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 crystallographic data revealed, the two phenyl groups in the same diphenylacetylene unit are highly coplanar and the conjugation planes of the two adjacent diphenylacetylene units are orthogonal to each other. Such molecular conformation and chromophore packing eliminate the intermolecular π–π interaction thereby greatly reducing the π–π interaction that caused fluorescence quenching, and meanwhile, the orthogonal packing allows the formation of C–H···π bonding between the adjacent phenyl groups to lock the intramolecular rotations and retain fluorescence. As a result, the FOEB single crystal shows evidently enhanced emission. Besides the CIEE property, FOEB demonstrates good liquid crystal property due to the diphenylacetylene mesogen and solvatochromic and mechnochromic effects due to the modification of diphenylacetylene with introduction of electron donor and acceptor functionalities.