Ultrafast photophysics and magnetisation dynamics of Cr(III)-based Prussian blue analogue thin films

Abstract

As reliance on the use of magnetism for technological applications increases, molecule-based magnetic materials have emerged as interesting candidates for study due to their immense chemical flexibility and tuneable physical properties. Of these, Prussian blue analogues (PBAs) are an interesting subclass which conform to a simple cubic structure and their composition can be readily modified to incorporate a variety of different transition metal species. This has given rise to a wide range of materials with different spectral characteristics and notably high magnetic ordering temperatures.

As such, PBAs provide useful model systems for investigations into how magnetic properties may be influenced through photoexcitation. This body of work aims to provide a description of photoinduced electronic and magnetisation dynamics in a Cr(III)-based PBA on ultrafast timescales using a combination of transient absorption (TA) spectroscopy and time-resolved magneto-optics (TR-MO) in transmission geometry.

The materials were synthesised in the form of thin films through electrodeposition from solution. While the greatest magneto-optical response is afforded in principle through the use of thicker films, these tend to scatter light heavily. A way to circumvent this is encapsulation of the films using a suitable agent of similar refractive index. For this purpose, two adhesives that are transparent in the visible region were tested for both mixed-valence Cr–Cr and Fe–Cr PBAs. A vast improvement in transmittance was observed, particularly for Cr–Cr PBA where spectral features that previously had been completely obscured by a broad background could be more clearly resolved. This was attributed to the reduction of air/film interfaces in our polycrystalline materials, which act as a major source of light scatter. Encapsulation became part of standard film preparation for TA and TR-MO measurements, which would typically use films of ∼1 µm thickness. Thus far, the dynamics of Cr(III)-based PBAs have been interpreted using the ultrafast photophysics observed in Cr(acac)3 (acac = acetylacetonate) as a model.

However, there are large differences in the structure and bonding observed within this complex compared to the hexacyanochromate(III) anion which forms the molecular “backbone” of said PBAs. Hence, the ultrafast photophysics in hexacyanochromate(III) in aqueous solution were investigated using TA to validate the existing interpretation.

A broad excited state absorption feature was observed upon ligand-field excitation, consistent with observations in Cr(acac)3 while also conforming to similar kinetics.

However, a narrow, secondary feature was also observed to grow in within 2 ps. While the origins of the two features remain unknown, absorption from the quartet manifold, the spin-flipped doublet manifold or from photoproducts have been presented as possible causes.

For the combined TA and TR-MO studies, thin films of Cr–Cr PBA were produced with magnetic ordering temperature, Tc, of ∼160 K. Broadband TA measurements revealed an excited state absorption which overlapped with a broad, complex feature that accounted for most of the spectrum. Kinetic analyses of these two spectral components determined that their intensities both evolved on different characteristic timescales. Complementary TR-MO experiments indicated that the change in magneto-optical response over time conformed to the same kinetics as the broad spectral feature, suggesting that both the electronic and magnetisation dynamics are linked. Due to the complex stoichiometry of the material, assignment of the origin of these features is ongoing.