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.