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Although similar to more commonly implemented single wavelength approaches, broadband pump‐probe or transient absorption microscopy presents unique experimental challenges due to the simultaneous requirements of a broadband probe pulse and a small sample volume. Here we provide an in‐depth analysis of broadband detection schemes and their common noise sources to provide strategies for balancing the conflicting needs of high sensitivity and low probe fluence. We show that broadband pump‐probe microscopy is atypically sensitive to laser shot noise and therefore, low pump on/off modulation frequencies, on the order of 100 s of Hz to a few kHz, are essential to measure small (~10^-3 - 10^-4) amplitude transient spectra while remaining in the perturbative limit. 

Using a variety of steady state and time-resolved microscopies, this work directly compares the excited state dynamics of two distinct morphologies of a hierarchical perylene diimide material: a kinetically trapped 1D mesoscale aggregate produced with a redox-assisted self-assembly process, and a thin film produced via conventional solution-processing techniques. Although the constituent monomer is identical for both materials, linear dichroism studies indicate that the kinetically trapped structures possess significantly higher long-range order than the conventional thin film. A comparison of the two systems with broadband pump–probe microscopy reveals distinct differences in their excited state dynamics. In the kinetically trapped structures, polarization-resolved kinetics, as well as a picosecond redshift of the ground state bleach provide evidence for rapid excited state delocalization, which is absent in the thin film. A comparison of transient spectra collected at 1 μs indicates the presence of long-lived charge separated states in redox treated samples, but not in the thin film. These results provide direct evidence that control of the supramolecular assembly process can be leveraged to affect the long-range order of derived PDI materials, thus enabling increased yield and lifetime of charge separated states for light harvesting applications. Furthermore, these results highlight the need for microscale broadband probes of organic materials to accurately capture the influence of local morphology on excited state functionality. 

Structural and functional heterogeneity is a consequence of the weak noncovalent interactions that direct the formation of organic materials from solution precursors. While covalent tethering of solution-phase assemblies provides a compelling strategy to enhance intermolecular order, the effects of this tethering strategy on the formed solid-state materials remain unestablished. This work uses pump–probe microscopy to compare excited-state dynamics in thin films fabricated from tethered perylene bisimide assemblies to those fabricated from noncovalent assemblies. On average, tethered films exhibit faster and more homogeneous excited-state lifetimes, consistent with stronger and more uniform intermolecular coupling. Optical measurements of excited-state diffusion show that the tethered film has ∼75% faster transport than the control film. Kinetic Monte Carlo modeling suggests that the reduction of site energetic disorder is sufficient to quantitatively explain the difference in diffusion coefficients. These results provide strong support that covalent tethering is a promising strategy to enhance the structural and energetic ordering in molecular materials.