Excitons in Organics Using Time-Dependent Density Functional Theory: PPV, Pentacene, and Picene.

Area of Science:

• Computational physics and chemistry
• Materials science
• Condensed matter physics

Background:

• Organic polymers and molecular crystals are crucial in materials science.
• Understanding their optical properties, particularly excitonic effects, is key to developing new electronic devices.
• Time-dependent density functional theory (TD-DFT) is a powerful tool for studying excited states.

Purpose of the Study:

• To investigate the optical absorption spectra of polyphenylenevinylene (PPV), picene, and pentacene using TD-DFT.
• To analyze the role of excitonic effects and local-field effects in these materials.
• To validate the bootstrap kernel approximation within TD-DFT for these systems.

Main Methods:

• Application of the bootstrap kernel approximation within time-dependent density functional theory (TD-DFT).
• Computational study of the one-dimensional polymer poly(p-phenylenevinylene) (PPV).
• Computational study of the molecular crystals picene and pentacene.

Main Results:

• The calculated absorption spectrum for PPV shows a well-reproduced bound excitonic peak.
• Remarkably different excitonic physics between electronically similar picene and pentacene is accurately captured.
• Inclusion of local-field effects significantly alters the spectra of picene and pentacene, but not PPV.

Conclusions:

• The bootstrap kernel TD-DFT method is effective for studying the optical properties of organic materials.
• Local-field effects play a critical role in the excitonic spectra of molecular crystals like picene and pentacene.
• The distinct excitonic behaviors of picene and pentacene can be computationally resolved.