Synthesis and Characterization of Electron-Deficient Asymmetrically Substituted Diarylindenotetracenes

Electron-deficient asymmetrically substituted diarylindenotetracenes were prepared via a series of Friedel–Crafts acylations, aryl–aryl cross-couplings, and an intramolecular oxidative cyclization to form the indene ring. Single-crystal X-ray experiments showed good π–π overlap with π–π distances ranging from 3.26 to 3.76 Å. Both thermogravimetric analysis and differential scanning calorimetry indicated that asymmetrically substituted indenotetracenes (ASIs) are stable at elevated temperatures. From cyclic voltammetry experiments, HOMO/LUMO energy levels of ASI derivatives were determined to be near −5.4/–4.0 eV. UV/visible absorption spectra showed strong absorption of light between 400 and 650 nm with molar attenuation coefficients from 10⁴ to 10⁵ M^–1 cm^–1. ASIs were also found to have very low fluorescence quantum yields, less than 4%. Using the solid-state packing determined from the single-crystal X-ray experiments, computational modeling indicated that ASI molecules should favor electron transport

Partial Fluorination as a Strategy for Crystal Engineering of Rubrene Derivatives

Through a close examination of the intermolecular interactions of rubrene (<b>1a</b>) and select derivatives (<b>1b</b>–<b>1p</b>), a clearer understanding of why certain fluorinated rubrene derivatives pack with planar tetracene backbones has been achieved. In this study we synthesized, crystallized, and determined the packing structure of new rubrene derivatives (<b>1h</b>–<b>p</b>). Previously, we proposed that introducing electron-withdrawing CF₃ substituents induced planarity by reducing intramolecular repulsion between the peripheral aryl groups (<b>1e</b>–<b>g</b>). However, we found that in most cases, further increasing the fluorine content of rubrene lead to twisted tetracene backbones in the solid state. To understand how rubrene (<b>1a</b>) and its derivatives (<b>1b</b>–<b>p</b>) pack in the solid state, we (re)­examined the crystal structures through a systematic study of the close contacts. We found that planar tetracene cores occur when close contacts organize to produce an <i>S</i> symmetry element about a given rubrene molecule. We report the first instance of rubrene derivatives (<b>1l</b> and <b>1n</b>) that pack in a two-dimensional brick motif. The prospects for new rubrene derivatives in semiconductors were estimated by calculating the reorganization energies of the monomers and transfer integrals of the dimers we observed. Our work allows for the rational design and improved crystal engineering of new rubrene derivatives