Reaction Mechanisms and
Kinetics Studies with Molecular Nanocrystals: A State in Transition Between Supramolecular Entities and Bulk Solids

The use of spectroscopic methods to document detailed mechanistic and kinetic processes taking place in polycrystalline solids, dry powders, and other bulk materials has always been challenging due to their complex optical properties. Problems typically arise from their high scattering and optical densities and sometimes from the large birefringence and dichroism. In this seminar I will describe the preparation of stable water suspensions of nanocrystalline compounds and demonstrate their use in the characterization of solid-state spectroscopic properties using simple transmission procedures. As a starting point we selected well-known photophysical probes with characteristic solid state properties to illustrate the fact that nanocrystals are a state of matter in transition between supramolecular structures and bulk solids. We gathered conclusive experimental evidence that the nanocrystals used in these experiments have the same morphology as the macroscopic specimens grown from solution. Analysis of size and size distribution by polarized microscopy, dynamic light scattering (DLS) and probe microscopy methods (AFM, SEM) indicates that optimal samples should be of the order or 200 nm in size, which is below the wavelength of light used in most experiments, and a key parameter for the method to work well. Examples will be shown to illustrate the use of nanocrystals to obtain detailed mechanistic information of excited state processes and highly selective reactions in crystals, including a novel strategy for signal amplification with a gain based on a very unusual excited state chain reaction.
Miguel Garcia-Garibay (UCLA)