One of the most basic molecular photophysical processes is that of spin transitions and intersystem crossing between excited states surfaces. The change in spin states affects the spatial distribution of electron density through the spin orbit coupling interaction. The subsequent nuclear reorganization reports on the full extent of the spin induced change in electron distribution, which can be treated similarly to intramolecular charge transfer with effective reaction coordinates depicting the spin transition. Here, single-crystal [Fe II (bpy) 3 ](PF 6 ) 2 , a prototypical system for spin crossover (SCO) dynamics, is studied using ultrafast electron diffraction in the single-photon excitation regime. The photoinduced SCO dynamics are resolved, revealing two distinct processes with a (450 ± 20)-fs fast component and a (2.4 ± 0.4)-ps slow component. Using principal component analysis, we uncover the key structural modes, ultrafast Fe-N bond elongations coupled with ligand motions, that define the effective reaction coordinate to fully capture the relevant molecular reorganization.

Given their nanoscale dimensions, colloidal semiconductor nanocrystals provide unique systems for investigating the dynamics controlling surface chemistry and fundamental issues regarding lattice reorganization upon changes in electron distribution. These systems are particularly amenable to ultrafast electron probes, offering an atomic level picture of the lattice reorganization involved following photoexcitation. Here, we study lead sulfide (PbS) quantum dots with ultrafast electron diffraction to characterize the atomic motions following high-intensity photoexcitation. Short-range nonthermal lattice distortions and increased atomic disorder were observed in PbS colloidal quantum dots ranging from 2.4 to 8.7 nm in size. These effects scaled inversely with size and were less pronounced in nanocrystals with a chloride-containing surface rather than only organic ligands, which is consistent with an effect arising at the surface. The anisotropic, nonthermal lattice disordering occurs preferentially along the (100) crystallographic directions, which could indicate an anisotropic distribution of localized charge between the differing lattice terminations of the {111} and {100} crystal facets. This is consistent with the larger anharmonicity for the lattice potential at lattice sites with reduced ligand coordination relative to the bulk, which has been shown to cause accelerated relaxation into dynamic and static surface trap sites. Through an exploration of quantum dot size and variation in surface termination, this work provides the missing structural details to advance our understanding and control of charge-carrier formation, trapping, and recombination processes in nanoscale semiconductor systems.

The mechanism of isomerization for azobenzene is a topic still to be completely elucidated. Here, we describe the ultrafast dynamics of a brominated dioxane-methoxy-azobenzene under single crystal conditions by means of femtosecond transient absorption (TA) spectroscopy. Upon excitation with 400 nm light, spectral components with decays of 0.72, 2.9, and >10 ps are observed. The fast components of the system correspond to vibrational cooling of the population on the S1 excited state, with a decay to a local minimum in the reaction coordinate, followed by a longer evolution to a dark intermediate state prior to relaxing to the ground state, S0. The long time constant can be used to describe the isomerization process, returning excited population to the ground state. Spectral frequencies observed at 33 and 82 cm−1 suggest that both rotation and inversion occur in the system, with a stronger contribution coming from the latter due to a weakened N–N double bond in the excited state. This information provides insight into the structural nature of modified azobenzene systems and sets the stage for future structural studies of the molecule’s isomerization dynamics.