The Role of Molecular Arrangement on the Dispersion in Strongly Coupled Metal-Organic Hybrid Structures

TL;DR

This study investigates how molecular arrangement and fluorination degree in zinc-phthalocyanine layers affect plasmon-exciton coupling in gold hybrid structures, revealing sensitive dependence on molecular packing and dipole orientation.

Contribution

It provides new insights into the influence of molecular fluorination and aggregation on surface plasmon coupling in metal-organic hybrids, supported by structural and theoretical analysis.

Findings

Anti-crossings correlate with fluorination degree and molecular phases.

Molecular dipole orientation affects anti-crossing energy and splitting.

Fluorination alters lattice spacing and dipole density, impacting coupling.

Abstract

Metal-organic hybrid structures have been demonstrated a versatile platform to study primary aspects of light-matter interaction by means of emerging states comprising excitonic and plasmonic properties. Here we are studying the wave-vector dependent photo-excitations in gold layers covered by molecular films of zinc-phthalocyanine and its fluorinated derivatives (FnZnPc, with n = 0,4,8,16). These layered metal-organic samples show up to four anti-crossings in their dispersions correlating in energy with the respective degree of ZnPc fluorination. By means of complementary structural and theoretical data, we attribute the observed anti-crossings to three main scenarios of surface plasmon coupling: i) to aggregated $\alpha $-phase regions within the FnZnPc layers at 1.75 eV and 1.85 eV , ii) to a coexisting F16ZnPc $\beta $-polymorph at 1.51 eV, and iii) to monomers, preferentially located at the metal interface, at 2.15 eV. Whereas energy and splitting of the monomer anti-crossings depend on strength and average tilting of the molecular dipole moments, the aggregate related anti-crossings show a distinct variation with degree of fluorination. These observations can be consistently explained by a change in FnZnPc dipole density induced by an increased lattice spacing due to the larger molecular van der Waals radii upon fluorination. The reported results prove Au/FnZnPc bilayers a model system to demonstrate the high sensitivity of exciton-plasmon coupling on the molecular alignment at microscopic length scales.