Letter pubs.acs.org/JPCL
Coherent High-Frequency Vibrational Dynamics in the Excited Electronic State of All-Trans Retinal Derivatives Jan Philip Kraack, Tiago Buckup, and Marcus Motzkus* Physikalisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg, D-69210 Heidelberg, Germany S Supporting Information *
ABSTRACT: Coherent vibrational dynamics of retinal in excited electronic states are of primary importance in the understanding of photobiology. Using pump-DFWM, we demonstrate for the first time the existence of coherent double-bond high-frequency modulations (>1300 cm−1) in the excited electronic state of different retinal derivatives. All-trans retinal as well as retinal Schiff bases exhibit a partial frequency downshift of the CC double-bond mode from ∼1580 cm−1 in the ground state to 1510 cm−1 in the excited state. In addition, a new vibrational band at ∼1700 cm−1 assigned to the CN stretching mode in retinal Schiff bases in the excited state is detected. The newly reported bands are observed only in specific spectral regions of excited-state absorption. Implications regarding the observation of vibrational coherences in naturally occurring retinal protonated Schiff bases in rhodopsins are discussed. SECTION: Spectroscopy, Photochemistry, and Excited States
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states of retinal chromophores, that is, all-trans retinal (ATR) and all-trans retinal Schiff bases (RSBs). The photophysics of ATR and its derivatives involve ultrafast internal conversion from an initially populated excited singlet state to lower-lying excited states. A comprehensive analysis has been recently presented.8 In brief, in polar solvents ATR undergoes a sub-100 fs internal conversion from a Bu+-like state to an Ag−-like state that is strongly coupled to an internal charge-transfer state. These two coupled states decay with a time constant around 1.8 ps. The RSB chromophore21 also undergoes a sub-100 fs IC from the Bu+-like state to an Ag−-like state, which afterward decays with a much longer time constant of ∼43 ps in polar solvents. Figure 1 shows ground-state absorption spectra and selected pump−probe difference spectra (ΔOD) of ATR (a) as well as RSB (b) in ethanol. The ΔOD spectra (orange, Figure 1 a,b) show the resonance conditions from excited electronic states on time scales longer than 500 fs, that is, when the initial relaxation is complete. Positive (negative) values indicate excited-state absorption (ESA) (excited-state stimulated emission (ESE)). For ATR, an ESA transition is observed around 19 000 cm−1. Additionally, ESE transitions are observed below 17 500 cm−1, overlapping with a second ESA transition centered around 13 000 cm−1, similar to RPSB22 and rhodopsins.23 These features decay with similar time constants of ∼1.8 ps. For RSB, analogous transitions in the spectral ranges described above are observed; however, the shape and relative intensities of the
he retinal chromophore is one of the most fascinating molecules in nature due to its performance as a keyelement of photoactive proteins (rhodopsins) to enable living organisms to convert light into chemical energy.1,2 Photonabsorption initiates an ultrafast (femtoseconds to picoseconds), efficient, and bond-selective isomerization in rhodopsins, which triggers a specific photocycle inside the protein.1,3 Hence, the importance of this reaction has motivated several studies of the dynamics of both rhodopsins as well as the isolated chromophore, the retinal (protonated)-Schiff-base (R(P)SB).3−13 A primary goal has always been the elucidation of transient changes in the chromophore structure. To resolve this, transient vibrational spectra including bands with high frequencies (>1000 cm−1) from the excited state are needed with high time- (fs) and spectral resolution (1300 cm−1), especially from double-bond stretching motion, could be that initially prepared coherences of these modes may undergo extremely rapid dephasing right after excitation, leaving no signatures in the dynamics on later times. An approach to overcome this issue is to re-excite coherences once the initially prepared coherence is lost. To this end, our group developed pump degenerate four-wave-mixing (pumpDFWM),13,18−20 a technique that fulfils all requirements stated above and that has been applied previously for the detection of excited-state high-frequency modes in, for example, carotenoids. Here we show that pump-DFWM is able to pave the way for the investigation of high-frequency coherences in excited © XXXX American Chemical Society
Received: December 4, 2012 Accepted: January 10, 2013
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dx.doi.org/10.1021/jz302001m | J. Phys. Chem. Lett. 2013, 4, 383−387
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Figure 2. Pump-DFWM transients for (a) ATR and (b) RSB in ethanol detected at 19 000 cm−1. Solid lines indicate the signal induced from pu/St- and pr-pulses exclusively. Open circles indicate pumpDFWM signal from all four-pulses (IP, pu/St-, and probe) for IP delays T = 1 (ATR, a) and 2 ps (RSB, b). Insets show oscillatory signals after the subtraction of an exponential fit to the population dynamics (τ23 > 80 fs). Transient signal contains oscillatory contribution from the solvent with a period of 37 fs (885 cm−1) (see the Supporting Information).
Figure 1. (a) Ground-state absorption of ATR in ethanol (black), UV excitation (25 000 cm−1, violet), DFWM spectra for ATR (cyan (18 000 cm−1) and red (13 500 cm−1)), and ΔOD spectrum (orange) of ATR in ethanol (T = 510 fs) after UV excitation. (b) Ground-state absorption of RSB in ethanol (black), UV excitation (violet), DFWM spectra (blue (20 000 cm−1) and green (16 500 cm−1)), and ΔOD spectrum of RSB (orange) in ethanol at T = 510 fs after UV excitation. Horizontal lines indicate the level of ΔOD = 0 mOD.
vector interactions.24 Oscillatory and nonoscillatory dynamics are observed for both ATR and RSB if the IP pulse precedes the DFWM-sequence (T = 1 and 2 ps, respectively). The nonoscillatory pump-DFWM dynamics reflect the evolution of photogenerated population in different electronic states (Sn, n > 0).13 For instance, pu/St-pulses of the DFWM sequence remove part of the nonstationary population from the state generated by the IP pulse (e.g., S1) to higher-lying optically allowed excited singlet states in spectral regions of ESA or to lower lying states in spectral regions of ESE. In a brief description of such dynamics, population generated in higherlying electronic states by the action of pu/St-pulses can relax back to the S1 state or to other electronic states depending on the characteristics of the involved PES. This gives rise to the short-lived nonoscillatory pump-DFWM dynamics that decay in the τ23 direction within the initial few hundreds of femtoseconds. Additionally, pu/St-pulses also prepare a “hole” in the S1 state that relaxes through the population decay of the respective electronic state (e.g., S1). A detailed description of the pump-DFWM population dynamics will be presented elsewhere. Selected vibrational dynamics after subtraction of an exponential fit to the nonoscillatory dynamics for probe delays larger than 80 fs are shown as insets. For details of the analysis of pump-DFWM data, the reader is referred to refs 13 and 18−20. For both samples, not only are low-frequency oscillations with 100 or more femtoseconds observed, but also specifically high-frequency modulations with periods of a few tens of femtoseconds are clearly resolved. The former ones are less pronounced for RSB (b) compared with ATR (a). This transient signal stems clearly from the excited state because no signal is observed if the DFWM precedes the IP pulse (see solid lines in Figure 2a,b). To analyze the vibrational frequencies that contribute to the oscillatory pump-DFWM dynamics, we calculated FFT spectra from the oscillatory residuals after subtraction of an exponential fit to the nonoscillatory pump-DFWM dynamics. Examples of
features show pronounced differences: two separated ESA bands are observed at 21 500 and 18 500 cm−1. Below 15 000 cm−1, a very shallow ESE transition is observed. By analogy to ATR, these features decay together with a time constant of ∼43 ps. The spectral features described above allowed pump-DFWM experiments on the two samples exploiting the single ESA and ESE transitions. Specifically, our goal was to resolve coherent high-frequency modes in the excited states to clarify whether these play a role in the excited-state dynamics and which kind of modes are coupled to which transition (ESE and ESA). First, we investigated the dependence of the vibrational dynamics in ATR’s excited state on spectral positions of the DFWM spectra after excitation at 25 000 cm−1. In particular, DFWM spectra at ∼18 000 cm−1 as well as 13 500 cm−1 were chosen (Figure 1a). For RSB, DFWM spectra were tuned to about 20 000 and 18 000 cm−1 to investigate dynamics associated with the two distinct ESA bands (Figure 1b). For both samples, the applied DFWM spectra are clearly off-resonance with respect to the ground-state absorption. Thus, the pump-DFWM signals report exclusively on dynamics initiated from a population in the excited state. Examples of pump-DFWM transients associated with ATR and RSB are depicted in Figure 2. The transients at several IP delays consist of different contributions, depending on resonance conditions (ESA or ESE) encountered by the DFWM spectra after the interaction of the IP pulse. No signal is observed when the probe pulse interacts with the system prior to the arrival of pu/St-pulses (τ23 < −50 fs) within each independent transient because all DFWM-spectra are offresonance with respect to the ground-state absorption spectra. Around the temporal overlap of pu/St and pr-pulses, a sharp spike is observed (the coherent artifact) for all transients. This feature originates from nonresonant multiphoton-pathways due to a series of permutations in the temporal order of the wave384
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Figure 3. FFT spectra of ground state DFWM dynamics (gray, ATR (a) and RSB (d) both in ethanol) and pump DFWM transients for ATR (b and c) and RSB (e and f). pump-DFWM data was acquired at T = 1 ps for ATR (b) (cyan DFWM-spectrum) and (c) (red DFWM-spectrum). PumpDFWM data were acquired at T = 2 ps for RSB (e) (blue DFWM spectrum) and (f) (green spectrum). Vibrational bands assigned to solvent dynamics are indicated with asterisks.
spectrum centered at 18 000 cm−1 (green spectrum Figure 1b) show only two high-frequency bands at 1510 and 1575 cm−1 (Figure 3f). For additional pump-DFWM measurements of RSB in hexane (see Supporting Information) and in the ESA -band centered around 22 000 cm−1, a single band is observed around 1680 cm−1. Furthermore, also the two bands below 1600 cm−1 are down-shifted to 1550 and 1500 cm−1, respectively. The comparison of pump-DFWM and ground-state spectra data unambiguously shows that the reported bands stem from the excited electronic states of ATR and RSB. This is in addition supported by the measurements of both samples in different solvents for which no Raman-active bands are known for the frequencies in question (see the Supporting Information). The above presented results thus report for the first time coherent high-frequency vibrational dynamics (>1500 cm−1) in excited singlet states of retinal derivatives. In particular, for RSB, specific bands can be detected only in certain spectral regions of ESA. Vibrational modes with frequencies above 1500 cm−1 reflect double-bond stretching motion in polyenes (CC and CN in the case of RSB).25,26 Therefore, we assign the newly reported bands at 1510/1580 and ∼1700 cm−1 to CC and CN double-bond stretching modes in the excited states, respectively. Because of the population of charge-separated states, especially in polyenes with heteroatoms conjugated to the polyene chain, changes in bond lengths occur during the transition from ground to excited states, thereby changing the vibrational frequencies in the excited state relative to the ground state.27,28 However, the direction of shifting (higher or lower) is challenging to predict. From our results, it is clear that parts of the CC bonds for ATR and RSB are weakened in the excited state, indicated by a splitting of the frequency at 1580 cm−1 into a second band at 1510 cm−1. Note that the band around 1580 cm−1 mode is preserved in the excited state, which indicates that part of the polyene chain retains its bond characters. The frequency of the CN double bond shifts from ∼1625 cm−1 in the ground (measured previously25) state to ∼1700 cm−1 in the excited state. This suggests that the charge density of this bond is increased upon excitation.
FFT spectra from a series of pump-DFWM experiments are shown in Figure 3 for ATR and RSB. A full number of highfrequency modes are observed. For a comparison, the vibrational spectra from the pure electronic ground states were also obtained by nonresonant DFWM experiments (DFWM spectrum at ∼21 000 cm−1, blocked IP pulse) (Figure 3a,d). To obtain measurable signals, experiments were performed with high sample concentrations (∼6 OD) and high excitation energies (∼90 nJ) to compensate for the nonresonant interaction. The measured ground-state vibrational band positions of ATR and RSB match very well with vibrational frequencies obtained from high-resolution frequency-domain Raman-spectra.25,26 Bands assignable to solvent contributions are marked with asterisks. For pump-DFWM data of ATR in ethanol (Figure 3b) (cyan spectrum, Figure 1a), we observe a clear doublet-structure above 1500 cm−1 with frequencies at about 1510 and 1580 cm−1. In particular, the band at 1510 cm−1 is not observable in the ground-state dynamics (Figure 3a). The frequency of the band at 1580 cm−1 is very similar to the frequency of the mode in the ground state (Figure 3a) within the spectral resolution of our experiment (±10 cm−1). If the DFWM spectrum (red spectrum in Figure 1a) is tuned to the near-infrared in the spectral region of ESE, then both bands are again found in the FFT spectrum (Figure 3c). However, the band at 1510 cm−1 is weaker relative to the second more intense band, which is down-shifted to 1560 cm−1 compared with probing in spectral regions of ESA (Figure 3b). For pump-DFWM experiments on RSB around 20 000 cm−1 (blue spectrum in Figure 1b) we observe multiple new bands in the FFT-spectra in the region above 1500 cm−1 (Figure 3e,f). Specifically, we observe a doublet structure with frequencies around 1510 and 1575 cm−1 (similar to ATR) from which only the mode at 1575 cm−1 is observable again in the ground state (Figure 3d). The 1510 cm−1 mode appears always with lower intensity compared with the 1575 cm−1 band. New bands appear for RSB also on the higher energy side of the 1575 cm−1 mode. For RSB in ethanol, we observe a band around 1700 cm−1 with multiple maxima (Figure 3e). In contrast, pumpDFWM experiments on RSB in ethanol with a DFWM385
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The Journal of Physical Chemistry Letters
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ATR was purchased from Sigma-Aldrich and used without further purification. RSB samples were prepared as previously described.12,13 All solvents were degassed and dried over molecular sieves (3 Å). Freshly prepared solutions were used for every independent scan (60 min). Samples were dissolved to a concentration that resulted ∼2 OD in appropriate solvents. Excitation energies were set to ∼200 nJ for the IP pulse (diameter of 100 μm), 20 nJ for (pu/St pulses), as well as 12 nJ for probe pulses (diameter of 70 μm).
Previous studies on polyenes such as carotenoids and carotenals revealed a frequency shift of the CC doublebond stretching mode up to 1800 cm−1 in the lowest-lying excited singlet state.18,19,29 An analogous frequency-shift is, however, not observed in this study for ATR and RSB (vide supra). To exclude possible origins of insufficient temporal resolution in the reported experiments, test scans on β-carotene samples were conducted that clearly revealed the existence of the well-known 1780 cm−1 mode in the excited state (see the Supporting Information). In this context, it has to be kept in mind that the frequency shift to ∼1800 cm−1 in carotenoids has been observed only for polyenes with higher conjugation length (n > 6) compared with ATR and RSB, which both exhibit an effective conjugation length n < 6 due to the nonplanarity of the β-ionone ring. It might hence not be expected that similar frequency upshifts occur for ATR and RSB as well. This difference in observations may be reasoned by the fact that for carotenoids such as lycopene or β-carotene the lowest-lying singlet state is of perfect Ag− symmetry while polyenes with conjugated heteroatoms exhibit a more complex electronic structure for the lowest-lying singlet state, possibly comprising PES with even mixed charge-transfer states.8 Thus, specific differences in the structure of the involved PES can directly influence the vibrational spectra of the samples. The observation of modes (e.g., ∼1700 cm−1) only for pump-DFWM spectra overlapping with the ESA band centered around 22 000 cm−1 in RSB is a major finding. It shows that the Franck−Condon factors of certain vibrational modes between different excited and ground electronic states are different throughout the spectral range of detection. To be able to detect such modes, the DFWM spectrum thus needs to be tuned to the correct position. This may have important implications regarding the detection of high-frequency coherences in excited states of RPSB in rhodopsins and in solution phase as well. For both samples, the most intense ESA band heavily overlaps with the ground-state absorption,10,14 and thus dynamics detected in this spectral range are always congested with ground-state modes that may mask weak excited-state features. Experiments currently performed in our lab will reveal whether the results presented here can be generalized to rhodopsins and RPSB. The protonation of the Schiff-base linkage induces a reversion of the relative energetic position of the Bu+- and Ag−-like states, which complicates the photophysics. If similar modes were not traceable in rhodopsins and RPSB, then this might give indications that, for example, vibrational dephasing in Bu+- and Ag−-like states is fundamentally different in retinal derivatives. However, the presented results give strong indications that analogous modes can live long enough to be traced with pumpDFWM even in samples with relatively short electronic lifetimes of
