Vibrational energy transfer dynamics in ruthenium polypyridine transition metal complexes.

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Vibrational energy transfer dynamics in ruthenium polypyridine transition metal complexes Marina Fedoseeva,a Milan Delor,a Simon C. Parker,a Igor V. Sazanovich,b Michael Towrie,b Anthony W. Parkerb and Julia A. Weinstein*a Understanding the dynamics of the initial stages of vibrational energy transfer in transition metal complexes is a challenging fundamental question which is also of crucial importance for many applications, such as improving the performance of solar devices or photocatalysis. The present study investigates vibrational energy transport in the ground and the electronic excited state of Ru(4,4 0 -(COOEt)2-2,2-bpy)2(NCS)2, a close relative of the efficient ‘‘N3’’ dye used in dye-sensitized solar cells. Using the emerging technique of ultrafast two-dimensional infrared spectroscopy, we show that, similarly to other transition-metal complexes, the central Ru heavy atom acts as a ‘‘bottleneck’’ making the energy transfer from small ligands with high energy vibrational stretching frequencies less favorable and thereby affecting the efficiency of vibrational energy flow in the complex. Comparison of the vibrational relaxation times in the electronic ground and excited state of Ru(4,4 0 -(COOEt)2-2,2-bpy)2(NCS)2 shows that it is dramatically faster in the latter. We propose to explain this observation by the intramolecular electrostatic interactions between the thiocyanate group and partially oxidised Ru metal center, which increase the degree of vibrational coupling

Received 16th September 2014, Accepted 19th November 2014

between CN and Ru–N modes in the excited state thus reducing structural and thermodynamic barriers

DOI: 10.1039/c4cp04166f

behavior was earlier observed in another transition-metal complex, Re(4,40 -(COOEt)2-2,20 -bpy)(CO)3Cl, we

that slow down vibrational relaxation and energy transport in the electronic ground state. As a very similar suggest that this effect in vibrational energy dynamics might be common for transition-metal complexes

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with heavy central atoms.

Introduction Photoinduced processes in transition metal complexes have been extensively studied over the last five decades owing to their efficient absorption of visible light. One of the most studied are the photoactive metal complexes from the family of Ru(II) polypyridine complexes and in particular the [Ru(bpy)3]2+ dication and its derivatives (bpy = 2,2 0 -bipyridine). In this class of compounds, the lowest excited state is of a metal-to-ligand charge-transfer (MLCT, Ru - bpy) character, with the corresponding intense electronic transition occurring in the visible part of the spectrum. Accordingly, these compounds have been utilized in many applications which rely on light-induced charge transport initiated by the formation of an optically prepared MLCT state. Major applications have been in dye-sensitized solar cells, where they serve to inject electrons into the semiconductor,

a

Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK. E-mail: Julia.Weinstein@sheffield.ac.uk b Central Laser Facility, Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, STFC, Chilton, Oxfordshire, OX11 0QX, UK

1688 | Phys. Chem. Chem. Phys., 2015, 17, 1688--1696

as chromophores in light-harvesting assemblies for artificial photosynthesis, or photocatalysts in a variety of processes including reduction of CO2.1–4 The electronic and structural dynamics of the MLCT state plays a crucial role in the ensuing chemical processes, and have been studied in much detail.5–8 One of the current challenges lies in the fundamental understanding of photoinduced processes occurring in the course of, and immediately after, population of an MLCT state, including ultrafast electron transfer from non-thermalized excited states.7,9–12 In particular, one of the key questions related to the electron transfer process in transition-metal complexes concerns the role of specific vibrational modes in the associated energy transport and reaction pathways.13–18 Vibrational energy transport in molecules in condensed phase is known to take place through intramolecular vibrational energy transfer (VET):15,19–24 this process drives the propagation of vibrational energy involving vibrational modes at different spatial locations in the molecule in question. In general, the rate of vibrational energy transport depends on both the nature of the modes, such as their energy and intermode coupling, and on the rate and amplitude of the solvent fluctuations.15,25–27 Despite its obvious importance,

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Fig. 1

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Chemical structure of Ru(4,4 0 -(COOEt)2-2,2-bpy)2(NCS)2 (1).

studying the initial stages of the VET process, i.e. the relaxation of a single excited high-frequency mode under non-equilibrium conditions remains challenging: the timescales of the VET process range from sub-picoseconds to tens of picoseconds14,15,24,28,29 thereby requiring ultrafast spectroscopic methods. Partly due to this reason, while numerous structural,30–32 spectroscopic,32–34 spectroelectrochemical34–37 and computational38,39 studies have been performed on ruthenium polypyridine complexes, there have been only few attempts to analyze VET in such or similar transition-metal complexes.8,15,40,41 The present study investigates VET processes in the ground and electronic excited states of Ru(4,4 0 -(COOEt)2-2,2bpy)2(NCS)2 (1), Fig. 1, a close analog of one of the most efficient solar cell sensitizers, the so-called ‘‘N3’’-dye Ru(4,4 0 (COOH)2-2,2 0 -bpy)2(NCS)2.42 The only difference between the two compounds is that the anchoring –COOH group of the ‘‘N3’’ is protected by an ethyl group. The high-frequency CQO (of the carboxylic ester, –COOEt groups) and CN (of the NCS groups) stretching vibrations are excellent infrared reporters, enabling interrogation of vibrational dynamics by infrared vibrational spectroscopy.15,18,24,28,43,44 In 1, the ester groups are attached to the bipyridine units, serving as

an infrared reporter on the opposite side of the molecule to the CN groups. n(CQO) and n(CN) are separated by B300 cm1 in energy and by B10 Å in space providing an opportunity to excite them selectively, and at the same time to probe long-range VET across the molecule with a single broadband pulse. To initiate and follow VET processes in the electronic ground and excited states of 1 we employ two-dimensional infrared (2D IR) spectroscopy which has the ability to cross correlate bond specific features across the selected molecular framework on ultrafast timescales.45 This method has been used to study a number of fundamental phenomena including chemical exchange, intramolecular vibrational energy redistribution, hydrogen bonding, protein conformation changes, and spectral diffusion.23,46–50 2D IR allows the direct measurement of the population and relaxation dynamics of vibrational modes in molecules, providing insight on relaxation pathways, energy flow and coupling mechanisms over significant distances.15,29

Experimental Frequency-domain 2D IR spectroscopy: basic principles Frequency-domain 2D IR spectroscopy is a variant of the 2D IR method in a pump–probe geometry. To construct a ground state 2D spectrum, we use a tunable narrowband IR pump to selectively excite vibrations in the molecule, and a broadband IR probe to follow the changes induced by the IR pump (Fig. 2A top). Repeating this experiment at different pump–probe delays allows one to monitor the dynamics of vibrationally excited states. An additional UV/Vis pump pulse allows accessing the electronic excited state of the molecule, so that the 2D IR spectrum of this excited state at different time delays can be measured (Fig. 2A bottom). A detailed description of both fundamental and practical aspects of 2D IR techniques can be found elsewhere.14,45,51 As an example, Fig. 2B shows a simplistic scheme of the VET process taking place in the molecule upon the IR perturbation of the CQO modes (green contours): the energy transfer is

Fig. 2 (A) 2D IR pulse sequence in ground (top) and excited state (bottom) configurations. (B) Simplistic scheme of VET in complex 1 with the initiated energy flow upon IR pumping n(CQO). (C) The typical 2D map contains diagonal peaks of the excited modes (green) and cross-peaks (purple). The distances between the solid and dashed contours are diagonal and off-diagonal anharmonicities.


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initiated by the IR pump selectively exciting the CQO modes. These modes subsequently relax and trigger energy propagation throughout the molecule, reaching other modes, for instance, in the vicinity of the NCS groups. The resulting 2D spectrum (Fig. 2C) displays diagonal peaks (green contours), which correspond to the directly excited CQO modes, and the off-diagonal cross-peaks (purple contours) that represent a response of the CN modes to the excitation. The diagonal peaks, in general, correspond to the transitions that would be seen in conventional spectroscopic experiments, although with several important modifications.52 The presence of cross-peaks is the essential feature of 2D IR spectroscopy as it implies that the transitions represented on the diagonal (CQO and CN in our case) are sensing one another. Therefore, the cross-peaks bear valuable information for characterization of the interactions between different parts of the molecule.51 Solid contour lines show negative peaks, which generally correspond to a bleach (loss of ground state population) and stimulated emission, whereas dashed contour lines show positive peaks, which (in case of population of higher lying vibrational levels) represent excited state absorption. Dashed contours are shifted to lower frequencies due to vibrational anharmonicity. Generally, the off-diagonal anharmonicity is a direct indication of the coupling strength between two vibrations and is at the origin of the observed cross-peaks, if the two modes are directly coupled.45 At relatively large distances between excited and probed modes, such as in the case of this study (the spatial separation between CQO and CN groups is B10 Å), the coupling between these modes mostly originates from anharmonically coupled intermediate modes, which drive the energy flow in the molecule and result in the cross-peak response (the so-called ‘‘relaxation-assisted’’ coupling).15,29 The direct through-space coupling contributes to the cross-peak to a much lesser extent (if any) as it scales as 1/r6 with the distance between the modes.

Experimental setup The time-resolved IR (TRIR) and 2D IR two/three-pulse experiments were performed on the ULTRA system at the STFC Central Laser Facility, Rutherford Appleton Laboratories, described in detail elsewhere.53 The three-pulse experiments use optical choppers that modulate the repetition rate of the UV pump at 5 kHz and IR pump at 2.5 kHz while the probe pulse is at 10 kHz, facilitating the simultaneous collection of background, UV pump–IR probe (TRIR), IR pump–IR probe (ground state 2D IR) and UV pump–IR pump–IR probe (excited state 2D IR or IR pre-vibrationally-excited TRIR depending on the order of the two pumps) spectra. The tunable IR pump (B12 cm1 bandwidth) and probe (400 cm1 bandwidth) pulses used for 2D IR spectroscopy were generated by two optical parametric amplifiers pumped by synchronized Ti:sapphire based regenerative amplifiers with B2 ps and B50 fs pulse duration, respectively. The 400 nm pump beam was generated from the second harmonic of the femtosecond laser. The probe spectrum was recorded via a HgCdTe array detector and spectrometer combination with a spectral resolution of 2 cm1.


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Pulse energies for both UV and IR pumps will be specified for each experiment individually. Beam sizes were approximately 80, 100 and 150 mm for the IR probe, IR pump and UV pump beams, respectively. The pump–probe polarization relationships were set to magic angle to exclude the effects of molecular rotation in all 2-pulse experiments. In the 3-pulse experiments, UV and IR pumps were set parallel to each other and the probe beam was set at magic angle with respect to these pumps. UV-Vis absorption and Fourier transform IR (FTIR) spectra were taken before and after every experiment to control the stability of the sample. Optical densities at the excitation wavelength (400 nm) were approximately 0.5. Synthesis RuCl33H2O (0.156 g, 0.597 mmol) and 2,2 0 -bipyridine-4,4 0 diethylester (0.350 g, 1.166 mmol) were degassed 3 times under Ar, after that dimethylformamide (DMF, Grubbs, Ar saturated, 20 mL) was added via syringe. The resulting solution was then heated to 130 1C and stirred for 2.5 h. The reaction temperature was then lowered to 80 1C and NH4NCS (0.9984 g, 13.116 mmol) was added to the reaction mixture, which was left stirring for another hour. The DMF was then removed under the vacuum, whilst keeping the reaction mixture at 80 1C. Afterwards MeOH (50 ml) was added and the resulting solution stirred overnight. The solvent was then removed on a rotary evaporator, yielding a red-purple solid product. The product was dissolved in dichloromethane (DCM), filtered and the remaining solid washed with more DCM until the washings became colorless; this procedure removed excess NH4NCS, which is insoluble in DCM. Excess DCM was removed from the filtrate using a rotary evaporator to approx. 10 ml. This solution was purified using a silica packed column with 5 : 1 DCM : ethyl acetate mobile phase. 149 mg (0.182 mmol, 31% yield) of dark purple product was obtained. 1 H NMR (400 MHz, DMSO-d6) d 9.46 (d, J = 5.8 Hz, 2H), 9.24 (s, 2H), 9.07 (s, 2H), 8.48–8.35 (m, 2H), 7.81 (d, J = 5.9 Hz, 2H), 7.60 (dd, J = 5.9, 1.3 Hz, 2H), 4.53 (q, J = 7.0 Hz, 4H), 4.37 (q, J = 7.0 Hz, 4H), 1.45 (t, J = 7.1 Hz, 6H), 1.31 (t, J = 7.1 Hz, 6H). m/z TOF MS ES+: 819.1027.

Results and discussion FTIR and TRIR spectroscopy The spectra in Fig. 3 show the ground state IR absorbance (FTIR) and the excited state absorbance (TRIR) of 1 in the 1650 to 2150 cm1 region. The two peaks in the FTIR spectrum of n(CQO) and n(CN) are positioned at 1732 cm1 and 2102 cm1 respectively.35,54 For the n(CN), symmetric and antisymmetric combinations are energetically close to each other and are barely resolved in the ground state spectra.35 The multipeak deconvolution of this band with two Voigt functions results in peak positions at 2097 cm1 and 2108 cm1. The TRIR spectrum shown is taken at 500 ps time delay, to ensure the solute is vibrationally relaxed and fully solvated. The photophysics of 1 is expected to be similar to the Ru N3 dye. The latter was studied in detail:8 upon 400 nm excitation,


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–NC-bond order. A similar pattern at 2040 cm1 and 2080 cm1 appears in the CN stretching mode region of the TRIR spectrum of Ru N3 dye8,55 and in the IR spectrum of a spectroelectrochemically obtained cation of 1.35


Ground state 2D IR spectroscopy

Fig. 3 Ground state Fourier Transform Infrared spectrum (FTIR, top) and transient IR (TRIR, bottom) spectrum recorded at 500 ps time delay after 400 nm excitation of 1 in DCM.

the instantly populated excited state manifold relaxes to its lowest lying thermally equilibrated 3MLCT (Ru - bpy) state in less than 100 fs and remains in this state for 50 ns.7 Density functional theory (DFT) calculations for 1 showed that the transition designated as an MLCT to some extent involves the –NCS-localized orbitals in the HOMO.35 Within the 1650–2150 cm1 window, the excited state absorption of 1 consists of two distinct bands in the 2000– 2100 cm1 region, and a band at B1690 cm1. The former are symmetric and antisymmetric combinations of the n(CN) of the two NCS ligands, which, in contrast to the ground state, are well resolved in the excited state due to a partial reduction in

Fig. 4 shows the ground state 2D IR maps obtained by scanning the pump frequency across the range from 1670 cm1 to 2170 cm1, covering the stretch frequencies of the main IR reporters, n(CQO) and n(CN), and plotting the probe frequency along the x-axis for a particular time delay (2, 5, 10, and 20 ps time delays are shown). Some basic qualitative information may be obtained by visual inspection of these maps. The map at 2 ps shows the diagonal peaks of n(CQO) and n(CN) centered at 1732 cm1 and 2102 cm1 pump–probe frequencies, respectively. The peaks associated with n(CQO) are only observable up to 20 ps time delay signifying that intramolecular vibrational redistribution (IVR) of n(CQO) occurs on shorter time scale, in agreement with our previous observations of rapid IVR of n(CQO) in Re(4,40 COOEt-2,20 -bpy)(CO)3Cl (abbreviated ReCOe).40 Here, IVR is defined as the relaxation of high-frequency modes to surrounding anharmonically coupled intramolecular lower-frequency modes. The maps also display the n(CN) cross-peak upon pumping n(CQO) on a timescale of 5–20 ps delay after the pump pulse, which reaches its maximum intensity at 10 ps delay. The presence of this cross-peak supports the fact that the excitation energy on n(CQO) reaches the lower-frequency modes anharmonically coupled to n(CN) on a time scale of B10 ps. We note

Fig. 4 Ground state 2D IR maps for 1 in DCM at 2 ps, 5 ps, 10 ps and 20 ps time delays between IR pump and IR probe. The colors in the main map area correspond to 3 mOD (red) and 3 mOD (blue). The colors of the top right insets of the maps correspond to 30 mOD (red) and 30 mOD (blue).



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Fig. 5 1D slices across the ground state 2D IR spectra of 1 in DCM at (A) 1732 cm1 pump frequency (pump intensity 200 nJ), reflecting CQO diagonal- and CN cross-peaks, and (B) 2102 cm1 pump frequency (pump intensity 900 nJ), reflecting CN diagonal- and CQO cross-peaks. All the signals on (A) and (B) are normalized by the maximum intensity of n(CQO) and n(CN) diagonal peaks, respectively, and corrected for the ratio of peak intensities in the FTIR spectra (Fig. 3, top). Each peak is accompanied by the inset at representative time delays. The inset graph intensities of the crosspeaks are zoomed by 10 (A) and 100 (B) for better visualization.

that direct energy transfer leading to population of the n(CN) cannot be the case here as the energy of the n(CN) vibration is considerably (B300 cm1) higher than n(CQO). Fig. 5 shows 1D slices across the ground state 2D IR maps from Fig. 4 at specific pump frequencies that correspond to the diagonal and cross-peaks of the n(CQO) and n(CN) modes. The representation of the n(CQO) and n(CN) diagonal and cross peak dynamics in Fig. 5 is helpful in extracting anharmonicity values, which in turn bear valuable information on the role of intermode coupling in the VET process. Notably, the extraction of anharmonicity values depends on the relationship

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between the anharmonic shift and the bandwidth of the transition: if the anharmonic shift is larger than the bandwidth of the transition and the peaks are well separated, the anharmonicity values can be directly obtained from the 2D IR spectra; otherwise, the 0 - 1 and 1 - 2 transitions overlap and partially cancel, so that the anharmonic shift has to be determined by lineshape deconvolution of the peaks.45 Fig. 6 shows anharmonicity values obtained by deconvolution and fit of the diagonal and cross-peak contributions to n(CQO) and n(CN) modes at different time delays. Fig. 6A shows the evolution of anharmonicity values for n(CQO): when these modes are excited, there is a clear decrease in its anharmonicity values from early to late time delays. This behavior is typical for vibrations undergoing rapid IVR to nearby anharmonically coupled low-frequency modes, which gives rise to a shift from diagonal to off-diagonal anharmonicity.14 The evolution of the n(CQO) diagonal contribution is anticipated to be exponential, however, in this case it cannot be resolved due to limitations of the spectral resolution. The change in anharmonicities is clearly different for n(CN) where at all time delays the anharmonicity values are of v = 1 excited state character (Fig. 6B). This indicates that the IVR of n(CN) is much slower than that of the low-frequency modes anharmonically coupled to it. In order to evaluate the dynamics of the diagonal and crosspeak signals from n(CQO) and n(CN) observed upon pumping at 1732 cm1 and 2102 cm1, respectively, the integrated peak intensities of the corresponding signals were plotted as a function of IR(pump)–IR(probe) time delay (Fig. 7). Kinetic data presented in Fig. 7 can be satisfactorily modeled with a biexponential function; the best fit parameters are given in Table 1. QO) diagonal peak dynamics. The short time constant n(CQ t1 = 2.2 ps of the n(CQO) diagonal signal can be attributed to ultrafast IVR to spatially close low-frequency modes.40 The energy transport through the low-frequency modes, which in turn undergo IVR, is expected to reach modes that are coupled to n(CN). This energy propagation pathway is manifested by the appearance of a n(CN) cross-peak which, due to multiple vibrational modes involved in the energy transport between n(CQO) and n(CN), appears with a slightly delayed response, with a rise time of 4.2 ps. The 12 ps time constant for the n(CQO) mode is assigned to the cooling to the solvent, which is a typical timescale for this process.13,40

Fig. 6 Anharmonic shifts as a function of time delay between IR pump and IR probe for the diagonal and cross-peak contributions for (A) CQO modes and (B) CN modes. The shift values were obtained by deconvolving and fitting the 1D spectra from Fig. 5 by Voigt function. The error of obtained values is B0.5 cm1 from the fit; the experimental spectral resolution is 2 cm1.



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Fig. 7 Ground state 2D IR time profiles of 1 in DCM plotted as integrated peak intensities of transient CQO and CN diagonal- and cross-peaks at different time delays. The profiles were fitted with the convolution of Gaussian (IRF B 1.5 ps) and biexponential functions. The results of the fits are presented in Table 1.

Table 1 The lifetimes and the corresponding relative amplitudes (in parentheses) obtained from the fit of the ground state 2D IR time profiles presented in Fig. 7. The lifetimes corresponding to a signal rise are indicated accordingly

Probed mode Pumped mode k n(CQO)

n(CN)

n(CQO)

t1 = 2.2 0.2 ps (99%) t2 = 12 3 ps (1%)

t1 = 4.2 0.7 ps (52%) rise t2 = 16 1 ps (48%)

n(CN)

t1 = 20 0 ps (41%) rise t1 = 1.9 0.5 ps (3%) t2 = 60 1 ps (97%) t2 = 42 0 ps (59%)

n(CN) diagonal peak dynamics. The n(CN) diagonal peak decays with 1.9 ps and 60 ps time constants (Table 1). These lifetimes are similar to the ones measured by Hunt and co-workers in (m-propanedithiolate)Fe2(CO)4(CN)22, where the CN group is attached to the Fe metal centre.24 In their report, the long component is assigned to the relaxation time of the CN stretching vibration, whereas the fast component represents equilibration of population occurring between the vibrationally excited CN mode and a group of modes in its vicinity. In our case, however, n(CN) is more isolated from the rest of the modes, both spatially and energetically. In general, the NCS ligand can be represented as a superposition of two resonant structures, Ru:NRC–S::: and Ru::NQCQS::, where the former was shown to dominate in the ground state configuration and the latter in the excited state.8 In these structures, the CQS stretch was reported to oscillate at 809 cm1,54 C–S – at 748 cm1,56 whereas the bridging Ru–N stretch between the metal center and the NCS ligand – at about 364 cm1.54 Therefore 1.9 ps in the n(CN) diagonal peak time profile is rather too short to be attributed to the equilibration within the NCS ligand due to significant difference in the intraligand frequencies. We assign the 1.9 ps component in the n(CN) diagonal peak to the vibrational population equilibration between the symmetric and antisymmetric n(CN) combinations taking place prior to IVR from the excited CN modes. Such assignment is consistent with the small magnitude of the n(CN) diagonal peak signal associated with the 1.9 ps time constant (3%, Table 1), as the almost complete spectral overlap of the n(CN) symmetric and antisymmetric modes would lead to their signals practically cancelling one another.


The 60 ps time constant of the n(CN) diagonal peak dynamics can be attributed to the IVR to low-frequency modes such as those aforementioned. As n(CN) is the only high-frequency mode amongst those in the NCS ligand and the surrounding solvent, it is anticipated that the intramolecular relaxation takes place through IVR as the first step in this relaxation. The intramolecular relaxation is favored over intermolecular pathways, i.e. cooling, as there are no resonant bands in the solvent.40 Such abnormally long IVR can be explained by the presence of the central Ru atom in the complex. The modes surrounding the Ru atom have low vibrational energies, due to its large mass, leading to low transition probability between the high-frequency CN modes and the low-frequency modes attached to the Ru atom. Therefore, the latter acts as a ‘‘bottleneck’’ in energy transport dramatically slowing down the IVR of the CN modes. We observed earlier a similar phenomenon for the ReCOe complex:40 it was shown that the presence of the Re heavy atom was significantly affecting VET rates. In the report of Rubtsov and co-workers,15 the presence of a Fe metal center in the tetraethylammonium bis(maleonitriledithiolate)iron(III) nitrosyl complex was similarly shown to ‘‘isolate’’ the NQO vibration from the rest of the molecule, indicating an absence of effective relaxation pathways for the NO group as the NO mode relaxation process was found to be the slowest in this compound. The similarity of these observations suggests that heavy atoms in transition metal complexes isolate some modes from the rest of the molecule (as CN in the case of the present study), and largely dictate energy transport rates within the complexes. The findings reported above, although consistent with the energy transport observations for the other complexes, are nevertheless surprising for the particular case of 1. On the one hand, the slow IVR from the excited CN mode unambiguously confirms that the energy transport takes place in the direction of the Ru atom that affects further energy propagation within the complex. On the other hand, it is unclear why the excitation of the CN mode does not initiate energy propagation to the spatially close CS mode. As mentioned above, the CS mode oscillates at around 800 cm1, and should bear some transition probability between the CN and the CS modes. Unfortunately, the spectral window of our experiment does not allow monitoring vibrational frequencies as low as 800 cm1,54,56 so no experimental evidence in favor or against of the interaction between the CN and the CS modes can be provided.


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n(CN) cross-peak dynamics. The dynamics of the n(CN) cross-peak is relatively straightforward. As mentioned above, the 4.2 ps rise time of this peak is the time required to transport energy from the initially excited n(CQO) through the sequence of low-frequency modes to eventually reach those modes that are coupled to n(CN). The decay time t2 of the cross-peak signal (16 ps) can be attributed to energy dissipation to the solvent thus regaining thermal equilibrium in the system. QO) cross-peak dynamics. The lifetime of the CN modes is n(CQ as long as 60 ps and is thus the rate-determining step in the vibrational dynamics of the complex. Therefore, we anticipate that it should affect the measured CQO cross-peak dynamics. The observed 15 ps arrival time (the time, at which the intensity of the peak reaches its maximum) of the n(CQO) cross-peak is not consistent with a 60 ps decay of the CN modes, which is the primary step in the intramolecular relaxation. Consequently, in order to obtain the absolute rates of energy transfer and cooling in the CN - CQO direction, further analysis of the experimental data is required. The n(CQO) cross-peak already has some signal immediately after n(CN) excitation. This initial signal is within our instrument response and indicates the presence of a direct through-space coupling between these modes. It was discussed above that such coupling should be considered negligible in molecules where the distance between the modes is as large as 10 Å, whereby coupling occurs primarily via an energy transfer cascade through spatially mediating anharmonically coupled modes (the ‘‘relaxation-assisted’’ coupling). At first sight, the initial rise in n(CQO) cross-peak shows just the opposite as the direct coupling contributes to B80% of the total cross-peak intensity. However, this is not intrinsically true. The ‘‘relaxation-assisted’’ cross-peak looks small because the decay of the CQO cross-peak through vibrational cooling is faster than its population through CN IVR and subsequent energy transfer, so the CQO cross-peak is essentially a dark state. We found that the energy transfer in the CN - CQO direction can be satisfactorily described using a simple sequential rate model:

(1)

Fig. 8 CQO cross-peak dynamics from Fig. 7 (blue markers), sequential rate model (red solid line) and rise-decay profile accounting for ‘‘relaxation-assisted’’ coupling only (green dashed line).

The solution of (2) for the hot CO modes (CQO cross-peak) was found to reproduce the data with kIVR, ktransfer, and kcooling rates corresponding to 60 ps, 3 ps and 10 ps lifetimes, respectively, and is presented in Fig. 8 (red line). The obtained lifetimes are the absolute rates of the energy transport in the CN - CQO direction: 3 ps rise time can be assigned to the energy transfer from the CN to the vicinity of CQO modes, which is close to the 4.2 ps energy transfer in the CQO to CN direction (Table 1 and discussion above). The 10 ps decay time is attributed to cooling to the solvent. Model (1) accounts for both direct and ‘‘relaxation-assisted’’ coupling and yields the relative coupling magnitudes of 0.4 and 1.8, respectively. The rise-decay profile that assumes only ‘‘relaxation-assisted’’ coupling is presented in Fig. 8 (green line). It can be seen that although the total coupling magnitude is dominated by ‘‘relaxation-assisted’’ coupling, the direct coupling contribution is as large as 20%. We note that this model does not account for cooling of the hot CN(h) modes or any low-frequency modes in between CN and CQO; this thus assumes that the excited modes favor intramolecular energy equilibration prior to cooling to the solvent. However, due to the complexity of the energy transfer pathways through a large number of vibrational modes (including overtones and combination bands), the possibility of energy redistribution through other relaxation pathways, i.e. those involving cooling or resonant energy transfer, cannot be excluded. Ground state vs. excited state energy transport

where CN(1) is the initial population of the CN modes, CN(h) – population of hot modes in spatial proximity to CN with the kIVR rate, CO(h) – population of hot modes in spatial proximity to the probed CO modes with the energy transfer ktransfer rate, and solvent is a bath of solvent low-frequency modes, to which the hot CO modes dump energy with the kcooling rate. In this case, the CO(h) population can be derived from the set of differential equations: dCNð1Þ ¼ kIVR CNð1Þ dt dCNðhÞ ¼ kIVR CNð1Þ ktransfer CNðhÞ dt dCOðhÞ ¼ ktransfer CNðhÞ kcooling COðhÞ dt


(2)

In order to investigate energy transport in the MLCT excited state, excited state 2D IR studies were performed by introducing a UV-pulse (400 nm, B50 fs) to populate the MLCT state, which is left to relax on a 500 ps timescale. The 2D IR experiment is then performed in the same way as for the ground state, on a fully relaxed and solvated 3MLCT state. The transient 2D IR signals were very small and the measured profiles were, in most cases, instrument limited with the only detectable signal from the n(CN) diagonal peak, which was expected to have the longest lifetime. Fig. 9 shows the comparison between temporal evolutions of the n(CN) diagonal peak in the ground and the excited state in DCM (Fig. 9). Fig. 9 demonstrates that the relaxation of n(CN) in DCM in the electronic excited state of 1 is significantly faster (time constants


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Fig. 9 Time profiles of 1 plotted as integrated peak intensity of n(CN) diagonal peak in the ground state (from Fig. 7) and in the excited state in DCM. The profiles were fitted with the convolution of Gaussian (IRF B 1.5 ps) and biexponential function.

0.9 ps and 7 ps) compared to the ground state (1.9 ps and 60 ps, Table 1). We have previously observed a similar effect in ReCOe, where the dynamics of CRO modes in the excited state was found to be B8 times faster than in the ground state.40 In ReCOe, such dramatic difference was explained in terms of intramolecular electrostatic interactions between the CRO and partially oxidized metal center in the MLCT state, that increases the degree of vibrational coupling between CO and Re–C vibrations thus overcoming structural and thermodynamic barriers that slow down vibrational relaxation and energy transport in the ground state.40 We suggest that a similar interaction between the thiocyanate group and the Ru metal center could be responsible for the acceleration of the excited state dynamics of the n(CN) mode in 1. The faster IVR of the n(CN) in the excited state can be also explained in terms of configurational changes in the NCS ligand. It was mentioned above that the NCS ligand can be viewed as a superposition of Ru:NRC–S:::(1) and Ru::N(1)QCQS:: structures, which are dominating in the ground and excited states, respectively.8 Upon excitation, the bond order between N and C atoms decreases, and the Ru–N bond shortens,57 which leads to a lower vibrational frequency of the CN stretch (as observed in the TRIR spectra, Fig. 3) and to a higher vibrational frequency of the Ru–N stretch, thereby bringing these two vibrations closer in energy. At the same time, the redistribution of electron density leads to the larger dipole moment in the excited state compared to the one in the ground state. These changes make the IVR process more favorable in the excited state than in the ground state, which can account, at least to some extent, for the shorter lifetime of the n(CN) mode in the excited state.

Conclusions Ultrafast two-dimensional IR spectroscopy has been applied to investigate vibrational dynamics in the electronic ground and excited states of an archetypal ruthenium polypyridine complex, Ru(4,4 0 -(COOEt)2-2,2-bpy)2(NCS)2, that may be considered as a model charge-transfer transition metal chromophore. Investigation of the rates of intramolecular vibrational redistribution (IVR) and vibrational energy transfer (VET) showed that in the electronic ground state the central heavy atom plays an isolating role in


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the vibrational dynamics: IVR from the Ru-bound CN stretch mode is remarkably slow (t = 60 ps) compared to the esterlocalized CQO (t = 2 ps). This is rationalized by the fact that the relaxation of vibrational energy contained in the highfrequency CN mode has to occur through low-frequency modes involving movement of the heavy Ru atom. As a similar phenomenon was observed in other transition metal complexes, it supports the notion that the presence of heavy atoms in molecules may commonly affect the efficiency, and potentially the direction, of vibrational energy flow. However, this study also demonstrates that IVR of the CN stretching mode in the charge-transfer electronic excited state of Ru(4,4 0 -(COOEt)2-2,2-bpy)2(NCS)2 is dramatically faster (t = 7 ps) than in its ground state. This phenomenon can be accounted for by the increased degree of vibrational coupling through intramolecular electrostatic interactions between the thiocyanate group and partially oxidised Ru metal center, which reduces structural and thermodynamic barriers that slow down vibrational relaxation and energy transport in the ground state. A similar effect has been recently observed by us in an analogous Re(I) complex. Thus the alleviation of the insulating effect of the heavy metal center on energy flow in the charge transfer excited state may be a general phenomenon for transition metal complexes. Understanding vibrational energy propagation pathways during and following electron transfer can provide new insights on energy flow and coupling mechanisms occurring in charge transfer molecules following light absorption, which ultimately determine photoreactivity.

Acknowledgements We thank the STFC, the EPSRC, the White Rose consortium, and the University of Sheffield for support. MF is grateful to the Swiss National Science Foundation for personal fellowship (Early Postdoc. Mobility grant P2GEP2_151832).

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