A computational study is presented in which two strategies of ligand modifications have been explored to invert the relative energy of the metal-to-ligand charge transfer (MLCT) and metal-centered (MC) state in Fe(II)-polypyridyl complexes. Replacing the bipyridines by stronger $\sigma$ donors increases the ligand-field strength and pushes the MC state to higher energy, while the use of ligands with a larger $\pi$ conjugation leads to lower MLCT energies.

First row transition metal complexes with d⁴ to d⁷ electronic configurations exhibit spin-crossover (SCO), which can be induced by external stimuli, such as temperature, pressure and light. The low-spin to high-spin transition has been widely studied, but very little is known about the reverse process. Here, we present a theoretical study of thermal and light-induced high-to-low spin crossover in prototypical Fe(II) complexes. The lifetime of the high-spin state in the thermal process is determined using Fermi’s golden rule. With this methodology, we have accurately computed the transfer rate of the HS state thermal relaxation at several time scales (from sub-nanosecond to a few seconds) in two different iron complexes. The use of quasi-degenerate perturbation theory (QDPT2) in the analysis of the LS–HS spin–orbit coupling has allowed us to identify ³T₁ as the main intermediate state coupling the LS and HS states. The light-induced process has been studied using wavepacket quantum dynamics along the main vibrational coordinates (one symmetric and two asymmetric Fe–N stretchings). The study suggests that after the initial excitation from the ⁵T_2g to the ⁵E_g state, the population is transferred back to a vibrationally hot ⁵T_2g state, from which a small amount of the population is transferred to the ¹A_1g state via the intermediate ³T_1g. Most of the population remains trapped in the HS state at the time scale of the simulation.

Cooperative photoinduced switching of molecular materials at the nanoscale is still in its infancy. Strongly cooperative spin‐crossover nanomaterials are arguably the best prototypes of photomagnetic and volume‐changing materials that can be manipulated by short pulses of light. Open questions remain regarding their non‐equilibrium dynamics upon light excitation and the role of cooperative elastic interactions in nanoscale systems that are characterized by large surface/volume ratios. Femtosecond‐resolved broadband spectroscopy is performed on nanorods of the strongly cooperative Fe‐triazole, which undergoes a reversible low‐spin to high‐spin (HS) phase transition ≈360 K. Supported by density functional theory and mechano‐elastic Monte Carlo simulations, a marked difference is observed in the photoswitching dynamics at the surface of the nanoparticles compared with the core. Surprisingly, under low excitation (<2%) conditions, there occurs a transient increase in the HS population at the surface on the picosecond time scale, while the HS population in the core decays concomitantly. These results shed light onto the importance of surface properties and dynamical size limits of nanoscale photoresponsive nanomaterials that can be used in a broad range of applications.