Ultrafast coherent exciton dynamics in size-controlled perylene bisimide aggregates

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.

Excitation transport in molecular aggregates with thermal motion

“…We however shall find that adi-arXiv:2002.09418v1 [physics.chem-ph] 21 Feb 2020 abatic excitation transport persists and thus intend to explore in the future to what extent adiabatic excitation transport can survive partial decoherence. Meanwhile, our results should already be applicable to some extent to particularly coherent molecular aggregates 22 such as those reported in 23,24 .…”

Real‐time Observation of Structural Dynamics Triggering Excimer Formation in a Perylene Bisimide Folda‐dimer by Ultrafast Time‐Domain Raman Spectroscopy

“…[44] Consequentially, we point out the crucial features of the excimer formation mechanism; Firstly, in numerous co-facial PBI stacks, including Bis-PBI, similar rate constants of excimer formation (k = ( � 200 fs) À 1 ) were observed irrespective of the external environment [21] (e.g., solvent polarity) and exciton coupling strengths. [13,18,20,23,29,45] Secondly, the time constant of the excimer formation corresponds to that of ultrafast structural rearrangement along the interchromophore coordinate. Thirdly, the excited states of Bis-PBI are not pure Frenkel or CT diabats (Tables S3-S4), but rather (quasi)adiabatic states which consist of different degrees of LE and CR configurations.…”

Real‐time Observation of Structural Dynamics Triggering Excimer Formation in a Perylene Bisimide Folda‐dimer by Ultrafast Time‐Domain Raman Spectroscopy

“…Consequentially, we point out the crucial features of the excimer formation mechanism; Firstly, in numerous co‐facial PBI stacks, including Bis‐PBI , similar rate constants of excimer formation ( k =(≈200 fs) −1 ) were observed irrespective of the external environment [21] (e.g., solvent polarity) and exciton coupling strengths [13, 18, 20, 23, 29, 45] . Secondly, the time constant of the excimer formation corresponds to that of ultrafast structural rearrangement along the interchromophore coordinate.…”

“…However, in contrast to the wealth of theoretical and experimental studies, the direct observation of the ultrafast structural dynamics triggering excimer formation has remained elusive so far. In previous works, several investigations on the structural information of the excimer state have been reported using femtosecond stimulated Raman spectroscopy (FSRS), [25] time‐resolved infrared spectroscopy (TR‐IR), [26–28] and TA spectroscopy [29–32] . However, these techniques have revealed some technical limits, such as non‐trivial background signals (FSRS) and a lack of target selectivity (TR‐IR and TA).…”