image: a. Schematic diagram of aggregate structure in NaCl crystal. b. Schematic diagram of chemical structure in zwitterionic aggregate. c. Visualization of the inter- and intramolecular motion in zwitterionic aggregate through fluorescent signal.
Credit: ©Science China Press
In physical science, molecules are considered the fundamental units that embody the properties of matter, continuously engaging in inter- and intramolecular motions. The dynamics of molecular motion are crucial in various processes, including thermal energy transfer, chemical reactions, and biological mechanisms. Aggregates represent the most common state of molecular existence, where constituent molecules undergo either vigorous or subtle inter- and intramolecular motion.
Ionic aggregates are prevalent in nature, industry, and biological systems, with anions and cations serving as the fundamental constituent units. However, these ions often coexist as ion pairs, making it challenging for isolated cations and anions to exist stably in isolation. This complicates the study of the structure and properties of individual ionic units. The presence of two different types of ions further complicates investigations, resulting in limited research on unit motion within ionic aggregates. “Simplifying the structure of ionic aggregates and introducing a signal that facilitates monitoring is key to revealing molecular motion within these aggregates,” says Dr. Jin Wang, the co-first author of this paper at HKUST.
In this research, they proposed a zwitterionic strategy, integrating sulfonate and ammonium ions into a fluorescent tetraphenyl ethylene framework. “The zwitterionic strategy establishes the zwitterionic molecule as the fundamental unit for constructing ionic aggregates, thereby simplifying the complex dual-unit systems typically found in conventional inorganic salts. Additionally, this strategy imparts fluorescent properties to the building units, providing a novel means for real-time tracking of molecular motion within ionic aggregates,” Jin Wang explains.
"Blue shifts in fluorescence wavelength correspond to aggregate state transitions caused by intermolecular motion, while intensity decay reflects conformational changes due to intramolecular motion," noted Dr. Zihao Deng, the co-first author. Thanks to the simplified system and the sensitive response of fluorescence signals to mechanical stimuli, the team successfully achieved in situ monitoring of two different molecular motion modes.
“The dynamic changes in aggregate structures and their interaction mechanisms are highly complex, posing significant challenges for traditional macroscopic experimental methods to uncover their microscopic dynamic processes. However, theoretical calculations provide a reliable approach to studying these processes.” adds Dr. Xinwen Ou, another co-first author. Through theoretical calculations, the research team successfully reconstructed the structural recovery process of ionic aggregates transitioning from a disordered state to a crystalline state via inter- and intramolecular molecular motions. Additionally, they confirmed that ionic interactions play a critical role in this process.
“Utilizing intense ionic interactions induced molecular motion, we achieved dynamic switching of the excited state energy decay pathway of molecules, leading to switchable color-light responses.” states Prof. Ryan T. K. Kwok, the corresponding author of this paper at HKUST. Interestingly, the dynamic transitions of photochromism and photoluminescence induced by scratching, along with subsequent self-recovery through inter- and intramolecular motion, suggest that mechanical action allows for convenient and repeatable switching between color and light activities of aggregates. The corresponding author, Professor Jacky W. Y. Lam, adds: "This intriguing phenomenon will undoubtedly inspire researchers to achieve functional transitions through structural regulation at the aggregate level, rather than relying solely on the molecules themselves."
“The world of aggregates is undeniably complex, yet it is equally rich with untapped potential.” Prof. Ben Zhong Tang, the corresponding author from CUHK-Shenzhen and HKUST, emphasizes. This study leverages ionic aggregates as a model to systematically elucidate the structure-property relationships and dynamic transformation mechanisms between the whole and its parts at the aggregate level. It represents another successful application of the aggregate science to organic ionic aggregates, following their previous publications titled “Is the whole equal to, or greater than, the sum of its parts? The similarity and difference between molecules and aggregates” and “Constructing flexible crystalline porous organic salts via a zwitterionic strategy.”