Unveiling ultrafast dynamics in bridged bimetallic complexes using optical and X-ray transient absorption spectroscopies

In photosynthetic systems employing multiple transition metal centers, the properties of charge-transfer states are tuned by the coupling between metal centers. Here, we use ultrafast optical and X-ray spectroscopies to elucidate the effects of metal–metal interactions in a bimetallic tetrapyridophenazine-bridged Os(ii)/Cu(i) complex. Despite having an appropriate driving force for Os-to-Cu hole transfer in the Os(ii) moiety excited state, no such charge transfer was observed. However, excited-state coupling between the metal centers is present, evidenced by variations in the Os MLCT lifetime depending on the identity of the opposite metal center. This coupling results in concerted coherent vibrations appearing in the relaxation kinetics of the MLCT states for both Cu and Os centers. These vibrations are dominated by metal–ligand contraction at the Cu/Os centers, which are in-phase and linked through the conjugated bridging ligand. This study shows how vibronic coupling between transition metal centers affects the ultrafast dynamics in bridged, multi-metallic systems from the earliest times after photoexcitation to excited-state decay, presenting avenues for tuning charge-transfer states through judicious choice of metal/ligand groups.


CuOs XTA -Long-Time Kinetic Traces
In order to determine the Cu and Os excited-state lifetimes in CuOs, kinetics traces were collected with 3 ps step size out to 150 ps time delay. Kinetics are shown in Figure S14. The Cu K-edge kinetics were fit with an exponential convoluted with a Gaussian, along with an additional Gaussian function (to model the sub-ps kinetics) and a shelf. A 49 ps lifetime is obtained for the excited-state decay, with a long-lived shelf. For Os, a bi-exponential convoluted Gaussian function is used; the first time constant models the <2 ps components shown in Figure 5a, while the second has a lifetime >>150 ps time range used in the measurement. This is consistent with the OTA measurements that expected a ~400 ps excited-state decay for CuOs at both 500 nm and 625 nm excitation.
CuOs XTA -500 nm vs. 625 nm Excitation XTA spectra of CuOs were measured with two excitation wavelengths: 500 nm, where a mixture of Cu and Os MLCT are excited, and 625 nm, where predominantly Os MLCT is excited. The XTA spectra at the Cu Kand Os L III -edges are shown in Figure S13. The same general difference signal observed with 500 nm excitation is observed here. Although absorption at 625 nm mostly originates from Os MLCT absorption, a small tail from the Cu MLCT band allows direct excitation of the Cu MLCT state. Kinetics traces were measured following 625 nm excitation, using the same incident energies used in Figure 5a. The kinetics at the two excitation wavelengths are plotted together in Figure S18. The Os kinetics at 625 nm overlays quite well with the 500 nm kinetics trace, showing that the MLCT dynamics are consistent between the two pump wavelengths. It is more difficult to judge the Cu MLCT kinetics because those obtained with the 625 nm excitation because the difference signal is much weaker, but the rise and decay of the kinetics appears consistent with the 500 nm excitation.

XTA Experimental Time Resolution
The time resolution of the XTA measurement, similar to optical measurements, is dependent on both temporal width of both the X-ray and optical laser pulses. However, due to the non-stochastic nature of the generation of the X-ray pulses, a timing tool is used to correct the arrival time for the X-rays, and the bin size chosen will further impact the time resolution. Additionally, the group velocity dispersion mismatch of optical and X-ray pulses through liquid media will introduce further temporal broadening. Assuming ~50 fs FWHM optical and X-ray pulses, 50 fs timing tool bin size, and 100 μm liquid jet, our predicted experimental time resolution is = 132 fs [1] = (50 2 ) + (50 2 ) + (50 2 ) + (100 2 ) To gain an experimental estimate of the instrument response, the derivative of the kinetics traces at the Cu K-and Os L III -edges was fit to a Gaussian response, shown in Figure S2. The fits yielded an IRF of ~120 fs at the Cu K-edge and ~140 fs at the Os L III -edge, showing that our effective time resolution is consistent with our predicted value. This is consistent with the method used by Katayama et al. 1

Cu K-edge XTA Residual Fitting
As discussed in the main text, oscillations are observed in the XTA residuals, assigned as coherent vibrational wavepackets. These are modelled using a sum of damped sine waves: [1] ∑ * ( * ( -, ) ) * ( where damped sine waves with dephasing time τ i and period w i are centered in time at t c,I with amplitude A i . Data in the main text are fit to a single component, with the total fitting results shown in Table S3. The weak amplitude of the oscillations makes it difficult to reliably extract a dephasing time; however, the 40 cm -1 frequency is consistent both with the ISRS measurements, and the rise time in the XTA (half the period of the 40 cm -1 mode.)

CuOs Computational Results -Molecular Orbitals and Vibrational Modes
DFT calculations were used to generate molecular orbitals (Mos) for CuOs, as well as determine the nature of the low frequency vibrational modes that appear in the ISRS/XTA. Computational details are shown in Experimental. MOs for the HOMO, LUMO, and LUMO + 1 are shown in Figure S16. The HOMO is largely localized on the Os center and the surrounding ligands, while the LUMO and LUMO + 1 are largely delocalized across the tpphz bridging ligand. This demonstrates that upon photoexcitation, electron density will ultimately be distributed across the bridging ligand in close proximity to both the Cu and Os centers.
ISRS measurements of CuOs revealed vibrational modes of ~50, 240 and 430 cm -1 . To assign these vibrations, ground-state Raman modes were calculated for CuOs, with representative modes shown in Figure S17 - Figure S19. The 430 cm -1 vibrational mode likely corresponds to breathing modes on the tpphz ligand at the Cu and Os centers, as shown in the 431 and 443 cm -1 modes, respectively. There are two calculated modes that could likely represent the experimental 240 cm -1 mode. At 232 cm -1 , distortions are seen in the rotation of the Cu HETPHEN center which involves modulating π-stacking interactions between the tpphz ligand and the mesityl group at the phenanthroline 2,9 positions. At 238 cm -1 , a large breathing mode in the tpphz ligand is observed, causing expansion of the tpphz ligands while altering the Os-N and Cu-N bond distances. Finally, the 60 cm -1 vibrational mode detected by ISRS likely corresponds to an additional tpphz breathing mode (calculated at 58 cm -1 ), similar to that at 238 cm -1 . Importantly, both the experimental 50 and 240 cm -1 modes can be ascribed to breathing modes in the tpphz ligands, which serve to cause alterations in the metal-ligand bonding distance, which should cause the vibrational modes to manifest in the XTA kinetics. The 240 cm -1 is difficult to observe via XTA, however, as the period of this vibrational mode is approaching the time-resolution of that measurement, and the 50 fs step size used would cause such an oscillatory feature to be under-sampled.

Cyclic Voltammetry
Cyclic voltammetry was used to determine the redox potentials for the Cu and Os centers in CuOs and OsOs, with the results plotted in Figure S15. CuOs and OsOs have nearly identical potentials for the Os +2 /Os +3 couple around 900 mV. CuOs exhibits a Cu +1 /Cu +2 couple around 540 mV. Therefore, CuOs should have ~360 mV driving force for hole transfer from the Os to the Cu. This is similar to the driving force for hole transfer observed in the previously published CuH 2 -CuMe 2 , 2 which is also a tpphz-bridged complex. Relevant redox potentials for CuOs, OsOs, and related complexes are shown in Table S2.

XTA -Laser Power Titration
Laser power scans were performed to determine the linear/non-linear optical absorption regimes, shown in Figure S1. Difference signal is linear in pulse energy until ~15 μJ; 14.37 μJ pulse energy used in experiments, giving fluence of ~80 mJ/cm 2 . Figure S1: Power titration curve measured at the Cu K-edge with 500 nm excitation. X-ray energy is 8.986 keV