Photoluminescent property, mechanistic and crystallographic studies on novel (Z)-5,6-dihydrodibenzo[b,f]azocine AIEgens

Abstract

A series of four novel (Z)-5,6-dihydrodibenzo[b,f]azocine AIEgens (DHDBA-1–4) featuring a flexible non-aromatic eight-membered ring fused with two rigid aromatic rings were designed and synthesized. All of these π-conjugated polycyclics exhibited typical AIE properties with excellent solid-state fluorescence quantum yields ranging from 33.2 % to 67.6 %. Theoretical calculations revealed that DHDBA-1–4 adopt a unique tub conformation in solutions and their excitations have both locally excited (LE) and charge transfer (CT) characters. As exemplified by DHDBA-1, the low molar absorption coefficient and energetically accessible S₀/S₁ minimum energy conical intersection (MECI) were identified as the two major causes for the poor photoluminescent properties of DHDBA compounds in solution phase. In solid state, DHDBA compounds was found to adopt more planar conformations. The more effective excitation and prohibition of structural deformation that leads to S₀/S₁ surface crossing open the radiative decay channel. Crystallographic analysis illustrated that the flexibility of the central dihydroazocine core enables the DHDBA compounds to exist in multiple conformations in crystal lattice. The absence of π–π stacking and presence of abundant weak intermolecular interactions in the single crystals well explain their high solid-state fluorescence quantum yields.