Vibronic Dynamics Localize Charge in Photoexcited CoFe Prussian Blue Analogue Nanoparticles

Prussian Blue analogues are prototype photoswitchable materials for understanding how electron spin and charge density evolve in nonequilibrium photoexcited states. Great interest in photomagnetism has driven substantial investigation of ultrafast electronic dynamics, but a sufficient understanding of how molecular-level vibronic dynamics influences spin-related photochemistry in these materials is needed. Here, the dynamics of CsCoFe Prussian Blue analogue nanoparticles are studied with transient infrared absorption spectroscopy probing the photoexcited CN stretch vibration using tunable excitation pulses spanning 350 to 700 nm, paired with first-principles density functional theory analysis to understand the molecular nature of the electronic excited states. Regardless of excitation wavelength, all initially prepared excited states converge to the same metal-to-metal charge-transfer manifold within 300 fs, which then efficiently populates a metal-to-ligand charge-transfer state within 500 fs, localizing the charge onto bridging CN ligands. This study highlights that the electronic, spin, and structural degrees of freedom work together to collectively determine how charge is distributed in CsCoFe nanoparticles.