Realizing long range π-conjugation in phenanthrene and phenanthrene-based molecular crystals for anomalous piezoluminescence.

Unlike the known aggregation-caused quenching (ACQ) that the enhancement of π-π interactions in rigid organic molecules usually decreases the luminescent emission, here we show that an intermolecular "head-to-head" π-π interaction in the phenanthrene crystal, forming the so-called "transannular effect", could result in a higher degree of electron delocalization and thus photoluminescent emission enhancement. Such a transannular effect is molecular configuration and stacking dependent, which is absent in the isomers of phenanthrene but can be realized again in the designed phenanthrene-based cocrystals. The transannular effect becomes more significant upon compression and causes anomalous piezoluminescent enhancement in the crystals. Our findings thus provide new insights into the effects of π-π interactions on luminescence emission and also offer new pathways for designing efficient aggregation-induced emission (AIE) materials to advance their applications.