Hydration of a Binding Site with Restricted Solvent Access

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Hydration of a Binding Site with Restricted Solvent Access – Solvatochromic Shift of the Electronic Spectrum of a Ruthenium Polypyridine Complex, One Molecule at a Time Shuang Xu, James E. T. Smith, and J. Mathias Weber J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07668 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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The Journal of Physical Chemistry

Hydration of a Binding Site with Restricted Solvent Access – Solvatochromic Shift of the Electronic Spectrum of a Ruthenium Polypyridine Complex, One Molecule at a Time

Shuang Xu,a James E. T. Smithb and J. Mathias Weberb*

a

JILA and Department of Physics, University of Colorado, 440 UCB, Boulder, CO 80309-0440, USA

b

JILA and Department of Chemistry and Biochemistry, University of Colorado, 440 UCB, Boulder, CO 80309-0440, USA

Abstract We report the electronic spectra of mass selected [(bpy)(tpy)Ru-OH2]2+·(H2O)n clusters (bpy = 2,2’-bipyridine, tpy = 2,2’:6’2”-terpyridine, n = 0 – 4) in the spectral region of their metal-toligand charge transfer bands. The spectra of the mono- and dihydrate clusters exhibit partially resolved individual electronic transitions. The water network forming at the aqua ligand leads to a rapid solvatochromic shift of the peak of the band envelope: addition of only four solvent water molecules can recover 78% of the solvatochromic shift in bulk solution. The sequential shift of the band shows a clear change in behavior with the closing of the first hydration shell. We compare our experimental data to density function theory (DFT) calculations for the ground and excited states.

*

Corresponding author; email: [email protected]; phone: ++1-303-492-7841

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INTRODUCTION The solvation of ions is one of the most fundamental processes in ion chemistry, and it can profoundly affect the properties of a solute ion. One of the effects where such changes can be observed is the solvatochromic shift of the electronic spectrum of the solute, i.e., the difference between the spectrum of a bare ion and the same ion in solution. Upon hydration of an ion, a water network forms around it incorporating the ion, and ion-water interactions can be discerned as far as to the third and fourth solvation shells.1-3 However, many molecular ionic systems do not expose their charged components uniformly to the hydration environment. The charge structure in such systems creates regions in the molecule that favor interaction with the solvent, while other areas of the molecule are less favorable for solvation and may even be hydrophobic. Many organometallic complexes and the active sites of many proteins are examples of such structures. In this context, it is an interesting question how the hydration of a site with restricted solvent access affects the electronic states of the molecule. This is particularly important for molecules where the interaction with the solvent directly leads to useful chemistry, such as the hydration of water oxidation catalysts. Since the discovery of Meyer’s blue dimer,4-5 ruthenium complexes with polypyridine ligands and their derivatives have attracted considerable attention as catalysts and photosensitizers for electrochemical and photoelectrochemical water oxidation (see, e.g.,

6-11

).

Early work mostly involved complexes with multiple metal cores,4-5 but since Zong and Thummel reported12 that mononuclear Ru complexes are also active catalysts for water oxidation, there has been considerable activity regarding these simpler species.9,

13

In such

complexes, the Ru ion is partially surrounded by an organic framework where it is usually coordinated to nitrogen atom(s) in pyridine, 2,2’-bipyridine (bpy), 2,2’:6’2”-terpyridine (tpy) or


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similar organic ligands. If not all six ligand positions are occupied in this way, the remaining positions can be filled with monodentate ligands, e.g. halide ions, carbonyl groups, or solvent molecules. The organic framework allows solvent access to the Ru2+ ion only in these “open” coordination positions. Since Ru complexes (RuII in particular) absorb photons in the visible, they can participate in redox photochemistry and exhibit considerable utility in light harvesting devices. This participation may be intentional or it may come as a side effect of their use, but it is of interest to characterize the photophysical and photochemical behavior of Ru complexes in this context. One can adjust the photophysical properties of a complex by modifying the structure of the ligands1415

and thereby tune its performance as a catalyst or photosensitizer. Excitation into the lowest

singlet states occurs by metal-to-ligand charge transfer (MLCT). Gaining detailed experimental information on the electronic structure of transition metal complexes in situ is often very difficult due to speciation effects in solution, and one has to rely on approximate computational approaches in many cases.

Studying the evolution of their

electronic structure under stepwise hydration is impossible in solution. Realized by coupling electrospray ionization mass spectrometry with laser spectroscopy, experiments on mass-selected ions in vacuo represent a very attractive alternative to study the electronic16-33 and vibrational34-39 structure of such species, since they offer the ability to circumvent the challenges of speciation. In such experiments, the solvation environment can be entirely removed to investigate the intrinsic properties of the molecular system under study, without the perturbative influence of the solvation environment. Additionally, the ions under study can be prepared in clusters with a well-defined number of solvent molecules to study how the microsolvation environment changes the properties of a solute ion one solvent molecule at a time.16-17, 30, 40-44



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The aqua-complex [(bpy)(tpy)Ru-OH2]2+ (abbreviated as [RuBT-OH2]2+, see Figure 1) is a prototypical key species in water oxidation catalysis and the starting point in the catalytic cycle, according to the proposed mechanism for water oxidation by RuII catalysts.9, 45-48 Its electronic absorption spectrum in aqueous solution shows a broad MLCT band with an onset at ca. 15300 cm-1 (1.91 eV) and a peak at about 21100 cm-1 (2.61 eV). This band is composed of multiple singlet transitions which are essentially unresolved in solution.49

Figure 1. Calculated minimum energy structures of [RuBT-OH2]2+·(H2O)n, showing the lowest energy structures for n = 0 and 1, and the two lowest energy structures for n = 2 – 4, respectively, together with their relative energies. Additional conformers are shown in Supporting Information. Carbon atoms are shown in brown, nitrogen in blue, ruthenium in gold, oxygen in red, and hydrogen in white.

Xu and Weber recently studied the electronic spectrum of the prototypical water oxidation catalysis complex [RuBT-OH2]2+ in vacuo by electronic photodissociation spectroscopy.50 The electronic spectrum of the isolated complex shows multiple partially resolved electronic bands that compose the broad singlet MLCT band in solution. Infrared photodissociation spectroscopy by Garand and coworkers39 on mass selected [RuBT-OH2]2+·(H2O)n clusters (n = 0 – 4) showed 4 ACS Paragon Plus Environment

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that the interaction between the aqua ligand and the Ru center is strongly affected by the presence of solvent molecules. In particular, the presence of H-bonded networks serves to induce charge transfer into the Ru-OH2 bond, anticipating proton transfer into the microhydration environment in the first step of the water oxidation reaction. The solvatochromic behavior of the MLCT band in aqueous solution is particularly interesting. The MLCT transitions involve the Ru2+ ion as well as the hydrophobic bpy and tpy ligands. Solvent access to Ru2+ is restricted to a single coordination site, which is occupied by an aqua ligand in aqueous solution. The polypyridine ligands have a small amount of positive charge in the ground state.51 Upon excitation, they gain partial negative charge, making them more hydrophobic. The onset of solvent induced proton transfer into the solvent environment39 can be expected to introduce a change in the electronic structure of the complex, and the MLCT band can be expected to report on this change. In the present work, we probe the effect of stepwise hydration on the electronic spectrum of [RuBT-OH2]2+ in clusters of the form [RuBT-OH2]+·(H2O)n (n = 0 – 4) by photodissociation spectroscopy. As found earlier by Garand and coworkers, the first few solvent water molecules form a network anchored on the aqua ligand,39 so we can selectively probe the solvatochromic shift induced by solvent effects on the Ru2+ environment, one solvent molecule at a time.

METHODS Experimental. The experimental setup has been described in detail in previous work.52 Briefly, solutions of [RuBT-OH2]2+ in ethanol/water mixture (4:1 by volume) were prepared as described in the literature9, 12-13, 46-48, 53 and used without purification. Ions were generated by electrospray ionization and transported by a series of octupole ion guides through several differential pumping stages and injected into a cryogenic 3D Paul trap held at 180 K. The 5 ACS Paragon Plus Environment


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temperature was chosen to allow condensation of water vapour onto the aqua complex, while avoiding freezing out the vapour onto the trap optics. In the trap, the ions were cooled by collisions with buffer gas with a trap residence time of 95 ms. In order to facilitate the formation of [RuBT-OH2]+·(H2O)n clusters, water vapor was used as the buffer gas. After storing and cooling, the contents of the ion trap were injected into a Wiley-McLaren time-of-flight mass spectrometer and the ions of interest were mass selected by a pulsed mass gate in the space focus of the mass spectrometer.54 A typical mass spectrum in the mass region of interest is shown in Supporting Information. The mass selected ions were irradiated with the output of a pulsed optical-parametric converter (GWU preciscan) pumped by the third harmonic of a Nd:YAG laser and operating in the visible and near UV. Photofragments were mass-analyzed using a reflectron and detected on a dual microchannel plate detector.

Photodissociation action spectra were obtained by

monitoring the photofragment ion intensity (IF) as a function of photon energy while tuning the light source. The light source was triggered on every second shot of the mass spectrometer (the latter operated at 10 Hz), allowing the fragment ion background signal without irradiation (IB) to be monitored as well. The relative photofragment action is then given by σ A = (I F − I B )·

hν

ε

,

(1)

where hν is the photon energy and ε the laser pulse energy, which was measured using a pyroelectric joulemeter. Alternatively, parent ion photodepletion spectra were measured by monitoring the parent ion intensity with (IP) and without (IP,0) irradiation as a function of photon energy. The photodissoiciation cross section σ can be extracted from depletion experiments using the modified Lambert-Beer law:55


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IP =1 − α + α ⋅ e−σF , I P,0

(2)

where α is the overlap between laser and ion packet, and F is the photon fluence. The overlap was determined using the intensity dependence of the depletion in the peak of the absorption profile. Depletion spectra are inherently noisier than action spectra, since IP and IP,0 are similar in magnitude, and calculating the depletion is equivalent to the comparison of two large numbers, both with statistical fluctuations. For all spectra reported here, several scans were acquired in each spectral range, averaged and reproduced on different days. Computational. Ground state structures for [RuBT-OH2]2+·(H2O)n (n = 0 – 4, multiple isomers) and related species were optimized employing density functional theory56 using the B3LYP57-58 functional and def2-TZVP basis sets for all atoms as implemented in the Turbomole suite of programs.59-60 Electronic excitation spectra were calculated using time-dependent61-63 DFT with the same functional in two different sets of calculations. In one series, we used def2TZVP basis sets again for all atoms. This approach yields a qualitative description of the bare complex,50 and we therefore judge it to be an appropriate tool for the clusters under study. All calculated excitation energies in this series reported here are shifted by -1210 cm-1 (-0.15 eV) to improve agreement with the experimental spectrum for n = 0. However, with our resources these calculations were only possible up to n = 3. In order to obtain calculated values for the solvatochromic shifts of all cluster sizes, we repeated the calculations starting from the same optimized ground state structures, but with def2-SV(P) basis sets for all atoms. The latter series was shifted by +2420 cm-1 (+0.3 eV) to improve agreement with the experimental spectrum for n = 0.



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RESULTS AND DISCUSSION The unsolvated aqua-complex. The photodissociation spectrum of [RuBT-OH2]2+ at room temperature and at 180 K (see Figure 2) has been discussed in detail in previous publications.50-51 We will therefore only briefly introduce it here in order to help understand the spectra of the solvated complex. The 1MLCT band, which is a mostly featureless broad band in solution,49 shows significant substructure in vacuo, even at room temperature.50 At 180 K,51 a signature corresponding to the first excited singlet state (S1) is observed at 17300 cm-1 (2.15 eV), followed by two discernible bands at 19300 cm-1 (2.39 eV) and 21400 cm-1 (2.65 eV), which we assign to the S2 and S3 excited states. The two most intense signatures are peaks at 22600 cm-1 (2.80 eV) and 23900 cm-1 (2.96 eV). TDDFT calculations qualitatively reproduce the experimentally observed pattern. Calculations predict that there are several weak bands below the main peak at 22600 cm-1 (2.80 eV), which are likely obscured by the envelope of the stronger electronic bands.

Figure 2. Photodissociation action spectrum of [RuBT-OH2]2+ at 180 K trap temperature. Circles are raw data, the line is a 15-point gliding average. Vertical bars are calculated transition energies (B3LYP/def2-TZVP), shifted by -1210 cm-1 (-0.15 eV). Data were taken from ref 51.


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We find no resolved vibrational structure in the electronic bands. The singlet excited states are thought to decay on ultrashort timescales via intersystem crossing to the triplet manifold,64 due to the large spin-orbit interaction typical for Ru compounds. The width of the observed bands is most likely caused by a combination of lifetime broadening and spectral congestion from the Franck-Condon envelopes of the excited electronic bands as well as hot bands.50 While we cannot observe resolved vibrational features, we expect a broad Franck-Condon envelope on the high energy side of each electronic band origin, similar to that recently observed in copperbipyridine complexes.52 The Franck-Condon envelopes of the lower energy bands are likely to be obscured by the more intense higher lying bands. This may be different for the most intense features at 22600 cm-1 (2.80 eV) and 23900 cm-1 (2.96 eV), which have been previously assigned to the S8 and S9 bands.50 Our TDDFT calculations indicate that the S9 state is relatively weak, and the strong feature at 23900 cm-1 (2.96 eV) may actually be composed of the Franck-Condon envelope belonging to the intense S8 band and the S9 electronic band, rather than stemming from the S9 band alone. In solution, the electronic bands blend together, with S8 dominating the spectrum.

Absorption, photodissociation action and photodepletion spectra. It is important to note the connections (and distinctions) between absorption, photodissociation action and photodepletion spectra. If absorption of a photon always leads to dissociation of the parent ion on the time scale of the experiment (here: ca. 15 µs), the shape of the photodepletion spectrum is identical to that of the photoabsorption spectrum. This means that any other fate of the excited parent ion leading to its survival (photon emission or metastability leading to kinetic shift effects) can be neglected. For an ion of the size of [RuBT-OH2]2+, this is not guaranteed, even 9 ACS Paragon Plus Environment


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though the binding energy of the aqua ligand (90 kJ/mol, 0.93 eV, see Table 1) is significantly lower than the onset of absorption (ca. 16000 cm-1, 2 eV). However, photon emission from this complex has been found to be very weak,49 and we are therefore justified in neglecting it. In addition, we have previously established that there are no significant kinetic shift effects for this complex under excitation in the visible and near UV.50-51

Finally, if there is only one

photofragment channel, then – in the absence of kinetic shift and photoemission effects, as discussed above – the photodissociation action spectrum, the photodepletion spectrum and the photoabsorption spectrum all contain the same information up to a multiplicative constant. This is the case for the unsolvated aqua-complex, but not for the solvated complexes (see Supporting Information). For the bare aqua complex, we assume that the loss of the aqua ligand proceeds via unimolecular decay of the complex in a vibrationally hot electronic ground state. The hot ion is formed by radiationless transitions from the excited 1MLCT states, presumably via ultrafast intersystem crossing to the triplet manifold and subsequent further radiationless transitions into the S0 ground state.64 The intensity dependence of the fragment signals implies that the observed signals are due to absorption of a single photon.50-51 A comparison of the photofragment action and photodepletion spectra for the bare aqua complex shows that the two spectra contain equivalent information (see Supporting Information). We therefore use the photodissociation action spectrum of the bare aqua complex throughout the present work, since it has a higher signal-to-noise ratio.


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Table 1. Zero-point corrected binding energies of water molecules in [RuBT-OH2]2+·(H2O)n clusters. The binding energy for n =2 is the same for both conformers (see text). The values for n = 3 and 4 are upper limits, based on the most strongly bound isomers found for these cluster sizes. water molecule

binding energy [kJ/mol]

aqua ligand

90

solvent, n = 1

58

solvent, n = 2

50

solvent, n = 3

47

solvent, n = 4

49

In principle, adding up the action spectra from all fragment channels should reconstitute the photodepletion spectrum. In practice, however, fragment channel specific behavior regarding fragment ion detection efficiency and background makes this reconstitution not exact, and for ions with multiple fragment channels only the photodepletion spectrum contains a faithful representation of the absorption spectrum (assuming photon emission and kinetic shift effects for depletion are negligible). For the clusters, we generally observe two dominant decay channels; (i) the loss of all solvent molecules at lower energies within the 1MLCT band and (ii) the loss of all water molecules (solvent and aqua ligand) at higher energies. Because of the strong kinetic shift effects in the photofragment action spectra (see Supporting Information), we will only discuss the photodepletion data for the clusters in detail here. The calculated binding energies of solvent 11 ACS Paragon Plus Environment


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water molecules in [RuBT-OH2]2+·(H2O)n clusters are substantially lower than that of the aqua ligand (see Table 1). In the monohydrate, the solvent water molecule is bound by ca. 65% of the binding energy of the aqua ligand. We therefore assume that photodissociation of the clusters upon photoexcitation is much faster than for the bare aqua complex, and kinetic shift effects for parent ion depletion are absent for all species under study. Consequently, the photodepletion spectra for the hydrated parent ions are comparable to the photoabsorption spectra.

The hydrated clusters. The calculated structure of the monohydrate, [RuBT-OH2]2+·H2O, is shown in Figure 1. Consistent with work by Garand and coworkers,39 only one low-lying stable isomer is found, and the solvent molecule binds directly to the aqua ligand. Structures where the water ligand binds to one of the polypyridine ligands are ca. 29 kJ/mol (0.3 eV) and more above this low-lying geometry. There is considerably less substructure observed in the 1MLCT band (see Figure 3), but all discernible electronic bands are red-shifted compared to the bare aqua complex by ca. 320 cm-1 (40 meV). This can be interpreted as the solvatochromic shift of each electronic band caused by the presence of a single solvent water molecule. Interestingly, the center of gravity and the envelope of the 1MLCT band (see Supporting Information for definitions) exhibit a much stronger red-shift of ca. 570 cm-1 (70 meV). This may be due to a shortening of the FranckCondon progressions on the high energy side of each band, which exacerbates the red-shift of the overall envelope. The TDDFT calculations qualitatively reflect the observed red-shift of the individual bands, but they show no drastic changes in oscillator strengths.


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Figure 3. Experimental parent ion depletion spectra for [RuBT-OH2]2+·(H2O)n with n = 1 (top) and n = 2 (bottom) together with calculated vertical excitation spectra (columns), the latter shifted by -1210 cm-1 (-0.15 eV). Circles are raw data points; the full lines are a 25-point gliding average to guide the eye. Open and solid columns in the bottom panel represent the 2-A and 2-B isomers of n = 2.

As found earlier by Garand and coworkers,39 there are two low lying structural motifs for the dihydrate, [RuBT-OH2]2+·(H2O)2 (see Figure 1). In one motif (2-A), the aqua ligand and the two solvent molecules form a linear chain, where the second solvent molecule can be viewed as the onset of the second solvation shell. This isomer is lowest in energy at 0 K and lowest in free energy at 180 K. In the other isomer (2-B), the two solvent molecules are equivalent, both acting as H-bond acceptors from the aqua ligand. The 2-B isomer is 0.1 kJ/mol higher in energy (zeropoint corrected) at 0 K and 0.7 kJ/mol higher at 180 K. The threshold energies for loss of the “second” water molecule are very similar for both isomers, and the two isomers can be viewed as isoenergetic. 13 ACS Paragon Plus Environment


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There is less resolved substructure in the 1MLCT region of the dihydrate spectrum than for n = 0 or 1 (see Figure 3). The 1MLCT band center of gravity and envelope shift by -720 cm-1 (90 meV) and -800 cm-1 (-100 meV), respectively. The two isomers of the dihydrate result in similar calculated TDDFT spectra (see Figure 3), and the differences between the calculated spectra are not sufficient to unambiguously assign the experimental spectrum to one particular isomer. Similar to the monohydrate, the TDDFT calculations qualitatively reflect the observed red-shift of the individual bands, but they do not recover the magnitude of the shift and underestimate the effect of solvation on the envelope of the spectrum. Some of the structural motifs of the tri- and tetrahydrates have been discussed by Duffy et al.39 The additional water molecules continue to build the H bonding network anchored at the aqua ligand (Figure 1 and Supporting Information), and the calculated lowest energy structures have four membered rings (isomers 3-A and 4-A). These structures are calculated to be significantly lower in energy than the others we found (ca. 7.8 kJ/mol for n = 3 and 11.3 kJ/mol for n = 4). However, similar to the case of the dihydrate, the experimental data do not allow assignment of the spectra to specific isomers. Parent ion intensities for [RuBT-OH2]2+·(H2O)3,4 were at the edge of the sensitivity of our photodissociation spectrometer, resulting in a low signal-to-noise ratio, but the spectra for these species can still be observed (see Figure 4). Excited state calculations for n = 3 are compatible with the observed spectrum, but do not allow to assign specific isomers. Excited state calculations for n = 4 could only be obtained with a small basis set, but together with calculations at the same level for smaller clusters they show that the general trends found in the higher level calculations continue.


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Figure 4. Photodissociation spectra for [RuBT-OH2]2+·(H2O)n and comparison with the absorption spectrum of the complex in aqueous solution (data taken from Ref.

49

). Circles are

raw data points, the full lines in the photodissociation spectra are 25-point gliding average to



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guide the eye. Arrows show the center of gravity for each MLCT band shown (numerical values are listed in Table 2).

Solvatochromic behavior. To determine the solvatochromic shift, one needs to compare the solution spectrum to the spectrum of the bare solute. However, the comparison becomes problematic if the spectra have qualitatively different shapes. In order to quantitatively evaluate the spectral shifts at different stages of solvation, a reference point that can be used for all spectra needs to be determined. Accurately determining solvatochromic shifts is challenging in the present case, since the spectra progressively lose spectroscopic detail as solvation increases (see Figure 4). A good numerical measure for the solvatochromic shift would be the peak shift of the envelope of each spectrum. However, since there is no well-grounded, simple physical model to describe the envelope shape, we cannot rely on fitting a peak shape function to the experimental data. To get a quantitative description of the solvatochromic shift, we chose two different approaches. In one approach, we determined the center of gravity of the MLCT band, in the other we employed a sufficiently broad smoothing function (using a gliding average) to determine the peak of the envelope of the spectrum (see Supporting Information for detailed procedures). The corresponding wavenumbers are listed in Table 2. Note that while the center of gravity is not necessarily the same as the envelope peak position, the solvatochromic shifts determined using the two procedures are consistent with each other (see Figure 5). Since both methods yielded values well within the error bars of each other, we define the overall solvatochromic shift by using the difference between the peak of the band envelope and the peak of the solution spectrum49 (-2380 cm-1, 0.295 eV).


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Table 2. Experimental and calculated energies (in cm-1) of the selected spectroscopic features of [RuBT-OH2]2+·(H2O)n clusters at 180 K trap temperature. ES8 [cm-1] c ECOG [cm-1] a

n

def2-SV(P)

def2-TZVP

0

23470 (80)

23390 (100)

22390

22850

1

22940 (100)

22700 (100)

22000

22530

2

22160 (100)

22030 (100)

21710

22350

3

21820 (160)

21660 (160)

21490

22120

4

21760 (240)

21550 (240)

21340

bulk

a

Eenv [cm-1] b

21010 (100)

Center of gravity of the 1MLCT band for each cluster size (see Supporting Information).

Uncertainties are estimated from the sensitivity of the procedure to the choice of integration interval. b

Peak of band envelope (see Supporting Information). The bulk value was extracted from the

data in Ref.

49

. Error bars are estimated from small variations in the peak position when using

different gliding averages. c

Calculated values for the most intense transition (S8) using the B3LYP functional with the basis

sets for all atoms as listed. Values are rounded to 10 cm-1 accuracy. For cluster sizes with more than one isomer, an average of all values is reported. All def2-TZVP results were shifted by 1210 cm-1. All def2-SV(P) values were shifted by +2420 cm-1.



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Figures 4 and 5 summarize the solvatochromic behavior of [RuBT-OH2]2+ from the unsolvated aqua complex through the first four water adducts to bulk hydration. There are two distinct size regions in the solvatochromic behavior. The solvatochromic shifts are ca. -650 cm-1 for each of the first two solvent water molecules, then the sequential shift slows to ca. -280 cm-1 per added solvent molecule. Together with the calculated structural motifs, this behavior suggests that the slower red shift with increasing solvation reflects saturation of the aqua ligand with H-bonds. In principle, this can be understood as closing the first solvation “shell” for the aqua ligand with the second solvent molecule, since the second solvation “shell” must begin at n = 3 for the lower energy isomers. The term “shell” is to be meant loosely in this context, since the water network begins to grow at the aqua ligand, and will presumably include the organic ligands only in much larger clusters.

Figure 5. Summary of solvatochromic behavior of [RuBT-OH2]2+·(H2O)n compared to bulk aqueous solution49 (horizontal line). The center of gravity positions of the MLCT bands are shown as filled blue squares, the envelope peaks (see text) as filled red triangles. Results from TDDFT calculations are shown as open circles (def2-TZVP) and open squares (def2-SV(P)). The dashed lines are meant to guide the eye. Numerical values are listed in Table 2, and error bars (where given) are determined from Gaussian error propagation of the errors listed in Table 2.


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The TDDFT calculations underestimate the solvatochromic shift significantly (see Figure 5), and the description does not improve with increasing basis set size from def2-SV(P) to def2TZVP. Experimentally, the overall envelope red shift reaches 78% of the full bulk solvatochromic shift with the addition of the fourth solvent water molecule. Following a linear extrapolation of the data for n = 2 – 4 to estimate the effects of continuing addition of solvent water molecules, one would expect that bulk solvatochromic shift is reached for ca. 6 solvent water molecules. We note that this extrapolation is to be taken with a grain of salt. First, the data show that the red shift per water molecule diminishes as the number of solvent molecules grows, and it is not unreasonable to expect that the shift per added solvent molecule will continue to diminish as cluster size increases. In addition, it is not straightforward to predict the effect of additional solvent once the water network spreads to interact with the organic ligands. In light of these arguments, the above extrapolation represents a lower limit rather than an accurate estimate. Nonetheless, the fact that the electronic structure is modified by just four water molecules to reach more than 75% of bulk behavior highlights the importance of the water network that incorporates the aqua ligand. It is instructive to put the above extrapolation in perspective to bulk aqueous solutions of atomic ions. Assuming that we reach bulk-like behavior for [RuBT-OH2]+(H2O)n at n ≈ 6, the total number of water molecules interacting with the Ru2+ ion at this size (including the aqua ligand) is 7. Using this number together with the fact that the solid angle on the Ru2+ ion accessible to hydration only amounts to ca. 15% of a full sphere, our extrapolation is roughly consistent with the number of the water molecules in a hydrated ion cluster that corresponds to a 1.2 M solution of an atomic ion (56 molecules per ion correspond to roughly 1 M). It is also in



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line with results by Williams and collaborators1, 3 on the behavior of large hydrated clusters in terms of water binding energy and vibrational spectroscopy. Garand and coworkers found that increasing levels of solvation result in a loosening of the H-bonded OH groups of the aqua ligand, suggesting an incipient proton or H transfer from the aqua ligand to the solvent. This raises the question whether the ligand in the solvated complex has already substantial OH or OH- character that would be encoded in the electronic spectum. Exploratory TDDFT calculations for [RuBT-OH]2+ and [RuBT-OH]+ (see Supporting Information) show that the simulated spectra for these ligands do not agree with the experimental data, and that the aqua ligand (while strained) is still intact. This is in fact consistent with the infrared spectra reported by Garand and coworkers which do not show that the proton or H transfer is complete. In order to gauge the excited state interactions of the aqua ligand with the Ru center, it is interesting to investigate the change in the Ru-O bond length in an MLCT excitation. While excited state geometry optimizations are too costly to carry out for the clusters, a simple way to gauge this geometry change is to compare the Ru-O bond distances of the dication and the trication for different cluster sizes, using oxidation of the whole complex in lieu of transient photo-oxidation of the Ru center. Our exploratory calculations indicate that the Ru-O bond distance change depends strongly on the level of hydration. For the bare aqua complex, the calculated Ru-O bond distance hardly changes at all upon oxidation (from 222.4 pm in [RuBTOH2]2+ to 221.9 pm in [RuBT-OH2]3+). In contrast, the linear chain lowest energy isomer for n = 3 (3-A) shows a marked change in this coordinate, from 216 pm in the dication to 207 pm in the trication.


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SUMMARY AND CONCLUSIONS Upon photoexcitation, [RuBT-OH2]2+·(H2O)n clusters sequentially lose water molecules, likely by unimolecular decay of a vibrationally hot molecule in its electronic ground state. Kinetic shift effects can be neglected for the photodepletion spectra of all ions under study. While the spectrum of the unsolvated aqua complex [RuBT-OH2]2+ shows extensive, partially resolved structure, the cluster spectra exhibit considerably less well defined structure as the number of solvent molecules increases, likely because of increased spectral congestion through the addition of weakly bound solvent molecules with large conformational freedom. The 1MLCT band of the complex shifts towards lower energies upon addition of solvent water molecules. The red shifts are stronger in magnitude for the addition of the first two solvent molecules than for the following. The slower red shift with increasing solvation reflects the saturation of the aqua ligand with H-bonds and the closing of the first solvation “shell”. In the dihydrate (n = 2), this shift recovers 55% of the solvatochromic shift in bulk aqueous solutions of [RuBT-OH2]2+, while the tetrahydrate spectrum shows 78% of the bulk solvatochromic shift. This observation highlights the importance of the first few solvent molecules for the electronic structure of this complex.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx.

Additional information mentioned in the text (PDF): Calculated minimum energy

structures of [RuBT-OH2]2+·(H2O)n for n = 0 – 4; mass spectrum of [RuBT-OH2]2+·(H2O)n cluster ions and other species in the same mass range; action and depletion spectra of [RuBT-



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OH2]2+ at 180 K; photodissociation spectra for [RuBT-OH2]2+·H2O; photodissociation spectra for [RuBT-OH2]2+·(H2O)2; procedures for determining the MLCT band envelope peak positions and centers of gravity; overview of TDDFT Spectra for all isomers.

AUTHOR INFORMATION Corresponding Author *[email protected]

Phone: ++1-303-492-7841 Notes The authors declare no competing financial interest.

ACKNEWLEDGEMENTS We gratefully acknowledge the National Science Foundation for support of this work under Grant CHE-1361814.


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TOC Figure



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Figure 1. Calculated minimum energy structures of [RuBT-OH2]2+·(H2O)n, showing the lowest energy structures for n = 0 and 1, and the two lowest energy structures for n = 2 – 4, respectively, together with their relative energies. Additional conformers are shown in Supporting Information. Carbon atoms are shown in brown, nitrogen in blue, ruthenium in gold, oxygen in red, and hydrogen in white. 338x190mm (96 x 96 DPI)

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Figure 2. Photodissociation action spectrum of [RuBT-OH2]2+ at 180 K trap temperature. Circles are raw data, the line is a 15-point gliding average. Vertical bars are calculated transition energies (B3LYP/def2TZVP), shifted by -1210 cm-1 (-0.15 eV). Data were taken from ref 51. 84x62mm (300 x 300 DPI)



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Figure 3. Experimental parent ion depletion spectra for [RuBT-OH2]2+·(H2O)n with n = 1 (top) and n = 2 (bottom) together with calculated vertical excitation spectra (columns), the latter shifted by -1210 cm-1 (0.15 eV). Circles are raw data points; the full lines are a 25-point gliding average to guide the eye. Open and solid columns in the bottom panel represent the 2-A and 2-B isomers of n = 2. 84x110mm (300 x 300 DPI)


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Figure 4. Photodissociation spectra for [RuBT-OH2]2+·(H2O)n and comparison with the absorption spectrum of the complex in aqueous solution (data taken from Ref. 49). Circles are raw data points, the full lines in the photodissociation spectra are 25-point gliding average to guide the eye. Arrows show the center of gravity for each MLCT band shown (numerical values are listed in Table 2). 84x206mm (300 x 300 DPI)



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Figure 5. Summary of solvatochromic behavior of [RuBT-OH2]2+·(H2O)n compared to bulk aqueous solution49 (horizontal line). The center of gravity positions of the MLCT bands are shown as filled blue squares, the envelope peaks (see text) as filled red triangles. Results from TDDFT calculations are shown as open circles (def2-TZVP) and open squares (def2-SV(P)). The dashed lines are meant to guide the eye. Numerical values are listed in Table 2, and error bars (where given) are determined from Gaussian error propagation of the errors listed in Table 2. 84x59mm (300 x 300 DPI)


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TOC Figure 85x64mm (150 x 150 DPI)


Hydration of a Binding Site with Restricted Solvent Access

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