Spiro-fused bicyclo[3,2,2] octatriene-cored triptycene: synthesis, molecular packing, and functional aggregates

Aggregate science has led to the existence of functional properties at the aggregate level that significantly differs from those of corresponding single-molecule species, leading to a number of novel conceptual materials with state-dependent functionality. For developing such materials, new molecular scaffolds that allow modulating the structural and functional properties of aggregates in a controlled manner could be extremely helpful. In this study, we report a class of spiro-fused bicyclo[3,2,2] octatriene triptycene derivatives with a unique 3D molecular scaffolding structure that display aggregation-induced emission (AIE) activity with bright solid-state blue luminescence. The core scaffold features a Y-shaped bicyclo[3,2,2]octatriene backbone and a vertical crossover arrangement of adjacent spiro-conjugated rings. Unlike classic bicyclo[2,2,2]octatrienes with high rigidity, this stereoscopic bicyclo[3,2,2]octatriene containing a seven-membered ring allows retaining certain flexibility in the molecular scaffold, which results in a weak fluorescence in dilute solutions (ΦF < 10%). However, the corresponding molecular aggregates emit intense fluorescence (ΦF = 50%–5%) and show prominent AIE activity because of the restriction of intramolecular scaffold motions. Moreover, in principle, these stereoscopic scaffold constructions with partially restricted flexibility facilitate the assembly of multiple different crystalline packing structures in both porous and nonporous forms depending on the growth conditions of crystalline aggregates. The obtained crystals with regular voids (12.85 × 6.89 A each) show reversible luminescence in response to the adsorption/desorption of iodine vapor. These materials based on an unusual 3D scaffold exhibit a high degree of structural modulation at the aggregate level and demonstrate potential applications in chemical sensing and flexible optoelectronics, which may open up the exploration of highly stereoscopic molecular building blocks for developing functional aggregates.