Large electronic bandwidth in solution-processable pyrene crystals: the role of close-packed crystal structure

Area of Science:

• Materials Science
• Solid-State Physics
• Organic Electronics

Background:

• Substituted pyrenes serve as model systems for understanding organic semiconductors.
• Crystal structure and electronic properties are interdependent in these materials.
• Steric effects of side groups influence molecular packing and electronic behavior.

Purpose of the Study:

• To investigate how different side groups (n-hexyl vs. trimethylsilyl) affect the crystal structure and electronic properties of 1,3,6,8-tetraalkynylpyrene derivatives.
• To elucidate the relationship between π-π overlap, crystal packing, and charge transport characteristics.
• To identify the role of photophysical defects in localized trap states.

Main Methods:

• Combined spectroscopic probes.
• Density-functional theory (DFT) calculations.
• Crystallographic studies of solution-grown crystals.

Main Results:

• Both derivatives form triclinic crystal structures.
• Significant differences in electronic band dispersion (around 0.45 eV) were observed due to varying π-π overlap.
• Dispersion was primarily along the π-stacking axis for trimethylsilyl-substituted pyrene, but less aligned for n-hexyl-substituted pyrene.
• Photophysical defects, attributed to excimer-like states, indicate the presence of localized trap states.

Conclusions:

• Side group sterics critically influence crystal packing and π-electron overlap in substituted pyrenes.
• The directionality of electronic dispersion is directly linked to the co-facial π overlap, impacting charge mobility.
• Localized trap states arising from defects are significant for the photophysical properties of these organic semiconductors.