Can Fluorenone-Based Compounds Emit in the Blue Region? Impact of the Conjugation Length and the Ground-State Aggregation

Fluorenone-based compounds usually emit light in the yellow-red region. While different origins have been assigned to these colors, only a few studies make a direct correlation between the characteristic emission and the lowest absorption energy band (LEB). Intriguingly, striking similarities can be observed in the position, shape, and intensity of the LEB for such compounds with different molecular sizes. This work is focused on these aspects of absorption and emission spectra, by means of density functional theory (DFT) calculations and experimental characterizations of a series of fluorenone-centered compounds of increasing size. Our results show that while the absorption LEB is intrinsically related to a π–π* transition between orbitals strongly localized on the fluorenone core, its position and intensity are affected by the tendency of these compounds to associate/aggregate in their ground state even in solution. The intermolecular CO···HC (aromatic) hydrogen bonds (H-bonds) redshift the absorption maximum position and reduce the intensity of the intramolecular π–π* transition. Under some circumstances these effects disappear, in close correlation with the disappearance of the characteristic absorption LEB. Bright and intense blue emission is then obtained in THF solution instead of the strongly quenched yellow-orange one, indicating that blue constitutes the intrinsic emission color of fluorenone-based compounds.