Mode Coupling Pattern Changes Drastically Upon Photoisomerization

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J. Phys. Chem. C 2010, 114, 16740–16745

Mode Coupling Pattern Changes Drastically Upon Photoisomerization in RuII Complex Christopher S. Keating,† Beth A. McClure,‡ Jeffrey J. Rack,‡ and Igor V. Rubtsov*,† Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118, and Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio UniVersity, Athens, Ohio 45701 ReceiVed: June 22, 2010

Photoinduced isomerization in a bis(4,4′-dimethyl-2,2′-bipyridyl)(o-methylsulfinylbenzoate) ruthenium(II), (RuBzSO) complex is investigated using linear infrared absorption and dual-frequency two-dimensional infrared (2DIR) spectroscopies. The changes in the linear absorption spectrum associated with the photoisomerization were compared to the changes in the mode coupling pattern observed in the 2DIR spectra. The comparison indicates that the coupling of the SO and CdO stretching modes is much smaller in the photoisomerized compound compared to that in the ground-state isomer that has not been exposed to incident light (relaxed). Transition-dipole interaction estimations indicate a large decrease of the electric coupling contribution to the CdO/SO mode anharmonicity in the O-bonded versus S-bonded states thus supporting the formation of the O-bonded state upon photoisomerization of the RuBzSO compound. 1. Introduction Recent studies have demonstrated that ruthenium complexes with a variety of ligands involving sulfoxides exhibit photochromic properties.1-4 Upon photoexcitation of the complex, a long-lived metastable state is formed that has a red-shifted absorption spectrum in the visible spectral region. It has been suggested that a switch of the coordination bond occurs in the excited state from sulfur-ruthenium (S-bonded) to oxygenruthenium (O-bonded).2,4,5 S to O linkage isomerization has been observed in pentaamine Ru (II) sulfoxide complexes,1 as well as in various polypyridyl Ru(II) sulfoxides.4,6,7 Experimental evidence and density functional theory (DFT) calculations have shown that in the electronic ground state, the S-bonded coordination is favored in the presence of σ-donor ligands; the Ru-S and S-O bond distances indicate that substantial π-backbonding occurs in the S-bonded isomer.8-10 While the coordination-bond switch is generally accepted as a main step of the photoinduced isomerization in ruthenium complexes involving sulfoxides, the detailed mechanism of ligand rearrangement is unclear and formation of several intermediates is plausible.4,6,11 Several spectroscopy methods were used to understand the conformational changes associated with photoisomerization, including UV/vis pump-probe and time-resolved fluorescence spectroscopies.5,7 In this work, we investigated the photoisomerization process in bis(4,4′-dimethyl-2,2′-bipyridyl)(o-methylsulfinylbenzoate) ruthenium(II) (RuBzSO Scheme 1, middle) using a combination of linear infrared absorption spectroscopy, DFT computations, and two-dimensional infrared (2DIR) spectroscopy. Since the conformational changes were expected at the sulfoxide site of the complex, we have selected a vibrational reporter there, the SO stretching mode having frequency at ca. 1000-1120 cm-1. Because of the difficulty of mode assignment in the fingerprint region, SO stretching modes have not been widely used as structural reporters, although its transition dipole strength is often comparable to that of the CdO stretching

mode.12,13 To assign the peaks in the absorption spectrum of RuBzSO in the fingerprint region, we used DFT calculations and normal-mode analysis. A substantial red shift of the SO stretching frequency is found in the irradiated state of RuBzSO. Dual-frequency 2DIR spectroscopy, focusing on the cross peaks between the CdO stretching mode and modes in the fingerprint region, including the SO stretching mode, was used to characterize the structures in the relaxed and irradiated compounds. The cross peak patterns differ significantly for the relaxed and metastable states of the complex. These changes were used to characterize the photoisomerized state. 2. Experimental Details 2.1. Dual-Frequency 2DIR Setup for Cross-Peak Measurements down to ca. 850 cm-1. Details of the experimental setup for dual-frequency 2DIR measurements with heterodyned detection have been reported in refs 13-15. Briefly, the pulses of a Ti:sapphire oscillator/regenerative amplifier tandem (804 nm wavelength, 44 fs duration, and 800 µJ/pulse energy) were used to pump two in-house built optical parametric amplifiers (OPA), which produced two Signal-Idler pulse pairs, each generating tunable mid-IR pulses via difference-frequency generation (DFG) among the Signal and Idler pulses in a 2 mm thick AgGaS2 crystal. Each mid-IR beam was split into two parts: three beams of ca. 1.2-2 µJ energy per pulse were focused onto the sample in the phase-matching geometry, whereas the fourth beam (ca. 0.1 nJ) was used as a local oscillator (LO). The spectrum of the two pulses, k1 and k2 interacting first with the sample, was centered at 1620 cm-1 to excite the CdO stretching mode. The spectra of the third pulse, k3, and LO were centered at 1060 cm-1 accessing the modes in the fingerprint SCHEME 1

* To whom correspondence should be addressed. E-mail: irubtsov@ tulane.edu. † Tulane University. ‡ Ohio University.

10.1021/jp105776z  2010 American Chemical Society Published on Web 09/02/2010

Photoisomerization in RuII Complex region, including the SO stretching mode. A third order signal generated in the sample in the phase matching direction (-k1 + k2 + k3) was focused onto a HgCdTe detector, where it was mixed with the local oscillator pulse, delayed by a time delay, t, referred to as the detection time. The delay between the first and the second pulses, τ, and the second and the third pulses, T, are referred to as the dephasing and waiting times, respectively. The durations of the pulses at 1620 and 1060 cm-1 were ca. 90 and 200 fs, respectively. Two dimensional (τ, t) data sets were recorded and double-Fourier transformed yielding the two-dimensional (ωτ, ωt) spectra. 2.2. Sample Preparation. The bis(4,4′-dimethyl-2,2′-bipyridyl)(o-methylsulfinylbenzoate) ruthenium(II), [Ru(dmb)2(BzSO)]+ (RuBzSO) and o-methylsulfinyl-benzoic acid (oMSBA) compounds were synthesized in accordance with literature procedures.11,13 The concentrations of the samples of RuBzSO compounds dissolved in dichloromethane were ca. 20 mM. For the Fourier transform IR (FTIR) and 2DIR measurements, the samples were held in an optical cell with 50 or 100 µm path length and 1.5 mm thick BaF2 windows. All measurements were performed at room temperature, 23.5 ( 0.5 °C. To prevent any unwanted photoisomerization by a room light, the sample preparation and experiments with RuBzSO were performed under red light or dark conditions. The sample irradiation was performed using a 250 W mercury arc lamp (Ealing) and a longwavelength-path filter with a cutoff wavelength of 400 nm. 2.3. DFT Calculations. Geometry optimization and peak assignment were performed by DFT calculations using the Gaussian 03 suite.16 The B3LYP hybrid functional was used with the 6-311+G(d,p) basis set for carbon, nitrogen, hydrogen and oxygen atoms, a triple-ζ basis set with two polarization functions (TZ2P) for the sulfur atoms,17,18 and a LANL2DZ basis set with effective core potential (ECP) for the ruthenium atom.19 The calculations also included a polarizable continuum model for dichloromethane (RuBzSO) and for acetonitrile (oMSBA). Because of the high computational time required for the Ru complex, the dmb ligands were truncated into bpy groups. 3. Results and Discussion 3.1. Linear Absorption Spectra of oMSBA and RuBzSO. The SO bond strength in sulfoxides is very sensitive to the molecular structure and to the solvent,9,13,20 which makes the SO stretching mode a valuable reporter on the structure of the molecule. To use the SO stretching mode as a structural reporter requires understanding the linear absorption spectrum of the compound in the region from 950 to 1150 cm-1, where the frequency of SO stretching modes are found. Detailed analysis of the absorption spectra of several sulfoxides using a combined DFT and 2DIR methods has recently been reported.13 Figure 1 compares the absorption spectra of the relaxed RuBzSO complex (Figure 1, top) with that of the ligand, oMSBA (Figure 1, bottom); the spectra differ substantially, reporting on the differences in the bond order and the polarization extent of the sulfinyl group in these compounds. Note that the DFT computed spectra describe reasonably well the main features of the experimental absorption spectra (Figure 1). For example, the frequency of the SO stretch in S-bonded RuBzSO is higher than that in oMSBA (Table 1) while the transition dipole is substantially smaller. These data indicate that the SO bond order is higher in RuBzSO (S-bonded) but the bond is less polarized, consistent with previous observations.20 Indeed, the DFT calculations support these observations (Scheme 2); the SO bond order is greater in RuBzSO while the partial charges of the S and O atoms are smaller than those in oMSBA. Lewis resonant structures shown in Scheme 2 assist in understanding these

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16741

Figure 1. Absorption spectra of the (top) relaxed RuBzSO and (bottom) oMSBA compounds (red). The DFT-computed spectra are also shown (blue).

TABLE 1: Experimental and Computed Frequencies of the CdO and SO Stretching Modes experimental peak frequency (peak areaa) oMSBA: ν(CdO) ν(SO) S-RuBzSO: ν(CdO) ν(SO) O-RuBzSO: ν(CdO) ν(SO)

1718.4 (677) 1034.1 (436) 1620.5 (577) 1100.9 (139) 1620.2 (995) 1027 (518)

DFT/peak frequency (IR intensity) 1742.0 (677) 1019.8 (246) 1680.6 (577) 1103.8 (83.3) 1630.5 (995) 920 (229)

a

The experimental peak areas in the absorption spectra were normalized so that the peak area of the CdO stretching mode equals to the computed IR intensity for the respective compound.

SCHEME 2: Lewis Resonant Structures for the Sulfoxide Sites in oMSBA and S- and O-Bonded RuBzSO Compoundsa

a The formal atomic charges are shown in parentheses. Note that the arrows between the structures vary in size, representing larger or smaller contributions of the respective structures to the actual charge distribution in the compound. The SO bond lengths and Mulliken atomic charges for the sulfur and oxygen atoms shown at the right were obtained via DFT calculations.

trends. In the ligand alone, the SO bond polarization can be described by almost equally weighted contributions from the two resonance structures shown; note that the partial charges on S and O are close to (0.5 of the charge of an electron. The coordination bond S-Ru in the S-bonded RuBzSO complex is formed by the lone-pair electrons originated from the sulfur atom; the coordination bond in Scheme 2 is represented by an arrow directed toward the Ru atom (for electron counting purposes, the arrow describes two electrons).

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Figure 2. Linear absorption spectra of the relaxed ground state (blue) and metastable irradiated state (green) of the RuBzSO compound and the spectrum of their difference, irradiated-relaxed (red).

The extent of donation of this pair to the ruthenium atom depends on the oxidation state of the metal atom and on the nature of the other ligands in the complex and is not the focus of this study. Note that while the oxidation state of Ru can be accurately determined using Mo¨ssbauer and K-edge absorption spectroscopies21 only a small electron donation to the ruthenium atom is expected by this ligand in this complex. The Lewis structures shown for the ligand describe well the changes between the oMSBA and the S-bonded RuBzSO compounds. The Lewis structure with a double-bond SO character (left) contributes more in S-bonded RuBzSO than in oMSBA, which explains the stronger SO bond and the smaller bond polarization in the complex. This is consistent with the findings of Amstutz et al., who concluded that in o-substituted phenyl sulfoxides, hydrogen-bonding interaction of the o-substituent with the sulfoxide moiety plays a role in the overall SO bond order.20 There are substantial differences in the linear absorption spectra of the ground state and irradiated RuBzSO compounds in the fingerprint region (Figure 2). The largest changes are observed for the SO stretching mode, which shifts from 1101 cm-1 in the relaxed compound to 1027 cm-1 in the irradiated compound. Note, that the latter is close to the SO stretching mode in oMSBA (1034 cm-1). 4,4-Dimethylbipyridine is both a σ-donor and π-acceptor ligand, where the electron donation from the nitrogen atom lone pair to ruthenium is compensated by the carbons within the dmb rings. While some of the ruthenium electron density is donated into the dmb ligands via π-backbonding interaction, significant electron density is also donated to the sulfoxide group. Evidence for this bonding model comes from electrochemical, spectroscopic, and DFT calculations. Metal-sulfur coordination increases the effective positive charge on the sulfur relative to the unbound ligand,9 which is stabilized due to donation from the attached phenyl ring and the methyl branch.22-24 The frequency shift of the SO stretching mode of ca. 70 cm-1 in the irradiated compound suggests that the Ru-S bond is either broken or severely elongated. This frequency shift is similar to that associated with the S to O coordination change in Ru-DMSO complexes.4,5,10 The absorption increase at 1027 cm-1 in the irradiated compound is similar to the absorption decrease at 1101 cm-1 in the relaxed compound, which suggests similar polarizations of the SO bond in both relaxed and metastable state compounds. The other changes upon irradiation include a decrease in absorption at 1155, 1122, 1048, 953, and 940 cm-1 and an increase at 1062

Keating et al. cm-1. Are these changes consistent with formation of the O-bonded state? The SO frequency shift and similar level of bond polarization to that for the S-bonded compound is in accord with formation of the O-bonded complex, as can be seen from the corresponding Lewis diagrams in Scheme 2. The absorption changes for other peaks are likely associated with the SO frequency shift. In the S-bonded compound, the SO stretching motion is coupled to the motions of several other groups, such as phenyl ring deformation motions (which include CC stretching and CH bending motions) and CH3 rocking motion.13 As a result of such coupling, several modes in the region have some contribution from the SO stretching motion. For example, the main SO stretch at 1101 cm-1 has ca. 24% SO stretching contribution to its potential energy distribution (PED), the largest among all the modes. The modes at 1122 and 1048 cm-1, while dominated by the phenyl-ring deformation motions, have ca. 0.2 and 0.6% of the SO contribution to them, respectively. These contributions, although small, enhance substantially the IR intensities of these modes.13 Therefore, when the local SO stretch shifts to lower frequencies, the IR intensity of several modes in the region decreases, although to a different extent. When shifted to lower frequencies, the SO stretching motion in the irradiated compound is likely contributing to several normal modes. In addition to the major peak at ca. 1027 cm-1, which certainly has the largest SO stretching contribution, another peak at 1062 cm-1 is enhanced, likely due to the contribution from the SO stretching motion. The decrease in the absorbance at 953 cm-1, assigned to the CH3 group rocking motion, is likely associated with the change of the sulfur atom partial charge (Scheme 2). We cannot say with certainty if the IR intensity of this mode has been strongly reduced or the frequency has been shifted out of the range of observation. Note that the conversion of the S-bonded state to the metastable state upon irradiating is not complete and the peak at 953 cm-1 for the irradiated compound has a contribution from the nonisomerized S-bonded species. To summarize, the changes in the linear absorption spectrum observed upon irradiation are consistent with formation of the O-bonded complex. 3.2. 2DIR Spectra for Relaxed and Irradiated RuBzSO. To study how the coupling pattern of vibrational modes changes among the relaxed ground state and metastable irradiated state of RuBzSO, we have measured their dual-frequency 2DIR spectra focusing on the interaction of the CdO stretching mode with the modes in the fingerprint region that include the SO stretching mode. First, the 2DIR spectrum was measured for a fresh sample prepared in the darkness (relaxed RuBzSO) (Figure 3 top). Then the sample was irradiated for 30 min with a mercury lamp through a long wavelength path filter with a cutoff wavelength of 400 nm and its 2DIR spectrum was recorded again. To minimize the recovery of the relaxed state, the 2DIR spectra were measured in a rapid fashion, which took ca. 10 min for acquiring each spectrum, which is significantly shorter than the reversion time (Figure 3 bottom).4 Rapid spectral acquisition was achieved by recording a small number of points along the τ direction. We have tested earlier that the cross peaks involving CdO stretching mode fully dominated the 2DIR spectra for the particular experimental conditions used for the measurements.13 Therefore, even though the cross peaks did not converge into narrow peaks along the ωτ direction, they still represent the cross peak features among the CdO stretch and the modes in the fingerprint region.15 Sufficiently long interferograms have been measured along the t direction to ensure high spectral resolution along ωt, which is ca. 2 cm-1.

Photoisomerization in RuII Complex

Figure 3. 2DIR spectra of the relaxed (top panel) and irradiated (bottom) RuBzSO compounds. The linear spectra of the respective compounds are attached. The acquisition time for each spectrum was ca. 10 min. Despite poor resolution along ωτ direction (only 10 points were recorded along the τ direction) the observed peaks represent the cross peaks with the CdO stretching mode, as those completely dominate the spectrum.

Figure 4. (top) Projections of the 2DIR spectra shown in Figure 3 onto ωt axis. The linear absorption spectrum of the relaxed compound is also shown. (bottom) Difference 2DIR spectrum (blue) and difference linear spectrum (red) are shown.

The 2DIR spectra of both compounds have multiple peaks, many of which occur at similar frequencies. Note that neither the CdO stretching frequency nor its IR intensity change much upon irradiation (Table 1, Figure 2). The 2DIR spectra were then integrated along the ωτ direction and plotted as ωtprojection spectra in Figure 4, top panel. By subtracting the ωt-projection spectrum for the relaxed compound from that of the irradiated compound a cross-peak difference spectrum was constructed. Figure 4, bottom panel compares the cross-peak difference spectrum with the FTIR difference spectrum; the former was scaled so that the amplitude at 1100 cm-1 matches to that of the FTIR difference spectrum. The peaks in the cross-peak difference spectrum are expected to occur at similar frequencies as the peaks in the FTIR difference spectrum, although the amplitude of the peaks might

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16743 vary in the two difference spectra. Indeed, the peaks of the same sign are seen at 953, 1037, 1048, 1101, 1122, and 1144 cm-1 in both spectra. The largest peaks in the cross-peak difference spectrum occur at the frequencies of three modes that bear the largest SO stretching contributions, the peaks at 1048, 1101, and 1122 cm-1. The shift of the frequency of the local SO stretching motion to smaller values in the irradiated compound reduces substantially the IR intensity of these modes, which can be seen in both the FTIR and the cross-peak difference spectra. The most striking feature in the cross-peak difference spectrum is the absence of the peak at 1026 cm-1, where the SO stretching mode is seen in the linear spectrum of the irradiated compound. In other words, although a clear and strong SO stretching transition is observed in the irradiated compound in the linear spectrum, its cross peak with the CdO stretching mode is negligible (Figure 4, bottom). The anharmonic mode coupling, as well as the width of the SO stretching transition, affects the cross peak amplitude. The SO stretching mode in the irradiated compound is ca. 26% broader than that in the relaxed compound (24 vs 19 cm-1), which is expected to provide only ca. 26% smaller cross peak. The extinction coefficient of the CdO stretching mode in the irradiated compound is smaller than that for the relaxed compound, but only by ca. 10%. A small CdO/SO anharmonic coupling value is thus the main reason of the small CdO/SO cross peak in the irradiated compound. Two main mechanisms of anharmonic coupling are generally considered, electric (through space) and mechanical (through bond) couplings.22-25 The anharmonic DFT calculations via Gaussian,16 which would automatically include both contributions, are currently prohibitively lengthy for the RuBzSO compound because of its large size (60 atoms). Therefore, we estimate separately the electric and mechanical coupling contributions. 3.3. Electric CdO/SO Mode Coupling. The electric coupling was first calculated in the framework of the exciton coupling model.26 Transition dipole moments were calculated using the equation |µ|2 ) [(3pc)/(4π2k)]∫σ(k)dk, where k is the frequency of the mode, c is the speed of light, and σ(k) in the absorption cross-section related to the extinction coefficient, ε(k), through ∫σ(k)dk ) [(ln(10) · 103)/NA]∫ε(k)dk.27 Here NA is the Avogadro’s number and the integration is taken over a respective absorption band in the linear absorption spectrum. The transition-dipole-transition-dipole coupling energies of the CdO and SO stretching modes were determined for the S- and O- bonded states of RuBzSO using eq 1 and 2 and the DFT-optimized geometries obtained. Inspecting the angle between SO and CdO bonds we found that it changes from ca. 24.4° in the relaxed state to ca. 97.3° in the irradiated state, which leads to a substantial decrease of the transition-dipole-transition-dipole interaction of the two modes. Equations 1 and 2 were used for calculating the interaction energy (β12) and the values of offdiagonal anharmonicity (∆12)24

β12 )

b1b µ2) - 3(µ b1bi )(µ b2bi ) 1 (µ 4πε0 R312

∆12 ) -

2(∆11 + ∆22)β212 (E1 - E2)2

(1)

(2)

Here b µj is the transition dipole of the mode j, bis ı the unit vector along the line connecting the centers of the two dipoles, R12 is

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the distance between the dipoles taken as the distance between the centers of the CdO and SO bonds, ε0 is the permittivity of vacuum, ∆jj and Ej are the diagonal anharmonicity and the transition energy for mode j, respectively, expressed using the same units as β12. The directions of the CdO and SO transition dipoles were taken as the directions of the respective bonds. The transition dipole approach predicts a large difference in the computed interaction energies for the S- and O-bonded compounds. If R12 is taken as the distance between the centers of the CdO and SO bonds, 4.5 (S-bonded) and 3.4 Å (O-bonded), the calculated couplings, β12, are -13.3 and 1.7 cm-1, respectively, which results in small anharmonicities of ca. 0.046 and 6 × 10-4 cm-1, respectively. While there is a big difference in the anharmonicity values for the S- and O-bonded compounds, the value for the former is ca. 2-3 times smaller than the expected one based on the comparison of the cross peak amplitudes in the relaxed RuBzSO and oMSBA compounds. Note that the anharmonicity ∆CdO/SO computed for oMSBA using the DFT method is 0.34 cm-1. Certainly, neither the point dipole approximation nor the partial-charge approach is expected to work well in the conditions where the vibrating atoms of interacting groups essentially overlap by their electron clouds. Indeed, the equilibrium distance between the carbon atom of the carboxylate and the sulfur atom of the sulfoxide in the S- and O-bonded RuBzSO are 3.33 and 3.31 Å, which are ca. 0.17 and 0.19 Å smaller than the sum of the respective atomic van-der-Waals radii (r(C) ) 1.7 Å, r(S) ) 1.8 Å).28 If in eq 1 we take R12 as the distance of the closest nuclei separation between the atoms of the CdO and SO groups (3.33 and 3.31 Å) the anharmonic CdO/SO shift is 0.28 and 7 × 10-4 cm-1, for the S- and O-bonded structures, respectively. The electric contribution to the anharmonic shift computed for the S-bonded compound (0.28 cm-1) is approximately the total CdO/SO anharmonicity expected, suggesting that the electric contribution might be dominant, at least for the S-bonded compound. It has been shown that even for modes with very small transition dipoles, such as CtN and C-C, the contribution from the dipole moment nonlinearity (the electric coupling) is dominant despite very large mismatch of their frequencies (ca. 1370 cm-1).22 A substantial electric contribution has been reported recently for the coupling of the OH stretching mode and lower-frequency modes.29 Thus, the calculations suggest that the electric coupling is sizable in the S-bonded state and drops significantly in the O-bonded state in agreement with the experiment, thus supporting the assignment of the photoisomerized metastable state as the O-bonded state of RuBzSO. 3.4. Mechanical CdO/SO Mode Coupling. The mechanical coupling is difficult to calculate without methods of quantum chemistry, but several observations can be made based on comparison of the structures of the S- and O-bonded compounds. The orthogonality of the transition dipoles in the O-bonded state is not expected to affect the mechanical coupling strength. Interestingly, the angle between the carboxylate group and the phenyl ring is small in the S-bonded compound (ca. 26 degrees) but is much larger in the O-bonded compound (ca. 78 degrees). Thus, the mechanical coupling of the CdO stretching mode with the in-plane modes at the phenyl ring is expected to be smaller in the O-bonded complex due to their almost orthogonal orientation. Reduced coupling of the CdO stretch to the modes at the phenyl ring can cause a reduction of its overall mechanical coupling to the SO stretching mode. The contributions to the overall ∆CdO/SO anharmonicity can be separated into a contribution involving third- and fourth-order potential energy deriva-

Keating et al. TABLE 2: Electric Contributions to the CdO/SO Anharmonicity Computed for the Indicated Compounds As Well As the Angles between the Respective Bonds compounds RuBzSO, S-bonded RuBzSO, O-bonded

θCdO/SO/ degrees

θC-O/SO/ degrees

β(CdO/SO)a/ cm-1

24.4

80.2

-32.8

97.3

27.3

1.9

∆(CdO/SO)/ cm-1 0.28 0.0007

a The R12 distance was taken as the distance of closest nuclei separation between the CdO and SO group nuclei, 3.33 and 3.31 Å for the S- and O-bonded compounds, respectively.

tives over the normal modes involved, such as Φiii, Φiij, and Φiijj, where modes i and j are the CdO and SO stretching modes, and to a mixed-mode contribution involving third-order derivatives involving other modes as well, such as Φiik and Φijk, where the mode k is different from the CdO and SO stretching modes.30,31 It has been shown for a series of peptides that the mixed-mode contribution can vary in a wide range from being less than 4% to more than 95%.31 Electronic delocalization and bond conjugation occurring through the bonding of the d-orbitals of the sulfur atom and π-orbitals of the phenyl ring12,32-34 facilitates the coupling of the modes at the sulfoxide site and those at the phenyl ring. Near orthogonality of the carboxylate group to the phenyl ring in the O-bonded compound may significantly reduce the mixed-mode contribution to the CdO/ SO anharmonicity. Thus, the mechanical coupling can also contribute to a decrease of the overall CdO/SO coupling in the O-bonded state. Interestingly, the angle between the C-O single bond and SO bond is ca. 80 to ca. 27° for the S- and O-bonded complexes, respectively (Table 2). Therefore, the electric coupling among the C-O and SO stretching modes is expected to be stronger in the O-bonded state. Also, the electric coupling contribution to the C-O/SO anharmonicity is expected to be larger that that to the CdO/SO anharmonicity in both S- and O-bonded states due to a smaller gap between the C-O and SO frequencies. Thus the C-O/SO cross peak could be used in future experiments to follow the isomerization process and to further characterize the photoisomerized metastable state. 4. Conclusions Photoisomerization in the RuBzSO compound is investigated using linear IR absorption and dual-frequency 2DIR spectroscopies. Significant differences are found between the spectral changes induced by photoexcitation in the linear absorption spectrum and the 2DIR spectrum of the compound. The observed cross-peak pattern supports the formation of the O-bonded state in the RuBzSO complex upon photoexcitation, although further experiments and computational efforts are needed to further characterize the intermediate states and the reaction coordinate. Observation of the real-time, dynamic changes of the C-O/SO cross peak may permit characterizing the structure of the photoisomerization product state with higher certainty. Acknowledgment. Support from the National Science Foundation (CHE-0750415 and CHE-0936133 to I.V.R. and CHE0809669 to J.J.R.) is gratefully acknowledged. C.S.K. recognizes support from Louisiana BOR Fellowship and Tulane Provost Scholarship. B.A.M. recognizes NDSEG for a graduate fellowship. Computational support was partially provided by the Tulane Center for Computational Science.

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