Revealing Excited-State Trajectories on Potential Energy Surfaces with Atomic Resolution in Real Time.

Photoexcited molecular trajectories on potential energy surfaces (PESs) prior to thermalization are intimately connected to the photochemical reaction outcome. The excited-state trajectories of a diplatinum complex featuring photo-activated metal-metal σ-bond formation and associated Pt-Pt stretching motions were detected in real time using femtosecond wide-angle X-ray solution scattering. The observed motions correspond well with coherent vibrational wavepacket motions detected by femtosecond optical transient absorption. Two key coordinates for intersystem crossing have been identified, the Pt-Pt bond length and the orientation of the ligands coordinated with the platinum centers, along which the excited-state trajectories can be projected onto the calculated PESs of the excited states. This investigation has gleaned novel insight into electronic transitions occurring on the time scales of vibrational motions measured in real time, revealing ultrafast nonadiabatic or non-equilibrium processes along excited-state trajectories involving multiple excited-state PESs.

Ultrafast branching in intersystem crossing dynamics revealed by coherent vibrational wavepacket motions in a bimetallic Pt(II) complex.

Ultrafast excited state processes of transition metal complexes (TMCs) are governed by complicated interplays between electronic and nuclear dynamics, which demand a detailed understanding to achieve optimal functionalities of photoactive TMC-based materials for many applications. In this work, we investigated a cyclometalated platinum(II) dimer known to undergo a Pt-Pt bond contraction in the metal-metal-to-ligand-charge-transfer (MMLCT) excited state using femtosecond broadband transient absorption (fs-BBTA) spectroscopy in combination with geometry optimization and normal mode calculations. Using a sub-20 fs pump and broadband probe pulses in fs-BBTA spectroscopy, we were able to correlate the coherent vibrational wavepacket (CVWP) evolution with the stimulated emission (SE) dynamics of the 1MMLCT state. The results demonstrated that the 145 cm-1 CVWP motions with the damping times of ∼0.9 ps and ∼2 ps originate from coherent Pt-Pt stretching vibrations in the singlet and triplet MMLCT states, respectively. On the basis of excited state potential energy surface calculations in our previous work, we rationalized that the CVWP transfer from the Franck-Condon (FC) state to the 3MMLCT state was mediated by a triplet ligand-centered (3LC) intermediate state through two step intersystem crossing (ISC) on a time scale shorter than a period of the Pt-Pt stretching wavepacket motions. Moreover, it was found that the CVWP motion had 110 cm-1 frequency decays with the damping time of ∼0.2 ps, matching the time constant of 0.253 ps, corresponding to a redshift in the SE feature at early times. This observation indicates that the Pt-Pt bond contraction changes the stretching frequency from 110 to 145 cm-1 and stabilizes the 1MMLCT state relative to the 3LC state with a ∼0.2 ps time scale. Thus, the ultrafast ISC from the 1MMLCT to the 3LC states occurs before the Pt-Pt bond shortening. The findings herein provide insight into understanding the impact of Pt-Pt bond contraction on the ultrafast branching of the 1MMLCT population into the direct (1MMLCT → 3MMLCT) and indirect ISC pathways (1MMLCT → 3LC → 3MMLCT) in the Pt(II) dimer. These results revealed intricate excited state electronic and nuclear motions that could steer the reaction pathways with a level of detail that has not been achieved before.

Exploring Potential Energy Surfaces Using Reinforcement Machine Learning.

Reinforcement machine learning is implemented to survey a series of model potential energy surfaces and ultimately identify the global minima point. Through sophisticated reward function design, the introduction of an optimizing target, and incorporating physically motivated actions, the reinforcement learning agent is capable of demonstrating advanced decision making. These improved actions allow the agent to successfully converge to an optimal solution more rapidly when compared to an agent trained without the aforementioned modifications. This study showcases the conceptual feasibility of using reinforcement machine learning to solve difficult environments, namely, potential energy surfaces, with multiple, seemingly, correct solutions in the form of local minima regions. Through these results, we hope to encourage extending reinforcement learning to more complicated optimization problems and using these novel techniques to efficiently solve traditionally challenging problems in chemistry.

General Design Rules for Bimetallic Platinum(II) Complexes.

A series of platinum(II) bimetallic complexes were studied to investigate the effects of ligands on both the geometric and electronic structure. Modulating the Pt-Pt distance through the bridging ligand architecture was found to dictate the nature of the lowest energy electronic transitions, localized in one-half of the molecule or delocalized across the entire molecule. By reducing the separation between the platinum atoms, the lowest energy electronic transitions will be dominated by the metal-metal-to-ligand charge transfer transition. Conversely, by increasing the distance between the platinum atoms, the lowest electronic transition will be largely localized metal-to-ligand charge transfer or ligand centered in nature. Additionally, the cyclometalating ligands were observed to have a noticeable stabilizing effect on the triplet excited states as the conjugation increased, arising from geometric reorientation and increased electron delocalization of the ligands. Such stabilization of the triplet state energy has been shown to alter the excited state potential energy landscape as well as the excited state trajectory.

Resolving the ultrafast intersystem crossing in a bimetallic platinum complex.

Bimetallic platinum complexes have interesting luminescent properties and feature long-lasting vibrational coherence and ultrafast intersystem crossing (ISC) after photoexcitation. Ultrafast triplet formation is driven by very strong spin-orbit coupling in these platinum (II) systems, where relativistic theoretical approaches beyond first-order perturbation theory are desirable. Using a fully variational relativistic theoretical method recently developed by the authors, we investigate the origins of ultrafast ISC in the [Pt(ppy) (µ-tBu2pz)]2 complex (ppy = phenylpyridine, pz = pyrazolate). Spin-orbit coupling values, evaluated along a Born-Oppenheimer molecular dynamics trajectory, are used to propagate electronic populations in time. Using this technique, we estimate ultrafast ISC rates of 15-134 fs in this species for the possible ISC pathways into the three low-lying triplet states.

Deciphering Charge Transfer Processes in Transition Metal Complexes from the Perspective of Ultrafast Electronic and Nuclear Motions.

Chemical transformations in charge transfer states result from the interplay between electronic dynamics and nuclear reorganization along excited-state trajectories. Here, we investigate the ultrafast structural dynamics following photoinduced electron transfer from the metal-metal-to-ligand charge transfer state of an electron donor, a Pt dimer complex, to a covalently linked electron acceptor group using ultrafast time-resolved wide-angle X-ray scattering and optical transient absorption spectroscopy methods to disentangle the interdependence of the excited-state electronic and nuclear dynamics. Following photoexcitation, Pt-Pt bond formation and contraction takes up to 1 ps, much slower than the corresponding process in analogous complexes without electron acceptor groups. Because the Pt-Pt distance change is slow with respect to excited-state electron transfer, it can affect the rate of electron transfer. These results have potential impacts on controlling electron transfer rates via structural alterations to the electron donor group, tuning the charge transfer driving force.

Ultrafast Excited-State Dynamics of Photoluminescent Pt(II) Dimers Probed by a Coherent Vibrational Wavepacket.

Intricate potential energy surfaces (PESs) of some transition metal complexes (TMCs) pose challenges in mapping out initial excited-state pathways that could influence photochemical outcomes. Ultrafast intersystem crossing (ISC) dynamics of four structurally related platinum(II) dimer complexes were examined by detecting their coherent vibrational wavepacket (CVWP) motions of Pt-Pt stretching mode in the metal-metal-to-ligand-charge-transfer excited states. Structurally dependent CVWP behaviors (frequency, dephasing time, and oscillation amplitudes) were captured by femtosecond transient absorption spectroscopy, analyzed by short-time Fourier transformation, and rationalized by quantum mechanical calculations, revealing dual ISC pathways. The results suggest that the ligands could fine-tune the PESs to influence the proximity of the conical intersections of the excited states with the Franck-Condon state and thus to control the branching ratio of the dual ISC pathways. This comparative study presents future opportunities in control excited-state trajectories of TMCs via ligand structures.