Thermal or Mechanical Stimuli‐Induced Photoluminescence Color Change of a Molecular Assembly Composed of an Amphiphilic Anthracene Derivative in Water

Abstract

Molecular assemblies that change photoluminescence color in response to thermal or mechanical stimulation without dissociation into the monomeric states in water are described herein. A dumbbell-shaped amphiphilic compound forms micellar molecular assemblies in water and exhibits yellow photoluminescence derived from excimer formation of the luminescent core, which contains a 2,6-diethynylanthracene moiety. Annealing of the aqueous solution induces a photoluminescence color change from yellow to green (λem, max =558→525 nm). The same photoluminescence color change is also achieved by rubbing the yellow-photoluminescence-emitting molecular assemblies adsorbed on glass substrates with cotton wool in water. The observed green photoluminescence is ascribed to micelles that are distinct from the yellow-photoluminescence-emitting micelles, on the basis of transmission electron microscopy observations, atomic force microscopy observations, and dynamic light scattering measurements. We examined the relationship between the structure of the molecular assemblies and the photophysical properties of the anthracene derivative in water before and after thermal or mechanical stimulation and concluded that thermal or mechanical stimuli-induced slight changes of the molecular-assembled structures in the micelles result in the change in the photoluminescence color from yellow to green in water.

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen‐Bonded Sheets

“…This lack of previous reports is likely because the combinationo fp-p stacking and multiple hydrogen bonds decreases the solubility of these molecules unless large and flexible substituents are attached. [8,14,21,23,[39][40][41][42] Here, we demonstrate that the molecular design strategy described above is significantly effective using as imple anthracene derivative. 9,10-Bis(phenylethynyl)anthracene derivative 1 (Figure 1a)f orms at wo-dimensional( 2D) molecular sheet constructed from intermolecular hydrogen bonds between the amide groups in the green-emissive crystals (G-form).…”

“…However, mechanoresponsive luminescent compounds in which several amide groups are directly introduced into one π‐extended aromatic moiety have rarely been reported. This lack of previous reports is likely because the combination of π–π stacking and multiple hydrogen bonds decreases the solubility of these molecules unless large and flexible substituents are attached …”

“…Because AIE‐active luminophores tend to form loosely‐packed assemblies, mechanical stimuli induces closed‐packing and/or conformational changes, which alter their photoluminescence properties. Another promising approach is the design of molecular structures in which different types of intermolecular interactions can compete against each other . Adopting this strategy, molecules can form several thermodynamically ( meta )stable assembled states and mechanical stimuli can induce interconversions among these states.…”

“…Adopting this strategy, molecules can form several thermodynamically ( meta )stable assembled states and mechanical stimuli can induce interconversions among these states. In particular, competition between hydrogen‐bonding and π–π stacking is effectively achieved by introducing amide groups directly into luminophores . This effectiveness is attributed to the fact that the length between amide groups forming hydrogen bonds is generally longer than the distance between aromatic groups forming π–π stacks.…”

Cyclophane‐Based Fluorescence Tuning Induced by Hydrostatic Pressure Changes

“…Grinding, crushing and tensile deformation significantly affect the photophysical properties of molecular assembled materials . Of several approaches to mechanochromic luminescent materials hitherto examined, cyclic, dendritic, and mechanically‐interlocked compounds, originated from particular structural features, show mechanoresponsive luminescence in the condensed states. Nevertheless, hydrostatic pressure‐induced fluorescence behavior of these structurally interesting molecules has not been examined yet.…”