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.
Bibliographical noteFunding Information:
The work at Yonsei University was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2016R1E1A1A01943379). The quantum mechanical calculations were supported by the National Institute of Supercomputing and Network (NISN)/Korea Institute of Science and Technology Information (KISTI) with supercomputing resources including technical support (No. KSC-2018-CRE-0051).
The research at the Universit€at Wu€rzburg was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within research unit No. FOR 1809. The authors declare no competing financial interests.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics