Article pubs.acs.org/JPCC
Vibrational Strong Coupling of Organometallic Complexes Shaelyn R. Casey and Justin R. Sparks* Department of Chemistry, Muhlenberg College, Allentown, Pennsylvania 18104, United States S Supporting Information *
ABSTRACT: Strong coupling of optical and molecular vibrational states to form polariton states is a promising route toward the modification of molecular bond properties without changing the constituent atoms or formal bonding structure. The transition from weak to strong coupling of ligand vibrational modes of aqueous organometallic complex ions to microfluidic Fabry−Pérot cavity modes is demonstrated using the Fe(CN)64− ion as a model system. It was found that the complex can exhibit tunable strong vibrational coupling while dissolved in solution at moderate concentrations, with a lower limit of approximately 15 mM, due to its large molar extinction coefficient and narrow resonance bandwidth. Combining the exquisite fluid control of microfluidic devices with the ability to modify ligand bond properties of transition metal complexes via vibrational strong coupling may lead to novel methods for the examination of catalytic reaction mechanisms and provide a new means for tailoring catalyst molecules.
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INTRODUCTION The physical and chemical properties of molecules are typically controlled by changing the constituent atoms and/or the way in which those atoms are bonded together. This fundamental tenet of chemistry has led to the synthesis of an immense library of molecular architectures with tunable structures and functions. Alternatively, using light-matter strong coupling, molecular properties can be altered without changing the atoms or formal bonding structure.1−5 This emerging field of chemistry, coined “quantum optical chemistry”,6 aims to exploit the formation of hybrid polariton states to access novel, currently unattainable, chemistries.7 When a colocalized optical state and molecular state resonate at the same frequency, coupling between the two states occurs spontaneously8 and two hybrid light-matter states are formed (Figure 1). The resulting two polariton states have energies that are shifted from the original resonant frequency by half of the Rabi splitting, Ω. The strong coupling regime is achieved if the magnitude of the Rabi splitting is larger than the full width at half max (fwhm) of both the molecular and optical cavity resonances. Strong coupling has been explored extensively for modifying electronic4,5 and plasmonic9 states using optical cavities with length scales on the order of the photon wavelengths involved in such transitions (hundreds of nanometers). The creation of hybrid states leads to the alteration of chemical and physical properties of the original molecule such as the workfunction,10 phase transitions,3 conductivity,11 and reaction energy landscapes.1,2,12 Recently, strong coupling has been investigated in the frequency range of molecular vibrational states.13−18 Strong coupling to vibrational states creates two new hybrid modes associated with the chemical bond; one at higher frequency and the other at lower frequency compared to the original resonance. This change in vibrational frequency is expected © XXXX American Chemical Society
to modify the strength of the chemical bond as well as the ground state potential well. Indeed, the resulting alteration of the properties of the bond has recently been shown to change the reaction dynamics involved in the breaking of that particular bond.19 Moreover, such a modification of the bond reactivities provides an additional tool for probing reaction mechanisms involving particular bonds to supplement techniques such as isotope substitution. The coupling of vibrational states to Fabry−Pérot cavities has been investigated in organic liquids,14,20 polymers,13,15,21 proteins,22 and solid mixtures of polymers,6 but it has been unexplored in aqueous solutions of organometallic complexes in the context of catalysis. Such compounds have properties that are significantly influenced by the chemical structure and symmetry of the ligands around the metal center and are important in traditional homogeneous catalysis, where they are used for producing polymers and pharmaceutical compounds, for example. Modification of the ligand bond properties in organometallic complexes via strong coupling has the potential to provide a new means for tuning ligand/metal interactions and tailoring catalyst molecules. Furthermore, many biological catalytic processes use metalloenzymes with organometallic centers. As an example, the hydrogenase enzymes are promising candidates for the production of H2 fuel due to their ability to catalyze the 2 H+ + 2 e− ⇌ H2 reaction with high efficiency and turnover frequencies in aqueous solutions.23 The active site of the [FeFe] class of these enzymes consists of a cubane [4Fe− 4S] linked to a 2Fe subunit that is coordinated by five CN− and CO ligands.24 These ligands are important in controlling the Fe(II)/Fe(I) redox reaction, where CO is thought to control Received: October 17, 2016 Revised: November 18, 2016 Published: November 22, 2016 A
DOI: 10.1021/acs.jpcc.6b10493 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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an Ar atmosphere. An approximately 5 nm thick film was used to provide moderate transmission and cavity finesse. FTIR Spectroscopy. Solutions of K4[Fe(CN)6] were prepared by dissolving the trihydrate solid in deionized water. Unpolarized FTIR spectra were collected using a PerkinElmer Spectrum 100 instrument with 4 cm−1 resolution. A single bounce diamond ATR accessory (Pike MIRacle) was used to collect spectra of solutions for optical constant determination. ATR correction and optical constant extraction were performed using Essential FTIR. Peak fitting was performed using the lmfit package29 (version 0.9.5) in Python 3.4. Theoretical Calculations. Transfer matrix method calculations were carried out using the tmm package30 (version 0.1.6) in Python 3.4. The system was modeled as two 4 mm thick CaF2 slabs with 5 nm Au films using reported optical constants of CaF231 and Au.32 Density functional theory (DFT) calculations were performed using NWChem version 6.533 and the results were visualized using Avogadro version 1.2.0.34 The M06-L functional and aug-cc-pVDZ basis set were used, which has been shown to produce accurate results for similar compounds.35
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RESULTS AND DISCUSSION A microfluidic flow cell design, consisting of two closely spaced CaF2 windows (Figure 1), was used to study the weak to strong coupling characteristics of K4[Fe(CN)6] solutions over the water solubility range of the compound. When coated with a reflective Au film, the two windows form a Fabry−Pérot cavity whose resonance can be tuned to match that of the molecular vibration of interest. Cavity lengths of approximately 7 μm were used such that the energy of the fourth order mode of the cavity (≈ 2000 cm−1) aligned with that of the v = 1 vibrational mode associated with the CN− ligand stretches (≈ 2039 cm−1) of the Fe(CN)64− ion. The feasibility of strong coupling of Fe(CN)64− was first assessed by determining the optical constants of 0.649 M (saturated), 0.502, 0.325, 0.201, 0.094, and 0.030 M aqueous solutions of K4[Fe(CN)6] from standard, uncoupled FTIR spectra. In order to avoid potential deviations from Beer’s law caused by the strong absorbance of the solutions, an attenuated total internal reflection (ATR) accessory with a shorter path length of approximately 2 μm was used to collect spectra of each K4[Fe(CN)6] solution. The spectra were then corrected to remove distortions caused by the ATR measurement and the complex refractive index for each solution was accurately determined using standard techniques.36,37 Figure 2 shows the ATR-FTIR spectra corrected to 7 μm path length transmission spectra for each of the K4[Fe(CN)6] solutions. It was confirmed that the corrected spectra overlay well with the transmission spectra from the lower concentration solutions that were measured directly in the microfluidic flow cell (Figure S1). The intense peak at 2038.6 cm−1, with a molar absorptivity of 2.66× 103 dm3 mol−1 cm−1, is associated with vibrational modes of the molecule that primarily involve the stretching of the CN− ligand. In order to investigate the vibrational modes of the molecule that are associated with this peak in more detail, the structure and vibrational modes of the Fe(CN)64− ion were analyzed using DFT calculations. The Fe−C and CN bond distances were determined to be 200 and 119 pm, respectively, which is in good agreement with previously reported theoretical and experimental results (190 pm for Fe−C and 117 pm for C N).38 The Fe(CN)64− ion belongs to the Oh point group and
Figure 1. Schematic of the microfluidic device used to investigate strong vibrational coupling in aqueous solutions. The two IR transparent windows are coated with Au films to create optical cavity states. When the spacing of the windows is such that one of the optical states resonates at the same frequency of the Fe(CN)64− ion vibrational states, strong coupling can occur. This leads to the creation of two polariton states, P+ and P−, separated by the Rabi splitting energy, Ω.
the occupation of frontier orbitals25 and CN− is expected to modify the frontier orbital energetics to enable electron transfer between the cubane [4Fe−4S] and 2Fe subunits.26 Importantly, the ligands display relatively strong infrared absorption bands,27,28 making them good candidates for the use of vibrational strong coupling as tool for further elucidation of the catalytic mechanism. The low energy separations of vibrational states allows for micron dimension optical cavities to be used for coupling experiments, which are amendable to liquid phase studies in microfluidic devices.14 In this work, vibrational strong coupling of this important class of molecules in aqueous solutions is demonstrated in a microfluidic device using the ferrocyanide ion, Fe(CN)64−, as a model system. Combining the exquisite fluid control properties of microfluidic devices with the ability to alter molecular bond properties may lead to novel methods for the examination of reaction mechanisms, aid in future catalyst design, and enable the exploration of vibrational mode selective reactions.
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METHODS Microfluidic Device Fabrication. Two rectangular CaF2 windows (Crystran Ltd.), both with dimensions of l = 38.5 mm, w = 19.0 mm, and h = 4.0 mm and one with 2 mm diameter holes drilled through, were used to construct the microfluidic device. The distance between the two windows was controlled using mylar film (Chemplex Industries, Inc.). The entire device was assembled in a demountable FTIR flow cell for measurements. The windows were coated with Au using a Cressington 108auto sputter coater with 20 mA of current and B
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the Lorentzian and Gaussian widths. The homogeneous line width of the molecular absorption was consistent between all the concentrations, giving a value of 14 ± 1 cm−1 centered at 2038.6 ± 0.4 cm−1 with Gaussian line widths of 7 ± 2 cm−1. The microfluidic Fabry−Pérot cavity modes had a 60 cm−1 bandwidth, which is larger than the 14 cm−1 bandwidth of the CN− stretches. Therefore, it was the fwhm of the cavity, Γcav, that determined the threshold for strong coupling. The value for eq 2 could then be determined for each solution (Table 1). Table 1. Prediction of Coupling Regime Based on Experimentally Determined Optical Constants of K4[Fe(CN)6] Aqueous Solutionsa Figure 2. ATR-FTIR spectra corrected to 7 μm path length transmission spectra for each of the K4[Fe(CN)6] solutions. The inset shows one of the triply degenerate vibrational motions associated with this absorption.
the triply degenerate CN stretching mode (T1u) consists of the antisymmetric stretching of opposite CN− pairs. The DFT calculations predicted the frequency of these modes to be 2039.0 cm−1 (a scaling factor was not applied), which is in excellent agreement with the experimentally determined 2038.6 cm−1. One of the triply degenerate, infrared active vibrations associated with the 2038.6 cm−1 peak is shown schematically in the inset of Figure 2. Since these vibrational modes primarily involve the CN bond stretching, strong coupling to this vibrational mode should alter the bond properties of the ferrocyanide ion ligands, which in turn could modify the ligand field splitting of the metal center. The coupling of the CN vibrational modes with the optical mode depends strongly on the total absorbance of the sample at the resonance frequency. The strength of splitting observed during transmission through a Fabry−Pérot cavity surrounding a molecular resonator, ΩT, can be expressed as39 ΩT =
α0 (cm−1)
eq 2
0.649 0.502 0.325
0.1615 0.1321 0.0885
4138 3386 2267
1.85 1.51 1.01
0.201 0.094 0.030
0.0615 0.0317 0.0141
1574 812 361
0.70 0.36 0.16
The empty row indicates the strong/weak coupling threshold for this work.
Thus, for the experimental concentrations investigated in this study, the 0.030, 0.094, and 0.201 M solutions were expected to exhibit weak coupling, where the absorption from the Fe(CN)64− ion merely overlaps with the cavity transmission mode, and the remaining solutions were expected to exhibit strong coupling. It is important to note that in the case of weak coupling, splitting of the transmission spectrum into two peaks is still observed, but the separation of these peaks is not greater than the fwhm of the individual resonances (Figure S2). The lower concentration limit was determined by the moderate finesse of the Fabry−Pérot optical cavity used in this study. However, it will be shown that due to the very sharp, intense absorption of the Fe(CN)64− ion, this concentration limit can be further decreased using cavities with higher finesse. The coupling behavior of the solutions was experimentally investigated by fabricating a Fabry−Pérot resonator within the microfluidic device by depositing thin Au films onto the CaF2 windows. When a K4[Fe(CN)6] solution was placed in the microfluidic flow cell with the spacing of the mirrors tuned to have the fourth order cavity mode overlap with the molecular resonance, coupling of the matter states and the optical states occurred and the spectral signature of two new polariton states, P+ and P−, was observed. Figure 3 shows the FTIR transmission spectrum at normal incidence of the saturated 0.649 M K4[Fe(CN)6] solution in a device with a 6.84 μm spacing between the Au films. The uncoupled absorption spectrum of this solution with the same path length is also shown in Figure 3. The original peak associated with the CN stretches at 2038.6 cm−1 splits into two polariton peaks separated by ≈90 cm−1. In order to compare the measured coupling strength to that predicted from the eq 2 estimation, the strength of the Rabi splitting, Ω, for each K4[Fe(CN)6] solution was determined. By changing the incidence angle of the IR light with respect to the plane of Fabry−Pérot cavity, the optical mode resonance can be swept through the molecular resonance. The Rabi splitting can be extracted from the minimum splitting observed in such an analysis, which was determined experimentally by measuring
⎛ α0 Γvib ⎞2 αΓ ⎜ ⎟ + 0 vib Γvib(Γvib + Γcav) − Γvib 2 πnB ⎝ 2πnB ⎠ (1)
2πnB Γcav 2
k
a
where nB is the background refractive index, Γvib is the line width of the molecular resonance, and Γcav is the line width of the cavity resonance. The α0 term (units of cm−1) is the maximum absorption coefficient which is equal to 4 πk ν̃0, where ν̃0 is the maximum absorption frequency and k is the imaginary part of the refractive index. The strong coupling condition is met when the observed splitting, ΩT, is greater than both Γvib and Γcav. For strongly absorbing species such as the Fe(CN)64− ion, the first term in eq 1 is expected to dominate. Also, since strong coupling occurs only when the Rabi splitting is greater than either resonance fwhm (Ω ≥ Γcav, Γvib), eq 2 can be derived as a criterion for strong coupling.6
α0 Γvib
concentration (M)
≥1 (2)
The applicability of strong coupling to the various concentrations of K4[Fe(CN)6] solutions could be determined once the complex refractive index of the solutions, Γvib, and Γcav were known. In order to determine Γvib for each solution, a Voigt model was used to fit each experimental peak and extract C
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are displayed as 2D colormaps in Figure 4. Good agreement is observed between the experimentally determined peak positions and those predicted by the transfer matrix calculations. For all of the solutions, anticrossing behavior is observed when the resonance of the empty cavity without resonators (modeled using the refractive index of the solution without the imaginary component associated with the molecular resonance) overlaps with the molecular vibrational mode (shown as dashed white lines in Figure 4). This causes a splitting of the transmission into an upper branch, P+, and a lower branch, P−. Since the strength of the Rabi splitting depends on the absorbance per unit length6 and the length of the cavity is relatively constant between the experiments, the splitting strength was observed to increase with concentration. The experimental Rabi splitting strengths for each concentration of K4[Fe(CN)6] were extracted from each plot in Figure 4. The experimental data were compared to the expected splitting using the full analytical expression in eq 1, which is plotted over the concentration range used in this study in Figure 5. Good agreement between the experimental splitting and that predicted by eq 1 is observed. A horizontal line is drawn that separates the weak and strong coupling regime (in this case, defined by Γcav). The initial assessment using the eq 2 approximation agrees with the observation that solutions with a concentration of 0.325 M K4[Fe(CN)6] or above exhibited strong coupling. Moreover, the strength of the coupling can be controlled by the concentration of Fe(CN)64−, potentially
Figure 3. Uncoupled FTIR transmission spectrum of the 0.649 M K4[Fe(CN)6] solution (purple) shows one peak associated with CN stretches, while the FTIR transmission spectrum of the same solution in the microfluidic optical cavity with Au films (green) shows two peaks, P+ and P−, associated with the polariton states formed via strong coupling.
FTIR spectra as a function of incidence angle from −5 to 30° in 5° increments. These results are shown as white data points for each concentration in Figure 4. The dispersion of the coupling was also modeled with standard transfer matrix methods for unpolarized light using the optical constants of the solutions and the experimental pathlengths determined from the analysis of the Fabry−Pérot resonator before the introduction of the solutions. The results of the calculations for each concentration
Figure 4. Experimentally determined polariton peak positions (white circles) overlaid onto 2D colormap calculations of angular dispersion for each concentration of K4[Fe(CN)6] in water. The dashed white lines are the peak positions of the uncoupled cavity modes (curves) and the fixed position of the molecular resonance (horizontal lines). D
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DFT calculations predict that two degenerate Eg modes and one A1g mode occur at 2046.5 and 2084.5 cm−1, respectively (Figure S3). Because of the different selection rules of Raman spectroscopy, the ferrocyanide modes that are coupled to the cavity in the FTIR measurements are not those that are probed by Raman spectroscopy. This mutually exclusive set of vibrational modes provides an interesting system to investigate in future studies to determine the impact of strongly coupled infrared active modes on Raman active (but not infrared active) modes involving the same bonds.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10493. Comparison of transmission and corrected ATR FTIR spectra, an example of a weak coupling spectrum, and Raman active modes of the Fe(CN)64− ion (PDF)
Figure 5. Comparison of experimentally determined Rabi splitting strengths (black circles) to the full analytical expression (purple line, eq 1) as a function of α0. In this work, the strong/weak threshold is determined by Γcav ≈ 60 cm−1 and is shown as the green dashed line.
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allowing for the properties of the ligand to be tuned to a desired property. The concentration limit for strong coupling of Fe(CN)64− can be further decreased using cavities with higher finesse. For example, employing an optical cavity with Γcav < 30 cm−1 would allow all of the concentrations in this study to exhibit strong coupling. The lower concentration limit would be reached with Γcav = Γvib ≈ 14 cm−1, allowing for concentrations of Fe(CN)64− as low as 15 mM to achieve strong coupling with a Rabi splitting of ≈20 cm −1 . Thus, relatively low concentrations of organometallic species can couple strongly to optical fields in moderate finesse cavities if the molecule has a large molar extinction coefficient and narrow resonance bandwidth. Consequently, vibrational strong coupling is amendable to the possible alteration of catalyst molecular properties in dilute solutions.
AUTHOR INFORMATION
Corresponding Author
*(J.R.S.) E-mail:
[email protected]. ORCID
Justin R. Sparks: 0000-0001-7001-0691 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. Elizabeth McCain for assistance with sputter coating and the G. N. Russel Smart and David Stehly Grant at Muhlenberg College for funding.
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REFERENCES
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CONCLUSIONS This work shows that the strongly absorbing Fe(CN)64− ion can exhibit tunable vibrational strong coupling while dissolved in aqueous solutions at moderate concentrations, down to approximately 15 mM. Altering the vibrational states of the ligands in organometallic molecules may lead to a modification of the ligand field splitting of the transition metal center to manifest in a change in the optical, electrical, and chemical properties of the molecule. Other common organometallic ligands such as CO are likely to display similar coupling characteristics, which could be used to modify hydroformylation catalysts, for example. Further experimental and theoretical studies are necessary to develop an understanding of such effects, which may have profound implications in the design of new catalysts and the elucidation of catalytic mechanisms. Finally, vibrational strong coupling can be readily exploited in channels on the order of micrometers, which suggests that the field of microfluidics is amendable to the investigation of quantum optical chemistry effects on the reactivity of catalytic molecules in dilute solutions. As a final note, the Raman scattering intensity from vibrational polariton states has been shown to be greater than that from the uncoupled vibrational modes.21,40 The vibrational modes of Fe(CN)64− studied in this work are not Raman active. However, the molecule possesses three additional Raman active modes involving CN stretches in the same frequency region. E
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