SUEMENTARY INFORMATION - Nature Research · Sub-50 fs photo-induced spin cross-over in [Fe(bpy) 3] 2+ Gerald Auböck and Majed Chergui * Ecole Polytechnique Fédérale de Lausanne,

NATURE CHEMISTRY | www.nature.com/naturechemistry 1

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.2305

Supplementary Information

Sub-50 fs photo-induced spin cross-over in [Fe(bpy)3]2+

Gerald Auböck and Majed Chergui*

Ecole Polytechnique Fédérale de Lausanne, Laboratoire de Spectroscopie Ultrarapide,Faculté des Sciences de Base, ISIC, CH-1015 Lausanne, Switzerland

[email protected]

Contents:S1. Experimental set-upS2. Measurement procedureS3. Data analysis and fits

S3.1 visible pump/UV-probe experimentsS3.2 Fits of the time tracesS3.3 visible pump/visible probe experiments

S4. Assignment of the vibrational modes

1

Sub-50-fs photoinduced spin crossover in [Fe(bpy)3]2+

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S1. Experimental set-up

The experimental setup used for the transient absorption measurements presented here is described in detail in Refs 1,2. Briefly, an amplified Ti:Al2O3 laser (KM labs Wyvern, 20 kHz repetition rate, 40 fs pulse duration, 750 mJ / pulse) is used to pump a commercial two-stage NOPA (Light Conversion TOPAS white). For probing in the UV spectral range, 1/3 of the light from the NOPA is sent to an achromatic frequency doubling3 via a delay line and then focused onto the sample with a 10 cm, 90° off-axis parabolic mirror. The remaining light from the NOPA is used to generate the pump beam after passing through a chopper locked to half the laser repetition rate. In the case of a visible pump beam, this light is attenuated and directly focused onto the sample with a 20 cm spherical mirror aligned slightly off-axis. For a UV pump, it passes a 200 µm-thick BBO frequency doubling crystal, thereafter the visible light is filtered out using two dichroic mirrors before sending the beam onto the same spherical mirror as in the case of a visible pump.Another fraction of the light from the Ti:Al2O3 amplifier passes a second delay line and is used to generate a white light (WL) probe by focusing it onto a 5 mm CaF2 plate using a lens (f=10 cm). The generated WL is collimated with a 5 cm, 90° off-axis parabolic mirror and sent onto the same parabolic mirror as used when probing the UV range.For all measurements presented here the relative polarizations of pump and probe beam were set to magic angle using an achromatic λ/2 plate in the probe beam path. After passing the sample the probe light (visible WL or broadband UV) is coupled into a 200 µmmultimode fibre and guided to a 0.5 m spectrograph. The dispersed spectrum is finally detected on a single shot basis using a CMOS linear array detector (Hamamatsu S11105).1

S2. Measurement procedureFor each measurement ~50 ml of [Fe(bpy)3]2+Cl- dissolved in water at a concentration of ~2 mM was prepared. All measurements were performed on a flat liquid jet produced from a 200 µm Sapphire nozzle. The typical probe beam size on the sample was ~30 µm for visible and ~40 µm for UV probe. The pump beam size was chosen much larger, ~110 µmfor 300 nm and 580 nm excitation. The pump pulse energy was ~0.1 µJ and ~0.3 µJ for 300 nm and 580 nm excitation, respectively.

To determine the instrumental response function (IRF) each measurement was accompanied by a measurement on a 200 µm-thick glass plate and the liquid jet operated with pure water under identical conditions to the measurement (see Supplementary Fig. 1). The duration of the IRF was then estimated by a fit of the transient absorption signal from the glass plate with a Gaussian function. As evident from Supplementary figure 1, this agrees well with the duration of the cross-phase modulation (CPM) signal in pure water within the uncertainty given by its irregular shape. The obtained values for the full width half maximum (FWHM) of the IRF, which are summarized in Supplementary table 1, are in excellent agreement with values obtained from the fit of the [Fe(bpy)3]2+ transient absorption data directly (see below).

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Figure 1: Signal measured on a 200 mm glass plate (black) and the 200 µm liquid jet with pure water (blue) for A: 580 nm pump, UV probe, B: 300 nm pump UV probe, C: 550 nm pump, visible white light probe. The red curves are fits of the glass plate signal with a Gaussian function.

Table 1: Duration of the instrument response function (IRF) determined as described in the text, corresponds to the FWHM value.

Experiment / fs

580 nm pump / UV probe 60±5300 nm pump / UV probe 58±5550 nm pump / vis probe 38±3

S3. Data analysis and fitsS3.1 Visible pump/UV-probe experiment:This section contains details on the data and their analysis for excitation at 580 nm and aprobe in the near UV. Figure 2b of the main article shows UV transient absorption (TA) spectra of aqueous [Fe(bpy)3]2+ at selected pump–probe delays. As discussed in the article and ref. 4, the transients are composed of a strong ground state bleach (GSB) signal just below ~305 nm and excited state absorption (ESA) from the HS state which is present already at the earliest accessible times (>305 nm) and peaks near 310 nm from 100 fs onwards. Indeed, at 50 fs time delay, despite the distortion due to CPM, the HS ESA is present somewhat red shifted and significantly broader than at later times, as a result of formation of a vibrationally hot HS state. This is also confirmed by the visible data (see below). Already by 100 fs, the transient resumes a shape that is basically the same as that




found at later times (except for small deviations due to wavepacket oscillations and CPM.A weak cooling kinetics is also observed.4

Figure 2: Time traces in the UV probe region of aqueous [Fe(bpy)3]2+ at selected probe wavelengths upon 580 nm excitation. The right column zooms into the early times.

Supplementary Figure 3 shows the separation of the transient absorption signal into a smooth kinetic (exponential, left) contribution and a coherent one (oscillations, right), as obtained from a spline interpolation of the transient absorption data. For the oscillatory part (right panel), only times >150 fs are considered, to avoid artefacts from CPM. It was Fourier transformed to give Fig. 2d of the main article.




Figure 3: Separation of the transient absorption data taken upon 580 nm excitation into a smooth kinetics contribution (left) and a coherent oscillation contribution (right).

S3.2 Fits of the time traces:Supplementary Figure 2 shows a set of time traces at different probe wavelengths in the region of the HS ESA (left), zooming into the early times (right). We fitted these traces with a model given by:

(S1)where

i) is a negative signal representing the GSB contribution:(S2)

Its amplitude for different probe wavelengths is fixed to a value corresponding to the static absorbance multiplied by the excitation yield estimated from the pump fluence. It isassumed to recover exponentially with a time constant τHS->LS = 650 ps, corresponding to the HS to LS relaxation time.5 G(t) is the instrument response function assumed to be Gaussian with a width (FWHM), and is identical to the one used to model the cross-phase modulation (see below)

ii) A positive ESA contribution:

(S3)with an amplitude that is a free parameter and a τHS->LS decay. In line with our previous report,4 we allow for bi-exponential kinetics (τC1 and τC2) that capture the cooling dynamics in the HS state. The latter contribution is superimposed with two damped cosine oscillations (frequencies ν1, ν2, damping times τD1, τD2) representing the vibrational wave-packets discussed above. We account for an eventual delayed onset of the HS population




(including the oscillation) ∆τHS, which is a free parameter of the fit. As will be seen later, the MLCT manifold is left on timescales much shorter than the Fe-N stretch vibrational period implying that the SCO process is a single curve crossing event and therefore the HS state is most probably populated impulsively. Contribution (ii) is convoluted with a Gaussian representing the instrument response function (IRF).Since the HS ESA and GSB contributions evolve on the same time scale ( ) and can consequently not be distinguished in the fit, we fixed the amplitudes of the GSBcontribution to values estimated from the excitation yield while those of the HS ESA contribution were adjusted. Additionally time-zero ( ) was a free fit parameter for each wavelength.

iii) Finally, we add a contribution limited to the duration of the IRF representing cross-phase modulation (CPM):

(S4)This function contains a Gaussian corresponding to the duration of the IRF and its first four derivatives. An eventual signal contribution on time-scales much shorter than the IRF(like excited state absorption from the MLCT states) would be absorbed in this modelling of the CPM.In the fits presented here, the initial phase of both oscillations was fixed to 0 (using a negative cosine function), and both, the delay in the onset of the HS state absorption (∆τHS)and time-zero, are free parameters for each individual trace. The fits are shown in figure 2cand Supplementary figure 2 and are in excellent agreement with the experimental traces. The obtained global and wavelength-dependent fit parameters are reported in Supplementary table 2 and Supplementary figure 4, respectively. Regarding the delayed onset, we also attempted alternative fits with an exponential rise of the quintet population but it resulted in rise times being much shorter than our time resolution.

Table 2: Global fit parameters for [Fe(bpy)3]2+ in water at 2 excitation wavelengths and probed in the near UV. Values marked with a star were fixed in the fit. are wavelength-dependent parameters for 580 nm excitation. The reported uncertainties correspond to one standard deviation as obtained from the least squares fitting procedure.

Excitation/nm / fs / ps / ps /ps

/cm-1

/cm-1

580 55±5 1.07±0.1 3.4±0.6 650* 127±10 157±5* Ref. 5




Figure 4: Wavelength-dependent fit parameters for aqueous [Fe(bpy)3]2+ excited at 580 nm.

The amplitudes of the exponential components form the decay associated spectra (DAS). As mentioned above, those of the GSB contribution where fixed allowing to extract the absorption profile of the HS state . and describe vibrational cooling in the HS state, namely a blue-shift and narrowing of the absorption band. Our cooling times of 1.1 and 3.4 ps are in good agreement with earlier work.4 Of course, these results do not necessarily imply a double exponential cooling, more likely the double exponential describes a non-exponential cooling behaviour. We actually also tried a stretchedexponential and the results are very satisfactory. However, whichever fit one uses has no incidence on the final outcome of the present study. Since most of the fits were done with biexponential functions, we have retained them in the paper.While one oscillation is sufficient to describe the data for wavelengths >308 nm, the bluer part requires two, which is in excellent agreement with Fig. 2 of the main article. Also the obtained frequencies agree well with the Fourier transform result and the de-phasing times are of comparable size as the latter cooling times. Finally the delayed onset of the HS absorption converges robustly to values around 50 fs for all wavelengths. This is, however, at the upper limit of our time-resolution.

S3.3 Visible pump/Visible probe experiment:Supplementary Figure 5 shows TA spectra in the visible region for excitation at 550 nm and selected pump probe delays. At the shortest delay time, the strong MLCT ESA, characteristic of the reduced bpy is clearly visible.5 It overlaps a GSB contribution. Within 100 fs, the transient spectrum turns to the inverted static absorption spectrum, indicating a




pure bleach signal. We also show the singlet MLCT fluorescence at time zero (inverted dashed trace) as measured by fluorescence up-conversion.5 At the redmost wing of the 30 fs transient, stimulated emission (SE) also contributes, yielding the first negative peak of the 567 nm kinetic trace in figure 3b of the main article.Supplementary Figure 6 shows a time trace integrated over the 450-550 nm wavelength range. It displays an oscillatory pattern with a weak beat pattern indicating that, similar to the UV, more than one vibrational mode is present in the time trace. This points to a weak HS ESA in the visible data.

Figure 5: (left) Transient spectra at selected pump-probe delays and (right) zoom into the >500 nm region. For comparison the inverted static absorption and fluorescence5,6 spectra are shown. The absorption spectrum is normalized to the amplitude of the transient spectrum at a long pump probe delay; the normalization of the fluorescence spectrum is arbitrary. Note the deviation of the red most wing of the 30 fs trace from the other traces, due to stimulated emission.

Figure 6: Time trace averaged over a wavelength range from 450 to 550 nm. The beat pattern points to more than one mode present.




Therefore the fit model used for the visible probe data is quite similar to the one used for the UV data (see above) except for some terms, which we describe hereafter. The signal is given by:

(S5)where the first term represents the ESA from the MLCT manifold of states and is given by

(S6)is the same as before. is the GSB contribution given by Eq. S2.

Finally is an ESA contribution of the vibrationally hot HS state (see discussion in the article and below) described by

(S7)Note that the HS state can only be seen in the visible probe range as long as it has sufficient vibrational energy (Figure 1 of the main article). Therefore corresponds to the vibrational relaxation time, shifting the “hot” HS ESA out of the probe window. In addition, the SE, which causes the first minimum in the 567 nm trace in fig. 3b of the main article, is included in the above equations via the term AMLCT (Negative values mean negative absorption, i.e. stimulated emission). The SE is also seen in Supplementary Fig. 5(right) where it causes the deviation of the 30 fs transient from the ones at later time in the red most part of the probe range.Finally, the cross-phase modulation signal was very small compared to the signal amplitude in these measurements and is thus not included in the fit. Small CPM contributions may of course affect . The results for the global and local fit parameters are reported in Supplementary table 3 and Supplementary figure 7, respectively.

Table 3: Global fit parameters for [Fe(bpy)3]2+ in water excited at 550 nm and probed in the visible. Quantities marked with a star were fixed in the fit. The reported uncertainties correspond to one standard deviation as obtained from the least squares fitting procedure.

/ fs / fs / fs / ps / cm-1 / cm-1 / fs / fs

40* 56±10 220±50 650* 127* 157* 400±100 400±100




Figure 7: Wavelength-dependent fit parameters for [Fe(bpy)3]2+ in water excited at 550 nm and probed in the visible. The black, dotted curve is the inverted static absorption spectrum scaled to the amplitude of the bleach contribution, the dark green, dotted curve the MLCT absorption spectrum of [Ru(bpy)3]2+ 7 red-shifted by 10 nm (the same shift as needed to overlap the ligand centred absorption bands of [Ru(bpy)3]2+ and [Fe(bpy)3]2+) and finally the light green dotted curve is the fluorescence spectrum of [Fe(bpy)3]2+.5

resembles the MLCT ESA with its characteristic absorption between 350 and 400 nm, typical of the reduced bpy.5,8 It overlaps the 1MLCT SE in the red-most part of the spectrum (compare also to Supplementary figure 5 and the time traces in figure 3b of the main article). The visible probe does not distinguish singlet from triplet MLCT states. We find a decay time of ~55 fs for this component, which corresponds to the time found for the onset of the HS ESA observed in the UV spectral range. This is much shorter than the vibrational period of the Fe-N stretch mode, implying that relaxation from the MLCT states is governed by the actual wave packet dynamics and is by no means exponential. The reported 55 fs thus provide only a rough estimate of the lifetime of the MLCT states.

describes the ground state bleach contribution recovering with .Regarding the occurrence of HS ESA in the visible data, for short pump-probe time delays vibrational energy (at least in Fe-N stretch modes, fig. 1 of the main article) allows accessing shorter Fe-N distances, which shifts to longer wavelengths the HS absorption,most likely to the 5MLCT state. We thus conclude that the 220 fs component together with the coherent oscillations are due to the ESA from the vibrationally ‘hot’ HS state, which shifts rapidly out of the probe region upon cooling and de-phasing, both of which lead to localization of the vibrational wave-function in the region around the equilibrium distance of the HS state. The 220 fs decay time is faster than the cooling times obtained from the UV probe data, and the somewhat quicker de-phasing times of the oscillations support the picture that the vibrationally hottest part of the HS population is observed. Note that due to quicker de-phasing the two oscillations are harder to distinguish




compared to the UV probe experiments and consequently the oscillation amplitudes ( ) and ) are strongly correlated and should be taken with caution. Fits with only one exponentially damped oscillation results in a frequency of ~130 cm-1 but fail to reproduce the time-evolution of the oscillatory envelope.Consistent with the experiments discussed above we also find a delay in the onset of this HS absorption of ~50 fs again confirming the correspondence between departure from the MLCT states and arrival in the HS state.

S4. Assignment of the vibrational modesWe now address the nature of the wave packets (excited state vs ground state ones) and their assignments.

- In the UV probe range, the oscillations are strong in the region of HS ESA but are weak or absent in the regions of ground state bleach (GSB). This speaks in favour of excited state wave packets.

- In the visible probe range, oscillations appear weakly, which could suggest that one (or more) may be on the ground state surface, as would be expected in an impulsive stimulated Raman scattering process. However, we would expect the oscillations to be more significant in the regions of GSB than in the ESA, as seen in other metal complexes.9 Here, this should have been the case particularly in the visible region of the MLCT transitions, but is not and as a matter of fact, in order to clearly unravel the oscillations in the visible range, we had to integrate the entire 450-550nm probe region to generate Supplementary fig. 6.

- The appearance of oscillations in the visible probe range is possible if a weak HS ESA is present as long as there is sufficient vibrational energy in the HS. This is reasonable if one looks at the PE surfaces in fig. 1.

- This ESA from the vibrationally hot corresponds to the weak component AHS in the analysis of the visible data (Supplementary Fig. 7). It also has the shape of an absorption by a vibrationally hot state.

- The damping time of a wave packet is not only due to its (intrinsic) dephasing time, but it also depends on the observation window due to the choice of the probe wavelengths. This is well documented in the early literature on wavepackets in small molecules, in the early days of femtosecond spectroscopy (see papers by A. H. Zewail, G. Gerber, N. Schwentner, A. Apkarian, etc.). These will need to match the difference potential between the lower and the higher state they reach. Therefore, damping times are probe wavelength-dependent, and they are shorter in the visible than in the UV, which makes sense, since they are observed when the HS state is hot, i.e. at the beginning of their relaxation towards to its minimum.

In summary, based on the above observations, we conclude that the wave packets occur on the HS surface.We now turn to the assignment of vibrational modes, which is not trivial due to the lack of studies (especially in the low frequency region of particular interest here) as well as conflicting assignments in the literature. By comparing a large set of Fe(II) SCO complexes,10 it was concluded that the Fe-N stretch modes are mostly concentrated in the




340-440 cm-1 region for the LS state and in the 210-250 cm-1 region for the HS. The modes we report in this study are at lower frequencies (127, 157 and 225 cm-1) and some studies have investigated this region, mainly for the case of the complex [Fe(phen)2(NCS)2]2+.Supplementary Table 4 compiles the values reported in the literature for the latter and for [Fe(bpy)3]2+. As can already be seen for the case of [Fe(phen)2(NCS)2]2+, assignments are conflicting between three reports. However, a number of modes occurring in the region of interest involve Fe-N bending or stretching modes. Of course, caution must be exerted in extending these frequencies to the case of [Fe(bpy)3]2+. Also, many of these modes arenon-totally symmetric ones, whenever their symmetry is known.For the wave packets we observe on the excited HS state:

- The 127 cm-1 mode may be attributed to an Fe-N bending mode (reported at 117cm-1 in ref. 11 and 132.7 cm-1 in ref. 12), a stretching mode (at 121.4 in ref.12) or a combination of stretching and bending modes (at 125 cm-1 in ref. 13).

- The 157 cm-1 mode may be attributed to bending modes reported between 149 and 159 cm-1 in refs 11-13.

- The 225 cm-1 mode is most likely the Fe-N stretching mode as reported in ref. 10.



Table S4: compilation of the low frequency vibrational mode of Fe(II) SCO complexes

Fe(phen)2(NCS)2, Ref. 14 Fe(phen)2(NCS)2, Ref. 11 Fe(phen)2(NCS)2, Ref. 13 Fe(bpy)3, Ref. 15 Fe(bpy)3, Ref. 12

LSExp (calc.)

HSExp. (calc.)

Assign. LS HS Assign. LSExp (calc.)

HS Assign. LS (sym.) Assign. LS (Sym.) HS Assign.

83 (106) 79 (75) 89 81 Ass. Fe-NbendPhen bend

86 (e) Bpy

118 (120) 96 (89) 121 99 Phen 118 NFeN 114(e)/128(a1) FeN bend154 (133) 99 (92) ligand 151 97 FeN breath. 156 94/95 FeN stretch 140(a1) FeN str. 144.6 (a1) 121.4 FeN str.163 (161) 102 ligand 107 Stretch 163 (e) FeN str.173 120 (118) 170 117 FeN bend 125 Str.+ bend 176 (a2) FeN

Bend+str.170.2 (e) 116.2 Str.

178 (176) 135 (127) 176 153 PhenFe-NCS bend

182 138 177 118 Ass. FeN6 bend

185 Str.+ bend 183 (a2) 132 Bend

189 (190) 154 (154) 184 149 Ass. Fe-NCS bend

183 (193) FeN str. 205.8 (e) 149.6/152.4

Bend

195 160 (160) 188 158 Phen. 156/159 Bend 198 (e) FeN216 (212) 163 (174) 204 164 Sym. FeNCS

bend198 (212) Bend+str.

219 185 (179) 212 131 Ass FeNCS bend

235 191 Phen 231.4(e) 195.1/198.6

Bend

235 240 Phen244 243 Phen246 182 Phen291 229 Fe-Nphen

stretch306 280 Fe-Nphen

stretch359 331 FeNCS stretch366 243 FeNCS stretch377 283 FeNCS stretch

13


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9 van der Veen, R. M., Cannizzo, A., van Mourik, F., Vlcek, A. & Chergui, M. Vibrational Relaxationand Intersystem Crossing of Binuclear Metal Complexes in Solution. J Am Chem Soc 133, 305-315, doi:Doi 10.1021/Ja106769w (2011).

10 Tuchagues, J. P., Bousseksou, A., Molnar, G., McGarvey, J. J. & Varret, F. The role of molecular vibrations in the spin crossover phenomenon. Spin Crossover in Transition Metal Compounds Iii235, 85-103, doi:Doi 10.1007/B95423 (2004).

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12 Sousa, C. et al. Ultrafast Deactivation Mechanism of the Excited Singlet in the Light-Induced Spin Crossover of [Fe(2,2 '-bipyridine)(3)](2+). Chem-Eur J 19, 17541-17551, doi:DOI 10.1002/chem.201302992 (2013).

13 Baranovic, G. & Babic, D. Vibrational study of the Fe(phen)(2)(NCS)(2) spin-crossover complex by density-functional calculations. Spectrochim Acta A 60, 1013-1025, doi:Doi 10.1016/S1386-1425(03)00333-0 (2004).

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SUEMENTARY INFORMATION - Nature Research · Sub-50 fs photo-induced spin cross-over in [Fe(bpy) 3] 2+ Gerald Auböck and Majed Chergui * Ecole Polytechnique Fédérale de Lausanne,