Modification of π–π Interaction and Charge Transfer in Ratiometric Cocrystals: Amplified Spontaneous Emission and Near-Infrared Luminescence

Photofunctional cocrystals have been paid much attention due to their applications in multicolor displays, two-photon emission, and sensors. It has been known that the selection of different assembled units in cocrystals may result in different intermolecular interactions and photophysics; however, how to tailor these interactions and properties based on the same assembled units has been scarcely detected to date. In this work, we report that the cocrystallization of 4,4′-bis( N -carbazolyl)-1,1′-biphenyl (CBP) and 1,2,4,5-tetracyanobenzene (TCNB) with different ratios (1:1 and 1:2) can lead to different π–π interactions and charge transfer effects, which further result in tunable fluorescence. Compared with the pristine CBP with blue emission, the cocrystal CBP@TCNB (1:1) exhibits typical red fluorescence, accompanied by amplified spontaneous emission, and CBP@TCNB (1:2) presents near-infrared luminescence. Crystal morphology prediction and scanning electron microscopy observations show that the ratiometric cocrystals present different crystal morphologies with typical two-dimensional sheet and one-dimensional rod structures. Therefore, this work supplies an effective means to tune photophysical properties and morphologies of molecular crystals, which further gives insights into intermolecular interactions between different ratios of self-assembled units in cocrystals.