Probing Excited State Delocalization and Charge Separation in Hierarchical Perylene Diimide Materials with Time-Resolved Broadband Microscopy

Abstract

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.

Electronic Couplings versus Thermal Fluctuations in the Internal Conversion of Perylene Diimides: The Battle to Localize the Exciton

“…Perylene diimide (PDI)-based oligomers feature highly efficient light absorption and tunable optical and electronic properties − resulting in their applications as materials for organic light emitting devices, organic field effect transistors, and organic solar cells, building blocks for functional supramolecular architectures, and nonfullerene materials among others. − Studies of excited state dynamics of PDI-related organic compounds in thin films provide important insights into the exciton formation and decay dynamics, revealing that initial photoinduced delocalized excitons experience ultrafast exciton decay within hundreds of femtoseconds into spatially confined trap sites . Moreover, PDI oligomers have been intensely studied, as the different association motifs and morphologies influence the optical and electronic properties. , Experimental and theoretical studies performed on PDI-based materials and oligomers have also analyzed how structural geometry relaxation and flexibility at ambient temperatures counteract initial exciton delocalization. − Spectroscopic properties of PDI-based materials can be modulated by introducing different substitution patterns or using different linkers connecting PDI units. − Effects of conformational disorder, solvation, energy, and charge-transfer excitations on absorption spectrum and photoinduced dynamics of PDI monomers and dimers have been previously explored using a linear vibronic coupling (LVC) Hamiltonians. , …”