Electronic Structure and Vibrational Properties of Indenotetracene-Based Crystal

Abstract

Asymmetrically substituted indenotetracene crystals are promising nonfullerene electron transport materials for organic photovoltaics, offering potential improvements in efficiency and stability. In this work, we present a first-principle investigation of the electronic and vibrational properties of a diarylindenotetracene system functionalized with two methoxy groups (hereafter DimethoxyASI). Single-crystal X-ray diffraction analysis [reported in J. Org. Chem. 2018, 83, 4, 1828] reveals a monoclinic ${\text{P2}}_1/\text{c}$ structure with an interplanar distance of 3.76 Å, providing insight into the molecular packing and intermolecular interactions that govern the solid-state organization. Notably, for the first time, in this work we identify two distinct dimeric species within the crystalline lattice by a structural and electronic analysis, each exhibiting different intermolecular arrangements that significantly influence both the electronic structure and vibrational properties of the material. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations provide insight into the molecular packing, electronic states, and vibrational characteristics of the crystal. The theoretical absorption spectrum, obtained from TDDFT calculations, features three main electronic transitions centered at 530, 360, and 275 nm, displaying a mixed character of localized excitations and charge-transfer contributions. The vibrational properties, investigated through phonon density of states calculations at the DFT level, highlight well-defined spectral features. While most vibrational modes remain consistent between monomeric and dimeric configurations, significant deviations emerge in the low-frequency region, where intermolecular interactions and crystal packing effects play a crucial role. Furthermore, the two dimeric species exhibit distinct electronic properties beyond their geometric differences. A key distinguishing factor is the transition electric dipole moments (TEDMs), which governs the probability and polarization of electronic transitions. Our analysis reveals that the TEDMs magnitude and orientation vary significantly between the two dimeric species, suggesting that they may interact differently with polarized light. These differences provide new insight into the role of molecular aggregation in shaping the optical response of organic semiconductors and highlight the impact of polymorphism on their electronic properties. Overall, this study underscores the intricate relationship between molecular packing, electronic structure, and vibrational properties in indenotetracene-based materials, contributing to a deeper understanding of their potential applications in optoelectronic devices.