Isosbestic behavior in transformations of colloidal semiconductor magic-size clusters via intermediates in dispersion

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 magic-size clusters (MSCs) from MSC-a to MSC-b display interrupted spectral shifts in optical absorption with or without isosbestic behavior. We demonstrate that a multicomponent model explains consistently the pathway. The model invokes precursor compounds (PCs) of MSCs as intermediates, with three key steps from MSC-a to PC-a (step 1), to PC-b (step 2), and to MSC-b (step 3). Monomer substitution assists step 2 (PC-a to PC-b). Based on the experimental result and theoretical study, we conclude that when step 1 or 2 or 3 is rate-determining, isosbestic behavior can be ideally perfect or distorted or absent, respectively. Our study provides a deeper understanding of isosbestic behavior and of MSC transformations that are assisted by PC intermediates and monomer substitution.