Effect of Methoxy Substitution on Supramolecular Arrangement of Borondifluorides of 2'-Hydroxy Chalcones and their Solid-State NIR Fluorescence

Abstract

This work describes the borondifluoride complexes of 2'-hydroxychalcone derivatives, 2 and 3, differently substituted with methoxy groups. Their optical study in dichloromethane reveals similar UV-vis absorption and fluorescence emission features in the visible spectral region that are close to those of the parent compound 1. While fluorescence quantum yields Φf are very low in solution, they are higher in the solid state, showing that aggregation-induced enhanced emission occurs. Remarkably, the solid-state emission spectra are strongly red-shifted reaching the near infrared (NIR) spectral range. Molecular structures of 2 and 3 were determined by single crystal X-ray diffraction. The data show the occurrence of tightly packed arrangements that favor strong π-π overlap between neighboring molecules. Such situation allows the generation of promising NIR emitting materials, such as compound 3 emitting at 784 nm with Φf = 0.07 that stands as one of the best NIR solidstate organic emitters.