Ultrafast Spectroscopy Investigating Aggregation Dynamics: Methylene Blue Photocatalysis and Zinc(II)-Tripyrrindione Fluorescence

Abstract

Ultrafast spectroscopies were employed to investigate aggregation and concentration-dependent effects in systems involving photocatalysis and the non-Kasha fluorescence of a relatively new metal–ligand complex. This thesis addresses how interactions between solute molecules and their environment alter optical properties, as our findings can inform strategies for catalyst loading concentrations and the selection of metal–ligand combinations for fluorescence-based sensing. Since the critical relaxation timescales in these systems occur on the femtosecond to hundreds of picoseconds scale, we use a variety of ultrafast methods to characterize these properties.The first two chapters contextualize the molecules, light sources, and experiments discussed in Chapters 3 through 5. Chapter 3 uses a combination of steady-state and transient absorption (TA) spectroscopy to verify the triplet-enabled photocatalytic mechanism of methylene blue in the oxidative hydroxylation of arylboronic acid. Kinetic modeling of its populations confirms the dimeric form undergoes rapid nonradiative decay, suggesting that increasing catalyst loading can lead to diminishing returns by promoting the formation of nonreactive aggregates. Building on this, Chapter 4 applies two-dimensional electronic spectroscopy (2DES) with a broadband probe to the same catalytic reaction, further resolving spectral features found in TA and revealing unexpected differences in the linewidths of the monomer and dimer ground-state bleaches (GSB). The final chapter explores the unusual fluorescence behavior of a pair of metal–ligand complexes, in which emission occurs from a state higher than the lowest-energy doublet, consistent with non-Kasha fluorescence. A subtle difference in ligand substituents results in dramatically different fluorescence decays: one complex exhibits strong concentration dependence in the coordinating solvent pyridine, while the other consistently shows biexponential decay. Altogether, this thesis demonstrates how ultrafast techniques can probe subtle, transient photophysical properties in systems undergoing aggregation—systems that hold promise for greener catalysis and biosensing applications, such as organic dyes and biologically compatible metals like zinc.