1,4-Dihydropyrrolo[3,2-b]pyrrole modified with dibenzooxazepine: A Highly Efficient Core for Charge-Transfer-Based OLED Emitters

The present work is focused on designing novel heteroaromatic systems, formally a hybrid of dibenzo[b,f]oxazepines and 1,4-dihydropyrrolo[3,2-b]pyrroles (DHPPs). Straightforward synthesis affords a family of rigid, centrosymmetric, π-expanded aromatic heterocycles amenable to facile post-functionalization. The rigidified molecular architecture is responsible for several key photophysical features including (1) the excellent blue colour purity (full width at half maximum parameter = 0.435 eV) and (2) stronger emission compared to analogous DHPPs capable of free rotation. Comparison of dyes possessing nitro groups at various positions reveals that if NO2 groups are located at distant positions the quadrupolar dye shows strong yellow fluorescence in non-polar solvents, whereas the same group at position 3 versus the DHPP core leads to a poorly emitting dye. Computational studies suggest that the key difference lies in relative energies of dark and bright excited states. It was shown that the DHPP core offers unique advantages as a high-emission energy system, serving as a foundation for charge-transfer (CT)-based efficient emitters. These features, combined with the ability to modulate electronic properties via peripheral functionalization, highlight the potential of the DHPP core in advanced optoelectronic devices, including new-generation OLEDs.