Ultrafast coherent exciton dynamics in size-controlled perylene bisimide aggregates.

Seongsoo Kang,Agnieszka Nowak-Król,Dongho Kim,Woojae Kim,Christina Kaufmann,Yongseok Hong,Frank Würthner

doi:10.1063/1.5124148

Abstract

For H-aggregates of perylene bisimide (PBI), it has been reported that upon photoexcitation, an initially delocalized Frenkel exciton is localized by excimer formation. However, in recent studies, the beforehand exciton dynamics preceding the excimer formation was suggested in larger aggregates consisting of at least more than 10-PBI subunits, which was not observed in small aggregates comprising less than four-PBI subunits. This feature implies that the size of molecular aggregates plays a crucial role in the initial exciton dynamics. In this regard, we have tried to unveil the initial exciton dynamics in PBI H-aggregates by tracking down the transient reorientations of electronic transition dipoles formed by interactions between the PBI subunits in systematically size-controlled PBI H-aggregates. The ultrafast coherent exciton dynamics depending on the molecular aggregate sizes can be distinguished using polarization-dependent femtosecond-transient absorption anisotropy spectroscopic measurements with a time resolution of ∼40 fs. The ultrafast decay profiles of the anisotropy values are unaffected by vibrational relaxation and rotational diffusion processes; hence, the coherent exciton dynamics of the PBI H-aggregates prior to the excimer formation can be directly revealed as the energy migration processes along the PBI H-aggregates.

Highlights

When perylene bisimide (PBI) aggregates are formed in highly concentrated solutions in a poorly solvating solvent, the absorption spectra of a series of PBI aggregates exhibit a hypsochromic shift and broad spectral features resulting from the electronic interactions between the transition dipoles along the long axis of the p-stacked monomer
The reversed intensities of the 0–0 and 0–1 bands in the absorption spectrum of the PBI aggregates well reflect the excitonic characteristics of the H-aggregates
The PBI H-aggregates clearly exhibit different spectral features in the steady-state absorption and emission spectra compared to the monomer PBI, a disparity according to the aggregate lengthdependence could hardly be distinguished by the steady-state spectral features except for the intensity ratio values of the A0–0 and A0–1 bands (A0–0/A0–1) in the absorption spectra

Summary

Introduction

Supramolecular dye assemblies are drawing considerable attention among numerous research groups owing to the possibilities of promising applications in the fields of photonics, optoelectronics, and organic photovoltaics. In particular, long-lived Frenkel excitons in H-aggregates forming a cofacile organization play an important role in the efficient energy transfer preceding energy loss through exciton relaxation processes. Among several representative molecular entities in supramolecular dye assemblies available as a medium for the efficient energy transfer, perylene bisimide (PBI) dyes have been considered as suitable candidates for artificial light-harvesting applications since PBI dyes show a strong thermodynamic driving force for p–p stacking into one-dimensional H-aggregates by monomeric p-p stacks as well as remarkable photostability and high fluorescence quantum yields. it has been challenging to reveal the fundamental exciton dynamics because of the formation of a low energy trapping state in PBI aggregates, which was commonly called as the excimer state, exciton localized state. Among several representative molecular entities in supramolecular dye assemblies available as a medium for the efficient energy transfer, perylene bisimide (PBI) dyes have been considered as suitable candidates for artificial light-harvesting applications since PBI dyes show a strong thermodynamic driving force for p–p stacking into one-dimensional H-aggregates by monomeric p-p stacks as well as remarkable photostability and high fluorescence quantum yields.. It has been challenging to reveal the fundamental exciton dynamics because of the formation of a low energy trapping state in PBI aggregates, which was commonly called as the excimer state, exciton localized state. Many studies reported that the localization processes of the initially generated exciton occur on an ultrafast time scale competing with the energy transfer processes.. Spectroscopic investigation on an ultrafast time scale prior to excimer formation can provide further insights into the fate of primarily generated excitons. Understanding the ultrafast exciton dynamics in molecular H-aggregates is highly important for advances in material performance

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