Features of Plasmon Enhancement of Fluorescence in the Case of Molecular Aggregates
Alexander Sorokin a, Irina Grankina a, Irina Bespalova a, Svetlana Yefimova a
a Institute for Scintillation Materials of NAS of Ukraine, 60 Nauky Ave., Kharkiv, Ukraine
Proceedings of nanoGe Fall Meeting 2021 (NFM21)
#LightMatter21. Light-Matter Interactions: From Fundamental Spectroscopy to Materials Design
Online, Spain, 2021 October 18th - 22nd
Organizers: Linn Leppert and Marina Filip
Poster, Alexander Sorokin, 266
Publication date: 23rd September 2021
ePoster: 

Supramolecular high-ordered assemblies, called J-aggregates, possess many unique spectral properties, which distinctly differ from those of the individual molecules: a narrow absorption band, near-resonant fluorescence, high oscillator strength, giant third-order susceptibility, effective resonant energy migration, etc. The specificity of J-aggregates optical properties is governed by the electronic excitations delocalized over molecular chains and molecular (Frenkel) excitons formation due to translational symmetry and strong dipole-dipole interaction between molecules in the J-aggregate chain. One of the J-aggregate characteristic features is the narrow red-shifted exciton band, called J-band with the width determined by the exciton coherence (or delocalization) length.

Unique spectral properties make J-aggregates excellent candidates for novel photonic materials especially in the form of thin films, particularly, polymer films. Indeed, while in solutions J-aggregates often possess low photostability, in polymer films their stability becomes much higher. However, J-aggregate formation in polymer films reveals also some drawbacks, such as the low fluorescence quantum yield of formed J-aggregates. One of the ways to manipulate J-aggregates optical properties in solid samples is using an effect of exciton-plasmon coupling via aggregates interaction with noble metal nanostructures.

Indeed, the J-aggregates fluorescence quantum yield enhancement can be achieved by placing the J-aggregates at the optimal distance from metal nanoparticles, i.e. in the weak regime of exciton-plasmon coupling. Particularly, for pseudoisocyanine J-aggregates in layered polymer films 8-fold fluorescence enhancement was achieved at a 16 nm distance between gold nanoparticles and the J-aggregates [1].

One of the non-radiative relaxation channels in J-aggregates is the exciton self-trapping. The latter appeared to be efficient enough for J-aggregates formed in polymer films due to increased exciton-phonon coupling [2]. It was revealed that the exciton-plasmon coupling leads to the exciton coherence length increasing due to an effect of the absorption cross-section enhancement [3]. It results in the weakening exciton-phonon coupling which in its turn, leads to the exciton self-trapping suppression. The latter enhances the J-aggregate fluorescence additionally to the direct plasmon fluorescence enhancement [3].

So, the exciton-plasmon interaction in the weak coupling regime enhances the J-aggregates fluorescence in two ways: directly by plasmon fluorescence enhancement effect and indirectly by the exciton self-trapping suppression.

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