The crucial role of planar π-conjugated segment in the molecular conformation of diphenyldibenzofulvene derivatives in crystals contributing to reducing emission energy

Aggregation-induced emission luminogens (AIEgens) have been treated as one kind of the most important materials utilized in applications including organic light-emitting diodes, sensors, biological therapies, and so on, owing to the intense emission in aggregated states. However, the optical performance of AIEgens seems do not simply abide by the empirical principle that the expanded π-conjugated molecular conformations acquire the emission with lower energy. The unexpected photophysical properties of AIEgens make molecular conformation design more difficult. Herein, to unveil the crucial factors dominating the optical performance of AIEgens, a series of diphenyldibenzofulvene (DPDBF) derivatives in crystals are utilized. After systematically analyzing the impact factors including molecular conformation parameters and intermolecular coupling, and further discussing with the calculation results by Gaussian 16, the emission energy of DPDBF derivatives in crystals is assigned to the tight connection with the conformation of the planar π-conjugated segment, but not the conformation torsion of phenyl blades or the intermolecular coupling. Although the expanded π-conjugated molecular conformation of the mentioned DPDBF derivatives does decline the energy gap between HOMO to LUMO to some degree, the Stokes shift effect which is dominantly impacted by the conformation of planar π-conjugated moiety can realize to adjust the emission energy in a much more efficient way.