Isosbestic behavior in multi-step transformations

Isosbestic behavior is a term that describes a specific wavelength with stable absorbance, when a reaction proceeds from a reactant (R) to a product (P) and is monitored by optical absorption spectroscopy, for example. Isosbestic behavior has been viewed as a distinctive indicator that a chemical reaction happens directly without any intermediates involved. However, researchers at Sichuan University (P. R. China) challenge this traditional view. Using colloidal semiconductor magic size clusters (MSCs) as a model system, the researchers demonstrate that relatively transparent intermediates are involved in the transformation from a reactant (MSC-a) to a product (MSC-b). The MSCs studied are small nanomaterials made of II-VI metal chalcogenide (ME) atoms with superior stability.  

 

Researchers published their results in Nano Research in May 2025.

 

“Isosbestic behavior has been traditionally deemed as a spectroscopic indicator of a chemical reaction, in which a reactant transforms directly to a product without intermediates. Room-temperature transformations of colloidal semiconductor MSCs from MSC-a (R) to MSC-b (P) can display interrupted spectral shifts in optical absorption with or without isosbestic behavior. The transformation pathway involves intermediates that are relatively transparent in optical absorption.” said Professor Kui Yu, researcher at Sichuan University and author of the study. “Isosbestic behavior is associated with a rate-determining step. This idea is noteworthy”.

 

The intermediates involve the precursor compounds of MSC-a and MSC-b, PC-a and PC-b, respectively. For the first time researchers identified how a rate-determining step in a three-step pathway affects isosbestic behaviors.

• The first step is isomerization from MSC-a to PC-a (a configuration change with the composition maintained); when this step is rate-determining, isosbestic behavior has no distortion and the “perfect” point is called an isosbestic point.
• The second step is the PC-a to PC-b transformation; when this step is rate-determining, isosbestic behavior can be distorted.
• The third step is isomerization from PC-b to MSC-b; when this step is rate-determining, isosbestic behavior is absent.

 

The three-step model has been used throughout the study with consistent results to explain isosbestic behavior observed during the MSC-a and MSC-b transformation. “Our findings provide a deeper understanding of isosbestic behavior (associated with a rate-determining step in a multi-component model with relatively transparent intermediates), and bring comprehensive insight into MSC-a to MSC-b transformations at room temperature that are assisted by intermediates (PC-a and PC-b) and monomer substitution,” said Professor Yu.

 

The traditional and standard explanation of isosbestic behavior, that a reactant (R) transforms into a product (P) directly without intermediates, had been accepted widely as a self-evident truth. However, indirect transformations via a multi-step process can be more energetically favored than direct transformations. The research team hopes their three-step pathway model involving intermediates advance the fundamental understanding of the transformation of various matter with or without isosbestic behavior. “Our study shines light into the probable pathway that involves intermediates that are relatively transparent, when molecules and metal clusters transform (displaying spectral shifts that are in a distinctly discontinuous pattern with and without isosbestic behavior)” said Professor Yu.

 

Yusha Yang, Kui Yu and Xiaoqin Chen of the Engineering Research Center in Biomaterials at Sichuan University, Andrei Sapelkin of the Department of Physics and Astronomy at Queen Mary University of London, Zifei Chen of the Laboratory of Nanochemistry for Energey at the Institute of Chemical Sciences and Engineering and Jiangli Lin and Chaoran Luan of the College of Biomedical Engineering at Sichuan University contributed to this research.

 

Funder

The National Key Research and Development Program of China, the National Natural Science Foundation of China, the Natural Science Foundation of Sichuan Province and the Open Project of Key State Laboratory for Supramolecular Structures and Materials of Jilin University made this research possible.

 

Reference

Title of original paper: Isosbestic behavior in transformations of colloidal semiconductor magic-size clusters via intermediates in dispersion Discursive Blueprinting in 2024

Journal: Nano Research

DOI: 10.26599/NR.2025.94907472

https://www.sciopen.com/journal/1998-0124

---

---