Substrate-Induced Shifts and Screening in the Fluorescence Spectra of Supramolecular Adsorbed Organic Monolayers

TL;DR

This study examines how substrates influence the fluorescence spectra of organic monolayers, revealing substrate-induced red-shifts and screening effects that depend on the substrate's refractive index, combining experimental and theoretical approaches.

Contribution

It provides new insights into substrate effects on molecular fluorescence, combining experimental measurements with theoretical modeling to quantify substrate-induced shifts and screening.

Findings

Substrate causes up to 0.37 eV red-shift in fluorescence spectra.

Interactions between molecules are screened by the substrate, reducing spectral shifts.

The magnitude of effects depends on the substrate's refractive index.

Abstract

We have investigated the influence of the substrate on the fluorescence of adsorbed organic molecules. Monolayer films of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI), a supramolecular network formed from PTCDI and melamine, and perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) have been deposited on hexagonal boron nitride (hBN). The principal peaks in the fluorescence spectra of these films were red-shifted by up to 0.37 eV relative to published measurements for molecules in helium droplets. Smaller shifts (~0.03 eV) arising from interactions between neighbouring molecules are investigated by comparing the fluorescence of distinct arrangements of PTCDI, which are templated by supramolecular self-assembly and determined with molecular resolution using atomic force microscopy under ambient conditions. We compare our experimental results with red-shifts calculated using a combination of a perturbative model and density functional theory which account for, respectively, resonant and non-resonant effects of a dielectric hBN substrate. We show that the substrate gives rise to a red-shift in the fluorescence of an adsorbed molecule and also screens the interactions between neighbouring transition dipole moments; both these effects depend on the refractive index of the substrate.