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Infrared and Visible Photodissociation Spectra of Rhodamine Ions at 3 K in the Gas Phase Published as part of The Journal of Physical Chemistry A virtual special issue “Spectroscopy and Dynamics of Medium-Sized Molecules and Clusters: Theory, Experiment, and Applications”. Juraj Jašík,‡ Rafael Navrátil,‡ Ivan Němec,† and Jana Roithová*,‡ ‡

Department of Organic Chemistry and †Department of Inorganic Chemistry, Faculty of Science, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 12843 Prague 2, Czech Republic S Supporting Information *

ABSTRACT: Helium-tagging predissociation spectroscopy in the visible spectral range (He@VisPD) is shown for Rhodamine 123, Rhodamine 110, and Rhodamine 110’s silver salt (silver carboxylate). It is shown that the spectra reflect single-photon absorption. The helium-tagged ions are in the ground vibrational state, and the He@VisPD spectra feature the Franck− Condon envelopes for the excitation to the first excited singlet state that agree very well with theoretical simulations. The S0 → S1 excitation energies are 2.712 ± 0.006 eV for Rhodamine 123, 2.700 ± 0.006 eV for Rhodamine 110, and 2.751 ± 0.006 eV for the silver salt of Rhodamine 110. The determined energies can be slightly blue-shifted due to the binding energy of helium. The Rhodamine ions were also characterized by helium-tagging infrared photodissociation spectroscopy. The distinctive spectral features of the individual derivatives are described and the spectra are compared to the classical solid-state IR spectra.

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INTRODUCTION The main applications of Rhodamine dyes are associated with their luminescence properties. Depending on their chemical makeup they act as efficient fluorescence labels with very high quantum yields. This feature led to their use in the singlemolecule spectroscopy.1,2 For some of the Rhodamine dyes, fluorescence properties can be tuned using different solvents or pH.3 Hence, it opens the door for their use as molecular switches.4 Rhodamine dyes are also used in the investigation of metal nanoparticles.1,5 Rhodamine fluorescence was also studied for gaseous ions.6−10 The results are interesting as benchmark data for solution studies and the investigation of solvent dependent behavior.11 Furthermore, Rhodamines were successfully used as fluorescence tags to determine the conformation of biomolecules.12 Experiments with carboxyl-containing Rhodamine dyes showed, for example, how salt formation affects the excitation energy and the photoabsorption cross section.13,14 Here, we will address the photoabsorption spectra of Rhodamine 123, Rhodamine 110, and Rhodamine 110’s silver salt (Scheme 1). In their recent publication, the group of Jockusch addressed absorption spectrum of Rhodamine 110 using detection by photofragmentation and by fluorescence excitation.15 They have shown that the absorption maximum for the photodissociation is at 462.5 nm. Absorption of a photon is associated with a gain in internal energy of 2.68 eV, which is usually sufficient for fragmentation. Results obtained by Wellman and Jockusch suggest that absorption of more than one photon is necessary to observe the fragmentation under © 2015 American Chemical Society

their experimental conditions. As a result, the photofragmentation spectrum has narrower bands than the linear fluorescence excitation spectrum. We will compare these results to the photoabsorption spectra of Rhodamine ions using our heliumtagging photodissociation technique.16 Photodissociation spectroscopy was initially developed for measurements of UV and UV−vis spectra of mass-selected ions.17,18 Absorption of a UV photon leads to the electronic excitation of the studied ion and during the internal energy conversion the ion can dissociate. The dissociation yield as a function of the photon wavelength gives the UV photodissociation (UVPD) spectrum of the ion.17 Later, this approach was broadened to include infrared spectroscopy.19−25 Absorption of one IR photon is usually not sufficient to induce ion fragmentation. Therefore, either absorption of multiple photons is required (infrared multiphoton dissociation spectroscopy) or a trick of formation of a complex between the ion of interest and an innocent loosely bound tag is used (infrared predissociation spectroscopy, IRPD). The tagging can be achieved either during supersonic expansion19 or in cold ion traps.26,27 The ultimately most innocent tag is helium.28 Recently, several laboratories demonstrated the advantageous use of helium tagging in IRPD spectroscopy of trapped ions.16,29−33 Received: August 31, 2015 Revised: November 23, 2015 Published: November 23, 2015 12648

DOI: 10.1021/acs.jpca.5b08462 J. Phys. Chem. A 2015, 119, 12648−12655

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The Journal of Physical Chemistry A Scheme 1. Investigated Rhodamine Ions

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EXPERIMENTAL DETAILS The experiments were performed with the ISORI instrument.16 ISORI is based on the combination of a low-temperature ion trap with a commercial TSQ 7000 instrument. In its original design TSQ 7000 has a quadrupole−octopole−quadrupole geometry.34 The original ion source vacuum chamber is connected to the main instrument via a customized flange. It preserves all of the ionization options provided by TSQ; here, the electrospray ionization equipment is used. The new ultrahigh vacuum chamber consists of three additional differentially pumped regions: (1) the region with the first quadrupole (4P1) and the quadrupole bender (QPB), (2) the region with octopole (8P), and (3) the region with the ion trap (w4PT), the second quadrupole (4P2), and the detector. The ion trap has a linear quadrupole geometry where the hyperbolic shape of each electrode is approximated by six wires (more details concerning the trap and the spatial confinement of the ions can be found in refs 16 and 35). It is mounted into a copper base, which is screwed onto a cold-head. The base reaches temperature of 2.6 K. Cooling of the ions is achieved by collisions with a helium buffer gas. The temperature of the trap base and of the surrounding heat shield is measured by silicondiode sensors. The buffer gas is injected by a custom-made piezo valve, situated in vacuum, directly into the trap with a straight Teflon tube. The presence of the He buffer gas leads to elevation of the trap temperature via convective heat transfer by several tens of Kelvin. The investigated ions were obtained by electrospray ionization of methanolic solutions of Rhodamine 110 chloride or Rhodamine 123 chloride (purchased from Sigma-Aldrich and used without further purification). The silver salt was obtained after addition of silver acetate to the solution of Rhodamine 110 chloride (Figure 1). The ions of interest were mass-selected by the 4P1, deflected by the QPB, and guided by the 8P to the w4PT. During the first 300 ms of the 1 Hz trapping cycle the ions were guided into the trap and, simultaneously, He buffer gas was injected. The maximum He number density was on the order of 1015 cm−3. During the

trapping time, the ions internally relaxed and formed heliumtagged complexes. Typically, we have 1−3% yield of He-tagged complexes for closed shell ions produced by electrospray ionization. After 300 ms the ion filling of the trap was stopped (He valve was closed and the ions were deflected by QPB). The trapped ions were then irradiated. At 990 ms, the exit electrode of the trap was opened, the ions were mass-analyzed by the 4P2, and detected by a Daly type detector operated in ion-counting mode. For IRPD spectroscopy, we used radiation of a pulsed (10 Hz repetition rate) OPO/OPA system (LaserVision, tuning range 700−4700 cm−1, fwhm ∼4 cm−1, 10 ns pulse length). The OPO is pumped by Nd:YAG laser (Surelite EX from Continuum). The photon beam was focused into the trap by CaF2 lenses and entered the vacuum chamber through the CaF2 window mounted on the detector side. Energy of the photon pulses was controlled by attenuation of the pump energy delivered to the OPA stage and measured by laser energy meter Coherent Fieldmax II with J-25MB-LE sensor. The pulse energy of the light passing through the instrument and coming out from the CaF2 window on QPB chamber was monitored routinely during acquisition of the spectra (Figure S1 in Supporting Information). For the VisPD spectroscopy, we used radiation generated by supercontinuum laser NKT Photonics SuperK Extreme (78 MHz seed laser repetition rate) filtered by acusto-optic tunable filter (AOTF) SuperK Select (range 400−650 nm, fwhm from 1.8 nm at low wavelengths to 8.5 nm at high wavelengths). The light from the filter delivered by single-mode optical fiber was focused into the trap region by CaF2 lens from the QPB side (diameter of the focal spot was approximately 1 mm). According to the manufacturer’s test report, the accuracy of the AOTF is in the range of ±0.2 nm of the nominal value over the whole wavelength domain. Power of the Vis laser was measured right before entrance window of the instrument (after focusing lens) using power meter Thorlabs PM100A with detector Thorlabs S120C. The trapped ions were irradiated in every other trapping cycle. The irradiation time was controlled either by mechanical shutter (in the case of IRPD experiments where the trapped ions interacted with six 10 Hz photon pulses) or by electrical gate signal (in the case of VisPD experiments, the trapped ion were irradiated from 300 to 900 ms from the beginning of the trapping period). The number of He complexes that were detected in both periods with and without irradiation are designated as N(ν̃) and N0, respectively. The vis and IR spectra are presented by plotting the reduced signal (1 − N(ν̃)/N0) as a function of photon energy. The infrared spectra of powder samples were recorded by ATR technique on a Thermo Nicolet iN10 FTIR microscope using Ge crystal in the 675−4000 cm−1 region (2 cm−1 resolution, Norton−Beer strong apodization, MCT/A detec-

Figure 1. Mass spectrum of the methanolic solution of Rhodamine 110 and silver acetate. Note that the ions of Rhodamine 123 (denoted as Rh123+) are formed by esterification of Rhodamine 110 (denoted as Rh110+) in situ. MeOH is methanol; ACN is acetonitrile present as impurity. 12649


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The Journal of Physical Chemistry A tor). Standard ATR correction (Thermo Nicolet Omnic 9.2 software) was applied to the recorded spectra.

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COMPUTATIONAL DETAILS The calculations were performed using the density functional theory (DFT) method B3LYP together with the SDD basis set for silver and 6-311G** for the remaining atoms as implemented in the Gaussian 09 suite.36 Computation of the Hessian matrix was performed for all optimized structures at the same level of theory to ensure that the structures correspond to genuine minima and also to calculate the thermochemical data and IR spectra. The optimized geometries as well as the calculated energies are listed in the Supporting Information section (Table S1). The Franck−Condon simulations were performed as implemented in Gaussian using Franck−Condon-Herzberg−Teller method.37 The first excited state of Rhodamine 123 and Rhodamine 110 was calculated by a simple broken symmetry approach, geometries were optimized and the obtained frequencies were used for the Franck−Condon simulations (see below and Supporting Information). Energy of the 0−0 transition was set to the experimental value, because the theoretical excitation energy is almost 0.5 eV underestimated (see the text).

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RESULTS AND DISCUSSION Helium-Tagging Visible Photodissociation (He@ VisPD) Spectroscopy. Direct measurement of photoabsorption of mass-selected ions in the gas phase is a challenging task, because of the small number of events to be monitored. Typically, we work with 104−105 numbers of the trapped ions per cycle. The fact that the experiments are performed with mass spectrometers can be used to circumvent this problem. Instead of photoabsorption, the ion fragmentation induced by the photon absorption is monitored. Usually, in the infrared spectra region, absorption of many photons is necessary to increase the internal energy of the absorbing ion above the dissociation channel. For the UV−vis spectroscopy, one photon is usually sufficient for exciting the ion above the dissociation limit. Nevertheless, the energy does not necessarily need to trigger dissociation. It can be also lost by radiative transition or in collisions, especially, if the experiments are done at higher buffer gas/background pressure. Hence, probing molecules with high fluorescent yield is particularly ambitious and challenging. Wellman and Jockusch showed that on average absorption of three photons is required to induce fragmentation of Rhodamine 110 in the visible spectral range in an ion trap with helium buffer gas.15 Alternative to the mere photofragmentation experiments are the experiments with loosely bound tags. The ultimate tag is a helium atom. It has been shown repeatedly that the effect of helium on the spectrum of the investigated ions is negligible.28,38−41 The binding energy of helium to a singly charged ion is at maximum on the order of kJ mol−1.33 To form such complexes, the internal energy of the ions must be very low. In return, the elimination of the helium atom is induced by absorption of single photon only. We will test this method for recording of the absorption spectrum of Rhodamine ions in the visible spectral range. Figure 2 shows visible helium-tagging photodissociation spectra of Rhodamine 123, Rhodamine 110 and the silver salt of Rhodamine 110. The spectra are vibrationally resolved with the most intense transition corresponding to the electronic

Figure 2. He@VisPD spectra of (a) Rhodamine 123, (b) Rhodamine 110, and (c) the silver salt derived from Rhodamine 110. The blue graph shows the power of the laser.

Figure 3. Depletion of helium complexes of Rhodamine 123 at λ = 457.0 nm as a function of laser power. The data are fitted with the function y = y0 + (1 − y0) exp(−x/x0), where y0 is the number of helium complexes not interacting with the laser beam and x0 is related to the photofragmentation cross section at the given wavelength.35

excitation from the ground vibrational state of the singlet ground state to the ground vibrational state of the first singlet excited state (0−0 transition). The excitation energy of Rhodamine 123 corresponds to 457.1 ± 1.0 nm and thus the S0 → S1 excitation energy (ES0→S1(Rhodamine 123)) is derived as 2.712 ± 0.006 eV. Similarly, we derive the excitation energies for Rhodamine 110 and its silver salt as ES0→S1(Rhodamine 110) = 2.700 ± 0.006 eV (459.2 ± 1.0 nm) and 12650


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Figure 4. Comparison of He@VisPD spectra with the simulated Franck−Condon envelopes for (a) Rhodamine 123 and (b) Rhodamine 110. The blue spectra show the simulated spectrum along with the blue scale at the right side; the calculated transitions were folded with Gaussian peaks with full width at half-maximum 85 cm−1. The positions of the most intense transitions are denoted with circles and marked. The red spectrum is simulated VisPD spectrum using eq 1.

Figure 6. (a) Infrared spectrum of solid Rhodamine 123 chloride measured with the ATR technique (blue trace). (b) He@IRPD spectrum of Rhodamine 123 measured with full laser pulse energy (the black trace) and with reduced (10%) pulse energy (red trace, also shown in gray in (a)). (c) and (d) Theoretical IR spectra (B3LYP/6311G**) of Rhodamine 123 in (c) harmonic approximation (scaling factor: 0.98) and (d) anharmonic approximation (only fundamental bands included in the black spectrum). The red stick spectrum in (d) shows combination bands.

ES0→S1(Rhodamine 110 Ag) = 2.752 ± 0.006 eV (450.6 ± 1.0 nm). It is important to note that the wavelength corresponding to the 0−0 transition for Rhodamine 110 (459.2 nm) is blueshifted by 5.8 nm (276 cm−1) with respect to the photodissociation spectra and by 3.3 nm (160 cm−1) with respect to the fluorescence excitation spectra published by Wellman and Jockusch.15 This corresponds to energy differences of 3.3 and 1.9 kJ mol−1, respectively. The red shifts in the previously published spectra could have resulted from a lower resolution of the experiment and probing an ensemble of ions with higher internal energy. Their experiments were performed at laboratory temperature, whereas we perform the experiments in an ion trap cooled to 3 K, which assures investigation of ions in their ground vibrational state. Alternatively, the energy difference can be an artifact of the helium-tagging method (see

Figure 5. He@IPRD spectra of (a) Rhodamine 123, (b) Rhodamine 110, and (c) the silver salt of Rhodamine 110. The experimental spectra are compared with theoretical spectra obtained via DFT (B3LYP/6-311G**; scaling factor: 0.957).

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Figure 8. (a) He@IRPD spectrum of the silver salt of Rhodamine 110 measured with full laser pulse energy (the black trace) and with reduced (10%) pulse energy (the red trace). (b) and (c) Theoretical IR spectra (B3LYP/6-311G**; scaling factor: 0.98) of the two possible isomers of the silver salt.

Figure 7. (a) Infrared spectrum of solid Rhodamine 110 chloride measured with the ATR technique (blue trace). (b) He@IRPD spectrum of Rhodamine 110 measured with full laser pulse energy (the black trace) and with reduced (10%) pulse energy (the red trace, also shown in gray in (a)). (c) Theoretical IR spectrum (B3LYP/6311G**) of Rhodamine 110 in harmonic approximation (scaling factor: 0.98).

The excitation energy increases starting from the acid via the ester to the silver salt. The effect of the salt formation on the excitation energy has been observed already earlier.13,14 For the series of alkali metal salts it was shown that the blue shift in the excitation energy increased with the increasing size of the metal cation. The effect was explained by a simple model in which the electrostatic field of an alkali metal ion results in Stark shifts of the electronic energy levels. The effect is destabilizing the ground as well as the excited state and it is bigger for the latter. Therefore, the stronger the field is, the higher the excitation energy is.13 Analogous reasoning accounts for the blue shift in the excitation energy observed here for the silver salt. In addition, we can also see a compression of vibrational levels from the Franck−Condon envelopes. Though the resolution of the spectra is not ultimate, it can be inferred that the energy separations between the favored transitions are decreasing from the acid via the methyl ester to the silver salt. We have analyzed the obtained spectra by simulation of the Franck−Condon envelopes. The first excited states were fully optimized using broken symmetry approach. The adiabatic excitation energy at 0 K is calculated as 2.11 eV for both Rhodamine ions. Vertical ionization energy amounts to 2.24 eV for Rhodamine 123 and 2.23 eV for Rhodamine 110. All values are significantly lower than the experimentally determined thresholds. For the Franck−Condon simulations, the 0−0 transitions were set to 2.71 eV (21 875 cm−1, 457.1 nm) for Rhodamine 123 and to 2.70 eV (21777 cm−1, 459.2 nm) for Rhodamine 110. The results of the simulation are shown in Figure 4 in blue along with the specification of the most intense

below). The absorption profiles for all investigated ions extend to about 420 nm, where the laser power drastically decreases (blue lines in Figure 2). It should be, however, noted that the previously published spectra also extended only to about 420 nm.15 The photodissociation of helium-tagged Rhodamine ions is very efficient. Clearly, we are working almost at the saturation regime (i.e., almost 100% of helium-tagged ions are fragmented). Figure 2a shows He@VisPD spectra recorded at full laser power (red line) and at 28% laser power (gray line), respectively. The full width at half-maximum of the 457.1 nm peek recorded with the reduced laser power is 1.5 nm. In comparison to the bandwidth of the acusto-optic tunable filter declared by the manufacturer, it is clear that the spectra resolution is limited by the line width of the probing light. We have also measured the photofragmentation yield as a function of the laser power (Figure 3). Clearly, the data can be fitted with an exponential function, which shows that the absorption is a single photon process and provides the characteristic laser power of 53 μW. The fit also shows that we had about 12% ions that were not interacting with the laser beam. The fitted characteristic laser power can be used to estimate the molar absorption coefficient ε which amounts to ε(457.1 nm) = 2.8 × 104 dm3 mol−1 cm−1 (see Supporting Information for details). This value is in good agreement with theoretical calculation (see below and Supporting Information). 12652


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Figure 9. Comparison of IRPD spectra of Rhodamine 123 (denoted as ester, in red), Rhodamine 110 (denoted as acid, in black), and the silver salt of Rhodamine 110 (denoted as COOAg, in green).

characterized by the O−H stretching band at 3579 cm−1, whereas the methyl ester group of Rhodamine 123 is revealed by the sharp bands at 2969, 3014, and 3046 cm −1 corresponding to the C−H vibrations of the methyl group. The silver salt of Rhodamine 110 does not show any additional band compared to the other two spectra. It allows us to state that silver is coordinated to the oxygen atoms of the carboxylate and thus to exclude the possibility that silver would be coordinated to one of the nitrogen atoms. Further structural information can be obtained from infrared spectra in the fingerprint region, which shows a straightforward correlation with classical IR spectra (Figure 6a,b).42 Comparison of the IR spectrum of Rhodamine 123 chloride measured with attenuated total reflectance (ATR) method on a germanium crystal with the He@IRPD spectrum shows that most of the bands are in very good agreement with predictions. The biggest differences are encountered for the double-bond stretches above 1600 cm−1. Theoretical methods predict that four fundamental bands should be observed. Analyzing the He@IRPD spectrum (Figure 6b), the most intense peak at 1600 cm−1 (theoretically predicted at 1616 cm−1) and a small peak at 1652 cm−1 (theoretically predicted at 1660 cm−1) correspond to the CC stretching vibrations of the dibenzopyrane core. The band at 1630 cm−1 (theoretically predicted at 1641 cm−1) corresponds to the NH2 bending mode. This band is not observed in the IR spectrum measured in the solid state probably due to the hydrogen bonding, which leads to a broadening and shifting of the bands associated with the NH2 groups. The carbonyl stretching is found at 1726 cm−1 in the He@IRPD spectrum and it is red-shifted to 1703 cm−1 in the solid-state IR spectrum. The red shift is probably associated with an interaction of the carbonyl within the crystal with the NH2 groups (i.e., N−H···O hydrogen bonding). As there is a partial mismatch between the DFT calculated IR spectrum using the harmonic approximation (Figure 6c) and the experimental He@IRPD spectrum, we have recalculated the spectra in anharmonic approximation also (Figure 6d). Nonetheless, we did not find a better agreement. The anharmonic calculations suggest rather intense combination bands. It could explain why the bands at 1600 and 1630 cm−1 have a larger width than the others. The He@IRPD spectra of Rhodamine 110 and its silver salt are rather similar to that of Rhodamine 123. The carbonyl stretching mode of the carboxylic group of Rhodamine 110 is found at 1745 cm−1 in the He@IRPD spectrum and at 1705 cm−1 in the solid-state IR spectrum (Figure 7). Hence, in comparison to the band for Rhodamine 123, the solid-state carbonyl band remained almost unchanged, whereas for the

transitions. All transitions can be found in the Supporting Information (Figure S2). From the calculated Franck−Condon spectrum, we have simulated VisPD spectrum at the saturation regime according to eq 1: A(λ) = 1 − exp[−σ(λ) ϕ(λ)]

(1)

where σ(λ) is the calculated Franck−Condon cross section and ϕ(λ) is the photon flux of our laser (Supporting Information). Nice agreement between the simulated spectrum based on the calculations of the naked Rhodamine ions with our He@ VisPD experiment points to a negligible effect of the helium tagging on the structure of the Rhodamine ions. The question remains, what the mechanism of the dissociation process is. The helium atom can be eliminated either at the excited state or after the fluorescence transition to the ground state. We consider as the more probable scenario that the helium is detached in the S1 excited state. Although helium elimination from vibrationally excited states is straightforward, two pathways are suggested for the most intense 0−0 transition. First, the observed 0−0 transition may in fact correspond to a transition to a hot state with the vibrational excitation energy in the range of the helium binding energy in the S1 state. Second, the 0−0 transition can lead to the ground vibrational state of S1, which is, however, dissociative for the helium atom. Both mechanisms would result in a blue shift of the 0−0 transition in the He@VisPD spectrum with respect to the fluorescence spectra.15 The blue shift would correspond either to the excitation in the S1 state or to the binding energy of the helium atom in the S0 state. Helium-Tagging Infrared Photodissociation (He@ IRPD) Spectroscopy. The structure of gaseous ions is best determined from their infrared spectra. We have again applied the helium-tagging method. The infrared photodissociation spectra of the helium-tagged ions in the X−H stretching region (X = C, N, O) are compared in Figure 5. The dominant band in all spectra corresponds to symmetric stretching of the N−H bonds located at 3452 cm−1. The antisymmetric stretching of the N−H bonds can be found at 3556 cm−1. The theoretical IR spectra predict the same features and the relative band positions agree reasonably well with the experiment. Note that the experimental spectra were obtained in the saturation regime. The relative intensity of the symmetric NH2 stretching band with respect to the remaining bands in the spectra would be much larger, if we would work in the linear regime as evidenced from the width of the peaks. Other features in the spectrum would, however, disappear. The individual ions can be recognized on the basis of their distinctive bands. The carboxylic group of Rhodamine 110 is 12653


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The Journal of Physical Chemistry A gaseous ion it is blue-shifted by ∼20 cm−1. This again points to strong effects of crystal packing that override the individual molecular features. The major distinction between the silver salt of Rhodamine 110 on the one hand and Rhodamine 110 and Rhodamine 123 on the other is a bidentate coordination of carboxylate to the silver ion. It is reflected in the fingerprint He@IRPD spectrum (Figure 8). Instead of the carbonyl stretching discussed above and the C−O stretching found for Rhodamine 123 at 1297 cm−1, we detect two new strong bands at 1394 and 1527 cm−1. These bands can be assigned as symmetric and antisymmetric stretching of the CO2 moiety. The direct comparison of all three spectra can be found in Figure 9.

(2) Moerner, W. E. Single-Molecule Spectroscopy, Imaging, and Photocontrol: Foundations for Super-Resolution Microscopy (Nobel Lecture). Angew. Chem., Int. Ed. 2015, 54, 8067−809. (3) Beija, M.; Afonso, C. A. M.; Martinho, J. M. G. Synthesis and Applications of Rhodamine Derivatives as Fluorescent Probes. Chem. Soc. Rev. 2009, 38, 2410−2433. (4) Bossi, M.; Belov, V.; Polyakova, S.; Hell, S. W. Reversible Red Fluorescent Molecular Switches. Angew. Chem., Int. Ed. 2006, 45, 7462−7465. (5) Sarkar, J.; Chowdhury, J.; Pal, P.; Talapatra, G. B. Ab initio, DFT Vibrational Calculations and SERRS Study of Rhodamine 123 Adsorbed on Colloidal Silver Particles. Vib. Spectrosc. 2006, 41, 90−96. (6) Greisch, J.-F.; Harding, M. E.; Kordel, M.; Klopper, W.; Kappes, M. M.; Schooss, D. Intrinsic Fluorescence Properties of Rhodamine Cations in Gas-phase: Triplet Lifetimes and Dispersed Fluorescence Spectra. Phys. Chem. Chem. Phys. 2013, 15, 8162−8170. (7) Kordel, M.; Schooss, D.; Neiss, C.; Walter, L.; Kappes, M. M. Laser-Induced Fluorescence of Rhodamine 6G Cations in the Gas Phase: A Lower Bound to the Lifetime of the First Triplet State. J. Phys. Chem. A 2010, 114, 5509−5514. (8) Forbes, M. W.; Jockusch, R. A. Gas-Phase Fluorescence Excitation and Emission Spectroscopy of Three Xanthene Dyes (Rhodamine 575, Rhodamine 590 and Rhodamine 6G) in a Quadrupole Ion Trap Mass Spectrometer. J. Am. Soc. Mass Spectrom. 2011, 22, 93−109. (9) Sagoo, S. K.; Jockusch, R. A. The fluorescence Properties of Cationic Rhodamine B in the Gas Phase. J. Photochem. Photobiol., A 2011, 220, 173−178. (10) Nagy, A. M.; Talbot, F. O.; Czar, M. F.; Jockusch, R. A. Fluorescence Lifetimes of Rhodamine Dyes in Vacuo. J. Photochem. Photobiol., A 2012, 244, 47−53. (11) Lopez Arbeloa, F.; Urrecha Aguirresacona, I.; Lopez Arbeloa, I. Influence of the Molecular Structure and the Nature of the Solvent on the Absorption and Fluorescence Characteristics of Rhodamines. Chem. Phys. 1989, 130, 371−378. (12) Talbot, F. O.; Rullo, A.; Yao, H.; Jockusch, R. A. Fluorescence Resonance Energy Transfer in Gaseous, Mass-Selected Polyproline Peptides. J. Am. Chem. Soc. 2010, 132, 16156−16164. (13) Greisch, J. F.; Harding, M. E.; Klopper, W.; Kappes, M. M.; Schooss, D. Effect of Proton Substitution by Alkali Ions on the Fluorescence Emission of Rhodamine B Cations in the Gas Phase. J. Phys. Chem. A 2014, 118, 3787−3794. (14) Chingin, K.; Balabin, R. M.; Barylyuk, K.; Chen, H.; Frankevich, V.; Zenobi, R. Rhodamines in the Gas Phase: Cations, Neutrals, Anions, and Adducts with Metal Cations. Phys. Chem. Chem. Phys. 2010, 12, 11710−11714. (15) Wellman, S. M.; Jockusch, R. A. Moving in on the Action: Experimental Comparison of Flueorescence Excitation and Photodissociation Action Spectroscopy. J. Phys. Chem. A 2015, 119, 6333− 6338. (16) Jašík, J.; Ž abka, J.; Roithová, J.; Gerlich, D. Infrared Spectroscopy of Trapped Molecular Dications Below 4 K. Int. J. Mass Spectrom. 2013, 354−355, 204−210. (17) Fu, E. W.; Dunbar, R. C. Photodissociation Spectroscopy and Structural Rearrangements in Ions of Cyclooctatetraene, Styrene, and Related Molecules. J. Am. Chem. Soc. 1978, 100, 2283−2288. (18) Altinay, G.; Citir, M.; Metz, R. B. Vibrational Spectroscopy of Intermediates in Methane-to-Methanol Conversion by FeO+. J. Phys. Chem. A 2010, 114, 5104−5112. (19) Duncan, M. A. Frontiers in the Spectroscopy of Mass-Selected Molecular Ions. Int. J. Mass Spectrom. 2000, 200, 545−569 and references therein.. (20) Jiang, L.; Wende, T.; Bergmann, R.; Meijer, G.; Asmis, K. R. Gas-Phase Vibrational Spectroscopy of Microhydrated Magnesium Nitrate Ions [MgNO3(H2O)1−4]+. J. Am. Chem. Soc. 2010, 132, 7398−7404. (21) Goebbert, D. J.; Wende, T.; Bergmann, R.; Meijer, G.; Asmis, K. R. Messenger-Tagging Electrosprayed Ions: Vibrational Spectroscopy of Suberate Dianions. J. Phys. Chem. A 2009, 113, 5874−5880.

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CONCLUSIONS We report on the first helium-tagging photodissociation spectra in the visible spectral range (He@VisPD). We demonstrate it on Rhodamine ions that are widely used as fluorescent agents. The high quantum yield of fluorescence of Rhodamine dyes makes analysis via classical photodissociation spectroscopy tricky, because dissociation events are in competition with radiation. The helium-tagging photodissociation spectrum clearly demonstrates single-photon absorbance. The heliumtagging technique enables measurements of Franck−Condon envelopes and determination of the 0−0 excitation energy. The spectra were measured for Rhodamine 123, Rhodamine 110, and Rhodamine 110’s silver salt; hence, an ester, carboxylic acid, and the salt of the same lead structure were probed. With respect to the previously published value of the 0−0 excitation energy Rhodamine 110 from fluorescence experiments,15 our excitation energy is slightly blue-shifted; this can be due to the effect of helium tagging. Next to the He@VisPD spectrum, also the He@IRPD spectra of the Rhodamine ions were measured. The distinctions of the derivatives were shown in the fingerprint as well as in the X−H (X = C, N, O) stretching regions.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b08462. OPO/OPA power dependence (Figure S1), He@VisPD spectra (Figure S2), discussion of Franck−Condon simulations (Figure S3), estimation of molar absorption coefficient, and computational results (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*J. Roithová. Phone: (420) 221951322. E-mail: roithova@ natur.cuni.cz. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the European Research Council (StG ISORI, No. 258299). REFERENCES

(1) Nie, S. M.; Emery, S. R. Probing Single Molecules and Single Nanoparticles by Surface-enhanced Raman Scattering. Science 1997, 275, 1102−1106. 12654


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