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

TL;DR

This study investigates how different substituents on pyrene crystals influence their crystal structure and electronic properties, revealing that molecular packing significantly affects electronic dispersion and potential charge mobility.

Contribution

It provides detailed insights into the relationship between crystal packing, electronic dispersion, and photophysical defects in substituted pyrene crystals, highlighting the role of co-facial π-electron overlap.

Findings

Both derivatives exhibit high valence band dispersion (0.40-0.45 eV).

Electronic dispersion aligns with crystal structure differences caused by substituents.

Localized trap states are linked to excimer-like photophysical defects.

Abstract

We examine the interdependence of structural and electronic properties of two substituted pyrene crystals by means of combined spectroscopic probes and density-functional theory calculations. One derivative features n-hexyl side groups, while the other one contains branched silanyl groups. Both derivatives form triclinic crystal structures when grown from solution, but the electronic dispersion behavior is significantly different due to differences in $\pi$-$\pi$ overlap along the $a$ crystal axis. Both systems display dispersion of 0.40-0.45 eV in the valence band, suggesting a high intrinsic hole mobility. However, the dispersion is primarily along the a-axis in the silanyl-substituted derivative, but less aligned with this crystal axis in the hexyl-substituted material. This is a direct consequence of the diferences in co-facial $\pi$ electron overlap revealed by the crystallographic studies. We find that photophysical defects, ascribed to excimer-like states, point to the importance of localized trap states. Substituted pyrenes are useful model systems to unravel the interplay of crystal structure and electronic properties in organic semiconductors.