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X-ray Crystallography

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Excimer formation in zinc-phthalocyanine revealed using ultrafast electron diffraction.

Sebastian Hammer^1,2, Tristan L Britt^1,2, Laurenz Kremeyer^1,2

¹Department of Physics, Centre for the Physics of Materials, McGill University, Montreal, QC H3A OB8, Canada.

Proceedings of the National Academy of Sciences of the United States of America

|December 10, 2024

Summary

Excited dimer (excimer) formation in organic semiconductors was studied using ultrafast electron diffraction. This technique revealed a two-step process involving dimerization and structural rearrangement, crucial for understanding excimer dynamics.

Keywords:

excimer organic semiconductors structural dynamics ultrafast electron diffraction zinc-phthalocyanine

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Area of Science:

Materials Science
Physical Chemistry
Organic Electronics

Background:

Excimers are long-lived, low-energy excited dimer states critical in organic molecular semiconductors.
Their formation involves significant intermolecular geometric reorganization, differentiating them from Frenkel exciton-polarons.

Purpose of the Study:

To investigate the dynamics of solid-state excimer formation in organic semiconductors.
To reveal the key reaction modes and final structure of the emitting state using ultrafast electron diffraction.
To study zinc-phthalocyanine (ZnPc) as a model excimeric system.

Main Methods:

Ultrafast electron diffraction (UED) was employed to probe the real-time dynamics of excimer formation.
Polycrystalline thin films of zinc-phthalocyanine (ZnPc) in its α-phase were used as the model system.
Partially fluorinated ZnPc derivatives were also studied to assess the impact of fluorination.

Main Results:

Excimer formation occurs in a two-step process: rapid dimerization (approx. 0.4 ps) followed by a slower shear-twist motion (14 ps).
This motion leads to π-system alignment and the final excimer structure, persisting beyond 300 ps.
Increasing fluorination in ZnPc derivatives slows down the excimer formation kinetics while maintaining the same excimer geometry.

Conclusions:

Ultrafast electron diffraction is a powerful tool for observing excimer formation dynamics in solid-state organic materials.
The identified two-step mechanism and associated timescales provide fundamental insights into excimer stabilization.
Fluorination level influences excimer formation rates, offering potential for tuning optoelectronic properties.