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Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter

Femtosecond Extreme Ultraviolet Photoelectron Spectroscopy of Organic Molecules in Aqueous Solution Johan Hummert, Geert Reitsma, Nicola Mayer, Evgenii Ikonnikov, Martin Eckstein, and Oleg Kornilov J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02937 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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The Journal of Physical Chemistry Letters

Femtosecond Extreme Ultraviolet Photoelectron Spectroscopy of Organic Molecules in Aqueous Solution Johan Hummert,†,‡ Geert Reitsma,† Nicola Mayer,† Evgenii Ikonnikov,† Martin Eckstein,† and Oleg Kornilov∗,† †Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Str. 2a, 12489 Berlin, Germany ‡Present address: Bioquant, Im Neuenheimer Feld 267, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany E-mail: [email protected]

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Abstract Time-resolved valence photoelectron spectroscopy is an established tool for studies of ultrafast molecular dynamics in the gas phase. Here we demonstrate time-resolved XUV photoelectron spectroscopy from dilute aqueous solutions of organic molecules paving the way to application of this method to photodynamics studies of organic molecules in natural environments, which so far have only been accessible to all-optical transient spectroscopies. We record static and time-resolved photoelectron spectra of a sample molecule, Quinoline Yellow WS, analyze its electronic structure and follow the relaxation dynamics upon excitation with 400 nm pulses. The dynamics exhibit three timescales, of which a 250 ± 70 fs timescale is attributed to solvent rearrangement. The two longer timescales of 1.3 ± 0.4 ps and 90 ± 20 ps can be correlated to the recently proposed ultrafast excited state intramolecular proton transfer in a closelyrelated molecule, Quinophthalone.

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Ultrafast relaxation dynamics of light-sensitive organic molecules upon photoexcitation are important in a vast number of biological processes such as photosynthesis and human vision. For example, relaxation via charge transfer 1 or isomerization 2 have been found to trigger the ensuing biological function with high quantum efficiency, in many cases controlled by couplings of chromophore molecules to their surroundings, such as in the case of the human vision chromophore, retinal. 3 Since the emergence of short pulse lasers, the relaxation dynamics have been studied directly in a number of all-optical time-resolved experiments routinely carried out in solution, thus including the influence of solvent environment. 4 However, time-resolved photoelectron spectroscopy, which brought many insights in surface science 5 and gas phase molecular physics, 6–8 remained out of reach for organic molecules in solutions until recently. 9 In the gas phase, development of XUV sources based on high-order harmonic generation (HHG) has brought particularly many advantages for application of photoelectron spectroscopy to molecular dynamics studies, 10 since XUV photoelectron spectroscopy yields absolute electron binding energies, admits no ”dark” states, and allows following the dynamics along the complete reaction coordinate with femtosecond time resolution. Photoelectron spectroscopy from liquids 11 was not widely applied until the development of the liquid jet technique. 12 Since then multiple studies of liquid water 13–15 improved the understanding of its electronic structure considerably. Time-resolved photoelectron spectroscopy using UV 16–18 and XUV 19 pulses helped to shed light on the dynamics of solvated electrons. However, photoelectron studies of organic molecules in solution remain rare and are conducted either at synchrotron facilities, obtaining only ground state ionization energies, 20,21 or in the UV photon energy range, lacking the sensitivity to the electronic ground state. 9,22–24 The combination of the liquid jet technique with time-resolved XUV photoelectron spectroscopy was recently used to investigate a dense ferrocyanide ion solution, 25,26 but at a concentration which is not accessible for most biologically relevant molecules. In this work we combine the XUV time-delay-compensating monochromator setup 27 with a commercial liquid jet assembly (Microliquids) and a ”magnetic bottle” time-of-flight pho-

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toelectron spectrometer 28 to study dilute samples of organic molecules (see Methods for details). The high throughput efficiency of the monochromator and the high collection efficiency of the spectrometer allow us to detect very low signals for sample concentrations below 1 mM (see Supplementary for details). Further, we implement a pump-probe experiment with 400 nm pump pulses and use time-resolved photoelectron spectroscopy to follow relaxation of an electronically excited organic molecule, Quinoline Yellow WS (QY). Quinoline Yellow WS is a sulfonated, water-soluble form of Quinophthalone, 29 an industrial yellow dye consisting of 1,3-Indandione and Quinoline connected by a bridge bond. Quinophthalone can assume three main tautomeric forms: one keto and two enol forms 30 (see fig. 1a). NMR spectroscopy, 31 vibrational spectroscopy 30 and computational studies 32 conclude that in solution Quinophthalone takes the enamino form and suggest that this tautomer is likely responsible for the photoabsorption properties of the molecule, owing to its extended conjugated system. 31 Recently, relaxation dynamics of Quinophthalone attracted interest possibly featuring an uncommon N-to-O excited state intramolecular proton transfer (ESIPT). 33 We first conduct XUV-only experiments to access the ground state photoelectron spectrum of the QY molecule in solution. fig. 1b shows photoelectron spectra of a 10 mM solution of QY in water (orange line) and photoelectron spectra of water without QY (blue line), both resulting from ionization by 26.5 eV XUV photons. 60mM of NaCl is added in both cases to suppress streaming currents and associated jet charging. 34,35 The XUV photon energy used here can ionize both solute and solvent molecules and therefore the photoelectron spectra in fig. 1 are dominated by electrons originating from water molecules. The three distinct peaks in the photoelectron spectrum can be assined to ionization from the 1b1 , 3a1 and 1b2 orbitals of the gas phase water molecules evaporated from the liquid jet. 36 The peak at a kinetic energy of 15.3 eV, corresponding to a binding energy of 11.2 eV, is assigend to the 1b1 band of liquid water. 13 When QY is added to the solution (orange line) no significant changes are observed in

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Figure 1: a) Three possible forms of Quinophthalone. b) Single color photoelectron spectra of a 10 mM Quinoline Yellow WS solution and a reference water spectrum recorded using XUV (26.5 eV, orange and blue) and UV (400 nm=3.1 eV, purple) photon energies. The inset shows the photoelectron spectra near the edge of the liquid water band, where two molecular bands (orange) can be clearly distinguished. UV pulses (1 · 1011 W/cm2 , 30 fs) produce no detectable ionization of the reference water sample (not shown).

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the part of the spectrum corresponding to water, but two new weak features appear at low binding energies, close to the edge of the liquid water 1b1 band. These features are visible in the inset of fig. 1. The ground state photoelectron spectrum of solvated QY is obtained by subtracting the reference water spectrum and presented in fig. 2. The data in this figure are recorded in the counting mode to reduce electronic noise (see Methods). To assign the origin of the two QY bands we perform TDDFT calculations (see Methods). The results suggest that the planar enamino structure, with the hydrogen located at the nitrogen atom, is favored in the Quinoline Yellow WS, similar to the results reported for Quinophthalone. 30,31 The influence of sulfonation on molecular structure appears to be negligible. Vertical ionization energies of the relevant molecular orbitals are shown in fig. 2 as sticks. The results of the calculations for the enamino tautomer fit well to the experimental photoelectron spectra, suggesting that the π molecular orbital (HOMO) is responsible for the lower band emerging at 6.0 eV, while the band at higher binding energies is composed of several orbitals. In contrast, the results for the keto tautomer are not compatible with the measured photoelectron spectra, thus indicating that the enamino form is stable in solution also for the sulfonated molecule. We now turn to investigation of the relaxation dynamics of QY upon photoexcitation. QY is a yellow dye with absorption peaking at 412 nm in water solutions and low fluorescence yield, 33 which indicates fast relaxation pathways for the bright electronic state. We follow these relaxation dynamics in a time-dependent pump-probe experiment carried out by introducing a UV pump pulse at 400 nm wavelength and scanning the relative delay between the UV pump pulse and the XUV probe pulse, while recording photoelectron spectra for each delay step. The UV pump beam alone may produce photoelectrons via multiphoton ionization of water or QY molecules. To quantify this signal photoelectron spectra of the QY solution upon ionization by the 400 nm beam only are recorded and are shown in fig. 1. No photoelectron signal originating from 400 nm ionization of water is observed (not shown). However, at the

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Figure 2: The ground state photoelectron spectrum of Quinoline Yellow WS in water. Experiment (orange) and TDDFT calculations (green and purple) on the M06-2x/def2-TZVP level of theory. The lower binding energy band (A-band) corresponds mainly to the delocalized π orbital of enamino-QY (green), while calculations for the keto-QY (purple) do not match the experiment. Additionally, the figure shows the electron densities of the HOMO orbitals for enamino-QY and keto-QY.

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Figure 3: a) Space charge-corrected difference map of the QY UV-XUV pump-probe photoelectron data with the signal at negative delays subtracted (see Methods). Red areas correspond to XUV signals enhanced by the UV pump pulse, while blue areas correspond to signal depletion. Note that the time axis switches from linear to logarithmic spacing of points. b) Dynamics of the excited state signal integrated between 2.5 eV and 5.5 eV binding energies (circles). Error bars are statistical uncertainties (1σ) calculated over several delay scans. A monoexponential decay (red line) fails to model the data, while a biexponential decay function (orange line) adequately describes the decaying signal. The extracted decay constants are 1.3 ± 0.4 ps and 90 ± 20 ps. The inset at the bottom shows fit residuals. c) Excited state photoelectron spectra averaged over short and long pump-probe delays as highlighted by boxes in the map of panel a. d) Delay-dependent ”center of mass” of the excited state photoelectron spectrum. The exponential fit yields a spectral shift of 0.28 ± 0.04 eV and an associated decay time of 250 ± 70 fs. Note that the time axis is logarithmic in the right part of the plots.

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same pump photon flux significant ionization is observed when switching to the QY solution (purple line). This indicates that multiphoton ionization of the solute is greatly enhanced by the lower ionization potential of the molecule and the broad absorption resonance at 412 nm. In the experiments the pump beam photon flux was chosen such as to keep the maximum kinetic energy in the pump-only signal below the kinetic energy of the photoelectrons originating from the liquid water and the solute bands. Therefore the pump-only signal produced no background in the region where molecular signals are found (binding energies of less than 10 eV). The time-resolved photoelectron spectra obtained for 10 mM aqueous solution of QY are shown in fig. 3a as a false color difference map. The difference map is obtained by subtracting the spectra for negative delays from all recorded spectra, which is equivalent to subtracting one color spectra for pump and probe beams, because no pump-probe effects are observed when the XUV pulse arrives before the UV pump pulse. The red areas in the map correspond to the signal enhanced by the 400 nm UV pump pulse, while blue areas correspond to depletion. Even at low pump photon flux the space charge of the electrons created by the pump pulse somewhat affects kinetic energies of the electrons created by the XUV probe pulse. 37 This effect is corrected for based on the delay-dependent position of the liquid water 1b1 band (see Supplementary for details). The difference map contains two features with a positive difference signal (red): an intense area around zero time delay and a broad decaying band between 2.5 and 5.5 eV. The feature at zero delay corresponds to the first positive sideband of the 1b1 liquid water peak, which was also observed using a reference water scan and corresponds to the pump-pulse-assisted ionization of liquid water. This signal is used as in situ characterization of the pump-probe cross correlation and calibration of the zero time delay with a precision of better than 10 fs. 38 The decaying signal between 2.5 and 5.5 eV corresponds to the relaxation of the molecular state excited by the 400 nm pump pulse. The time-dependent photoelectron yield integrated over this energy range is shown in fig. 3b. It is best modeled by a biexponential decay,

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revealing the decay timescales of 1.3 ± 0.4 ps and 90 ± 20 ps. The monoexponential decay function is also shown in the figure for comparison. The plot of residuals demonstrates, that the monoexponential decay function fails to reproduce the experimental data. While the total electron yield demonstrates dynamics on ps timescales, analysis of photoelectron spectra in different time ranges reveals a faster process. fig. 3c shows photoelectron spectra integrated over three time ranges: -30 - 500 fs, 0.6 - 10 ps, and 12 - 65 ps. While the two spectra for longer timescales are similar, a clear shift of about 0.3 eV to lower binding energies is observed for the short timescale (dark green curve). This shift corresponds to an adjustment of the photoelectron spectra within the first few hundreds of femtoseconds after UV excitation. To assess this shift quantitatively we plot the weighted average, or ”center of mass”, of the photoelectron spectra as a function of delay time in the fig. 3d. One immediately sees that the center of the photoelectron band shifts from 4.15 eV to 4.40 eV on a 250 fs timescale, as given by the exponential decay fit also plotted in the figure. The fast adjustment of the photoelectron spectrum without corresponding change in the total photoelectron yield suggests a process, whereby the molecule is quickly stabilized but remains in the initially excited electronic state. A processes with such characteristics is solvent rearrangement. 39 When the molecule is excited, the dipole moment in the excited state may differ from the one in the ground state. The excitation is then followed by ultrafast rearrangement of the polar solvent molecules, which stabilizes the excited state. Water is one of the fastest solvents 39 and exhibits rearrangement timescales compatible with the one observed here. To assess the origin for the two slower timescales, 1.3 ps and 90 ps, we refer to the recent publication of Han and coworkers, 33 who investigated relaxation of Quinophthalone and deuterated Quinophthalone in cyclohexane and concluded that the first relaxation process in the excited state is the intramolecular proton transfer (ESIPT) taking place between the N atom of Quinoline and the O atom of 1,3-Indandione. The fast decay component they observed is 3.3 ps and depends on the deuteration of the molecule at the NH site.

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The authors argue that, although Quinoline is known for its photobasicity (i.e. it attracts protons in the excited state), in the particular case of Quinophthalone the accepting part is even more photobasic thus triggering the proton transfer. Han et al 33 also observe a slower 84 ps timescale, which they attribute to internal conversion to the ground state. Based on the correspondence between the timescales observed here and those reported by Han et al, 33 the same relaxation pathways could be considered to explain the dynamics of QY in water. The 1.3 ps timescale would then correspond to the ESIPT process, while the 90 ps timescale would be the internal conversion to the ground state. This inference should however be taken with caution. Water is a strongly protic solvent and can build intermolecular hydrogen bonds with the QY molecule at the oxygen sites as well as to the NH group. Such hydrogen bonds are known to slow down ESIPT in some cases by weakening the intramolecular hydrogen bond, though this may depend on the particular type of the ESIPT process. At present it is not possible to establish with certainty whether ESIPT takes place in aqueous QY or other possible relaxation processes should be considered. The 90 ps timescale observed in our experiment is too long to be ascribed to solvent rearrangement or intramolecular vibrational cooling. It can indicate an intersystem crossing in the excited state. We see however no sign of long-lived triplet states formed in our spectra, in agreement with the observations of Han et al. 33 The other option is the internal conversion to the ground state. If ESIPT does take place, the transfer of the proton from the N atom of Quinoline (enamino) to the O atom of the 1,3-Indandione (ketoenol) transforms the bridge bond of the QY molecule to a single bond 33 and thus unlocks the rotation around the bridge bond. 40 The molecule can then be expected to undergo internal conversion along the rotational coordinate. The observed timescale of approximately 90 ps is commensurate with such rotation mechanism. Any relaxation back to the enamino ground state has to also include the proton transfer back to nitrogen, possibly aided by the solvent. This could explain the deuteration dependence of the long timescale found by Han and coworkers 33 which is consistent with the long timescale found in the present experiment.

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Our results demonstrate that combination of an efficient XUV monochromator beamline with the liquid jet and efficient photoelectron detector allows for investigation of timeresolved dynamics of organic molecules in solution even at millimolar concentrations. The molecular concentration used in the experiment is comparable to concentrations used in previous photoemission experiments at synchrotron facilities. 21 Investigation of a selected molecule, Quinoline Yellow WS, demonstrates that high quality XUV-only photoelectron spectra can be quickly recorded for molecular binding energies lower than the ionization energy of liquid water. Comparison of the experimental photoelectron spectra with ab initio TDDFT-based calculations support the claim that QY is found in the enol-form in solution and provide tentative assignments for the origins of the observed photoelectron bands. Time-resolved photoelectron spectra upon excitation by UV laser pulses (400 nm) reveal an ultrafast (250 ± 70 fs) stabilization of the excited state and a biexponential decay with timescales of 1.3 ± 0.4 ps and 90 ± 20 ps. The fast timescale suggests an ultrafast solvent rearrangement in the excited state. The 1.3 ps timescale, in analogy with the recent work of Han et al, 33 can be tentatively assigned to ultrafast excited state intramolecular proton transfer between the N and O atoms in the molecule, however further research is needed to reliably assert the possibility of ESIPT in aqueous QY. The 90 ps timescale is compatible with ensuing internal conversion along the rotational coordinate about the bridge bond. The results presented here establish the feasibility of time-resolved valence photoelectron spectroscopy with XUV photons of organic molecules in solutions. In the future, the experiments will be extended to solvents other than water, which will open the possibility to investigate solvent effects on electronic structure of ground and excited states of organic chromophores.

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Methods The time-delay-compensating monochromator setup was previously described in detail. 27,41 In short, a commercial Ti:Sapp laser system (Aurora, Amplitude Technologies) provides 30 fs laser pulses at the central wavelength of 795 nm and 1 kHz repetition rate. Approximately 2 mJ of pulse energy are used to produce XUV pulses via high-order harmonic generation (HHG) in argon. 10 The XUV light is sent through the time-delay-compensating monochromator to select a well-defined wavelength region from the XUV spectrum. XUV pulses with photon energies of 26.5 eV (harmonic 17 of the HHG spectrum) and durations of less than 20 fs are used in the present experiments. The spectral bandwidth is set to approximately 0.5 eV. In the pump arm the second harmonic of the fundamental 795 nm light is generated in a 50 µm thin BBO crystal. Both the pump and the probe beams are focused to an approximately 200 µm diameter spot in the interaction region. In experiments the pump beam energy is limited to 1 µJ leading to pump intensities of about 1 · 1011 W/cm2 in the focus. A newly designed liquid jet photoelectron spectroscopy endstation is connected to the XUV setup to carry out time-resolved experiments with solvated molecules. It consists of a micro liquid jet assembly and a ”magnetic bottle” time-of-flight spectrometer for photoelectron detection. The liquid jet assembly (microliquids GmbH ) delivers sample solutions through a fused silica 18 µm diameter nozzle at a typical backing pressure of 30 bar and a flow rate of 0.5 ml/min. The outer surface of the fused silica nozzles is coated with a conductive graphite layer to prevent charging, which otherwise distorts the photoelectron spectra. 35 About 5 mm downstream from the nozzle the liquid jet exits the vacuum through a 100 µm hole of a heated copper catcher. The liquid is collected in an evacuated cooled bottle. The evaporation from the 5 mm section of the jet is frozen out by a liquid nitrogen filled cold trap maintaining the pressure below 10−4 mbar. The liquid jet is intercepted by the pump and probe laser beams in the focus of the photoelectron spectrometer 1-2 mm downstream from the nozzle tip, where the jet flow remains laminar. 12,42 The spectrometer is separated from the liquid jet chamber by a 0.5 mm 13



diameter skimmer, which helps to maintain the pressure difference necessary to operate the photoelectron detector. The photoelectrons are detected by a microchannel plate detector in ”chevron” stack configuration and a phosphor screen. The latter is capacitively coupled to a fast current amplifier (Philips 6954-B-10 ) and a fast PCI digitizer card (Acqiris AP240 ), which detects the arrival times of the photoelectrons. The digitizer signals are read out for each laser shot and then averaged in the acquisition software, giving what is referred to as the ”averaging mode” spectra. Simultaneously, a software algorithm detects individual hit times in the single shot spectra which are summed up in the ”counting mode” spectra. The counting mode provides very efficient noise reduction in TOF ranges where the signal is low enough to distinguish individual hits. This allows collection of data for several organic molecules (sulfonated dyes) in the aqueous solution at concentrations down to 1 mM. The XUV-only spectra for Tartrazine, Quinoline Yellow WS, Metanil Yellow, and Methyl Orange (purchased from Sigma Aldrich and used without further modification) are given in the Supplementary. The detected signals show negative correlation to the sample solubility. The photoelectron spectrometer is calibrated against known ionization potentials of gas phase water 36,43 and nitrogen. 44 Harmonics from 13 to 21 are used to cover the required range of electron kinetic energies. Since the averaging mode spectra containing the photoelectron bands of liquid and gas phase water are acquired simultaneously with the solute signals, any shifts due to charging are monitored in the experiment. The DFT calculations were carried out using version 4.0.1 of the ORCA program system. 45 For all calculations of the water-soluble form the sulfonic acid groups (SO3 Na) are replaced with the protonated form SO3 H. 46 The initial geometry is obtained via a force field optimization of the molecules in the Avogadro software (version 1.2.0). 47 The molecular geometry is then optimized with a density functional (DFT) calculation using the meta-GGA functional TPSS 48 and the def2-TZVP basis set. 49 The aqueous solution is included in the calculation via the conductor-like continuum polarization model (CPCM) 50 implemented in the ORCA package. Single point energy calculations show that the enol form is lower in

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energy both for Quinophthalone and several sulfonated forms. The energy difference ranges from 0.1 eV to 1.1 eV. For the Quinophthalone and its disulfonic acid the tautomers with hydrogen bound to nitrogen (enamino) and to oxygen (ketoenol) were compared. In both cases the structure with hydrogen attached to nitrogen is energetically favored in agreement with the literature. 29,30 All optimized geometries and single point energies are given in the Supplementary. Following a recent benchmark study, 51 ionization energies are calculated by TDDFT using the Minnesota functional M06-2x 52 and the def2-TZVP basis set.

Acknowledgement We are very grateful to Kathrin Lange-Aziz and Reinhard Großer for their contributions to the construction of the experimental endstation and to Boris Peev, Katrin Herrmann and Roman Peslin for their help with experiments. The research has been funded by the grant of Deutsche Forschungsgemeinschaft (KO 4920/1-1).

Supporting Information Available Total XUV-only photoelectron yields for several sulfanted dyes and their molecular structures. Correction procedure for the light-induced space charge effect (including the data before correction). TDDFT ionization energies. DFT geometry optimization.

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