Crystallization-Induced Emission Enhancement of a Simple Tolane-Based Mesogenic Luminogen

The photophysical properties of a diphenylacetylene derivative (FOEB) were investigated. The fluorescence quantum efficiency (ΦF, %) of FOEB changes from moderate (49%) to low (9%) and to evidently enhanced (60%) in dilute solution, amorphous solid, and crystal state, respectively. This is a typical phenomenon of crystallization-induced emission enhancement (CIEE). The diphenylacetylene luminogen can be viewed as a propeller-like molecule with two blades (phenyl groups). In solution, the intramolecular rotations of the two blades around the ethynyl unit are activated and partially dissipate the energy in the photoexcited state, thereby leading to emission quenching. Consequently, FOEB exhibits moderate ΦF in solution. In amorphous solid, the intramolecular rotations are restricted, and a higher ΦF could be expected. But in fact, the strong π–π interactions between the luminogens cause heavier emission quenching, and the ΦF of the molecular aggregates is as low as 9%. In the single crystal, as the crystall...