Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5296-5300
pubs.acs.org/JPCL
Infrared-Enhanced Fluorescence-Gain Spectroscopy: ConformationSpecific Excited-State Infrared Spectra of Alkylbenzenes Daniel M. Hewett,† Daniel P. Tabor,‡ Joshua L. Fischer,† Edwin L. Sibert, III,§ and Timothy S. Zwier*,† †
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States § Department of Chemistry, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States ‡
S Supporting Information *
ABSTRACT: An ultraviolet−infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S1 origin levels of different conformational isomers. The shorter lifetimes of these IR-excited molecules, combined with their red-shifted emission, provides a way to discriminate the fluorescence due to the infrared-excited molecules from the S1 origin fluorescence, resulting in spectra with high signalto-noise ratios. Spectra for a series of phenylalkanes and a capped phenylalanine derivative (AcPhe-NHMe) demonstrate the potential of the method. The excited-state spectrum in the alkyl CH stretch region of ethylbenzene is well-fit by an anharmonic model developed for the ground electronic state, which explicitly takes into account stretch−bend Fermi resonance.
S1 ← S0 origin transition of a single conformation of the molecule of interest, providing a constant fluorescence signal that monitors the ground-state zero-point level population of that conformer. The IR laser is then tuned over the region of interest, in this case the alkyl C−H stretch region (2800−3000 cm−1). When the IR frequency is on resonance with a vibrational transition from a state being probed by the UV laser, some of the population is removed from the conformational zero-point level by absorption. This results in a partial depletion of the fluorescence signal generated by the UV laser. When the IR laser is set to fire at half the repetition rate of the UV laser, an alternating set of IR-on/IR-off data points is generated. When these data are collected through a gated integrator set in active baseline subtraction mode, the difference signal generated by the IR source can be recorded and plotted against infrared frequency. This generates a single conformation infrared spectrum for the ground-state conformer being probed by the UV laser. This technique has been utilized in many groups in the past with great success.10−12 Analogous experiments can be performed using resonant two-photon ionization techniques, which allow for mass-selectivity.13 Excited-state infrared spectra can be acquired with a similar scheme, but instead of the infrared laser temporally preceding the UV laser, the IR laser arrives after the UV laser, within the molecule’s excited-state lifetime. When the IR laser is on
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ingle conformer infrared spectroscopy of cold, isolated molecules provides structural information that enables determination of the most stable conformations taken by the molecule of interest.1−5 When the molecules possess O−H and N−H groups, the local hydrogen bonding environment of each hydride group can be determined by comparing single conformation experimental spectra with the predictions of scaled, harmonic frequencies and infrared intensities.1,6,7 Until recently, the C−H stretch region of the infrared spectrum had been of limited use as a spectroscopic tool because of the anharmonic coupling of the C−H stretch fundamentals with the overtones of the CH2 and CH3 bending modes, also known as stretch−bend Fermi resonances. This coupling renders standard harmonic calculations ineffective for predicting the spectra in this region; however, the recent development by the Sibert group of a local mode approach to the C−H stretch region that accounts for these couplings has proven successful in matching single-conformer spectra in this region.3−5,8,9 This method has been shown to accurately predict the C−H stretch region of the infrared spectrum of a number of alkyl-containing hydrocarbons,4,5 including straight-chain alkylbenzenes from ethylbenzene through decylbenzene.3,8 The high-quality, single conformation experimental spectra required to develop, test, and refine this model were collected using infrared−ultraviolet (IR-UV) double-resonance techniques. To obtain single-conformer infrared spectra in the ground electronic state, the IR laser is spatially overlapped but temporally precedes the UV laser, typically by 200 ns. The UV laser is fixed at a frequency that is resonant with the © 2017 American Chemical Society
Received: August 28, 2017 Accepted: October 10, 2017 Published: October 10, 2017 5296
DOI: 10.1021/acs.jpclett.7b02276 J. Phys. Chem. Lett. 2017, 8, 5296−5300
Letter
The Journal of Physical Chemistry Letters
Figure 1. Fluorescence lifetime scans of ethylbenzene (a) and the C5 conformer of Ac-Phe-NHMe (b), both with the IR laser off (blue) and with the IR laser on (red). When a gain spectrum is acquired, the gate is set over the enhanced signal (red box). When a depletion spectrum is acquired, the gate is set after the enhanced signal (blue box).
lying high up in the vibrational manifold in the S1 excited states of these molecules. During the course of a recent study of the conformer-specific CH stretch infrared spectra of the straight-chain alkylbenzenes, we pursued excited-state IR spectra with the goal of testing whether the local mode anharmonic model developed for the ground electronic state would transfer successfully to the excited-state spectra. The alkylbenzenes are well-suited to obtaining conformer-specific IR spectra in the S1 electronic excited state, because the excited-state lifetime of the phenyl ring is long (∼130 ns) compared to the 6−8 ns pulse duration of the Q-switched Nd:YAG lasers used to pump the dye laser and infrared optical parametric converter used for UV-IR double resonance. Figure 1 shows the time-dependent fluorescent signals with the UV laser fixed on the S0−S1 origin transitions of ethylbenzene and one of the main conformers of Ac-PheNHMe,19,20 with the IR OPO delayed by 45 ns and resonant with an infrared transition in the excited electronic state. While the expected depletion in fluorescence signal is observed at long times (>90 ns), a distinct gain in fluorescence signal is observed coincident with IR excitation. This gain phenomenon has been observed in previous work by Ebata et al.15 and Matsumoto et al.,14 in which S1 gain signal was observed in aniline and 2-naphthol, respectively. In these cases the additional light generated by the introduction of the infrared laser was detected using a monochromator, where they observed a broad gain in signal higher in energy than the UV excitation wavelength. This study further explores this gain phenomenon using techniques complementary to those utilized in the previous work. The additional fluorescence was present only when the IR laser was on resonance with a vibrational transition in the excited state. As the inset in Figure 1a summarizes, the timedependent fluorescence signal of ethylbenzene can be fit by adding to the fluorescence decay signal (τ = 130 ns) an additional component with an 18 ns decay that begins with IR excitation. A similar effect is observed in Ac-Phe-NHMe,
resonance with a vibrational transition out of the monitored excited-state zero-point level, absorption occurs to levels well above the origin (2800−3000 cm−1 for the CH stretch region). When the molecules are isolated in the gas phase, these highlying levels will typically undergo intramolecular vibrational relaxation to the dense manifold of S1 vibrational levels at that energy, conserving the overall energy. In addition, in the vast majority of molecules, population in these energy levels, far above the electronic origin, undergoes fast nonradiative decay processes, leading to a drop in the fluorescence quantum yield. This results in a depletion of the fluorescence decay profile at times after the IR laser fires. Because depletion in fluorescence signal is observed whenever an excited-state IR transition of the monitored conformer is encountered, tuning the IR laser provides a conformer-specific excited-state IR spectrum. Conformer-specific excited-state IR spectra provide structural and dynamic information that is complementary to their ground-state counterparts.14,15 First, comparison of the excitedstate IR spectrum of a particular conformer to that of the ground state shows perturbations when the frequencies of the IR transition(s) are shifted by electronic excitation of the UV chromophore responsible for the excited state.16 This constitutes a structural clue that the moiety responsible for the IR absorption (e.g., the XH bond in the XH stretch fundamental region) is in close proximity to the UV chromophore. Second, by choosing different vibronic levels during the UV excitation step, it is possible to record IR spectra at well-defined energies above the electronic origin. Changes in the spectrum with increasing vibrational energy can indicate the onset of various excited-state dynamical processes, such as intramolecular vibrational redistribution (IVR) or isomerization.17 Finally, there are times when particular IR transitions are selectively missing from the excited-state spectrum, indicating the coupling of the group to a dissociative channel.18 In the present Letter, we report a novel scheme for recording excited-state IR spectra in alkylbenzenes and related molecules, which also sheds light on the photophysics of vibrational levels 5297
DOI: 10.1021/acs.jpclett.7b02276 J. Phys. Chem. Lett. 2017, 8, 5296−5300
Letter
The Journal of Physical Chemistry Letters
Figure 2. Infrared spectra taken while gating over the increased fluorescence (red) versus the spectrum obtained while gating over the dip (blue) in the CH stretch region of ethylbenzene (a) and the NH stretch region of Ac-Phe-NHMe (b).
Figure 3. (a) Difference of the IR on and IR off scans plotted for each timing (solid lines) versus the fluorescence trace of a UV-only scan (dashed line) and (b) fluorescence traces taken using different long-pass filters. The filters used were 280 nm (A), 305 nm (B), 340 nm (C), and 360 nm (D).
fluorescence decay profile of the S1 origin of ethylbenzene and hence the population of the S1 state origin. As a result, IR excitation occurs out of the S1 zero-point level of each conformer and not some other state. A second means of characterizing the source of the IRenhanced fluorescence is to determine its emission spectrum so that it can be compared to that of the S1 origin. However, we were unable to record a meaningful emission spectrum for the IR-enhanced fluorescence using the monochromator and intensified CCD we use for dispersed fluorescence spectra because of the wavelength breadth of the emission. Instead, we employed a simpler test by recording fluorescence lifetime traces using a set of wavelength long-pass filters with increasing wavelength cut-offs from 280 to 360 nm. This enabled a comparison of the fraction of the UV-only fluorescence that occurs to the red of a given wavelength relative to the IRenhanced fluorescence. As Figure 3b shows, as the wavelength of the long-pass filter was increased, the ratio of IR-enhanced fluorescence to UV-only fluorescence also increased, indicating that a greater proportion of the IR-enhanced fluorescence is shifted well to the red of the excitation wavelength than the emission arising from the S1 origin. By contrast, Ebata et al.
although smaller in magnitude, indicating that this process can be generalized to a range of functionalized alkylbenzenes. Spectra taken by gating over this gain peak are identical to those collected when gating over the depletion but with a significantly improved signal-to-noise ratio in the case of ethylbenzene, as shown in Figure 2a. As a result, conformationspecific excited-state infrared spectra of a representative sample of spectra of the conformations of the alkylbenzenes were recorded using the fluorescence gain signal. In order to understand better the excited-state processes responsible for the additional fluorescence, the following tests were devised. One possibility is that the IR-enhanced fluorescence arises from a different state than S1 following a nonradiative process (Figure 1 inset, in brown) that produces an electronic state or photoproduct that subsequently fluoresces when IR-excited. To test this possibility, a series of ethylbenzene fluorescence decay profiles were recorded with IR excitation delayed by 25−185 ns from the UV pulse, in 10 ns increments. The fluorescence signal in the absence of IR was subtracted from each trace to obtain a difference signal due to the IR-enhanced fluorescence. The result is shown in Figure 3a. The intensity of the infrared-induced fluorescence follows the 5298
DOI: 10.1021/acs.jpclett.7b02276 J. Phys. Chem. Lett. 2017, 8, 5296−5300
Letter
The Journal of Physical Chemistry Letters
fundamentals and the scissor overtones are shifted from their ground-state values, producing a spectrum significantly different than its ground-state counterpart (Figure 4c). The success of the model bodes well for future studies of excited-state IR spectra in the alkyl CH stretch region. In summary, an alternative scheme for recording conformation-specific infrared spectra of molecules in their electronically excited states has been developed. In alkylbenzenes, infrared excitation out of the S1 state zero-point level of a given conformer leads to an increase in fluorescence rather than the fluorescence depletion characteristic of many molecular excited states. This IR-enhanced fluorescence is almost 10times shorter-lived than the S1 origin fluorescence and can be selectively detected through its red-shifted emission tail. The body of evidence points to the emission as arising from S1(v) levels following intramolecular vibrational redistribution (IVR) to the low-frequency modes of the alkyl chain tail. Excitation in the alkyl CH stretch region places 2800−3000 cm−1 of vibrational energy in the conformer. While initial excitation is to CH stretch/scissors overtones of the conformer out of which excitation occurs, excess energies in this regime are well above the anticipated barriers to isomerization of the alkyl chain. In ethylbenzene, the ethyl group prefers an out-of-plane geometry for the methyl group; however, geometries in which the ethyl chain is in-plane are energetically accessible and may contribute to the red-shifted emission produced by IR excitation. The presence of this IR-enhanced fluorescence has been observed in the n-alkylbenzenes but also appears more weakly in the model phenylalanine-containing peptide, Ac-Phe-NHMe (with structure in Figure 1). In Ac-Phe-NHMe, the phenyl ring is attached to the peptide backbone through a two-carbon alkyl chain in which C(α) is a part of the peptide backbone and is thus representative of Phe-containing peptides in general. While the magnitude of the effect is diminished, the presence of a detectable IR-enhanced excited-state fluorescence in Ac-PheNHMe suggests that alkylbenzene derivatives of a much wider variety may produce a similar effect. Because the phenyl ring is the prototypical aromatic, phenyl derivatives are pervasive in combustion, astrochemistry, and atmospheric chemistry, opening the door to utilizing this technique much more broadly than directly demonstrated here. Furthermore, in Ac-Phe-NHMe, the excited-state amide NH stretch fundamentals were detected in this way (Figure 2b), indicating that the effect is still present after initial excitation occurs via infrared excitation of other hydride stretch vibrations beyond the alkyl CH stretch region which played a primary role in the present work.
observed a broad, higher-energy emission, albeit at very low intensity. It should be noted that the total fluorescence decreased significantly as the cutoff wavelength of the longpass filters increased, indicating that a majority of the IRenhanced fluorescence is at wavelengths similar to the UV-only fluorescence with an additional component that is at longer wavelengths. We surmise on this basis that the IR-excited fluorescence is S1-like in its overall emission profile but with a somewhat more intense tail at longer wavelength. Using this fluorescence gain scheme, we recorded S1 excitedstate IR spectra of C6H5−CH2−CH3, C6H5−CD2-CH3, the two conformers of propylbenzene (trans and gauche), the two main conformers of butylbenzene, and the three observed conformers of pentyl and hexylbenzene in the alkyl CH stretch region (Figure S1). The experimental spectra for ethylbenzene and C6H5−CD2−CH3 are shown in Figure 4, where they are
Figure 4. Theoretical fits for the excited-state infrared spectra of C6H5CD2CH3 (a) and ethylbenzene (b) shown in dashed red. This theoretical fit uses the ωB97X-D/6-311++G(d,p) model Hamiltonian. The ground-state spectrum of the hydrogenated ethylbenzene is shown in trace c, with the shifts in the CH2 groups shown in blue.
compared with the predictions of the anharmonic local mode model used by Tabor et al. to assign the ground-state spectra.3 Following excited-state geometry optimization, the groundstate model coupling parameters were used without change after incorporating the local mode site frequencies for the excited state. The scaling factors that enter the model Hamiltonian were originally parametrized for three different density functional methods: B3LYP, B3LYP-D3, and ωB97XD, all with the 6-311++G(d,p) basis set, and also for MP2/augcc-pVDZ. These levels of theory have been successfully applied to other ground-state molecules containing alkyl chains.8,21 The results of the model at the ωB97X-D/6-311++G(d,p) level of theory are shown in Figure 4b, which account for the experimental excited-state infrared spectra in significant detail. Notably, the model parameters at this level of theory also appear to transfer from the ground state to the excited state with high fidelity. The spectrum for the partially deuterated ethylbenzene sample has absorptions due solely to the terminal methyl group (Figure 4a). In the fully hydrogenated isotopomer (Figure 4b), the local site frequencies of the benzylic CH2 group are shifted lower in frequency by 13 cm−1 upon ππ* excitation, while the local frequencies of the scissor overtones also drop in frequency by about 18 cm−1. The triplet of bands due to symmetric and antisymmetric stretch
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EXPERIMENTAL SECTION The samples were entrained in ∼3 bar of helium and were pulsed into a vacuum through an 800 μm diameter orifice (Parker General Valve Series 9) to produce a supersonic jet expansion. A laser-induced fluorescence detection scheme was used to record the S1 ← S0 excitation spectrum for each molecule. The alkylbenzenes were purchased from SigmaAldrich; Ac-Phe-NHMe was purchased from VWR. Ethylbenzene through pentylbenzene were studied without heating, while hexylbenzene was heated to 45 °C, and Ac-Phe-NHMe was heated to 160 °C in order to generate sufficient vapor pressure for our studies. An IR-UV double-resonance scheme was then utilized to record the excited-state single-conformation C−H stretching spectra for ethylbenzene. This experiment was performed by fixing the wavelength of the 20 Hz UV laser on the S1 ← S0 origin band for ethylbenzene. The 20 Hz UV 5299
DOI: 10.1021/acs.jpclett.7b02276 J. Phys. Chem. Lett. 2017, 8, 5296−5300
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The Journal of Physical Chemistry Letters
Conformation-specific IR and UV spectroscopy of cyclically constrained β/γ-peptides. J. Phys. Chem. B 2014, 118, 8246−8256. (7) Kusaka, R.; Zhang, D.; Walsh, P. S.; Gord, J. R.; Fisher, B. F.; Gellman, S. H.; Zwier, T. S. Role of ring-constrained γ-amino acid residues in α/γ-peptide folding: Single-conformation UV and IR spectroscopy. J. Phys. Chem. A 2013, 117, 10847−10862. (8) Hewett, D. M.; Bocklitz, S.; Tabor, D. P.; Sibert Iii, E. L.; Suhm, M. A.; Zwier, T. S. Identifying the first folded alkylbenzene via ultraviolet, infrared, and Raman spectroscopy of pentylbenzene through decylbenzene. Chem. Sci. 2017, 8, 5305−5318. (9) Sibert, E. L.; Tabor, D. P.; Kidwell, N. M.; Dean, J. C.; Zwier, T. S. Fermi resonance effects in the vibrational spectroscopy of methyl and methoxy groups. J. Phys. Chem. A 2014, 118, 11272−11281. (10) Derro, E. L.; Pollack, I.; Dempsey, L. P.; Greenslade, M. E.; Lei, Y.; Radenović, D. C.; Lester, M. I. Fluorescence-dip infrared spectroscopy and predissociation dynamics of OH A(2)Sigma(+) (v = 4) radicals. J. Chem. Phys. 2005, 122, 244313. (11) Frost, R. K.; Hagemeister, F.; Schleppenbach, D.; Laurence, G.; Zwier, T. S. Fluorescence-dip infrared spectroscopy of jet-cooled 5hydroxytropolone. J. Phys. Chem. 1996, 100, 16835−16842. (12) Biswal, H. S.; Wategaonkar, S. Nature of the N−H···S hydrogen bond. J. Phys. Chem. A 2009, 113, 12763−12773. (13) Stearns, J. A.; Das, A.; Zwier, T. S. Hydrogen atom dislocation in the excited state of anthranilic acid: probing the carbonyl stretch fundamental and the effects of water complexation. Phys. Chem. Chem. Phys. 2004, 6, 2605−2610. (14) Matsumoto, Y.; Ebata, T.; Mikami, N. Photofragment-detected IR spectroscopy (PFDIRS) for the OH stretching vibration of the hydrogen-bonded clusters in the S1 state — Application to 2Naphthol-B (B = H2O and CH3OH) clusters. J. Phys. Chem. A 2001, 105, 5727−5730. (15) Ebata, T.; Minejima, C.; Mikami, N. A new electronic state of aniline observed in the transient IR absorption spectrum from S1 in a supersonic jet. J. Phys. Chem. A 2002, 106, 11070−11074. (16) Kidwell, N. M.; Mehta-Hurt, D. N.; Korn, J. A., III; Sibert, E. L.; Zwier, T. S. Ground and excited state infrared spectroscopy of jetcooled radicals: Exploring the photophysics of trihydronaphthyl and inden-2-ylmethyl. J. Chem. Phys. 2014, 140, 214302. (17) Mukamel, S.; Smalley, R. E. Fluorescence of supercooled molecules as a probe for intramolecular vibrational redistribution rates. J. Chem. Phys. 1980, 73, 4156−4166. (18) Dian, B. C.; Longarte, A.; Zwier, T. S. Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives: Direct evidence for the 1πσ* state. J. Chem. Phys. 2003, 118, 2696− 2706. (19) Buchanan, E. G., III; James, W. H.; Choi, S. H.; Guo, L.; Gellman, S. H.; Müller, C. W.; Zwier, T. S. Single-conformation infrared spectra of model peptides in the amide I and amide II regions: Experiment-based determination of local mode frequencies and intermode coupling. J. Chem. Phys. 2012, 137, 094301. (20) Gerhards, M.; Unterberg, C.; Gerlach, A.; Jansen, A. [small beta]-sheet model systems in the gas phase: Structures and vibrations of Ac-Phe-NHMe and its dimer (Ac-Phe-NHMe)2. Phys. Chem. Chem. Phys. 2004, 6, 2682−2690. (21) Korn, J. A.; Tabor, D. P., III; Sibert, E. L.; Zwier, T. S. Conformation-specific spectroscopy of alkyl benzyl radicals: Effects of a radical center on the CH stretch infrared spectrum of an alkyl chain. J. Chem. Phys. 2016, 145, 124314.
laser preceded a 10 Hz IR laser by 50 ns while the IR laser was scanned over the alkyl C−H stretching region of the infrared spectrum (2800−3000 cm−1). A difference spectrum was recorded by passing the total fluorescence signal through a gated integrator working in active baseline subtraction mode. Whenever the infrared laser was resonant with a vibrational transition of the excited state being pumped by the UV laser, a gain in total fluorescence was observed, which, after passing through the gated integrator, produced the infrared spectrum of interest. A dip spectrum of the excited state was also obtained by setting the gate after the enhanced fluorescence (Figure 1).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02276. Brief description of the theoretical model used in this study and the local mode Hamiltonians calculated for the ground and excited states of ethylbenzene; additional excited-state infrared spectra showing the conformerspecific nature of the conducted experiments (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Daniel M. Hewett: 0000-0002-0039-5968 Daniel P. Tabor: 0000-0002-8680-6667 Timothy S. Zwier: 0000-0002-4468-5748 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS D.P.T. and E.L.S. acknowledge support from NSF Grant No. CHE-1566108. D.M.H., J.L.F., and T.S.Z. acknowledge support form DOE Office of Sciences Basic Energy Sciences under Grant DE-FG02-96ER14656.
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REFERENCES
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