Calculating optical absorption spectra of thin polycrystalline films: Structural disorder and site-dependent van der Waals interaction

TL;DR

This paper introduces a novel method to calculate how molecular absorption spectra shift when transitioning from gas phase to crystalline form, emphasizing the role of structural disorder and site-dependent van der Waals interactions.

Contribution

The authors develop an extended dipole approximation to predict gas-to-crystal spectral shifts based on molecular position and dispersion effects, applicable to polycrystalline films.

Findings

Accurately predicts absorption spectrum shifts in PTCDI films.

Separates dispersion effects into molecule-specific and site-dependent factors.

Demonstrates the method's effectiveness with experimental data.

Abstract

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift $\Delta{\cal E}_m$ is mainly caused by dispersion effects and depends sensitively on the molecule's specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide $\Delta{\cal E}_m= -Q W_{m}$ in two factors where $Q$ depends only on the molecular species and accounts for all non-resonant electronic transitions contributing to the dispersion, while $W_m$ is a sum running over the position of all molecules expressing the site-dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylene-tetracarboxylic-diimide (PTCDI).