Chiral Porphyrin Monolayers on Ferromagnetic Thin Films: Ultrafast Spectroscopy of Hybrid Interfaces

TL;DR

This study explores how chiral porphyrin monolayers on ferromagnetic thin films affect ultrafast photoinduced electron dynamics, revealing surface effects but no spin-dependent dynamics or charge transfer signatures.

Contribution

It provides new insights into the spectroscopic behavior of chiral organic monolayers on ferromagnetic surfaces, especially regarding their excited-state lifetimes and spin filtering effects.

Findings

Adsorption reduces the porphyrin excited-state lifetime.

No long-lived charge-transfer states are observed.

No dependence on magnetization or chirality in dynamics.

Abstract

Hybrid ferromagnetic metal/organic interfaces (spinterfaces) exhibit unique properties, including spin filtering. In parallel, chiral organic molecules can themselves induce efficient spin filtering, leading to unexpectedly high spin polarizations. Here, we investigate how the proximity of gold-capped Co/Ni ferromagnetic multilayers influences the spectroscopic properties and photoinduced electron dynamics of chiral oligopeptides bearing a porphyrin chromophore. The molecules are covalently attached to the gold cap via a chiral linker, forming a self-assembled monolayer. The porphyrin macrocycles adopt an orientation parallel to the surface, resulting in the formation of J-like aggregates. Photoinduced dynamics are probed using femtosecond pump-probe transient absorption spectroscopy. Despite excitation of only a single molecular layer, a clear transient absorption signal of the porphyrin singlet excited state is observed. Adsorption on the metal surface leads to a pronounced reduction of the excited-state lifetime. However, no signatures of long-lived photoinduced charge-transfer products are detected. Furthermore, no dependence of the excited-state dynamics on either the magnetization direction of the ferromagnetic layer or the molecular chirality is observed.