SUPPLEMENTARY INFORMATION

Transcription

1 In the format provided by the authors and unedited. SUPPLEMENTARY INFORMATION DOI: 1.138/NCHEM.2793 Directing the path of light-induced electron transfer at a molecular fork using vibrational excitation Supporting Information Milan Delor, 1#* Stuart A. Archer, 1 Theo Keane, 1 Anthony J.H.M. Meijer, 1 Igor V. Sazanovich, 2 Gregory M. Greetham, 2 Michael Towrie, 2 and Julia A. Weinstein 1* 1. Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K. 2. Central Laser Facility, Research Complex at Harwell, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 QX, U.K. #. Current address: Department of Chemistry, University of California, Berkeley, CA9472, USA. Index. S1. Methods S2. Ultrafast transient absorption data (Supplementary Figures 1-3) S3. Computational Investigations (Supplementary Figure 4, Supplementary Tables 1 and 2) S4. Additional TRIR and IR-control data and analysis (Supplementary Figures 5-9) S5. Two-dimensional infrared spectroscopy data (Supplementary Figures 1 and 11). S6. Numerical modelling NATURE CHEMISTRY Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

2 S1. Methods The synthesis and characterization of 1* are reported in detail in ref 1. All ultrafast experiments were performed at the ULTRA facility 2 at the Rutherford Appleton Laboratories, UK. Two synchronized 1 khz titanium sapphire oscillator/regenerative amplifiers (Thales) pump a range of optical parametric amplifiers (TOPAS). For IR experiments, the instrument uses optical choppers modulating the repetition rate of the UV pump at 5 khz and IR pump at 2.5 khz while the probe pulse is at 1 khz, facilitating the simultaneous collection of background, {UV pump IR probe } (TRIR); {IR pump IR probe }; and {UV pump IR pump IR probe } spectra. The UV pump pulse (4 nm) was the second harmonic of an 8 nm, 5 fs, 3 cm -1 pulsewidth titanium sapphire laser; the IR pump pulsewidth was 1.5 ps, ~12 cm -1 ; the IR probe pulsewidth was ca. 1 fs, ~35-4 cm -1, depending on the detection region. The resolution was 2 cm -1 per pixel. For the data shown in the text, beam energies were approximately.1 μj/pulse,.5 μj/pulse and.8 μj/pulse with spot sizes of approximately 8, 12 and 16 μm for IR probe, IR pump and UV pump respectively. All experiments were performed well within the linear excitation regime (section S5). UV and IR pumps were in parallel polarization to each other, while the probe beam was set at magic angle with respect to these pumps. For ultrafast transient absorption experiments, the broadband supercontinuum probe pulse was generated using a ~2 µj portion of the 4 fs Ti:Sapph beam focused into a 2-mm thick calcium fluoride plate mounted on a mechanical raster system. No reference beam was used in these experiments as the shot-to-shot stability of the probe light was not the limiting factor in obtaining high signal-to-noise. After passing through the sample, the probe beam was collimated and focused into a spectrograph (.25 m f/4 DK24, Spectral Products). The probe light was detected on a shot-by-shot basis on a 512-element silicon array (Quantum Detectors, QD) which has 1-mm-high active area and.5-mm pitch and is cooled to a temperature of -2 C using a Peltier system. The 4 nm beam from the second harmonic of the Ti:Sapph laser was used for sample excitation. The probe and pump beam diameters in the sample were about 8 and 2 μm, for probe and pump respectively. The pump energy at the sample was.8 μj and the relative polarisation was set to magic angle. Samples were flown and rastered in Harrick cells with path lengths of 5-64 μm. The molecules are very photostable, and UV-Vis absorption and FTIR spectra were taken before and after every experiment to ensure no photo-decomposition affected the results. The collection of data was repeated through cycling of the experimental parameters. Optical densities at the excitation wavelength (4 nm, ε = 546 M -1 cm -1 ) were kept at.8. Global and target analyses of ultrafast spectroscopy data was performed using Glotaran. 3 Although measurable thanks to the very high sensitivity of our experimental setup, a significant challenge is posed by the small number of molecules that are both UV and IR excited in the probed volume (e.g..6-.7% for the data shown in Figure 3c), somewhat inhibiting precise quantification of the effect. This is mainly due to the very low extinction coefficients (23-28 M -1 cm -1 ) of the bridge vibrations in the D-A model studied; designing molecular systems with parallel pathways where vibrations coupled to the electronic process of interest have high IR absorption cross-sections could be the focus of future developments in this area. 2

3 S2. Ultrafast transient absorption data.5 ps MLCT 3 ns CSS Wavelength (nm) Supplementary Figure 1. Ultrafast transient absorption spectra of 1* in CH 2 Cl 2 following 4 nm, 5 fs excitation, clearly showing the decay of an MLCT-associated transient at 64 nm and grow-in of the PTZ-cation in the CSS at 516 nm, corresponding closely to previously-reported transient absorption spectra of Pt((CO 2 Et) 2 bpy)(c C-p-C 6 H 4 CH 2 (PTZ)) DAS (mod) 5.6 ps 3.6 ps 14.7 ps 3.7 ns SAS (mod) MLCT 3 MLCT* 3 MLCT CSS CSS grow-in on at least two timescales 5 6 Wavelength (nm) 5 6 Wavelength (nm) Supplementary Figure 2. Results of target analysis 7 of the ultrafast TA data shown in Supplementary Figure 1 using the model shown in Figure 1 of the main text. Global analysis of the data reveals a minimum of 4 exponentials is required to satisfactorily fit the data, with lifetimes.6 ps, 3.6 ps, 14.7 ps and 3.7 ns. The left panel shows the Decay Associated Spectra (DAS), which represent the amplitudes of each of the labelled 4 exponential decay components in the spectra. When fitting these exponential components to the compartmental model of Figure 1 using target analysis, one can extract the species associated spectra (SAS, right panel), i.e. the spectral characteristic of the populated excited states. The small oscillatory features around 52 nm in the SAS of the MLCT states suggests that the model could be improved by adding a hot CSS state, but we found that the current model still allows very good reproduction of the spectral evolution and is also significantly more constrained (especially for the purposes of modelling the IR-control effect) than if an additional state is added. 3

4 ΔAbs. (a.u.) Singly labelled Unlabelled Doubly labelled 516 nm 634 nm Delay (ps) Supplementary Figure 3. Kinetics traces from ultrafast TA data in CH 2 Cl 2 at 516 nm (PTZ-cation, characteristic of the CSS) and 634 nm (characteristic of the MLCT states) for singly labelled, doubly labelled and unlabelled versions of Pt((CO 2 Et) 2 bpy)(c C-p-C 6 H 4 CH 2 (PTZ)) 2. These kinetic traces, as well as a full global analysis of the dynamics of the ultrafast TA data for each complex, reveals that within instrumental error, there is no difference in the kinetic evolution of the three complexes. 4

5 S3. Computational Investigations To further characterize the excited states of Pt((CO 2 Et) 2 bpy)(c C-p-C 6 H 4 CH 2 (PTZ)) 2, Density Functional Theory (DFT) calculations were performed using the Gaussian 9 package, revision D.1. 8 These calculations were carried out using the B3LYP functional and a basis set consisting of SDD for Pt and 6-311G(d,p) for all other atoms. Dichloromethane solvent was included implicitly using the IEF-PCM method. Time-dependent DFT (TD-DFT) calculations were also performed. The results are summarized below. More substantial investigations are forthcoming. Supplementary Figure 4. Comparison of experimental and calculated electronic absorption spectra. The experimental absorption spectra of the three isotopomers of Pt((CO 2 Et) 2 bpy)(c C-p- C 6 H 4 CH 2 (PTZ)) 2 are shown in grey. Calculated absorption peaks are shown as sticks and convoluted spectra (Gaussian lineshapes, FWHM=3 cm -1 ). The raw calculated data are displayed in black alongside a spectrum empirically adjusted to best fit the experimental data, shown in red, that is shifted in energy by cm -1 and whose intensity is multiplied by

6 Supplementary Table 1: Summary of significant calculated transitions of Pt((CO 2 Et) 2 bpy)(c C-p- C 6 H 4 CH 2 (PTZ)) 2 as calculated by TD-B3LYP from first 1 transitions. All transitions with λ 4 nm are included, below which only transitions with oscillator strength.4 are documented. Orbital excitations contributing 1% of the character of each transition are documented. Significant frontier molecular orbitals are shown in Supplementary Table 2. Wavelength / Index Energy Osc.Strength Contributions 1% / cm -1 nm % HOMO LUMO % H-1 LUMO % H-2 LUMO % H-3 LUMO % H-4 LUMO % HOMO L % H-1 L % H-7 LUMO % H-2 L % HOMO L % H-1 L % H-3 L+1, 17.6% H-2 L % H-5 LUMO % H-6 LUMO % H-2 L+2, 17.2% H-3 L % H-8 LUMO % H-14 LUMO, 22.% H-12 LUMO, 14.8% H-16 LUMO, 11.8% H-11 LUMO % H-2 L+3, 27.9% H-17 LUMO % H-16 LUMO, 14.8% H-14 LUMO % H-17 LUMO, 31.7% H-2 L % H-1 L+1, 14.1% HOMO L+11, 11.% H- 1 L % H-3 L % H-2 L+12, 24.1% H-4 L+3, 19.2% H-2 L % H-2 L+4, 17.9% H-4 L+3, 16.7% H-2 L % H-2 L+5, 29.3% H-3 L+4, 1.3% H-2 L % H-2 L+5, 28.5% H-3 L+6, 2.8% H-3 L % H-2 L+7, 17.7% H-13 L % H-1 L % H-22 LUMO, 18.2% H-13 L+2, 11.% H- 11 L % H-3 L+7, 16.7% H-3 L % H-14 L+2, 18.5% H-12 L+2, 14.7% H-17 L % H-3 L+1, 13.9% H-2 L % H-1 L+2, 18.% H-6 L+3, 12.1% H-6 L+4 6

7 Supplementary Table 2. Significant frontier molecular orbitals, contributing to calculated transitions with λ 4 nm. Isovalue plotted =.2 e bohr. Details of method are supplied in Supplementary Section 3. L+2 H-3 L+1 H-4 LUMO H-5 HOMO H-6 H-1 H-7 H-2 H-8 7

8 S4. Additional TRIR and IR-control data and analysis (a) 12C/12C Unlabelled 2 1 ps 3.4 ns cm cm cm (b) 13C/13C Fully labelled ps 3.4 ns cm cm cm ps 4 (c) 12C/13C 1* ns cm cm cm cm Wavenumber (cm -1 ) Delay (ps) Supplementary Figure 5. TRIR spectra following 4 nm, 5 fs excitation of the unlabelled (top, OD 4 =.5), doubly labelled (middle, OD 4 =.5), and singly labelled (bottom OD 4 =.8) versions of Pt((CO 2 Et) 2 bpy)(c C-p-C 6 H 4 CH 2 (PTZ)) 2 in CH 2 Cl 2, along with corresponding kinetics at representative frequencies. The global kinetic fits use the above-quoted lifetimes of 3.6 ps, 14.7 ps and 3.7 ns as measured using target analysis of ultrafast TA data in Supplementary Figure 2 (minus the initial.6 ps component which is not resolved in our TRIR data), which fit the data very satisfactorily for all three complexes. 8

9 1 1 DAS (mod) 3.6 ps ( 3 MLCT*) 14.7 ps ( 3 MLCT) 3.7 ns (CSS) SAS (mod) 3.6 ps ( 3 MLCT*) 14.7 ps ( 3 MLCT) 3.7 ns (CSS) Supplementary Figure 6. Decay-associated and species-associated spectra (DAS and SAS) extracted from target analysis of fingerprint region TRIR spectra of 1* in CH 2 Cl 2 following 4 nm, 5 fs excitation, using the model and lifetime parameters derived from target analysis of the ultrafast TA data shown in Supplementary Figure 2 of 3.6 ps, 14.7 ps and 3.7 ns. These spectra are used to identify which features are associated with the MLCT and CSS, with the most unambiguous ones annotated in Supplementary Figure 8 to help in the analysis of IR-control experiments. kinetics at 1781 cm -1 (MLCT signature) 1 Unlabelled 25C Unlabelled -2C Monolabelled 25C Monolabelled -2C ΔAbs. (a.u.) Wavenumber (cm -1 ) Wavenumber (cm -1 ) 2 4 Delay (ps) Supplementary Figure 7. Kinetic traces at 1781 cm -1, a spectral signature of the MLCT state, for the unlabelled 12 C- 12 C compound 1 and monolabelled 12 C- 13 C 1*, at two different temperatures (-2 o C and 25 o C). The two pairs of traces for the two different complexes are offset for clarity. It is clear from this data that there is negligible difference in the charge separation kinetics at these two temperatures, which represent a 9% change in kt. This helps us deduce that the rate acceleration from 3 MLCT* compared to 3 MLCT is due to population of medium-frequency intramolecular modes that have energies between the thermally-accessible phonon bath (<3 cm -1 ) and the high-frequency ν(c C) stretch at 2 cm -1. 9

10 Pump ν( 13 C) Pump ν( 12 C) (a) ΔABs. (mod) 1 1 (b) ΔΔABs. (μod) MLCT features CSS features -1 3 MLCT features CSS features Wavenumber (cm -1 ) Wavenumber (cm -1 ) Supplementary Figure 8. Fingerprint TRIR and IR-control experiments on 1* in CH 2 Cl 2. These spectra accompany those shown in Figure 3 of the main text, and are used to identify signals associated with changes in the electronic behavior of the molecule introduced by IR excitation, without hindrance from vibrational signals. The highlighted traces are at 2 ps, 15 ps and 5 ps (red, blue and magenta respectively) with 3 MLCT*, 3 MLCT and CSS being the respective dominant states at these time delays. Using the largest signal at 1567 cm -1, unambiguously identified as a 3 MLCT signature, we find that the 3 MLCT* decay rate is accelerated by 14 ± 2% for both pump wavelengths when normalized for the number of IR-excited molecules (see Supplementary section 6). 1

11 1 Off-diagonal transient intensity Diagonal bleach intensity ΔΔAbs. (μod) UV - IR pump delay Supplementary Figure 9. Dependence of the diagonal bleach intensity (red) and off-diagonal (electronic) transient intensity (black) on the UV-IR pump delay when pumping ν( 13 C) in 3 MLCT*. The results in the main text are shown for a UV-IR pump delay of 2 ps, which is where the off-diagonal transient intensity is maximized. At 3 ps, the bleach signal becomes contaminated by excitation of ν( 13 C) in other states ( 3 MLCT and CSS) which are significantly populated by this time. 11

12 S5. Two-dimensional infrared spectroscopy data Supplementary Figure 1. This figure is an extended version of Figure 3 in the main text. It contains an additional panel, d. Summary of excited state dynamics with and without vibrational perturbation in CH 2 Cl 2, with the left panel showing results for ν( 13 C) excitation and the right panel for ν( 12 C) excitation. (a) FTIR of 1*. (b) TRIR spectra at 2 and 5 ps time delay following 4 nm excitation, exhibiting the frequencies of ν( 12 C) and ν( 13 C) in 3 MLCT* and CSS, respectively. (c) IRcontrol experiments (see main text for details). (d) T-2DIR experiments on the two CSSs, which confirm lack of coupling between ν( 12 C) and ν( 13 C) in the CSS. The IR pump is introduced 1 ps after the UV pump, when all population is in the CSS state. The spectra shown are recorded with IR probe at 1 ps, 3 ps and 5 ps after the IR pump. The bleach-transient peak pairs are signatures of de-population of the v = and population of the v = 1 state of the excited mode. Off-diagonal signals are not present. Diagonal population dynamics are given in Fig. S11. 12

13 -1-2 Ground State τ(13c) = 2.5 +/-.4 ps τ(12c) = 5.6 +/-.2 ps CSS τ(13c) = 1.4 +/-.3 ps τ(12c) = 4. +/-.2 ps MLCT* τ(13c) = 1. +/-.4 ps τ(12c) = 3.6 +/-.4 ps 2 4 Delay (ps) Supplementary Figure dimensional IR and T-2DIR kinetics for the diagonal (self-response) bleaches in the ground state (top), CSS (middle) and 3 MLCT* (bottom) for the ν( 13 C) and ν( 12 C) modes. The vibrational lifetimes are annotated in each graph. ν( 13 C) is consistently shorter-lived than ν( 12 C), indicating that an additional vibrational relaxation pathway through a resonant combination mode is present in all tested electronic states. Both ν( 13 C) and ν( 12 C) also possess shorter vibrational lifetimes in the excited states compared to the ground state, which may be due to a higher density of states or electronic and structural environments more favorable to intramolecular vibrational redistribution. 13

14 S6. Numerical modelling Modelling of the kinetics was performed using the model from Figure 1 in the main text, and assuming that we only pump vibrations in 3 MLCT* and that any rate acceleration observed is from that state, i.e. during the 3 MLCT* CSS decay. First the number of molecules excited by UV and IR pumps must be estimated. The number of UVexcited molecules is easily estimated from the ratio of the magnitudes of bleached vibrations in TRIR data to FTIR data. Using this method we find that 6.5% of molecules in the probed volume are UVexcited. This corresponds very closely to the value found using a Beer-Lambert approximation: = (1 1 ) = (1 1 ) Where f BL is the excitation yield using this model, N ph is the number of photons in the beam of area F, ε is the extinction coefficient at the excitation wavelength corresponding to absorbance A (OD) at the actual concentration and path length used. Using the beam and sample parameters cited in the Methods section, this yields 6.6%. Using this value, we can then estimate the extinction coefficient (or cross-section) of the pumped ν( 13 C) and ν( 12 C) vibrations in 3 MLCT*, yielding 28 M -1 cm -1 and 23 M -1 cm -1, respectively. Using again a Beer-Lambert approximation with the parameters quoted in the Methods section, we estimate that 11% and 9.2% of UV-excited molecules are IR-excited when pumping ν( 13 C) and ν( 12 C), respectively. This can be roughly verified with the diagonal vibrational response of ν( 12 C) the vibrational lifetime is long enough to allow us to compare the size of the v = 1 bleach following the IR pump to the transient in the TRIR data. This yields an estimate of 9.4% of IR-excited molecules, very close to the Beer-Lambert calculation. We therefore use the calculated parameters for the numerical modelling. We can also use Poisson statistics to estimate the probability of a molecule being excited by more than one photon, i.e. 9 ( =! Where k is the number of absorption occurrences (k = 1 for single excitation, k = 2 for double excitation etc.) and f BL is the excitation yield calculated using the Beer-Lambert model above, which is related to the ratios of the excitation and absorption cross-sections. We find that there is a.2% chance of two-photon excitation with the UV pump, and.6% chance of two-photon excitation with the IR pump, confirming that our experiments are performed well within the linear excitation regime. The increase in 3 MLCT* rate is measured using the fingerprint region data shown in Figure S8, whereby the bleach of the 3 MLCT* band at 1567 cm -1 in the IR-control data acts as an indicator free of any vibrational signals caused by the IR pump. The rate increase is measured by comparing the signal size of a particular band between the TRIR (y 1 ) and TRIR (IR pump ON) (y 2 ) spectra (the latter is obtained by adding the TRIR (IR pump ON-OFF) signal normalized for the number of IR-excited molecules to the TRIR spectra at each time delay). We then use the relationship = ln At all early time delays. The average value across all values of t is used to calculate k 2. Using this method for the spectra in Figure S8 and with the above-calculated normalization factors for the number of IR-excited molecules in each case, we find that the 3 MLCT* decay to the CSS is increased by 14 ± 2% for both pump wavelengths. 14

15 Optimization of the fit is done through a reduced-χ 2 analysis on the experimental data with only two parameters to vary: the equilibration rate between the two CSSs (kept fixed across all datasets for both pump wavelengths), and the relative rate increase of one CSS pathway over another. To fit the numerical model to the data, a precise normalization factor needs to be applied to account for the number of IR-excited molecules, as well as the relation between signal size and population. The former is calculated above. For the latter, we relate the integrated CSS band in the TRIR data to population (known from target analysis) at any given time delay. References 1. Archer, S. A., Keane, T., Delor, M., Meijer, A. J. H. M. & Weinstein, J. A. ( 13 C) or Not ( 13 C): Selective synthesis of Asymmetric Carbon-13-Labeled Platinum (II) cis-acetylides. Inorg. Chem., 55, (216). 2. Greetham, G. et al. ULTRA: A unique instrument for time-resolved spectroscopy. Appl. Spectrosc. 64, (21). 3. Snellenburg, J. J., Laptenok, S. P., Seger, R., Mullen, K. M. & van Stokkum, I. H. M. Glotaran : A Java -Based Graphical User Interface for the R Package TIMP. J. Stat. Softw. 49, 1 22 (212). 4. McGarrah, J. E. & Eisenberg, R. Dyads for photoinduced charge separation based on platinum diimine bis(acetylide) chromophores: synthesis, luminescence and transient absorption studies. Inorg. Chem. 42, (23). 5. McGarrah, J. E., Hupp, J. T. & Smirnov, S. N. Electron transfer in platinum(ii) diiminecentered triads: mechanistic insights from photoinduced transient displacement current measurements. J. Phys. Chem. A 113, (29). 6. Sazanovich, I. V. et al. Ultrafast photoinduced charge transport in Pt(II) donor acceptor assembly bearing naphthalimide electron acceptor and phenothiazine electron donor. Phys. Chem. Chem. Phys. 16, (214). 7. van Stokkum, I. H. M., Larsen, D. S. & van Grondelle, R. Global and target analysis of timeresolved spectra. Biochim. Biophys. Acta 1657, (24). 8. Frisch, M. J. et al. Gaussian 9, Revision A.2. (29). 9. Prokhorenko, V. I. et al. Coherent control of retinal isomerization in bacteriorhodopsin. Science 313, (26). 15