Less is more: on the effect of benzannulation on the solid-state emission of difluoroborates

Abstract

We investigate the emission properties of four organic dyes containing a strong electron-donating N(CH₃)₂ group and an NBF₂O-bearing heterocyclic moiety acting as the electron-accepting group. The four studied compounds differ in the number and positions of the fused benzo rings included in the heterocyclic moiety. They exhibit strong emission in solution, with fluorescence quantum yields (Φ_f) systematically exceeding 0.8 at least in one of the solvents used, regardless of the benzannulation architecture. The strong dipolar character, achieved by substitution with the N(CH₃)₂ group and benzannulation, enhances the photoinduced charge transfer and appears to be an effective strategy to tune the photophysical properties of these dyes in solution. Indeed, red-shifted absorption spectra are obtained without deteriorating the emission properties. However, the present joint theory–experiment study clearly demonstrates that such a molecular design is not effective for solid-state applications, as only one derivative still exhibits significant emission in the crystalline form, namely the most compact one. We show that the combination of benzannulation in the presence of the strong amino donor leads to substantial changes in crystal packing and that a different network of intermolecular interactions can be found in the crystal. More specifically, going from the parent compound to its benzannulated derivatives induces a stronger π⋯π stacking combined with multiple CH⋯π interactions involving the fused benzo rings and the hydrogen atoms of the dimethylamino group, which impedes efficient emission of the crystals.