Electronic Spectroscopy of Molecules in Superfluid Helium Nanodroplets

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Electronic Spectroscopy of Molecules in Superfluid Helium Nanodroplets: An Excellent Sensor for Intramolecular Charge Redistribution D. Pentlehner,† R. Riechers, A. Vdovin, G. M. P€otzl, and A. Slenczka* Institut f€ur Physikalische und Theoretische Chemie Universit€at Regensburg, 93040 Regensburg, Germany ABSTRACT: Electronic spectra of molecules doped into superfluid 4He nanodroplets reveal important details of the microsolvation in superfluid helium. The vibrational fine structure in the electronic spectra of phthalocyanine derivatives and pyrromethene dye molecules doped into superfluid helium droplets have been investigated. Together with previous studies on anthracene derivatives [J. Chem. Phys. 2010, 133, 114505] and 3-hydroxyflavone [J. Chem. Phys. 2009, 131, 194307], the line shapes vary between two limiting cases, namely, sharp Lorentzians and nonresolved vibrational fine structure. All different spectral signatures are initiated by the same effect, namely, the change of the electron density distribution initiated by the electronic excitation. This change can be quantified by the difference of the electrostatic moments of the molecule in the electronic ground state and the corresponding Franck
Condon point in the excited state. According to the experimental data, electronic spectroscopy suffers from drastic line broadening when accompanied by significant changes of the charge distribution, in particular, changes of the dipole moment. Vice versa, the vibrational fine structure in electronic spectra of molecules doped into helium droplets is highly sensitive to changes of the electron density distribution.

1. INTRODUCTION Helium droplets generated via supersonic expansion of helium into a vacuum chamber1 are unique because of the superfluid phase. In the course of the expansion, 4He condensates to liquid droplets, which afterward cool to a temperature of 0.37 K by evaporative cooling.2 Under the given vacuum conditions, this is far below the lambda point of the bosonic isotope of helium. Superfluidity is accompanied by negligible viscosity and high thermal conductivity.3 Thus, in many experiments, helium droplets have been found to serve as a very gentle cryomatrix and to be well-suited as a host system for molecular spectroscopy and photochemistry.1,4
6 In contrast with solid matrices, superfluid helium does not exhibit an intrinsic structure given by the local arrangement of individual atoms. The superfluid is described by one global wave function whose amplitude increases with the number of atoms.3 As a result, the solvent
solute interface is mainly determined by the shape of the dopant molecule as seen by the helium environment. In most cases, the dopant
helium interaction is stronger than the helium
helium interaction.7,8 Consequently, a dopant molecule is surrounded by a layer of nonsuperfluid helium fixed to the dopant molecule. Because of the liquid phase of the surrounding helium droplet body, the number of different configurations of the solvent
solute complex is small compared with solid host systems such as argon. Therefore, inhomogeneous line broadening is greatly reduced. Indeed, vibrational and electronic spectra consist of very sharp transitions hardly obtainable in Doppler-free supersonic jet spectroscopy.9,10 Vibrational spectra of molecules in helium droplets exhibit rotational resolution, which is observed neither in a normal fluid11,12 nor in a solid r 2011 American Chemical Society

matrix. The spectral shape of the phonon wing (PW) in the electronic excitation spectrum of glyoxal revealed the spectrum of elementary excitations of superfluid helium.13 In addition to these outstanding and remarkable experimental observations, the unique properties of helium droplets as host system have been demonstrated in numerous examples of high-resolution spectra in the microwave, the infrared (IR), and the visible-ultraviolet (vis
UV) wavelength range.4,6,10,14
16 Zooming in vibrational or electronic spectra, the transition frequencies and line shapes in these spectra reveal details of the solvation of molecules in superfluid helium. Most of the rotationally resolved IR spectra revealed a significantly increased moment of inertia indicative for a helium solvation layer rigidly bound to the rotating dopant molecule.9,15 This is supported by the spectral shape of most of the PWs obtained in electronic spectra, which deviate from the spectral shape for bulk superfluid helium. In addition to a solvent shift of about (1% in the electronic transition energy, multiplet splitting was observed for several molecules. Tetracene is an example for line doubling throughout the whole electronic excitation spectrum. As deduced from pump
probe spectra17 and dispersed emission spectra,18 the doubling reflects the presence of two species, which are assigned to different configurations of a tetracene
helium solvation complex. More sophisticated, the asymmetric line shape at the electronic origin of Special Issue: A: J. Peter Toennies Festschrift Received: December 29, 2010 Revised: May 5, 2011 Published: May 26, 2011 7034

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The Journal of Physical Chemistry A phthalocyanine was shown to reveal information on the size distribution of the helium droplets.19,20 Other rather surprising effects upon the variation of the droplet size distribution have been reported for the line shape of single transitions in the rotationally resolved IR spectra of cyanoacetylene,21 CO2, and N2O.22 In piecemeal fashion, we have summarized a few examples that reveal details of the microsolvation in superfluid helium droplets. For most of these observations, empirical explanations sound reasonable; however, we are far from a quantitative treatment even of individual experimental observations, not to mention a general theoretical model that predicts microsolvation in superfluid helium droplets. The aim of our work is to collect further information on microsolvation in superfluid helium droplets from spectroscopic data, in the present case from electronic spectra. In particular, we focus on the vibrational fine structure. New data will be presented for phthalocyanine (Pc) derivatives and several pyrromethene dye molecules. In addition, electronic spectra of several anthracene derivatives23 and 3-hydroxyflavone (3-Hf)24 will be reanalyzed. All in all, the vibrational fine structure varies from sharp vibronic transitions of Lorentzian shape to broad electronic bands without vibrational substructure. The study provides evidence of the change of the electron density distribution being responsible for the corresponding signature in the electronic spectra. According to the selected set of molecular species discussed below, the change of the dipole moment appears to be a significant quantity for line broadening. The proposed model also holds for line broadening reported for the electronic spectrum of aniline in helium droplets.25 The phenomenon of line broadening is of far-reaching consequence for photochemical studies in helium droplets by means of electronic spectroscopy. As a key conclusion, we state that electronic spectra of molecules in helium droplets are highly sensitive to a change in the electron density distribution of the dopant molecule.

2. EXPERIMENT The experimental results discussed in the following have been obtained using two different helium droplet machines. They differ mainly in the helium droplet nozzle. One machine is equipped with a continuous flow nozzle built according to the design developed by J. P. Toennies in G€ottingen, Germany.1 The other machine carries a pulsed helium droplet nozzle based on the Even-Lavie valve, which is superior in the reliability producing short droplet pulses at high repetition rates up to 1 kHz. This system has been modified for operation under cryogenic conditions, as described recently in ref 26. The two droplet nozzles are operated under different expansion conditions. Those are temperatures between 10 and 15 K and stagnation pressures between 15 and 30 bar for the continuous nozzle with an orifice of 5 μm. The pulsed valve with an orifice of 60 μm was operated at temperatures between 20 and 25 K and at a stagnation pressure of 80 bar. In both machines, the droplet beam enters a second vacuum chamber by passing through a trombone-shaped skimmer. In the continuous beam machine, the skimmer was placed at a distance of 2 cm behind the nozzle with an aperture of 2 mm, whereas that in the pulsed machine with a diameter of 6 mm was placed at a distance of 5 cm. The design of the second vacuum chamber was identical for both machines. About 10 cm behind the skimmer the droplet beam passes through a pick-up oven where solid samples of the dopant species are sublimated by resistive heating. The

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Figure 1. (a) Lorentzian line widths (left axis) and the corresponding decay rates (right axis) of vibronic transitions of Pc in helium droplets versus vibrational energy. The horizontal line marks the average value. (b) Branching ratio in the decay path of electronically excited Pc to a relaxation of the solvation complex prior to radiative decay. (For details, see the text.) Fluorescence excitation spectrum of (c) Pc and (d) TTPc. Wavenumbers are scaled to the corresponding electronic origin, which is given in the two spectra.

temperature of the oven was adjusted to obtain a partial pressure appropriate for single-molecule doping. The entire pick-up unit is thermally shielded by a liquid-nitrogen-cooled copper cylinder. About 7 cm behind the oven, the doped droplet beam is intersected by a laser beam. Laser-induced fluorescence is collected and imaged by lenses either onto the photocathode of a photomultiplier when recording fluorescence excitation spectra or into the entrance slit of a spectrograph equipped with a charge coupled device (CCD) camera when recording dispersed emission spectra. A continuous-wave (cw) ring-dye-laser with a bandwidth of ∼500 kHz in combination with the continuous flow droplet nozzle or pulsed dye lasers with a bandwidth of 6 GHz (0.2 cm
1) in combination with the pulsed droplet nozzle were applied for recording electronic spectra. For excitation spectra, the signal intensities were corrected for the power profile of the dye laser. For free-base Pc, a pump
probe experiment was performed in the continuous flow droplet beam with two cw lasers. The time delay was accomplished by shifting the pump laser (Arþ ion laser operated in the multi-line UV wavelength range) along the droplet beam axis while keeping the probe laser (frequency-tunable cw-dye-laser) locally fixed crossing the 7035

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Figure 2. Selected vibronic transitions (black line) of Pc in helium droplets and the corresponding Lorentzian fit (red line). The vibrational energy (νrel) and the line width (Δν) are given in each panel.

detection volume. Lock-in technique was applied to probe only the population generated by the pump laser, whereby the pump laser was modulated. Calculations of molecular structures and charge distributions as well as dipole moments have been performed for 3-Hf, 2-methylanthracene, and 9-methylanthracene by means of the TURBOMOLE program package.27 Using DFT and TDDFT with B3LYP functional and an (aug)-cc-pVDZ basis set with diffuse functions added to atoms forming a hydrogen bond, the

data for 3-Hf were calculated. For the anthracene derivatives, DFT and TDDFT were applied on the B3LYP/def-TZVP level.

3. EXPERIMENTAL RESULTS 3.1. Phthalocyanine in Superfluid Helium Droplets. The fluorescence excitation spectrum of phthalocyanine (Pc) in superfluid helium droplets is shown in Figure 1c. As previously reported, a solvent shift of 42 cm
1 to the red is the most 7036

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Figure 3. Model potential deduced from line splitting in the emission spectrum of Pc in helium droplets addressing the electronic ground state S0 and the excited S1 state. Electronic excitation (black arrow upward) is followed either by radiative decay (blue dashed arrow downward) or by relaxation (red dotted arrow to the right) of the configuration of the solvation complex prior to radiative decay (blue dashed arrow downward), followed by back relaxation into the global minimum of S0 (red dotted arrow to the left).

remarkable effect caused by the helium environment.28 Within the experimental accuracy, the vibrational fine structure is identical to the gas-phase data.29
32 The most intense transition is the electronic origin whose inhomogeneous line shape reflects the droplet size distribution.19,20 Slightly larger in the widths, nevertheless very sharp, are vibronic transitions showing perfect Lorentzian line shapes. For a few examples, the Lorentzian fit to the measured transition is shown in Figure 2. The difference in the line shape for vibronic transitions in comparison with the electronic origin is due to the dissipation of vibrational energy into the helium droplet. This process is obviously faster than radiative decay. Consequently, the line shape of vibronic transitions is dominated by a broader homogeneous contribution, whereas the electronic origin is dominated by inhomogeneous processes. So far, Pc behaves as expected for a cold molecular sample of isolated molecules. In the top panel of Figure 1, the Lorentzian line widths (left axis) deduced from individual vibronic transitions of the excitation spectrum are plotted as a function of the vibrational frequency. A transformation into decay rates is given by the right axis of the same panel. Obviously, there is no correlation between the lifetime of the excited state and the vibrational energy. The dispersed emission spectra recorded upon excitation at any of the vibronic transitions revealed an important feature that was discussed in detail in refs 28, 33, and 34. A doublet splitting appeared in such a manner that the spectrum could be interpreted as two spectra that differed only in the solvent shift and the integral intensity while exhibiting identical vibrational frequencies and Franck
Condon factors. As reported in refs 28 and 33 and depicted in Figure 3, the spectra revealed a four-level system representing electronic transitions of two different configurations of a helium
Pc solvation complex. The two configurations are present in both electronic states, S0 and S1. However, energetically, the global minimum in S0 changes to a local one

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Figure 4. Fluorescence intensity of the metastable solvation complex of Pc in helium droplets as a function of the spatial shift between pump and probe laser. The spatial shift (bottom axis) corresponds to a delay time (top axis) calculated with the help of the droplet velocity of 280 m/s.35 Experimental data (squares) are fitted by an exponential decay function convoluted with a Gaussian overlap function of both lasers (red line). The least-squares fit reveals a decay time of 5 μs.

in S1 and vice versa. Therefore, upon electronic excitation, the decay path branches out into either radiative decay or relaxation to the second configuration prior to radiative decay. The latter path is finished by relaxation of the metastable configuration back to the global minimum configuration, as depicted in Figure 3. The branching to the second decay path, as reflected by the relative intensity of the red-shifted part of the emission spectrum (Ired), correlates monotonously with the vibrational excitation energy, as depicted in Figure 1b. Additional data in the green and UV spectral range confirm the convergence of Ired to almost 100%. This correlation contrasts with results obtained for the line widths (cf. Figure 1a). Therefore, we conclude that relaxation of the electronically excited complex is preceded by internal vibrational redistribution (IVR) with a rate constant as deduced from the corresponding Lorentzian line width (cf. Figure 1a). Without an intermediate decay step, one would expect a similarity in the energy dependence of the branching (Figure 1b) and the excited state decay rate (Figure 1a). The rate constant for relaxation of the solvent complex from the local to the global minimum in S0 was measured by means of a pump
probe experiment. As revealed by the data in Figure 1b, preferential population of the metastable configuration was accomplished by pumping in the UV frequency range. Simultaneously, the probe laser was frequencytuned across the electronic origin of the metastable species. The time delay between pump and probe laser was accomplished by shifting the beam of the pump laser along the droplet beam axis while keeping that of the probe laser fixed, irradiating the detection volume of the photomultiplier. With the help of the experimentally determined droplet velocity of 280 m/s,35 the spatial distance of both lasers can be transformed into a delay time. The resulting time profile shown in Figure 4 represents the exponential decay of the metastable complex convoluted with an overlap function of the pump and probe laser beams. The leastsquares fit (line) to the experimental data (squares) reveals a relaxation rate of 0.2 MHz, which corresponds to an exponential decay time of 5 μs. The relaxation of the solvation complex is surprisingly slow compared with the picosecond time regime for 7037

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Figure 5. Excitation spectra of pyrromethene dyes in helium droplets (a) borondipyrromethene (BDP), (b) 1,2,3,5,6,7-hexamethyl-8-cyanopyrromethenedifluoroborat (PM650), (c) 8-phenylpyrromethenedifluoroborat (8-PhPM), and (d) 1,3,5,7,8-pentamethyl-2,6-diethylpyrromethenedifluoroborat (PM567). Wavenumber scale is related to the corresponding origin. Intensity is normalized to the corresponding maximum. Gas-phase spectra are added in gray in panels b
d.

thermal relaxation of the dopant species inside the helium droplet. The obtained relaxation rate is indicative for tunneling as the relaxation mechanism of the solvation configuration. 3.2. Substituted Phthalocyanine in Superfluid Helium Droplets. To investigate the influence of the substituent on the dopant
helium interaction, the electronic spectra of various Pc derivatives and porphyrin derivatives have been measured. As an example, the fluorescence excitation spectrum of 2,9,16,23tetra-tert-butyl-phthalocyanine (TTPc) is shown in the bottom panel of Figure 1. In comparison with the spectrum of free-base Pc, the substituents are responsible for additional low-energy transitions coupled to the normal modes of Pc. However, the fine structure shows exclusively sharp transitions representing the ZPL accompanied by a PW with rather weak intensity at the chosen laser power. Because of the floppy substituent, several transitions group together, which overlap with the accompanying

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PWs. As a result, the sharp ZPLs reside on top of the PWs, which appear as an asymmetric baseline with a steep rising edge on the red side and an extending tail to the blue. For the study, it is crucial that despite the presence of floppy substituents exhibiting low-energy torsional and bending modes, the electronic spectra show sharp transitions, similar to those obtained for the nonsubstituted compound. It should be noted that the vibrational mode pattern of Pc (Figure 1c) is similarly present for the substituted compound (Figure 1d); however, it is split into a multiplet fine structure as a result of substitution. Only the modes below 300 cm
1 are significantly reduced in intensity. This might be due to IVR, which is greatly facilitated by the low-energy modes of the substituent. In this respect, the results obtained for TTPc are representative of additional phthalocyanine and porphyrin derivatives investigated in our laboratory. 3.3. Pyrromethene Dyes in Helium Droplets. Another class of molecular compounds that has been studied to provide insight into the dopant
helium interaction is pyrromethene dye molecules. A quasi-continuous emission of solvated pyrromethene dyes in the yellow-red frequency range qualifies for serving as laser dye.36
39 The core common to all of these compounds is borondipyrromethene (BDP). The position, number, and type of substituents are responsible for the shift of the electronic transitions from yellow to red. In addition, rich progressions of low-energy modes in the electronic spectra of cold pyrromethene dyes generated by supersonic jet expansion are observed. These progressions reveal a rearrangement of substituents such as methyl, ethyl, phenyl, or cyano groups upon electronic excitation. This is shown as gray lines in Figure 5b
d. When doped into superfluid helium droplets, the unsubstituted BDP shows exclusively sharp transitions and a rather sparse vibrational fine structure, as shown in Figure 5a. In contrast, the substituted pyrromethene dyes show drastic line broadening in helium droplets (cf. Figure 5b
d, black lines), which is counterintuitive to what is expected upon cooling to 0.37 K. At a closer look (cf. Figure 6), the spectra show sharp ZPL transitions, some even with a rich substructure, with line widths of