Local Circular Dichroism and Polarization Coupling in Phthalocyanine Molecular Assemblies Revealed by Photoinduced Force Microscopy

TL;DR

This study uses photoinduced force microscopy to visualize and analyze nanoscale polarization coupling and chiral optical responses in molecular assemblies, revealing collective excitation modes and local circular dichroism.

Contribution

It provides a theoretical framework for interpreting PiFM images of molecular assemblies, highlighting polarization coupling effects and chiral optical responses at the nanoscale.

Findings

Bonding and antibonding polarization modes produce characteristic PiFM images.

Circularly polarized light reveals enhanced local circular dichroism in assemblies.

PiFM can spatially resolve collective polarization and chiral optical modes.

Abstract

Photoinduced force microscopy (PiFM) enables nanoscale visualization of optical responses by directly detecting photoinduced forces without relying on luminescence. In molecular assemblies, intermolecular polarization coupling can generate collective excitation modes and chiral optical responses that are absent in isolated molecules. In this study, we theoretically investigate PiFM images of ZnPc molecular assemblies using the discrete dipole approximation combined with nonlocal molecular susceptibilities. Under linearly polarized illumination, bonding and antibonding polarization-coupling modes are found to produce characteristic spatial distributions in both the optical force spectra and PiFM images of molecular dimers and tetramers. Furthermore, under circularly polarized illumination, intermolecular coupling and asymmetric molecular packing generate enhanced local circular dichroism distributions characterized by large asymmetric factors and spatially varying chiral contrasts. These results demonstrate that PiFM can spatially resolve collective polarization modes and local chiral optical responses in molecular assemblies, providing insight into nanoscale intermolecular optical interactions.