.5 ps. The directly excited peak was fitted with a double-exponential decay exhibiting the same long time component and a second ultrafast time component less than our time resolution of 150 fs. The fact that all of the peaks could be fit with a common time scale supports our assignment of the NH vibrational 15717
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the OH and NH vibrations to relax with a common time scale of ∼2.5 ps, which is significantly longer than those of the corresponding homodimers. The rich transient features are not fully understood and will benefit from theoretical investigations to model the observed spectral changes in this very anharmonic and coupled system. Since the strong coupling is caused by the cyclic hydrogen-bonded structure, it is likely a general property of such systems. Given the abundance of similar structures in biological systems, we believe that the strong coupling observed here could be important for understanding the mechanism of biochemical energy dissipation and transfer as well as proton transfer reactions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by startup funding from Cornell University. C.C. acknowledges support from the Cornell Center for Materials Research with funding through IGERT: A Graduate Traineeship in Materials for a Sustainable Future (DGE-0903653) from the National Science Foundation.
Figure 5. Center frequency and peak width (fwhm) of the sixth Gaussian peak that was allowed to vary as a function of the pump− probe time delay. The dashed vertical line at 0.3 ps indicates the start of the single-exponential fits with time constants of 0.6 and 1.3 ps, respectively, as described in the text.
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dynamics to a frequency shift upon excitation of the OH vibration. If the NH vibration were excited through energy transfer, different time scales would likely be observed because of different decay times of the OH and NH vibrations. The previous investigations of the 7AI and acetic acid homodimers14−20 and DNA base pairs21−24 observed a shorter population decay time of less than 1 ps. In those systems, the OH/NH bands are on top of the Fermi resonances, resulting in very strong coupling to the bend overtones. In the 7AI-Ac heterodimer, the NH and OH bands are shifted to either side of the Fermi resonances, resulting in a weaker coupling to the bend overtones and a correspondingly a longer decay time. In addition to the vibrational relaxation dynamics, the directly excited high-frequency part of the OH stretching mode exhibits spectral diffusion dynamics that could be reproduced by allowing the center frequency and bandwidth of this spectral feature to vary with the time delay, as shown in Figure 5. These spectral diffusion dynamics occur because the pump intensity is not constant across the high-frequency part of the OH spectrum, resulting in a nonequilibrium excitation that relaxes to the equilibrium center frequency and peak width in 0.6 and 1.3 ps, respectively. Within our resolution, no noticeable spectral diffusion dynamics were observed in the low-frequency part of the OH mode or the NH mode.
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CONCLUSIONS We have performed ultrafast continuum mid-IR spectroscopy measurements on the 7AI-Ac heterodimer. Excitation of a part of the OH stretching band leads to spectral changes across 1750 cm−1. The broad transient spectrum is attributed to excitation of the whole OH band within the time resolution of the experiment accompanied by a frequency shift in the NH stretching band. This indicates a very strong intermolecular coupling between the OH vibration of the acetic acid and the NH vibration of the 7-azaindole mediated through the cyclic hydrogen-bonded structure. The strong coupling further causes 15718
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