UC Irvine

Chapter 1. Fluorescence lifetime imaging microscopy (FLIM) is an emerging tool to characterize ongoing chemical reactions in synthetic chemistry and catalysis. Initially applied to biological systems, FLIM now reveals spatially resolved chemical reaction species and system-wide physiochemical changes that accompany ongoing reactions. FLIM combines the advantage of environmental sensitivity with high signal sensitivity (as sensitive as single molecules) and has the key ability to operate under synthetic conditions (e.g., high concentrations of reagents, in organic solvents, under ambient temperature and pressure, in opaque mixtures, and in multiphasic systems). Chemical reactions inherently induce changes in the reaction medium, neighboring compounds, surface compositions, and/or bonding structure of the compounds involved, resulting in environmental changes. The FLIM methods recently developed harness and interpret these changes in ways that lead to characterizing compounds and enhancing a mechanistic understanding. Here, current advantages and limitations of FLIM methods are considered, common factors influencing fluorescence lifetime in chemical systems are discussed in a tutorial format, and seven research case studies are strategically analyzed, chosen to highlight how FLIM provided complementary information to understand chemical reaction mechanisms, intermediates, product distributions, partitioning, roles of reagents, and catalyst behaviors. These data and insights obtained from FLIM assist in the rational design and optimization of synthetic and catalytic methods. Reprinted with permission from López, P. A.; Blum, S. A. Fluorescence Lifetime Imaging Microscopy (FLIM) as a Tool to Understand Chemical Reactions and Catalysis. ACS Catal. 2024, 14, 17132–17147. Copyright 2024 American Chemical Society.

Chapter 2. An autofluorescence technique to characterize polymerization progress in real time/in line was developed, which functioned in the absence of typical fluorogenic groups on the monomer or polymer. The monomer dicyclopentadiene and polymer polydicyclopentadiene are hydrocarbons that lack traditional functional groups for fluorescence spectroscopy. Here, the autofluorescence of formulations containing this monomer and polymer during ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) was harnessed for reaction monitoring. The methods fluorescence recovery after photobleaching (FRAP) and here-developed fluorescence lifetime recovery after photobleaching (FLRAP) characterized polymerization progress in these native systems—without requiring exogenous fluorophore. (Auto)fluorescence lifetime recovery changes during polymerization correlated linearly to degree of cure, providing a quantitative link with reaction progress. These changing signals also provided relative rates of background polymerization, enabling comparison of 10 different catalyst–inhibitor-stabilized formulations. Multiple-well analysis demonstrated suitability for future high-throughput evaluation of formulations for thermosets. The central concept of the combined autofluorescence and FLRAP/FRAP method may be extendable to monitoring other polymerization reactions previously overlooked for lack of an obvious fluorescence handle. Reprinted with permission from López, P. A.; Pham, V. H. B.; Blum, S. A. A General Autofluorescence Method to Characterize Polymerization Progress. Angew. Chem., Int. Ed. 2023, e202304168. Copyright 2023 Wiley-VCH GmbH.

Chapter 3. An understanding of block-specific response to stimuli in self-assembled copolymers is essential for the effective design of responsive materials. Here, correlation of solvation behaviors at the individual-block level with the processing methods of the same ring-opening-metathesis-generated block copolymer is achieved, for film, powder, and in situ reaction precipitate. Data show that these different processing methods result in distinct solvent-response behavior. Tagging of the polar or nonpolar block separately, with a covalently incorporated, viscosity-sensitive fluorescent molecular rotor, provided the method to measure changing tightness or looseness of assembly through fluorescence lifetime imaging microscopy (FLIM). The polar blocks in the polymer powder and reaction precipitate exhibit loosening and disassembly behavior upon treatment with DMSO; but unexpectedly, this block fails to become solvated in the polymer film. These different solvation responses in solvent suggest a model of progressive block-independence. Tightness of long-range strand arrangement correlated with these different behaviors, as determined by DSC, providing ex situ Tg of dried samples. The distinct in situ behaviors reveal that the preparation method is paramount to self-assembled polymer solvation response, providing a future handle for tailoring solvent-triggered assembly/disassembly behavior. Initial data for this project were obtained by postdoctoral scholar Dr. Or Eivgi.

Chapter 4. Traditional techniques for measuring polymer molecular weight are not amenable to insoluble polymers, do not have spatial resolution, and lack in situ measurement capability. Here, the ruthenium-catalyzed ring-opening metathesis polymerization of norbornene and dicyclopentadiene was found to give rise to an autofluorescent signal. This signal proved sufficiently bright for fluorescence lifetime measurements, and was also sensitive to changes in molecular weight. A mathematical correlation between fluorescence lifetime and MW enabled measurement through spectroscopic means. The method was amenable to ongoing precipitation-polymerization reactions of polynorbornene and polydicyclopentadiene, with Mw ranging from 60–500 kg/mol. The origin of the unexpected autofluorescence appears to originate from the aggregation of the precipitated polymer solids, and the changes correlated in part with restriction of the rotational motion of the autofluorescent species. This method allows for in situ measurement of molecular weight without the need for isolation or dissolution of the sample. Fluorescence lifetime versus Mw data for the polynorbornene molecular weight ladder, shown in Figure 4.3a, were obtained by Pegah Honarmand.