A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline–Amorphous Transitions

Abstract

A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one-pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial π-π interactions, which caused concentration self-quenching. The crystalline-to-amorphous transition induced by grinding removed cofacial π-π stacking, which led to intensive emission. Crystallizing processes recovered the cofacial π-π stacking, resulting in elimination of the emission. Theoretical calculations and X-ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline-to-amorphous transition.

Mechanoresponsive Fluorescence On‐off Switching of Two‐component Emitters Composed of Mechanochromically Luminescent Dyes and Non‐emissive Pigments

“…When the amount of Cu−Pc was reduced to 0.05 equivalents, the fluorescence quantum yield was improved to 0.24. Although the emission intensity ratio between heated and ground 1 a / Cu−Pc (1:0.05) decreased to 25 : 1, this intensity ratio is still comparable to that of previously reported mechanoresponsive fluorescence on‐off switching [5e,7b,8f] . Mean fluorescence lifetimes of heated and ground 1 a / Cu−Pc (1:0.05) were 3.1 ns and 2.5 ns, respectively (Figure S4 and Table S1).…”

One-Pot Synthesis and Structure–Property Relationship of Aminomaleimides: Fluorescence Efficiencies in Monomers and Aggregates Easily Tuned by Switch of Aryl and Alkyl

“…Recently, 2-arylamino-N-arylmaleimides have been found to have AIE characteristics 32,34 and mechanochromic luminescence. 35 All reported maleimides contain aromatic moieties except two alkylaminomaleimides and four alkylaminobromomaleimides, as reported in 2015 by the O'Reilly group. 36 However, they only studied the optical properties of these aminomaleimids (AMIs) in different solutions.…”

“…Maleimides, a class of organic fluorophores with many interesting properties such as tunable emission wavelength, sensitivity to solvent polarity, , and high selective reactivity to thiol, , have been used as OLED materials, , metal ion sensors, , pH sensors, cell imaging dyes, ,, etc. Recently, 2-arylamino- N -arylmaleimides have been found to have AIE characteristics , and mechanochromic luminescence . All reported maleimides contain aromatic moieties except two alkylamino­maleimides and four alkylamino­bromo­maleimides, as reported in 2015 by the O’Reilly group .…”

Access to Multifunctional AEEgens via Ru(II)-Catalyzed Quinoxaline-Directed Oxidative Annulation

“…This twisted conformation of 3aa and the presence of multiple intermolecular interactions are responsible for the restriction of π–π stacking interactions, making them highly fluorescent in the solid and aggregated states. ,,,, From the crystal structure of 3aa , it is evident the AEEgen possesses a stator–rotor structure having a twisted 3D conformation held by multiple weak interactions. Thus, they are expected to be more sensitive toward mechanical forces, which may perturb the structure leading to chromic response. ,− …”

“…In the past few years, AIE/AEEgens have also served as mechanochromic luminogens, which are a special class of materials that undergo morphological transformations from crystalline to the amorphous state. This is associated with a noticeable change in their photophysical properties, thereby exhibiting new luminescence behavior under the influence of external mechanical stimuli including crushing, grinding, and hydrostatic pressure. ,− This phenomenon is termed as mechanochromic luminescence or piezochromic luminescence. Currently, the progress of AIE/AEE-based mechanochromic luminescent materials is quite remarkable due to their use in mechanical sensors, optical data storage devices, security links, and various biological applications .…”