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.

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 PtÀ Pt stretching motions were detected in real time using femtosecond wide-angle Xray 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 PtÀ 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.

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.

Bimetallic transition metal complexes have gained increasing attention because of their versatile functions in solar energy conversion and photonics applications arising from intermetal electronic coupling. In bimetallic platinum (Pt) complexes, electronic communication between the Pt-centered and ligand-centered moieties has been shown to be critical for defining their excited-state dynamic trajectories undergoing either localized ligand-centered (LC)/metal-to-ligand charge-transfer (MLCT) transitions or delocalized metal−metal-to-ligand charge-transfer (MMLCT) transitions. The branching of the excited-state intersystem crossing (ISC) trajectories is modulated through structural factors that alter the relative energies of the different states. In this study, we investigated the correlation of the structural factors influencing the excited-state trajectories. With the use of femtosecond broad-band transient absorption (fs-BBTA) spectroscopy, ultrafast dynamics in the excited state of two select Pt(II) dimers have been mapped out using their coherent vibrational wavepacket signatures in the corresponding transient absorption spectra. To examine how the ligand moieties of the Pt(II) dimers influence excited-state dynamics and the coherent vibrational wavepacket behavior, we carried out comparative studies on two pyrazolate-bridged Pt(II) dimers of the general formula [Pt( t Bu 2 Pz)(N^C)] 2 [ t Bu 2 Pz is 3,5-di-tert-butylpyrazole; N^C is 7,8benzoquinoline (bzq, 1) or 1-phenylisoquinoline (piq, 2)]. We found that photoexcitation into the low-energy absorption bands of 1 and 2, respectively, induces the formation of 1 MMLCT states from which ultrafast ISC proceeds, resulting in stimulated emission quenching and decoherence of the vibrational wavepacket motions. The results obtained in this study suggest that both the energetics and the structural rigidity of the aromatic cyclometalating ligands in 1 and 2 can significantly influence the dynamics along the excited-state trajectory characterized by dephasing of the coherent oscillations. The collective results provide direct evidence of how ligand structure alters electronic dynamics along excited-state trajectories associated with ISC processes, providing insight into using ligand design to steer photochemical processes.

Herein, we investigate the coherent vibrational wavepacket dynamics during the intersystem crossing processes in a bimetallic Pt(ii) complex.