Couplings Across the Vibrational Spectrum Caused by Strong

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Couplings Across the Vibrational Spectrum Caused by Strong Hydrogen Bonds: A Continuum 2D IR Study of the 7‑Azaindole− Acetic Acid Heterodimer Ashley M. Stingel and Poul B. Petersen* Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States ABSTRACT: Strongly hydrogen-bonded motifs provide structural stability and can act as proton transfer relays to drive chemical processes in biological and chemical systems. However, structures with medium and strong hydrogen bonds are difficult to study due to their characteristically broad vibrational bands and large anharmonicity. This is further complicated by strong interactions between the high-frequency hydrogen-bonded vibrational modes, fingerprint modes, and low-frequency intradimer modes that modulate the hydrogen-bonding. Understanding these structures and their associated dynamics requires studying much of the vibrational spectrum. Here, mid-IR continuum spectroscopy of the cyclic 7-azaindole−acetic acid (7AI−AcOH) heterodimer reveals the vibrational relaxation dynamics and couplings of this complex hydrogen-bonded system. Within this dimer, the NH bond of 7AI exhibits a band at 3250 cm−1 caused by a medium strength hydrogen bond, while the strongly hydrogen-bonded OH modes of acetic acid exhibit a broad double-peaked vibrational feature spanning 1750 to 2750 cm−1. Transient IR and 2D IR experiments were performed using three excitation frequencies, centered on the high-frequency OH and NH modes, and probed with a mid-IR continuum to measure the spectral response from 1000 to 3500 cm−1. While the NH stretch is observed to relax in 300 fs, the strongly hydrogen-bonded OH modes relax within the time resolution of the experiment (sub-100 fs). The difference in the strength of the hydrogen bonds is also reflected in the coupling pattern in the fingerprint region observed with 2D IR spectroscopy. Here the NH is strongly coupled to fingerprint modes involving the 7AI monomer, while the OH vibrations are strongly coupled to vibrational modes across the entire dimer. Together, the results show strong coupling and rapid energy transfer across the hydrogen-bonded interface and through the structure of the 7-azaindole−acetic acid heterodimer, highlighting the need to study the full vibrational spectrum for strongly hydrogen-bonded systems.

1. INTRODUCTION Hydrogen-bonding in biological systems is crucial for structure and biological function. A classic example of this is the hydrogen-bonding between the base pairs of DNA, which provides support for its double helix structure.1 Additionally, these hydrogen bonds may provide a pathway for energy dissipation from the excited state2−5 and are important for proton-coupled electron transfer reactions in many chemical and biological systems.6−9 Cyclic hydrogen-bonded structures like those found in DNA and those formed by carboxylic acid− amidine interfaces are capable of mediating multiproton transfer.10−12 Vibrational spectroscopy is a sensitive probe of the molecular structure, and ultrafast vibrational spectroscopy is a valuable tool for investigating the vibrational energy transport at the natural time scales of the system. Here, we examine the vibrational dynamics and couplings of a strongly hydrogenbonded dimer in the electronic ground state, which is important to developing an understanding of both the structure itself and energy transport through such systems. Vibrational spectroscopy probes the chemical bonds directly, but the vibrations of structures with medium and strong © 2016 American Chemical Society

hydrogen bonds are complicated by the very broad spectral features associated with such systems.13 Because of this, studying the vibrational characteristics of such structures requires probing a large portion of the vibrational spectrum. While linear infrared spectroscopy utilizes broadband, incoherent light sources that cover the whole vibrational range, most ultrafast vibrational spectroscopy relies on infrared pulses generated by optical parametric amplifiers (OPAs). These pulses are typically 100−500 cm−1 broad, which can span only a fraction of the broad hydrogen-bonded vibrational spectra. Traditional transient IR and two-dimensional infrared (2D IR) spectroscopy experiments use replicates of a single mid-IR pulse as both pump and probe pulses,14,15 while two-color experiments use different IR frequencies to excite and probe different spectral regions. These experiments require knowing where to look for new spectral features and are still limited to probing several hundred cm−1 in a single experiment.16−19 Full Received: May 18, 2016 Revised: September 17, 2016 Published: September 27, 2016 10768

DOI: 10.1021/acs.jpcb.6b05049 J. Phys. Chem. B 2016, 120, 10768−10779

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The Journal of Physical Chemistry B

bonded in the heterodimer, resulting in that peak being further red-shifted by about 500 cm−1 relative to the AcOH homodimer, to nearly 1000 cm−1 lower frequency than the free OH of the unbound monomer. The third peak at 1900 cm−1 results from a Fermi resonance between the OH stretch and the overtone of the OH bend.56 Due to the very anharmonic potential of, and high degree of coupling within, hydrogen-bonded dimers, complex calculations are required to replicate these spectra.17,46,48,56 The substructure of the broad high-frequency peaks is due to Fermi resonances between the XH modes and overtones and combination bands of various fingerprint modes. Accurate modeling of the experimental spectra of these systems requires the inclusion of these Fermi resonances. As such, coupling between the high-frequency hydrogen-bonded modes and lower frequency fingerprint modes plays an important role in the transfer and dissipation of vibrational energy. The bridging hydrogen bonds are also important to the spectra and dynamics of these systems, but occur at very low frequencies (5 ps) time constant. This is evident in the decay of the peaks at approximately 1360 and 1430 cm−1 and the corresponding induced absorbances. In contrast, excitation at 3200 cm−1 results in different dynamics among the fingerprint modes. Some vibrations, such as the very strong 1290 cm−1 mode, are excited immediately, but others, such as those at 1360 and 1430 cm−1, show the same rise time of about 300 fs seen in the highfrequency OH modes. The instantaneously excited vibrations are located predominantly on the 7AI monomer, while the modes with a clear rise time involve more AcOH motion. This indicates that excitation of the NH stretch results in a response in the fingerprint modes on the 7AI monomer due to vibrational coupling, but the excitation takes longer to spread across the hydrogen-bonded interface into modes with high AcOH character. This likely results from the conjugated heterocycle of the 7AI monomer having many delocalized ring bending and deformation modes that provide more pathways for energy dissipation than the smaller AcOH monomer. Additionally, the modulation caused by the intermolecular hydrogen-bonding mode at 140 cm−1 is much stronger following excitation of the OH stretch, illustrating that the OH stretch is more strongly coupled to the intradimer stretch mode than the NH, reflecting the strengths of the corresponding hydrogen bonds. Concerning the overall vibrational relaxation, we note that regardless of the excitation frequency, the populated fingerprint modes remain excited for several ps showing that a hot ground state is not reached within that time scale. Upon exciting the NH stretch mode, vibrational relaxation into the OH modes occurs in about 300 fs. This leads to the transient features in the fingerprint region growing in on this time scale in addition to the instantaneous responses due to vibrational couplings. As described above, it is hard to experimentally determine whether the instantaneous response across the spectrum upon exciting the OH mode is due to vibrational couplings or very fast vibrational relaxation. Previous investigations of the homodimers revealed an ultrafast decay of the OH and NH stretch modes in less than 200 fs.17,44,45,47 The OH stretch in the heterodimer is more strongly hydrogenbonded than either the NH stretch in the heterodimer or the OH/NH modes in the homodimers, causing a much broader transition and stronger couplings to other modes. As a result, a faster relaxation time of the heterodimer OH modes is expected. The observed instantaneous response across the spectrum and the absence of a rise time in any of the fingerprint modes leads to the conclusion that the ultrafast vibrational relaxation of the OH modes occurs within the time resolution of the experiment. The excitation of multiple fingerprint modes within the sample ensemble then leads to a quasi-thermal response in the high-frequency OH and NH modes due to vibrational couplings, thereby shifting the spectrum. This is similar to hot ground state effects commonly observed in transient IR experiments but is mechanistically different since the fingerprint modes remain excited for several ps. By observing the whole vibrational spectrum we are thus able to distinguish this quasi-thermal response from a true hot ground state response.

Figure 7. Transient response of the 7AI−AcOH fingerprint modes following the excitation of (top) the acid OH stretch centered at 2500 cm−1 and (bottom) the 7AI NH stretch at 3250 cm−1. The change in absorbance is presented in 15 evenly spaced contours ranging from −5 mOD (blue) to 5 mOD (red).

expands the transient spectrum from 1200 to 1650 cm−1 following the 2500 and 3200 cm−1 excitations. This spectral region is dominated by a series of ring-bending and deformation modes and was not included in the earlier studies. Numerous fingerprint modes have a transient response following both high frequency excitations. Generally, these modes appear as the bleach and induced absorbance pairs typically expected in transient IR spectroscopy, but the congestion of the spectrum adds some additional complexity. Figure 8 shows the low-frequency dynamics following excitation of both the OH and NH stretches. The short (2.5 ps) and long (13 ps) dynamics of each of three modes centered at approximately 1290, 1360, and 1430 cm−1 are shown.

Figure 8. Dynamics of several fingerprint modes following excitation of the OH stretch (top) and the NH stretch (bottom) over 2.5 ps (left) and over 13 ps (right). 10773

DOI: 10.1021/acs.jpcb.6b05049 J. Phys. Chem. B 2016, 120, 10768−10779

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The Journal of Physical Chemistry B

Figure 9. Full 2D IR spectrum obtained using three excitation frequencies and probed with the CIR pulse. The FTIR of the corresponding spectral regions of the heterodimer are shown at top and right. The three pump spectra are overlaid on the FTIR shown at the top. The change in absorbance is presented in 25 evenly spaced contours, where blue contours denote negative bleach features and yellow contours denote positive induced absorbances.

2D IR Results and Discussion. The IR pumpCIR probe experiments revealed interesting mode-specific dynamics across the vibrational spectrum of the 7AI−AcOH heterodimer that are dependent on the excitation frequency. To gain further insight into the strong couplings within the dimer, 2D IR spectra at a waiting time of 200 fs were measured at each of the three pump frequencies, 1900, 2500, and 3200 cm−1, and probed with the mid-IR continuum. Since the OH modes have decayed significantly in 200 fs, the resulting peaks in the 2D spectrum following excitation of the OH modes are a combination of the quasi-thermal response in the highfrequency modes and the response in the subsequently excited fingerprint modes. The quasi-thermal response results from anharmonic couplings between the vibrationally populated fingerprint modes and the high-frequency modes. Vibrational coupling also facilitates the rapid energy transfer between the OH and the fingerprint modes. Thus, even though the OH has decayed at this time delay, the peaks in the 2D spectrum are still illustrative of vibrational couplings, although by mechanis-

tically different processes than the direct vibrational coupling typically observed in 2D IR spectra. The NH mode has decayed about 50% at 200 fs. Accordingly the features in the 2D spectrum are a combination of direct vibrational couplings as well as energy transfer. Like the pump−probe experiments, the implementation of the CIR probe uncovers information across nearly the entire vibrational spectrum of 7AI−AcOH, as shown in Figure 9. Coupling of High-Frequency Modes. The CIR-probe 2D IR spectrum reveals couplings between the modes within the excited regions and across the entire vibrational spectrum. Bleaching of the high-frequency region is observed along the diagonal of all three excitations, spanning from the carbonyl at 1700 cm−1 to above the NH stretch at 3250 cm−1. Scattered pump results in a narrow, elongated bleach along the diagonal of the 2D spectrum. Additionally, the three main hydrogenbonded modes are bleached regardless of the excitation frequency, illustrating the strong coupling between the OH modes, the NH stretch mode, and the fingerprint modes. 10774

DOI: 10.1021/acs.jpcb.6b05049 J. Phys. Chem. B 2016, 120, 10768−10779

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Figure 10. 2D IR spectrum of the fingerprint mode cross peaks obtained using three excitation frequencies and probed with the CIR pulse. For comparison, the FTIR of the heterodimer is shown at the top and to the right. The pump spectra are overlaid on the FTIR at the top. The change in absorbance is presented in 25 evenly spaced contours.

The induced absorbances in the high-frequency region of the full 2D spectrum exhibit additional pump-dependence. The feature around 1800 cm−1 appears strongly correlated with both OH excitations, but not with the high-frequency NH excitation. The most notable difference in the high-frequency spectrum is the induced absorbance observed in the pump−probe experiments around 3000 cm−1. In those results, this induced feature was observed regardless of the pump frequency, but the bandwidth and substructure of that feature varied. In the 2D spectra, a similar pattern is observed as this induced cross peak varies significantly across the pump axis. A very strong induced absorbance cross peak occurs with the NH stretch excitation. Like the transient spectra, the cross peak with the NH has the Fermi resonance substructure, but the cross peak with the OH stretch is much narrower (in ω3) and is mostly featureless. The different substructure and coupling of these peaks suggests there are multiple overlapping features in this range, including the 1→2 transition of the NH stretch. Previously, the 1→2 or very anharmonic 1→3 transitions of the OH stretch were suggested as a possible assignments for the induced feature.25

This study further supports that the OH and NH cross peaks at 3000 cm−1 have different origins, and shows that there are features at both 1800 and 3000 cm−1 that couple with the OH bands but not the NH. This is consistent with the assignment of these modes as one or both of the 1→2 or 1→3 transitions of the OH stretch. Coupling to Fingerprint Modes. As was observed in the pump−probe results, the much narrower fingerprint modes are difficult to see in the full 2D surfaces. The fingerprint region from 1200 to 1625 cm−1 is expanded in Figure 10. Many of the fingerprint modes, particularly those with energies in the range of 1250 to 1600 cm−1, couple differently with the NH and Fermi resonances as compared to the OH modes. These are of particular interest, as the overtones and combination modes of these fingerprint vibrations correspond to energies that could feasibly form Fermi resonances with the OH and NH modes (0→2 frequencies in the range of 2500−3200 cm−1). These Fermi resonances provide a pathway for rapid energy dissipation from the high-frequency modes into the fingerprint modes, which was observed in the pump−probe dynamics, and 10775

DOI: 10.1021/acs.jpcb.6b05049 J. Phys. Chem. B 2016, 120, 10768−10779

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Figure 11. Carbonyl diagonal and cross peaks in perpendicular (top) and parallel (bottom) polarizations measured with excitation pulses centered at 1900 (left), 2500 (middle), and 3200 cm−1 (right). The change in absorbance is presented in 25 evenly spaced contours.

facilitate strong interactions between nearly all of the modes. The coupled modes provide further evidence for the energy dissipation to fingerprint modes associated with the 7AI monomer prior to spreading across the whole dimer. The vibration at about 1290 cm−1, which has strong CN and CO character, displays an immediate and very strong bleach regardless of excitation frequency in the pump−probe results. Correspondingly, the cross peak of this vibration is the only fingerprint bleach that couples strongly across the entire pump axis. Since this vibration greatly influences the intermolecular cyclic hydrogen-bonded structure and has strong COH and CNH bend character in each monomer, it follows that any perturbation of the hydrogen-bonded modes also induces a strong response from this mode. Additionally, the overtone (0→2) of this mode may be strongly coupled to both the OH stretch and the NH through the Fermi resonances. This mode is clearly important for the dissipation of vibrational energy, and will likely also be important for proton transfer across this interface. Several fingerprint modes from about 1300 to 1450 cm−1 couple differently to the different high-frequency modes. This includes the vibrations at 1360 and 1430 cm−1 that exhibited different dynamics following the different excitations in the pump−probe experiments. Both of these modes are predominately associated with the AcOH monomer, with little contribution from 7AI. Accordingly, each of these modes couples with both OH modes and the Fermi resonances, but neither couples with the NH stretch. The different coupling with the Fermi resonances and the NH stretch also explains the different dynamics of these modes. Since these modes are not coupled with the NH stretch directly, the transient response at these frequencies occurs only after vibrational energy transfers from the NH stretch into the Fermi resonances and OH modes. Nearly all of the fingerprint modes form induced absorbances with the NH excitation. This is clear in the shape of the cross peaks, many of which span only the width of the NH stretch in the excitation axis, while many other modes appear as continuous bands across much of the spectrum. This hints at a broadband induced absorbance at low frequencies that couples with only the NH, which is likely due to a very anharmonic excited state absorption or overtone of the NH stretch. Additionally, the modes from 1300 to 1400 cm−1

couple differently with the NH pump, where there are bleached cross peaks with the Fermi resonance excitation but induced features with the NH stretch excitation. The strong positive peak at (3250, 1315) does not appear at any other excitation frequencies. In contrast, the cross peaks across the OH region consist of an induced absorbance at 1355 cm−1 with broad bleaches on either side from below 1300 to almost 1400 cm−1. The differences in the coupling pattern of the NH and OH modes reflect the difference in the strength of the hydrogenbond to the NH and OH modes. While the medium strength hydrogen-bonded NH mode couples to the subset of fingerprint modes located in the 7AI monomer at 200 fs, the strongly hydrogen-bonded OH is coupled to fingerprint modes across the entire dimer. Carbonyl. The acid carbonyl stretch appears at 1710 cm−1 in the heterodimer FTIR and falls within the bandwidth of the 1900 cm−1 pump pulse. In the pump−probe results, the carbonyl peak is only weakly bleached regardless of the pump frequency, suggesting it is only weakly involved in the transfer of energy through the dimer. This is surprising considering the strong linear absorbance of that mode and its role in the doubly hydrogen-bonded intermolecular ring structure. In contrast to the very delocalized fingerprint modes, the localization of this mode on only the CO moiety contributes to the slower transfer of energy following excitation of the NH. However, even when directly excited by the 1900 cm−1 pump, the carbonyl bleach appears weak, likely due to the spectral overlap with the strong induced absorbance at 1800 cm−1. Similarly, there is very little signal from the acid carbonyl in the 2D spectra probed in a parallel polarization, again suggesting that the carbonyl stretch is largely decoupled from the rest of the delocalized cyclic hydrogen-bonded structure. To understand this in more detail, the 2D IR spectra of the carbonyl diagonal and cross peaks in both parallel and perpendicular polarizations are presented in Figure 11. Since the polarized pump beam only excites dipoles that are aligned with the polarization of the pump, the weak signal in the parallel polarization may be due in part to the transition dipole angle. However, a diagonal peak is observed when the carbonyl is directly excited by the pump centered at 1900 cm−1. In contrast, nearly all of the other modes exhibit broad cross peaks across the pump axis. Besides the diagonal bleach, the carbonyl also couples with the NH stretch and Fermi resonances as a 10776

DOI: 10.1021/acs.jpcb.6b05049 J. Phys. Chem. B 2016, 120, 10768−10779

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Fingerprint modes that involve mainly the 7AI monomer are very strongly coupled with the NH stretch and exhibit an instantaneous response, while fingerprint modes centered on the AcOH monomer exhibit a delayed response on the same time scale as the NH relaxes into the OH. In contrast, upon excitation the OH stretch and bend modes relax into several fingerprint modes within the time scale of the experiment. This causes a quasi-thermal response observed as a bleach in the high-frequency modes and the involved fingerprint fundamentals. That the response in the fingerprint modes is caused by vibrational excitation and not due to hot ground state effects is evident from the symmetric fundamental and excited state absorbances. This quasi-thermal response then decays with a slower time scale of several ps as the fingerprint modes relax and energy is dissipated. Differences in the coupling strength between the highfrequency modes and the fingerprint modes are governed by the differing hydrogen-bond strength and are resolved in the 2D IR spectrum. The NH stretch couples strongly with fingerprint modes that are located on the 7AI monomer, while the OH modes couple strongly to fingerprint modes across the dimer. An exception is the carbonyl stretch, which couples much more weakly to the NH and OH modes than the fingerprint modes. This difference is caused by the carbonyl stretch being much more localized than the fingerprint modes, where delocalization promotes the couplings. Additionally, the modulation caused by the intermolecular hydrogen-bonded intradimer mode at 140 cm−1 is much stronger following excitation of the OH stretch, illustrating that the strength of the coupling to the low-frequency dimer stretch modes is also governed by the strength of the hydrogen bond. Ultrafast midIR continuum spectroscopy allows us to observe the rapid energy transfer and strong couplings across the whole vibrational spectrum induced by the strong hydrogen bonds within the dimer.

pair of slightly displaced cross peaks following the NH excitation. The splitting of the carbonyl into two peaks is also apparent in the linear infrared spectrum, and the NH and Fermi resonances couple separately with these two peaks of the carbonyl. The 2D IR spectrum resulting from the 1900 cm−1 shows that the narrow carbonyl diagonal bleach is overwhelmed by the broad induced absorbance at (1900, 1800) cm−1. In parallel polarization, only a very small cross peak following the OH stretch excitation is observed. When probed in the perpendicular polarization, however, the carbonyl cross peaks show up more strongly at all three excitation wavelengths, while the diagonal peak is significantly weaker. This polarization dependence again indicates the carbonyl is not delocalized like the other modes directly involved in the bridging hydrogen bonds. Furthermore, Lim and Hochstrasser43 observed that the anharmonicity shifted the 1→2 transition only 8 cm−1 from the fundamental CO stretch in the acetic acid homodimer. Here we observe a carbonyl anharmonicity of nearly 30 cm−1, a result of the significantly stronger hydrogen-bond in the heterodimer compared to the homodimer.

4. CONCLUSIONS Ultrafast mid-infrared continuum probe pulses have been incorporated in transient IR and 2D IR spectroscopies and used to study the vibrational dynamics of a cyclically hydrogenbonded heterodimer. Unlike traditional ultrafast vibrational spectroscopy, which is limited to a few hundred cm−1 of bandwidth in a single experiment, ultrafast mid-IR continuum spectroscopy has the bandwidth to study the very broad and complex vibrational spectra of strongly hydrogen-bonded structures. By capturing the dynamics and couplings across the spectrum, this comprehensive study of the 7AI−AcOH heterodimer illustrates the value of mid-IR continuum probes in ultrafast spectroscopy. From the IR pump−CIR probe and 2D IR presented here, a detailed picture of the vibrational relaxation dynamics and couplings within the 7AI−AcOH heterodimer emerges and is summarized in Figure 12. Upon directly exciting the NH stretch mode at 3250 cm−1, the vibrational energy relaxes into the OH modes in 300 fs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: +001 (607) 255-4303. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Arnold and Mabel Beckman Foundation in supporting this work through a Young Investigator Award.

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REFERENCES

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