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Dynamic Raman Lineshapes on an Evolving Excited State Landscape: Insights from Tunable Femtosecond Stimulated Raman Spectroscopy Breland G. Oscar, Cheng Chen, Weimin Liu, Liangdong Zhu, and Chong Fang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04404 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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The Journal of Physical Chemistry

Dynamic Raman Lineshapes on an Evolving Excited State Landscape: Insights from Tunable Femtosecond Stimulated Raman Spectroscopy

Breland G. Oscar,†,§ Cheng Chen,†,§ Weimin Liu,†,¶ Liangdong Zhu,‡ and Chong Fang*,†,‡

†

Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331,

United States ‡

Department of Physics, Oregon State University, 301 Weniger Hall, Corvallis, Oregon 97331,

United States

§

B.G.O. and C.C. contributed equally to this work.

Corresponding Author *E-mail: [email protected]. Phone: 541-737-6704.

1 ACS Paragon Plus Environment


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ABSTRACT. Tracking molecular motions in real time remains a formidable challenge in science and engineering fields because the experimental methodology requires simultaneously high spatial and temporal resolutions. Building on early successes and future potential of femtosecond stimulated Raman spectroscopy (FSRS) as a structural dynamics technique, we present a comprehensive study of stimulated Raman lineshapes of a photosensitive molecule in solution with tunable Raman pump and probe pulses. Following femtosecond 400-nm electronic excitation, the model photoacid pyranine exhibits dynamic and mode-dependent Raman lineshapes when the Raman pump is tuned from the red side toward and across the excited-state absorption (ESA) band (e.g., from S1) with varying resonance conditions. On the anti-Stokes FSRS side, low-frequency modes below ~1000 cm-1 exhibit a lineshape change from gain, dispersive, to loss, while the dispersive intermediate is much less notable for high-frequency modes. The characteristic mode frequency blueshift involving vibrationally hot states in S1 with time constants of ~9.6, 58.6 picoseconds reveals the sensitivity of anti-Stokes FSRS to vibrational cooling and solvation. This work lays the foundation for expanding tunable FSRS technology on both the Stokes and anti-Stokes sides to investigate a variety of photoinduced processes in solution with sufficient resolution to expose functional motions and increased sensitivity to monitor vibrational cooling.


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I. INTRODUCTION Femtosecond stimulated Raman spectroscopy (FSRS) is a relatively new technique for collecting time-resolved vibrational signatures about molecular structural evolution in the electronic excited state.1-7 A great variety of ultrafast chemical reactions have been studied ranging from excited state proton transfer,3,8-10 isomerization,2,11 charge transfer,12-15 internal conversion,5,16 and bond dissociation.17,18 In brief, the typical FSRS technique employs three pulses: an actinic pump initiates photochemistry by creating a population and sometimes coherence in an electronic excited state, followed by a narrowband Raman pump and broadband probe pair which generates the stimulated Raman scattering photon.1,19,20 The broadband probe simultaneously samples vibrational modes in a wide spectral range (ca. 200—2000 cm-1), and the multiple pulses which drive the stimulated Raman process result in much stronger signals compared to the spontaneous counterpart. In the stimulated Stokes region, when the lower frequency probe is used with the pump to create transient vibrational coherences in the electronic ground state (S0), the dominant off-resonance contribution to sharp gain signal is depicted in Scheme 1 (left). In contrast, when a higher frequency probe is used the inverse Raman loss scattering can occur (Scheme 1, right), which we liken to the anti-Stokes process used in the conventional spontaneous Raman terminology mainly because the emitting Raman scattering photon has higher frequency than the Raman pump photon, although the exact use of terminology has been discussed in literature.21-25 Multi-wave mixing diagrams with different pulse sequences and interaction schemes contribute to the observed Raman signal from a realistic ensemble average system, and the situation is further complicated by the fact that all three laser pulses can be tuned to overlap with electronic absorption bands in the ground and/or excited state. Unlike vibrational spectroscopy using IR transitions, the resonance Raman effect can be exploited in wavelength-tunable FSRS to greatly



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improve the signal-to-noise ratio (SNR),10,26-28 but additional signal generation pathways become readily available which could complicate the observed Raman lineshape.4,21,25,29-31

Scheme 1. An off-resonance four-wave mixing process involving the Raman pump (red) and probe (black) generates the stimulated Raman scattering photons (blue) collinear with the probe. The SRS(I) process on the left leads to sharp gain peaks in the electronic ground state (S0) with a Stokes Raman shift (ωpr < ωpu). The IRS(I) process on the right can generate sharp loss peaks with an anti-Stokes shift (ωpr > ωpu). The dashed and solid lines denote the interactions of incident light fields with the bra and ket side of the density matrix of the molecular system under study. “Vs” stands for a virtual state (grey dashed line) and two vibrational levels in S0 (black) are depicted. Note that the FSRS scattering photon has the same energy as the Raman probe photon.

The resonance Raman effect has been characterized for both Stokes and anti-Stokes stimulated Raman peaks in the electronic ground state, S0. Frontiera et al. showed the emergence of evolving dispersive anti-Stokes Raman loss peaks as the pump pulse was tuned toward the rhodamine 6G (R6G) absorption band from pre-resonance (on the red side) to resonance.21 These results agreed with the theoretical predictions of Sun et al. who attributed the loss signal to the inverse Raman process (IRS(I), see Scheme 1, right panel) and additional contributions from the 4 ACS Paragon Plus Environment

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hot luminescence pathways available near resonance which lead to dispersive lineshapes.22,32 Similarly, Umapathy et al. observed the –1620 cm-1 mode (i.e., the Raman probe is on the blue side of the Raman pump) of crystal violet in ethanol changing from gain to loss through a dispersive intermediate, using ultrafast Raman loss spectroscopy (URLS) with a range of Raman pump and bluer probe wavelengths, which is experimentally equivalent to the anti-Stokes FSRS approach.23,33 Despite efforts to characterize the Raman pump dependence of the ground state spectra, a thorough experimental study of the effect of Raman pump and probe wavelengths on the excited state Raman lineshape has not been undertaken. Recently, the excited state time-resolved URLS spectra of bis(phenylethynyl)benzene (BPEB) were collected with a Raman pump at the excited state absorption (ESA) peak maximum of 621 nm.31 It is notable that only the Raman loss signal beyond ca. –1100 cm-1 was recorded, which means that the vibrational modes as well as the Raman probe photons closer to resonance with the ESA maximum are not available for comparative analysis. On the other hand, we recently reported the FSRS signal sign change with non-dispersive lineshapes for a photoacid undergoing excited state proton transfer (ESPT) reaction in aqueous solution. The Raman pump wavelength, however, was fixed at 580 nm and fortunately achieved pre-resonance enhancement conditions for the photoreactant with an ESA band and the photoproduct with a stimulated emission (SE) band within the same spectral window.25 However, it remains unclear how the Raman lineshapes change when the Raman pump and probe pulses are scanned across a transient electronic band that also evolves after photoexcitation, which is generally applicable for the real-time analysis of dynamic molecular systems and the relevant excited state potential energy surface (PES) in condensed phase.



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In this work, we examine the intricate relationship between the Raman pump and probe wavelengths and the excited state resonance condition by systematically collecting the excited state Stokes and anti-Stokes FSRS spectra of the photoacid pyranine (8-hydroxypyrene-1,3,6trisulfonic acid, HPTS) in neat methanol across a broad spectral window. HPTS is a functional model photoacid that is unable to undergo ESPT in methanol,25,34-36 so we track only the electronic and vibrational features of the photoexcited protonated chromophore (PA*) without complication features originating from the photobase form (PB*). This system is of particular interest because there are several isolated Raman modes below 800 cm-1 as well as clusters of peaks at higher frequencies serving as marker bands, so we can assess different regions of the Raman probe profile as well as obtain structural dynamics insights into the multidimensional photophysical and photochemical reaction coordinate. The ESA band of HPTS spans the visible range, which is ideally suited for the ca. 480—720 nm wavelength tunability of our home-built picosecond Raman pump pulse.10,25,27,28 We show that as the Raman pump is tuned closer to resonance with the ESA band, the FSRS signal becomes dispersive on the anti-Stokes side but mostly for the low-frequency modes below ~1000 cm-1. This mode dependence of the Raman lineshape can be understood by different resonance conditions achieved by the Raman pump and probe in reference to the ESA band maximum. More importantly, the Raman mode dynamics exhibit time dependence which is related to the ultrafast electronic dynamics and non-stationary change of resonance conditions. We also propose guidelines in selecting optimal conditions for excited state FSRS on the basis of the routinely acquired electronic spectra in both the equilibrium and non-equilibrium state.

II. MATERIALS AND METHODS


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A. Solution Sample Preparation. The 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS, >85%) was purchased from TCI America and used without further purification. No detectable spectral difference was found when using the higher purity (>97%) HPTS from Aldrich.10,20,36 Sample solutions were made by dissolving HPTS powder in anhydrous methanol. UV/visible (UV/Vis) spectra were collected before and after each ultrafast spectroscopic measurement to monitor photostability and confirm sample integrity after laser pulse irradiation. The solution sample in a 1 mm thick quartz cell was constantly stirred during FSRS experiments with a micro magnetic stir bar wrapped by a thin layer of parafilm to minimize sample degradation and provide a fresh spot for consecutive laser shots and spectral data acquisition.

B. Ultrafast Raman Spectroscopic Methods. A full description of our wavelength-tunable FSRS optical setup can be found in our previous reports.9,27,28,37 Two femtosecond (fs) pulses and a picosecond (ps) pulse are required for FSRS. Briefly, ~2 W of the 35 fs, 800 nm fundamental output of a mode-locked Ti:sapphire oscillator (Mantis-5) and regenerative laser amplifier (Legend Elite-USP-1K-HE, Coherent, Inc.) is sent through a home-built second harmonic bandwidth compressor and two-stage noncollinear optical parametric amplifier system to produce the intense, tunable ps Raman pump. Between 4—6 mW of Raman pump power was used to collect both the ground and excited state Raman spectra. The fs supercontinuum white light (SCWL) Raman probe is generated in either a 2 mm thick sapphire crystal or a 2 mm pathlength cuvette filled with flowing deionized water depending on the laser amplifier condition and output laser pulse compression and stability. We found that the two different sources of white light generation do not introduce a significant effect on the reported FSRS data,10,25,37 while the main benefit of using a water-based white light is that bluer photons are generated in



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greater quantity so we could extend the spectral detection window more to the blue side (i.e., toward the UV region, see Figure 2 for example). In our experiments we typically attenuate the incident 800 nm light to obtain the most stable white light with sufficient photons. The 400 nm actinic pump is generated by frequency doubling a portion of the fundamental laser output with a 0.3 mm thick type-I BBO crystal, and 0.8—1.2 mW power is used for excited state FSRS after all the incident laser beams are synchronized spatially and temporally. The cross correlation between the fs pulses yields an instrument response time of ~150 fs on the basis of optical Kerr effect measurement in a 1 mm thick solvent standard such as methanol. Notably, one of the most important research strategies in FSRS is to perform systematic, comparative analysis on an array of stimulated Raman modes.10,20,25 For ground state FSRS, 25 mM HPTS in methanol was used to enhance the SNR,10,37 compared to 1.0 mM HPTS in methanol for the excited state FSRS due to different resonance conditions (e.g., in the first singlet excited state S1) as well as an additional incident pulse for photoexcitation. Steady-state electronic absorption spectra were collected on a dilute solution sample using a Thermo Scientific Evolution 201 spectrophotometer while emission spectra were measured on a Hitachi F-2500 fluorescence spectrophotometer. Femtosecond transient absorption (fs-TA) spectra were collected by focusing ~0.4 mW of a 400 nm, fs pump pulse via second harmonic generation and a much weaker, fs SCWL probe pulse generated in a 2 mm thick water cuvette onto the sample solution (i.e., 0.1 mM HPTS in methanol) in a 1 mm thick quartz cuvette. The solution sample was constantly stirred during the fs-TA experiments as well (see above). Because the use of a femtosecond pulse as the broadband Raman probe can be considered as an indispensable pillar for the FSRS methodology in general, we specifically describe our equilibrium and transient Raman data as from the ground state FSRS and excited


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state FSRS experiments, respectively. In addition, we choose different sample concentrations in our experimental series to (1) increase the SNR of ground state Raman peaks because the Raman pump is far away from the ground state absorption peak, (2) compensate for the much increased intensity of excited state Raman peaks because of the strong resonance Raman enhancement factor, and (3) act as a control to corroborate the fs-TA, ground and excited state FSRS experimental data in tracking the electronic and atomic motions from S0 to S1. Notably, we have reported the ground state FSRS data of HPTS in methanol from 15 mM (solute) using 800 nm Raman pump,36 40 mM using 580 nm Raman pump37 to 25 mM using tunable Raman pump in the visible region for this work. Based on the consistency of Raman peak positions and lineshapes in S0, it is quite reasonable to infer that at 25 mM concentration there is no photoacid aggregation in neat solvent because that would introduce significant change of peak frequency and/or lineshape which is not observed experimentally. Furthermore, we also recorded ground state spectra of 1.0 and 1.5 mM HPTS in methanol.10,37 For example with the 605 nm Raman pump (see below), the anti-Stokes ground state we collected during the excited state FSRS (at 1.0 mM) shows the same peak frequencies as the 25 mM sample spectrum in the region above 1000 cm-1. The relative peak intensities are largely preserved (e.g., the 1632/1617 cm-1 pair is the most intense, the modes between ~1100 and 1500 cm-1 increase in intensity as frequency goes up). We report the high concentration spectra to accurately monitor shoulder peaks and low-frequency modes. The low-frequency modes disappear within the SNR in dilute HPTS solutions. Since a main focus of our work is to track low-frequency modes where both Raman pump and probe are near resonance, we need to see those modes in the ground state.



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III. RESULTS AND DISCUSSION A. Ground and Excited State Stimulated Raman. Figure 1a shows ground state FSRS peaks of HPTS in methanol using a 550 nm Raman pump, which is far from the S0 electronic absorption peak at 403 nm (see inset). The Raman gain (Stokes) and loss (Anti-Stokes) features are sharp and symmetric with respect to frequency and relative intensity, and the peak positions of the solvent-subtracted ground state modes are tabulated in Table 1. The observed Raman frequencies are reproduced to ±3 cm-1 in the electronic ground state on the Stokes and AntiStokes side at three different Raman pump (Rpu) wavelengths (see Table S1 in the Supporting Information for two representative conditions on both sides). Table 1 lists the observed Raman marker bands in S0 at all three Rpu conditions with Raman probe (Rpr) photons on the Stokes side as well as the major vibrational normal mode assignment aided by ab initio quantum calculations.36,37 On a related note, we consider that various subpopulations of the sample may have different dependence on the Raman pump pulse, which could enhance certain subpopulations more than the others.15,38 That way we may observe the Raman-pump-dependent slight variation of the center frequency of a relatively broad, weak Raman band (e.g., the 817 cm1

mode shifts to 823 cm-1 when the Raman pump is tuned from 580 to 605 nm, see Table 1)

representing an ensemble average of an intrinsically inhomogeneous sample system in solution phase.


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Figure 1. Ground and excited state Raman features are affected by different resonance conditions. (a) Baseline-subtracted ground state Stokes (red) and anti-Stokes (blue) spectra of 25 mM HPTS in methanol collected with an off-resonance 550 nm Raman pump pulse (vertical grey line). The steady-state UV/Vis absorption (solid) and emission (dashed) spectra are shown in the upper inset. The chemical structure of HPTS in one H-bonding configuration with methanol is depicted (lower inset) with the black curved arrows showing the electron migration induced by photoexcitation. (b) Excited state Stokes (red) and anti-Stokes (blue) traces of 1.0 mM HPTS in methanol at 100 fs after 400 nm photoexcitation using an on-resonance 550 nm Raman pump. The solvent peak (*) at 1034 cm-1 survives ground state subtraction. The ESA band (black solid) is shown in the inset.

Table 1. Selected ground state Raman peak positions of HPTS in methanol from the Stokes FSRS with 550, 580, and 605 nm Raman pump and mode assignments 11 ACS Paragon Plus Environment


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Freq. (cm-1)a Freq. (cm-1)a Freq. (cm-1)a with 550 nm Rpu with 580 nm Rpu with 605 nm Rpu

a

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Vibrational mode assignment (major)b

429

428

430

4-ring in-plane deformation

462

460

462

4-ring out-of-plane (OOP) deformation

665

663

665

In-plane ring symmetric deformation

821

817

823

Asymmetric ring deformation

1049

1045

1050

Ring deformation and H-rocking

1148

1148

1146

Ring deformation

1189

1191

1189

Ring-H rocking with minor ring asymmetric breathing

1223

1225

1222

Ring asymmetric breathing

1277

1280

1277

Ring stretching and H-scissoring with C–O stretching and COH rocking

1367

1367

1366

In-plane H-rocking and C=C stretching

1591

1594

1591

Asymmetric C=C stretching with COH rocking

1629

1633

1631

Symmetric C=C stretching

Ground state peak positions are reproduced well at each Raman pump wavelength with a lower

frequency Raman probe. See Figure 1a (red) for the experimental trace at 550 nm Raman pump. b

The vibrational normal mode assignments are based on the density functional theory (DFT)

calculations at the B3LYP level using 6-311G++(d,p) basis sets in Gaussian 09 software.39 We keep the Raman pump and probe powers unchanged for comparative measurement of Stokes and anti-Stokes FSRS signals of HPTS in methanol, and the magnitude of the stimulated gain signal is about half of the loss intensity on the anti-Stokes side. A similar phenomenon has been observed in power-dependent URLS experiments and demonstrated computationally.30,40 Within our spectral window (–1500 cm-1 to 1500 cm-1) the Raman probe does not directly overlap the S0 absorption band, but the proximity of the anti-Stokes probe photons may contribute to the larger signal intensity, especially given that shoulder peaks in some higher 12 ACS Paragon Plus Environment

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frequency modes are weak on the Stokes side but enhanced on the anti-Stokes side (see Table S1) because the pertaining Raman probe photons are closest to the ground state absorption peak within the particular spectral window. Addition of a 400 nm actinic pump, however, results in excited state Raman spectra with a broad background attributed to the transient electronic profile (see Figure 1b) affected by the Raman pump.10,41 The features on the Stokes side exhibit a gain signal except for a solvent peak at 1034 cm-1, which appears as a dip in the spectrum due to solvent and ground state subtraction. This is expected because the addition of solute will induce changes to bulk solvent and its Hbonding network particularly at the vicinity of the solute (e.g., first solvation shell).35,36,42 The solvent modes are specifically kept in the spectra throughout this work to validate frequency shifts of the solute molecules. Conversely this solvent feature becomes positive in the anti-Stokes spectrum, while the Raman features attributed to HPTS are dispersive in the low-frequency region (see Figure 1, dashed ellipse) and loss in the region above 1000 cm-1. We note that the solvent concentration (e.g., ~24.7 M of methanol) is much higher than the solute, but stoichiometry alone cannot be responsible for the observed spectral dip. This reproducible experimental result for HPTS in methanol implies that (1) a rather large, collective force could exist after photoexcitation of the solute via the H-bonding network to exert significant enough effect on the Raman signal of the solvent, likely involving a change of the nonlinear Raman polarizability that is electronic in nature,20 and (2) some transient, non-negligible heating effect due to ultrafast laser pulse irradiation may decrease the observed solvent signal strength to a certain degree.5,37 Notably, no clear ground state bleach was observed for 1.0 mM HPTS because the ground state Raman peaks are small to start with due to the off-resonance condition of the 550 nm Raman



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pump (i.e., in reference to the ground state absorption peak at ~400 nm, see Figure 1a inset) while the photoexcitation typically elevates 540 nm and Rpr>582 nm, Figure 4b except the top trace) for a mode with strong vibronic coupling to an electronic transition (e.g., ESA from S1→S2), the Raman mode intensity is small. Furthermore, the 678 cm-1 in-plane ring symmetric deformation mode37 on the Stokes side exhibits interesting lineshape changes (likely due to its vibronic structure) as the Raman pump is tuned across the ESA band. Once the Raman probe photons responsible for the transition are on the higher energy side of ESA the mode begins to take on dispersive characteristics (see Figure S2). In Figure 4a, the FSRS peak intensity of the ~1520 cm-1 mode (as the loss signal) on the antiStokes side significantly drops when the Raman pump is tuned from 580 to 550 nm (i.e., the ESA peak). This observation confirms that the mode intensity is significantly affected by the Raman excitation profile of the corresponding vibration,45,51,52 and in this case, masked under the broad ESA band (see Figure 2). The Raman probe wavelength at those conditions is ~533 and 508 nm, respectively, which is very close to the ESA peak position and away from it (i.e., almost reaching the ESA band intensity at half maximum). This result demonstrates the dominance of Rpr resonance condition in determining the Raman peak intensity, in contrast to conventional wisdom that Raman pump alone in resonance with an electronic band can exploit the resonance enhancement effect, irrespective of the probe wavelength. In comparison, the intensity variation for low-frequency modes below 800 cm-1 is much less notable since resonance conditions remain largely unchanged with the Raman probe wavelength (e.g., varying from ~566 to 538 nm for a 420 cm-1 mode). Raman peak frequencies are determined from the baseline-subtracted spectra shown in Figure 4 and tabulated in Table 2 for Rpu = 580 nm in conjunction with Rpr = 590—640 nm (Stokes side) and Rpr = 530—570 nm (anti-Stokes side). Tables S2–S4 list additional excited state



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marker band frequencies at other Raman pump wavelengths. In general, the anti-Stokes modes appear at lower frequencies compared to the Stokes counterpart with the same Raman pump wavelength after photoexcitation. For example, the ring C=C stretching mode at ~1530 cm-1 on the Stokes side appears at ~1520 cm-1 (negative sign omitted) on the anti-Stokes side. With Raman pump and probe wavelengths approaching the ESA peak resonance (i.e., anti-Stokes FSRS with Rpu = 580, 550 nm in Figure 4a) the 420 cm-1 in-plane ring deformation mode is dispersive. The “zero crossing point” is defined where the spline baseline intersects the raw spectrum before subtraction (see Figure 3a) and it is redshifted by 7—10 cm-1 relative to the intense Stokes mode frequency, which indicates that vibrationally hot states in S1 are more involved in the anti-Stokes FSRS signal generation (see below for more details).25 We note that the FSRS peak intensity difference in S0 (see Figure 1a and Scheme 1) does not apply in S1 mainly due to the excited state resonance conditions dominating the generation of Stokes and anti-stokes FSRS signals (see Figure S3 for the latter case). Because the methanol peak consistently appears at 1034 cm-1 as an internal solvent standard, these frequency variations are attributed to higher vibrational levels of HPTS.25

Table 2. Excited state Stokes and anti-Stokes Raman peak frequencies of HPTS in methanol with a 580 nm Raman pump at 500 fs after photoexcitation Stokes frequency (cm-1)

Anti-Stokes frequency (cm-1)

422

– 414 (– 405)a

673

– 666 (– 648)a

949

– 943 (– 930)a

1034b

– 1033b

1132

– 1128 (1117)


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a

1488

– 1481

1528 (sh: 1570)

– 1524

The numbers outside and inside the parentheses indicate the frequency of “zero crossing

point” of the dispersive feature and the peak of the positive-going portion, respectively, from the anti-Stokes FSRS experimental data. b

Solvent peak (i.e., methanol C–O stretching motion) that is dominant in this work.

C. Time-Resolved Changes in the Excited State Raman Lineshapes and Frequencies. The Raman pump and probe positions relative to the excited state resonance conditions exert a large impact on the recorded Raman spectrum. Figures 3 and 4 show excited state FSRS spectra at only one time point (i.e., a snapshot), but the ESA band of HPTS in methanol undergoes a pronounced, ~13 nm blueshift on ultrafast timescales attributed to ultrafast solvation events (see Figure 2). Though the Raman pump is experimentally stationary (i.e., with a fixed wavelength during one excited state FSRS data scan), the resonance conditions can evolve in time which is characteristic of most of the solute-solvent systems under study. In other words, the Raman pump and probe wavelengths close to resonance (e.g., 530—580 nm) may complicate the Raman mode dynamics especially for an evolving ESA band. Below we focus on the dynamic Raman lineshapes in S1.



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Figure 5. Time-resolved anti-Stokes FSRS spectrum of 1.0 mM HPTS in methanol with a 580 nm Raman pump following 400 nm photoexcitation. (a) The ground-state-subtracted spectra track the blueshift of the ESA band in time (blue arrow). Broad spectral baselines are shown in black dashed curves. Prominent Raman modes are labeled by vertical dotted lines. (b) The baseline-subtracted spectra manifest dispersive low-frequency modes at early times that evolve into purely gain signals at later times (within the dashed box). High-frequency modes are Raman loss and their frequencies blueshift in time (denoted by black arrows pointing to the left). For both (a) and (b) the methanol solvent peak at 1033 cm-1 is marked by the vertical grey line and an asterisk. The Raman intensity magnitude is indicated by the double-headed arrows on the lower right corner of each panel. 24 ACS Paragon Plus Environment

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Figure 5 presents the time-resolved anti-Stokes FSRS spectra with a 580 nm Raman pump up to 600 ps after 400 nm photoexcitation. The excited state Stokes FSRS spectra have been reported at this condition37 so an informative comparison can be made. The broader electronic dynamics are manifested in the ultrafast shift of the ground-state subtracted FSRS data traces (see Figure 5a, blue arrow),10,41,53 and because they are plotted against the anti-Stokes Raman shift axis in Figure 5b, the corresponding Raman probe wavelengths are decreasing (e.g., a blueshift of ~12 nm from 548 to 536 nm) with the increase of the Raman mode frequency (e.g., a blueshift of ~400 cm-1 from –1000 to –1400 cm-1). We note that these ground-state-subtracted experimental Raman traces largely mirror the profile of the solute ESA band. This feature could thus serve as a convenient experimental validation for the proper chopper phase in the Raman pump and/or actinic pump beampaths to collect the accurate difference FSRS signal induced by photoexcitation.25 Furthermore, we subtract the smooth baselines in Figure 5a and plot the transient Raman spectra as a time-stacked progression in Figure 5b. Interestingly, the dispersive features in the low-frequency modes transition into gain signals after hundreds of ps (highlighted by the dashed box) while Rpu remains fixed at 580 nm. This result confirms that an increased gap between the Rpr (also fixed for the specific low-frequency modes here) and the blueshifting ESA band maximum (Figure 2) is responsible for the diminution of dispersive features. In other words, the specific resonance condition of the Raman probe could play an important role in the origin of dispersive features in anti-Stokes FSRS while the Raman pump is largely off or pre-resonance. In our previous work, the time-resolved anti-Stokes spectra of HPTS in water were presented below ~900 cm-1 but the low-frequency modes therein (e.g., ~426 cm-1 on the Stokes side and 410 cm-1 on the anti-Stokes side) exhibit non-dispersive lineshapes25 because the solute ESA 25 ACS Paragon Plus Environment


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band of HPTS in aqueous solution is centered at ~530 nm.10 This wavelength represents a 20 nm blueshift from the ESA band maximum of HPTS in methanol at 550 nm (Figure 2) which is sufficient to cause the observed Raman lineshape variation in S1. Particularly with the Raman pump at 580 nm, the corresponding anti-Stokes Raman probe photons in generating the ca. 414 and 666 cm-1 modes of HPTS in methanol (Figure 5b and Table 2) are at ca. 566 and 558 nm, respectively. It is clear that these wavelengths of the Raman probe are ~30 nm to the red side of the ESA peak in water but this gap decreases to ~10 nm in methanol, and as a result, clearly dispersive Raman peaks are only observed in methanol due to an effective on-resonance condition of the Raman probe. Note that the Raman pump remains at a fixed pre-resonance condition in both cases. We suspect that various four-wave mixing pathways in the electronic excited state with different Raman pump and probe pulse ordering, resonance conditions, and the stimulated Raman nature of the time-resolved FSRS signal photons4,25,47 are responsible for the specific sensitivity of the observed transient Raman lineshape and dynamics to the Raman probe frequency. Figure 6 presents a detailed analysis of the time-resolved frequency and intensity dynamics of a high-frequency marker band observed in Figure 5. Further discussion on Figure 6a and the improved sensitivity of anti-Stokes FSRS on vibrational cooling from a combined experimental and theoretical perspective can be found in the next section. Since the time-resolved Stokes FSRS maintains a pre-resonance condition for the Raman pump-probe pair, the biexponential intensity decay in Figure 6b with time constants of 620 fs (45%) and 320 ps (55%) reports on the PA* relaxation dynamics within a “trapped” electronic state because ESPT reaction is inhibited in methanol.34,36,37 The hundreds of ps time constant is faster than typical fluorescence lifetime on the ns timescale, which suggests that other non-radiative relaxation pathways from S1→S0 exist 26 ACS Paragon Plus Environment

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for HPTS in methanol. Using Coumarin 102 dye in ethanol as an internal standard with 387 nm excitation,28,54 we obtain the fluorescence quantum yield of 0.79 for HPTS in methanol.

Figure 6. The time-resolved (a) frequency and (b) intensity dynamics of an excited state marker band of HPTS in methanol from both Stokes and anti-Stokes (see Figure 5) FSRS after 400 nm photoexcitation in a semi-logarithmic plot. The Raman pump is set at 580 nm. In (a), the timeresolved Stokes spectra (red circles) in S1 show a largely constant mode frequency at ~1528 cm-1 (grey dashed line). The corresponding mode frequency on the anti-Stokes side (blue squares) exhibit a pronounced double-exponential blueshift with time constants of 9.6 and 58.6 ps with amplitude weights in parentheses and noted in the inset. The mode intensity decay in (b) lengthens on the hundreds of ps timescale due to the solvation-induced blueshift of the ESA band and the consequent dynamic increase of the resonance enhancement factor for this highfrequency mode. 27 ACS Paragon Plus Environment


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At later times with a 580 nm Raman pump on the anti-Stokes side, the Raman probe (e.g., at ~533 nm corresponding to a ~1530 cm-1 Raman mode) is more on resonance with the ESA peak (e.g., ~543 nm at 50 ps) than early times (e.g., ~555 nm at 0 fs) after actinic photoexcitation. As a result, the observed vibrational intensity decay dynamics in the high-frequency region are conflated with the additional resonance enhancement effect. Figure 6b highlights this essential result by contrasting the Stokes and anti-Stokes data points as the multi-exponential least-squares fit of the latter dynamics yields a first decay time constant of ~500 fs (similar to the Stokes side signal dynamics), an additional ~5 ps rise component (likely affected by solvation events) for the vibrational mode, and the lengthening of the second decay time constant to ~3 ns (a combination of hundreds of picoseconds relaxation similar to the Stokes side and resonance enhancement). In consequence, we can understand why the ~1524 cm-1 mode frequency blueshifts to ~1533 cm-1 which represents an “overshoot” by 5 cm-1 at 600 ps when compared to the Stokes mode frequency at ~1528 cm-1 (Figure 6a). This may be due to the Raman probe pulse and the way the time-resolved FSRS data are collected. The difference here between the Stokes and anti-Stokes probing goes beyond energy level difference within the same electronic state, instead, the fourwave mixing in the electronic excited state and multidimensional PES results in various pathways (and potentially adjacent electronic excited states) being probed. Notably, due to bluer probe photons in the anti-Stokes measurement, the Raman probe has a significant overlap with the ESA band of HPTS in methanol (e.g., with the 580 nm Raman pump, the ~1524 cm-1 mode corresponds to the anti-Stokes Raman probe at ~533 nm but Stokes Raman probe at ~636 nm), which makes the anti-Stokes FSRS much more susceptible to solvation events due to the lightmatter interaction with those relevant pulse sequences in time. Interestingly, the ~1524 cm-1


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mode frequency blueshift dynamics are consistent with the reported ~407 cm-1 mode frequency blueshift dynamics25 and the anti-Stokes method enables us to access the solvated electronic state that governs the observed ESA blueshift presented in Figure 2. On the Stokes side, when the Raman probe frequency is far away from the ESA peak, the effect of solvation (i.e., intermolecular interaction by nature) on the observed Raman mode is much less significant and as a result, we observe largely unchanged mode frequency and intensity dynamics insensitive to solvation (e.g., ~9 ps longitudinal relaxation time of methanol).25,34,37,55 This interpretation is corroborated by the ~1524 cm-1 mode intensity dynamics on the Stokes and anti-Stokes side (Figure 6b), wherein only the anti-Stokes data manifest the effect of ps solvation event (i.e., an ~5 ps peak intensity rise due to the ESA band blueshift following photoexcitation and the consequent better match to the corresponding Raman probe frequency, see Figure 5). Furthermore, we examine the blueshift of the similar ~1520 cm-1 mode on the anti-Stokes side using the 625 nm Raman pump (Figure 7a, also see Table S2). At this condition, the observed highest frequency mode is pre-resonant with the ESA band before the solvation-induced blueshift, which pushes the ESA peak further away from the Raman probe. This characteristic Raman mode still blueshifts by ~8 cm-1 from 100 fs to 600 ps, however, the final frequency we observed (1522 cm-1) is still redshifted compared to the Stokes mode. In this Raman pump-probe configuration, the solvated S1 state is not being probed as effectively as in the 580 nm Raman pump case (Figures 5 and 6). This comparison reveals that when the Raman probe required for the anti-Stokes FSRS measurement better overlaps with ESA band tracking solvation, the antiStokes vibrational modes become more sensitive to the intermolecular cooling events starting from the first solvation shell.



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Figure 7. Stack plot of the time-resolved excited state FSRS data of HPTS in methanol on the (a) anti-Stokes and (b) Stokes side. The Raman pump is set at 625 nm. The blue dashed lines highlight the mode frequency blueshift as the system evolves in S1 after 400 nm photoexcitation. The ~1135 cm-1 mode that is sensitive to the solute-solvent complex is marked by the magenta dashed line. The methanol C–O stretching mode at ~1034 cm-1 (green solid line) exemplifies that the frequency is invariant as time delay increases. The arrows point to the direction for the mode frequency shift.

D. Vibrational Cooling Dynamics Reported by the Anti-Stokes Excited State Raman Mode Frequency Shift. In addition to dispersive features evolving in time, several modes blueshift. While Raman frequencies are known to shift from their ground state (S0) frequencies to excited state (S1) frequencies due to photoinduced electronic redistribution,3,11,20 dynamic frequency shifts can be associated with both vibrational cooling and conformational dynamics of the photoexcited molecule. For example, the ring C=C stretch is seen at 1594 (1592) cm-1 in the 30 ACS Paragon Plus Environment

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S0 Stokes (anti-Stokes) spectrum using a 580 nm Raman pump. In the Franck-Condon region of S1, the mode redshifts to ~1530 cm-1 on the Stokes side following electron migration from the HPTS hydroxyl back into the largely co-planar four-ring system, and this mode remains unchanged in frequency within the S1 lifetime.36,37 However, the same mode is initially observed at ~1524 cm-1 as Raman loss on the anti-Stokes side and blueshifts nearly 10 cm-1 within 600 ps (see Figures 5b and 6a). This vibrational marker band frequency exhibits blueshift dynamics in systems where ESPT reaction is allowed,10,37 such as HPTS in aqueous solution, but to see this behavior for HPTS in methanol only on the anti-Stokes side is notable (Figure 6a, blue trace). Vibrational cooling from higher quantum states, however, can explain this mode frequency blueshift on the ps timescale.46,56 To further examine and validate this key observation regarding vibrational cooling in S1, we collect the time-resolved Stokes and anti-Stokes excited state FSRS data sets using a 625 nm Raman pump, which represents a further away-from-resonance condition and where the probe is completely encompassed on the red side of the solute ESA band (Figure 7). In this “strictly” preresonance condition, we avoid dispersive lineshapes and dynamic changes in signal signs because only the Raman gain signals are observed within our broad detection spectral window. On the Stokes side (see Figure 7b), only the 1132 cm-1 mode, which has been attributed to combined HTPS and solvent motions,36 blueshifts by ~5 cm-1 due to vibrational cooling and structural motions within the first solvation shell. The same ~1135 cm-1 mode also blueshifts on the anti-Stokes side (see Figure 7a), but the frequency shift magnitude is greater (8 cm-1) and it is also accompanied by frequency shifts in other low- and high-frequency modes. It is apparent in Figure 7a that the high-frequency anti-Stokes Raman modes shift more than the low-frequency modes within our detection time window of 100 ps. Taken together, the results indicate that



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vibrational cooling processes are uncovered in the anti-Stokes stimulated Raman.25,31 Since the experimentally observed excited state anti-Stokes modes are generally redshifted from their Stokes counterparts and blueshift in time, especially when both the Raman pump and probe wavelengths are close to an electronic resonance condition, we suspect that (1) vibrationally hot states are more involved in generating the anti-Stokes FSRS signal due to the shorter wavelength Raman probe (see Figure S3 for the pertaining four-wave-mixing energy-level diagrams under pre- and on-resonance conditions), and (2) vibrational coherences close in frequency (i.e.,

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