Solution Structure and Ultrafast Vibrational Relaxation of the PtPOP Complex Revealed by ΔSCF-QM/MM Direct Dynamics Simulations

Abstract

Recent ultrafast experiments have unveiled the time scales of vibrational cooling and decoherence upon photoexcitation of the diplatinum complex [Pt2(P2O5H2)4]4– in solvents. Here, we contribute to the understanding of the structure and dynamics of the lowest lying singlet excited state of the model photocatalyst by performing potential energy surface calculations and Born–Oppenheimer molecular dynamics simulations in the gas phase and in water. Solvent effects were treated using a multiscale quantum mechanics/molecular mechanics approach. Fast sampling was achieved with a modified version of delta self-consistent field implemented in the grid-based projector-augmented wave density functional theory code. The known structural parameters and the PESs of the first singlet and triplet excited states are correctly reproduced. Besides, the simulations deliver clear evidence that pseudorotation of the ligands in the excited state leads to symmetry lowering of the Pt2P8 core. Coherence decay of Pt–Pt stretching vibrations in solution was found to be governed by vibrational cooling, which is in agreement with previous ultrafast experiments. We also show that the flow of excess Pt–Pt vibrational energy is first directed toward vibrational modes involving the ligands, with the solvent favoring intramolecular vibrational energy redistribution. The results are supported by thorough vibrational analysis in terms of generalized normal modes.

Theoretical Evidence of Solvent-Mediated Excited-State Dynamics in a Functionalized Iron Sensitizer

“…The solvent structural changes in the excited state are dominated by reorganization of the first coordination shell, the change in the number of solvent molecules in the first shell is negligible. This result is in agreement with the fact that electronic transitions to MLCT states remove electronic density from the Fe, making the metallic center more positive and hence more prone to attract the negatively charged nitrogen moiety of the solvent.We use the obtained equilibrium RDFs as references for subsequent out-of-equilibrium MD simulations, aimed to probe the excited-state solvent dynamics 69,70. These simulations yield a ∼300 fs timescale for the above-reported rotation of ACN towards theFe center, identified by the increase of n Fe-N(ACN) and n Fe-N(ACN) /n Fe-CH 3 (ACN) as a function of time (Figures S23 and S24).…”

Ultrafast X-Ray Scattering Measurements of Coherent Structural Dynamics on the Ground-State Potential Energy Surface of a Diplatinum Molecule

“…Our structural analysis of the X-ray data is compared with Born-Oppenheimer Molecular Dynamics (BOMD) simulations using quantum mechanics/molecular mechanics (QM/MM) calculated forces [24][25][26]. Following Fleming and co-workers [72], the simulations are used to model the dynamics of a ground-state non-equilibrium density created by the pump pulse through propagation of a so-called hole in the classical ground-state equilibrium distribution mirroring at time zero the distribution promoted to the excited state.…”

“…Low-temperature optical spectroscopy in the crystal phase [14] and Raman spectroscopy in solution [29,30] determined the ground-state potential to be also highly harmonic but slightly softer than the singlet-and tripletstate potentials with a Pt-Pt oscillation with period T gs = 0.285 ps. Whereas much effort has been devoted towards investigating the energy dissipation mechanisms and structural dynamics of the excited-state structure(s) of PtPOP [26,[31][32][33], no studies have directly addressed the ground-state dynamics. Here, we utilize off-resonance excitation at 395 nm to selectively excite solute molecules near the excited-state equilibrium geometry, see Figure 1.…”

“…To complement the experiments, the structural evolution following off-resonance excitation of PtPOP was also investigated via hybrid QM/MM BOMD simulations. Full descriptions of the methods are given in refer-ences [25] and [26]. Briefly, PtPOP was modelled using DFT with the BLYP functional [34,35], and a representation of the Kohn-Sham (KS) orbitals in terms of tzp basis set for the Pt atoms and dzp for the rest of the atoms [36].…”

“…From these, a subset with sufficiently short Pt-Pt distances to allow excitation to the singlet excited state by a 395 nm (≈3.14 eV) photon was selected. Photoexcitation to the S 1 singlet state of PtPOP was modelled by starting 50 independent trajectories from this subset of ground-state configurations using the ∆SCF method [26,42]. The procedure thus established two sets of trajectories, representing propagation of a depleted ground-state ensemble and of an excited-state ensemble.…”