Towards understanding ligand-nanoparticle photochemistry with ultrafast electronic-vibrational spectroscopy

Photochemical processes in complex molecules and materials are central to controlling the movement of charge and energy on the microscopic level. The ability to tailor the motion of charge through synthetic modification enables new material properties to be realized. Dynamic couplings and correlations occurring on ultrafast timescales between the electronic motions of the system and the microscopic structure of the system are key aspects to understand in order to discover new photoinduced and optoelectronic behaviors in molecules and materials. In nanoparticle systems and colloidal suspensions, the capping ligands can be used both to stabilize the material and to tune optoelectronic properties of the materials. This paper discusses studies using femtosecond transient-IR absorption spectroscopy and two-dimensional electronic-vibrational (2D EV) spectroscopy on Co-CN-Fe (i.e., Prussian blue analogue) nanoparticles with polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) stabilizing ligands. These mixed-valence, mixed-metal nanoparticle systems contain broad and complex electronic absorption features with different charge transfers accessible throughout the UV and visible spectral region. The cyano bridging ligand stretching vibration is a sensitive reporter of charge transfer dynamics that is used in this study to directly probe how photoinduced charge motion in 11 nm Co-CN-Fe nanoparticles occurs. The initial data discussed in this paper show different CN stretching dynamics and spectral features are observed that are capping ligand dependent. The results of this study suggest that ultrafast electronic-vibrational spectroscopies will be crucial methods to understand vibronic coupling dynamics in complex systems, such as nanoparticle-ligand materials.