Crystal growth, HOMO-LUMO engineering and charge-transfer emission of TCNB-based donor-acceptor cocrystals

Abstract

Using carbazole (CARB), dibenzothiophene (DBZP) and dibenzofuran (DBZF) as electron donors, 1,2,4,5-tetracyanobenzene (TCNB) as electron acceptor, three typical types of luminescent organic cocrystals were synthesized by a simple supramolecular assembly strategy. The cocrystals exhibit interesting optical properties for its charge transfer (CT) interactions. Meanwhile, a series of three-component (CARB)x (DBZP)1-x (TCNB) crystalline microtubes comprising organic CT interactions have been prepared by the self-assembly with poor solvent co-precipitating method. DFT calculations were used to predict the CT interactions and the binding patterns of donor and acceptor, which suggest that the CT interactions can largely influence the energy level and orbital distributions. Moreover, the SXRD analyses verified the cocrystals are constructed by π-π stacking with a face-to-face model. The solid-state cocrystals and microtubes were well characterized by PXRD, UV absorption spectra, emission spectra, fluorescence lifetime and quantum yield analyses. The obvious red-shift in UV–vis absorption spectra and emission spectra with respect to the free monomer indicate the formation of CT interactions. The emission colors of the mixed microtubes can be tailored from green to orange as well as associated photophysical properties with the increasing of dopant concentrations. Therefore, by the combination of experimental and theoretical studies on the cocrystals and microtubes, this work not only reports the new types of crystalline micro-nano materials, but also provides an in-depth understanding of the relationship between structure and fluorescence effect.