Tuning Luminescence Through Molecular Packing—Insights into Halogenated Emitters for Advanced Optoelectronics

We report on the photophysical properties of two polymorphs of 2,5-dioctyloxy-4-bromobenzaldehyde (Br8), focusing on their potential applications in organic light-emitting diodes (OLEDs). These materials can exhibit a combination of thermally activated delayed fluorescence (TADF) and phosphorescence, characteristic of halide-containing compounds. The fluorophores incorporate bromine atoms, whose high atomic number induces significant electron density enhancement in their vicinity, (the heavy atom effect), which can amplify the luminescence properties of organic molecules. Using a combination of experimental techniques, including UV-visible spectroscopy, photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), PL quantum yield (QY), and PL lifetime measurements, we thoroughly characterized the structure and optical properties of both polymorphs. The Br8-J polymorph adopts a triclinic crystal structure, while Br8-H adopts a monoclinic crystal structure, as determined by XRD analysis. These distinct polymorphic forms exhibit different packing arrangements, with J-aggregates displaying a head-to-tail arrangement and H-aggregates a face-to-face arrangement, significantly impacting their optical properties through variations in intermolecular distance and offset. The chirality of the molecular packing in Br8 polymorphs further influences their photophysical properties. The PL spectra show a single emission peak at 420 nm for Br8-H (H-aggregates) and 467 nm for Br8-J (J-aggregates). The PL QY of Br8-H (<5%) is significantly lower than that of Br8-J, which has a PL QY of up to 38%. Additionally, the valence band and optical conduction band measurements indicate that the energy levels are well-suited for the fabrication of blue OLEDs. These findings offer valuable insights into the structure and optimization of halogenated emitters, showcasing their suitability for advanced optoelectronic applications, including efficient blue OLEDs.