Polymer‐Bound Supramolecular J‐Aggregates: Optical Properties and Applications

Abstract

Luminescent molecular aggregates, called J‐aggregates, are an interesting example of supramolecular structures with a number of unique spectral properties. Their optical properties are governed by the excitonic nature resulting from high‐order molecular packing in the J‐aggregate chains. The J‐aggregate characteristic feature is the bathochromically shifted narrow excitonic band termed the J‐band. The optical properties of J‐aggregates are markedly distinct from those of the individual molecules forming the aggregate, namely narrow absorption band, high oscillator strength, giant third‐order susceptibility, resonant luminescence, and effective exciton diffusion. Although solutions of J‐aggregates often possess low photostability for practical usage, their solid samples are more convenient, especially different polymer films due to their ease of preparation and availability of application. J‐aggregate formation in polymer films has both advantages and disadvantages: Some spectral characteristics could degrade, whereas composites creation could modify and improve their optical properties. This article aims to demonstrate how the interaction of J‐aggregates with polymers in both solution and solid film state affects their spectral properties. In addition, examples of composites and devices based on polymer‐bound J‐aggregates are discussed.

Plasmon-Induced Suppression of Exciton Self-Trapping in Polymer-Bound Pseudoisocyanine J-Aggregates

“…The aggregate morphology change in combination with a more rigid environment in the polymer film compared with the water solution results in the spectral property transformation typical for J-aggregate formation in polymer films. , The main changes are the following: in LbL films, the J-band becomes wider (Δ ν J Lb L = 330 cm –1 ), the fluorescence quantum yield strongly decreases (η LbL ≈ 0.005), and the lifetime is also significantly shortened (τ av film ≈ 40 ps) (Figure ). , For comparison, in an aqueous NaCl (0.2 M) solution, PIC J-aggregates at the used dye concentration ( C = 0.5 mM) demonstrate such characteristics: Δ ν J = 180 cm –1 , η ≈ 0.38, and τ ≈ 1.39 ns .…”

“…Supramolecular high-ordered assemblies, called J-aggregates, possess a number of 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. − Specificity of J-aggregate optical properties is governed by the electronic excitations delocalized over molecular chains and molecular (Frenkel) exciton formation due to translational symmetry and strong dipole–dipole interactions between molecules in the J-aggregate chain. − One of the J-aggregate characteristic features is the narrow red-shifted exciton band, called J-band, which width is determined by the exciton coherence (or delocalization) length. − Thus, J-aggregates are examples of molecular nanocrystals formed by cyanines, porphyrins, merocyanines, perylenes, and other dyes. − However, the exciton properties of J-aggregates often differ from those of typical molecular crystals . First of all, it is associated with the predominant one-dimensional J-aggregate geometry in solutions or two-dimensional geometry in films and on surfaces, while molecular crystals typically exhibit three-dimensional ordering .…”

“…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 a low fluorescence quantum yield of formed J-aggregates .…”

“…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 a low fluorescence quantum yield of formed J-aggregates . One of the possible reasons is exciton self-trapping in a more rigid environment. ,,, The exciton self-trapping appears when the excitons localize themselves in the self-induced potential well caused by the large lattice distortion under the condition of strong exciton–phonon coupling …”

“…However, J-aggregate formation in polymer films reveals also some drawbacks, such as a low fluorescence quantum yield of formed J-aggregates . One of the possible reasons is exciton self-trapping in a more rigid environment. ,,, The exciton self-trapping appears when the excitons localize themselves in the self-induced potential well caused by the large lattice distortion under the condition of strong exciton–phonon coupling …”